U.S. patent application number 11/849542 was filed with the patent office on 2008-03-06 for process for the production of diaryl carbonates and treatment of alkalichloride solutions resulting therefrom.
This patent application is currently assigned to Bayer Material Science AG. Invention is credited to Andreas Bulan, Marc Buts, Pieter Ooms, Johann Rechner, Johan Vanden Eynde, Rainer Weber.
Application Number | 20080053836 11/849542 |
Document ID | / |
Family ID | 38779525 |
Filed Date | 2008-03-06 |
United States Patent
Application |
20080053836 |
Kind Code |
A1 |
Bulan; Andreas ; et
al. |
March 6, 2008 |
PROCESS FOR THE PRODUCTION OF DIARYL CARBONATES AND TREATMENT OF
ALKALICHLORIDE SOLUTIONS RESULTING THEREFROM
Abstract
Processes comprising: (a) reacting phosgene and a monohydroxyl
aryl compound in the presence of a suitable catalyst to form a
diaryl carbonate and a solution comprising an alkali chloride; (b)
separating the diaryl carbonate from the solution; (c) adjusting
the pH of the solution to a value of less than or equal to 8 to
form a pH-adjusted solution; (d) treating the pH-adjusted solution
with an adsorbent to form a treated solution; (e) subjecting at
least a portion of the treated solution to electrochemical
oxidation to form chlorine and an alkali hydroxide solution; and
(f) recycling at least a portion of one or both of the chlorine and
the alkali hydroxide solution.
Inventors: |
Bulan; Andreas; (Langenfeld,
DE) ; Ooms; Pieter; (Krefeld, DE) ; Weber;
Rainer; (Odenthal, DE) ; Rechner; Johann;
(Kempen, DE) ; Buts; Marc; (Duffel, BE) ;
Vanden Eynde; Johan; (Zwijnaarde, BE) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ, LLP
P O BOX 2207
WILMINGTON
DE
19899
US
|
Assignee: |
Bayer Material Science AG
Leverkusen
DE
|
Family ID: |
38779525 |
Appl. No.: |
11/849542 |
Filed: |
September 4, 2007 |
Current U.S.
Class: |
205/338 ;
205/439 |
Current CPC
Class: |
C07C 68/02 20130101;
C25B 1/34 20130101; C25B 1/46 20130101; C25B 15/08 20130101; C01D
3/06 20130101; C07C 68/02 20130101; C01D 3/04 20130101; C07C 69/96
20130101 |
Class at
Publication: |
205/338 ;
205/439 |
International
Class: |
C25B 3/02 20060101
C25B003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 2, 2006 |
DE |
DE102006041465.9 |
Claims
1. A process comprising: (a) reacting phosgene and a monohydroxyl
aryl compound in the presence of a suitable catalyst to form a
diaryl carbonate and a solution comprising an alkali chloride; (b)
separating the diaryl carbonate from the solution; (c) adjusting
the pH of the solution to a value of less than or equal to 8 to
form a pH-adjusted solution; (d) treating the pH-adjusted solution
with an adsorbent to form a treated solution; (e) subjecting at
least a portion of the treated solution to electrochemical
oxidation to form chlorine and an alkali hydroxide solution; and
(f) recycling at least a portion of one or both of the chlorine and
the alkali hydroxide solution.
2. The process according to claim 1, wherein recycling at least a
portion of the chlorine comprises feeding the portion to a reaction
with carbon monoxide to form at least a portion of the phosgene
reacted with the monohydroxyl aryl compound.
3. The process according to claim 1, wherein recycling at least a
portion of the alkali hydroxide solution comprises feeding the
portion to the reaction of the phosgene and the monohydroxyl aryl
compound.
4. The process according to claim 1, further comprising subjecting
one or more of the solution, the pH-adjusted solution and the
treated solution to a separation to remove an amount of residual
solvent.
5. The process according to claim 4, wherein the separation
comprises steam stripping.
6. The process according to claim 1, wherein the electrochemical
oxidation is carried out with a cathode comprising a gas diffusion
electrode.
7. The process according to claim 1, further comprising feeding a
portion of the treated solution to a brine circuit of a membrane
electrolysis process.
8. The process according to claim 1, further comprising adding
additional alkali chloride to the electrochemical oxidation.
9. The process according to claim 1, wherein the pH of the solution
is adjusted to a value of less than or equal to 7.
10. The process according to claim 1, wherein adjusting the pH of
the solution comprises adding hydrogen chloride.
11. The process according to claim 1, wherein the treated solution
prior to electrochemical oxidation has an alkali chloride
concentration of 100 to 280 g/L.
12. The process according to claim 1, wherein the alkali hydroxide
solution has an alkali hydroxide concentration of 13 to 33 wt.
%.
13. The process according to claim 1, wherein separating the diaryl
carbonate from the solution comprises: (i) separating a diaryl
carbonate-containing organic phase and an aqueous alkali
chloride-containing waste water solution; and (ii) washing the
diaryl carbonate-containing organic phase at least once and
separating the wash liquid.
