U.S. patent application number 09/901223 was filed with the patent office on 2001-12-20 for process for the non-oxidative preparation of formaldehyde from methanol.
This patent application is currently assigned to Ticona GmbH, corporation of Germany. Invention is credited to Haubs, Michael, Kaiser, Thomas, Rosenberg, Michael, Schweers, Elke.
Application Number | 20010053865 09/901223 |
Document ID | / |
Family ID | 27545075 |
Filed Date | 2001-12-20 |
United States Patent
Application |
20010053865 |
Kind Code |
A1 |
Schweers, Elke ; et
al. |
December 20, 2001 |
Process for the non-oxidative preparation of formaldehyde from
methanol
Abstract
In a process for preparing formaldehyde from methanol by
dehydrogenation in a reactor in the presence of a catalyst at a
temperature in the range from 300 to 1000.degree. C., a carrier gas
stream which has a temperature above the dehydrogenation
temperature is fed to the reactor.
Inventors: |
Schweers, Elke; (Bad Soden,
DE) ; Kaiser, Thomas; (Kelkheim, DE) ; Haubs,
Michael; (Bad Kreuznach, DE) ; Rosenberg,
Michael; (Niedernhausen, DE) |
Correspondence
Address: |
CONNOLLY BOVE LODGE AND HUTZ LLP
1220 Market Street
P.O. Box 2207
Wilmington
DE
19899
US
|
Assignee: |
Ticona GmbH, corporation of
Germany
|
Family ID: |
27545075 |
Appl. No.: |
09/901223 |
Filed: |
July 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09901223 |
Jul 9, 2001 |
|
|
|
09445082 |
Feb 22, 2000 |
|
|
|
Current U.S.
Class: |
568/471 ;
422/198 |
Current CPC
Class: |
C07D 323/06 20130101;
C07C 47/04 20130101; C07C 45/002 20130101; C08G 2/10 20130101; C07C
45/002 20130101; C01B 3/22 20130101; C08G 2/08 20130101 |
Class at
Publication: |
568/471 ;
422/198 |
International
Class: |
C07C 045/29; F28D
001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 1998 |
DE |
19814283.8 |
Jun 30, 1997 |
DE |
19727520.6 |
Jun 2, 1997 |
DE |
19722774.0 |
Sep 30, 1997 |
DE |
19743145.3 |
Mar 31, 1998 |
DE |
19814884.6 |
Claims
1. A process for preparing formaldehyde from methanol by
dehydrogenation in a reactor in the presence of a catalyst at a
temperature in the range from 300 to 1000.degree. C., wherein a
carrier gas stream which has a temperature above the
dehydrogenation temperature is fed to the reactor.
2. The process as claimed in claim 1, wherein the temperature
difference is at least 20.degree. C.
3. The process as claimed in claim 1 or 2, wherein the catalyst is
generated physically separately from the reactor.
4. The process as claimed in one or more of the preceding claims,
wherein a primary catalyst is used for generating the catalyst and
further amounts of this primary catalyst are introduced
continuously.
5. The process as claimed in one or more of the preceding claims,
wherein the carrier gas stream consists essentially of the
by-products of the dehydrogenation.
6. The process as claimed in one or more of the preceding claims,
wherein part of the by-products of the dehydrogenation is used as
fuel for heating the reactor.
7. An apparatus for carrying out a process as claimed in claim 1,
comprising one or more heat exchangers for preheating the starting
materials, a vessel for superheating a carrier gas stream, a heated
reactor for carrying out the dehydrogenation, one or more heat
exchangers for cooling the product mixture, a unit for separating
off the formaldehyde and also an apparatus for introduction of the
methanol and for further introduction of a catalyst.
8. The use of formaldehyde prepared by a process as claimed in one
or more of claims 1 to 6 for preparing polyoxymethylene and/or
trioxane.
9. The process as claimed in one or more of claims 1 to 6, wherein
the hydrogen formed as by-product is separated off and passed to a
further use.
10. The use of hydrogen prepared by a process as claimed in claim 9
for preparing methanol and/or as hydrogenation gas.
11. A process for preparing trioxane, which comprises converting
methanol into formaldehyde by dehydrogenation in a reactor at a
temperature in the range from 300 to 1000.degree. C. in the
presence of a catalyst, where a carrier gas stream having a
temperature above the dehydrogenation temperature is fed to the
reactor, and the formaldehyde prepared in this way is trimerized to
give trioxane.
