U.S. patent application number 13/933732 was filed with the patent office on 2013-10-31 for process for preparing acrylic acid from ethanol and formaldehyde.
The applicant listed for this patent is BASF SE. Invention is credited to Stefan ALTWASSER, Stefanie HERZOG, Frank HUETTEN, Markus OTTENBACHER, Alexander SCHAEFER, Annebart Engbert WENTINK.
Application Number | 20130289305 13/933732 |
Document ID | / |
Family ID | 44658736 |
Filed Date | 2013-10-31 |
United States Patent
Application |
20130289305 |
Kind Code |
A1 |
HERZOG; Stefanie ; et
al. |
October 31, 2013 |
PROCESS FOR PREPARING ACRYLIC ACID FROM ETHANOL AND
FORMALDEHYDE
Abstract
A process for preparing acrylic acid from ethanol and
formaldehyde, in which, in a reaction zone A, the ethanol is
partially oxidized to acetic acid in a heterogeneously catalyzed
gas phase reaction, the product gas mixture A obtained and a
formaldehyde source are used to obtain a reaction gas input mixture
B which comprises acetic acid and formaldehyde and has the acetic
acid in excess over the formaldehyde, and the formaldehyde present
in reaction gas input mixture B is aldol-condensed with acetic acid
present in reaction gas input mixture B to acrylic acid under
heterogeneous catalysis in a reaction zone B, and unconverted
acetic acid still present along-side the acrylic acid target
product in the product gas mixture B obtained is removed therefrom,
and the acetic acid removed is recycled into the production of
reaction gas input mixture B.
Inventors: |
HERZOG; Stefanie; (Berlin,
DE) ; ALTWASSER; Stefan; (Wachenheim, DE) ;
OTTENBACHER; Markus; (Wilhelmsfeld, DE) ; HUETTEN;
Frank; (Mannheim, DE) ; WENTINK; Annebart
Engbert; (Mannheim, DE) ; SCHAEFER; Alexander;
(Limburgerhof, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BASF SE |
Ludwigshafen |
|
DE |
|
|
Family ID: |
44658736 |
Appl. No.: |
13/933732 |
Filed: |
July 2, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13232380 |
Sep 14, 2011 |
8507721 |
|
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13933732 |
|
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|
61383358 |
Sep 16, 2010 |
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Current U.S.
Class: |
562/598 |
Current CPC
Class: |
C07C 51/235 20130101;
C07C 57/04 20130101; C07C 51/353 20130101; C07C 51/235 20130101;
C07C 57/04 20130101; C07B 2200/05 20130101; C07C 53/08 20130101;
C07C 51/353 20130101 |
Class at
Publication: |
562/598 |
International
Class: |
C07C 57/04 20060101
C07C057/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 16, 2010 |
DE |
102010040923.5 |
Claims
1-21. (canceled)
22. Acrylic acid for which the ratio V of the molar amount
n.sup.14C of .sup.14C atomic nuclei present in this acrylic acid to
the molar amount n.sup.12C of .sup.12C atomic nuclei present in the
same acrylic acid, V=n.sup.14C:n.sup.12C, is greater than 0 and
less than the corresponding molar ratio V* of .sup.14C atomic
nuclei to .sup.12C atomic nuclei present in the carbon dioxide in
the earth's atmosphere.
23. Acrylic acid according to claim 22, wherein V=(2/3)V*.
24. A liquid phase P comprising at least 1 kg of acrylic acid,
wherein the acrylic acid present is an acrylic acid according to
claim 22.
Description
[0001] The present invention relates to a process for preparing
acrylic acid from ethanol and formaldehyde. The present invention
also relates to the preparation of conversion products from acrylic
acid thus obtained.
[0002] At present, acrylic acid is prepared on the industrial scale
essentially exclusively by heterogeneously catalyzed two-stage
partial oxidation of propylene (see, for example, DE-A 103 36
386).
[0003] One advantage of this procedure is that it has a
comparatively high target product selectivity based on propylene
converted, which, in the case of recycling of propylene unconverted
in single pass, enables high acrylic acid yields from the propylene
used. Furthermore, propylene has extremely economically viable
backward integration to the base fossil raw material, mineral oil
(i.e. propylene can be produced from mineral oil with comparatively
low production costs), which enables inexpensive acrylic acid
preparation overall.
[0004] In view of the foreseeable shortage in the fossil resource
of mineral oil, there will, however, be a need in the future for
processes for preparing acrylic acid from raw materials, which can
be performed in a comparatively economically viable manner even
without backward integration thereof to the base fossil raw
material, mineral oil, and which at the same time have backward
integration of the raw materials thereof to base raw materials
whose lifetimes extend beyond that of mineral oil.
[0005] WO 2005/093010 considers propylene itself to be such a raw
material. It proposes continuing, in the future, with the two-stage
heterogeneously catalyzed partial gas phase oxidation of propylene
to acrylic acid, but obtaining the propylene required from
methanol. The advantage of such a procedure is that methanol is
obtainable both proceeding from base fossil raw materials such as
coal, for example brown coal and hard coal; cf., for example, WO
2010/072424) and natural gas (cf., for example, WO 2010/067945),
both of which have a much longer lifetime than mineral oil, and
proceeding from the renewable base raw material of biomass, and
also directly from the carbon dioxide present in the earth's
atmosphere (in each case optionally with additional use of steam or
molecular hydrogen) (cf., for example, G. A. Olah et al., Beyond
Oil and Gas; The Methanol Economy, Wiley-VCH, 2009).
[0006] However, a disadvantage of the procedure proposed in WO
2005/093010 is that the selectivity of obtaining propylene
proceeding from methanol with the preparation processes currently
known, based on methanol converted, is less than 70 mol %, which is
unsatisfactory (in addition to propylene, for example, ethylene and
butylene are also formed).
[0007] In this document, base fossil raw materials shall be
understood to mean base raw materials which, like brown coal, hard
coal, natural gas and mineral oil, for example, originate from
degradation products of dead plants and dead animals in geological
prehistory.
[0008] In contrast, in this document, renewable raw materials shall
be understood to mean those raw materials which are obtained from
fresh biomass, i.e. from (new) vegetable and animal material which
is being newly grown (in the present) and will be grown in the
future.
[0009] There have also already been proposals (for example in WO
2008/023040) to prepare acrylic acid and the conversion products
thereof proceeding from the renewable raw material glycerol.
[0010] However, a disadvantage of such a procedure is that glycerol
is obtainable economically as a renewable raw material essentially
only as a coproduct of biodiesel production. This is
disadvantageous in that the current energy balance of biodiesel
production is unsatisfactory.
[0011] Furthermore, the prior art has proposed the preparation of
acrylic acid from propane (for example in DE-A 102006024901), which
constitutes a raw constituent of natural gas. However, a
disadvantage of such a method of preparation of acrylic acid is
firstly the comparatively high unreactiveness of propane, and the
fact that propane also constitutes a sought-after energy carrier
with good manageability.
[0012] It was therefore an object of the present invention to
provide an alternative process for preparing acrylic acid, which
does not have the described disadvantages of the prior art
processes, and especially has a satisfactory selectivity of target
product formation proceeding from the raw materials used for
preparation thereof.
[0013] Accordingly, a process for preparing acrylic acid from
ethanol and formaldehyde is provided.
[0014] The appeal of such a procedure is firstly that ethanol is
the most sustainable renewable raw material (ethanol is formed by a
natural route in the fermentation of glucose-comprising biomass;
however, it is also possible to proceed from starch- and
cellulose-comprising biomass, by using an upstream enzymatic or
acidic conversion process which converts the starch and cellulose
types to glucose; cf., for example, WO 2010/092819).
[0015] In principle, ethanol, however, is also obtainable on the
industrial scale by reacting water with ethylene with addition of
catalysts such as sulfuric acid, or phosphoric acid applied to
starch, at temperatures of about 300.degree. C. and pressures of
around 70 bar, with the advantage that ethylene has close backward
integration to fossil resources such as natural gas and coal, which
have a longer lifetime than mineral oil (e.g. Chemie Ingenieur
Technik--CIT, Volume 82, pages 201-213, Issue 3, Published Online
on Feb. 9, 2010, Wiley--VCH Verlag Weinheim, "Alternative
Synthesewege zum Ethylen" ["Alternative synthesis routes to
ethylene"], A. Behr, A. Kleyentreiber and H. Hertge).
[0016] A further advantage of the inventive procedure is based on
the fact that formaldehyde is obtainable by partial oxidation of
methanol (e.g. Catalysis Review 47, pages 125 to 174, 2004; EP-A
2213370; WO 2010/022923; DE-A 2334981; DE-A 1618413; WO
2009/149809; DE-A 2145851; WO 2005/063375; WO 03/053556; WO
2010/034480; WO 2007/059974; DE-A 102008059701 and Ullmann's
Encyclopedia of Industrial Chemistry, Fifth, Completely Revised
Edition, Volume A 11, 1988 pages 624 to 627), and methanol can be
obtained via synthesis gas (gas mixtures of carbon monoxide and
molecular hydrogen) in principle from all carbonaceous base fossil
fuels and all carbonaceous renewable raw materials (as in the case
of methane, the molecular hydrogen required (a process for
obtaining methane from biogas or biomass is described, for example,
by DE-A 102008060310 or EP-A 2220004) may already be present in the
carbon carrier; an alternative hydrogen source available is water,
from which molecular hydrogen can be obtained, for example, by
means of electrolysis; the oxygen source is generally air; cf., for
example, WO 10-060236 and WO 10-060279). A suitable renewable
carbonaceous raw material is, for example, lignocellulose for
synthesis gas production (cf., for example, WO 10-062936). It is
also possible to obtain synthesis gas by coupling the pyrolysis of
biomass directly with steam reforming.
[0017] The present invention thus provides a process for preparing
acrylic acid from ethanol and formaldehyde, which comprises the
following measures: [0018] a stream of a reaction gas input mixture
A comprising the ethanol and molecular oxygen reactants and at
least one inert diluent gas other than steam is conducted through a
first reaction zone A charged with at least one oxidation catalyst
A and, in the course of passage through reaction zone A, ethanol
present in the reaction gas input mixture A is oxidized under
heterogeneous catalysis to acetic acid and steam so as to form a
product gas mixture A comprising acetic acid, steam, molecular
oxygen and at least one inert diluent gas other than steam, and a
stream of product gas mixture A leaves reaction zone A, it
optionally being possible to supply further molecular oxygen and/or
further inert diluent gas to the reaction gas mixture A flowing
through reaction zone A on its way through reaction zone A, [0019]
a stream of a reaction gas input mixture B which comprises acetic
acid, steam, molecular oxygen, at least one inert diluent gas other
than steam and formaldehyde and in which the molar amount n.sub.HAc
of acetic acid present is greater than the molar amount n.sub.Fd of
formaldehyde present therein is obtained from the stream of product
gas mixture A leaving reaction zone A (without having to carry out
a separation process thereon beforehand) and at least one further
stream comprising at least one formaldehyde source, [0020] the
stream of reaction gas input mixture B is passed through a second
reaction zone B charged with at least one aldol condensation
catalyst B and formaldehyde present in reaction gas input mixture
B, as it flows through reaction zone B, is condensed with acetic
acid present in reaction gas input mixture B under heterogeneous
catalysis to give acrylic acid and H.sub.2O, so as to form a
product gas mixture B comprising acrylic acid, acetic acid, steam,
molecular oxygen and at least one inert diluent gas other than
steam, and a stream of product gas mixture B leaves reaction zone
B, it optionally being possible to supply further molecular oxygen
and/or further inert diluent gas to the reaction gas mixture B
flowing through reaction zone B on its way through reaction zone B,
[0021] the stream of product gas mixture B leaving reaction zone B
is fed to a separation zone T and separated in separation zone T
into at least three streams X, Y and Z, [0022] the acrylic acid
flow present in stream X being greater than the acrylic acid flow
present in streams Y and Z together, [0023] the acetic acid flow
present in stream Y being greater than the acetic acid flow present
in streams X and Z together, [0024] the flow of inert diluent gas
other than steam present in stream Z being greater than the flow of
inert diluent gas other than steam present in streams X and Y
together, and [0025] stream Y is recycled into reaction zone B and
used to obtain reaction gas input mixture B.
[0026] Preferably in accordance with the invention, reaction zone A
is charged with at least one oxidation catalyst A comprising at
least one vanadium oxide.
[0027] More preferably, reaction zone A is charged with at least
one oxidation catalyst A which comprises at least one vanadium
oxide and, as well as the at least one vanadium oxide, additionally
comprises at least one oxide from the group of the oxides of
titanium, of aluminum, of zirconium and of tin.
[0028] Most preferably, reaction zone A is charged with at least
one oxidation catalyst A which comprises at least one vanadium
oxide and, as well as the at least one vanadium oxide, additionally
comprises at least one oxide of titanium.
[0029] Advantageously in accordance with the invention, the
oxidation catalysts A comprise the vanadium in the +5 oxidation
state. In other words, the oxidation catalysts A comprising at
least one vanadium oxide comprise, appropriately in accordance with
the invention, the (vanadium oxide) unit of V.sub.2O.sub.5
(vanadium pentoxide).
[0030] In contrast, the aforementioned oxidation catalysts A, in
the case of the elements Ti, Zr and Sn, comprise them
advantageously in the +4 oxidation state. In other words, the
oxidation catalysts comprising, as well as a vanadium oxide,
additionally at least one oxide from the group of titanium oxide,
zirconium oxide and tin oxide, appropriately in accordance with the
invention, comprise at least one unit (the element dioxide) from
the group of TiO.sub.2, ZrO.sub.2 and SnO.sub.2, the TiO.sub.2 unit
being very particularly advantageous within this group for the
inventive purposes, especially when it is present in the anatase
polymorph.
[0031] Quite generally, oxidation catalysts A preferred in
accordance with the invention are mixed oxide catalysts A
comprising at least one vanadium oxide, the term "mixed oxide"
expressing the fact that the catalytically active oxide comprises
at least two different metal elements.
[0032] Suitable oxidation catalysts A comprising at least one
vanadium oxide are, in accordance with the invention, for example,
all oxidation catalysts disclosed in EP-A 294846. These are
especially those oxidation catalysts of EP-A 294846 which,
neglecting the oxygen present, have the stoichiometry
Mo.sub.xV.sub.yZ.sub.z in which Z may be absent or may be at least
one particular metal element.
[0033] Further oxidation catalysts A which comprise at least one
vanadium oxide and are suitable in accordance with the invention
are the mixed oxide catalysts which are disclosed in U.S. Pat. No.
5,840,971 and whose active material consists of the elements
vanadium, titanium and oxygen.
[0034] The supported catalysts which comprise vanadium pentoxide
and titanium dioxide and are prepared for use for the partial
oxidation of o-xylene to phthalic anhydride in DE-A 1642938 are
also suitable in accordance with the invention as oxidation
catalysts A.
[0035] Very particular preference is given to using, for the
process according to the invention, as oxidation catalysts A
comprising at least one vanadium oxide, those which are recommended
in this regard in the priority-establishing EP application number
09178015.5.
[0036] Mixed oxide catalysts A which comprise at least one vanadium
oxide and are suitable in accordance with the invention are
obtainable, for example, by the preparation process described in
U.S. Pat. No. 4,048,112. This proceeds from a porous oxide of at
least one of the elements Ti, Al, Zr and Sn. The latter is
impregnated with a solution of a vanadium compound. Subsequently,
the solvent used to prepare the solution is advantageously
substantially removed (generally by the action of heat and/or
reduced pressure), and the resulting catalyst precursor is
subsequently calcined.
[0037] This involves decomposing the vanadium compound, generally
in an atmosphere comprising molecular oxygen, to vanadium oxide.
The porous oxide to be impregnated may have any desired geometric
three-dimensional form. Appropriate three-dimensional forms in
application terms for the process according to the invention
include spheres, rings (hollow cylinders), extrudates, tableted
pellets and monolithic forms. Advantageously, the longest dimension
of the aforementioned geometric shaped bodies is 1 or 2 to 10 mm
(the longest dimension of a shaped body in this document is
generally understood to mean the longest direct line connecting two
points on the surface of the shaped body). For the solution which
comprises at least one dissolved vanadium compound and is to be
used for the impregnation, suitable vanadium compounds are, for
example, vanadium pentoxide, vanadyl chloride, vanadyl sulfate and
ammonium metavanadate. The solvent used is preferably water, to
which complexing agents, for example oxalic acid, are
advantageously added as dissolution promoters. The removing of the
solvent which is to be undertaken after the impregnation and the
calcination which is to be performed may be operations which merge
seamlessly into one another or else overlap. Advantageously,
however, the solvent is first removed at a temperature of 100 to
200.degree. C. This is then followed by calcination at a
temperature of 400 to 800.degree. C., or of 500 to 700.degree. C.
The calcination can be effected in an atmosphere comprising
molecular oxygen, for example under air, or under inert gas. The
calcination atmosphere may be stationary above the precursor
material to be calcined or flow over and through the precursor
material. The calcination time generally varies within the range
from 0.5 h to 10 h. Higher calcination temperatures are normally
associated with shorter calcination times. When comparatively low
calcination temperatures are employed, the calcination generally
extends over a longer period.
[0038] Optionally, the procedure of impregnation-drying-calcination
can also be repeated several times in order to achieve the desired
loading with vanadium oxide.
[0039] In the case of inventive mixed oxide catalysts A comprising
vanadium oxide, advantageously in accordance with the invention,
quite generally 0.1 to 6% by weight, preferably 1 to 50% by weight
or 3 to 40% by weight, particularly advantageously 5 to 30% by
weight, based in each case on the total weight of the active
material, is accounted for the by the V.sub.2O.sub.5 unit.
[0040] Furthermore, in the case of inventive mixed oxide catalysts
A comprising vanadium oxide and titanium oxide, advantageously 40
to 99.9% by weight, preferably 50 to 99% by weight or 60 to 97% by
weight, particularly advantageously 70 to 95% by weight, based in
each case on the total weight of the active material, is accounted
for by the TiO.sub.2 unit.
[0041] An alternative process for preparing mixed oxide catalysts A
comprising vanadium oxide and titanium dioxide is described by U.S.
Pat. No. 3,464,930. This involves treating finely divided titanium
dioxide together with a vanadium compound. The resulting
composition can then be shaped to the corresponding catalyst
geometry even before or after the calcinations thereof. In
principle, the calcined composition can, however, also be used in
powder form as the catalyst for the relevant partial oxidation. The
shaping can, for example, be effected by compacting (for example by
tableting or extruding) the pulverulent active material or the
uncalcined precursor material thereof to the desired catalyst
geometry to produce unsupported catalysts or unsupported catalyst
precursors, and the shaping may be preceded by optional addition of
assistants, for example graphite or stearic acid as lubricants
and/or shaping assistants and reinforcing assistants such as
microfibers of glass, asbestos, silicon carbide or potassium
titanate. Unsupported catalyst geometries suitable in accordance
with the invention are (and quite generally in the case of
oxidation catalysts A (especially in the case of corresponding
unsupported catalysts A (they consist only of active material))),
for example, solid cylinders and hollow cylinders having an
external diameter and a length of 1 or 2 to 10 mm. In this case of
the hollow cylinder, a wall thickness of 1 to 3 mm is appropriate
in application terms. Otherwise, the tableting can advantageously
be undertaken as described in documents WO 2008/152079, WO
2008/087116, DE-A 102008040094, DE-A 102008040093 and WO
2010/000720. All geometries detailed in the aforementioned
documents are also suitable for inventive oxidation catalysts
A.
[0042] The finely divided titanium dioxide can be treated with a
vanadium compound, for example with a sparingly soluble vanadium
compound such as V.sub.2O.sub.5, under hydrothermal conditions. In
general, it is, however, undertaken by means of a solution
comprising a dissolved vanadium compound (for example in water or
in an organic solvent (for example formamide or mono- and
polyhydric alcohols)). The vanadium compounds used may be vanadium
pentoxide, vanadyl chloride, vanadyl sulfate and ammonium
metavanadate. The dissolution promoters added to the solution may
be complexing agents, for example oxalic acid.
[0043] Alternatively to the unsupported catalyst, the shaping can,
however, also be undertaken in the form of an eggshell catalyst.
This involves using the pulverulent active material obtained or the
pulverulent, as yet uncalcined precursor material, to coat the
surface of an inert shaped support body using a liquid binder (when
coating with uncalcined precursor material, the calcination follows
the coating and generally the drying). Inert shaped support bodies
differ from the catalytic active material (in this document,
"catalytically active material" is quite generally also used as a
synonym thereof) normally in that it has a much lower specific
surface area. In general, the specific surface area thereof is less
than 3 m.sup.2/g of shaped support body. At this point, it should
be emphasized that all figures in this document for specific
surface areas relate to determinations according to DIN 66131
(determinations of the specific surface area of solids by means of
gas adsorption (N.sub.2) according to Brunauer-Emmett-Teller
(BET)).
[0044] Suitable materials for aforementioned inert shaped support
bodies are, for example, quartz, silica glass, sintered silica,
sintered or fused alumina, porcelain, sintered or fused silicates
such as aluminum silicate, magnesium silicate, zinc silicate,
zirconium silicate, and especially steatite (e.g. C 220 steatite
from CeramTec). The geometry of the inert shaped support bodies may
in principle have an irregular shape, preference being given in
accordance with the invention to regularly shaped support bodies,
for example spheres or hollow cylinders. Appropriately in
application terms, the longest dimension of the aforementioned
inert shaped support bodies for the inventive purposes is 1 to 10
mm.
[0045] The coating of the inert shaped support bodies with the
particular finely divided powder is generally executed in a
suitable rotatable vessel, for example in a coating drum.
Appropriately in application terms, the liquid binder is sprayed
onto the inert shaped support bodies and the binder-moistened
surface of the shaped support bodies being moved within the coating
drum is dusted with the particular powder (cf., for example, EP-A
714 700). Subsequently, the adhering liquid is generally removed at
least partly from the coated shaped support body (for example by
passing hot gas through the coated shaped support bodies, as
described in WO 2006/094766). In principle, however, it is also
possible to employ all other application processes acknowledged as
prior art in EP-A 714700 to produce the relevant eggshell
catalysts. Useful liquid binders include, for example, water and
aqueous solutions (for example of glycerol in water). For example,
the coating of the shaped support bodies can also be undertaken by
spraying a suspension of the pulverulent material to be applied in
liquid binder (for example water) onto this surface of the inert
shaped support bodies (generally under the action of heat and a
drying entraining gas). In principle, the coating can also be
undertaken in a fluidized bed system or powder coating system.
[0046] The layer thickness of the active material applied to the
surface of the inert shaped support body is, appropriately in
application terms, selected within the range from 10 to 2000 .mu.m,
or 10 to 500 .mu.m, or 100 to 500 .mu.m, or 200 to 300 .mu.m.
Eggshell catalysts of the type described which are suitable in
accordance with the invention for charging the reaction zone A
include those whose inert shaped support body is a hollow cylinder
with a length in the range from 3 to 6 mm, an external diameter in
the range from 4 to 8 mm and a wall thickness in the range from 1
to 2 mm. Additionally suitable for the inventive purposes in the
reaction zone A are all ring geometries disclosed in DE-A
102010028328 and in DE-A 102010023312 and all of those disclosed in
EP-A 714700 for possible inert shaped support bodies for annular
eggshell oxidation catalysts A.
[0047] Further processes for producing mixed oxide catalysts A
which comprise vanadium oxide and titanium dioxide and are suitable
in accordance with the invention are disclosed by U.S. Pat. No.
4,228,038 and U.S. Pat. No. 3,954,857. The basis of the procedure
of U.S. Pat. No. 4,228,038 is a treatment of titanium dioxide with
water and vanadium oxychloride until the desired vanadium content
has been attained. The basis of the procedure of U.S. Pat. No.
3,954,857 is the neutralization of a solution of vanadium pentoxide
and titanium tetrachloride in hydrochloric acid, which leads to a
precipitation reaction. The oxidic active materials which result
from the two procedures, or from the precursor materials thereof,
can be used as described above to produce unsupported and eggshell
catalysts suitable for reaction zone A.
[0048] A further procedure suitable in accordance with the
invention for preparation of mixed oxide active materials
comprising vanadium oxide and titanium dioxide for oxidation
catalysts A is disclosed by U.S. Pat. No. 4,448,897. The basis of
this preparation method is the mixing of a vanadyl alkoxide with a
titanium alkoxide in an aqueous solution and subsequent drying and
calcining of the precipitate which forms. The shaping to give
unsupported or eggshell catalysts can proceed from the
corresponding active material powder or the precursor material
powder thereof in the manner already described (above). Finally, it
should be emphasized that the catalysts which comprise vanadium
oxide and are disclosed in WO 2008/110468 and in DE-A 19649426 are
also suitable as oxidation catalysts A for reaction zone A of the
process according to the invention. In addition, inert shaped
support bodies for the eggshell oxidation catalysts A (in contrast
to supported catalysts A) are preferably nonporous or low in pores.
In the case of supported catalysts A, the active material is
introduced into the pore structure of the shaped support body.
[0049] In addition to the oxides already mentioned, the active
materials of oxidation catalysts A suitable in accordance with the
invention may additionally (in addition to the element oxides
already mentioned) comprise one or more oxides of the metals from
the group of boron, silicon, hafnium, niobium, tungsten, lanthanum,
cerium, molybdenum, chromium, antimony, alkali metal and alkaline
earth metal, and also the elements of main groups 5 and 6 of the
Periodic Table and other transition metals. In many cases, the
total content of the aforementioned oxides, based on the total
weight of the active material, is 1 to 15% by weight. An essential
feature of the invention is that the term "element oxide" also
comprises metalates. These are negatively charged anions formed
only from the metal and oxygen.
[0050] The ethanol content in reaction gas input mixture A in the
process according to the invention will generally be 0.3 to 20% by
volume, preferably in application terms 0.5 to 15% by volume, more
preferably 0.75 to 10% by volume and most preferably 1 to 5% by
volume.
