U.S. patent application number 11/484253 was filed with the patent office on 2007-01-18 for process for producing isocyanates.
This patent application is currently assigned to H. C. Starck GmbH. Invention is credited to Andreas Bulan, Friedhelm Kamper, Berthold Keggenhoff, Wolfgang Lorenz, Gerhard Moormann, Rainer Weber.
Application Number | 20070012577 11/484253 |
Document ID | / |
Family ID | 37563461 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070012577 |
Kind Code |
A1 |
Bulan; Andreas ; et
al. |
January 18, 2007 |
Process for producing isocyanates
Abstract
An isocyanate is produced by: (a) reacting chlorine with carbon
monoxide to form phosgene, (b) reacting the phosgene with an
organic amine to form an isocyanate and hydrogen chloride, (c)
separating the isocyanate and hydrogen chloride, (d) optionally,
purifying the hydrogen chloride, (e) preparing an aqueous solution
of the hydrogen chloride, (f) optionally, purifying the aqueous
solution of hydrogen chloride, (g) subjecting the aqueous hydrogen
chloride solution to electrochemical oxidation to form chlorine,
and (h) returning at least a portion of the chlorine produced in
(g) to (a).
Inventors: |
Bulan; Andreas; (Langenfeld,
DE) ; Weber; Rainer; (Odenthal, DE) ; Lorenz;
Wolfgang; (Dormagen, DE) ; Moormann; Gerhard;
(Brunsbuttel, DE) ; Kamper; Friedhelm; (Krefeld,
DE) ; Keggenhoff; Berthold; (Krefeld, DE) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ, LLP
P O BOX 2207
WILMINGTON
DE
19899
US
|
Assignee: |
H. C. Starck GmbH
Goslar
DE
|
Family ID: |
37563461 |
Appl. No.: |
11/484253 |
Filed: |
July 11, 2006 |
Current U.S.
Class: |
205/431 |
Current CPC
Class: |
C07C 263/20 20130101;
C07C 263/10 20130101; C07C 265/14 20130101; C07C 265/14 20130101;
C25B 1/26 20130101; C07C 263/20 20130101; C07C 263/10 20130101 |
Class at
Publication: |
205/431 |
International
Class: |
C25B 3/06 20070101
C25B003/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 13, 2005 |
DE |
10 2005 032 663.3 |
Claims
1. A process for the production of an isocyanate comprising: (a)
reacting chlorine with carbon monoxide to produce phosgene, (b)
Reacting the phosgene formed in (a) with at least one organic amine
to form an isocyanate and hydrogen chloride, (c) separating the
isocyanate from the hydrogen chloride, (d) optionally, purifying
the hydrogen chloride, (e) preparing an aqueous solution of the
hydrogen chloride, (f) optionally, purifying the aqueous solution
of hydrogen chloride, (g) subjecting at least a portion of the
aqueous hydrogen chloride solution to electrochemical oxidation to
form chlorine, and (h) returning at least a portion of the chlorine
produced in (g) to (a).
2. The process of claim 1 in which phosgene is separated from the
hydrogen chloride in (d) by liquefaction.
3. The process of claim 1 in which the hydrogen chloride is
purified in (d) by freezing.
4. The process of claim 1 in which the aqueous hydrogen chloride
solution formed in (e) is formed by absorption in an aqueous
solution of hydrogen chloride having a concentration of 15 to 20
wt. %.
5. The process of claim 1 in which purification in (f) is carried
out by stripping the aqueous hydrogen chloride solution with
steam.
6. The process of claim 1 in which purification in (f) is carried
out with a chelating ion exchanger to remove iron, silicon and/or
aluminum compounds from the aqueous hydrogen chloride solution.
7. The process of claim 6 in which the aqueous hydrogen chloride
solution is an at least 8 wt. % solution.
8. The process of claim 1 in which (g) is carried out in an
electrolytic cell having an anode chamber and a cathode chamber
separated by an ion-exchange membrane.
9. The process of claim 1 in which (g) is carried out in an
electrolytic cell having an anode chamber and a cathode chamber
separated by a diaphragm.
10. The process of claim 8 in which the anode and/or the cathode
comprise graphite.
11. The process of claim 10 in which the cathode comprises graphite
and has a coating which contains iridium.
12. The process of claim 10 in which the anode and/or cathode has
vertically arranged grooves.
13. The process of claim 9 in which the anode and/or cathode
comprise graphite.
14. The process of claim 13 in which the cathode comprises graphite
and has a coating which comprises iridium.
15. The process of claim 9 in which the anode and/or cathode has
vertically arranged grooves.
16. The process of claim 1 in which platinum group metal ions are
added to the aqueous hydrogen chloride solution before (e).
17. The process of claim 1 in which platinum and/or palladium ions
are added to the aqueous hydrogen chloride solution before (e).
18. The process of claim 1 in which a gas diffusion electrode is
used as cathode in (e).
19. The process of claim 18 in which the gas diffusion electrode
comprises an electrically conductive woven fabric, an interwoven
fabric, a knitted fabric, a lattice or a non-woven fabric made from
carbon which is positioned between a carbon-containing catalyst
layer and a gas diffusion layer.
20. The process of claim 19 in which the catalyst layer comprises
rhodium, a rhodium sulfide or a mixture of rhodium and a rhodium
sulfide.
21. The process of claim 1 in which an anode comprising titanium
and having a coating of at least one noble metal oxide is used in
(e).
22. The process of claim 1 in which an anode comprising titanium
and having a ruthenium oxide coating is used in (e).
23. The process of claim 1 in which an electrolytic cell comprising
a titanium and/or a titanium alloy is used in (e).
24. The process of claim 1 in which (i) the carbon monoxide used in
(a) is produced by reacting methane with water in a steam reformer,
(ii) hydrogen produced during the reaction of water with methane is
reacted with an organic nitro compound to form an amine, and (iii)
the amine produced in (ii) is used in (b).
25. The process of claim 1 in which chlorine containing bromine
and/or iodine is used in (a) to form phosgene which is reacted with
TDA in (b).
Description
BACKGROUND OF THE INVENTION
[0001] The present invention is directed to an integrated process
for producing isocyanates from phosgene and at least one amine in
which chlorine generated by electrochemical oxidation of the
hydrogen chloride produced in the course of the phosgenation
process is recycled to produce phosgene.
[0002] Chlorine is very commonly used as an oxidizing agent in the
production chain in the preparation of many organic compounds and
in the preparation of raw materials for the production of polymers.