12. A process for preparing polyoxymethylene, which comprises
converting methanol into formaldehyde by dehydogenation in a
reactor at a temperature in the range from 300 to 1000.degree. C.
in the presence of a catalyst, where a carrier gas stream having a
temperature above the dehydrogenation temperature is fed to the
reactor, and if desired, purifying the formaldehyde obtained in
this way, polymerizing the formaldehyde, capping the end groups of
the polymer prepared in this way and if desired, homogenizing the
polymer in the melt and/or providing it with suitable
additives.
13. A process for preparing polyoxymethylene copolymers, which
comprises converting methanol into formaldehyde by dehydrogenation
in a reactor at a temperature in the range from 300 to 1000.degree.
C. in the presence of a catalyst, where a carrier gas stream having
a temperature above the dehydrogenation temperature is fed to the
reactor, and trimerizing the formaldehyde obtained in this way to
give trioxane, if desired, purifying the trioxane, copolymerizing
the trioxane with cyclic ethers or cyclic acetals, if desired,
removing unstable end groups and if desired, homogenizing the
polymer prepared in this way in the melt and/or admixing it with
suitable additives.
14. A process for preparing polyoxymethyene copolymers, which
comprises converting methanol into formaldehyde by dehydrogenation
in a reactor in the presence of a catalyst at a temperature in the
range from 300 to 1000.degree. C., where a circulating gas stream
comprising by-products of the dehydrogenation is passed through the
reactor, and if desired, purifying the formaldehyde obtained in
this way, copolymerizing the formaldehyde with cyclic ethers or
cyclic acetals, is if desired, removing unstable end groups and if
desired, homogenizing the polymer prepared in this way in the melt
and/or admixing it with suitable additives.
15. The process as claimed in one or more of claims 1 to 6, wherein
the catalyst used is one or more sodium compounds which are
selected from the group consisting of: sodium alkoxides, sodium
carboxylates, sodium salts of C--H acid compounds, sodium oxide,
sodium hydroxide, sodium nitrite, sodium acetylide, sodium carbide,
sodium hydride and sodium carbonyl.
Description
[0001] A number of processes for preparing formaldehyde from
methanol are known (see, for example, Ullmann's Encyclopedia of
Industrial Chemistry). The processes carried out industrially are
predominantly the oxidation
CH.sub.3OH+1/2O.sub.2.fwdarw.CH.sub.2O+H.sub.2O
[0002] over catalysts comprising iron oxide and molybdenum oxide at
from 300.degree. C. to 450.degree. C. (Formox process) and the
oxidative dehydrogenation (silver catalyst process) according
to:
CH.sub.3OH.fwdarw.CH.sub.2O+H.sub.2
H.sub.2+1/2O.sub.2.fwdarw.H.sub.2O
[0003] at from 600.degree. C. to 720.degree. C. In both processes,
the formaldehyde is first obtained as an aqueous solution.
Particularly when used for the preparation of formaldehyde polymers
and oligomers, the formaldehyde obtained is way has to be subjected
to costly dewatering. A further disadvantage is the formation of
corrosive formic ad, which has an adverse effect on the
polymerization, as by-product.
[0004] The dehydrogenation of methanol enables these disadvantages
to be avoided and enables, in contrast to the abovementioned
processes, virtually water-free formaldehyde to be obtained
directly: 1
[0005] In order to achieve an ecologically and economically
interesting industrial process for the dehydrogenation of methanol,
the following prerequisites have to be met: the strongly
endothermic reaction should be carried out at high temperatures so
that high conversions are achieved. Competing secondary reactions
have to be suppressed in order to achieve sufficient selectivity
for formaldehyde (without catalysis, the selectivity for the
formation of formaldehyde is less than 10% at conversions above
90%). The residence times have to be short or the cooling of the
reaction products has to be rapid in order to minimize the
decomposition of the formaldehyde which is not thermodynamically
stable under the reaction conditions
CH.sub.2O.fwdarw.CO+H.sub.2.
[0006] Various methods of carrying out this reaction have been
proposed; thus, for example, DE-A-37 19 055 describes a process for
preparing formaldehyde from methanol by dehydrogenation in the
presence of a catalyst at elevated temperature. The reaction is
carried out in the presence of a catalyst comprising at least one
sodium compound at a temperature of from 300.degree. C. to
800.degree. C.