14. The process according to claim 1, wherein the electrochemical
oxidation is carried out using an ion exchange membrane having a
water transport value greater than 4 moles H.sub.2O per mole of
alkali chloride.
15. The process according to claim 1, wherein the electrochemical
oxidation is carried out using an ion exchange membrane having a
water transport value of 5.5 to 6.5 moles H.sub.2O per mole of
alkali chloride.
16. The process according to claim 1, wherein the electrochemical
oxidation is carried out at a current density of 2 to 6 kA per
m.sup.2 of membrane.
17. The process according to claim 15, wherein the electrochemical
oxidation is carried out at a current density of 2 to 6 kA per
m.sup.2 of membrane.
18. The process according to claim 1, wherein the electrochemical
oxidation is carried out at a temperature of 70 to 100.degree.
C.
19. The process according to claim 1, wherein the electrochemical
oxidation is carried out at an absolute pressure of 1.0 to 1.4
bar.
20. The process according to claim 1, wherein the electrochemical
oxidation is carried out at a differential pressure between an
anode compartment and a cathode compartment of 20 to 150 mbar.
21. The process according to claim 1, wherein the electrochemical
oxidation is carried out using an anode having a coating comprising
ruthenium oxide and a compound of an element selected from the
group consisting of Group 4 elements, Group 7 elements, Group 8
elements, and combinations thereof.
22. The process according to claim 1, wherein the electrochemical
oxidation is carried out using an electrolysis cell having an anode
and a membrane, and wherein the anode has a surface area greater
than a surface area of the membrane.
23. The process according to claim 1, wherein the electrochemical
oxidation is carried out at a current density of 2 to 6 kA per
m.sup.2 of membrane; a temperature of 70 to 100.degree. C.; an
absolute pressure of 1.0 to 1.4 bar; and a differential pressure
between an anode compartment and a cathode compartment of 20 to 150
mbar.
24. The process according to claim 1, wherein the monohydroxyl aryl
compound comprises a phenol compound of the general formula (I):
##STR3## wherein each R independently represents a substituent
selected from the group consisting of hydrogen, halogens, C.sub.1-9
alkyl groups, C.sub.1-9 alkoxy groups, C.sub.1-9 carbonyl groups,
and C.sub.1-9 alkoxycarbonyl groups; and n represents an integer of
0 to 5.
25. A process comprising: (a) reacting phosgene and a phenol
compound of the general formula (I) in the presence of a suitable
catalyst to form a diaryl carbonate and a solution comprising an
alkali chloride; ##STR4## wherein each R independently represents a
substituent selected from the group consisting of hydrogen,
halogens, C.sub.1-9 alkyl groups, C.sub.1-9 alkoxy groups,
C.sub.1-9 carbonyl groups, and C.sub.1-9 alkoxycarbonyl groups; and
n represents an integer of 0 to 5; (b) separating the diaryl
carbonate from the solution; (c) adjusting the pH of the solution
with hydrogen chloride to a value of less than or equal to 7 to
form a pH-adjusted solution; (d) treating the pH-adjusted solution
with an adsorbent to form a treated solution; (e) subjecting at
least a portion of the treated solution to electrochemical
oxidation to form chlorine and an alkali hydroxide solution; and
(f) recycling at least a portion of one or both of the chlorine and
the alkali hydroxide solution; wherein recycling at least a portion
of the chlorine comprises feeding the portion to a reaction with
carbon monoxide to form at least a portion of the phosgene reacted
with the monohydroxyl aryl compound; wherein recycling at least a
portion of the alkali hydroxide solution comprises feeding the
portion to the reaction of the phosgene and the monohydroxyl aryl
compound; and wherein the electrochemical oxidation is carried out
at a current density of 2 to 6 kA per m.sup.2 of membrane; a
temperature of 70 to 100.degree. C.; an absolute pressure of 1.0 to
1.4 bar; and a differential pressure between an anode compartment
and a cathode compartment of 20 to 150 mbar.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates, in general, to a combined process for
the production of diaryl carbonate and electrolysis of alkali
chloride-containing process wastewater. The invention also relates,
more particularly, to processes for the treatment of alkali
chloride-containing waste solutions for further use in a diaryl
carbonate production process, and even more particularly, a
diphenyl carbonate production process ("DPC process").
[0002] The production of diaryl carbonates, and more particularly
diphenyl carbonates, generally takes place by a continuous process,
by the production or introduction of phosgene and subsequent
reaction of monophenols and phosgene in an inert solvent in the
presence of alkali and a nitrogen catalyst at the reaction
interface, according to the following general reaction scheme:
##STR1##
[0003] The production of diaryl carbonates, e.g., by interfacial
polycondensation, is described in various literature sources, e.g.,
in "Chemistry and Physics of Polycarbonates", POLYMER REVIEWS, H.
Schnell, Vol. 9, John Wiley and Sons, Inc. (1964) pp. 50-51, the
entire contents of which are incorporated herein by reference.
[0004] Processes for the production of diaryl carbonates operated
at temperatures of >65.degree. C. are known. The pH can be
adjusted initially to a low value (pH.about.8 to 9) in such
processes and then to a higher value (pH.about.10 to 11.