Hydrogen chloride is frequently produced as a by-product. For
example, chlorine is used in isocyanate production, hydrogen
chloride being formed as a by-product. Additional use can be made
of the hydrogen chloride, for example by marketing the aqueous
solution (hydrochloric acid) or by using it in syntheses of other
chemical products. The full amounts of hydrogen chloride that are
produced cannot always be marketed or used for other syntheses,
however. Furthermore, hydrogen chloride can only be used for
syntheses if it has first been purified by appropriate means. On
the other hand, its marketing is generally only cost-effective if
the hydrogen chloride or hydrochloric acid does not have to be
transported over long distances. One of the most common possible
uses for the hydrogen chloride that is formed is its use as a raw
material in PVC production, wherein ethylene is oxychlorinated with
hydrogen chloride to form ethylene dichloride. Disposal of the
hydrogen chloride, e.g. by neutralization with alkaline solution,
is unappealing from an economic and ecological perspective.
[0003] A recycling process for the hydrogen chloride and the return
of the chlorine and/or hydrogen to the production process in which
the hydrogen chloride is produced is therefore the desired mode of
operation. Recycling processes include the catalytic oxidation of
hydrogen chloride, by the Deacon process for example, the
electrolysis of gaseous hydrogen chloride and the electrolysis of
an aqueous solution of hydrogen chloride (hydrochloric acid). Thus
an integrated process for producing isocyanates and catalytic
oxidation of hydrogen chloride by the Deacon process is disclosed
in WO 04/14845, for example, and an integrated process for
producing isocyanates and gas phase electrolysis of hydrogen
chloride is disclosed in WO 97/24320.
[0004] A review of electrochemical recycling processes is given in
the article "Chlorine Regeneration from Anhydrous Hydrogen
Chloride" by Dennie Turin Mah, published in "12.sup.th
International Forum Electrolysis in Chemical Industry--Clean and
Efficient Processing Electrochemical Technology for Synthesis,
Separation, Recycle and Environmental Improvement, Oct. 11-15,
1998, Sheraton Sand Key, Clearwater Beach, Fla.".
[0005] Catalytic hydrogen chloride oxidation by the Deacon process
as a recycling method, as described in WO 04/014845 for example,
has a number of processing disadvantages. For instance, the
heterogeneously catalyzed hydrogen chloride oxidation can only be
adjusted to different load states within certain limits. The Deacon
process is markedly more sensitive to load changes than
electrolysis. Changing the capacity of an industrial plant for
catalytic hydrogen chloride oxidation is also complicated.
[0006] A further disadvantage of catalytic hydrogen chloride
oxidation is that the catalyst used for the reaction is
exceptionally sensitive to impurities in the hydrogen chloride. The
recycling capacity falls dramatically due to a loss of activity of
the catalyst. At the same time, the lower conversion of hydrogen
chloride oxidation in the reactor makes it more difficult to
recover the reaction gases emerging from the reactor (oxygen,
hydrogen chloride, chlorine, water). Taken as a whole, this reduces
the cost-effectiveness of the catalytic oxidation process
significantly.
[0007] A process is described in WO 97/24320 and EP 876 335 A in
which the hydrogen chloride formed during isocyanate production is
converted to chlorine by gas phase electrolysis and the chlorine is
returned to phosgene production for preparation of the isocyanate.
In the special case of the preparation of toluene diisocyanate
(TDI), hydrogen is also returned to the production of toluene
diamine (TDA). The conversion of hydrogen chloride into chlorine by
electrolysis in the gas phase has not yet been tried on an
industrial scale and has the disadvantage that industrial
performance places increased technical demands on the plant
components, in terms of their resistance to pressure for example,
and is also associated with increased safety costs. A further
disadvantage is that if the hydrogen chloride is not completely
converted, a further process step has to be performed in which the
chlorine that is formed is separated from excess hydrogen chloride.
According to EP 1 106 714 A, oxygen is added to the gaseous
hydrogen chloride to improve conversion in gas phase electrolysis.
The disadvantage here is that with incomplete oxygen conversion,
the chlorine that is formed must be freed from hydrogen chloride
and additionally from oxygen, by, e.g., total liquefaction.
[0008] Furthermore, according to WO 97/24320 and others, so-called
solid electrolyte systems, e.g. Nafion.RTM. membranes in which the
anode and cathode are positioned on either side of the ion-exchange
membrane can be used. The anode and cathode can be gas diffusion
electrodes, for example. Alternatively, the catalytically active
material acting as the anode or cathode can be incorporated into
the ion-exchange membrane or applied to the ion-exchange membrane.
The disadvantage here is that if the ion-exchange membrane or the
catalytically active material is contaminated or damaged, the
entire unit, comprising the ion-exchange membrane and the
catalytically active material of the electrodes, must be
replaced.
[0009] The electrochemical oxidation of an aqueous solution of
hydrogen chloride using a gas diffusion electrode as the cathode is
described for example in WO 00/73538 and WO 02/18675. In these
disclosed processes, rhodium sulfide is used as the catalyst for
oxygen reduction at the cathode. According to WO 02/18675, this
catalyst is largely resistant to organic constituents which can be
present in the hydrochloric acid as impurities and which derive
from upstream synthesis steps, for example. The organic
constituents travel from the anode chamber to the cathode chamber
via the ion-exchange membrane. Over an extended electrolysis
running time, organic compounds lead to a rise in voltage, which
has a negative impact on the cost-effectiveness of the process. In
order to remove organic constituents, purification of the
hydrochloric acid using activated carbon and optionally
additionally using an ion-exchange resin, e.g. a molecular sieve,
is proposed in WO 02/18675.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is therefore to provide a
process for producing isocyanates, with recycling of the hydrogen
chloride produced during isocyanate production, which is simple and
reliable to operate. In particular, a process which offers rapid
start up and shutdown and simple operation under varying load
states. Increased capacity should also be easy to achieve.
[0011] This and other objects which will be apparent to those
skilled in the art are accomplished by electrochemical oxidation of
hydrogen chloride generated during phosgenation of an amine to
produce chlorine which is then used to produce phosgene for use in
a subsequent phosgenation reaction.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0012] The present invention provides a process for producing
isocyanates which includes the following steps:
a) reacting chlorine with carbon monoxide to produce phosgene,
b) reacting phosgene, preferably, the phosgene formed in step (a),
with at least one organic amine to form at least one isocyanate and
hydrogen chloride,
c) separating and recovering the isocyanates formed in step
(b),
d) separating and, optionally, purifying the hydrogen chloride
formed in step (b),
e) preparing an aqueous solution of the hydrogen chloride,
f) optionally, purifying the aqueous solution of hydrogen
chloride,
g) converting at least a part of the aqueous hydrogen chloride
solution by electrochemical oxidation to chlorine, and
h) returning at least a part of the chlorine produced in step (g)
to the production of phosgene in step (a).
[0013] The process according to the invention is an integrated
process for the production of isocyanates and for the electrolysis
of an aqueous solution of hydrogen chloride to recover chlorine for
the synthesis of phosgene as a starting product for isocyanate
production.