[0007] J. Sauer and G. Emig (Chem. Eng. Technol. 1995, 18, 284-291)
were able to set free a catalytically active species, which they
presumed to be sodium, from a catalyst comprising NaAlO.sub.2 and
LLAlO.sub.2 by means of a reducing gas mixture (87% N.sub.2+13%
H.sub.2). This species was able to catalyze the dehydrogenation of
methanol introduced at a downstream point in the same reactor, i.e.
not coming into contact with the catalyst bed, to give
formaldehyde. When using non-reducing gases, only a low catalytic
activity was observed.
[0008] According to J. Sauer and G. Emig and also results from more
recent studies (see, for example, M. Bender et al., paper presented
to the 30th annual meeting of German catalyst technologists, Mar.
21-23, 1997), sodium atoms and NaO molecules were identified as
species emitted into the gas phase and their catalytic activity for
the dehydrogenation of methanol in the gas phase was described. In
the known processes, the starting material methanol is always
diluted with nitrogen and/or nitrogen/hydrogen mixtures for the
reaction.
[0009] Although good results are achieved with the known processes,
there is nevertheless considerable room for improvement from a
technical and economic point of view, particularly because the
catalysts employed become exhausted or inactivated over time and
the formaldehyde yields are still capable of improvement.
[0010] It has surprisingly been found that the yield in the
dehydrogenation can be increased if a carrier gas stream which has
been brought to a temperature above the actual reaction temperature
by heating is introduced into the reactor. By means of such a
superheated carrier gas stream, at least part of the heat required
for the endothermic dehydrogenation reactor can be introduced.
[0011] An advantage here is that the heat of reaction does not have
to be transferred to the gas stream via a hot wall, i.e. one having
a temperature above the reaction temperature, in the reaction zone,
but can be introduced directly and more gently for the reaction
gases by means of the separate heating and intensive mixing of the
various substreams. Decomposition of the unstable formaldehyde and
secondary reactions at the high temperatures in the reactor, in
particular in the zones close to the wall, can thus be reduced.
[0012] The invention accordingly provides a process for preparing
formaldehyde from methanol by dehydrogenation in a reactor in the
presence of a catalyst at a temperature in the range from 300 to
1000.degree. C., wherein a carrier gas stream which has a
temperature above the dehydrogenation temperature is fed to the
reactor.
[0013] The temperature difference between carrier gas stream and
dehydrogenation temperature is preferably at least 20.degree. C.,
particularly preferably from 40 to 250.degree. C.
[0014] The superheated gas stream can be fed directly into the
reaction zone or all or part of it can be brought into contact with
a primary catalyst (see below) beforehand.
[0015] The preferred temperatures for the superheated gas stream
are from 600 to 1000.degree. C. particularly preferably from 700 to
900.degree. C. Preferred temperatures for the dehydrogenation of
the methanol are from 500 to 900.degree. C.; particular preference
is given to temperatures of from 600 to 800.degree. C.
[0016] The carrier gas stream or streams can consist of a reducing
or non-reducing gas, for example H.sub.2/CO mixtures or nitrogen,
preferably the by-products of the dehydrogenation.
[0017] FIG. 1 shows a schematic overview of a preferred variant of
the process of the invention.
[0018] The carrier gas stream 1 is heated in the heat exchanger 2.
Together with the catalyst 4 coming from a reservoir 3, the total
stream is introduced into the reactor 5. Methanol 7 is conveyed
from a reservoir 6, vaporized in a heat exchanger 8 and likewise
fed to the reactor 5. The product gases from the reactor 5 are
cooled in the heat exchanger 9 and fed to a unit 10 for separating
off the formaldehyde.
[0019] The invention further provides an apparatus for carrying out
the abovementioned process comprising one or more heat exchangers
for preheating the starting materials, a vessel for superheating a
carrier gas stream, a heated reactor for carrying out the
dehydrogenation, one or more heat exchangers for cooling the
product mixture, a unit for separating off the formaldehyde and an
apparatus for introduction of the methanol and for further
introduction of a catalyst
[0020] For the purposes of the invention, dehydrogenation is a
non-oxidative process according to the equation: 2
[0021] Suitable catalysts are known, for example, from the
literature, see, for example, Chem. Eng. Technol. 1994, 17, 34.