[0005] Optimizations of such a process by improved intermixing,
maintaining a narrow temperature and pH profile, and/or isolation
of the product, are also known.
[0006] In such known processes, however a high residual phenol
value in the wastewater, which can pollute the environment and
confront the sewage works with increased wastewater problems, makes
costly purification operations necessary. Thus, removal of organic
impurities in the wastewater by an extraction with methylene
chloride has been suggested in the literature. The alkali
chloride-containing waste solution is generally freed of solvents
and organic residues and is then disposed of.
[0007] It is also known, however, that the sodium
chloride-containing wastewaters can be purified by ozonolysis and
are then suitable for use in sodium chloride electrolysis. A
disadvantage of ozonolysis is that such processes can be very
cost-intensive.
[0008] It is also known that a sodium chloride-containing
wastewater stream can be evaporated until all the water has been
removed and the remaining salt, together with the organic
impurities, can be subjected to a heat treatment as a result of
which the organic components are destroyed. The use of infrared
radiation for such processes can be preferred. A disadvantage of
such processes is that the water has to be completely evaporated,
and so the processes cannot be carried out economically.
[0009] It is also known that the wastewater from a DPC production
can be purified by extraction and then fed into sodium chloride
electrolysis. However, only a maximum of 26% of the sodium chloride
is known to be recoverable from the wastewater from DPC production
by such processes, since if greater quantities were recovered, the
water introduced into the electrolysis with the wastewater would
bring the water balance of the sodium chloride electrolysis out of
equilibrium.
[0010] Sodium chloride-containing solutions formed during DPC
production typically have a sodium chloride content of 13 to 17 wt.
%. Thus, the sodium chloride present in the solutions can never be
completely recovered by known processes. With a sodium chloride
concentration of 17 wt. %, in a standard sodium chloride
electrolysis using a commercial ion exchange membrane having a
water transport of 3.5 moles of water per mole of sodium, only
approx. 23% of the sodium chloride from the sodium
chloride-containing solutions is successfully used. Even by
concentrating up to a saturated sodium chloride solution of approx.
25 wt. %, only a recycling quota of 38% of the sodium chloride
contained in the sodium chloride-containing solution would be
achieved. No complete recycling of the sodium chloride-containing
solution has become known. It has also been suggested that the
sodium chloride-containing solution can be evaporated by means of
thermal processes in such a way that a highly concentrated sodium
chloride solution can be fed into an electrolytic cell. However,
the evaporation is energy-intensive and costly.
[0011] Thus, there is a need in the art to provide a diaryl
carbonate production process which provides products in high purity
and good yield and, at the same time, represents a reduction in
environmental pollution or wastewater problems at the production
area's sewage works by maximized recycling of alkali chloride from
alkali chloride-containing process wastewater solutions obtained
from diaryl carbonate production.
[0012] Moreover, there is a need, during the recycling, to provide
processes which require minimal energy input, thus also conserving
resources.
SUMMARY OF THE INVENTION
[0013] It has been found that the alkali chloride-containing
wastewater solutions forming during the continuous production of
diaryl carbonates, such as by reaction of monophenols and phosgene
in an inert solvent in the presence of alkali and amine catalyst at
the interface, can be fed directly into an electrochemical
oxidation of the alkali chloride obtained to form chlorine, alkali
hydroxide solution and optionally hydrogen without costly
purification. It has been found that this can be accomplished by
introduction of the wastewater solution into an electrochemical
oxidation after adjustment of the pH of the solution to a value
less than or equal to 8 and simple treatment with an adsorbent,
such as activated carbon. The chlorine obtained from the
electrochemical oxidation can be recycled into the production of
phosgene. Moreover, the alkali hydroxide solution can be recycled
into the diaryl carbonate production reaction as basic
catalyst.
[0014] One embodiment of the present invention includes a process
comprising: (a) reacting phosgene and a monohydroxyl aryl compound
in the presence of a suitable catalyst to form a diaryl carbonate
and a solution comprising an alkali chloride; (b) separating the
diaryl carbonate from the solution; (c) adjusting the pH of the
solution to a value of less than or equal to 8 to form a
pH-adjusted solution; (d) treating the pH-adjusted solution with an
adsorbent to form a treated solution; (e) subjecting at least a
portion of the treated solution to electrochemical oxidation to
form chlorine and an alkali hydroxide solution; and (f) recycling
at least a portion of one or both of the chorine and the alkali
hydroxide solution.