[0014] In the first step (a) of the process according to the
invention, phosgene is produced by reacting chlorine with carbon
monoxide. The synthesis of phosgene is known and is described for
example in Ullmanns Enzyklopadie der industriellen Chemie, 3.sup.rd
Edition, Volume 13, page 494-500. Other processes for producing
isocyanates are described in U.S. Pat. No. 4,764,308 and WO
03/072237, for example. On a technical scale, phosgene is
predominantly produced by reacting carbon monoxide with chlorine,
preferably on activated carbon as the catalyst. The highly
exothermic gas phase reaction takes place at temperatures from at
least 250.degree. C. to a maximum of 600.degree. C., generally in
multitube fixed-bed reactors. The reaction heat can be dissipated
in various ways, for example using a liquid heat-exchanging medium,
as described for example in WO 03/072237, or by hot cooling via a
secondary cooling circuit with simultaneous use of the reaction
heat to generate steam, as disclosed in U.S. Pat. No. 4,764,308,
for example.
[0015] In step (b), at least one isocyanate is formed from the
phosgene produced according to step (a) by reaction with at least
one organic amine or a mixture of two or more amines. The process
step (b) is also referred to below as phosgenation. The reaction
takes place with formation of hydrogen chloride as a
by-product.
[0016] The synthesis of isocyanates is likewise well known from the
prior art, in which phosgene is generally used in a stoichiometric
excess, based on the amine. Phosgenation according to (b)
conventionally takes place in the liquid phase, wherein the
phosgene and the amine can be dissolved in a solvent. Preferred
solvents are chlorinated aromatic hydrocarbons, such as
chlorobenzene, o-dichlorobenzene, p-dichlorobenzene,
trichlorobenzenes, the corresponding chlorotoluenes or
chloroxylenes, chloroethylbenzene, monochlorodiphenyl, .alpha.- or
.beta.-naphthyl chloride, ethyl benzoate, dialkyl phthalate,
diisodiethyl phthalate, toluene and xylenes. Further examples of
suitable solvents are known to those skilled in the art. As is also
known from the prior art, e.g. WO 96/16028, the isocyanate which is
formed can itself also act as a solvent for phosgene. In another,
preferred embodiment, the phosgenation, in particular of suitable
aromatic and aliphatic diamines, takes place in the gas phase, i.e.
above the boiling point of the amine. Gas phase phosgenation is
described in EP 570 799 A, for example. Advantages of this process
in comparison with the otherwise conventional liquid phase
phosgenation lie in the energy saving due to the minimizing of a
complex solvent and phosgene circuit.
[0017] In principle, all primary amines having one or more primary
amino groups, which can react with phosgene to form one or more
isocyanates having one or more isocyanate groups, are suitable as
organic amines. The amines have at least one, preferably two, or
optionally three or more primary amino groups. Thus aliphatic,
cycloaliphatic, aliphatic-aromatic, aromatic amines, diamines
and/or polyamines are suitable as organic primary amines. Specific
examples of suitable organic primary amines include: aniline;
halogen-substituted phenylamines such as 4-chlorophenylamine;
1,6-diaminohexane; 1-amino-3,3,5-trimethyl-5-aminocyclohexane;
2,4-, 2,6-diaminotoluene and mixtures thereof; 4,4'-, 2,4'-,
2,2'-diphenylmethane diamine and mixtures thereof; and also
higher-molecular-weight isomeric, oligomeric or polymeric
derivatives of such amines and polyamines. Other possible amines
are known to those skilled in the art. Preferred amines for the
present invention are the amines of the diphenylmethane diamine
series (monomeric, oligomeric and polymeric amines); 2,4- and
2,6-diaminotoluene; isophorone diamine and hexamethylene diamine.
The corresponding isocyanates, i.e., diisocyanatodiphenyl methane
(MDI, monomeric, oligomeric and polymeric derivatives), toluene
diisocyanate (TDI), hexamethylene diisocyanate (HDI) and isophorone
diisocyanate (IPDI) are obtained during phosgenation.
[0018] The amines can be reacted with phosgene in a single-stage, a
two-stage or optionally, a multistage reaction. A continuous or
discontinuous mode of operation is possible.
[0019] If a single-stage phosgenation in the gas phase is chosen,
the reaction takes place above the boiling point of the amine,
preferably within an average contact time of 0.5 to 5 seconds and
at temperatures of from 200 to 600.degree. C.
[0020] For phosgenation in the liquid phase, temperatures of from
20 to 240.degree. C. and pressures of 1 to about 50 bar are
conventionally used. Phosgenation in the liquid phase can be
performed as a single-stage or multistage process in which phosgene
may be used in a stoichiometric excess. Here the amine solution and
the phosgene solution are combined using a static mixing element
and then passed from bottom to top through one or more reaction
towers, for example, where the mixture reacts to form the desired
isocyanate. In addition to reaction towers, which are equipped with
suitable mixing elements, reaction vessels with a stirrer can also
be used. As well as static mixing elements, special dynamic mixing
elements can also be used. Suitable static and dynamic mixing
elements are known to those skilled in the art.
[0021] Continuous liquid-phase isocyanate production is generally
performed in two stages on an industrial scale. In the first stage,
carbamoyl chloride is formed from amine and phosgene and amine
hydrochloride from amine and eliminated hydrogen chloride,
generally at temperatures of a maximum of 220.degree. C.,
preferably a maximum of 160.degree. C. This first stage is highly
exothermic. In the second stage, the carbamoyl chloride is cleaved
to form isocyanate and hydrogen chloride and the amine
hydrochloride is reacted to give carbamoyl chloride. The second
stage is generally performed at temperatures of at least 90.degree.
C., preferably from 100 to 240.degree. C.
[0022] After the phosgenation according to step (b), the
isocyanates formed during phosgenation are separated off according
to the invention in step (c). This is done by first separating the
reaction mixture from the phosgenation into a liquid and a gaseous
product stream in a manner known to the person skilled in the art.
The liquid product stream substantially contains the isocyanate or
isocyanate mixture, the solvent and a small amount of unreacted
phosgene. The gaseous product stream is substantially composed of
hydrogen chloride gas, excess phosgene, and small amounts of
solvent and inert gases (e.g., nitrogen and carbon monoxide). The
liquid stream from the separation of step (c) then also undergoes
processing, preferably distillation, to separate the phosgene and
the solvent in succession. A further processing of the isocyanates
that are formed optionally also takes place in accordance with step
(c). This is done, for example, by fractionating the isocyanate
product obtained in a manner known to the person skilled in the
art.
[0023] The hydrogen chloride obtained from the reaction of phosgene
with an organic amine generally contains organic constituents which
can disrupt the electrochemical oxidation of an aqueous hydrogen
chloride solution according to step (g). These organic constituents
include the solvents used in the isocyanate production, such as
chlorobenzene, o-dichlorobenzene or p-dichlorobenzene. If
electrolysis is carried out by the membrane process, the function
of the ion-exchange membrane could be damaged by these organic
constituents or by inorganic impurities, such as iron, silicon or
aluminum compounds. The impurities can be deposited on the
ion-exchange membrane, thereby increasing the voltage of the
electrolysis. If a gas diffusion electrode is used as the cathode
for the electrolysis, the catalyst of the gas diffusion electrode
can also be deactivated by the inorganic or organic impurities.