[0051] The molar amount n.sub.O of molecular oxygen present in
reaction gas input mixture A will, in the process according to the
invention, appropriately in application terms, be such that it is
greater than the molar amount n.sub.Et of ethanol present in
reaction gas input mixture A. In general, the n.sub.O:n.sub.Et
ratio in the process according to the invention will be at least
1.3, better at least 1.5, preferably at least 1.75 and more
preferably at least 2. Normally, the n.sub.O:n.sub.Et ratio will,
however, not be more than 10, and usually not more than 5.
[0052] The above conditions are essential for the process according
to the invention especially when they relate to n.sub.O*:n.sub.Et*,
where n.sub.O* is the total molar amount of molecular oxygen
supplied to reaction zone A within a period t, and n.sub.Et* is the
total molar amount of ethanol supplied as a constituent of reaction
gas input mixture A to reaction zone A within the same period
t.
[0053] An excess of molecular oxygen over ethanol reactant viewed
over reaction zone A is found to be advantageous for the process
according to the invention, both for the service life of the
catalyst charge of reaction zone A and for the service life of the
catalyst charge of reaction zone B, since this excess molecular
oxygen is introduced into reaction gas input mixture B in the
process according to the invention.
[0054] In this document, an inert diluent gas shall be understood
to mean a reaction gas input mixture constituent which behaves
inertly under the conditions in the particular reaction zone A or B
and--viewing each inert reaction gas constituent alone--remains
chemically unchanged in the particular reaction zone to an extent
of more than 95 mol %, preferably to an extent of more than 97 mol
%, or to an extent or more than 98 mol %, or to an extent of more
than 99 mol %. The above definition also applies correspondingly to
inert diluent gases in reaction gas input mixture C and with
reference to reaction zone C, which will be introduced later in
this document.
[0055] Examples of inert diluent gases both for reaction zone A and
reaction zones B and C are H.sub.2O, CO.sub.2, N.sub.2 and noble
gases such as Ar, and mixtures of the aforementioned gases. One
task assumed by the inert diluent gases is that of absorbing heat
of reaction released in the reaction zone A, thus limiting what is
called the hotspot temperature in reaction zone A and having a
favorable effect on the ignition behavior of reaction gas mixture
A. The hotspot temperature is understood to mean the highest
temperature of reaction gas mixture A on its way through reaction
zone A.
[0056] Steam as an inert diluent gas in the two reaction zones A
and B assumes a special role compared to other possible inert
diluent gases. This is attributable to the fact that presence of
steam in reaction gas input mixture A in reaction zone A
facilitates the desorption of the desired partial oxidation product
from the catalyst surface, which has a positive effect on the
selectivity of acetic acid formation. In addition, steam has an
increased molar heat capacity compared, for example, to N.sub.2 and
noble gases.
[0057] Advantageously in accordance with the invention, reaction
gas input mixture A may therefore comprise 1 to 40% by volume of
H.sub.2O, Since presence of steam in reaction zone B, however,
generally reduces the desired aldol condensation to a certain
extent and also increases the energy expenditure required to remove
a stream X comprising enriched acrylic acid from product gas
mixture B in separation zone T (acrylic acid has an elevated
affinity for H.sub.2O), steam contents of 1 to 20% by volume in
reaction gas input mixture A are preferred in accordance with the
invention. This also takes account of the fact that H.sub.2O is
formed as a by-product both in reaction zone A and in reaction zone
B. Particularly advantageously in accordance with the invention,
the steam content in reaction gas input mixture A will be 5 to 15%
by volume or 7.5 to 12.5% by volume.
[0058] The inert diluent gas other than steam used in the process
according to the invention, both in reaction zone A and in reaction
zone B, is preferably molecular nitrogen. This is favorable not
least in that molecular nitrogen occurs in air as a natural
companion of molecular oxygen, which makes air a preferred source
of the molecular oxygen required in reaction zone A. It will be
appreciated, however, that it is also possible in accordance with
the invention to use pure molecular oxygen, or air enriched with
molecular oxygen, or another gas mixture of molecular oxygen and
inert diluent gas, as the oxygen source.
[0059] Advantageously in accordance with the invention, at least
80% by volume, preferably at least 90% by volume, frequently at
least 95% by volume and sometimes 100% by volume of the inert
diluent gas other than steam present in reaction gas input mixture
A is accounted for by molecular nitrogen. The same also applies to
the inert gas ratios in reaction gas input mixture B. In general,
reaction gas input mixture A comprises, as well as an inert diluent
gas other than steam, additionally steam as an inert diluent gas.
Normally, the content of inert diluent gases other than steam in
reaction gas input mixture A will be at least 5% by volume, but
generally not more than 95% by volume. Typical contents of inert
diluent gas other than steam in reaction gas input mixture A are 10
to 90% by volume, preferably 30 to 90% by volume, more preferably
50 to 90% by volume and even more preferably 60 to 90% by volume,
or 70 to 90% by volume, in particular 75 to 85% by volume.
[0060] Thus, the molecular nitrogen content in reaction gas input
mixture A may be at least 5% by volume, preferably at least 10% by
volume, more preferably at least 20 or at least 30% by volume or at
least 40% by volume, but generally not more than 95% by volume.
Typical molecular nitrogen contents in reaction gas input mixture A
may be 10 to 90% by volume, preferably 30 to 90% by volume, more
preferably 50 to 90% by volume and even more preferably 60 to 90%
by volume, or 70 to 90% by volume, in particular 75 to 85% by
volume.
[0061] The boiling point of the inert diluent gases other than
steam (based on a pressure of 10.sup.5 Pa=1 bar) is normally
distinctly below that of steam (based on the same pressure), and
stream Z in the process according to the invention therefore
comprises the inert diluent gases other than steam (e.g. N.sub.2
and CO.sub.2) generally in enriched form.
[0062] As a source for inert diluent gas other than steam, it is
therefore appropriate in application terms also to recycle a
substream of stream Z into reaction zone A for configuration of
reaction gas input mixture A (cycle gas mode). Advantageously in
application terms, the separation of product gas mixture B in
separation zone T will be performed such that stream Z also has an
appropriate content of steam and therefore, in the case of use of
the above-described cycle gas mode, can also function as a source
for steam which is advantageously used in addition in reaction gas
input mixture A (or C). It will be appreciated that substreams of
stream Z can be recycled not only into reaction zone A but also
into reaction zone B (and C).
[0063] The temperature of reaction gas mixture A (the term
"reaction gas mixture A" in the present application comprises all
gas mixtures which occur in reaction zone A and are between
reaction gas input mixture A and product gas mixture A; in an
entirely corresponding manner, the term "reaction gas mixture B"
comprises all gas mixtures which occur in reaction zone B and are
between reaction gas input mixture B and product gas mixture B) in
the process according to the invention within reaction zone A will
normally be within the range from 100.degree. C. to 450.degree. C.,
preferably within the range from 150.degree. C. to 400.degree. C.
and more preferably within the range from 150.degree. C. to
350.degree. C. or 150.degree. C. to 300.degree. C. The
aforementioned temperature range may of course also be 200.degree.
C. to 300.degree. C. The term "temperature of reaction gas mixture
A" (also referred to in this document as the reaction temperature
in reaction zone A) means primarily that temperature that reaction
gas mixture A has on attainment of a conversion of the ethanol
present in reaction gas input mixture A of at least 5 mol % until
attainment of the corresponding final conversion of the ethanol
within reaction zone A.
[0064] Advantageously in accordance with the invention, the
temperature of reaction gas mixture A over the entire reaction zone
A is within the aforementioned temperature ranges. Advantageously,
reaction gas input mixture A is also supplied to reaction zone A
already with a temperature within the range from 100.degree. C. to
350.degree. C. Frequently, however, at the inlet into reaction zone
A, upstream of the actual catalytically active catalyst charge of
reaction zone A in flow direction, there is a charge of reaction
zone A with solid inert material, or of catalytically active
catalyst charge highly diluted with such inert material. In the
course of flow through such an upstream charge of reaction zone A,
the temperature of reaction gas input mixture A supplied to
reaction zone A can be adjusted in a comparatively simple manner to
the value with which reaction gas mixture A should enter the actual
catalytically active catalyst charge of reaction zone A. In
principle, the charge of reaction zone A with at least one
oxidation catalyst A can be configured as a fluidized bed.
Advantageously in application terms, the charge of reaction zone A
with oxidation catalyst A is, however, configured as a fixed
bed.
[0065] In principle, reaction gas mixture A can be either forced
through or sucked through reaction zone A. Accordingly, the working
pressure (=absolute pressure) within reaction zone A may be either
.gtoreq.10.sup.5 Pa or <10.sup.5 Pa. Appropriately in
application terms, the working pressure in reaction zone A will be
.gtoreq.10.sup.5 Pa. In general, the working pressure in reaction
zone A will be in the range from 1.210.sup.5 Pa to 5010.sup.5 Pa,
preferably in the range from 1.510.sup.5 to 2010.sup.5 Pa and more
preferably in the range from 210.sup.5 to 10.sup.6 Pa or in the
range from 210.sup.5 to 610.sup.5 Pa.
[0066] The configuration of reaction zone A can, appropriately in
application terms, be undertaken in the form of what is called a
"heat exchanger reactor". The latter has at least one primary space
and at least one secondary space, which are separated from one
another by a dividing wall. The catalyst charge positioned in the
at least one primary space comprises at least one oxidation
catalyst A, and reaction gas mixture A flows through it. At the
same time, a fluid heat carrier flows through the secondary space
and heat exchange takes place between the two spaces through the
dividing wall, which pursues the purpose of monitoring and
controlling the temperature of reaction gas mixture A on its way
through the catalyst bed (of controlling the temperature of
reaction zone A).
[0067] Examples of heat exchanger reactors suitable in accordance
with the invention for the implementation of reaction zone A are
the tube bundle reactor (as disclosed, for example, in EP-A 700714
and the prior art cited in that document) and the thermoplate
reactor (as disclosed, for example, in documents EP-A 1651344, DE-A
10361456, DE-A 102004017150 and the prior art acknowledged in these
documents). In the case of the tube bundle reactor, the catalyst
bed through which reaction gas mixture A flows is preferably within
the tubes thereof (the primary spaces), and at least one heat
carrier is conducted through the space surrounding the reaction
tubes (the secondary space). Useful heat carriers for the heat
exchanger reactors are, for example, salt melts, heat carrier oils,
ionic liquids and steam. In general, tube bundle reactors used on
the industrial scale comprise at least 3000 up to several tens of
thousands of reaction tubes connected in parallel (reactor tubes).
It will be appreciated that the configuration of reaction zone A
can also be implemented in a fluidized bed reactor or a
microreactor.
[0068] Conventional reactors and microreactors differ by their
characteristic dimensions and especially by the characteristic
dimensions of the reaction space which accommodates the catalyst
bed through which the reaction gas mixture flows.
[0069] The inventive partial oxidation of ethanol to acetic acid
proceeds in two successive steps. In the first step the ethanol is
partially oxidized to acetaldehyde, and in the second step (which
follows the first) the acetaldehyde is partially oxidized to acetic
acid.
[0070] It has now been found that, surprisingly, it is advantageous
for the target product formation in reaction zone A to use, for the
second reaction step, oxidation catalysts A whose active materials,
as well as a vanadium oxide, also comprise a molybdenum oxide. In
other words, for the second step of the partial oxidation of
ethanol to acetic acid, it is advantageous in accordance with the
invention to use multimetal oxide catalysts A whose active
materials, as well as oxygen, also comprise at least the elements
Mo and V.
[0071] Oxidation catalysts of this type are known from the
literature as catalysts for the heterogeneously catalyzed partial
gas phase oxidation of acrolein to acrylic acid. It has now been
found that, surprisingly, all multimetal oxide catalysts of the
aforementioned type which are known from the prior art as catalysts
for the heterogeneously catalyzed partial gas phase oxidation of
acrolein to acrylic acid are also advantageously suitable as
oxidation catalysts for the second step of the partial oxidation of
ethanol to acetic acid, the partial oxidation of acetaldehyde to
acetic acid, in reaction zone A.
[0072] Such multimetal oxide active materials comprising Mo and V,
including the catalysts comprising them, can be found, for example,
in documents U.S. Pat. No. 3,775,474, U.S. Pat. No. 3,954,855, U.S.
Pat. No. 3,893,951, U.S. Pat. No. 4,339,355, EP-A 614872, EP-A
1041062, WO 03/055835 and WO 03/057653.
[0073] Especially suitable for the second step in reaction zone A
of the present invention are, however, also the multimetal oxide
active materials comprising Mo and V, including the catalysts
comprising them, as disclosed in documents DE-A 10325487, DE-A
10325488, EP-A 427508, DE-A 2909671, DE-C 3151805, DE-B 2626887,
DE-A 4302991, EP-A 700893, EP-A 714700, DE-A 19736105, DE-A
19927624, DE-A 102010028328 and DE-A 10360057. This is especially
true of the exemplary embodiments of EP-A 714700, of DE-A 19736105,
of DE-A 19927624, of DE-A 10360057 and of DE-A 102010028328.
[0074] Particularly suitable for the relevant second reaction step
in reaction zone A are those oxidation catalysts A whose active
material is at least one multimetal oxide which, as well as V and
Mo, additionally comprises at least one of the elements W, Nb, Ta,
Cr and Ce, and at least one of the elements Cu, Ni, Co, Fe, Mn and
Zn.
[0075] Suitable among these for catalysis of the second step of the
partial oxidation of ethanol to acetic acid are in particular
multimetal oxide, active materials of the general formula I
Mo.sub.12V.sub.aX.sup.1.sub.bX.sup.2.sub.cX.sup.3.sub.dX.sup.4.sub.eX.su-
p.5.sub.fX.sup.6.sub.gO.sub.n
in which the variables are each defined as follows: [0076]
X.sup.1=W, Nb, Ta, Cr and/or Ce, [0077] X.sup.2=Cu, Ni, Co, Fe, Mn
and/or Zn, [0078] X.sup.3=Sb and/or B.sup.1, [0079] X.sup.4=one or
more alkali metals, [0080] X.sup.5=one or more alkaline earth
metals, [0081] X.sup.6=Si, Al, Ti and/or Zr, [0082] a=1 to 6,
[0083] b=0.2 to 4, [0084] c=0.5 to 1.8 [0085] d=Oto 40, [0086]
e=Oto 2, [0087] f=Oto 4, [0088] g=0 to 40, and [0089] n=the
stoichiometric coefficient of the element oxygen, which is
determined by the stoichiometric coefficients of the non-oxygen
elements and the charge numbers thereof in I.
[0090] In embodiments very particularly preferred in accordance
with the invention, the variables of the general formula I are each
defined as follows: [0091] X.sup.1=W, Nb and/or Cr, [0092]
X.sup.2=Cu, Ni, Co, Fe, Mn and/or Fe, [0093] X.sup.3=Sb, [0094]
X.sup.4=Na and/or K, [0095] X.sup.5=Ca, Sr and/or Ba, [0096]
X.sup.6=Si, Al and/or Ti, [0097] a=1.5 to 5, [0098] b=0.5 to 2,
[0099] c=0.5 to 3, [0100] d=0 to 2, [0101] e=0 to 0.2, [0102] f=0
to 1, [0103] g=0 to 1, and [0104] n=the stoichiometric coefficient
of the element oxygen, which is determined by the stoichiometric
coefficients of the non-oxygen elements and the charge numbers
thereof in I.
[0105] The multimetal oxide active materials comprising V and Mo,
especially those of the general formula I, can be used either in
powder form or shaped to particular catalyst geometries as
unsupported catalysts for catalysis of the partial oxidation of
acetaldehyde to acetic acid. With regard to the catalyst geometries
suitable in accordance with the invention, the statements already
made in this document with regard to the possible geometries of
unsupported oxidation catalysts A apply correspondingly.
[0106] Preferably in accordance with the invention, the described
multimetal oxide active materials comprising V and Mo are, however,
employed in the form of eggshell catalysts (i.e. applied to the
outer surface of preshaped inert catalyst supports (shaped support
bodies)) in the catalysis of the relevant second reaction step.
With regard to the geometries of the shaped support bodies suitable
in accordance with the invention, the statements already made in
this document in connection with eggshell oxidation catalysts A
apply correspondingly. Preferred geometries of the shaped support
bodies here too are spheres and rings, the longest dimension of
which may be 1 to 10 mm, frequently 2 to 8 mm or 3 to 6 mm. The
ring geometries favorable in accordance with the invention are
hollow cylindrical shaped support bodies having a length of 2 to 10
mm, an external diameter of 4 to 10 mm and a wall thickness of 1 to
4 mm. The hollow cylindrical shaped support bodies preferably have
a length of 3 to 6 mm, an external diameter of 4 to 8 mm and a wall
thickness of 1 to 2 mm. An illustrative geometry is the 7
mm.times.3 mm.times.4 mm ring geometry (external
diameter.times.length.times.internal diameter). The thickness of
the eggshell of catalytically active oxide material applied to the
shaped support bodies in the aforementioned eggshell catalysts is,
appropriately in application terms, generally 10 to 1000 .mu.m.
This eggshell thickness is preferably 10 to 500 .mu.m, more
preferably 100 to 500 .mu.m and most preferably 200 to 300 .mu.m.
Advantageously, the eggshell thickness is very substantially
homogeneous viewed over a single eggshell catalyst. In the case of
production of a relatively large production batch of the eggshell
catalysts, the eggshell thickness is likewise very substantially
homogeneous viewed over several shaped eggshell catalyst bodies.
Useful materials for the inert shaped support bodies include the
inert materials already mentioned in this document. Such possible
insert materials once again include aluminum oxide, silicon
dioxide, silicates such as clay, kaolin, steatite, pumice, aluminum
silicate and magnesium silicate, silicon carbide, zirconium dioxide
and thorium dioxide (an inert material particularly preferred in
accordance with the invention for shaped support bodies is steatite
of the C 220 type from CeramTec). Shaped support bodies with
distinct surface roughness (for example hollow cylinders with a
grit layer as described in Research Disclosure Database Number
532036 (published August 2008)) are preferred for production of the
shaped eggshell catalyst bodies. Otherwise, the shaped support
bodies are preferably very substantially nonporous.
[0107] To produce the shaped eggshell catalyst bodies, the
catalytically active oxide material of the general formula I can
first be prepared as such. This is typically done by obtaining,
from sources of the elemental constituents of the catalytically
active oxide material, a very intimate, preferably finely divided,
dry mixture with a composition corresponding to the stoichiometry
thereof (a precursor material), and calcining (thermally treating)
it at temperatures of 350 to 600.degree. C. The calcinations can be
effected either under inert gas or under an oxidizing atmosphere,
for example air (or another mixture of inert gas and oxygen), or
else under reducing atmosphere (for example mixtures of inert gas
and reducing gases such as H.sub.2, NH.sub.3, CO, methane and/or
acrolein, or any of the reducing gases mentioned alone). The
calcination time may be a few minutes to a few hours and typically
decreases with the level of the calcination temperature. A
calcination process of good suitability in accordance with the
invention is described, for example, by WO 95/11081.
[0108] Useful sources for the elemental constituents of the
catalytically active oxide material of the general formula I
include (as is generally the case for oxidation catalysts A),
especially those compounds which are already oxides and/or those
compounds which can be converted to oxides by heating, at least in
the presence of oxygen. The starting compounds (sources) can be
intimately mixed in dry or wet form. When it is done in dry form,
the starting compounds are appropriately used in the form of fine
powders and, after mixing and optional compaction, subjected to
calcination. However, preference is given to effecting the intimate
mixing in wet form. Typically, this involves mixing the starting
compounds with one another in the form of an aqueous solution
and/or suspension.
[0109] Particularly intimate dry mixtures are obtained in the
mixing process described when the starting materials are
exclusively sources of the elemental constituents present in
dissolved form.
[0110] The solvent used is preferably water. Subsequently, the
liquid (e.g. aqueous) material obtained is dried, the drying
operation preferably being effected by spray-drying the liquid
(e.g. aqueous) mixture with exit temperatures of 100 to 150.degree.
C. Appropriately in application terms, the drying gas stream is air
or molecular nitrogen.
[0111] The catalytically active oxide material obtained after
calcining is subsequently converted to a fine powder, for example
by grinding, which can then normally be applied to the outer
surface of the shaped support body with the aid of a liquid binder.
The fineness of the catalytically active oxide material to be
applied to the surface of the shaped support body is of course
adjusted to the desired eggshell thickness as described in the
prior art (cf., for example, EP-A 714700).
[0112] For example, the shaped support bodies are moistened with
the liquid binder in a controlled manner, for example by spraying,
and the thus moistened shaped support bodies are dusted with the
finely divided catalytically active oxide material (cf., for
example, EP-A 714 700 and DE-A102010023312). Subsequently, the
adhering liquid is at least partly removed from the moistened
shaped support body coated with active oxide material (for example
passage of hot gas; cf. WO 2006/094766). However, it is also
possible to use all other application processes acknowledged as
prior art in EP-A 714700 for production of eggshell catalysts
suitable in accordance with the invention with multimetal oxide I
active materials. Examples of useful liquid binders include water
and aqueous solutions.
[0113] in principle, however, the procedure for production of
inventive eggshell catalysts may also be first to apply finely
divided precursor material to the surface of the support body and
to perform the calcination of the precursor material to the
catalytically active oxide material of the general formula I only
subsequently, i.e. already on the surface of the shaped support
body.
[0114] Useful element sources for the preparation of catalytically
active oxide materials of the general formula I include, as well as
the element oxides, quite generally in particular halides,
nitrates, formates, oxalates, acetates, carbonates and hydroxides.
Appropriately in application terms, the molybdenum source used is
ammonium heptamolybdate tetrahydrate. The preferred vanadium source
is ammonium metavanadate and, in the case of additional use of the
element W, ammonium paratungstate hydrate is the preferred element
source. Useful sources for the elemental constituent Cu include,
for the preparation of catalytically active oxide materials of the
general formula I, especially copper(II) sulfate pentahydrate,
copper(II) nitrate hydrate (Cu content=21.6% by weight) and
copper(II) acetate monohydrate, among which the latter is
preferred. The process for thermal treatment of the precursor
material of a catalytically active oxide material of the general
formula I will, advantageously in accordance with the invention, be
performed according to the procedure described and executed by way
of example in DE-A 10360057. In the case of annular shaped support
bodies, the preferred process for applying catalytically active
oxide material of the general formula I to the surface thereof is
the process described in EP-A 714700. An aqueous solution of 75% by
weight of water and 25% by weight of glycerol is then the preferred
binder. Otherwise, the procedure may be as described in working
examples 1 and 2 of DE-A 10360057. In the case of spherical shaped
support bodies, water is the preferred binder.
[0115] When at least one oxidation catalyst A whose active material
is a multimetal oxide (e.g. one of the general formula I) which, as
well as oxygen, also comprises at least the elements Mo and V is
used in the process according to the invention in reaction zone A
for the second step of the heterogeneously catalyzed partial
oxidation of ethanol to acetic acid, it is advantageous in
accordance with the invention to use, for the first step of the
partial oxidation of ethanol to acetic acid, at least one oxidation
catalyst A whose active material is a mixed oxide which, as well as
oxygen and V, also comprises at least one of the elements Ti
(preferable), Zr and Al (and generally no Mo) (i.e. at least one
oxidation catalyst A which comprises, in addition to a vanadium
oxide (preferably V.sub.2O.sub.5), also at least one oxide of Ti
(preferable: preferably TiO.sub.2), Zr and Al).
[0116] Such a charge of reaction zone A with oxidation catalyst A
is advantageous in accordance with the invention in that it ensures
particularly high target product selectivities for acetic acid at
high conversions of ethanol based on a single pass of reaction gas
mixture A through reaction zone A.
[0117] In other words, advantageously in accordance with the
invention, the charge of reaction zone A with at least one
oxidation catalyst A comprises two sections 1 and 2 which are
spatially successive (in the numerical sequence mentioned) in flow
direction of reaction gas mixture A (between the two there may
optionally be a section charged only with inert shaped bodies,
which, however, is less preferred in accordance with the
invention).
[0118] The active material of the at least one oxidation catalyst A
of section 1 (in section 1), which is also referred to here as
catalytically active material 1, is at least one mixed oxide which,
as well as oxygen and V, also comprises at least one of the
elements Ti (preferable), Zr and Al (and generally no Mo), while
the active material of the at least one oxidation catalyst A of
section 2 (in section 2), which is also referred to here as
catalytically active material 2 (and is different than
catalytically active material 1), is at least one mixed oxide
comprising the elements V and Mo (preferably one of the general
formula I). In other words, the at least one oxidation catalyst A
of section 1 (the first section in flow direction of reaction gas
mixture A) comprises, as well as a vanadium oxide (preferably
V.sub.2O.sub.5), also at least one oxide of Ti (preferable:
preferably TiO.sub.2), Zr and Al (and generally no oxide of Mo),
whereas the at least one oxidation catalyst A of section 2, as well
as a vanadium oxide, also comprises at least one molybdenum
oxide.
[0119] More preferably, the active material of the at least one
oxidation catalyst A of section 1 comprises or consists of 1 to 50%
by weight of V.sub.2O.sub.5 and 50 to 99% by weight of TiO.sub.2,
advantageously 3 to 40% by weight of V.sub.2O.sub.5 and 60 to 97%
by weight of TiO.sub.2, and very particularly advantageously 5 to
30% by weight of V.sub.2O.sub.5 and 70 to 95% by weight of
TiO.sub.2. The TiO.sub.2 in the aforementioned cases is preferably
in the anatase polymorph.
[0120] Advantageously in accordance with the invention, the
temperature of section 1 is controlled independently of the
temperature of section 2. Advantageously in application terms, the
temperatures of sections 1 and 2 are controlled such that the
temperature of reaction gas mixture A averaged arithmetically over
the length of the (catalytically active) section 1 (this is the
temperature T.sup.1, which is also referred to in this document as
the reaction temperature in section 1 averaged arithmetically over
the length of section 1) is 150 to 250.degree. C. and preferably
170 to 220.degree. C., whereas the temperature of reaction gas
mixture A averaged arithmetically over the length of the
(catalytically active) section 2 (this is the temperature T.sup.2
which is also referred to in this document as the reaction
temperature of section 2 averaged over the length of section 2) is,
appropriately in application terms, 180 to 260.degree. C.,
preferably 200 to 240.degree. C. and particularly advantageously
210 to 230.degree. C.