[0015] Another embodiment of the present invention includes a
process comprising: (a) reacting phosgene and a monohydroxyl aryl
compound in the presence of a suitable catalyst to form a diaryl
carbonate and a solution comprising an alkali chloride; (b)
separating the diaryl carbonate from the solution; (c) adjusting
the pH of the solution with hydrogen chloride to a value of less
than or equal to 7 to form a pH-adjusted solution; (d) treating the
pH-adjusted solution with an adsorbent to form a treated solution;
(e) subjecting at least a portion of the treated solution to
electrochemical oxidation to form chlorine and an alkali hydroxide
solution; and (f) recycling at least a portion of one or both of
the chlorine and the alkali hydroxide solution; wherein recycling
at least a portion of the chlorine comprises feeding the portion to
a reaction with carbon monoxide to form at least a portion of the
phosgene reacted with the monohydroxyl aryl compound; wherein
recycling at least a portion of the alkali hydroxide solution
comprises feeding the portion to the reaction of the phosgene and
the monohydroxyl aryl compound; and wherein the electrochemical
oxidation is carried out at a current density of 2 to 6 kA per
m.sup.2 of membrane; a temperature of 70 to 100.degree. C.; an
absolute pressure of 1.0 to 1.4 bar; and a differential pressure
between an anode compartment and a cathode compartment of 20 to 150
mbar.
DETAILED DESCRIPTION OF THE INVENTION
[0016] As used herein, the singular terms "a" and "the" are
synonymous and used interchangeably with "one or more."
Accordingly, for example, reference to "a diaryl carbonate" herein
or in the appended claims can refer to a single carbonate or more
than one carbonate. Additionally, all numerical values, unless
otherwise specifically noted, are understood to be modified by the
word "about."
[0017] Preferred monohydroxyl aryl compounds which are suitable for
use in various embodiments of the processes according to the
invention include phenol compounds of the general formula (I):
##STR2## wherein each R independently represents a hydrogen, a
halogen or a branched or unbranched C.sub.1 to C.sub.9 alkyl group,
alkoxy group, carbonyl group or alkoxycarbonyl group, and n
represents an integer of 0 to 5.
[0018] Preferred phenol compounds of the general formula (I) which
can be used in processes according to the invention include phenol
alkylphenols such as cresols, p-tert.-butylphenol, p-cumylphenol,
p-n-octylphenol, p-isooctylphenol, p-n-nonylphenol and
p-isononylphenol, halophenols such as p-chlorophenol,
2,4-dichlorophenol, p-bromophenol and 2,4,6-tribromophenol or
methyl salicylate are preferred. Phenol is particularly
preferred.
[0019] Suitable catalysts for the reaction of phosgene and a
monohydroxyl aryl compound to form a phenolate include basic
compounds, such as for example, alkali hydroxides. Suitable alkali
hydroxides include sodium hydroxide, potassium hydroxide and
lithium hydroxide. Suitable hydroxides, most preferably sodium
hydroxide, can be used as a solution, and are preferably used as a
10 to 55 wt. % solution.
[0020] The reaction of phosgene and a monohydroxyl aryl compound
can be accelerated by additional catalytic components, such as
tertiary amines, N-alkylpiperidines and/or onium salts.
Tributylamine, triethylamine and N-ethylpiperidine are preferably
used.
[0021] The amine catalyst used can be open-chained or cyclic,
triethylamine and ethylpiperidine being particularly preferred. The
amine catalyst is preferably used as a 1 to 55 wt. % solution.
[0022] The term "onium salts" as used herein refers to compounds
such as N.sub.4X, wherein R can be an alkyl and/or aryl group
and/or an H, and X is an anion.
[0023] Phosgene to be reacted with a monohydroxyl aryl compound can
be introduced to the reaction in liquid or gaseous form or
dissolved in an inert solvent.
[0024] Inert organic solvents that can preferably be used in the
processes of the invention include, for example, dichloromethane,
toluene, the various dichloroethanes and chloropropane compounds,
chlorobenzene and chlorotoluene. Dichloromethane is preferably
used.
[0025] The reaction of phosgene and a monohydroxyl aryl compound is
preferably carried out continuously and particularly preferably in
a plug flow without any significant back mixing. This can therefore
take place, e.g., in tubular reactors. The intermixing of the two
phases (aqueous and organic phase) can be achieved, for example, by
inbuilt pipe baffles, static mixers and/or e.g. pumps.
[0026] The reaction of phosgene and a monohydroxyl aryl compound
particularly preferably takes place in two stages.
[0027] In a first stage of such preferred embodiments, the reaction
is started by bringing together the feedstocks of (i) phosgene,
(ii) an inert solvent, which preferably acts first as a solvent for
the phosgene, and (iii) a monohydroxyl aryl compound, which is
preferably already previously dissolved in the alkali hydroxide
solution. The residence time in the first stage can typically be 2
seconds to 300 seconds, particularly preferably 4 seconds to 200
seconds. The pH during the first stage is preferably adjusted by
the alkali lye/monophenol/phosgene ratio such that the pH is 11 to
12, preferably 11.2 to 11.8, particularly preferably 11.4 to 11.6.
The reaction temperature during the first stage, via cooling, is
preferably <40.degree. C., particularly preferably
<35.degree. C.