[0022] Suitable metals are, for example, Li, Na, K, Cs, Mg, Al, In,
Ga, Ag, Cu. Zn, Fe, Ni, Co, Mo, Ti, Pt or their compounds. Also
suitable are, for example, S, Se, phosphates of transition metals
such as V and Fe, and heteropoly-acids such as molybdophosphorc
acid. Examples of specific catalysts are:
[0023] sodium or sodium compounds (DE-A-37 19 055 and DE-A-38 11
509)
[0024] aluminum oxide, alkali metal aluminate and/or alkaline earth
metal aluminate (EP-A-04 05 348)
[0025] silver oxide (JP-A 60/089 441, Derwent Report 85-15 68
91/26)
[0026] a catalyst comprising copper, zinc and sulfur (DE-A 25 25
174)
[0027] a catalyst comprising copper, zinc and selenium (U.S. Pat.
No. 4,054,609)
[0028] a catalyst comprising zinc and/or Indlum (EP-A 0 130
068)
[0029] silver (U.S. Pat. No. 2,953,602)
[0030] silver, copper and silicon (U.S. Pat. No. 2,939,883)
[0031] compounds containing zinc, cadmium, selenium, tellurium or
indium.
[0032] Preference is given to using sodium or sodium compounds.
[0033] The form in which such a catalyst, for example a
sodium-containing catalyst, is used can vary widely: metallic, e.g.
also as an alloy with at least one other alloy constituent, as
compound or salt, where at least one nonmetallic element is
chemically combined with Na (binary compounds and salts). If more
than one element is present in chemically combined form in the
compound, a binary, ternary or quaternary compound or salt is
present. Use of the catalyst in supported form, for example on an
inorganic support, is likewise preferred.
[0034] If sodium is used in metallic form, it can be used as solid,
liquid or preferably as vapor. Preferred alloys are those with
other alkali metals and/or alkaline earth metals, e.g. Ba, Sr, Ca,
Cs, Rb, K or particularly preferably Li and/or magnesium.
[0035] Furthermore, alloys with B, Al, Si and Sn can also be used.
This also applies, in particular, to alloys which can comprise
compounds such as sodium boride NaB.sub.2, sodium silicide NaSi or
NaSn.
[0036] Examples of suitable binary sodium compounds and salts are
sodium carbides such as Na.sub.2C.sub.2, NaC.sub.8, sodium halides
such as NaF, sodium oxides such as Na.sub.2O, sodium azide, sodium
phosphide, sodium sulfide, sodium polysuffides, preferably also
sodium hydrides such as NaH.
[0037] Examples of suitable ternary sodium compounds and salts are
sodium borates such as borax, sodium phosphates or
hydrogenphosphates, sodium phosphates, sodium (meta)silicates and
aluminosilicates, e.g. water glass, Na.sub.3AlF.sub.6 (cryolite),
sodium (hydrogen)sulfate, sodium sulfite, sodium nitrite, sodium
nitrate, sodium amide, sodium acetylide NaCCH, sodium cyanide,
sodium thiocyanate, the sodium salt of methyl thiol, sodium
thiosulfate, but preferably NaOR where R.dbd.H or an organic
radical (=salts of organic acids, alkoxides, phenoxides,
acetylacetonate, acetoacetic ester salt, salts of salicylic acid or
of salicylaldehyde), sodium carbonate and sodium hydrogencarbonate
and mixtures thereof, for example soda, thermonatrite, trona,
pirssonite, natrocalcite. The use of anhydrous, i.e. dried, salts
is generally preferred. Particular preference is given to NaOH,
NaOOC--R.sup.- (preferably formate, acetate, lactate, oxalate),
NaOR' (R' is an organic radical having from 1 fo 4 carbon atoms)
and sodium carbide. Very particular preference is given to NaOH,
sodium formate, sodium methoxide, sodium acetate and sodium
carbides such as Na.sub.2C.sub.2.
[0038] Examples of suitable quaternary compounds are
sodium-containing aluminosilicates which can be prepared
synthetically or can also occur in a wide variety as natural
minerals and rocks (e.g. sodium feldspar or albite and
calcium-sodium feldspar or oligoclase). They can additionally be
laden with Na by ion exchange.
[0039] Use can also advantageously be made of double salts of the
alum type or thenardite, glauberite, astrakanite, glaserite,
vanthoffite.
[0040] The sodium compounds and salts mentioned here can
advantageously also be in the form of mixtures. In particular, it
is quite possible to use contents of <50%. preferably <30%,
of cations of other alkali metals and/or alkaline earth metals,
e.g. Ba, Sr, Ca, Cs, Rb, K or preferably Ll and/or magnesium.