Moreover, these impurities can be deposited on the current
collector, thereby diminishing the contact between the gas
diffusion electrode and the current collector, leading to a voltage
rise. If the diaphragm cell electrolysis process is used for
electrolysis of the hydrochloric acid, the cited organic and
inorganic constituents can be deposited on the graphite electrodes
and/or the diaphragm, thereby increasing the electrolysis
voltage.
[0024] Accordingly, separation of the hydrogen chloride produced in
the phosgenation according to step (b) from the gaseous product
stream takes place in a further process step (d). The gaseous
product stream which is obtained during separation of the
isocyanate according to step (c) is treated in step (d) in such a
way that the phosgene can be sent back to the phosgenation reaction
and the hydrogen chloride is subjected to an electrochemical
oxidation.
[0025] The separation of the hydrogen chloride in step (d) is
achieved by first separating phosgene from the gaseous product
stream. The phosgene is separated by liquefying phosgene, for
example at one or more condensers connected in series. The
liquefaction preferably takes place at temperatures in the range
from -15 to -40.degree. C., depending on the solvent used. Parts of
the solvent residues can also be removed from the gaseous product
stream through this deep cooling.
[0026] Phosgene can additionally or alternatively be washed out of
the gas stream in one or more stages with a cold solvent or
solvent-phosgene blend. The solvents already used in the
phosgenation, chlorobenzene and o-dichlorobenzene, are suitable as
solvents for this purpose, for example. The temperature of the
solvent or solvent-phosgene blend is generally in the range from
-15 to -46.degree. C.
[0027] The phosgene separated out of the gaseous product stream can
be returned to the phosgenation in step (b). In addition to inert
gases such as nitrogen and carbon monoxide, the hydrogen chloride
obtained after separation of the phosgene and part of the solvent
residue can also contain from 0.1 to 1 wt. % of solvent and from
0.1 to 2 wt. % of phosgene.
[0028] A purification of the hydrogen chloride to reduce the
proportion of solvent then optionally takes place in accordance
with step (d). This can be done by freezing, for example, by
passing the hydrogen chloride through one or more cryogenic traps,
depending on the physical properties of the solvent.
[0029] In a preferred embodiment, the hydrogen chloride is purified
by passing it through two heat exchangers connected in series, in
which the solvent to be removed is frozen out at -40.degree. C.,
for example, depending on the fixed point. The heat exchangers are
run alternately, so that the heat exchanger through which the gas
stream first passes thaws out the previously frozen solvent. The
solvent can be reused to produce a phosgene solution. In the second
heat exchanger connected downstream, which contains a conventional
heat-exchanging medium for refrigerating machines, e.g. a compound
from the series of freons, the gas is cooled to below the fixed
point of the solvent so that the latter crystallizes out. At the
end of the thawing and crystallization process, the gas stream and
the refrigerant stream are switched so that the function of the
heat exchanger is reversed. In this way the gas stream containing
hydrogen chloride can be depleted to a solvent content of
preferably a maximum of 500 ppm, more preferably a maximum of 50
ppm, most preferably a maximum of 20 ppm.
[0030] Alternatively, purification of the hydrogen chloride can
take place in two heat exchangers connected in series, as described
in U.S. Pat. No. 6,719,957. Here the hydrogen chloride is
compressed to a pressure of 5 to 20 bar, preferably 10 to 15 bar,
and the compressed gaseous hydrogen chloride is passed to a first
heat exchanger at a temperature of 20 to 60.degree. C., preferably
30 to 50.degree. C. The hydrogen chloride is cooled with cold
hydrogen chloride at a temperature of -10 to -30.degree. C., which
comes from a second heat exchanger. Organic constituents condense
in this process and can be sent for disposal or recycling. The
hydrogen chloride supplied to the first heat exchanger leaves it at
a temperature of -20 to 0.degree. C. and is cooled in the second
heat exchanger to a temperature of -10 to -30.degree. C. The
condensate formed in the second heat exchanger is composed of
additional organic constituents and small amounts of hydrogen
chloride. To avoid a loss of hydrogen chloride, the condensate
discharged from the second heat exchanger is sent to a separation
and evaporator unit. This can be a distillation column, for
example, in which the hydrogen chloride is stripped from the
condensate and returned to the second heat exchanger. The stripped
hydrogen chloride can also be returned to the first heat exchanger.
The hydrogen chloride cooled in the second heat exchanger and freed
from organic constituents is passed to the first heat exchanger at
a temperature of from -10 to -30.degree. C. After being heated to
10 to 30.degree. C., the hydrogen chloride freed from organic
constituents leaves the first heat exchanger.
[0031] In an alternative process, the purification of the hydrogen
chloride optionally provided according to step (d) takes place by
adsorption of organic impurities, such as solvent residues, on
activated carbon. Here, after removal of excess phosgene at a
pressure of from 0 to 5 bar, preferably 0.2 to 2 bar, the hydrogen
chloride is passed over or through an activated carbon bed, for
example. The flow rates and residence times are adjusted to the
content of impurities in a manner known to the person skilled in
the art. The adsorption of organic impurities is just as possible
on other suitable adsorbents, such as zeolites.
[0032] In a further alternative process, the hydrogen chloride can
be purified by distillation. This takes place after condensation of
the gaseous hydrogen chloride. In the distillation of the condensed
hydrogen chloride, the purified hydrogen chloride is removed as the
overhead product of the distillation, the distillation taking place
under the conventional conditions of pressure, temperature, etc.,
for such a distillation known to the person skilled in the art.
[0033] In step (e), an aqueous hydrogen chloride solution is
prepared from the hydrogen chloride separated off and optionally
purified in step (d). To this end, the hydrogen chloride is
preferably sent for adiabatic hydrogen chloride absorption, which
takes place in an absorption column with addition of a suitable
absorbent. In a preferred embodiment, the absorbent is an aqueous
hydrogen chloride solution (hydrochloric acid) in the concentration
range up to 20 wt. %, preferably 16 to 18 wt. %. Alternatively, a
hydrochloric acid of a lower concentration or deionized water or a
steam condensate can also be used. The adiabatic absorption of
hydrogen chloride in aqueous hydrochloric acid to produce
concentrated hydrochloric acid is already known from the prior art.
The absorption takes place, for example, by introducing the stream
of hydrogen chloride into the lower section of an absorption
column, the absorption column being equipped with material exchange
elements, such as sieve plates or packing. The absorbent is
introduced into the upper section of the absorption column, above
the material exchange elements. The hydrogen chloride gas is
absorbed, i.e. dissolved, countercurrently at the material exchange
elements in the absorbent.