[0028] In a second stage of such preferred embodiments, the
reaction to form a diaryl carbonate is completed. The residence
time in the second stage can typically be 1 minute to 2 hours,
preferably 2 minutes to 1 hour, especially preferably 3 minutes to
30 minutes. The second stage can be regulated by constant
monitoring of the pH value (preferably measured online in the
continuous process by known methods) and corresponding adjustment
of the pH value by addition of alkali hydroxide. The quantity of
alkali hydroxide introduced can preferably be adjusted such that
the pH of the reaction mixture in the second stage is 7.5 to 10.5,
preferably 8 to 9.5, especially preferably 8.2 to 9.3. The reaction
temperature of the second stage is preferably <50.degree. C.,
particularly preferably <40.degree. C., especially preferably
<35.degree. C., by cooling.
[0029] Broad ranges, preferred ranges, more preferred ranges and
most preferred ranges described herein for various process
parameters can be combined with one another as desired, in other
words, general conditions for one parameter can be employed with
preferred values for another parameter, and more preferable values
for another parameter.
[0030] In various preferred embodiments of the invention, phosgene
can be reacted with a monohydroxyl aryl compound in a molar ratio
of phosgene to monohydroxyl aryl compound of 1:2 to 1:2.2. Solvent
can be mixed into the reaction such that the diaryl carbonate is
present in a 5 to 60% solution, preferably a 20 to 45% solution,
after the reaction.
[0031] The concentration of amine catalyst is preferably 0.0001 mol
to 0.1 mol, based on the monophenol used.
[0032] After the reaction of phosgene and a monohydroxyl aryl
compound, the organic phase containing the diaryl carbonate is
preferably washed, generally with an aqueous liquid and optionally
repeatedly, and after each washing operation it is separated as far
as possible from the aqueous phase. The washing preferably takes
place with deionized water. The diaryl carbonate solution is
generally cloudy after the washing and separation of the washing
liquid. Aqueous liquids can be used as washing liquid for the
separation of the catalyst, e.g., a dilute mineral acid such as HCl
or H.sub.3PO.sub.4, and deionized water for the further
purification. The concentration of HCl or H.sub.3PO.sub.4 in the
washing liquid can be, e.g., 0.5 to 1.0 wt. %. The organic phase is
washed, for example and preferably, twice.
[0033] As phase-separating devices for separating the washing
liquid from the organic phase, separating vessels, phase
separators, centrifuges or coalescers or combinations of these
devices that are known in principle can be used.
[0034] At this stage of processes according to various embodiments
of the invention, without taking into account the solvent still to
be separated off, surprisingly high degrees of purity of the diaryl
carbonate of >99.85% can be obtained.
[0035] In such preferred embodiments of the process according to
the invention, the wash liquids separated in the separation of the
diaryl carbonate from the solution (b) can, optionally after
separating catalyst residues and/or organic solvent residues, be
recycled to reaction b) of the process according to the
invention.
[0036] In this regard, the separation and working-up of the diaryl
carbonate formed in the reaction of phosgene and monohydroxyl aryl
compound can include, as the separation according to (b),
preferably at least the following steps: [0037] (aa) separation of
diaryl carbonate-containing organic phase and aqueous alkali
chloride-containing waste water solution [0038] (bb) washing the
diaryl carbonate-containing organic phase obtained in step (aa) at
least once, preferably at least twice, particularly preferably
twice, and separating the respective wash liquid.
[0039] It may possibly be necessary to separate at least one of the
wash liquid(s) obtained according to (bb) from catalyst residues
and possibly organic solvent residues by adjusting the pH value to
at least 9, preferably at least 10, particularly preferably 10 to
11, by adding at least one basic compound, and then extract the
solution with at least one inert organic solvent, or preferably
subject the solution to a subsequent stripping with stream.
Suitable basic compounds for the adjustment of the pH value are for
example alkali or alkaline earth metal hydroxides or carbonates.
The basic compounds may be used in solid form or in the form of
their aqueous solutions. Alkali metal hydroxides, particularly
preferably sodium hydroxide, are preferably used.
[0040] Preferably, at least part of the wash liquid(s) from (bb)
can be used as a partial replacement of the water for the
preparation of the sodium hydroxide for reaction of phosgene and a
monohydroxyl aryl compound according to (a), in particular for
adjusting the concentration of the sodium hydroxide for reaction of
phosgene and a monohydroxyl aryl compound according to (a).
Preferably, at least part of the wash liquid(s) from (bb) can be
used to dilute the alkali metal hydroxide prepared according to
electrochemical oxidation (c), before this is recycled to the
production of diaryl carbonate according to reaction of phosgene
and a monohydroxyl aryl compound according to (a). Such preferred
embodiments of the process according to the invention, in which the
wash liquid separated in separation according to (b) are recycled
to the process according to the invention, have the additional
advantage of a lower waste water discharge.
[0041] After the synthesis of the diaryl carbonate, the diaryl
carbonate is separated off in the form of its solution in an
organic solvent which may be used during the synthesis, e.g.
methylene chloride.
[0042] To obtain a highly pure diaryl carbonate, the solvent can be
evaporated. The evaporation can take place in several evaporator
steps. This takes place, e.g., by one or more distillation columns
arranged in series, in which the solvent is separated from the
diaryl carbonate.