Industrially available, complex mixtures such as soda lime, ground
basic slag and cements, e.g. Portland cement, if desired after
enrichment with sodium by storage in sodium containing solutions
(NaCl, sea water) are particularly advantageous.
[0041] Particular preference is given to sodium compounds selected
from the group consisting of:
[0042] a) sodium alkoxides,
[0043] b) sodium carboxylates,
[0044] c) sodium salts of C--H acid compounds,
[0045] d) sodium oxide, sodium hydroxide, sodium nitrite, sodium
acetylide, sodium carbide, sodium hydride and sodium carbonyl.
[0046] The abovementioned catalysts will hereinafter be referred to
as primary catalyst.
[0047] In the process of the invention, the abovementioned
compounds give formaldehyde yields of over 60% and low water
concentrations of less than 5 mol % of H.sub.2O per mole of
formaldehyde even at reaction temperatures of from 600 to
1000.degree. C.
[0048] The liberation of the catalytically active species from the
primary catalyst is preferably carried out by thermal decomposition
of the latter.
[0049] The primary catalyst can, for example, be introduced
initially or afterwards, in each case continuously or
discontinuously, as solid, dissolved in a solvent, as a liquid or
as a melt.
[0050] The subsequent introduction of the primary catalyst as a
solid, e.g. In powder form, particulate or compacted, is generally
carried out by means of solids metering, e.g. using a reciprocating
or rotary piston, a cellular wheel feeder, a screw or a vibrating
chute.
[0051] If the primary catalyst is added in dissolved form,
particularly suitable solvents are those having a chemical
composition consisting of only the elements already present in the
process (C, H, O). Particular preference is given to MeOH as
solvent. The addition is carried out, for example, via a nozzle
which can be cooled in order to avoid evaporation of the solvent or
crystallization or deposition of the solid primary catalyst in the
nozzle.
[0052] The addition of the primary catalyst as a melt can be
carried out, for example, via a nozzle. The melt can then be
vaporized or decomposed directly in the gas stream.
[0053] For all possible ways of introducing further primary
catalyst, this is advantageously carried out in such a way that the
material is in intimate contact with flowing gas. This can be
achieved, for example, by applying the catalyst material by the
above-described methods onto a suitable surface through or over
which the gas flows. This can be the surface of a support material
which is present in a fixed bed. Suitable materials are, for
example, SiC, SiO.sub.2 and Al.sub.2O.sub.3 in a suitable geometric
form, e.g. as granules, pellets or spheres. The support material is
preferably arranged vertically in a fixed bed, preferably with
metering-in from above. The substance which is introduced deposits
on the support material and the catalytically active species goes
into the gas phase during the process.
[0054] Another possibility is placing the primary catalyst in a
fluidized bed through which the carrier gas stream is passed. Here,
the fluidized material comprises at least some of the supported or
unsupported primary catalyst The loss of active substance can be
made up by introducing further fresh primary catalyst; exhausted
material can, if desired, be taken off. This can be realized in the
continuous case, for example, by means of a circulating fluidized
bed.
[0055] Further introduction of the primary catalyst can also be
carried out by alternating secondary catalyst generation in
different vessels in which the primary catalyst can be located, for
example as a fixed bed or a fluidized bed, in each case supported
or unsupported. The advantage of using a plurality of units for the
discontinuous introduction of further catalyst is that it is also
possible to use primary catalysts for which, e.g. owing to material
properties such as melting point, viscosity or decomposition
temperature, continuous feeding would be impossible or possible
only with great difficulty.
[0056] In a preferred variant of the process of the invention, the
secondary catalyst is generated physically separately from the
reaction zone in which the actual dehydrogenation takes place and
at a temperature above the dehydrogenation temperature. The
temperature difference between the site of catalyst generation and
the reaction zone is preferably at least 20.degree. C.,
particularly preferably from 40 to 250.degree. C.
[0057] On thermal treatment of the primary catalysts according to
the invention in the primary catalyst decomposition zone and on
passing a reducing or non-reducing gas such as molecular nitrogen
over them at temperatures which may be different from the reaction
temperature for the dehydrogenation and may be higher or lower, one
or more catalytically active species which are able to catalyze the
dehydrogenation of methanol are released or generated and/or
generated on them (secondary catalyst). Such a fluid catalyst can
be transported over considerable distances without suffering an
appreciable loss of effectiveness in the dehydrogenation. This
separate setting of temperatures makes it possible, in particular,
to lower the reaction temperature by matching to the respective
conditions for catalyst liberation/vaporization or generation of a
catalytically active species (secondary catalyst) on the one hand
and to the reaction on the other hand. This reduces the
decomposition of the formaldehyde, which is unstable under the
reaction conditions, as a result of secondary reactions and
increases the yield.