[0121] Advantageously, T.sup.2 is at least 5.degree. C., preferably
at least 10.degree. C., more preferably at least 15.degree. C. or
at least 20.degree. C. and most preferably at least 25.degree. C.
or at least 30.degree. C. greater than T.sup.1. In general, T.sup.2
is, however, not more than 80.degree. C. and frequently not more
than 60.degree. C. greater than T.sup.1.
[0122] The length of the two charge sections 1 and 2 is, in
accordance with the invention, normally such that the conversion of
ethanol achieved in a single pass of reaction gas mixture A through
section 1 is at least 90 mol %, regularly even at least 95 mol %,
and the conversion of acetaldehyde achieved in section 2 is
likewise at least 90 mol % and regularly even at least 95 mol %.
The conversion of ethanol achieved in a single pass of reaction gas
mixture A through sections 1 and 2 is regularly .gtoreq.97 mol %,
in many cases .gtoreq.98 mol % and frequently .gtoreq.99 mol %. The
selectivity of the associated acetic acid formation is normally
.gtoreq.85 mol %, frequently .gtoreq.86 mol % or .gtoreq.87 mol %
and in many cases even .gtoreq.88 mol % or .gtoreq.90 mol %.
[0123] The implementation of the two sections 1 and 2 of reaction
zone A is possible in a simple manner, for example in two heat
exchanger reactors connected in series (for example two tube bundle
reactors), through whose particular secondary spaces an independent
fluid heat carrier flows in each case. The at least one primary
space of the first of the two reactors in flow direction
accommodates section 1, while the at least one primary space of the
second of the two reactors in flow direction accommodates section
2.
[0124] The two sections 1 and 2 of reaction zone A can, for
example, also be implemented in what is called a two-zone reactor,
as disclosed by way of example in DE-A 2830765. In this case, the
two sections 1 and 2 are accommodated in spatial succession in the
same primary space, and the secondary space adjoining the primary
space is divided into two subspaces, one of which extends over
section 1 and the other over section 2, through both of which
separately flow heat carriers having different inlet temperatures.
Two-zone reactors are also covered by documents DE-A 10313210, DE-A
10313209, DE-A 19948523, DE-A 19948523, DE-A 19948241, DE-A
10313208 and WO 2007/082827. Preferably in accordance with the
invention, two-zone tube bundle reactors are employed.
[0125] When the two sections 1 and 2 are implemented in a two-zone
reactor, reaction gas input mixture A must already have (comprise)
all constituents required for the partial oxidation of ethanol to
acetic acid to the extent required for the reaction. When they are
implemented in two heat exchanger reactors connected in series, it
is also possible, for example, to meter molecular oxygen and/or
inert gas to reaction gas mixture A between the two reactors.
[0126] Advantageously in accordance with the invention, oxidation
catalysts A whose active material comprises at least one vanadium
oxide (including those of the general formula I), in the case of
use for the heterogeneously catalyzed partial oxidation of ethanol
to acetic acid, have a completely satisfactory service life even
when reaction gas input mixture A is produced using bioethanol,
i.e. ethanol which is obtained from the renewable base raw material
biomass. As a result of production, bioethanol generally comprises
at least one chemical compound comprising the element sulfur in
chemically bound form as an impurity. Based on the weight of the
ethanol present in bioethanol and expressed via the weight of the
sulfur present in such sulfur compounds, the content in bioethanol
of such sulfur compounds is generally 1 ppm by weight, frequently
.gtoreq.2 ppm or .gtoreq.3 ppm by weight. In general, the
aforementioned sulfur content in bioethanol is .ltoreq.200 ppm by
weight, or .ltoreq.150 ppm by weight or in some cases .ltoreq.100
ppm by weight.
[0127] Examples of sulfur-comprising impurities of this kind
include dimethyl sulfate and dimethyl sulfoxide. The content of
sulfur compounds can be determined by gas chromatography.
Remarkably, oxidation catalysts A whose active material comprises
at least one vanadium oxide are apparently substantially resistant
to such sulfur compounds as a constituent of reaction gas input
mixture A, and so corresponding contents of sulfur compounds based
on the ethanol content of reaction gas input mixture A can be
tolerated in reaction gas input mixture A in the process according
to the invention. It will be appreciated, however, that bioethanol
with a corresponding content of sulfur compounds which has been
lowered to values of <1 ppm by weight is also suitable as the
ethanol source for the process according to the invention.
[0128] For example, for the process according to the invention, it
is possible to use bioethanol which satisfies the following
specification:
TABLE-US-00001 Ethanol >99.8% by volume DIN 12803 Water <1500
ppm DIN EN ISO 12937 Methanol <100 ppm GC Sum of C.sub.3- to
C.sub.5-alcohols <1500 ppm GC Esters, calculated as ethyl
<250 ppm GC acetate Aldehydes, calculated as <250 ppm GC
acetaldehyde Acetone <10 ppm GC Neutralization number <0.028
mg KOH/g ASTM D 1613-03 Acid number, calculated <50 ppm ASTM D
1613 as acetic acid Chlorine-comprising <0.5 ppm ASTM 4929 B
compounds as Cl Sulfur-comprising <1 ppm DIN EN ISO 11885
compounds as S (E22) Iron-comprising <0.1 ppm DIN EN ISO 11885
constituents as Fe (E22) Nitrogen-comprising 0 ppm ASTM D 1614-03
compounds as N
[0129] It is essential to the invention that sulfur present in
chemically bound form in corresponding impurities present in
reaction gas input mixture A is introduced into reaction zone B as
a constituent of reaction gas input mixture B in the process
according to the invention. It is surprising that the aldol
condensation catalysts for use in accordance with the invention in
reaction zone B, especially those preferred in accordance with the
invention, have a completely satisfactory tolerance to compounds
comprising sulfur in chemically bound form.
[0130] To obtain reaction gas input mixture A, bioethanol used as a
raw material is converted as such to the vapor phase and introduced
into reaction gas input mixture A. It will be appreciated that it
is also possible in this way to use aqueous bioethanol solutions in
the process according to the invention. In principle, the ethanol
source used for the process according to the invention may also be
aqueous slurry which comprises bioethanol in dissolved form and is
obtained in bioethanol production. This is subjected to filtration
and solids present therein are filtered off. The filtrate is
converted to the vapor phase and sent to the production of reaction
gas input mixture A.
[0131] Otherwise, the space velocity on the fixed catalyst bed
which is present in reaction zone A and comprises at least one
oxidation catalyst A of ethanol present in reaction gas input
mixture A in the process according to the invention may, for
example, be 20 to 500, preferably 30 to 100 and more preferably 50
to 100 l (STP)/lh. The term "space velocity" is used as defined in
DE-A 19927624.
[0132] Useful sources for the formaldehyde required in reaction gas
input mixture B for the process according to the invention include
various raw materials. One possible source is aqueous solutions of
formaldehyde (cf., for example, DE-A 102008059701) which can be
purchased commercially as formalin, for example with a formaldehyde
content of 35 to 50% by weight (e.g. 49-2015 formaldehyde from BASF
SE). Typically, formalin also comprises small amounts of methanol
as a stabilizer. These may, based on the weight of the formalin, be
0.5 to 20% by weight, advantageously 0.5 to 5% by weight and
preferably 0.5 to 2% by weight. Converted to the vapor phase, the
formalin can be used directly to produce reaction gas input mixture
B. However, a disadvantage of formalin as the formaldehyde source
is that it also comprises water as well as formaldehyde, which has
an unfavorable effect on the position of the reaction equilibrium
in reaction zone B.
[0133] A useful alternative formaldehyde source is trioxane.
Trioxane is a heterocyclic compound from the group of the acetals,
which forms as a result of trimerization of formaldehyde. It is
solid at standard pressure (10.sup.5 Pa) at 25.degree. C., melts at
62.degree. C. and boils at 115.degree. C. When heated to 150 to
200.degree. C., it depolymerizes again to form monomeric
formaldehyde. Since the reaction temperature in reaction zone B is
normally above 250.degree. C., trioxane is thus a formaldehyde
source favorable in accordance with the invention for production of
reaction gas input mixture B. Since trioxane also has comparatively
good solubility in water and in alcohols such as methanol, it is
also possible to use corresponding trioxane solutions as the
formaldehyde source suitable in accordance with the invention for
the process according to the invention. Presence of 0.25 to 0.50%
by weight of sulfuric acid in trioxane solutions promotes
redissociation to formaldehyde. Alternatively, the trioxane can
also be dissolved in a liquid stream Y consisting principally of
acetic acid, and the resulting solution can be evaporated for the
purposes of producing reaction gas input mixture B, and the
trioxane present therein can be redissociated to formaldehyde at
the elevated temperature.
[0134] In addition, the formaldehyde source used for the process
according to the invention may be paraformaldehyde.
Paraformaldehyde is the short-chain polymer of formaldehyde, the
degree of polymerization of which is typically 8 to 100. This is a
white powder which is dissociated back to formaldehyde at low pH
values or when heated.
[0135] It decomposes when paraformaldehyde is heated in water to
obtain an aqueous formaldehyde solution which is likewise a source
suitable in accordance with the invention. It is sometimes referred
to as aqueous "paraformaldehyde solution" in order to delimit it
for terminology purposes from aqueous formaldehyde solutions which
are obtained by diluting formalin. In fact, paraformaldehyde as
such is, however, essentially insoluble in water.
[0136] A further formaldehyde source suitable for the process
according to the invention is methylal (dimethoxymethane). This is
a reaction product of formaldehyde with methanol, which forms a
colorless liquid at standard pressure to 25.degree. C. It is
hydrolyzed in aqueous acids to again form formaldehyde and
methanol. Converted to the vapor phase, it is suitable for
production of reaction gas input mixture B.
[0137] It is also possible to employ the processes disclosed in
Chemie Ingenieur Technik-CIT, Volume 66, Issue 4, pages 498 to 502,
Published Online 2004 for continuous metered addition of
formaldehyde.
[0138] On the industrial scale, formaldehyde is prepared by
heterogeneously catalyzed partial gas phase oxidation of methanol.
A formaldehyde source particularly preferred in accordance with the
invention for formation of reaction gas input mixture B is
therefore the product gas mixture of a heterogeneously catalyzed
partial gas phase oxidation of methanol to formaldehyde, optionally
after a portion or the entirety of any unconverted methanol present
therein has been removed.
[0139] Reaction gas input mixture B can be produced from the stream
of product gas mixture A leaving reaction zone A and the
formaldehyde source which has been converted to the vapor phase as
at least one further stream, and stream Y and optionally further
streams, for example additional steam or additional inert gas other
than steam (=inert diluent gas).
[0140] The temperature of reaction gas mixture B in the process
according to the invention within reaction zone B will normally be
within the range from 260 to 400.degree. C., preferably within the
range from 270 to 390.degree. C., more preferably within the range
of 280 to 380.degree. C., advantageously within the range of 300 to
370.degree. C. and particularly advantageously within the range of
300 to 340.degree. C.
[0141] The term "temperature of reaction gas mixture B" (also
referred to in this document as reaction temperature in reaction
zone B) means primarily that temperature that reaction gas mixture
B has from attainment of a conversion of the formaldehyde present
in reaction gas input mixture B of at least 5 mol % until
attainment of the appropriate final conversion of the formaldehyde
within reaction zone B. Advantageously in accordance with the
invention, the temperature of reaction gas mixture B over the
entire reaction zone B is within the aforementioned temperature
ranges. Advantageously, reaction gas input mixture B is already
supplied to reaction zone B with a temperature within the range
from 260 to 400.degree. C. Frequently, however, a charge of
reaction zone B with solid inert material or of catalytically
active catalyst charge highly diluted with such inert material is
present at the inlet into reaction zone B in flow direction
upstream of the actual catalytically active catalyst charge of
reaction zone B. As it flows through such a primary charge of
reaction zone B, the temperature of the reaction gas input mixture
B supplied to reaction zone B can be adjusted in a comparatively
simple manner to the value with which reaction gas mixture B is to
enter the actual catalytically active catalyst charge of reaction
zone B. In general, the temperature of the product gas mixture A
leaving reaction zone A is different than this temperature. Against
this background, the stream of product gas mixture A, on its way
from reaction zone A into reaction zone B, can flow through an
indirect heat exchanger in order to approximate its temperature to
the inlet temperature envisaged for reaction gas input mixture B
into reaction zone B, or to bring it to this temperature.
[0142] In principle, the charge of reaction zone B with at least
one aldol condensation catalyst B can be configured as a fluidized
bed. Advantageously in application terms, the charge of reaction
zone B with aldol condensation catalyst B is, however, configured
as a fixed bed.
[0143] With regard to the working pressure which exists in reaction
zone B, the same applies correspondingly as has already been stated
for the working pressure which exists in reaction zone A. In
general, the working pressure in reaction zone B, due to the
pressure drop which occurs as reaction gas mixture A flows through
reaction zone A, is lower than the working pressure in reaction
zone B. It is also possible to configure reaction zone B in
corresponding heat exchanger reactors to reaction zone A, in which
case the same rules of preference apply.
[0144] The formaldehyde content in reaction gas input mixture B
will, in the process according to the invention, generally be 0.5
to 10% by volume, preferably 0.5 to 7% by volume and more
preferably 1 to 5% by volume.
[0145] The ratio n.sub.Hac:n.sub.Fd of molar amount of acetic acid
present in reaction gas input mixture B (n.sub.HAc) to molar amount
of formaldehyde present therein (n.sub.Fd) in the process according
to the invention is greater than 1 and may be up to 10 (n.sub.Fd is
understood to mean the sum of formaldehyde units present in
monomeric form (preferred) and in oligomeric and polymeric form in
reaction gas input mixture B, since further redissociation to
monomeric formaldehyde can also be established only in the course
of flow of reaction gas mixture B through the catalyst charge of
reaction zone B). Advantageously in accordance with the invention,
the ratio n.sub.HAc:n.sub.Fd in reaction gas input mixture B is 1.1
to 5 and more preferably 1.5 to 3.5. Frequently, the acetic acid
content of reaction gas input mixture B will vary within the range
from 1 or from 1.5 to 20% by volume, advantageously within the
range from 2 to 15% by volume and particularly advantageously
within the range from 3 to 10% by volume. The molecular oxygen
content of reaction gas input mixture B varies, in the process
according to the invention, appropriately in application terms,
within the range from 0.5 to 5% by volume, preferably within the
range from 1 to 5% by volume and more preferably within the range
from 2 or from 3 to 5% by volume. Presence of molecular oxygen in
reaction gas input mixture B has an advantageous effect on the
service life of the catalyst charge of reaction zone B. When the
oxygen content of reaction gas mixture B is too high, however,
there is unwanted carbon oxide formation in reaction zone B.
[0146] The steam content of reaction gas input mixture B in the
process according to the invention should not exceed 30% by volume
since presence of steam in reaction gas mixture B has an
unfavorable effect on the equilibrium position of the aldol
condensation. Appropriately in application terms, the steam content
of reaction gas input mixture B will therefore generally not exceed
25% by volume and preferably 20% by volume. In general, the steam
content of reaction gas input mixture B will be at least 1.5% or at
least 2% by volume. Advantageously, the steam content of reaction
gas input mixture B is 5 to 15% by volume and, taking account of
the effect thereof and formation thereof in reaction zone A, in
particular 10 to 15% by volume. The proportion by volume of inert
diluent gases other than steam in reaction gas input mixture B will
normally be at least 30% by volume. Preferably, the aforementioned
inert gas content is at least 40% by volume or at least 50% by
volume. In general, the proportion of inert diluent gas other than
steam in reaction gas input mixture B will not exceed 95% by volume
or usually 90% by volume. Particularly advantageously in
application terms, reaction gas input mixture B comprises 60 to 90%
by volume, particularly advantageously 70 to 80% by volume, of
inert diluent gas other than steam. An inert diluent gas other than
steam which is preferred in accordance with the invention is, in
reaction gas input mixture B2, molecular nitrogen (N.sub.2).
[0147] Thus, the molecular nitrogen content of reaction gas input
mixture B may be at least 30% by volume, preferably at least 40% by
volume or at least 50% by volume. In general, reaction gas input
mixture B comprises not more than 95% by volume and usually not
more than 90% by volume of molecular nitrogen. Advantageously,
reaction gas input mixture B comprises 60 to 90% by volume,
particularly advantageously 70 to 80% by volume, of molecular
nitrogen.
[0148] Useful catalysts for charging of reaction zone B include,
for example, those disclosed in I & EC PRODUCT RESEARCH AND
DEVELOPMENT, vol. 5, No. 1, March 1966, pages 50 to 53. This group
of basic catalysts comprises firstly zeolites (aluminosilicates)
with anionic structural charge, on the inner and outer surfaces of
which at least one cation type from the group of the alkali metal
ions and alkaline earth metal ions is present (preferably Na.sup.+,
K.sup.+, Ca.sup.2+ and/or Mg.sup.2+), in order to balance out (to
neutralize) the negative structural charge. However, it also
comprises hydroxide applied to inert supports (e.g. amorphous
silicon dioxide (silica gel)), from the group consisting of alkali
metal hydroxides, alkaline earth metal hydroxides and aluminum
hydroxide (preferably KOH, NaOH, Ca(OH).sub.2 and
Mg(OH).sub.2).
[0149] However, also suitable for charging reaction zone B are the
acidic catalysts disclosed in EP-A 164614.
[0150] These are catalysts which comprise [0151] as constituent a),
at least one oxide of at least one of the elements Si, Al, Ti, Zr,
Cd, Sn, Ga, Y and La and/or zeolite, [0152] and [0153] as
constituent b), at least one oxide selected from boron oxide and
phosphorus oxide, [0154] and optionally [0155] as constituent c)
one or more than one oxide of at least one of the elements V, Cr,
Co, Ni, Mo and Pb and/or one or more than one heteropolyacid with
at least one poly atom selected from V, Mo and W.
[0156] Preferred boron oxide is B.sub.2O.sub.3, and preferred
phosphorus oxide is P.sub.2O.sub.5.
[0157] Preference is given to catalysts whose boron oxide content
(calculated as B.sub.2O.sub.3 (based on the amount of B present))
is 1 to 50% by weight. However, catalysts favorable in accordance
with the invention are also those whose phosphorus oxide content
(calculated as P.sub.2O.sub.5 (based on the amount of P present))
is 1 to 50% by weight. However, useful aldol condensation catalysts
B for the process according to the invention also include those
among the aforementioned catalysts whose total content of
phosphorus oxide (calculated as P.sub.2O.sub.5) and of boron oxide
(calculated as B.sub.2O.sub.3) is 1 to 50% by weight. The
aforementioned contents of phosphorus oxide and/or boron oxide are
preferably 5 to 30% by weight.
[0158] In addition, constituent a) is preferably at least one oxide
of at least one of the elements Si, Al, Ti and Zr.
[0159] Particularly favorable in accordance with the invention are
the combinations of titanium oxide as constituent a) and boron
oxide or phosphorus oxide as constituent b), or silicon
dioxide-aluminum oxide as constituent a) and boron oxide as
constituent b), or aluminum oxide as constituent a) and boron oxide
or phosphorus oxide as constituent b). When the catalysts detailed
above additionally comprise a heteropolyacid, it preferably
comprises at least one of the elements P, B and Si as heteroatom.
When the aforementioned catalysts comprise a constituent c), the
amount thereof is normally 0.01 to 10 mmol per gram of catalyst and
in many cases 0.03 to 5 mmol per gram of catalyst. It is favorable
when the catalysts have, as constituent c), both at least one of
the oxides and at least one of the heteropolyacids.
[0160] More preferably in accordance with the invention, reaction
zone B is, however, charged with aldol condensation catalysts B
whose active material is a vanadium-phosphorus oxide and/or a
vanadium-phosphorus oxide doped with elements other than vanadium
and phosphorus (also referred to collectively in the literature as
V--P--O catalysts).
[0161] Such catalysts have been described before in the literature
and are recommended there especially as catalysts for the
heterogeneously catalyzed partial gas phase oxidation of
hydrocarbons having at least four carbon atoms (especially
n-butane, n-butene and/or benzene) to maleic anhydride.
[0162] Surprisingly, these catalysts known from the prior art for
aforementioned partial oxidations are suitable in principle as
aldol condensation catalysts B for charging reaction zone B. They
are notable for particularly high selectivities of target product
formation (of acrylic acid formation) (with simultaneously high
formaldehyde conversions).
[0163] Accordingly, the aldol condensation catalysts B used in the
process according to the invention may, for example, be all of
those disclosed in documents U.S. Pat. No. 5,275,996, U.S. Pat. No.
5,641,722, U.S. Pat. No. 5,137,860, U.S. Pat. No. 5,095,125,
DE-69702728 T2, WO 2007/012620, WO 2010/072721, WO 2001/68245, U.S.
Pat. No. 4,933,312, WO 2003/078310, Journal of Catalysis 107, pages
201-208 (1987), DE-A 102008040094, WO 97/12674, "Neuartige Vanadium
(IV)-phosphate fur die Partialoxidation von kurzkettigen Koh
lenwasserstoffen-Synthesen, Kristallstrukturen, Redox-Verhalten and
katalytische Eigenschaften [Novel vanadium(IV) phosphates for the
partial oxidation of short-chain hydrocarbon syntheses, crystal
structures, Redox behavior and catalytic properties], thesis by
Ernst Benser, 2007, Rheinische Friedrichs-Wilhelms-Universitat
Bonn", WO 2010/072723, "Untersuchung von V--P--O-Katalysatoren fur
die partielle Oxidation von Propan zu Acrylsaure [Study of V--P--O
catalysts for the partial oxidation of propane to acrylic acid],
thesis by Thomas Quandt, 1999, Ruhr-Universitat Bochum", WO
2010/000720, WO 2008/152079, WO 2008/087116, DE-A 102008040093,
DE-A 102005035978 and DE-A 102007005602, and the prior art
acknowledged in these documents. In particular, this applies to all
exemplary embodiments of the above prior art, especially those of
WO 2007/012620.
[0164] The phosphorus/vanadium atomic ratio in the undoped or doped
vanadium-phosphorus oxides is, advantageously in accordance with
the invention, 0.9 to 2.0, preferably 0.9 to 1.5, more preferably
0.9 to 1.2 and most preferably 1.0 to 1.1. The arithmetic mean
oxidation state of the vanadium therein is preferably +3.9 to +4.4
and more preferably 4.0 to 4.3. These active materials also
advantageously have a specific BET surface area of .gtoreq.15
m.sup.2/g, preferably of .gtoreq.15 to 50 m.sup.2/g and most
preferably of .gtoreq.15 to 40 m.sup.2/g. They advantageously have
a total pore volume of .gtoreq.0.1 ml/g, preferably of 0.15 to 0.5
ml/g and most preferably of 0.15 to 0.4 ml/g. Total pore volume
data in this document relate to determinations by the method of
mercury porosimetry using the Auto Pore 9220 test instrument from
Micromeritics GmbH, DE-4040 Neuss (range from 30 .ANG. to 0.3 mm).
As already stated, the vanadium-phosphorus oxide active materials
may be doped with promoter elements other than vanadium and
phosphorus. Useful such promoter elements include the elements of
groups 1 to 15 of the Periodic Table other than P and V. Doped
vanadium-phosphorus oxides are disclosed, for example, by WO
97/12674, WO 95/26817, U.S. Pat. No. 5,137,860, U.S. Pat. No.
5,296,436, U.S. Pat. No. 5,158,923, U.S. Pat. No. 4,795,818 and WO
2007/012620.
[0165] Promoters preferred in accordance with the invention are the
elements lithium, potassium, sodium, rubidium, cesium, thallium,
molybdenum, zinc, hafnium, zirconium, titanium, chromium,
manganese, nickel, copper, iron, boron, silicon, tin, niobium,
cobalt and bismuth, among which preference is given not only to
iron but especially to niobium, molybdenum, zinc and bismuth. The
vanadium-phosphorus oxide active materials may comprise one or more
promoter elements. The total content of promoters in the catalytic
active material is, based on the weight thereof, generally not more
than 5% by weight (the individual promoter element calculated in
each case as the electrically uncharged oxide in which the promoter
element has the same charge number (oxidation number) as in the
active material).
[0166] Useful active materials for aldol condensation catalysts B
for charging reaction zone B are thus especially multielement oxide
active materials of the general formula II
V.sub.1P.sub.bFe.sub.cX.sup.1.sub.dX.sup.2.sub.eO.sub.n (II),
in which the variables are each defined as follows: [0167]
X.sup.1=Mo, Bi, Co, Ni, Si, Zn, Hf, Zr, Ti, Cr, Mn, Cu, B, Sn
and/or Nb, preferably Nb, Mo, Zn and/or Hf, [0168] X.sup.2=Li, K,
Na, Rb, Cs and/or Tl, [0169] b=0.9 to 2.0, preferably 0.9 to 1.5,
more preferably 0.9 to 1.2 and most preferably 1.0 to 1.1, [0170]
c=.gtoreq.0 to 0.1, [0171] d=.gtoreq.0 to 0.1, [0172] e=.gtoreq.0
to 0.1, and [0173] n=the stoichiometric coefficient of the element
oxygen, which is determined by the stoichiometric coefficients of
the non-oxygen elements and the charge numbers thereof in II.
[0174] Irrespective of the stoichiometric coefficients d, e and b,
the stoichiometric coefficient C is, advantageously in accordance
with the invention, in active materials of the general formula II,
0.005 to 0.1, preferably 0.005 to 0.05 and particularly
advantageously, 0.005 to 0.02.
[0175] The aldol condensation catalysts B may comprise the
multimetal oxide active materials of the general formula II, for
example, in pure, undiluted form, or diluted with an oxidic,
essentially inert dilution material in the form of unsupported
catalysts. Inert dilution materials suitable in accordance with the
invention include, for example, finely divided aluminum oxide,
silicon dioxide, aluminosilicates, zirconium dioxide, titanium
dioxide or mixtures thereof. Undiluted unsupported catalysts are
preferred in accordance with the invention. The unsupported
catalysts may in principle be of any shape. Preferred shaped
unsupported catalyst bodies are spheres, solid cylinders, hollow
cylinders and trilobes, the longest dimension of which in all cases
is advantageously 1 to 10 mm.