[0043] Such purification of the diaryl carbonate can be carried out
continuously, for example, in such a way that the bottom
temperature in the distillation is 150.degree. C. to 310.degree.
C., preferably 160 to 230.degree. C. The pressure applied to
perform this distillation is particularly 1 to 1000 mbar,
preferably 5 to 100 mbar.
[0044] Diaryl carbonates purified in such a manner can be
distinguished by particularly high purities (GC>99.95%) and
extremely good transesterification performance, so that a
polycarbonate of excellent quality can be produced therefrom.
[0045] The use of diaryl carbonates for the production of aromatic
oligo/polycarbonates by the melt transesterification process is
known from the literature and is described, e.g., in the
Encyclopedia of Polymer Science, Vol. 10 (1969), Chemistry and
Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9,
John Wiley and Sons, Inc. (1964) and U.S. Pat. No. 5,340,905, the
entire contents of each of which are incorporated herein by
reference.
[0046] The remaining aqueous solution, after separation of the
diaryl carbonate, can be advantageously freed of highly volatile
organic impurities, such as, e.g., residues of the organic solvent
used in the synthesis and optionally catalyst, for example by
distillation or steam stripping. A wastewater solution then remains
with a high content of dissolved sodium chloride (.about.10-20 wt.
%) and dissolved sodium carbonates (.about.0.3-1.5 wt. %). The
carbonates can form, e.g., by hydrolysis of the phosgene as a
secondary reaction of diaryl carbonate production. In addition, the
wastewater can be contaminated with organic compounds, e.g., with
phenols (e.g. unsubstituted phenol, and/or alkylphenols).
[0047] The treatment of the previously purified wastewater solution
with adsorbents can then preferably take place with activated
carbon.
[0048] According to various preferred embodiments of the processes
according to the invention, the adjustment (e.g., lowering) of the
pH can be performed with hydrochloric acid or hydrogen chloride.
The use of less expensive sulfuric acid, which is conceivable in
principle but less desirable in the present process, can lead to
the formation of sodium sulfate during the lowering of the pH,
which would then become concentrated in the anolyte circulation in
the subsequent electrolysis. Since, e.g., ion exchange membranes
can generally only be operated up to a certain sodium sulfate
concentration in the anolyte, according to manufacturer's
instructions, more anolyte would have to be discharged than when
using hydrochloric acid or hydrogen chloride, the reaction product
of which is the desired sodium chloride.
[0049] The alkali chloride electrochemical oxidation (electrolysis)
is described in more detail below. The following description is
provided as an example relating to the electrolysis of sodium
chloride, although, as already stated above, in principle any
alkali chloride can be used in the process (particularly LiCl,
NaCl, KCl). The use of sodium is preferred in various embodiments
of the processes according to the invention.
[0050] Membrane electrolysis processes, conventionally used e.g.
for the electrolysis of sodium chloride-containing solutions, such
as described in Peter Schmittinger, CHLORINE, Wiley-VCH Verlag,
2000, the entire contents of which are incorporated herein by
reference, can be used for electrochemical oxidation in accordance
with the various embodiments of the present invention. In such
processes, an electrolytic cell divided into two compartments,
namely an anode compartment with an anode and a cathode compartment
with a cathode, is used. The anode and cathode compartments are
separated by an ion exchange membrane. A sodium chloride-containing
solution with a sodium chloride concentration of generally more
than 300 g/l is introduced into the anode compartment. At the
anode, the chloride ion is oxidized to form chlorine, which is
passed out of the cell with the depleted sodium chloride-containing
solution (approx. 200 g/l). Under the influence of the electrical
field, the sodium ions migrate through the ion exchange membrane
into the cathode compartment. During this migration, each mole of
sodium entrains between 3.5 and 4.5 moles of water, depending on
the membrane. This leads to the anolyte becoming depleted of water.
In contrast to the anolyte, on the cathode side water is consumed
by the electrolysis of water to hydroxide ions and hydrogen. The
water entering the catholyte with the sodium ions is sufficient to
keep the concentration of the sodium hydroxide solution in the
discharge at 31-32 wt. %, with an intake concentration of 30% and a
current density of 4 kA/m.sup.2. In the cathode compartment, water
is electrochemically reduced resulting in the formation of
hydroxide ions and hydrogen.
[0051] Alternatively, a gas diffusion electrode can also be used as
the cathode, at which oxygen is converted to hydroxide ions with
electrons, no hydrogen being formed. With the sodium ions entering
the cathode compartment through the ion exchange membrane, the
hydroxide ions form sodium hydroxide. A sodium hydroxide solution
with a concentration of 30 wt. % is generally fed into the cathode
compartment and a sodium hydroxide solution with a concentration of
31-32 wt. % is discharged. The aim is to achieve the highest
possible concentration of sodium hydroxide solution, since sodium
hydroxide solution is generally stored or transported as a 50 wt. %
lye. However, commercial membranes are not at present resistant to
a lye with a concentration higher than 32 wt. %, and so the sodium
hydroxide solution has to be concentrated by thermal
evaporation.