[0058] Preferred temperatures for generating the secondary catalyst
from the primary catalyst are from 300 to 1100.degree. C.;
particular preference is given to temperatures of from 400 to
1000.degree. C.
[0059] In addition, the residence times in the dehydrogenation
reactor and vessels for primary catalyst addition or for generating
the secondary catalyst can be set separately by dividing the
carrier gas stream. This achieves a targeted loading of the gas
stream passed through the catalyst addition unit with the active
species.
[0060] Preferred residence times for generating the secondary
catalyst are from 0.01 to 60 sec, particularly preferably from 0.05
to 3 sec.
[0061] Commercial methanol can be used for the reaction; it should
preferably be low in water and contain no substances which poison
the catalyst
[0062] To carry out the dehydrogenation, the fluid, preferably
gaseous, methanol is preferably diluted with carrier gas.
[0063] The molar proportion of methanol is generally from 5 to 90%,
preferably from 10 to 50%, particularly preferably from 10 to
40%.
[0064] The pressure is not critical in the process of the
invention. The dehydrogenation of the methanol can be carried out
at subatmospheric pressure, atmospheric pressure or subatmospheric
pressure. A range from about 0.1 to 10 bar, preferably from 0.5 to
2 bar, is particularly suitable. Preference is given to atmospheric
pressure. The process of the invention can be carried out
discontinuously or continuously, with the latter being preferred.
The temperature is generally from 300.degree. C. to 950.degree. C.,
preferably from 500 to 900.degree. C., particularly preferably from
600 to 850.degree. C.
[0065] If the secondary catalyst is generated physically separately
from the reaction zone, the temperatures in the reaction zone are
generally from 200 to 1000.degree. C., preferably from 300.degree.
C. to 980.degree. C. Preference is given to reacting from 0.01 to 1
kg of methanol per hour and per gram of catalyst used. In the case
of a continuous process, further catalyst has to be introduced
continuously or discontinuously. The amounts here are generally
from 10 milligrams to 5 grams, preferably from 10 mg to 1 g,
particularly preferably from 50 to 1000 mg, very particularly from
50 mg to 500 mg, per kg of methanol reacted.
[0066] For the dehydrogenation of the methanol, residence times in
the reaction zone are preferably from 0.005 to 30 sec, particularly
preferably from 0.01 to 15 sec, very particularly preferably from
0.05 to 3 sec.
[0067] Suitable reactors are well known to those skilled in the
art. Essentially, it is possible to use reactor types and
assemblies as are known from the literature for dehydrogenation
reactions. Such apparatuses are described, for example, in
Winnacker/Kuchler. Chemische Technologie, 4th edition, chapter
"Technik der Pyrolyse" Hanser Verlag, Munich 1981-86. Suitable
reactors are, for example, tube reactors; suitable reactor
materials are, for example, ceramic materials such as
.alpha.-alumina but also iron- and nickel-based alloys which are
resistant to carbonization, heat and scale, e.g. Inconel 600.RTM.
or Hasteloy.RTM..
[0068] If the reactor 5 or the vessel 2 is heated by means of a
combustion reaction, externally fired tubes, for example, are
suitable.
[0069] Preference is likewise given to heating the reactor by means
of microwaves.
[0070] In a further preferred variant of the process of the
invention, a circulating gas stream consisting essentially of
by-products of the dehydrogenation is passed through the
reactor,
[0071] Preference is also given to bleeding off part of the
by-products from the circulating gas process and using this for
firing the reactor.
[0072] The formaldehyde can be separated from the reaction mixture
by methods known per se with which those skilled in the art are
familiar, for example by polymerization, condensation or physical
or chemical absorption or adsorption.
[0073] An industrially proven method is the formation of
hemiacetals from formaldehyde and an alcohol. The hemiacetals are
subsequently dissociated thermally, giving very pure formaldehyde
vapor. The alcohol used is usually cyclohexanol since its boiling
point is sufficiently far above the decomposition temperature of
the hemiacetal. The hemiacetals are usually dissociated in failing
film or thin film evaporators at temperatures of from 100 to
160.degree. C. (see, for example, U.S. Pat. No. 2,848,500 of Aug.