[0176] In the case of shaped unsupported catalyst bodies, the
shaping is advantageously effected with precursor powder which is
calcined only after the shaping. The shaping is effected typically
with addition of shaping assistants, for example graphite
(lubricant) or mineral fibers (reinforcing aids). Suitable shaping
processes are tableting and extrusion.
[0177] The external diameter of cylindrical unsupported catalysts
is, appropriately in application terms, 3 to 10 mm, preferably 4 to
8 mm and in particular 5 to 7 mm. The height thereof is
advantageously 1 to 10 mm, preferably 2 to 6 mm and in particular 3
to 5 mm. The same applies in the case of hollow cylinders. In
addition, the internal diameter of the orifice running through from
the top downward is advantageously 1 to 8 mm, preferably 2 to 6 mm
and most preferably 2 to 4 mm. A wall thickness of 1 to 3 mm is
appropriate in application terms in the case of hollow cylinders.
It will be appreciated that the doped or undoped
vanadium-phosphorus oxide active material can also be used in
powder form, or as eggshell catalysts with an active material
eggshell applied to the surface of inert shaped support bodies, as
aldol condensation catalysts B in reaction zone B. The preparation
of the eggshell catalysts, the eggshell thickness and the geometry
of the inert shaped support bodies may be selected as described in
the case of the eggshell catalysts for reaction zone A.
[0178] Otherwise, doped or undoped vanadium-phosphorus oxide active
materials and unsupported catalysts manufactured therefrom can be
produced as described in the documents of the prior art, to which
reference is made in this patent application.
[0179] These are especially the documents WO 2007/012620, WO
2010/07273, WO 2010/000720 and WO 2010/000764.
[0180] For example, the procedure may be as follows: [0181] a)
reaction of a pentavalent vanadium compound (e.g. V.sub.2O.sub.5)
with an organic reducing solvent (e.g. isobutanol) in the presence
of a pentavalent phosphorus compound (e.g. ortho- and/or
pyrophosphoric acid) with heating to 75 to 205.degree. C.,
preferably to 100 to 120.degree. C.; [0182] b) cooling of the
reaction mixture to advantageously 4 to 90.degree. C.; [0183] c)
optional addition of compounds comprising doping elements, for
example iron(III) phosphate; [0184] d) reheating to 75 to
205.degree. C., preferably 100 to 120.degree. C.; [0185] e)
isolation of the solid precursor material formed, comprising V, P,
O and, for example, Fe (for example by filtering); [0186] f) drying
and/or thermal pretreatment of the precursor material (optionally
until commencement of performing by elimination of water from the
precursor material); [0187] g) addition of shaping assistants, for
example finely divided graphite or mineral fibers, and subsequent
shaping to give the shaped unsupported catalyst precursor body by,
for example, tableting; [0188] h) subsequent thermal treatment of
the shaped catalyst precursor bodies formed by heating in an
atmosphere which comprises oxygen, nitrogen, noble gases, carbon
dioxide, carbon monoxide and/or steam (for example as described in
WO 2003/078310 at page 20, line 16 to page 21, line 35). The
temperature of the thermal treatment generally exceeds 250.degree.
C., in many cases 300.degree. C. or 350.degree. C., but normally
not 600.degree. C., preferably not 550.degree. C. and most
preferably not 500.degree. C.
[0189] The space velocity on the catalyst charge of reaction zone B
of formaldehyde present in reaction gas input mixture B may, in
accordance with the invention, be for example 1 to 100, preferably
2 to 50 and more preferably 3 to 30 or 4 to 10 l (STP)/lh. The term
"space velocity" is used as defined in DE-A 19927624. Both in
reaction zone A and in reaction zone B, the particular fixed
catalyst bed (including the case of a section 1/section 2 division
in reaction zone A) may consist only of catalysts comprising active
material, or else of a mixture of catalysts comprising active
material and inert shaped bodies.
[0190] Especially in the case of use of V--P--O catalysts as aldol
condensation catalysts in reaction zone B, in the process according
to the invention, based on a single pass of reaction gas mixture B
through reaction zone B, at least 95 mol %, usually at least 98 mol
%, of the formaldehyde present in reaction gas input mixture B is
converted. The selectivity of acrylic acid formation, based on
formaldehyde converted, is generally .gtoreq.95 mol %, frequently
.gtoreq.98 mol %.
[0191] Suitable in accordance with the invention for configuration
of reaction zone B are those heat exchanger reactors which have
already been recommended for implementation of reaction zone A.
[0192] As already mentioned, formaldehyde is prepared on the
industrial scale by heterogeneously catalyzed partial gas phase
oxidation of methanol. A formaldehyde source which is particularly
preferred in accordance with the invention for formation of
reaction gas input mixture B is therefore the product gas mixture
of a heterogeneously catalyzed partial gas phase oxidation of
methanol to formaldehyde.
[0193] Appropriately in application terms, the process according to
the invention therefore comprise a further, third reaction zone C
which is charged with at least one oxidation catalyst C and
advantageously comprises the following additional measures: [0194]
a stream of a reaction gas input mixture C comprising the methanol
and molecular oxygen reactants and at least one inert diluent gas
other than steam is passed through a third reaction zone C charged
with at least one oxidation catalyst C and methanol present in
reaction gas input mixture C, as it passes through reaction zone C,
is oxidized under heterogeneous catalysis to formaldehyde and
steam, so as to form a product gas mixture C comprising
formaldehyde, steam and at least one inert diluent gas other than
steam, and a stream of product gas mixture C leaves reaction zone
C, it optionally being possible to add further molecular oxygen
and/or further inert diluent gas to reaction gas mixture C flowing
through reaction zone C on its way through reaction zone C.
[0195] The formaldehyde-comprising stream of product gas mixture C
leaving reaction zone C can then be used as such (i.e. without
performing a removal process thereon beforehand) in order to obtain
reaction gas input mixture B. In general, for this purpose, product
gas mixture C will first be cooled (quenched) on leaving reaction
zone C, in order to reduce undesired further reactions in product
gas mixture C before introducing it into reaction gas input mixture
B. Typically, it will be cooled very rapidly to temperatures of 150
to 350.degree. C., or 200 to 250.degree. C.
[0196] Optionally, however, it is also possible first to remove, in
a separation zone T*, a portion or the entirety of any methanol
which is still present in product gas mixture C and has not been
converted in reaction zone C therefrom, and then to use the
remaining formaldehyde-comprising product gas mixture C* (which can
pass through the liquid state in the course of removal) to obtain
reaction gas input mixture B. Advantageously in application terms,
the removal will be undertaken by rectificative means. For this
purpose, product gas mixture C, optionally after preceding direct
or indirect cooling, can be supplied in gaseous form to the
appropriate rectification column provided with cooling circuit. It
is of course, however, possible first to convert, from product gas
mixture C, those constituents whose boiling point at standard
pressure (10.sup.5 Pa) is less than or equal to the boiling point
of formaldehyde to the liquid phase (for example by condensation)
and to undertake the rectification from the liquid phase. In
general, such a methanol removal is also accompanied by a removal
of steam present in product gas mixture C. For the purpose of the
aforementioned direct cooling, it is possible, for example, to use
liquid phase which has been withdrawn from the bottom region of the
rectification column and optionally additionally cooled by indirect
heat exchange, which is sprayed by means of appropriate nozzles
into fine droplets which provide the large heat exchange area
required for the hot product gas mixture C. The methanol removed
will, appropriately in accordance with the invention, be recycled
into reaction zone C and used to obtain reaction gas input mixture
C (cf. DE-A 1618413). A removal of methanol from product gas
mixture C prior to the use thereof to obtain reaction gas input
mixture B is generally undertaken when reaction zone C is
configured such that the resulting conversion of methanol in
reaction zone C, based on a single pass of product gas mixture C
through reaction zone C, is not more than 90 mol %. It will be
appreciated that such a methanol removal can, however, also be
employed in the case of corresponding methanol conversions of not
more than 95 mol %. For example, such a methanol removal can be
undertaken as described in Ullmann's Encyclopedia of Industrial
Chemistry, vol. A11, 5th Ed., VCH Weinheim on pages 626 ff.
[0197] The oxidation catalysts C which are particularly suitable
for charging reaction zone C can be divided essentially into two
groups.
[0198] The first of the two groups comprises what are called the
silver catalysts, which have, as the active material, elemental
silver whose purity is preferably .gtoreq.99.7% by weight,
advantageously .gtoreq.99.8% by weight, preferably .gtoreq.99.9% by
weight and most preferably .gtoreq.99.99% by weight. The
corresponding processes for heterogeneously catalyzed partial gas
phase oxidation of methanol to formaldehyde over these "silver
catalysts" are described in the prior art as silver processes (cf.,
for example, "A. Nagy, G. Mestl: High temperature partial oxidation
reactions over silver catalysts, Appl. Catal. 188 (1999), p. 337 to
353", "H. Schubert, U. Tegtmayr, R. Schlogl: On the mechanism of
the selective oxidation of methanol over elemental silver, Catalyst
Letters, 28 (1994), p. 383 to 395", "L. Lefferts, Factors
controlling the selectivity of silver catalysts for methanol
oxidation, thesis, University of Twente (1987)" and DE-A
2334981).
[0199] Silver oxidation catalysts C advantageous in accordance with
the invention for charging of reaction zone C are disclosed, for
example, in Ullmann's Encyclopedia of Industrial Chemistry, vol.
A11, 5th ed., VCH, Weinheim, p. 619 to 652, or in Encyclopedia of
Chemical Technology, vol. 11, 4th ed., Wiley & Sons, New York,
p. 929 to 949, in DE-B 1231229, in DE-B 1294360, in DE-A 1903197
and in BE patent 683130. Typically, these comprise crystals (the
shape of which may also be round) of elemental silver (preferably
of the abovementioned purity) which have been deposited by
electrolysis of aqueous silver salt solutions and which can be
poured as a fixed catalyst bed onto a perforated base (for example
a perforated plate, a sieve or a mesh network (preferably likewise
manufactured from silver)) (typical bed heights are 10 to 50 mm,
frequently 15 to 30 mm). The total content of metals present in
elemental form other than silver in the catalytically active silver
(e.g. Cu, Pd, Pb, Bi, Fe, Pt and Au) is advantageously .ltoreq.2000
ppm by weight, better .ltoreq.1000 ppm by weight, preferably
.ltoreq.100 ppm by weight and more preferably .ltoreq.50 ppm by
weight or .ltoreq.30 ppm by weight. The longest dimension of the
silver crystals is typically in the range from 0.1 to 5 mm and
preferably increases in flow direction of reaction gas mixture C.
The fixed silver bed is preferably configured as a two-layer bed,
in which case the lower layer has a thickness, for example, of 15
to 40 mm, preferably 20 to 30 mm, and consists to an extent of at
least 50% by weight of silver crystals of particle size 1 to 4 mm,
preferably 1 to 2.5 mm. The upper layer may have, for example, a
thickness (layer thickness) of 0.75 to 3 mm, preferably 1 to 2 mm,
and consist of crystals having particle sizes (longest dimensions)
of 0.1 to 1 mm, preferably 0.2 to 0.75 mm. In this case, reaction
gas input mixture C flows in from the top downward.
[0200] In order to counteract sintering of the silver crystals with
increasing operating time (at comparatively high reaction
temperatures), which reduces the performance of the fixed catalyst
bed, WO 2010/022923 recommends coating the silver crystals with a
thin porous layer of oxidic material of at least one of the
elements Al, Si, Zr and Ti (the layer thickness may be 0.3 to 10
.mu.m, preferably 1.0 to 5.0 .mu.m, more preferably 2.0 to 4.0
.mu.m and at best about 3 .mu.m), and in this way achieving
prolonging of the service life of the fixed catalyst bed.
[0201] The methanol content in reaction gas input mixture C is, in
the silver process, normally at least 5% by volume, usually at
least 10% by volume, and may extend up to 60% by volume. The
aforementioned methanol content in the silver process is preferably
15 to 50% by volume and more preferably 20 to 40 or to 30% by
volume.
[0202] In addition, the ratio of the molar amount of molecular
oxygen present in reaction gas input mixture C (n.sub.O) to the
molar amount of methanol present in reaction gas input mixture C
(n.sub.Me), n.sub.O:n.sub.Me, in the silver process is normally
less than 1 (<1), preferably .ltoreq.0.8. It will more
preferably be 0.2 to 0.6 and most preferably 0.3 to 0.5 or 0.4 to
0.5. In general, n.sub.O:n.sub.Me in the silver process will not be
less than 0.1.
[0203] The statements made for the inert diluent gases for reaction
zone A essentially also apply to reaction zone C in the case of the
silver process. Examples of inert diluent gases usable in reaction
gas input mixture C in the case of the silver process are H.sub.2O,
CO.sub.2, N.sub.2 and noble gases such as Ar, and mixtures of the
aforementioned gases.
[0204] A preferred inert diluent gas other than steam in the case
of the silver process is molecular nitrogen for reaction gas input
mixture C too. The advantage thereof is based not least on the fact
that molecular nitrogen occurs in air as a natural companion of
molecular oxygen, which makes air a preferred source of the
molecular oxygen required in reaction zone C. It will be
appreciated that, in the case of the silver process, it is,
however, also possible in accordance with the invention to use pure
molecular oxygen, or air enriched with molecular oxygen, or another
mixture of molecular oxygen and inert diluent gas, as the oxygen
source.
[0205] Typically, reaction gas input mixture C comprises, in the
case of the silver process, 20 to 80% by volume, or 30 to 70% by
volume, or 40 to 60% by volume, of inert diluent gas. The latter
may be entirely free of steam. In other words, reaction gas input
mixture C in the case of the silver process may comprise 20 to 80%
by volume, or 30 to 70% by volume, or 40 to 60% by volume, of
molecular nitrogen. For similar reasons to those for reaction gas
input mixture A, however, in the case of the silver process, steam
is generally also used as an inert diluent gas in reaction gas
input mixture C.
[0206] In principle, reaction gas input mixture C in the case of
the silver process may comprise >0 to 50% by volume of H.sub.2O:
For comparable reasons to those in the case of reaction gas input
mixture A, it is, however, advantageous in accordance with the
invention to comparatively restrict the H.sub.2O content of
reaction gas input mixture C in the case of the silver process.
[0207] In other words, reaction gas input mixture C preferably
comprises, in the case of the silver process, .gtoreq.5 to 45% by
volume of H.sub.2O, advantageously .gtoreq.10 to 40% by volume and
particularly advantageously 15 to 35% by volume or 20 to 30% by
volume of H.sub.2O. The inert gas source used in the case of the
silver process, for reaction gas input mixture C too, may be the
stream Z obtained in separation zone T. Appropriately in
application terms, in the case of the silver process, a substream
of stream Z will therefore be recycled into reaction zone C to
obtain reaction gas input mixture C.
[0208] In other words, reaction gas input mixtures C suitable in
accordance with the invention may, in the case of the silver
process, comprise, for example, 10 to 50% by volume of H.sub.2O and
20 to 60% by volume of inert diluent gas other than steam (e.g.
N.sub.2, or N.sub.2+CO.sub.2, or N.sub.2+noble gas (e.g. Ar), or
N.sub.2+CO.sub.2+noble gas (e.g. Ar).
[0209] It will be appreciated that reaction gas input mixtures C,
in the case of the silver process, may also comprise 10 to 40% by
volume of H.sub.2O and 30 to 60% by volume of inert diluent gases
other than steam (for example those mentioned above).
[0210] Of course, reaction gas input mixture C, in the case of the
silver process, may also comprise 20 to 40% by volume of H.sub.2O
and 30 to 50% by volume of inert diluent gases other than steam
(for example those mentioned above).
[0211] In principle, in the case of the silver process, reaction
gas mixture C can be even forced or sucked through reaction zone C.
Accordingly, the working pressure in the case of the silver process
within reaction zone C may be either .gtoreq.10.sup.5 Pa or
<10.sup.5 Pa. Appropriately in application terms, the working
pressure in the case of the silver process in reaction zone C will
be 10.sup.3 to 10.sup.6 Pa, preferably 10.sup.4 to 510.sup.5 Pa,
more preferably 10.sup.4 to 210.sup.5 Pa and most preferably
0.510.sup.5 Pa to 1.810.sup.5 Pa.
[0212] The temperature of reaction gas mixture C (the term
"reaction gas mixture C" comprises, in the present application, all
gas mixtures which occur in reaction zone C and are between
reaction gas input mixture C and product gas mixture C) will, in
the case of the silver process, within reaction zone C, normally be
within the range from 400 to 800.degree. C., preferably within the
range from 450 to 800.degree. C. and more preferably within the
range from 500 to 800.degree. C. The term "temperature of reaction
gas mixture C" (also referred to in this document as reaction
temperature in reaction zone C) means primarily that temperature
which reaction gas mixture C has from attainment of a conversion of
the methanol present in reaction gas input mixture C of at least 5
mol % until attainment of the corresponding final conversion of the
methanol within reaction zone C.
[0213] Advantageously in accordance with the invention, the
temperature of reaction gas input mixture C in the case of the
silver process is within the aforementioned temperature ranges over
the entire reaction zone C.
[0214] Advantageously, in the case of the silver process, reaction
gas input mixture C is also supplied to reaction zone C already
with a temperature within the aforementioned range. Frequently, in
the case of the silver process, a charge of reaction zone C with
solid inert material or of catalytically active catalyst charge
highly diluted with such inert material is present at the inlet
into reaction zone C upstream in flow direction of the actual
catalytically active catalyst charge (which may also be diluted
with inert shaped bodies). As it flows through such an upstream
charge of reaction zone C, the temperature of the reaction gas
input mixture C supplied to reaction zone C in the case of the
silver process can be adjusted comparatively easily to the value
with which reaction gas mixture C in the case of the silver process
is to enter the actual catalytically active catalyst charge of
reaction zone C.
[0215] When the temperature of reaction gas mixture C in the case
of the silver process within reaction zone C is limited to values
of 450 to 650.degree. C., preferably 500 to 600.degree. C., the
conversion of methanol will generally be .ltoreq.90 mol %,
frequently .ltoreq.85 mol % or .ltoreq.80 mol %, while the
selectivity of formaldehyde formation will be at values of
.gtoreq.90 mol %, in many cases .gtoreq.93 mol % or .gtoreq.95 mol
%. In this case (in which the steam content of the reaction gas
input mixture is preferably <10% by volume), it is appropriate
in accordance with the invention, from product gas mixture C, to
remove at least a portion of unconverted methanol prior to the use
thereof for obtaining reaction gas input mixture B, and to recycle
it into the production of reaction gas input mixture C.
[0216] Advantageously in accordance with the invention, the
temperature of reaction gas mixture C in the case of the silver
process within reaction zone C will therefore be 550 to 800.degree.
C., preferably 600 to 750.degree. C. and more preferably 650 to
750.degree. C.
[0217] At the same time, the steam content of reaction gas input
mixture C in the case of the silver process is advantageously
adjusted to values of .gtoreq.10% by volume, preferably .gtoreq.15%
by volume and particularly advantageously .gtoreq.20% by volume.
Both the elevated temperature and the elevated steam content of
reaction gas input mixture C, in the case of the silver process,
have an advantageous effect on the methanol conversion (based on a
single pass of reaction gas mixture C through reaction zone C). In
general, this conversion will be >90 mol %, in many cases
.gtoreq.92 mol %, or .gtoreq.95 mol % and frequently even
.gtoreq.97 mol % (cf., for example, Ullmann's Encyclopedia of
Industrial Chemistry, vol. A 11, 5th ed., VCH Weinheim on pages 625
ff.) (the high methanol conversions which are to be achieved in the
case of the silver process in spite of the comparatively low
n.sub.O:n.sub.Me ratios in reaction gas input mixture C are
attributable in particular to the fact that, with increasing
temperature of reaction gas mixture C in reaction zone C, the
exothermic partial oxidation
CH.sub.3OH+0.5O.sub.2.fwdarw.HCHO+H.sub.2O is increasingly
accompanied by the endothermic dehydrogenation
CH.sub.3OH.revreaction.HCHO+H.sub.2). In this way, in the case of
the silver process, it is regularly possible to achieve yields of
formaldehyde of .gtoreq.85 mol %, usually .gtoreq.87 mol % and in
many cases .gtoreq.89 mol % based on a single pass of reaction gas
mixture C through reaction zone C and the molar amount of methanol
converted. Otherwise, the silver process can be performed as
described in the prior art documents already mentioned in this
regard, or as described in documents U.S. Pat. No. 4,080,383, U.S.
Pat. No. 3,994,977, U.S. Pat. No. 3,987,107, U.S. Pat. No.
4,584,412 and U.S. Pat. No. 4,343,954. It will be appreciated that,
in the case of the silver process described, it is possible not
only to use comparatively pure methanol as the raw material
(source). Methanol raw materials suitable in accordance with the
invention in this regard are also aqueous methanol solutions and
technical-grade methanol, which can be used after appropriate
evaporation to obtain reaction gas input mixture C.
[0218] Suitable reactors for execution of the silver process in
reaction zone C include not only those recommended in the
aforementioned prior art but also those heat exchanger reactors
which have already been recommended for implementation of reaction
zone A. The space velocity of methanol present in reaction gas
input mixture C on the reactor charged with silver crystals will
generally be (0.5 to 6)10.sup.3 kg of methanol per m.sup.2 of
reactor cross section or cross section of the fixed catalyst
bed.
[0219] Preferably in accordance with the invention, the
heterogeneously catalyzed partial gas phase oxidation of methanol
to formaldehyde will, however, be performed by the FORMOX
process.
[0220] In contrast to the silver process, the FORMOX process is
performed over oxidation catalysts C whose active material is a
mixed oxide which has at least one transition metal in the oxidized
state (cf., for example, WO 03/053556 and EP-A 2213370). The term
"transition metals" means the chemical elements of the Periodic
Table with atomic numbers 21 to 30, 39 to 48 and 57 to 80.
[0221] Preferably in accordance with the invention, aforementioned
mixed oxide active materials comprise at least one of the
transition metals Mo and V in the oxidized state. Most preferably
in accordance with the invention, the aforementioned active
materials are mixed oxides having at least one of the elements Fe
and Mo in the oxidized state (cf., for example, U.S. Pat. No.
3,983,073, U.S. Pat. No. 3,978,136, U.S. Pat. No. 3,975,302, U.S.
Pat. No. 3,846,341, U.S. Pat. No. 3,716,497, U.S. Pat. No.
4,829,042, EP-A 2213370 and WO 2005/063375, U.S. Pat. No.
3,408,309, U.S. Pat. No. 3,198,753, U.S. Pat. No. 3,152,997, WO
2009/1489809, DE-A 2145851, WO 2010/034480, WO 2007/059974 and
"Methanol Selective Oxidation to Formaldehyde over Iron-Molybdate
Catalysts, Ana Paula Vieira Soares and Manuel Farinha Portela and
Alain Kiennemann in Catalysis Review 47, pages 125 to 174 (2004)"
and the prior art cited in these documents).
[0222] A further difference between the silver process and the
FORMOX process is that the ratio of the molar amount of molecular
oxygen present in reaction gas input mixture C (n.sub.O) to the
molar amount of methanol present in reaction gas input mixture C
(n.sub.Me), n.sub.O:n.sub.Me, is normally at least 1 or greater
than 1 (.gtoreq.1), preferably .gtoreq.1.1. In general, the
n.sub.O:n.sub.Me ratio in reaction gas input mixture C in the
FORMOX process will, however, be not more than 5, frequently not
more than 4. n.sub.O:n.sub.Me ratios which are advantageous in
accordance with the invention in reaction gas input mixture C are
1.5 to 3.5, preferably 2 to 3. In addition, the methanol content of
reaction gas input mixture C in the FORMOX process will typically
be not more than 15% by volume, usually not more than 11% by
volume. This is because gas mixtures of molecular nitrogen,
molecular oxygen and methanol with a molecular oxygen content of
not more than approx. 11% by volume of molecular oxygen are outside
the explosion range. Normally, the methanol content in reaction gas
input mixture C in the case of the FORMOX process will be
.gtoreq.2% by volume, preferably 4 to 10% by volume and more
preferably 6 to 9% by volume or 5 to 7% by volume. Gas mixtures of
molecular nitrogen, molecular oxygen and methanol whose methanol
content is .ltoreq.6.7% by volume are, irrespective of the
molecular oxygen content therein, outside the explosion range,
which is why particularly high n.sub.O:n.sub.Me ratios in reaction
gas input mixture C can be employed within this concentration
range.
[0223] However, the FORMOX process also differs from the silver
process in that the methanol conversions achieved by this process,
based on a single pass of reaction gas mixture C through reaction
zone C, essentially irrespective of the inert diluent gas used in
reaction gas input mixture C, are regularly >90 mol %, typically
.gtoreq.92 mol %, usually .gtoreq.95 mol % and in many cases even
.gtoreq.97 mol % or .gtoreq.98 mol %, or .gtoreq.99 mol %. The
accompanying selectivities of formaldehyde formation are regularly
.gtoreq.90 mol %, usually .gtoreq.92 mol % and in many cases
.gtoreq.94 mol %, and frequently even .gtoreq.96 mol %.
[0224] According to the invention, useful inert diluent gases in
reaction gas input mixture C for the FORMOX process in reaction
zone C are likewise gases such as H.sub.2O, N.sub.2, CO.sub.2 and
noble gases such as Ar, and mixtures of aforementioned gases. A
preferred inert diluent gas other than steam in the case of the
FORMOX process too in reaction gas input mixture C is molecular
nitrogen.
[0225] The inert diluent gas content (the definition of an inert
diluent gas for reaction zone C is analogous to that for reaction
zones A and B) in reaction gas input mixture C may, in the case of
the FORMOX process, be 70 to 95% by volume, frequently 70 to 90% by
volume and advantageously 70 to 85% by volume. In other words, the
molecular nitrogen content of reaction gas input mixture C may, in
the case of employment of the FORMOX process, in reaction gas input
mixture C, be 70 to 95% by volume, or 70 to 90% by volume, or 70 to
85% by volume. Advantageously in accordance with the invention,
reaction gas input mixture C in the case of the FORMOX process may
be free of steam. Appropriately in application terms, reaction gas
input mixture C, in the case of employment of a FORMOX process in
reaction zone C, may have a low steam content for the same reasons
as in the case of reaction gas input mixture A. In general, the
steam content of reaction gas input mixture C in the FORMOX process
in reaction zone C is .gtoreq.0.1% by volume and .ltoreq.20% by
volume or .ltoreq.10% by volume, advantageously .gtoreq.0.2% by
volume and .ltoreq.7% by volume, preferably .gtoreq.0.5% by volume
and .ltoreq.5% by volume.