[0052] In the case of sodium chloride electrolysis, additional
water can be introduced into the anolyte by this sodium
chloride-containing solution but only water is discharged into the
catholyte through the membrane. If more water is introduced by the
sodium chloride-containing solution than can be transported to the
catholyte, the anolyte is depleted of sodium chloride and the
electrolysis cannot be operated continuously. With very low sodium
chloride concentrations, the secondary reaction of oxygen formation
would start.
[0053] In order to feed maximum quantities of sodium
chloride-containing solutions to the sodium chloride electrolysis
economically, it may be useful for the water transport through the
membrane to be increased. This can take place by selecting suitable
membranes, such as described in U.S. Pat. No. 4,025,405, the entire
contents of which are incorporated herein by reference. The effect
of increased water transport is that the otherwise conventional
addition of water to maintain the lye concentration can be
omitted.
[0054] According to U.S. Pat. No. 3,773,634, the entire contents of
which are incorporated herein by reference, with a high level of
water transport through the membrane, the electrolysis can be
operated when a lye concentration of 31 to 43 wt. % and a sodium
chloride concentration of 120 to 250 g/l is used.
[0055] A disadvantage of such processes disclosed in the U.S.
patents is the lower current efficiency of the processes.
[0056] According to various preferred embodiments of the processes
according to the invention, the separation of the dialkyl carbonate
from the solution includes phase separation, subsequent removal of
solvent and optionally used catalyst by stripping with steam, and
the separation is followed by the adjustment of the solution pH,
and subsequent treatment with activated carbon. Following this
sequence, the alkali chloride-containing wastewater solution (i.e.,
the pH-adjusted, adsorbent-treated solution) can be fed directly
into the electrolysis.
[0057] The processes according to various embodiments of the
present invention in which more than 26% of an alkali chloride can
be recovered from a wastewater solution by electrolysis is an
improvement over known prior art processes in which no more than
26% of the sodium chloride present in the wastewater from DPC
production could be used in NaCl electrolysis.
[0058] In various preferred embodiments of the processes of the
present invention, water can be removed from the alkali
chloride-containing wastewater solution by a concentration process.
A process is therefore preferred, characterised in that the alkali
chloride-containing solution is concentrated before the
electrolysis by membrane distillation processes or reverse
osmosis.
[0059] Reverse osmosis for example or, particularly preferably,
membrane distillation or membrane contactors can be used, such as
described in MELIN; RAUTENBACH, Membranverfahren; SPRINGER, BERLIN,
2003, the entire contents of which are incorporated herein by
reference. By combing operation of the electrolytic cells according
to the invention and concentration processes, theoretically up to
100% of the sodium chloride can be recovered from the
wastewater.
[0060] Processes according to the invention can also be carried out
with an alkali chloride electrolysis in which no hydrogen is
produced at the cathode but the cathode is replaced by a gas
diffusion electrode at which oxygen is reduced to form hydroxide
ions.
[0061] If, for example, at an integrated production site which can
directly receive one or more of the electrolysis products, no
hydrogen is required for chemical reactions, it is possible to omit
the forced production of hydrogen. One advantage is an energy
saving during electrolysis, which is attributable to the lower
electrolysis voltage when using a gas diffusion electrode.
[0062] A sodium chloride-containing solution coming from DPC
production generally has a sodium chloride content of up to 17 wt.
%, in so far as it is the wastewater from the reaction. If the
wastewater from the reaction is combined with the washing water,
the NaCl concentration is, for example, approx. 13 wt. %. If the
electrolysis supplies the chlorine and the sodium hydroxide
solution exclusively for DPC production, only a small part of the
sodium chloride-containing wastewater can be used in the
electrolysis. Thus, with the conventional ion exchange membranes
and the standard operating parameters of sodium chloride
electrolysis, only a maximum of 26% of the sodium chloride from a
17 wt. % sodium chloride-containing DPC wastewater can be used. The
standard operating parameters of NaCl electrolysis are a brine
concentration in the discharge of 200 to 240 g/l and an NaOH
concentration of 31-32 wt. %. Up to now, therefore, complete
recycling of the sodium chloride formed has been impossible.
Concentration by thermal evaporation of the water is not currently
economical, since sodium chloride is available as a very
inexpensive product.
[0063] With a process according to the invention, significantly
more than 26% of the sodium chloride from wastewaters formed with a
concentration of 17 wt. % can be recycled, in so far as the sodium
chloride electrolysis exclusively provides the chlorine and the
sodium hydroxide solution for DPC production. Sodium chloride
electrolyses are generally operated at integrated chemical
production sites in connection with a variety of chlorine
consumers, such that a sodium chloride-containing wastewater
solution is not necessarily available for recycling from each of
the various chlorine consumers. The proportion of reusable sodium
chloride from the wastewater can be increased where the sodium
chloride electrolysis is not used exclusively to provide the sodium
hydroxide solution and chlorine for diaryl carbonate
production.
[0064] Another preferred variant of the processes of the invention
is that the wastewater from diaryl carbonate production is
concentrated by solid alkali chloride and fed into the alkali
chloride electrolysis. As a result, more than 50% of the alkali
chloride from the DPC wastewater could be reused.