[0034] In the conventional process temperature range of from 90 to
120.degree. C., preferably 105 to 109.degree. C., the gas stream
(i.e., the vapors) emerging at the head of the absorption column is
substantially made up of water vapor. In addition, hydrogen
chloride, inert gases such as nitrogen and carbon monoxide,
phosgene which has not yet reacted with water and residual amounts
of solvent are still included. To separate off condensable
components, such as water, hydrochloric acid and solvent residues,
and to dissipate the heat of condensation, the gaseous overhead
stream is preferably passed to a condensation unit. This
condensation unit can be made up of one or more shell-and-tube heat
exchangers connected in series and run on cooling water, for
example. The liquid runoff from this condensation system is then
preferably sent to a separator to separate off the condensed-out
solvent components from the aqueous hydrochloric acid phase. This
separator is preferably a static phase separator. The separation of
the organic and aqueous phase can be supported by corresponding
separating elements in this separator. The separated organic phase
is sent for appropriate recovery. The solvent-depleted hydrochloric
acid phase can be returned to the upper section of the absorption
column.
[0035] The aqueous hydrogen chloride solution (hydrochloric acid)
leaving the lower section of the absorption column can, if
necessary, be cooled with a suitable cooler, optionally purified
according to step (f) and then sent for electrochemical oxidation
in accordance with step (g). This solution is generally about 24 to
30 wt. %, preferably 27 to 30 wt. % hydrochloric acid (also
referred to below as concentrated hydrochloric acid) and contains
solvent proportions of preferably a maximum of 0.05 wt. %, most
preferably a maximum of 0.005 wt. %. The phosgene content of the
hydrochloric acid is preferably from about 0.1 to 0.0001 wt. %, but
can also be less than 0.0001 wt. %.
[0036] The aqueous hydrogen chloride solution optionally undergoes
a purification in a step (f), in particular to further reduce the
solvent proportion and the phosgene content. This can take place by
stripping in a column in a manner known to the person skilled in
the art, for example, by introducing the concentrated hydrochloric
acid into a packed column which is fitted with either a circulation
evaporator or a steam inlet. While the vapors from the stripper
column can be returned to the absorption column, the liquid output
from the column in the form of purified concentrated hydrochloric
acid can be sent for hydrochloric acid electrolysis according to
step (g), optionally via a cooler. Instead of carrying out the
stripping in a separate stripper column, it can also take place in
the absorption column itself by direct injection of steam,
preferably in the stripping section located below the absorption
column. Instead of stripping in the absorption column, the solvent
content in the hydrogen chloride can also be reduced by partial
distillation with the aid of a heat exchanger connected downstream
from the absorption column.
[0037] In optional step (f), the aqueous hydrogen chloride solution
undergoes a purification to remove iron, aluminum and/or silicon
compounds. The removal of iron, aluminum and/or silicon compounds
preferably takes place using chelating ion exchangers. Such ion
exchangers are available commercially.
[0038] Thus the removal of iron compounds, for example, can be
accomplished by using ion exchangers such as those which are
commercially available under the name Amberjet 4400CI from Rohm
& Haas or Lewatit M500 from LANXESS. The concentration of
hydrochloric acid for removal of iron is preferably at least 8 wt.
%.
[0039] Precipitation in the form of poorly soluble compounds and
subsequent filtration can also be used to remove iron-containing
compounds.
[0040] After preparing an aqueous hydrogen chloride solution
according to step (e) and optionally after purification of the
aqueous hydrogen chloride solution according to step (f), the
hydrochloric acid is passed to an electrolytic cell. The
electrochemical oxidation of the hydrochloric acid according to
step (g) can be performed by the membrane process or by the
diaphragm cell electrolysis process in a two-chamber electrolytic
cell composed of an anode chamber and a cathode chamber or in a
three-chamber electrolytic cell composed of an anode chamber, a
cathode chamber and an electrolyte chamber between the anode and
cathode chamber. A two-chamber electrolytic cell is preferred. In
the membrane process, the anode chamber is separated from the
cathode chamber by an ion-exchange membrane (also simply referred
to below as a membrane), in particular a cation-exchange membrane
in the diaphragm cell electrolysis process, the anode chamber is
separated from the cathode chamber by a diaphragm. The distance of
the electrodes (anode and cathode) from the diaphragm or membrane
is preferably from 0 to 3 mm, more preferably from 0 to 2 mm.
Suitable ion-exchange membranes are available commercially. One
such suitable single-layer ion-exchange membrane with sulfonic acid
groups is a Nafion.RTM. 117 membrane which is commercially
available from DuPont.
[0041] As the diaphragm, a woven diaphragm according to DE 3 321
159 A can be used, for example. Plastic threads can be used for
this. Polyvinyl chloride (PVC) or polyvinylidene fluoride (PVDF)
fabrics, or mixed fabrics with PVC and PVDF threads are examples of
thread materials which can be used to make suitable woven
diaphragms. Warp or weft threads can be made up of multifilament
threads, as described in DE 3 321 159 A, as well as monofilament
threads. After the diaphragm has been woven, the fabric can be
compressed, e.g. by calendering, to optimize the gas
permeability.
[0042] Electrodes containing graphite, the anode and/or the cathode
preferably being substantially of graphite, can be used in the
electrolysis of hydrochloric acid by the diaphragm cell
electrolysis process or the membrane process. Bipolar graphite
electrodes are most preferably used. According to DE 4 417 744 A, a
particularly advantageous design of cathode and/or anode is a
graphite cathode and/or anode with a noble metal-containing
coating, for example, an iridium-containing coating.
[0043] The graphite anodes have in particular a geometrical shape,
as is known from DE 3 041 897 A. The cathodes preferably have a
similar structure to the anodes. The shape of the anode and/or
cathode preferably exhibits vertically arranged grooves, flutes,
notches, or indentations. These grooves substantially serve to
carry off the gas which is formed during electrolysis, i.e.
chlorine and hydrogen, upwards out of the narrow gap between the
electrode and the diaphragm or membrane. The grooves preferably
have a depth of 5 to 35 mm, most preferably 15 to 25 mm, and a
width of preferably 1 to 5 mm. The distance between two adjacent
grooves substantially positioned parallel to each other is
generally from 4 to 6 mm. In another embodiment, the depth and/or
width of the grooves varies along their length. Thus the depth of
the grooves can be from 12 to 15 mm at the lower end of the grooves
and from 20 to 30 mm at the upper end of the grooves.
[0044] Hydrochloric acid is used as the electrolyte in both the
anode chamber and the cathode chamber. During electrolysis,
chlorine is produced at the anode, hydrogen at the cathode.