[0226] A further advantage of the employment of a FORMOX process in
reaction zone C, in accordance with the invention, results from the
fact that the high methanol conversions described are established
as significantly lower reaction temperatures compared to the use of
a silver process.
[0227] The temperature of reaction gas mixture C in the case of the
FORMOX process in reaction zone C will normally be in the range
from 250 to 500.degree. C., preferably in the range from 300 to
450.degree. C. and frequently within the range from 270 to
400.degree. C. The meaning of the term "temperature of reaction gas
mixture C" corresponds in the case of the FORMOX process to that
which has already been given in this document for the silver
process.
[0228] Advantageously in accordance with the invention, the
temperature of reaction gas mixture C (also referred to in this
document as the reaction temperature in reaction zone C) in the
case of the FORMOX process, over the entire reaction zone C, is
within the aforementioned temperature ranges. Advantageously, in
the case of the FORMOX process too, reaction gas input mixture C is
supplied to reaction zone C already with a temperature within the
aforementioned range. Frequently, in the case of the FORMOX
process, a charge of reaction zone C with solid inert material or
of catalytically active catalyst charge highly diluted with such
inert material is present at the inlet into reaction zone C
upstream in flow direction of the actual catalytically active
catalyst charge (which may also be diluted with inert shaped
bodies). As it flows through such an upstream charge of reaction
zone C, the temperature of reaction gas input mixture C supplied to
reaction zone C in the FORMOX process can be adjusted in a
comparatively simple manner to the value with which reaction gas
mixture C in the FORMOX process is to enter the actual
catalytically active catalyst charge.
[0229] With regard to the working pressure in reaction zone C, the
statements made for the silver process apply correspondingly to the
FORMOX process.
[0230] Mixed oxide active materials particularly suitable for the
FORMOX process are those of the general formula III
[Fe.sub.2(Mo.sub.4).sub.3].sub.1[M.sup.1.sub.mO.sub.n].sub.q
(III),
in which the variables are each defined as follows: [0231] M.sup.1
Mo and/or Fe, or [0232] Mo and/or Fe and, based on the total molar
amount of Mo and Fe, a total molar amount of up to 10 mol % (e.g.
0.01 to 10 mol %, or 0.1 to 10 mol %), preferably not more than 5
mol %, of one or more elements from the group consisting of Ti, Sb,
Sn, Ni, Cr, Ce, Al, Ca, Mg, V, Nb, Ag, Mn, Cu, Co, Si, Na, K, Tl,
Zr, W, Ir, Ta, As, P and B, [0233] q=0 to 5, or 0.5 to 3, or 1 to
2, [0234] m=1 to 3, and [0235] n=1 to 6, with the proviso that the
contents of both sets of square brackets are electrically
uncharged, i.e. do not have any electrical charge.
[0236] Advantageously in accordance with the invention, mixed oxide
active materials III comprise less than 50 mol %, more preferably
less than 20 mol % and more preferably less than 10 mol % of the Fe
present in the mixed oxide active material III in the +2 oxidation
state, and the remaining amount of the Fe present therein in each
case in the +3 oxidation state. Most preferably, the mixed oxide
active material III comprises all of the Fe present therein in the
+3 oxidation state.
[0237] The n.sub.Mo:n.sub.Fe ratio of molar amount of Mo present in
a mixed oxide active material III (n.sub.Mo) to molar amount of Fe
present in the same mixed oxide active material (n.sub.Fe) is
preferably 1:1 to 5:1.
[0238] In addition, it is advantageous in accordance with the
invention when M.sup.1=Mo and m=1 and n=3. Mixed oxide active
materials advantageous in accordance with the invention also exist
when M.sup.1=Fe and m=2 and n=3.
[0239] Mixed oxide active materials III favorable in accordance
with the invention are also those with such a stoichiometry that
they can be considered (represented) in a formal sense as a mixture
of MoO.sub.3 and Fe.sub.2O.sub.3, and the MoO.sub.3 content of the
mixture is 65 to 95% by weight and the Fe.sub.2O.sub.3 content of
the mixture 5 to 35% by weight.
[0240] Mixed oxide active materials III can be prepared as
described in the prior art documents cited.
[0241] In general, the procedure will be to obtain, from sources of
the catalytically active oxide material III, a very intimate,
preferably finely divided, dry mixture of composition corresponding
to the stoichiometry of the desired oxide material III (a precursor
material), and calcining (thermally treating) it at temperatures of
300 to 600.degree. C., preferably 400 to 550.degree. C. The
calcination can be performed either under inert gas or under an
oxidative atmosphere, for example air (or another mixture of inert
gas and oxygen), or else under a reducing atmosphere (for example a
mixture of inert gas and reducing gases such as NH.sub.3 and CO).
The calcination time will generally be a few hours and typically
decreases with the magnitude of the calcination temperature.
[0242] Useful sources for the elemental constituents of the mixed
oxide active materials III are especially those compounds which are
already oxides and/or those compounds which can be converted to
oxides by heating, at least in the presence of oxygen. The intimate
mixing of the starting compounds (sources) can be effected in dry
or in wet form. Where it is effected in dry form, the starting
compounds are appropriately used in the form of fine powders and,
after mixing and optional compaction, subjected to calcination.
However, preference is given to effecting the intimate mixing in
wet form. In this case, the starting compounds are typically mixed
with one another in the form of aqueous suspensions and/or
solutions. Particularly intimate dry mixtures are obtained in the
mixing process described when the starting materials are
exclusively sources of the elemental constituents present in
dissolved form.
[0243] The solvent used is preferably water. Preference is given to
preparing, from the starting compounds, at least two aqueous
solutions, at least one of which is an acidic solution and at least
one of which an ammoniacal (basic) solution.
[0244] Combination of the aqueous solutions generally results in
precipitation reactions in which precursor compounds of the
multimetal oxide active material III form.
[0245] Subsequently, the aqueous material obtained is dry, and the
drying operation can be effected, for example, by spray drying.
[0246] The catalytically active oxide material obtained after the
calcining of the dry material can be used to charge reaction zone C
for the FORMOX process in finely divided form as such, or applied
with the aid of a liquid binder to an outer surface of a shaped
support body in the form of an eggshell catalyst. However, eggshell
catalysts can also be produced by applying, with the aid of a
liquid binder, fine precursor powder to the outer surface of shaped
support bodies, and calcining the precursor substance only after
completion of application and drying.
[0247] The multimetal oxide active materials III can, however, also
be used in reaction zone C in pure, undiluted form, or diluted with
an oxidic, essentially inert diluent material, in the form of what
are called unsupported catalysts (this is preferred in accordance
with the invention). Examples of inert diluent materials suitable
in accordance with the invention include finely divided aluminum
oxide, silicon dioxide, aluminosilicates, zirconium dioxide,
titanium dioxide or mixtures thereof. Undiluted unsupported
catalysts are preferred in accordance with the invention.
[0248] In the case of shaped unsupported catalyst bodies, the
shaping is advantageously effected with precursor powder which is
not calcined until after the shaping. The shaping is effected
typically with addition of shaping assistants, for example graphite
(lubricant) or mineral fibers (reinforcing aid). Suitable shaping
processes are tableting and extrusion. It will be appreciated that
the shaping may, however, also be performed, for example, with a
mixture of active material powder and precursor powder, to which
shaping assistants and optionally inert diluent powders are again
added prior to the shaping. Shaping is followed by another
calcination. In principle, the shaping to unsupported catalysts can
also be performed only with already prefabricated active material
powder and optionally the aids mentioned. This procedure is less
advantageous. The shaping here too is generally followed by another
calcination.
[0249] A favorable Mo source is, for example, ammonium
heptamolybdate tetrahydrate
(NH.sub.4).sub.6(Mo.sub.7O.sub.24).4H.sub.2O. Advantageous iron
sources are, for example, iron(III) nitrate [Fe(NO.sub.3).sub.3],
iron(III) chloride [FeCl.sub.3] or hydrates of iron(III) nitrate,
for example Fe(NO.sub.3).sub.3.9H.sub.2O.
[0250] With regard to the geometries and materials suitable in
accordance with the invention for the shaped support bodies and the
coating processes for producing eggshell catalysts of the mixed
oxide active materials III, the statements already made in this
document in connection with eggshell oxidation catalysts A apply
correspondingly. Preferred geometries of the shaped support bodies
here too are spheres and rings, the longest dimension of which is 1
to 10 mm, frequently 2 to 8 mm or 3 to 6 mm. Ring geometries
favorable in accordance with the invention have hollow cylindrical
shaped support bodies with a length of 2 to 10 mm, an external
diameter of 4 to 10 mm and a wall thickness of 1 to 4 mm. The
hollow cylindrical shaped support bodies preferably have a length
of 3 to 6 mm, an external diameter of 4 to 8 mm and a wall
thickness of 1 to 2 mm.
[0251] The thickness of the eggshell of catalytically active oxide
material applied to the shaped support bodies in the aforementioned
eggshell catalysts is, in the case of the mixed oxide active
materials III too, appropriately in application terms, generally 10
to 1000 .mu.m. The eggshell thickness is preferably 10 to 500
.mu.m, more preferably 100 to 500 .mu.m and most preferably 200 to
300 .mu.m.
[0252] Preferred shaped unsupported catalyst bodies comprising
mixed oxide active materials III are solid cylinders, hollow
cylinders and trilobes. The external diameter of cylindrical
unsupported catalysts is, appropriately in application terms, 3 to
10 mm, preferably 4 to 8 mm and in particular 5 to 7 mm. The height
thereof is advantageously 1 to 10 mm, preferably 2 to 6 mm and in
particular 3 to 5 mm. The same applies in the case of hollow
cylinders. In addition, the internal diameter of the orifice
passing through from the top downward is advantageously 1 to 8 mm,
preferably 2 to 6 mm and most preferably 2 to 4 mm. Appropriately
in application terms, the wall thickness of hollow cylinders is 1
to 3 mm.
[0253] However, mixed oxide active material III oxidation catalysts
C can also be employed in reaction zone C in the form of supported
catalysts. In this case, the starting materials are comparatively
porous shaped support bodies which, for example, are impregnated
successively with the at least two solutions of the precursor
compounds. The precipitation reaction described proceeds in the
pores of the shaped support body, and the precursor compounds which
form therein can subsequently be converted to the desired mixed
oxide active material III by calcination. Alternatively, it is also
possible to impregnate with a solution comprising all sources
required in dissolved form, to dry and then to calcine (cf., for
example, DE-A 2442311). Otherwise, the procedure for preparation of
the mixed oxide active material III oxidation catalysts may be as
in the prior art documents to which reference is made in this
regard in this application.
[0254] These are especially documents U.S. Pat. No. 3,716,497, U.S.
Pat. No. 3,846,341, EP-A 199359, DE-A 2145851, U.S. Pat. No.
3,983,073, DE-A 2533209, EP-A 2213370 and Catalysis Review, 47,
pages 125-174 (2004).
[0255] It will be appreciated that, in the FORMOX process too, it
is not only possible to use comparatively pure methanol to obtain
reaction gas input mixture C. Methanol raw materials suitable in
this regard in accordance with the invention are also aqueous
methanol solutions and technical-grade methanol, which can be used
after appropriate evaporation to obtain reaction gas input mixture
C.
[0256] It is also possible to charge reaction zone C with a fixed
catalyst bed which comprises FORMOX oxidation catalysts C in a form
diluted with inert shaped bodies.
[0257] The space velocity on the fixed catalyst bed present in
reaction zone C of reaction gas input mixture C will, in the case
of a FORMOX process employed in accordance with the invention,
generally be 3500 l (STP)/lh to 75 000 l(STP)/lh, preferably 25 000
l (STP)/lh to 35 000 l (STP)/lh. The term "space velocity" is used
as defined in DE-A 19927624.
[0258] Suitable reactors for execution of the FORMOX process in
reaction zone C are especially also the heat exchanger reactors
which have already been recommended for implementation of reaction
zone A (cf., for example, WO 2005/063375).
[0259] According to the invention, especially advantageous
processes for preparing acrylic acid are those in which reaction
gas input mixture B is obtained using, as the formaldehyde source,
the product gas mixture C which leaves reaction zone C and is the
product of a FORMOX process performed in reaction zone C. This is
also because such a product gas mixture C, in contrast to a product
gas mixture C after the silver process, is free of molecular
hydrogen.
[0260] In other words, the product gas mixture C of a
heterogeneously catalyzed partial gas phase oxidation of methanol
to formaldehyde after the FORMOX process is (without subjecting it
to a removal process beforehand, without performing a removal
process thereon beforehand) the ideal formaldehyde source for
formaldehyde required in reaction gas input mixture B.
[0261] Frequently, product gas mixture C is obtained in the FORMOX
process with a temperature with which it can be used without
further thermal pretreatment for production of reaction gas input
mixture B. In many cases, the temperature of the product gas
mixture C leaving reaction zone C, both in the case of the silver
process and in the case of the FORMOX process, however, is
different from that temperature with which it is to be used to
obtain reaction gas input mixture B. Against this background, the
stream of product gas mixture C, on its way from reaction zone C
into reaction zone B, can flow through an indirect heat exchanger
in order to match its temperature to the addition temperature
envisaged for production of reaction gas input mixture B.
[0262] In principle, the charge of reaction zone C with at least
one oxidation catalyst C can be configured as a fluidized bed.
Advantageously in application terms, the charge of reaction zone C
with oxidation catalyst C is, however, configured as a fixed
bed.
[0263] For the sake of completeness, it should also be added that,
in the case of employment of the FORMOX process in reaction zone C
too, the stream Z obtained in separation zone T in the process
according to the invention constitutes a suitable inert gas source
for the inert gas required in reaction gas input mixture C and,
appropriately in application terms, a substream of stream Z is
recycled into reaction zone C to obtain reaction gas input mixture
C.
[0264] The product gas mixture B which leaves reaction zone B and
which comprises acrylic acid formed, unconverted acetic acid, at
least one inert diluent gas other than steam, molecular oxygen and
steam can be separated in a manner known per se into the at least
three streams X, Y and Z in a separation zone T.
[0265] For example, the separation can be effected by fractional
condensation, as recommended in documents DE-A 102007004960, DE-A
10 2007055086, DE-A 10243625, DE-A 10235847 and DE-A 19924532. In
this procedure, the temperature of product gas mixture B is
optionally first reduced by direct and/or indirect cooling, and
product gas mixture B is then passed into a condensation column
equipped with separating internals (for example mass transfer
trays) and optionally provided with cooling circuits, and
fractionally condensed ascending into itself within the
condensation column. Appropriate selection of the number of
theoretical plates in the condensation column allows streams X, Y
and Z to be conducted out of the condensation column as separate
fractions with the desired degree of enrichment in each case.
[0266] Appropriately in application terms, stream X is generally
removed with an acrylic acid content of .gtoreq.90% by weight,
preferably .gtoreq.95% by weight, and conducted out of the
condensation column. In the event of an increased purity
requirement, stream X can, advantageously in application terms, be
purified further by crystallization (preferably suspension
crystallization) (cf. the aforementioned prior art documents and WO
01/77056). It will be appreciated that the stream X conducted out
of the condensation column can also be purified further by
rectification. It is possible in both ways, with a comparatively
low level of complexity, to achieve acrylic acid purities of
.gtoreq.99.9% by weight, which are suitable for production of
water-absorbing resins by free-radical polymerization of monomer
mixtures comprising acrylic acid and/or the sodium salt
thereof.
[0267] The water-absorbing resins can be prepared, for example, as
described in documents WO 2008/116840, DE-A 102005062929, DE-A
102004057874, DE-A 102004057868, DE-A 102004004496 and DE-A
19854575.
[0268] In a corresponding manner, stream Y is normally also
conducted out of the condensation column with an acetic acid
content of .gtoreq.90% by weight, preferably .gtoreq.95% by weight.
The stream Y thus removed can be recycled as such into reaction
zone B to obtain reaction gas input mixture B. It will be
appreciated that it is also possible, prior to the recycling of the
stream Y removed as described into reaction zone B, to enrich the
acetic acid content thereof by rectificative and/or crystallizative
means (for example to acetic acid contents of .gtoreq.99% by
weight), or to remove stream Y in the condensation column directly
with such elevated purity by increasing the number of theoretical
plates therein. Stream Z normally leaves the condensation column
overhead.
[0269] Alternatively, it is also possible to proceed as recommended
in documents DE-A 102009027401 and DE-A 10336386. After optionally
preceding direct and/or indirect cooling, product mixture B in this
procedure, in an absorption column advantageously equipped with
separating internals, is conducted in countercurrent to an organic
solvent having a higher boiling point than acrylic acid at standard
pressure (10.sup.5 Pa) (useful examples of these are the organic
solvents specified in DE-A 102009027401 and in DE-A 10336386), and
the acetic acid and acrylic acid present in product gas mixture B
are absorbed into the organic solvent, while a stream Z leaves the
absorption column at the top thereof. From the absorbate comprising
acetic acid and acrylic acid, it is possible to remove streams X
and Y with the desired degree of enrichment in each case by
rectification (fractional distillation) in a rectification column
in a manner known per se through appropriate selection of the
number of theoretical plates. In general, this degree of enrichment
of acrylic acid or acetic acid will be at least 90% by weight,
preferably at least 95% by weight. A subsequent crystallizative
further purification of the stream X removed (for example as
disclosed in WO 01/77056) leads with a comparatively low level of
complexity to acrylic acid purities of .gtoreq.99.9% by weight,
which are suitable for production of water-absorbing resins by
free-radical polymerization of monomer mixtures comprising acrylic
acid and/or the sodium salt thereof. The stream Y removed by
rectification as described can be recycled as such, or after
optional crystallizative and/or rectificative further purification
(for example to acetic acid contents of .gtoreq.99% by weight) into
reaction zone B to obtain reaction gas input mixture B. By
appropriately increasing the number of theoretical plates, it is
also possible to remove stream Y from the absorbate by
rectification directly with such a degree of enrichment.
[0270] Instead of using an organic absorbent, following the
teaching of EP-A 551111 or EP-A 778255, it is also possible to
absorb the acrylic acid and acetic acid present in product gas
mixture B therefrom into an aqueous absorbent in an absorption
column, while a stream Z leaves the absorption column at the top
thereof. Subsequent rectificative separation of the aqueous
absorbate, with optional inclusion of an azeotropic entraining
agent, gives the desired streams X and Y.
[0271] The conversion of the acetic acid and acrylic acid present
in reaction gas mixture B to the condensed phase to leave a gaseous
stream Z can also be effected, for example, by one-stage
condensation of those constituents present in reaction gas mixture
B whose boiling points at standard pressure are not above that of
acetic acid. Subsequently, the condensate comprising acrylic acid
and acetic acid can be separated again, in the degree of enrichment
desired in each case, into at least one stream Y and at least one
stream X.
[0272] Appropriately in application terms, in the process according
to the invention, at least 90 mol %, preferably at least 95 mol %,
more preferably at least 98 mol % or at least 99 mol % of the
acetic acid present in product gas mixture B is recycled into
reaction zone B to obtain reaction gas input mixture B.
[0273] Instead of the process according to the invention being
followed by a process in which acrylic acid present in stream X or
a mixture of acrylic acid and one or more at least
monoethylenically unsaturated monomers other than acrylic acid
present in stream X are polymerized to polymers (for example by
free-radical means; the polymerization may, for example, be a
solution polymerization or an aqueous emulsion polymerization or a
suspension polymerization), the process according to the invention
may also be followed by a process in which acrylic acid present in
stream X is esterified with at least one alcohol having, for
example, 1 to 8 carbon atoms (for example an alcohol such as
methanol, ethanol, n-butanol, tert-butanol and 2-ethyl-hexanol) to
give the corresponding acrylic esters (acrylate). The process for
acrylic ester preparation may then again be followed by a process
in which the acrylic ester prepared or a mixture of the acrylic
ester prepared and one or more at least monoethylenically
unsaturated monomers other than the acrylic ester prepared are
polymerized to polymers (for example by free-radical means; the
polymerization may, for example, be a solution polymerization or an
aqueous emulsion polymerization or a suspension
polymerization).
[0274] For the sake of good order, it should also be emphasized
that a deactivation of the different catalysts in the different
reaction zones of the process according to the invention can be
counteracted by correspondingly increasing the reaction temperature
in the particular reaction zone (in order to keep the reactant
conversion based on a single pass of the reaction gas mixture
through the catalyst charge stable). It is also possible to
regenerate the oxidic active materials of reaction zones A, B and C
in a manner corresponding to that described for comparable oxidic
catalysts in WO 2005/042459, by passing over an oxidizing
oxygen-comprising gas at elevated temperature.
[0275] Reliable operation, especially in reaction zones A and C,
can be ensured in the process according to the invention by an
analogous application of the procedure described in WO
2004/007405.
[0276] The process according to the invention is notable firstly
for its broad and wide-ranging raw material basis in terms of time.
Secondly, it is a process which, in contrast to the prior art
processes, enables a smooth transition from "fossil acrylic acid"
to "renewable acrylic acid" while maintaining the procedure.
[0277] "Fossil acrylic acid" is understood to mean acrylic acid for
which the ratio of the molar amount of .sup.14C atomic nuclei
present in this acrylic acid to the molar amount of .sup.12C atomic
nuclei present in the same acrylic acid, n.sup.14C:n.sup.12C, is
vanishingly small.
[0278] "Renewable acrylic acid" is understood to mean acrylic acid
for which the n.sup.14C:n.sup.12C ratio corresponds to the ratio V*
of n.sup.14C:n.sup.12C present in the CO.sub.2 in the earth's
atmosphere, the n.sup.14C:n.sup.12C ratio being determined by the
procedure developed by Willard Frank Libby
(http://de.wikipedia.orgn/wiki/Radikohlenstoffdatierung).
[0279] The terms "renewable carbon" and "fossil carbon" are used
correspondingly in this document.
[0280] The process developed by Libby is based on the fact that,
compared to the two carbon atom nuclei .sup.12C and .sup.13C, the
third naturally occurring carbon nucleus .sup.14C is unstable and
is therefore also referred to as radiocarbon (half-life=approx.
5700 years).
[0281] In the upper layers of the earth's atmosphere, .sup.14C is
constantly being newly formed by nuclei reaction. At the same time,
.sup.14C decomposes with a half-life of 5700 years by
.beta.-decomposition. An equilibrium forms in the earth's
atmosphere between constant new formation and constant degradation,
and so the proportion of the .sup.14C nuclei in the carbon in the
atmosphere on earth is constant over long periods; a stable ratio
V* is present in the earth's atmosphere.
[0282] The radiocarbon produced in the atmosphere combines with
atmospheric oxygen to give CO.sub.2, which then gets into the
biosphere as a result of photosynthesis. Since life forms (plants,
animals, humans), in the course of their metabolism, constantly
exchange carbon with the atmosphere surrounding them in this way,
the same distribution ratio of the three carbon isotropes and hence
the same n.sup.14C:n.sup.12C ratio is established in living
organisms as is present in the surrounding atmosphere.
[0283] When this exchange is stopped at the time of death of the
life form, the ratio between .sup.14C and .sup.12C in the dead
organism changes because the decomposing .sup.14C atomic nuclei are
no longer replaced by new ones (the carbon present in the dead
organism becomes fossil).
[0284] If the death of the organism (life form) was more than 50
000 years ago, the .sup.14C content thereof is below the detection
limit. Present and future biological ("renewable") raw materials
and chemicals produced therefrom have the particular current
.sup.14C concentration in the CO.sub.2 in the atmosphere on the
earth (this n.sup.14C:n.sup.12C ratio=V*). Fossil carbon sources
such as coal, mineral oil or natural gas, however, have already
lain "dead" in the earth for several million years, and they
therefore, just like chemicals produced therefrom, no longer
comprise any .sup.14C.
[0285] When fossil ethanol (ethanol obtained from fossil raw
materials) and renewable formaldehyde (formaldehyde obtained from
renewable raw materials) are used in the process according to the
invention, an acrylic acid is obtained whose n.sup.14C:n.sup.12C
ratio is only (1/3).times.V.
[0286] When, in the process according to the invention, in
contrast, ethanol obtained from renewable raw materials and
formaldehyde obtained from fossil raw materials are used, an
acrylic acid is obtained whose n.sup.14C:n.sup.12C
ratio=(2/3).times.V*.
[0287] When, in the process according to the invention, both fossil
(renewable) ethanol and fossil (renewable) formaldehyde are used,
an acrylic acid is obtained whose n.sup.14C:n.sup.12C ratio=0
(=V*).
[0288] When the possibility of blending renewable and fossil
starting materials (raw materials) is additionally considered in
the process according to the invention, the manufacturer of acrylic
acid, when employing the inventive procedure, is thus able, without
altering the preparation process (i.e. with one and the same
production plant), in accordance with customer requirements (for
example the manufacturer of superabsorbents (=water-absorbing
resins)), to adjust the "renewable level" of the acrylic acid to be
supplied to this customer (the n.sup.14C:n.sup.12C ratio desired by
the customer for the acrylic acid to be supplied) as required.
[0289] By esterifying an acrylic acid for which V=V* with
biomethanol or bioethanol, it is possible to obtain acrylic esters
whose n.sup.14C to n.sup.12C ratio is likewise V*.
[0290] A further advantage of the inventive procedure is that
neither the target product of reaction zone A nor the target
product of reaction zone C require removal from product gas mixture
A or C in order to be able to be employed for production of
reaction gas input mixture B. This ensures both high economy and an
efficient energy balance for the process according to the
invention. Furthermore, in the case of condensation of acetic acid
with formaldehyde, neither glyoxal nor propionic acid is formed as
a by-product, as is necessarily the case for a heterogeneously
catalyzed partial oxidation of propylene, propane, acrolein,
propionaldehyde and/or glycerol to acrylic acid.
[0291] Furthermore, the process according to the invention ensures
a high space-time yield coupled with simultaneously high target
product selectivity based on the reactants converted.