19, 1958 "Preparation of Purified Formaldehyde" and U.S. Pat. No.
2,943,701 of Jul. 6, 1960 "Process for purification of gaseous
formaldehyde", or JP-A 62/289 540). The formaldehyde vapors which
are liberated in such a process still contain small amounts of
impurities which are usually removed by means of a countercurrent
scrub using alcohol such as cyclohexanol hemiformal, by
condensation or also by targeted prepolymerization.
[0074] Particularly preferred methods of purifying the formaldehyde
prepared according to the invention are described in the German
Patent Applications 19 747 647.3 and 19 748 380.1.
[0075] A further method of separating formaldehyde from the
reaction mixture is the formation of trioxane in a catalytic
gas-phase process (see, for example, Appl. Catalysis A 1997, 150,
143-151 and EP-A 0 691 338). Trioxane can then, for example, be
condensed out.
[0076] Possible uses of the by-products of the reaction, in
particular hydrogen, are, for example, the synthesis of methanol or
the isolation of pure hydrogen which can be separated off, for
example, by means of membranes.
[0077] Hydrogen obtained in this way is suitable, for example, for
me synthesis of ammonia, in refinery processes for producing
gasoline and petrochemical cracking products, for the synthesis of
methanol, for hardening fats and for other hydrogenations, as
reducing agent for producing W, Mo, Co and other metals, as
reducing protective gas in metallurgical processes, for autogenous
welding and cutting, as fuel gas in admixture with other gases
(town gas, water gas) or in liquefied form as fuel in aerospace
applications.
[0078] The formaldehyde prepared by the process of the invention is
suitable for all known fields of application, for example corrosion
protection, production of mirrors, electrochemical coatings, for
disinfection and as a preservative, likewise as an intermediate for
producing polymers, for example polyoxymethylenes, polyacetals,
phenolic resins, melamines, aminoplastics, polyurethanes and casein
plastics, and also 1,4-butanol, trimethylolpropane, neopentyl
glycol, pentaerythritol and trioxane, for methanolic formaldehyde
solutions and methylal, for producing dyes such as fuchsin,
acrydine, for producing fertilizers and for treating seed.
[0079] Since the process of the invention usually produces
formaldehyde having a low water content, formaldehyde prepared in
this way is particularly suitable for polymerization to give
polyoxymethylene and trioxane, since water-free formaldehyde has to
be used for this purpose.
[0080] The invention also relates to plastics such as
polyoxymethylene and polyacetals, trioxane, dyes, fertilizers and
seed produced in such a way.
[0081] The invention further provides a process for preparing
trioxane, which comprises
[0082] 1. converting methanol into formaldehyde by dehydrogenation
in a reactor at a temperature in the range from 300 to 1000.degree.
C. in the presence of a catalyst, where a carrier gas stream having
a temperature above the dehydrogenation temperature is fed to the
reactor, and
[0083] 2. the formaldehyde prepared in this way is trimerized to
give trioxane
[0084] Details of the preparation of trioxane are well known to
those skilled in the art. They are described, for example, in
Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd edition,
volume 10, pp. 83, 89, New York Interscience 1963-1972.
[0085] The invention likewise provides a process for preparing
polyoxymethylene, which comprises
[0086] 1. converting methanol into formaldehyde by dehydrogenation
in a reactor at a temperature in the range from 300 to 1000.degree.
C. in the presence of a catalyst, where a carrier gas stream having
a temperature above the dehydrogenation temperature is fed to the
reactor, and
[0087] 2. if desired, purifying the formaldehyde obtained in this
way,
[0088] 3. polymerizing the formaldehyde.
[0089] 4. capping the end groups of the polymer prepared in this
way and
[0090] 5. if desired, homogenizing the polymer in the melt and/or
providing it with suitable additives.
[0091] The preparation of polyoxymethylene from formaldehyde is
well known to those skilled in the art Details may be found, for
example, in Ullmann's Encyclopedia of Industrial Chemistry, volume
21, 5th edition, Weinheim 1992, and the literature cited
therein.
[0092] The invention further provides a process for preparing
polyoxymethylene copolymers, which comprises
[0093] 1. converting methanol into formaldehyde by dehydrogenation
in a reactor at a temperature in the range from 300 to 100.degree.