[0045] A preferred mode of operation of the electrochemical
oxidation of hydrochloric acid involves adding metal ions from the
group of platinum metals, preferably platinum and/or palladium, to
the hydrochloric acid which serves as the electrolyte in the
cathode chamber. Solutions of hexachloroplatinate(IV) acid
(H.sub.2PtCi.sub.6) or solutions of disodium
tetrachloropalladate(II) (Na.sub.2PdCl.sub.4) or mixtures thereof
can thus be added, for example. The addition can take place
continuously or discontinuously. The addition of metal ions to the
hydrochloric acid in the cathode chamber serves to maintain a low
electrolysis voltage in the range from 1.6 to 2.1 V, compared with
2.2 to 2.3 V without addition of metal ions, at 5 kA/m.sup.2 and 70
to 80.degree. C. and with a preferably 15 to 25%, more preferably
approx. 20%, hydrochloric acid. A quantity of metal ions is which
is sufficient to maintain the electrolysis voltage in the range
from 1.8 to 2.1 is generally added. This means that the addition of
metal ions is increased as the electrolysis voltage rises during
operation.
[0046] The electrolysis of step (g) is preferably performed at a
temperature of from 50 to 90.degree. C. The concentration of the
aqueous solution of hydrogen chloride that is used is preferably 15
to 25 wt. %. The electrolysis can be performed at an absolute
pressure of 1 bar or at a higher pressure of up to 2 bar. Higher
pressures are generally possible but require a correspondingly
greater complexity in the design of the electrolytic cell. The
differential pressure between the anode chamber and the cathode
chamber is preferably 0 to 10 mbar, most preferably approx. 1 mbar,
so that, due to the higher pressure on the anode side, traces of
the chlorine gas that is formed pass through the diaphragm to the
cathode side and can therefore mix with the hydrogen formed at the
cathode.
[0047] In an alternative embodiment, the electrochemical oxidation
of the aqueous solution of hydrogen chloride in step (g) is
conducted by the membrane process with a gas diffusion electrode as
the cathode. In this case, the electrolytic cell can be composed
either of two chambers or of three chambers, but preferably two
chambers. An oxygen-containing gas, e.g. oxygen, air or oxygenated
air, is supplied to the cathode half cell. The oxygen is reduced at
the gas diffusion electrode, forming water. The aqueous hydrogen
chloride solution is supplied to the anode half cell, the hydrogen
chloride being oxidized to chlorine at the anode. The anode half
cell and the cathode half cell are separated from each other by a
cation-exchange membrane. The electrolysis of hydrochloric acid
using a gas diffusion electrode as the cathode is described in WO
00/73538, for example.
[0048] The electrolytic cell can be made up of either a
non-metallic material (disclosed, e.g., in DE 103 47 703 A) or a
metallic material. Titanium or a titanium alloy, such as a
titanium-palladium alloy, is a suitable metallic material for the
electrolytic cell. In this case, the shells for the anode and
cathode half cell, the current distributor and the supply leads are
made from titanium or a titanium alloy.
[0049] The anode can be designed in accordance with DE 102 34 806
A, for example. In this case, the anode is composed of a metal
(preferably titanium) with a coating of noble metal oxide (e.g.,
ruthenium oxide). Furthermore, in accordance with DE 102 00 072 A,
the titanium anode can have an interlayer of titanium carbide or
titanium boride, which is applied to the titanium anode by plasma
spraying or flame spraying before the noble metal oxide coating is
applied. According to DE 102 34 806 A, the metal anode has openings
for the passage of the gas formed during electrolysis, the openings
preferably having guide structures which lead the gas that is
formed to the side of the metal anode facing away from the
ion-exchange membrane. Here the total cross-sectional area of the
openings should be in the range from 20% to 70% of the area which
is formed by the height and width of the anode. The metal anode can
moreover have an undulated, zigzag or rectangular cross-section.
The depth of the anode should be at least 1 mm. The ratio of
electrochemically active area of the metal anode to the area formed
by the height and width of the metal electrode should be at least
1.2. In a special embodiment, the metal anode can be made up of two
adjacent expanded metal meshes, the expanded metal mesh facing the
ion-exchange membrane having a finer structure than the expanded
metal mesh facing away from the ion-exchange membrane. Furthermore,
the more finely structured expanded metal mesh is rolled flat and
the more coarsely structured expanded metal mesh is positioned so
that the mesh strands are inclined towards the cathode and serve as
guide structures. Alternatively, the anode can also be made up of
an expanded metal mesh. In principle, the anode should have a free
surface area of from 15 to 70%. The thickness of the expanded metal
meshes should be chosen so that no additional electrical resistance
occurs with a bipolar connection of the individual electrolytic
cells (cell elements) to an electrolyzer. The electrical resistance
substantially depends on the electrical contacting of the anode,
such as the number of current-supplying connecting elements between
the anode and the back wall of the anode half cell.
[0050] In the case of electrolysis using a gas diffusion electrode,
the anode chamber and cathode chamber can be separated by a
commercial ion-exchange membrane. Nafion.RTM. 324 or Nafion.RTM.
117 ion-exchange membranes from DuPont can be used, for example. A
membrane is preferably used which, as described in WO 05/12596, has
a smooth surface texture on the side facing the gas diffusion
electrode. The smooth surface texture of the membrane allows the
gas diffusion electrode and the membrane to lie against each other
in such a way that under a pressure of 250 g/cm.sup.2 and at a
temperature of 60.degree. C. the contact area is at least 50% of
the geometrical surface area of the membrane.
[0051] The cathodic current distributor to which the gas diffusion
electrode is applied is preferably designed in accordance with DE
102 03 689 A. This has a free surface area of less than 65% but
more than 5%. The thickness of the current distributor is at least
0.3 mm. It can be composed of an expanded metal mesh, lattice,
woven fabric, foam, nonwoven fabric, slotted plate or perforated
plate made from metal. The cathodic current distributor is
preferably an expanded metal mesh with a mesh length of 4 to 8 mm,
a mesh width of 3 to 5 mm, a strand width of 0.4 to 1.8 mm and a
thickness of 0.4 to 2 mm. The cathodic current distributor can
additionally have a second expanded metal mesh as a support for the
first expanded metal mesh. The second expanded metal mesh as the
support preferably has a mesh length of 10 to 40 mm, a mesh width
of 5 to 15 mm, a strand width of 2 to 5 mm and a thickness of 0.8
to 4 mm. A lattice which preferably has a wire thickness of 1 to 4
mm and a mesh size of 7 to 25 mm can also be used as a support.
Furthermore, a perforated plate or slotted plate which preferably
has an open area of less than 60% and a thickness of 1 to 4 mm can
be used as a support. Titanium or a noble metal-containing titanium
alloy, such as titanium-palladium, can be used as the material for
the cathodic current distributor. If the current distributor is an
expanded metal mesh, it is preferably rolled.
[0052] A commercial gas diffusion electrode equipped with a
suitable catalyst can be used as the gas diffusion electrode.