[0292] Thus, the present application especially comprises the
following embodiments of the invention: [0293] 1. A process for
preparing acrylic acid from ethanol and formaldehyde, which
comprises the following measures: [0294] a stream of a reaction gas
input mixture A comprising the ethanol and molecular oxygen
reactants and at least one inert diluent gas other than steam is
conducted through a first reaction zone A charged with at least one
oxidation catalyst A and, in the course of passage through reaction
zone A, ethanol present in the reaction gas input mixture A is
oxidized under heterogeneous catalysis to acetic acid and steam so
as to form a product gas mixture A comprising acetic acid, steam,
molecular oxygen and at least one inert diluent gas other than
steam, and a stream of product gas mixture A leaves reaction zone
A, it optionally being possible to supply further molecular oxygen
and/or further inert diluent gas to the reaction gas mixture A
flowing through reaction zone A on its way through reaction zone A,
[0295] a stream of a reaction gas input mixture B which comprises
acetic acid, steam, molecular oxygen, at least one inert diluent
gas other than steam and formaldehyde and in which the molar amount
n.sub.HAc of acetic acid present is greater than the molar amount
n.sub.Fd of formaldehyde present therein is obtained from the
stream of product gas mixture A leaving reaction zone A and at
least one further stream comprising at least one formaldehyde
source, [0296] the stream of reaction gas input mixture B is passed
through a second reaction zone B charged with at least one aldol
condensation catalyst B and formaldehyde present in reaction gas
input mixture B, as it flows through reaction zone B, is condensed
with acetic acid present in reaction gas input mixture B under
heterogeneous catalysis to give acrylic acid and H.sub.2O, so as to
form a product gas mixture B comprising acrylic acid, acetic acid,
steam, molecular oxygen and at least one inert diluent gas other
than steam, and a stream of product gas mixture B leaves reaction
zone B, it optionally being possible to supply further molecular
oxygen and/or further inert diluent gas to the reaction gas mixture
B flowing through reaction zone B on its way through reaction zone
B, [0297] the stream of product gas mixture B leaving reaction zone
B is fed to a separation zone T and separated in separation zone T
into at least three streams X, Y and Z, [0298] the acrylic acid
flow present in stream X being greater than the acrylic acid flow
present in streams Y and Z together, [0299] the acetic acid flow
present in stream Y being greater than the acetic acid flow present
in streams X and Z together, [0300] the flow of inert diluent gas
other than steam present in stream Z being greater than the flow of
inert diluent gas other than steam present in streams X and Y
together, [0301] and [0302] stream Y is recycled into reaction zone
B and used to obtain reaction gas input mixture B. [0303] 2. The
process according to embodiment 1, wherein the at least one
oxidation catalyst A has a catalytically active material which
comprises at least one vanadium oxide. [0304] 3. The process
according to embodiment 1 or 2, wherein the at least one oxidation
catalyst A has a catalytically active material which comprises at
least one vanadium oxide and additionally at least one oxide from
the group of the oxides of titanium, of aluminum, of zirconium and
of zinc. [0305] 4. The process according to embodiment 2 or 3,
wherein the at least one vanadium oxide comprises the vanadium in
the +5 oxidation state. [0306] 5. The process according to
embodiment 3, wherein the at least one vanadium oxide comprises the
vanadium in the +5 oxidation state and the at least one oxide from
the group of the oxides of titanium, of zirconium and of tin
comprises the element from the group of titanium, zirconium and tin
in the +4 oxidation state. [0307] 6. The process according to any
of embodiments 2 to 5, wherein the catalytically active material
comprises V.sub.2O.sub.5 as at least one vanadium oxide. [0308] 7.
The process according to embodiment 6, wherein the catalytically
active material comprises 0.1 to 60% by weight of V.sub.2O.sub.5.
[0309] 8. The process according to embodiment 6, wherein the
catalytically active material comprises 1 to 50% by weight of
V.sub.2O.sub.5. [0310] 9. The process according to embodiment 6,
wherein the catalytically active material comprises 3 to 40% by
weight of V.sub.2O.sub.5. [0311] 10. The process according to
embodiment 6, wherein the catalytically active material comprises 5
to 30% by weight of V.sub.2O.sub.5. [0312] 11. The process
according to any of embodiments 2 to 10, wherein the catalytically
active material comprises T.sub.iO.sub.2. [0313] 12. The process
according to any of embodiments 2 to 7, wherein the catalytically
active material comprises 40 to 99.9% by weight of T.sub.iO.sub.2.
[0314] 13. The process according to embodiment 8, wherein the
catalytically active material comprises 50 to 99% by weight of
T.sub.iO.sub.2. [0315] 14. The process according to embodiment 9,
wherein the catalytically active material comprises 60 to 97% by
weight of T.sub.iO.sub.2. [0316] 15. The process according to
embodiment 10, wherein the catalytically active material comprises
70 to 95% by weight of T.sub.iO.sub.2. [0317] 16. The process
according to any of embodiments 11 to 15, wherein the catalytically
active material consists of V.sub.2O.sub.5 as at least one vanadium
oxide and of T.sub.iO.sub.2. [0318] 17. The process according to
any of embodiments 11 to 16, wherein at least a portion of the
T.sub.iO.sub.2 is present in the anatase polymorph. [0319] 18. The
process according to any of embodiments 11 to 16, wherein 50 to
100% by weight of the T.sub.iO.sub.2 is present in the anatase
polymorph. [0320] 19. The process according to any of embodiments 2
to 18, wherein the at least one oxidation catalyst A is an
unsupported catalyst. [0321] 20. The process according to
embodiment 19, wherein the unsupported catalyst is a sphere, a ring
or a solid cylinder. [0322] 21. The process according to embodiment
20, wherein the longest dimension of the unsupported catalyst is 1
to 10 mm. [0323] 22. The process according to any of embodiments 2
to 18, wherein the at least one oxidation catalyst A is an eggshell
catalyst which has the catalytically active material as an eggshell
on the surface of an inert shaped support body. [0324] 23. The
process according to embodiment 22, wherein the shaped support body
is a sphere or a ring. [0325] 24. The process according to
embodiment 22 or 23, wherein the longest dimension of the shaped
support body is 1 to 10 mm. [0326] 25. The process according to any
of embodiments 22 to 24, wherein the inert shaped support body
consists of steatite. [0327] 26. The process according to any of
embodiments 22 to 25, wherein the thickness of the eggshell of
catalytically active material is 10 to 2000 .mu.m, or 10 to 500
.mu.m, or 100 to 500 .mu.m, or 200 to 300 .mu.m. [0328] 27. The
process according to embodiment 1, wherein the charge of the
reaction zone A with at least one oxidation catalyst A comprises
two sections 1 and 2 which are spatially successive in flow
direction of reaction gas input mixture A in numerical sequence
thereof and are charged with different oxidation catalysts A,
[0329] said at least one oxidation catalyst A of section 1 having a
catalytically active material 1 which comprises at least one
vanadium oxide and at least one oxide from the group of the oxides
of titanium, of aluminum, of zirconium and of tin, and [0330] said
at least one oxidation catalyst A of section 2 having a
catalytically active material 2 which is a multimetal oxide which,
as well as V and Mo, additionally comprises at least one of the
elements W, Mo, Ta, Cr and Ce, and at least one of the elements Cu,
Ni, Co, Fe, Mn and Zn. [0331] 28. The process according to
embodiment 27, wherein the catalytically active material 1
comprises or consists of 1 to 50% by weight of V.sub.2O.sub.5 as
the at least one vanadium oxide and 50 to 99% by weight of
T.sub.iO.sub.2 as the oxide of titanium (preferably in the anatase
polymorph). [0332] 29. The process according to embodiment 27,
wherein the catalytically active material 1 comprises or consists
of 3 to 40% by weight of V.sub.2O.sub.5 as the at least one
vanadium oxide and 60 to 97% by weight of T.sub.iO.sub.2 as the
oxide of titanium (preferably in the anatase polymorph). [0333] 30.
The process according to embodiment 27, wherein the catalytically
active material 1 comprises or consists of 5 to 30% by weight of
V.sub.2O.sub.5 as the at least one vanadium oxide and 70 to 95% by
weight of T.sub.iO.sub.2 as the oxide of titanium (preferably in
the anatase polymorph). [0334] 31. The process according to any of
embodiments 27 to 30, wherein the at least one oxidation catalyst A
of section 1 is an unsupported catalyst. [0335] 32. The process
according to embodiment 31, wherein the geometry of the unsupported
catalyst is selected from the group consisting of sphere, ring and
solid cylinder, and has a longest dimension in the range from 1 to
10 mm. [0336] 33. The process according to any of embodiments 27 to
30, wherein the at least one oxidation catalyst A of section 1 is
an eggshell catalyst which has the catalytically active material 1
applied as an eggshell to the surface of an inert shaped support
body. [0337] 34. The process according to embodiment 33, wherein
the shaped support body is a sphere or a ring. [0338] 35. The
process according to embodiment 33 or 34, wherein the longest
dimension of the shaped support body is 1 to 10 mm. [0339] 36. The
process according to any of embodiments 33 to 35, wherein the inert
support body consists of steatite. [0340] 37. The process according
to any of embodiments 33 to 36, wherein the thickness of the
eggshell of catalytically active material 1 is 10 to 2000 .mu.m, or
10 to 500 .mu.m, or 100 to 500 .mu.m, or 200 to 300 .mu.m. [0341]
38. The process according to any of embodiments 27 to 37, wherein
the catalytically active material 1 does not comprise any Mo.
[0342] 39. The process according to any of embodiments 27 to 38,
wherein the catalytically active material 2 is at least one
multimetal oxide of the general formula I
[0342]
Mo.sub.12V.sub.aX.sup.1.sub.bX.sup.2.sub.cX.sup.3.sub.dX.sup.4.su-
b.eX.sup.5.sub.fX.sup.6.sub.gO.sub.n (I) [0343] in which the
variables are each defined as follows: [0344] X.sup.1=W, Nb, Ta, Cr
and/or Ce, [0345] X.sup.2=Cu, Ni, Co, Fe, Mn and/or Zn, [0346]
X.sup.3=Sb and/or B.sup.1, [0347] X.sup.4=one or more alkali
metals, [0348] X.sup.5=one or more alkaline earth metals, [0349]
X.sup.6=Si, Al, Ti and/or Zr, [0350] a=1 to 6, [0351] b=0.2 to 4,
[0352] c=0.5 to 1.8 [0353] d=0 to 40, [0354] e=0 to 2, [0355] f=0
to 4, [0356] g=0 to 40, and [0357] n=the stoichiometric coefficient
of the element oxygen, which is determined by the stoichiometric
coefficients of the non-oxygen elements and the charge numbers
thereof in I. [0358] 40. The process according to any of
embodiments 27 to 38, wherein the catalytically active material 2
is at least one multimetal oxide of the general formula I
[0358]
Mo.sub.12V.sub.aX.sup.1.sub.bX.sup.2.sub.cX.sup.3.sub.dX.sup.4.su-
b.eX.sup.5.sub.fX.sup.6.sub.gO.sub.n (I) [0359] in which the
variables are each defined as follows: [0360] X.sup.1=W, Nb and/or
Cr, [0361] X.sup.2=Cu, Ni, Co, Fe, Mn and/or Fe, [0362] X.sup.3=Sb,
[0363] X.sup.4=Na and/or K, [0364] X.sup.5=Ca, Sr and/or Ba, [0365]
X.sup.6=Si, Al and/or Ti, [0366] a=1.5 to 5, [0367] b=0.5 to 2,
[0368] c=0.5 to 3, [0369] d=0 to 2, [0370] e=0 to 0.2 [0371] f=0 to
1 [0372] g=0 to 1, and [0373] n=the stoichiometric coefficient of
the element oxygen, which is determined by the stoichiometric
coefficients of the non-oxygen elements and the charge numbers
thereof in I. [0374] 41. The process according to any of
embodiments 27 to 40, wherein the at least one oxidation catalyst A
of section 2 is an eggshell catalyst which has the catalytically
active material 2 applied as an eggshell to the surface of an inert
shaped support body. [0375] 42. The process according to embodiment
41, wherein the shaped support body is a sphere or a ring. [0376]
43. The process according to embodiment 41 or 42, wherein the
longest dimension of the shaped support body is 1 to 10 mm. [0377]
44. The process according to any of embodiments 41 to 43, wherein
the inert shaped support body consists of steatite. [0378] 45. The
process according to any of embodiments 41 to 44, wherein the
thickness of the eggshell of catalytically active material 2 is 10
to 2000 .mu.m, or 10 to 500 .mu.m, or 100 to 500 .mu.m, or 200 to
300 .mu.m. [0379] 46. The process according to any of embodiments 1
to 45, wherein reaction zone A charged with at least one oxidation
catalyst A has been charged with a fixed catalyst bed. [0380] 47.
The process according to any of embodiments 1 to 46, wherein the
reaction temperature in reaction zone A is in the range from 100 to
450.degree. C. [0381] 48. The process according to any of
embodiments 1 to 46, wherein the reaction temperature in reaction
zone A is in the range from 100 to 350.degree. C. [0382] 49. The
process according to any of embodiments to 27 to 45, wherein the
reaction temperature averaged arithmetically over the length of
section 1, T.sup.1 is 150 to 250.degree. C., and the reaction
temperature averaged arithmetically over the length of section 2,
T.sup.2 is 180 to 260.degree. C. [0383] 50. The process according
to embodiment 49, wherein T.sup.2 is at least 5.degree. C. greater
than T.sup.1. [0384] 51. The process according to embodiment 49,
wherein T.sup.2 is at least 10.degree. C. greater than T.sup.1.
[0385] 52. The process according to embodiment 49, wherein T.sup.2
is at least 20.degree. C. greater than T.sup.1. [0386] 53. The
process according to any of embodiments 49 to 52, wherein T.sup.2
is not more than 80.degree. C. greater than T.sup.1. [0387] 54. The
process according to any of embodiments 27 to 46 and 49 to 53,
wherein the ethanol present in reaction gas input mixture A is
converted to an extent of at least 90 mol % in the course of
passage through section 1. [0388] 55. The process according to any
of embodiments 27 to 46 and 49 to 53, wherein the ethanol present
in reaction gas input mixture A is converted to an extent of at
least 95 mol % in the course of passage through section 1. [0389]
56. The process according to any of embodiments 27 to 46 and 49 to
55, wherein the ethanol present in reaction gas input mixture A is
converted to an extent of at least 97 mol % in the course of
passage through sections 1 and 2. [0390] 57. The process according
to any of embodiments 27 to 46 and 49 to 56, wherein the ethanol
present in reaction gas input mixture A is converted to an extent
of at least 99 mol % in the course of passage through sections 1
and 2. [0391] 58. The process according to any of embodiments 1 to
57, wherein reaction gas input mixture A comprises 0.3 to 20% by
volume of ethanol. [0392] 59. The process according to any of
embodiments 1 to 57, wherein reaction gas input mixture A comprises
0.5 to 15% by volume of ethanol. [0393] 60. The process according
to any of embodiments 1 to 57, wherein reaction gas input mixture A
comprises 0.75 to 10% by volume or 1 to 5% by volume of ethanol.
[0394] 61. The process according to any of embodiments 1 to 60,
wherein reaction gas input mixture A comprises the molecular oxygen
in a molar amount n.sub.O and the ethanol in a molar amount
n.sub.Et, and the n.sub.O:n.sub.Et ratio is at least 1.3. [0395]
62. The process according to embodiment 61, wherein
n.sub.O:n.sub.Et is at least 1.5. [0396] 63. The process according
to embodiment 61, wherein n.sub.O:n.sub.Et is at least 1.75. [0397]
64. The process according to any of embodiments 61 to 63, wherein
n.sub.O:n.sub.Et is not more than 10. [0398] 65. The process
according to any of embodiments 61 to 63, wherein n.sub.O:n.sub.Et
is not more than 5. [0399] 66. The process according to any of
embodiments 1 to 65, wherein reaction gas input mixture A comprises
1 to 40% by volume of H.sub.2O. [0400] 67. The process according to
any of embodiments 1 to 65, wherein reaction gas input mixture A
comprises 1 to 20% by volume of H.sub.2O. [0401] 68. The process
according to any of embodiments 1 to 65, wherein reaction gas input
mixture A comprises 5 to 15% by volume of H.sub.2O. [0402] 69. The
process according to any of embodiments 1 to 65, wherein reaction
gas input mixture A comprises 7.5 to 12.5% by volume of H.sub.2O.
[0403] 70. The process according to any of embodiments 1 to 69,
wherein at least 80% by volume of the inert diluent gas other than
steam present in reaction gas input mixture A is molecular
nitrogen. [0404] 71. The process according to any of embodiments 1
to 69, wherein at least 90% by volume of the inert diluent gas
other than steam present in reaction gas input mixture A is
molecular nitrogen. [0405] 72. The process according to any of
embodiments 1 to 69, wherein at least 95% by volume of the inert
diluent gas other than steam present in reaction gas input mixture
A is molecular nitrogen. [0406] 73. The process according to any of
embodiments 1 to 72, wherein reaction gas input mixture A
comprises, as at least one inert diluent gas other than steam, at
least 10% by volume of molecular nitrogen. [0407] 74. The process
according to any of embodiments 1 to 72, wherein reaction gas input
mixture A comprises, as at least one inert diluent gas other than
steam, at least 30% by volume of molecular nitrogen. [0408] 75. The
process according to any of embodiments 1 to 72, wherein reaction
gas input mixture A comprises, as at least one inert diluent gas
other than steam, at least 40% by volume of molecular nitrogen.
[0409] 76. The process according to any of embodiments 1 to 72,
wherein reaction gas input mixture A comprises, as at least one
inert diluent gas other than steam, not more than 90% by volume of
molecular nitrogen. [0410] 77. The process according to any of
embodiments 1 to 76, wherein the working pressure in reaction zone
A is 1.210.sup.5 Pa to 5010.sup.5 Pa. [0411] 78. The process
according to any of embodiments 1 to 77, wherein the source used
for ethanol present in reaction gas input mixture A is bioethanol.
[0412] 79. The process according to embodiment 78, wherein reaction
gas input mixture A, based on the weight of the ethanol present
therein, has at least one 1 ppm by weight of a chemical compound
comprising the element sulfur, calculated as the amount of sulfur
present. [0413] 80. The process according to embodiment 78, wherein
reaction gas input mixture A, based on the weight of the ethanol
present therein, has 2 to 200 ppm by weight of a chemical compound
comprising the element sulfur, calculated as the amount of sulfur
present. [0414] 81. The process according to any of embodiments 1
to 78, wherein reaction gas input mixture A, based on the weight of
the ethanol present, has 0 to <1 ppm by weight of a chemical
compound comprising the element sulfur, calculated as the amount of
sulfur present. [0415] 82. The process according to any of
embodiments 78 to 81, wherein the source for the ethanol present in
reaction gas input mixture A is an aqueous solution of bioethanol.
[0416] 83. The process according to embodiment 82, wherein the
source used for the ethanol present in reaction gas input mixture A
is the filtrate of an aqueous slurry which comprises dissolved
bioethanol and is obtained in bioethanol production. [0417] 84. The
process according to any of embodiments 1 to 83, wherein the
formaldehyde source used for the formaldehyde present in reaction
gas input mixture B is at least one of the sources trioxane,
paraformaldehyde, formalin, methylal, aqueous paraformaldehyde
solution, aqueous formaldehyde solution, and the product gas
mixture of a heterogeneously catalyzed partial gas phase oxidation
of methanol to formaldehyde from which any unconverted methanol
present therein has optionally been removed. [0418] 85. The process
according to any of embodiments 1 to 84, wherein the reaction
temperature in reaction zone B is 260 to 400.degree. C. [0419] 86.
The process according to any of embodiments 1 to 84, wherein the
reaction temperature in reaction zone B is 280 to 380.degree. C.
[0420] 87. The process according to any of embodiments 1 to 84,
wherein the reaction temperature in reaction zone B is 300 to
370.degree. C. [0421] 88. The process according to any of
embodiments 1 to 87, wherein the working pressure in reaction zone
B is 1.210.sup.5 Pa to 5010.sup.5 Pa. [0422] 89. The process
according to any of embodiments 1 to 88, wherein the formaldehyde
content in reaction gas input mixture B is 0.5 to 10% by volume.
[0423] 90. The process according to any of embodiments 1 to 88,
wherein the formaldehyde content in reaction gas input mixture B is
0.5 to 7% by volume. [0424] 91. The process according to any of
embodiments 1 to 88, wherein the formaldehyde content in reaction
gas input mixture B is 1 to 5% by volume. [0425] 92. The process
according to any of embodiments 1 to 91, wherein reaction gas input
mixture B comprises acetic acid in a molar amount n.sub.HAc and
formaldehyde in a molar amount n.sub.Fd, and the n.sub.HAc:n.sub.Fd
ratio is greater than 1 and .ltoreq.10. [0426] 93. The process
according to embodiment 92, wherein the n.sub.HAc:n.sub.Fd ratio is
1.1 to 5. [0427] 94. The process according to embodiment 92,
wherein the n.sub.HAc:n.sub.Fd ratio is 1.5 to 3.5. [0428] 95. The
process according to any of embodiments 1 to 94, wherein the acetic
acid content of reaction gas input mixture B is 1.5 to 20% by
volume. [0429] 96. The process according to any of embodiments 1 to
94, wherein the acetic acid content of reaction gas input mixture B
is 2 to 15% by volume. [0430] 97. The process according to any of
embodiments 1 to 94, wherein the acetic acid content of reaction
gas input mixture B is 3 to 10% by volume. [0431] 98. The process
according to any of embodiments 1 to 97, wherein the molecular
oxygen content of reaction gas input mixture B is 0.5 to 5% by
volume. [0432] 99. The process according to any of embodiments 1 to
97, wherein the molecular oxygen content of reaction gas input
mixture B is 2 to 5% by volume. [0433] 100. The process according
to any of embodiments 1 to 99, wherein the steam content of
reaction gas input mixture B does not exceed 30% by volume and is
not less than 1.5% by volume. [0434] 101. The process according to
any of embodiments 1 to 99, wherein the steam content of reaction
gas input mixture B does not exceed 20% by volume and is not less
than 2% by volume. [0435] 102. The process according to any of
embodiments 1 to 99, wherein the steam content of reaction gas
input mixture B is 5 to 15% by volume or 10 to 15% by volume.
[0436] 103. The process according to any of embodiments 1 to 102,
wherein the content of inert diluent gas other than steam in
reaction gas input mixture B is at least 30% by volume or at least
40% by volume. [0437] 104. The process according to any of
embodiments 1 to 102, wherein the content of inert diluent gas
other than steam in reaction gas input mixture B is at least 50% by
volume. [0438] 105. The process according to any of embodiments 1
to 104, wherein reaction gas input mixture B comprises, as at least
one inert diluent gas other than steam, at least 30% by volume or
at least 40% by volume of N.sub.2. [0439] 106. The process
according to any of embodiments 1 to 104, wherein reaction gas
input mixture B comprises, as at least one inert diluent gas other
than steam, at least 50% by volume of N.sub.2. [0440] 107. The
process according to any of embodiments 1 to 106, wherein the at
least one aldol condensation catalyst B is a zeolite with anionic
structural charge, on whose inner and outer surfaces at least one
cation type from the group of the alkali metal ions and alkaline
earth metal ions is present, in order to neutralize the negative
structural charge. [0441] 108. The process according to any of
embodiments 1 to 106, wherein the at least one aldol condensation
catalyst B is hydroxide from the group consisting of alkali metal
hydroxides, alkaline earth metal hydroxides and aluminum hydroxide
applied to amorphous silicon dioxide. [0442] 109. The process
according to embodiment 108, wherein the hydroxide applied to the
amorphous silicon dioxide is KOH, NaOH, Ca(OH).sub.2 or
Mg(OH).sub.2. [0443] 110. The process according to any of
embodiments 1 to 106, wherein the at least one aldol condensation
catalyst B is a catalyst which comprises [0444] as constituent a),
at least one oxide of at least one of the elements Si, Al, Ti, Zr,
Cd, Sn, Ga, Y and La and/or zeolite, and [0445] as constituent b),
at least one oxide selected from boron oxide and phosphorus oxide,
and optionally [0446] as constituent c) one or more than one oxide
of at least one of the elements V, Cr, Co, Ni, Mo and Pb and/or
more than one heteropolyacid with at least one poly atom selected
from V, Mo and W. [0447] 111. The process according to embodiment
110, wherein the at least one aldol condensation catalyst B
comprises 1 to 50% by weight of boron oxide, or 1 to 50% by weight
of phosphorus oxide, or 1 to 50% by weight of boron oxide and
phosphorus oxide, where the boron oxide, based on the amount of B
present, is always calculated as B.sub.2O.sub.3 and the phosphorus
oxide, based on the amount of P present, is always calculated as
P.sub.2O.sub.5. [0448] 112. The process according to any of claims
1 to 106, wherein the at least one aldol condensation catalyst B
has a catalytically active material which is a vanadium-phosphorus
oxide or a vanadium-phosphorus oxide doped with elements other than
vanadium and phosphorus. [0449] 113. The process according to
embodiment 112, wherein the catalytically active material is a
multielement oxide active material of the general formula II
[0449] V.sub.1P.sub.bFe.sub.cX.sup.1.sub.dX.sup.2.sub.eO.sub.n (II)
[0450] in which the variables are each defined as follows: [0451]
X.sup.1=Mo, Bi, Co, Ni, Si, Zn, Hf, Zr, Ti, Cr, Mn, Cu, B, Sn
and/or Nb, [0452] V=Li, K, Na, Rb, Cs and/or Tl, [0453] b=0.9 to
2.0 [0454] c=.gtoreq.0 to 0.1, [0455] d=.gtoreq.0 to 0.1, [0456]
e=.gtoreq.0 to 0.1, and [0457] n=the stoichiometric coefficient of
the element oxygen, which is determined by the stoichiometric
coefficients of the non-oxygen elements and the charge numbers
thereof in II. [0458] 114. The process according to embodiment 113,
wherein [0459] X.sup.1=Nb, Mo, Zn and/or Hf. [0460] 115. The
process according to embodiment 113 or 114, wherein [0461] b is 0.9
to 1.5. [0462] 116. The process according to embodiment 113 or 114,
wherein b is 0.9 to 1.2. [0463] 117. The process according to any
of embodiments 113 to 116, wherein X.sup.1=Mo. [0464] 118. The
process according to any of embodiments 113 to 117, wherein c is
0.005 to 0.1. [0465] 119. The process according to any of
embodiments 113 to 117, wherein c is 0.005 to 0.05 or 0.005 to
0.02. [0466] 120. The process according to embodiment 112, wherein
the ratio [0467] n.sub.p:n.sub.v of the molar amount n.sub.p of
phosphorus present in the catalytically active material to the
molar amount n.sub.v of V present in the catalytically active
material is 0.09 to 2.0, preferably 0.9 to 1.5 and more preferably
0.9 to 1.2. [0468] 121. The process according to either of
embodiments 112 or 120, wherein the elements other than vanadium or
phosphorus present in the catalytically active material are one or
more than one element from the group consisting of lithium,
potassium, sodium, rubidium, cesium, thallium, molybdenum, zinc,
hafnium, zirconium, titanium, chromium, manganese, nickel, copper,
iron, boron, silicon, tin, niobium, cobalt and bismuth. [0469] 122.