C. in the presence of a catalyst, where a carrier gas stream having
a temperature above the dehydrogenation temperature is fed to the
reactor, and
[0094] 2. trimerizing the formaldehyde obtained in this way to give
trioxane,
[0095] 3. if desired, purifying the trioxane,
[0096] 4. copolymerizing the trioxane with cyclic ethers or cyclic
acetals,
[0097] 5. if desired, removing unstable end groups and
[0098] 6. if desired, homogenizing the polymer prepared in this way
in the melt and/or admixing it with suitable additives.
[0099] The invention further providese a process for preparing
polyoxymethyene copolymers, which comprises
[0100] 1. converting methanol into formaldehyde by dehydrogenation
in a reactor in the presence of a catalyst at a temperature in the
range from 300 to 1000.degree. C., where a circulating gas stream
comprising by-products of the dehydrogenation is passed through the
reactor, and
[0101] 2. if desired, purifying the formaldehyde obtained in this
way,
[0102] 3. copolymerizing the formaldehyde with cyclic ethers or
cyclic acetals,
[0103] 4. if desired, removing unstable end groups and
[0104] 5. if desired, homogenizing the polymer prepared in this way
in the melt and/or admixing it with suitable additives.
[0105] The preparation of polyoxymethylene copolymers is well known
to those skilled in the art Details may be found, for example, in
Ullmann's Encyclopedia of Industrial Chemistry, volume 21, 5th
edition, Weinheim 1992 and the literature cited therein, and also
in the Russian documents SU 436067, 740715 and SU 72-1755156,
720303.
[0106] The contents of the priority-establishing German Patent
Applications 197 22 774.0, 197 27 519.2 and 19743145.3 and also the
Abstract of the present application are expressly incorporated by
reference into the present description.
[0107] The invention is illustrated by the examples without being
restricted thereby.
EXAMPLES
[0108] FIG. 2 schematically shows the configuration of the
experimental apparatus by means of a flow diagram.
[0109] The dehydrogenation of the methanol is carried out in a tube
reactor 26 which is indirectly heated by means of an electric tube
furnace 12. A catalyst addition unit is formed by a metal tube 11
which is indirectly heated by the electric tube furnace 12. In the
tube 11, there is a bed 13 of support material on which the primary
catalyst (0.1-5.0 g) is located. A part 14 of a superheated carrier
gas stream 15 which has been preheated beforehand by means of
heated feed lines is introduced into this tube 11. In addition,
further primary catalyst is fed as a solution via a nozzle 16 into
this tube 11. The primary catalyst deposits on the bed 13. The
carrier gas substream 14 is passed through the bed in order to load
the carrier gas substream with an active catalyst species which
forms. The total stream is subsequently introduced into the
reaction space 19.
[0110] Methanol 17 is preheated, conveyed in a further part 18 of
the carrier gas stream 15 and likewise introduced into the reaction
space 19.
[0111] A third gas stream 20 consisting of pure carrier gas 15 is
superheated 21, i.e. brought to a temperature which is above the
dehydrogenation temperature, and likewise introduced into the
reaction space 19.
[0112] The reaction space 19 is formed by a tube having a length of
200-450 mm, internal diameter 4.21 mm. In a cooler 22, the product
gases leaving the reaction space 19 are quickly cooled to a
temperature below 200.degree. C. and are analyzed by means of a gas
chromatograph. In a column 23, the reaction products are scrubbed
with alcohol 24 (e.g. cycohexanol at 20-80.degree. C.) in order to
remove the formaldehyde 25. The primary catalyst used is sodium
methoxide, the carrier gas used is H.sub.2/CO or nitrogen. The
total flow is 20-500 l/h, at least 50% of the carrier gas stream is
fed directly to the reactor after superheating. The methanol feed
rate is such that a methanol concentration of about 5-20 mol % is
established.
[0113] The formaldehyde yield is calculated as follows: 1 Yield (
in % ) = formaldehyde formed ( mol ) methanol fed in ( mol )
100
1 Furnace tem- perature for Furnace Example/ catalyst Temperature
temperature for Comparative decom- of carrier gas reactor, Yieid of
Example position stream dehydrogenation formaldehyde Example 1
900.degree. C. 870.degree. C. 750.degree. C. 76% Example 2
880.degree. C. 870.degree. C. 750.degree. C. 74% CE 1 900.degree.
C. 820.degree. C. 750.degree. C. 72%
* * * * *