According to WO 00/73538, suitable catalysts contain rhodium and/or
at least one rhodium sulfide or a mixture of rhodium and at least
one rhodium sulfide. According to EP 931 857 A, rhodium and/or
rhodium oxide or mixtures thereof can also be used. The gas
diffusion electrode is preferably composed of an electrically
conductive woven fabric, paper or nonwoven fabric made from carbon
with the woven fabric, paper or nonwoven fabric having a
carbon-containing catalyst layer on one side and a gas diffusion
layer on the other side. The catalyst is preferably applied to a
support, preferably composed of carbon in which
polytetrafluoroethylene particles are integrated. The gas diffusion
layer is preferably composed of carbon and polytetrafluoroethylene
particles, the ratio of carbon to PTFE being 50:50, for example.
The gas diffusion electrode can be positioned so that it is not
permanently connected to the ion-exchange membrane. The contacting
of the gas diffusion electrode with the current distributor and the
ion-exchange membrane is preferably made by press contact, i.e. the
gas diffusion electrode, the current distributor and the membrane
are pressed against one another. The gas diffusion electrode can be
connected to the current collector as described in DE 101 48 600
A.
[0053] The electrolysis of hydrochloric acid by the membrane
process with a gas diffusion electrode is conventionally performed
at a temperature of from 40 to 70.degree. C. The concentration of
the aqueous solution of hydrogen chloride in the anode chamber is
from 10 to 20 wt. %, preferably 12 to 17 wt. %. The cell can be
operated, for example, in such a way that the pressure in the anode
chamber is higher than the pressure in the cathode chamber. In this
way, the cation-exchange membrane is pressed against the gas
diffusion electrode and this in turn is pressed against the current
distributor. Alternatively, an electrolytic cell design as
described in DE 101 38 214 A can be chosen. The anode and/or the
current distributor are elastically supported, for example by being
connected by springs to the back wall of the relevant half cell. A
so-called zero gap configuration occurs when the cell is assembled,
wherein the anode is in direct contact with the ion-exchange
membrane, which in turn is in direct contact with the gas diffusion
electrode and this in turn is in direct contact with the current
distributor. The elastic support causes the anode, membrane, gas
diffusion electrode and current distributor to be pressed
together.
[0054] In a preferred embodiment of the electrolysis process, when
the electrolytic cell according to DE 10 152 275 A is started, the
anode half element is filled with a 5 to 20 wt. % hydrochloric
acid, the hydrochloric acid containing at least 10 ppm of free
chlorine and the concentration of the hydrochloric acid during
startup being more than 5 wt. %. The volumetric flow rate of the
hydrochloric acid through the anode chamber is adjusted so that at
the start of electrolysis, the hydrochloric acid in the anode
chamber flows at a rate of 0.05 to 0.15 cm/s. The electrolysis is
started with a current density of 0.5 to 2 kA/m.sup.2 and increased
in time intervals of 5 to 25 minutes by 0.5 to 1.5 kA/m.sup.2 each
time. Once a predefined current density of preferably 4 to 7
kA/m.sup.2 is reached, the volumetric flow rate of the hydrochloric
acid is adjusted so that the hydrochloric acid in the anode half
element flows at a rate of 0.2 to 0.4 cm/s.
[0055] A particularly advantageous mode of operation of the
electrolytic cell can take place in accordance with DE 101 38 215 A
which teaches operation of the electrolytic cell with an elevated
pressure in the cathode chamber to lower the cell voltage. The
differential pressure between the anode chamber and cathode chamber
should be 0.01 to 1000 mbar and the oxygen pressure in the cathode
chamber at least 1.05 bar absolute.
[0056] In accordance with the present invention, in process step
(h), at least a part of the chlorine produced in step (g) is
returned to phosgene production in step (a). Before being returned,
the chlorine is preferably cooled in a single-stage or multistage
cooling process by means of a cooler, e.g. a tubular heat
exchanger, and dried. Drying can take place with the aid of a
suitable desiccant in an absorption column equipped with material
exchange elements, for example. In addition to molecular sieves or
hygroscopic adsorbents, a suitable desiccant can be sulfuric acid
for example, as described e.g. in DE 10 235 476. Drying can take
place in one or more stages. Drying preferably takes place in two
stages, by bringing the chlorine to be dried into contact in a
first stage with a sulfuric acid of reduced concentration,
preferably 70 to 80%, most preferably 75 to 80%. In a second stage,
the residual moisture is removed from the chlorine by means of a
more highly concentrated sulfuric acid of preferably 88 to 96%,
most preferably 92 to 96%. The chlorine dried in this way having a
residual moisture of preferably a maximum of 100 ppm, more
preferably a maximum of 20 ppm, can be passed through a droplet
separator to remove any sulfuric acid droplets still remaining
therein.
[0057] The circulatory mode of operation of the process of the
present invention requires addition of chlorine in addition to the
chlorine produced by electrolysis in step (g) to the phosgene
production in step (a), because losses of chlorine and hydrogen
chloride occur in the chlorine-hydrogen chloride circuit. A portion
of the added chlorine can be in the form of elemental chlorine from
an external source, for example the electrolysis of an aqueous
sodium chloride solution. The losses of chlorine and hydrogen
chloride that occur can, however, also be balanced out by providing
a portion of hydrogen chloride from an external source. A portion
of hydrogen chloride in the form of an aqueous hydrogen chloride
solution from an external source (e.g., from a production process
in which an aqueous hydrogen chloride solution is produced as a
by-product) is preferably supplied as an approx. 30 wt. %
hydrochloric acid in step (e) to produce the aqueous hydrogen
chloride solution for electrolysis in step (g). A hydrochloric acid
of lower concentration can alternatively be supplied to the
absorption of hydrogen chloride according to step (e).
[0058] If the missing amount is replaced by chlorine, this
chlorine, which is produced by rock salt electrolysis, for example,
may contain small amounts of bromine or iodine. If this chlorine is
used for the production of MDI, a discoloration of the polyurethane
products produced from MDI can occur with a certain concentration
of bromine and iodine compounds, as described for example in DE 10
235 476 A. By contrast, the chlorine returned to the process
according to the invention is largely free from bromine and iodine,
so that a certain proportion of bromine and iodine in the chlorine
supplied from outside to the recycled chlorine may be present. A
preferred embodiment of the process according to the invention thus
involves using some chlorine from a source other than the
electrochemical oxidation of the process of the present invention
in the production of phosgene to be used for TDA phosgenation,
while the low-bromine and low-iodine chlorine from the electrolysis
according to step (g) is used in the production of phosgene for the
phosgenation of MDA (diphenylmethane diamine). In the production of
TDI by phosgenation of TDA, bromine and iodine are bound in the TDI
and are thus removed from the hydrogen chloride circuit. During
recovery of TDI by distillation, bromine and iodine are separated
from the TDI, however, and remain in the residue.