The process according to embodiment 121, wherein the total content
of elements other than vanadium and phosphorus in the catalytically
active material, based on the weight thereof, is not more than 5%
by weight, calculating the particular element other than vanadium
and phosphorus as the electrically neutral oxide in which the
element has the same charge number as in the active material.
[0470] 123. The process according to any of embodiments 112 to 122,
wherein the arithmetic mean oxidation state of vanadium in the
catalytically active material is +3.9 to +4.4 or +4.0 to +4.3.
[0471] 124. The process according to any of embodiments 112 to 123,
wherein the specific BET surface area of the catalytically active
material is .gtoreq.15 to 50 m.sup.2/g. [0472] 125. The process
according to any of embodiments 112 to 124, wherein the total pore
volume of the catalytically active material is 0.1 to 0.5 ml/g.
[0473] 126. The process according to any of embodiments 112 to 125,
wherein the total pore volume of the catalytically active material
is 0.15 to 0.4 ml/g. [0474] 127. The process according to any of
embodiments 112 to 126, wherein the at least one oxidation catalyst
B is an unsupported catalyst or a supported catalyst. [0475] 128.
The process according to embodiment 127, wherein the geometry of
the unsupported catalyst is selected from the group consisting of
sphere, ring and solid cylinder, and has a longest dimension in the
range from 1 to 10 mm. [0476] 129. The process according to
embodiment 127, wherein the geometry of the unsupported catalyst is
a ring (a hollow cylinder) with an external diameter in the range
from 3 to 10 mm, a height of 1 to 10 mm, an internal diameter of 1
to 8 mm and a wall thickness of 1 to 3 mm. [0477] 130. The process
according to any of embodiments 112 to 126, wherein the at least
one aldol condensation catalyst B is an eggshell catalyst which has
the catalytically active material as an eggshell applied to the
surface of an inert shaped support body. [0478] 131. The process
according to embodiment 130, wherein the shaped support body is a
sphere or a ring. [0479] 132. The process according to embodiment
130 or 131, wherein the longest dimension of the shaped support
body is 1 to 10 mm. [0480] 133. The process according to any of
embodiments 130 to 132, wherein the inert shaped support body is
composed of steatite. [0481] 134. The process according to any of
embodiments 130 to 133, wherein the thickness of the eggshell of
active material is 10 to 2000 .mu.m, or 10 to 500 .mu.m, or 100 to
500 .mu.m, or 200 to 300 .mu.m. [0482] 135. The process according
to any of embodiments 1 to 134, which comprises a further, third
reaction zone C which has been charged with at least one oxidation
catalyst C and comprises the following additional measures: [0483]
a stream of a reaction gas input mixture C comprising the methanol
and molecular oxygen reactants and at least one inert diluent gas
other than steam is passed through a third reaction zone C charged
with at least one oxidation catalyst C and methanol present in
reaction gas input mixture C, as it passes through reaction zone C,
is oxidized under heterogeneous catalysis to formaldehyde and
steam, so as to form a product gas mixture C comprising
formaldehyde, steam and at least one inert diluent gas other than
steam, and a stream of product gas mixture C leaves reaction zone
C, it optionally being possible to add further molecular oxygen
and/or further inert diluent gas to reaction gas mixture C flowing
through reaction zone C on its way through reaction zone C, [0484]
optionally, from product gas mixture C, any unconverted methanol
still present in product gas mixture C is removed from product gas
mixture C in a separation zone T* to leave a
formaldehyde-comprising product gas mixture C*, and [0485] product
gas mixture C or product gas mixture C* is conducted into reaction
zone B to obtain reaction gas input mixture B. [0486] 136. The
process according to embodiment 135, wherein methanol removed in
separation zone T* is recycled into reaction zone C to obtain
reaction gas input mixture C. [0487] 137. The process according to
embodiment 135 or 136, wherein the methanol is removed by
rectification in the separation zone T*. [0488] 138. The process
according to any of embodiments 135 to 137, wherein the at least
one oxidation catalyst C has a catalytically active material which
comprises at least elemental silver. [0489] 139. The process
according to embodiment 138, wherein the purity of the elemental
silver is .gtoreq.99.7% by weight. [0490] 140. The process
according to embodiment 138, wherein the purity of the elemental
silver is .gtoreq.99.9 or .gtoreq.99.99% by weight. [0491] 141. The
process according to any of embodiments 138 to 140, wherein the at
least one oxidation catalyst C comprises silver crystals whose
longest dimension is in the range from 0.1 to 5 mm. [0492] 142. The
process according to embodiment 141, wherein the silver crystals
have been coated with a porous layer of oxidic material of at least
one of the elements Al, Si, Zr and Ti, the thickness of which is in
the range of 0.3 to 10 .mu.m. [0493] 143. The process according to
any of embodiments 138 to 142, wherein the methanol content of
reaction gas input mixture C is at least 5% by volume. [0494] 144.
The process according to embodiment 143, wherein the methanol
content of reaction gas input mixture C is not more than 60% by
volume. [0495] 145. The process according to any of embodiments 138
to 142, wherein the methanol content of reaction gas input mixture
C is 15 to 50% by volume. [0496] 146. The process according to any
of embodiments 138 to 142, wherein the methanol content of reaction
gas input mixture C is 20 to 40% by volume or 20 to 30% by volume.
[0497] 147. The process according to any of embodiments 138 to 146,
wherein the reaction gas input mixture C comprises the molecular
oxygen in a molar amount n.sub.O and the methanol in a molar amount
n.sub.Me, and the n.sub.O:n.sub.Me ratio is less than 1. [0498]
148. The process according to embodiment 147, wherein
n.sub.O:n.sub.Me is 0.1 to 0.8 or 0.2 to 0.6. [0499] 149. The
process according to any of embodiments 138 to 148, wherein
n.sub.O:n.sub.Me is 0.3 to 0.5. [0500] 150. The process according
to any of embodiments 138 to 149, wherein reaction gas input
mixture C comprises .gtoreq.0 to 50% by volume of H.sub.2O. [0501]
151. The process according to embodiment 150, wherein reaction gas
input mixture C comprises 15 to 35% by volume or 20 to 30% by
volume of H.sub.2O. [0502] 152. The process according to any of
embodiments 138 to 151, wherein reaction gas input mixture C
comprises, as at least one inert diluent gas other than steam,
N.sub.2. [0503] 153. The process according to embodiment 152,
wherein reaction gas input mixture C comprises 20 to 80% by volume
of N.sub.2. [0504] 154. The process according to embodiment 152 or
153, wherein reaction gas input mixture C comprises 30 to 70% by
volume of N.sub.2. [0505] 155. The process according to any of
embodiments 152 to 154, wherein reaction gas input mixture C
comprises 40 to 60% by volume of N.sub.2. [0506] 156. The process
according to any of embodiments 138 to 155, wherein the methanol is
oxidized in the reaction zone C at a reaction temperature in the
range from 400 to 800.degree. C. to formaldehyde and water. [0507]
157. The process according to any of embodiments 138 to 156,
wherein the methanol is oxidized in the reaction zone C at a
reaction temperature in the range from 500 to 800.degree. C. to
formaldehyde and water. [0508] 158. The process according to any of
embodiments 138 to 156, wherein the methanol is oxidized in the
reaction zone C at a reaction temperature in the range from 450 to
650.degree. C., or from 500 bis 600.degree. C. to formaldehyde and
water. [0509] 159. The process according to any of embodiments 138
to 156, wherein the methanol is oxidized in the reaction zone C at
a reaction temperature in the range from 600 to 750.degree. C. to
formaldehyde and water. [0510] 160. The process according to any of
embodiments 138 to 159, wherein the methanol is oxidized in
reaction zone C at a working pressure in the range from 10.sup.3 to
10.sup.6 Pa or from 10.sup.4 to 210.sup.5 Pa to formaldehyde and
water. [0511] 161. The process according to any embodiments 135 to
137, wherein the at least one oxidation catalyst C has a
catalytically active material which is a mixed oxide which has at
least one transition metal in the oxidized state. [0512] 162. The
process according to embodiment 161, wherein the at least one
transition metal comprises Mo and/or V. [0513] 163. The process
according to embodiment 161, wherein the at least one transition
metal comprises Mo and Fe. [0514] 164. The process according to
embodiment 161, wherein the catalytically active material is a
mixed oxide of the general formula III
[0514] [Fe.sub.2(MoO.sub.4).sub.3].sub.1
[M.sup.1.sub.mO.sub.n].sub.q (III) [0515] in which the variables
are each defined as follows: [0516] M.sup.1=Mo and/or Fe, or [0517]
Mo and/or Fe and, based on the total molar amount of Mo and Fe, a
total molar amount of up to 10 mol % (eg. 0.01 to 10 mol % or 0.1
to 10 mol %), preferably not more than 5 mol %, of one or more
elements from the group consisting of Ti, Sb, Sn, Ni, Cr, Ce, Al,
Ca, Mg, V, Nb, Ag, Mn, Cu, Co, Si, Na, K, Tl, Zr, W, Ir, Ta, As, P
and B, [0518] q=0 to 5, [0519] m=1 to 3, [0520] n=1 to 6. [0521]
165. The process according to embodiment 164, wherein q=0.5 to 3.
[0522] 166. The process according to embodiment 164 or 165, wherein
q=1 to 2. [0523] 167. The process according to any of embodiments
164 to 166, wherein M.sup.1=Mo, m=1 and n=3. [0524] 168. The
process according to any of embodiments 164 to 166, wherein
M.sup.1=Fe, m=2 and n=3. [0525] 169. The process according to any
of embodiments 164 to 168, wherein less than 50 mol % of the Fe
present in the mixed oxide III is present in the +2 oxidation
state. [0526] 170. The process according to any of embodiments 164
to 168, wherein less than 20 mol % of the Fe present in the mixed
oxide III is present in the +2 oxidation state. [0527] 171. The
process according to any of embodiments 164 to 168, wherein less
than 10 mol % of the Fe present in the mixed oxide III is present
in the +2 oxidation state. [0528] 172. The process according to any
of embodiments 164 to 168, wherein the entirety of the Fe present
in the mixed oxide III is present in the +3 oxidation state. [0529]
173. The process according to any of embodiments 164 to 172,
wherein the n.sub.Mo:n.sub.Fe ratio, formed from the molar amount
of Mo present in the mixed oxide III and the molar amount of Fe
present in the same mixed oxide III, is 1:1 to 5:1. [0530] 174. The
process according to any of embodiments 164 to 172, wherein the
catalytically active material can be represented in a formal sense
as a mixture of MoO.sub.3 and Fe.sub.2O.sub.3, the content in the
mixture of MoO.sub.3 being 65 to 95% by weight and the content in
the mixture of Fe.sub.2O.sub.3 being 5 to 35% by weight. [0531]
175. The process according to any of embodiments 161 to 174,
wherein the at least one oxidation catalyst C is an unsupported
catalyst. [0532] 176. The process according to embodiment 175,
wherein the geometry of the unsupported catalyst is selected from
the group consisting of sphere, ring and solid cylinder. [0533]
177. The process according to embodiment 176, wherein the longest
dimension of the unsupported catalyst is 1 to 10 mm. [0534] 178.
The process according to embodiment 175, wherein the unsupported
catalyst has the geometry of a ring with an external diameter of 3
to 10 mm, a height of 1 to 10 mm and an internal diameter of 1 to 8
mm. [0535] 179. The process according to embodiment 178, wherein
the ring has a wall thickness of 1 to 3 mm. [0536] 180. The process
according to any of embodiments 161 to 174, wherein the at least
one oxidation catalyst C is an eggshell catalyst which has the
catalytically active mixed oxide as an eggshell applied to the
surface of an inert shaped support body. [0537] 181. The process
according to embodiment 180, wherein the shaped support body is a
sphere or a ring. [0538] 182. The process according to embodiment
181, wherein the longest dimension of the shaped support body is 1
to 10 mm. [0539] 183. The process according to embodiment 180,
wherein the inert shaped support body is a ring with a length of 2
to 10 mm, an external diameter of 4 to 10 mm and a wall thickness
of 1 to 4 mm. [0540] 184. The process according to any of
embodiments 180 to 183, wherein the inert shaped support body is
composed of steatite. [0541] 185. The process according to any of
embodiments 180 to 184, wherein the eggshell of catalytically
active mixed oxide has a thickness of 10 to 2000 .mu.m or 10 to 500
.mu.m, or 100 to 500 .mu.m, or 200 to 300 .mu.m. [0542] 186. The
process according to any of embodiments 161 to 185, wherein
reaction gas input mixture C comprises not more than 15% by volume
of methanol. [0543] 187. The process according to any of
embodiments 161 to 185, wherein reaction gas input mixture C
comprises not more than 11% by volume of methanol. [0544] 188. The
process according to any of embodiments 161 to 187, wherein
reaction gas input mixture C comprises 2 to 10% by volume of
methanol. [0545] 189. The process according to any of embodiments
161 to 188, wherein reaction gas input mixture C comprises 6 to 9%
by volume of methanol. [0546] 190. The process according to any of
embodiments 161 to 189, wherein reaction gas input mixture C
comprises the molecular oxygen in a molar amount n.sub.O and the
methanol in a molar amount n.sub.Me, and the n.sub.O:n.sub.Me ratio
is at least 1 or greater than 1. [0547] 191. The process according
to embodiment 190, wherein the n.sub.O:n.sub.Me ratio is 1.1 to 5.
[0548] 192. The process according to embodiment 190 or 191, wherein
the n.sub.O:n.sub.Me ratio is 1.5 to 3.5. [0549] 193. The process
according to any of embodiments 161 to 192, wherein reaction gas
input mixture C comprises, as at least one inert diluent gas other
than steam, N.sub.2. [0550] 194. The process according to
embodiment 193, wherein reaction gas input mixture C comprises 70
to 95% by volume of N.sub.2. [0551] 195. The process according to
any of embodiments 161 to 194, wherein reaction gas input mixture C
comprises 0 to 20% by volume of H.sub.2O. [0552] 196. The process
according to embodiment 195, wherein reaction gas input mixture C
comprises 0.1 to 10% by volume of H.sub.2O. [0553] 197. The process
according to embodiment 195 or 196, wherein reaction gas input
mixture C comprises 0.2 to 7% by volume of H.sub.2O. [0554] 198.
The process according to any of embodiments 194 to 196, wherein
reaction gas input mixture C comprises 0.5 to 5% by volume of
H.sub.2O. [0555] 199. The process according to any of embodiments
161 to 198, wherein the methanol is oxidized in reaction zone C at
a reaction temperature in the range from 250 to 500.degree. C. to
formaldehyde and water. [0556] 200. The process according to
embodiment 199, wherein the methanol is oxidized in reaction zone C
at a reaction temperature in the range from 250 to 400.degree. C.
to formaldehyde and water. [0557] 201. The process according to any
of embodiments 161 to 200, wherein the methanol is oxidized in
reaction zone C at a working pressure in the range from 10.sup.3 to
10.sup.6 Pa or from 10.sup.4 to 210.sup.5 Pa to formaldehyde and
water. [0558] 202. The process according to any of embodiments 1 to
201, wherein a portion of stream Y is recycled into reaction zone A
to obtain reaction gas input mixture A. [0559] 203. The process
according to any of embodiments 135 to 202, wherein a portion of
stream Y is recycled into reaction zone C to obtain reaction gas
input mixture C. [0560] 204. The process according to any of
embodiments 1 to 203, wherein product gas mixture B is separated in
separation zone T by passing product gas mixture B, optionally
after direct and/or indirect cooling thereof, into a condensation
column equipped with separating internals and fractionally
condensing it within the condensation column and conducting streams
X, Y and Z out of the condensation column as separate fractions.
[0561] 205. The process according to any of embodiments 1 to 203,
wherein product gas mixture B is separated in separating zone T by
passing product gas mixture B, optionally after direct and/or
indirect cooling thereof, into an absorption column equipped with
separating internals in countercurrent to an organic solvent with a
higher boiling point than acrylic acid at standard pressure, and
absorbing the acetic acid and acrylic acid present in product gas
mixture B into the solvent to obtain an absorbate, while a stream Z
leaves the absorption column at the top thereof, and then removing
streams X and Y from the absorbate as separate fractions by
fractional distillation thereof in a rectification column. [0562]
206. The process according to any of embodiments 1 to 203, wherein
product gas mixture B is separated in separating zone T by passing
product gas mixture B, optionally after direct and/or indirect
cooling thereof, into an absorption column equipped with separating
internals in countercurrent to an aqueous solution as an absorbent,
and absorbing the acetic acid and acrylic acid present in product
gas mixture B into the solvent to obtain an absorbate, while a
stream Z leaves the absorption column at the top thereof, and then
removing streams X and Y as separate fractions from the absorbate
by fractional distillation thereof in a rectification column.
[0563] 207. Acrylic acid for which the ratio V of the molar amount
n.sup.14C of .sup.14C atomic nuclei present in this acrylic acid to
the molar amount n.sup.12C of .sup.12C atomic nuclei present in the
same acrylic acid, V=n.sup.14C:n.sup.12C, is greater than 0 and
less than the corresponding molar ratio V* of .sup.14C atomic
nuclei to .sup.12C atomic nuclei present in the carbon dioxide in
the earth's atmosphere. [0564] 208. Acrylic acid according to
embodiment 207, wherein V=(1/3)V*. [0565] 209. Acrylic acid
according to embodiment 207, wherein V=(213)V*. [0566] 210. A
liquid phase P comprising at least 1 kg of acrylic acid, wherein
the acrylic acid present is an acrylic acid according to any of
embodiments 207 to 209.
EXAMPLES
[0566] [0567] I) Preparation of different catalysts [0568] A)
Preparation of a mixed oxide catalyst for the heterogeneously
catalyzed partial gas phase oxidation of methanol to formaldehyde
by the FORMOX process. [0569] 530 g of ammonium heptamolybdate
tetrahydrate ((NH.sub.4).sub.6Mo.sub.7O.sub.24.4H.sub.2O) were
dissolved in a mixture, at a temperature of 60.degree. C., of 800
ml of water and 250 g of a 25% by weight aqueous ammonia solution
while maintaining the 60.degree. C. This gave a solution 1 at
60.degree. C. [0570] 808 g of iron(III) nitrate nonahydrate
(Fe(NO.sub.3).sub.3.9H.sub.2O) were dissolved in 1000 ml of water
at 60.degree. C. while maintaining the 60.degree. C. This gave a
solution 2 at 60.degree. C. [0571] Within 20 min, solution 2 was
stirred continuously into solution 1 while maintaining the
60.degree. C. Subsequently, the mixture was stirred at 60.degree.
C. for another 5 min. The aqueous suspension obtained was
subsequently spray-dried at an inlet temperature of 340.degree. C.
and an outlet temperature of 120.degree. C. in an air stream within
1.5 h (spray tower of the Mobile Minor 2000 (MM-I) type from Niro
A/S, Gladsaxevej 305, 2860 Soborg, Denmark, with a centrifugal
atomizer of the FOIA type and an atomizer wheel of the SL24-50
type). During the spray drying, stirring of the proportion of the
suspension which was yet to be sprayed in each case was continued
while maintaining the 60.degree. C. [0572] The spray powder thus
obtained was, based on its weight, mixed homogeneously with 1% by
weight of TIMREX.RTM. T44 graphite from Timcal AG (cf. WO
2008/087116) (in a drum hoop mixer; wheel diameter: 650 mm, drum
volume: 5 l, speed: approx. 30 rpm, mixing time: 30 min). The
resulting mixture was then compacted in a roll compactor of the RCC
100.times.20 type from Powtec with 2 contrarotary steel rolls with
a pressure of 12 bar, and then forced through a screen with square
meshes of mesh size 0.8 mm. The resulting compactate (which had a
bulk density of 1050 g/l and an essentially homogeneous particle
size of .gtoreq.0.4 mm and .ltoreq.0.8 mm) was subsequently mixed
in the aforementioned drum hoop mixer at a speed of approx. 30 rpm
within 30 min with, based on its weight, 3% by weight of the same
graphite, and then compacted as described in DE-A 10 2008040093 to
annular shaped unsupported catalyst precursor bodies of geometry 5
mm.times.3 mm.times.3 mm (external
diameter.times.height.times.internal diameter) with a side crushing
strength of 22.+-.5 N and a mass of 130 mg (fill height: 7.5-9 mm;
pressing force: 3.0-3.5 kN; Kilian rotary tableting press (9-die
tableting machine) of the S100 type (from Kilian, D-50735 Cologne).
The tableting was effected under a nitrogen atmosphere. [0573] For
the final thermal treatment, the shaped catalyst precursor bodies
were divided homogeneously between 4 grids arranged alongside one
another with a square base area of in each case 150 mm.times.150 mm
(bed height: approx. 40 mm) and treated in a forced-air oven with
air flow (from Heraeus Instruments GmbH, D-63450 Hanau, model: K
750/2) as follows: the air flow was 100 l (STP)/h and initially had
a temperature of 120.degree. C. The 120.degree. C. was first
maintained for 10 h. Then the temperature was increased to
460.degree. C. in an essentially linear manner within 10 h, and the
460.degree. C. was then maintained for a further 4 h. This was
followed by cooling to 25.degree. C. within 5 h. [0574] An annular
unsupported oxidation catalyst C was thus obtained, the mixed oxide
active material of which had the stoichiometry
Fe.sub.2(MoO.sub.4).sub.3. Subsequently, the unsupported catalyst
rings were forced through a screen with square meshes of mesh size
0.8 mm. The material passing through the screen (spall of oxidation
catalyst C) had an essentially homogeneous particle size (longest
dimension) of .gtoreq.0.4 mm and .ltoreq.0.8 mm. [0575] B)
Production of a mixed oxide catalyst for charging of section 1 of a
reaction zone A for the heterogeneously catalyzed partial gas phase
oxidation of ethanol to acetic acid. [0576] 380 g of water were
initially charged in a 2 l beaker and heated to 55.degree. C. Then
220 g of oxalic acid dihydrate were added and dissolved while
stirring and maintaining the 55.degree. C. After complete
dissolution of the oxalic acid dihydrate, 116 g of finely divided
V.sub.2O.sub.5 were added, which formed a deep blue vanadium
complex. After the addition of the V.sub.2O.sub.5 had ended, the
mixture was heated while stirring to 80.degree. C. in an
essentially linear manner within 20 min. This temperature was
maintained while stirring for a further 10 min. Thereafter, the
aqueous solution formed was cooled in an essentially linear manner
to 25.degree. C. within 60 min. [0577] To 135 ml of the aqueous
solution prepared as described were added 97.5 g of finely divided
titanium dioxide (manufacturer: Fuji; type: TA 100C; anatase
polymorph; specific surface area (BET); 20 m.sup.2/g; 2200 ppm of
sulfur; 1600 ppm of niobium; particle size distribution (stir 1 g
of TiO.sub.2 in 100 ml of water with a magnetic stirrer for 10 min,
then determine the particle diameter distribution by means of
dynamic light scattering in a mastersizer S from Malvern
Instruments with an MS1 small volume wet dispersing unit):
d.sub.10=0.6 .mu.m, d.sub.50=1.1 .mu.m, d.sub.90=2.1 .mu.m; d.sub.x
means that X % of the total particle volume consists of particles
with this or a smaller diameter). Subsequently, the mixture was
homogenized at 8000 rpm for 3 minutes with an Ultra Turax stirrer
from Janke & Kunkel GmbH and Co. KG-IKA Labortechnik (shaft
type: S 50KR-G45 F, shaft tube diameter: 25 mm, stator diameter: 45
mm, rotor diameter: 40 mm). The resulting aqueous dispersion was
used to coat 150 g of spall of C220 steatite from CeramTec with a
longest dimension in the range from 1 mm to 1.5 mm. [0578] For this
purpose, the 150 g of steatite spall were introduced into a
rotating coating drum (with an internal radius of 15 cm and an
internal volume of 1800 cm.sup.3) as a coating system. The speed of
the coating drum was 200 rpm. The aqueous dispersion was applied to
the surface of the steatite spall by spraying the aqueous
dispersion with the aid of a two-substance atomization nozzle.
[0579] The atomization medium used was 250 l (STP)/h of compressed
air (10 bar gauge) at a temperature of 25.degree. C. The dispersion
flow rate was 100 ml/h at a temperature of the aqueous dispersion
of 25.degree. C. The atomization was in the form of a circular
solid cone (10.degree.-40.degree.). The atomization type was mist,
with a droplet size of <50 to 150 .mu.m. The diameter of the
flow cross section for the dispersion was 1 mm. The internal
temperature of the coating drum was kept at 120.degree. C. in the
course of the coating operation. After the spraying operation had
ended, drying was effected for another 30 min in the coating drum
under otherwise unchanged conditions. Subsequently, the coated
support particles were introduced into a porcelain dish and
calcined therein in a muffle furnace (capacity 1.5 l) under air
(stationary). To this end, the temperature in the calcination
material was increased with a heating rate of 3.degree. C./min from
100.degree. C. to 500.degree. C., and then kept at this temperature
for 3 h. Finally, the calcination material was cooled within the
muffle furnace essentially in a linear manner to 25.degree. C.
within 12 h. [0580] The oxidation catalyst A(1) thus obtained was
an eggshell catalyst whose catalytically active mixed oxide
eggshell had an average stoichiometry of
[0580] (V.sub.2O.sub.5), (TiO.sub.2).sub.y, where x=18.3% by weight
and y=81.7% by weight. [0581] C) Production of a mixed oxide
catalyst for charging of section 2 of a reaction zone A for the
heterogeneously catalyzed partial gas phase oxidation of ethanol to
acetic acid [0582] 127 g of copper(II) acetate monohydrate were
dissolved in 2700 g of water at a temperature of 25.degree. C.,
while maintaining this temperature, to give a solution 1. [0583]
860 g of ammonium heptamolybdate tetrahydrate were dissolved in
5500 g of water at a temperature of 95.degree. C., while
maintaining this temperature, within 5 min. Subsequently, while
maintaining 95.degree. C., 143 g of ammonium metavanadate were
added and the resulting solution was stirred at 95.degree. C. for a
further 30 min. Thereafter, 126 g of ammonium paratungstate
heptahydrate were added and the resulting solution was stirred at
95.degree. C. for a further 40 min. Subsequently, the solution 2
thus obtained was cooled to 80.degree. C. within 10 min. [0584]
Solution 1 at 25.degree. C. was then stirred into solution 2 at
80.degree. C., while maintaining the 80.degree. C., within 5 min.
The resulting solution was subsequently spray-dried in an air
stream at an inlet temperature of 350.degree. C. and an outlet
temperature of 110.degree. C. within 3 h (spray tower of the Mobile
Minor 200 (MM-I) type from Niro A/S, Gladsaxevej 305, 2860 Soborg,
Denmark, with a centrifugal atomizer of the F0 A1 type and an
atomizer wheel of the SL24-50 type). During the spray drying, the
stirring of the proportion of the suspension which was yet to be
sprayed in each case was continued while maintaining the 80.degree.
C. In each case 1 kg of the resulting spray powder was kneaded with
0.15 kg of water at a temperature of 25.degree. C. (kneader from
Werner & Pfleiderer of the ZS1-80 type; kneading time: approx.
2 hours; kneading temperature: 30 to 35.degree. C.). Subsequently,
the kneading material was calcined with a layer thickness of
approx. 2 cm in a forced-air oven charged with an oxygen/nitrogen
mixture. The molecular oxygen content of the oxygen/nitrogen
mixture was adjusted such that the O.sub.2 content at the outlet of
the forced-air oven was 1.5% by volume. In the course of the
calcination, the kneading material was first heated at a heating
rate of 10.degree. C./min from 30.degree. C. to 300.degree. C. (in
each case, temperature of the calcination material) and then kept
at this temperature for 6 h. Then the calcination material was
heated with a heating rate of 10.degree. C./min to 400.degree. C.
and this temperature was subsequently maintained for 1 h.
Subsequently, the temperature of the calcination material was
cooled in an essentially linear manner to 25.degree. C. within 12
h. The oven loading for the calcination was 250 g/l (g of kneading
material per l of capacity of the forced-air oven). The input
volume flow of the oxygen/nitrogen mixture was 80 l (STP)/h and the
residence time of the oxygen/nitrogen charge (calculated as the
ratio of the capacity of the forced-air oven and the volume flow of
the nitrogen/oxygen mixture supplied at a temperature of 25.degree.
C.) was 135 sec. The internal volume of the forced-air oven was 3
l. [0585] The resulting catalytically active mixed oxide had the
stoichiometry Mo.sub.12V.sub.3W.sub.1.2Cu.sub.1.6O.sub.n. [0586]
The catalytically active oxide material was subsequently ground in
a ZM 200 mill from Retsch to a fine powder whose particle diameter
(longest dimension) was essentially in the range from 0.1 to 50
.mu.m (a diameter distribution favorable in accordance with the
invention for the fine powder is shown by FIG. 2 of DE-A
102010023312). This active material powder was used, in a rotary
drum, in analogy to the coating process described in EP-A 714 700,
to coat rough-surface spheres of C220 steatite from CeramTec with a
sphere diameter of 2 to 3 mm with addition of water as a binder.
Finally, the coated spheres were dried with air at 110.degree. C.
The active material content of the spherical eggshell catalysts
A(2) thus obtained was adjusted to a homogeneous 20% by weight. The
residual water content of the active material after drying was 0.2%
by weight. [0587] D) Production of an aldol condensation catalyst B
whose active material is a vanadium-phosphorus oxide doped with
Fe(III) [0588] A stirred tank which was heatable externally with
molecular nitrogen (N.sub.2 content 99.99% by volume) and by means
of pressurized water, had an internal volume of 8 m.sup.3, was
enameled on the inner surface thereof and was equipped with baffles
and an impeller stirrer was initially charged with 4602 kg of
isobutanol. After the three-level impeller stirrer had been
started, the isobutanol was heated to 90.degree. C. under reflux.
While maintaining this temperature, continuous supply of 690 kg of
finely divided vanadium pentoxide via a conveying screw was then
commenced. Once 460 kg of vanadium pentoxide (V.sub.2O.sub.5) had
been added within 20 min, while continuing the supply of vanadium
pentoxide, the continuous pumped addition of 805 kg of 105% by
weight phosphoric acid (cf. DE-A 3520053) was commenced, in the
course of which the temperature of 90.degree. C. was maintained
(pumping rate=160 kg/h). [0589] After the addition of the
phosphoric acid had ended, the mixture was heated while stirring
and under reflux to temperatures in the range from 100 to
108.degree. C. and kept within this temperature range for 14 hours.
Subsequently, the hot suspension obtained was cooled to 60.degree.
C. in an essentially linear manner within 75 minutes, and 22.7 kg
of iron(III) phosphate hydrate were added (Fe content: 29.9% by
weight; supplier: Dr. Paul Lohmann; free of Fe(II) impurities).
Then the mixture was heated, again under reflux, to temperatures of
100 to 108.degree. C., and the suspension was kept at this
temperature while continuing to stir for 1 hour. Subsequently, the
suspension at approx. 105.degree. C. was discharged into a pressure
suction filter which had been inertized beforehand with nitrogen
and heated to 100.degree. C., and filtered at a pressure above the
suction filter of approx. 0.35 MPa abs. The filtercake obtained was
blown dry by constantly introducing nitrogen while stirring and at
100.degree. C. within one hour. After blowing dry, the filtercake
was heated to 155.degree. C. and evacuated to a pressure of 15 kPa
abs. (150 mbar abs.). [0590] Under these conditions, the drying was
performed down to a residual isobutanol content of 2% by weight in
the dried precursor material. This comprised Fe and V in a molar
Fe/V ratio of 0.016. [0591] Subsequently, the dry powder was
treated further in an inclined stainless steel rotating tube
through which 100 m.sup.3/h of air (the inlet temperature of which
was 150.degree. C.) flowed and which had internal spiral winding.
The tube length was 6.5 m, the internal diameter 0.9 m and the
rotating tube wall thickness 2.5 cm. The speed of rotation of the
rotating tube was 0.4 rpm. The dry powder was supplied to the
rotating tube interior in an amount of 60 kg/h at the upper end
thereof. The spiral winding ensured homogeneous flowing motion
(downward) of the dry powder within the rotating tube. The rotating
tube length was divided into five heating zones of the same length,
the temperature of which was controlled from the outside. The
temperatures of the five heating zones measured on the outer wall
of the rotating tube were, from the top downward, 250.degree. C.,
300.degree. C., 345.degree. C., 345.degree. C. and 345.degree. C.
[0592] 400 g of the powder leaving the rotating tube were, based on
the weight thereof, mixed homogeneously with 1% by weight of
graphite (Asbury 3160, from Timcal Ltd., cf. WO 2008/087116) (in a
drum hoop mixer of wheel diameter 650 mm, drum volume 5 l, speed:
30 rpm, mixing time: 30 min). The resulting mixture was then
compacted with the aid of a Powtec roll compactor with 2
contrarotary steel rolls at an applied pressure of 9 bar, and then
forced through a screen with square screen meshes of size 1 mm. The
resulting compactate had a bulk density of 1100 g/l and an
essentially homogeneous particle size of .gtoreq.0.7 mm and
.ltoreq.0.8 mm. 30 ml of bed volume of the granules were charged
into a vertical tube furnace (internal tube diameter: 26 mm; in the
center of the tube, thermowell running from the top downward with
an external diameter of 4 mm to accommodate a thermocouple). 25 l
(STP)/h of air with an inlet temperature of 160.degree. C. were
conducted through the tube furnace. At a heating rate of 5.degree.
C./min, the temperature of the calcination material present in the
tube furnace was increased from 25.degree. C. to 250.degree. C. On
attainment of the temperature of 250.degree. C., the temperature of
the calcination material was raised at a heating rate of 2.degree.
C./min to 330.degree. C. This temperature was maintained over a
period of 40 min. [0593] Then, while maintaining a volume flow rate
of 25 l (STP)/h, air flow was switched to a flow with a mixture of
50% by volume of N.sub.2 and 50% by volume of steam (the inlet
temperature of which was 160.degree. C.) through the tube furnace,
and the temperature of the calcination material was raised at a
heating rate of 3.degree. C./min to 425.degree. C. This temperature
was maintained over a period of 180 min. Then, while maintaining
the volume flow rate of 25 l (STP)/h, the flow was switched again
to an air flow (the inlet temperature of the air stream was
25.degree. C.). Then the temperature of the calcination material
was cooled to 25.degree. C. within 120 min. [0594] The
stoichiometry of the unsupported Fe(III)-doped vanadium-phosphorus
oxide aldol condensation catalyst B prepared as described was
V.sub.1P.sub.1.05Fe.sub.0.016O.sub.n. [0595] II) Performance of
processes A) to D) according to the invention for preparing acrylic
acid from ethanol and formaldehyde using the catalysts prepared in
I) (the contents of all reaction gas input mixtures and reactants
were determined by gas chromatography) [0596] A) 1. Configuration
of reaction zone A [0597] Reaction zone A was implemented in a
tubular reactor A (internal diameter: 12 mm; wall thickness: 1.5
mm; length: 2000 mm; material: stainless steel, DIN material
1.4541), which was electrically heatable externally in sections.
The catalyst charge in tubular reactor A was configured as
follows:
TABLE-US-00002 [0597] Section 1: 300 mm of a preliminary bed of
steatite spall (longest dimension 1 to 1.5 mm; C220 steatite from
CeramTec) at the reactor inlet; 59 mm of a homogeneous mixture of
3.3 ml of oxida- tion catalyst A(1) and 3.3 ml of the steatite
spall used for the preliminary bed; Section 2: 15 mm with 1.7 ml of
eggshell catalyst A(2); and 300 mm of a downstream bed of the
steatite spall used for the preliminary bed.
[0598] Sections 1 and 2 directly adjoin one another. [0599] The
temperature of tubular reactor A was set to 185.degree. C. in the
region of section 1 and to 220.degree. C. in the region of section
2 (in each case the outer wall temperature of tubular reactor A).
41.8 l (STP)/h of reaction gas input mixture A were supplied to the
preliminary bed of steatite spall with an inlet temperature of
150.degree. C. The pressure at the inlet into tubular reactor A was
2.0 bar abs. [0600] Reaction gas input mixture A had the following
contents: [0601] 1.6% by vol. of ethanol, [0602] 9.97% by vol. of
steam, [0603] 5.96% by vol. of O.sub.2, and [0604] 82.47% by vol.
of N.sub.2. [0605] The last 1326 mm of tubular reactor A were
unheated. The space velocity of reaction gas input mixture A on the
catalyst charge was approx. 5000 h.sup.-1. The space velocity of
ethanol was 80 h.sup.-1 (l (STP)/lh). [0606] The product gas
mixture A leaving the tubular reactor A (41.9 l (STP)/h) had the
following contents (online GC analysis): [0607] 11.65% by vol. of
water, [0608] 81.54% by vol. of N.sub.2, [0609] 4.12% by vol. of
O.sub.2, [0610] 1.42% by vol. of acetic acid, [0611] 0.05% by vol.
of acetaldehyde, [0612] 0.22% by vol. of CO.sub.2, and [0613]
<0.01% by vol. of ethanol. [0614] 2. Configuration of reaction
zone B [0615] Reaction zone B was implemented in a tubular reactor
B (internal diameter: 15 mm; wall thickness: 1.2 mm; length: 2000
mm; material: stainless steel, DIN material 1.4541), which was
electrically heatable externally. The catalyst charge in tubular
reactor B was configured as follows:
TABLE-US-00003 [0615] 1000 mm of a preliminary bed of steatite
spall (as in reaction zone A) at the reactor inlet; and 753 mm with
133 ml of the unsupported aldol condensation catalyst B.
[0616] The 41.9 l (STP)/h of product gas mixture A, 2.0 l (STP)/h
of an aqueous solution of formaldehyde in water which has been
converted to the vapor phase and 1.3 l (STP)/h of acetic acid
converted to the vapor phase (which formed stream Y) were used to
obtain 45.1 l (STP)/h of reaction gas input mixture B. [0617] The
contents of the vaporized formaldehyde solution were: [0618] 0.70%
by vol. of methanol, [0619] 37.23% by vol. of formaldehyde and
[0620] 62.06% by vol. of water. [0621] The contents of the
vaporized acetic acid were: [0622] 99% by vol. of acetic acid.
[0623] The contents of reaction gas input mixture B were: [0624]
0.03% by vol. of methanol, [0625] 1.64% by vol. of formaldehyde,
[0626] 13.54% by vol. of water, [0627] 75.65% by vol. of N.sub.2,
[0628] 3.82% by vol. of O.sub.2, [0629] 4.07% by vol. of acetic
acid, [0630] 0.04% by vol. of acetaldehyde, and [0631] 0.20% by
vol. of CO.sub.2. [0632] Reaction gas input mixture B was supplied
to the preliminary bed of steatite spall with an inlet temperature
of 350.degree. C. The pressure at the inlet into tubular reactor B
was 1.5 bar abs. The temperature of tubular reactor B was set to
375.degree. C. over the length of the fixed bed charge thereof
(outer wall temperature of tubular reactor B). The remaining length
of tubular reactor B was unheated. The space velocity of reaction
gas input mixture B on the catalyst charge was 340 h.sup.-1. [0633]
The product gas mixture B leaving tubular reactor B (45.1 l
(STP)/h) had the following contents (online GC analysis): [0634]
0.02% by vol. of methanol, [0635] 0.02% by vol. of formaldehyde,
[0636] 15.22% by vol. of water, [0637] 75.64% by vol. of N.sub.2,
[0638] 3.41% by vol. of O.sub.2, [0639] 1.29% by vol. of acrylic
acid, [0640] 2.78% by vol. of acetic acid, [0641] 0.59% by vol. of
CO.sub.2, and [0642] 0.01% by vol. of methyl acetate. [0643] Based
on the molar amount of ethanol supplied to reaction zone A, the
yield of acrylic acid achieved was 87.0 mol %. [0644] B) The
reaction zone A and reaction zone B were configured as in II)A) and
charged with the same catalyst beds. However, the bed length of the
catalytically active portion in section 1 was 62 mm (homogeneous
mixture of 3.5 ml of oxidation catalyst A(1) and 3.5 ml of steatite
spall). The performance of the heterogeneously catalyzed partial
oxidation of ethanol to acetic acid was performed as in II)A), but
the feed flow rate of reaction gas input mixture A was 43.3 l
(STP)/h. 43.4 l (STP)/h of product gas mixture A were obtained. The
composition of product gas mixture A corresponded to that in II)A).
The space velocity of reaction gas input mixture A on the catalyst
charge in tubular reactor A was approx. 5000 h.sup.-1. [0645] The
43.4 l (STP)/h of product gas mixture A, a formaldehyde stream
obtained by vaporizing and redissociating trioxane of 0.7 l
(STP)/h, and 1.0 l (STP)/h of acetic acid converted to the vapor
phase (which formed stream Y) were used to obtain 45.1 l (STP)/h of
reaction gas input mixture B. [0646] The contents of the
formaldehyde stream were: [0647] 100% by vol. of formaldehyde.
[0648] The contents of the vaporized acetic acid were: [0649] 99%
by vol. of acetic acid. [0650] The contents of reaction gas input
mixture B were: [0651] 1.48% by vol. of formaldehyde, [0652] 11.20%
by vol. of water, [0653] 78.38% by vol. of N.sub.2, [0654] 3.96% by
vol. of O.sub.2, [0655] 3.72% by vol. of acetic acid, [0656] 0.05%
by vol. of acetaldehyde, and [0657] 0.21% by vol. of CO.sub.2.
[0658] Reaction gas input mixture B was supplied to the preliminary
bed of steatite spall with an inlet temperature of 320.degree. C.
The pressure at the inlet into tubular reactor B was 1.5 bar abs.
The temperature of tubular reactor B was set to 340.degree. C. over
the length of its fixed bed (outer wall temperature of tubular
reactor B). The remaining length of tubular reactor B was unheated.
The space velocity of reaction gas input mixture B on the catalyst
charge was 340 h.sup.-1 (l (STP)/lh). [0659] The product gas
mixture B leaving the tubular reactor B (45.1 l (STP)/h) had the
following contents (online GC analysis): [0660] 0.02% by vol. of
formaldehyde, [0661] 12.72% by vol. of water, [0662] 78.37% by vol.
of N.sub.2, [0663] 3.76% by vol. of O.sub.2, [0664] 1.35% by vol.
of acrylic acid, [0665] 2.38% by vol. of acetic acid, and [0666]
0.39% by vol. of CO.sub.2. [0667] Based on the amount of ethanol
supplied to reaction zone A, the yield of acrylic acid achieved was
88 mol %. [0668] C) Reaction zone A and reaction zone B were
configured as in II)A) and charged with the same catalyst beds. The
heterogeneously catalyzed partial oxidation of ethanol to acetic
acid was performed as in II)A), but reaction gas input mixture A
had the following contents: [0669] 3.00% by vol. of ethanol, [0670]
10.01% by vol. of water, [0671] 7.41% by vol. of O.sub.2, and
[0672] 79.57% by vol. of N.sub.2. [0673] The product gas mixture A
leaving the tubular reactor A (41.9 l (STP)/h) had the following
contents (online GC analysis): [0674] 0.01% by vol. of ethanol,
[0675] 13.26% by vol. of water, [0676] 78.58% by vol. of N.sub.2,
[0677] 3.99% by vol. of O.sub.2, [0678] 2.66% by vol. of acetic
acid, [0679] 0.09% by vol. of acetaldehyde, ad [0680] 0.41% by vol.
of CO.sub.2. [0681] The 41.9 l (STP)/h of product gas mixture A, a
formaldehyde stream obtained by vaporizing and redissociating
trioxane of 1.2 l (STP)/lh, and 2.0 l (STP)/h of acetic acid
converted to the vapor phase (which formed stream Y) were used to
obtain 45.1 l (STP)/h of reaction gas input mixture B. [0682] The
contents of the formaldehyde stream were: [0683] 100% by vol. of
formaldehyde. [0684] The contents of the vaporized acetic acid
were: [0685] 99% by vol. of acetic acid. [0686] The contents of
reaction gas input mixture B were: [0687] 0.01% by vol. of ethanol,
[0688] 2.69% by vol. of formaldehyde, [0689] 12.32% by vol. of
water, [0690] 73.01% by vol. of N.sub.2, [0691] 3.70% by vol. of
O.sub.2, [0692] 6.80% by vol. of acetic acid, [0693] 0.08% by vol.
of acetaldehyde, and [0694] 0.39% by vol. of CO.sub.2. [0695]
Reaction gas input mixture B was supplied to the preliminary bed of
steatite spall with an inlet temperature of 320.degree. C. The
pressure at the inlet into tubular reactor B was 1.5 bar abs. The
temperature of tubular reactor B was set to 340.degree. C. over the
length of its fixed bed (outer wall temperature of tubular reactor
B). The remaining length of tubular reactor B was unheated. The
space velocity of reaction gas input mixture B on the catalyst
charge was 340 h.sup.-1. [0696] The product gas mixture B leaving
the tubular reactor B (45.1 l (STP)/h) had the following contents:
[0697] 0.03% by vol. of formaldehyde, [0698] 15.1% by vol. of
water, [0699] 73.0% by vol. of N.sub.2, [0700] 3.34% by vol. of
O.sub.2, [0701] 2.45% by vol. of acrylic acid, [0702] 4.36% by vol.
of acetic acid, and [0703] 0.71% by vol. of CO.sub.2. [0704] Based
on the amount of ethanol supplied to reaction zone A, the yield of
acrylic acid achieved was 88 mol %. [0705] D) Reaction zone A and
reaction zone B were configured as in II)A) and were charged with
the same catalyst beds. However, the bed length of the
catalytically active part in section 1 was 43 mm (homogeneous
mixture of 2.4 ml of oxidation catalyst A(1) and 2.4 ml of steatite
spall) and the bed length of the catalytically active part in
section 2 was only 11 mm (1.2 ml of eggshell catalyst A(2)). The
heterogeneously catalyzed partial oxidation of ethanol to acetic
acid was performed as in II)A), except that reaction gas input
mixture A had the following contents: [0706] 3.00% by vol. of
ethanol, [0707] 10.01% by vol. of water, [0708] 7.41% by vol. of
O.sub.2, and [0709] 79.57% by vol. of N.sub.2. [0710] In addition,
the feed flow rate of reaction gas input mixture A was only 30.6 l
(STP)/h. The product gas mixture A leaving the tubular reactor A
(30.6 l (STP)/h) had the following contents (online GC analysis):
[0711] 0.01% by vol. of ethanol, [0712] 13.26% by vol. of water,
[0713] 78.58% by vol. of N.sub.2, [0714] 3.99% by vol. of O.sub.2,
[0715] 2.66% by vol. of acetic acid, [0716] 0.09% by vol. of
acetaldehyde, and [0717] 0.41% by vol. of CO.sub.2. [0718] The 30.6
l (STP)/h of product gas mixture A, 1.8 l (STP)/h of acetic acid
converted to the vapor phase (which formed stream Y) and 12.7 l
(STP)/h of a formaldehyde-comprising product gas mixture C which
had been obtained by heterogeneously catalyzed partial gas phase
oxidation of methanol by the FORMOX process in a reaction zone C
were used to obtain 45.1 l (STP)/h of reaction gas input mixture B.
[0719] The contents of the vaporized acetic acid were: [0720] 99%
by vol. of acetic acid. [0721] The contents of product gas mixture
C were: [0722] 0.07% by vol. of methanol, [0723] 8.03% by vol. of
formaldehyde, [0724] 11.84% by vol. of water, [0725] 73.71% by vol.
of N.sub.2, [0726] 4.83% by vol. of O.sub.2, [0727] 0.40% by vol.
of CO.sub.2, [0728] 0.09% by vol. of formic acid, and [0729] 0.04%
by vol. of dimethyl ether. [0730] The contents of reaction gas
input mixture B were: [0731] 0.02% by vol. of methanol, [0732]
2.26% by vol. of formaldehyde, [0733] 12.34% by vol. of water,
[0734] 74.10% by vol. of N.sub.2, [0735] 4.07% by vol. of O.sub.2,
[0736] 5.73% by vol. of acetic acid, [0737] 0.06% by vol. of
acetaldehyde, [0738] 0.39% by vol. of CO.sub.2, [0739] 0.02% by
vol. of formic acid, and [0740] 0.01% by vol. of dimethyl ether.
[0741] Reaction gas input mixture B was supplied to the preliminary
bed of steatite spall with an inlet temperature of 320.degree. C.
The pressure at the inlet into tubular reactor B was 1.5 bar abs.
The temperature of tubular reactor B was set to 340.degree. C. over
the length of its fixed bed (outer wall temperature of tubular
reactor B). The remaining length of tubular reactor B was unheated.
The space velocity of reaction gas input mixture B on the catalyst
charge was 340 h.sup.-1. [0742] The product gas mixture B leaving
the tubular reactor B (45.1 l (STP)/h) had the following contents:
[0743] 0.01% by vol. of methanol, [0744] 0.02% by vol. of
formaldehyde, [0745] 14.66% by vol. of water, [0746] 74.09% by vol.
of N.sub.2, [0747] 3.50% by vol. of O.sub.2, [0748] 1.78% by vol.
of acrylic acid, [0749] 3.96% by vol. of acetic acid, [0750] 0.93%
by vol. of CO.sub.2, [0751] 0.01% by vol. of methyl acetate, [0752]
0.02% by vol. of formic acid, and [0753] 0.01% by vol. of dimethyl
ether. [0754] Based on the amount of ethanol supplied to reaction
zone A, the yield of acrylic acid achieved was 88 mol %. [0755]
Reaction zone C was implemented in a tubular reactor C (internal
diameter: 8 mm; wall thickness: 1 mm; length: 100 mm; material:
stainless steel DIN material 1.4541), which was electrically
heatable externally. The catalyst charge in tubular reactor C was
configured as follows: [0756] 50 mm of a preliminary bed of
steatite spall (as in reaction zone A) at the reactor inlet; and
[0757] 37 mm with 1.87 ml of the oxidation catalyst C in spall
form. [0758] The contents of reaction gas input mixture C were:
[0759] 9.15% by vol. of methanol, [0760] 3.04% by vol. of water,
[0761] 77.76% by vol. of N.sub.2, and [0762] 10.05% by vol. of
O.sub.2. [0763] Reaction gas input mixture C (12.2 l (STP)/h) was
supplied to the preliminary bed of steatite spall with an inlet
temperature of 265.degree. C. The pressure at the inlet into
tubular reactor C was 2 bar abs. The temperature of tubular reactor
C was set to 265.degree. C. over the length of the fixed bed charge
thereof (outer wall temperature of tubular reactor C). The
remaining length of tubular reactor C was unheated. The space
velocity of reaction gas input mixture C on the catalyst charge was
6500 h.sup.-1. [0764] U.S. Provisional Patent Application No.
61/383,358, filed Sep. 16, 2010, is incorporated into the present
patent application by literature reference. With regard to the
abovementioned teachings, numerous changes and deviations from the
present invention are possible. It can therefore be assumed that
the invention, within the scope of the appended claims, can be
performed differently than the way described specifically
herein.
* * * * *
References