[0059] In another preferred embodiment of the process of the
present invention, the carbon monoxide used in the production of
phosgene according to step (a) is produced by reacting methane with
water in a steam reformer and reacting the hydrogen produced in
that process with at least one organic nitro compound to form at
least one amine, which is used in the production of the isocyanate
according to step (b). The production of carbon monoxide by
reacting methane with water in a steam reformer has long been
known. The reaction of hydrogen with an organic dinitro compound to
produce an amine (hydrogenation) is likewise known. If a steam
reformer is used to produce carbon monoxide, the stoichiometrically
required amount of carbon monoxide for phosgene production and the
stoichiometric amount of hydrogen for hydrogenation of the dinitro
compounds are available. Nitrobenzene and dinitrotoluene (DNT) can
be used as the nitro compounds, for example. Nitrobenzene and
dinitrotoluene are hydrogenated to form aniline and toluene diamine
(TDA). Aniline is processed further to produce polyamines of the
diphenylmethane series. In addition to other amines, MDA and TDA
can be used for isocyanate production according to step (c). An
assessment of the cost-effectiveness of the overall process for
producing isocyanates also includes the production of carbon
monoxide, the carbon monoxide preferably being produced from
natural gas in a steam reformer. If other reformer processes are
used, e.g. coal gasification or cracking of petroleum fractions,
different ratios of carbon monoxide to hydrogen are obtained. The
higher the ratio of carbon monoxide to hydrogen, the less
cost-effective the overall process, since the missing hydrogen for
hydrogenation of the dinitro compound to form the homologous
diamines has to be supplied from another source. The missing
hydrogen can be provided by the electrolysis of hydrochloric acid
by the diaphragm cell electrolysis process, for example.
[0060] The advantages of the integrated process of the present
invention for producing isocyanates with electrochemical oxidation
of an aqueous solution of the hydrogen chloride produced during
isocyanate production to recover chlorine for the synthesis of
phosgene lie in the fact that the electrochemical oxidation can be
operated more simply than a catalytic oxidation by the Deacon
process. The simpler operation relates to the startup and shutdown
of the electrolytic cells and the adjustment to variable load
states and to a higher or lower capacity of the plant. Furthermore,
the electrochemical oxidation of an aqueous solution of hydrogen
chloride can be operated more simply than a gas phase electrolysis
of hydrogen chloride, since the process is performed not in the gas
phase but with a solution.
[0061] Through the production of a concentrated hydrochloric acid
of about 30% from a hydrochloric acid of about 17% in step (f), the
production of isocyanates in conjunction with the electrochemical
oxidation of hydrochloric acid also offers the possibility of
removing concentrated hydrochloric acid from the circuit for other
applications if required. One possible use of this concentrated
hydrochloric acid lies in the food sector. For this purpose, a
sufficiently high purity for the food industry can be achieved for
the concentrated hydrochloric acid produced by the process
according to the invention, e.g. by absorptive post-purification on
an activated carbon bed, as is known from the prior art.
Additionally, the concentrated hydrochloric acid may be used as a
catalyst in the production of MDA. The production of polyamines of
the diphenylmethane series conventionally takes place by reacting
aniline and formaldehyde in the presence of acid catalysts, as is
common knowledge from the prior art. Hydrochloric acid is
conventionally used as the acid catalyst.
[0062] Although the invention has been described in detail in the
foregoing for the purpose of illustration, it is to be understood
that such detail is solely for that purpose and that variations can
be made therein by those skilled in the art without departing from
the spirit and scope of the invention except as it may be limited
by the claims.
EXAMPLE
[0063] This example uses as a starting material hydrogen chloride
gas which still contains phosgene and has been obtained from an MDI
production process after the MI has been removed.
[0064] After removing phosgene from the HCl gas, 44,660 kg/h of HCl
are compressed to 13.4 bar, the temperature of the HCl is about
40.degree. C., the content of mono-chlorobenzene (MCB) is 200 ppm
and the content of orthodichlorobenzene (ODB) is 70 ppm. This HCl
stream is passed to a first heat exchanger and cooled to about
-9.2.degree. C. In this process a portion of the hydrogen chloride
is condensed out together with ODB and MDB. This portion of about
62 kg/h, of which the temperature is -9.2.degree. C., is passed to
a separator/evaporator unit. The purified residual stream of 44,598
kg/h is passed to a second heat exchanger, where it is cooled to
-23.4.degree. C. During this cooling a partial stream of hydrogen
chloride containing the impurities MCB, ODB and possible other
high-boiling components is again condensed out. This partial stream
is 998 kg/h and is passed to the separator/evaporator unit. The
remaining twice-purified residual HCl stream of 44,573 kg/h and
-23.4.degree. C. is recycled to the first heat exchanger for
cooling the abovementioned crude HCl gas stream and is heated
therein to about 21.degree. C. and then passed for HCl absorption
in water. The heated HCl stream issuing from the first heat
exchanger has an MCB and ODB content of less than 1 ppm.
[0065] The HCl partial streams (62 mg/h and 998 kg/h) from the
first and second heat exchangers are passed to a
separator/evaporator unit for freeing hydrogen chloride from
impurities (the high-boiling components). 87 kg/h of HCl of a
temperature of about -12.2.degree. C. are discharged from the
separator/evaporator unit and the remaining quantity of 973 kg/h is
passed to the second heat exchanger. The separator/evaporator unit
can for example be a distillation column with an evaporator at the
base of the column.
[0066] The twice purified hydrogen chloride is absorbed in water,
as described, whereupon 30% hydrochloric acid is produced.
[0067] The 30% hydrochloric acid is passed to hydrochloric acid
electrolysis in which an oxygen depletion cathode is used as the
cathode. The anode and cathode chambers of the electrolysis are
separated by an ion exchanger membrane from DUPONT Nafion 324. The
temperature of the anolyte is 50.degree. C. and the current density
is 5 kA/m.sup.2 at an electrolysis voltage of 1.39V. The anode
consists of titanium which is provided with a noble metal coating
from DENORA. The oxygen depletion cathode used is a rhodium
sulphide-containing gas diffusion electrode from ETEK, which rests
on a current distributor of titanium which is stabilized with
palladium. The differential pressure between the anode chamber and
the cathode chamber is adjusted so that the membrane is pressed on
the oxygen depletion cathode and the current distributor. The
differential pressure is 200 mbar. The electrolytic cell is
operated at an absolute pressure of 1.01 bar. The anode chambers of
the electrolysis are charged with 1466 t/h of hydrochloric aicd of
a concentration of 14% by weight and hydrochloric acid of a
concentration of 12.2% by weight is removed from the anode
chambers. A purging stream of 96 t/h of 12.2% by weight
hydrochloric acid is continuously removed and the remaining current
is strengthened with 148.5 t/h of the purified 30% hydrochloric cid
and recycled to the electrolysis.
[0068] 32.4 t/h of chlorine are able to be removed from the anode
chambers.
[0069] 1.2 mol of water are transported through the membrane per
mol of proton, so that 19.7 t/h of water enter the cathode chamber
through the membrane. In this chamber this water is removed
together with the reaction water from the oxygen reduction in the
form of a condensate. 28.1 t/h of condensate containing 0.8% by
weight of HCl are obtained. 7.29 t/h of pure oxygen are introduced
into the cathode chambers.
* * * * *