[0065] However, this assumes that the chlorine and the alkali lye
are not used exclusively for diaryl carbonate production.
[0066] An alkali chloride-containing wastewater with a pH of less
than 7 is particularly preferably used in or fed to the
electrolysis. The pH adjustment preferably takes place with
hydrochloric acid, but can also take place with gaseous hydrogen
chloride.
[0067] According to another preferred process embodiment, the NaCl
electrolysis can be operated in such a way that an NaCl solution
coming from the electrolysis cell has an NaCl concentration of less
than 200 g/l. In parallel to this, the sodium hydroxide
concentration discharged from the cell can be less than 30 wt.
%.
[0068] The water transport through the membrane depends not only on
the operating parameters but also on the type of membrane used. In
the processes of the invention, those ion exchange membranes that
permit a water transport through the membrane of more than 4.5
moles of water per mole of alkali, preferably sodium, under the
conditions of alkali chloride and alkali hydroxide concentration
according to the invention are preferably used.
[0069] The current density is calculated based on the area of the
membrane and is preferably 2 to 6 kA/m.sup.2. Anodes with a greater
surface area are particularly preferably used. Anodes with a
greater surface area are to be understood as those in which the
physical surface area is distinctly larger than the projected
surface area. Anodes with a greater surface area are, e.g.,
electrodes with a foam- or felt-like construction. As a result, a
very large anodic electrode surface area is offered and the local
current density is markedly reduced. The surface area of the anode
should preferably be selected such that the local current density
based on the physical surface area of the electrode is less than 3
kA/m.sup.2. The larger the surface area and the lower the local
current density, the lower the sodium chloride concentration that
can be selected in the brine and the higher the proportion of
sodium chloride from the wastewater that can be recycled.
[0070] The pH of the alkali chloride-containing wastewater should
preferably be less than 7, particularly preferably from 0.5 to 6,
before the electrolysis.
[0071] The alkali chloride electrolysis should be operated in such
a way that the alkali chloride concentration of the alkali chloride
solution coming from the cell is between 100 and 280 g/l sodium
chloride and/or that the concentration of the alkali hydroxide
coming from the cell is 13 to 33 wt. %.
[0072] Concentrations enabling the cell to be operated at lower
voltages are particularly preferred. For this purpose, the
concentration of the alkali chloride solution coming from the cell
should preferably be between 110 and 220 g/l alkali chloride and/or
the concentration of the alkali hydroxide coming from the cell
should be 20 to 30 wt. %.
[0073] The ion exchange membranes used in the electrolysis should
preferably have a water transport per mole of sodium of more than
4.0 moles H.sub.2O/mole alkali, particularly preferably 5.5 to 6.5
moles H.sub.2O/mole alkali.
[0074] The process can preferably be operated in such a way that
the electrolysis is operated at a temperature of 70 to 100.degree.
C., preferably at 80 to 95.degree. C.
[0075] The electrolysis can be operated at an absolute pressure of
1 to 1.4 bar, preferably at a pressure of 1.1 to 1.2 bar.
[0076] The pressure ratios between anode and cathode compartments
are preferably selected such that the pressure in the cathode
compartment is higher than the pressure in the anode compartment.
The differential pressure between cathode and anode compartments
should preferably be 20 to 150 mbar, more preferably 30 to 100
mbar.
[0077] With lower alkali chloride concentrations, special anode
coatings can also be used. In particular, the coating of the anode
can contain, in addition to ruthenium oxide, other precious metal
components from subgroups 7 and 8 of the periodic table of
elements. For example, the anode coating can be doped with
palladium compounds. Coatings based on diamond can also be
used.
[0078] The following examples illustrate an embodiment of a process
according to the invention based on the sodium chloride-containing
wastewater formed during the production of diphenyl carbonate, and
are only a reference point rather than a limitation.
EXAMPLES
Example 1
Addition of Sodium Chloride-Containing Reaction Wastewater to
Sodium Chloride Electrolysis
Addition of a 17 Wt. % Sodium Chloride Solution from DPC
Production
[0079] A mixture of 145.2 kg/h of 14.5% sodium hydroxide solution
and 48.3 kg/h of phenol are brought together with a solution of
86.2 kg/h of methylene chloride and 27.5 kg/h of phosgene (8 mole %
excess based on phenol) in a vertically standing, cooled tubular
reactor. This reaction mixture is cooled to a temperature of
33.degree. C. and, after an average residence time of 15 seconds, a
pH of 11.5 is measured. In the second step of the process, 5.4 kg/h
of 50% NaOH are then metered into this reaction mixture so that the
pH of the second reaction step is 8.5 after a further residence
time of 5 minutes. In the continuously operated reaction, metering
fluctuations that occur are offset by adjusting the additions of
NaOH in each case. In the second step of the process, the reaction
mixture is continuously mixed by passing through a pipe provided
with constrictions. After renewed addition of NaOH, the reaction
temperature is adjusted by cooling to 30.degree. C. After
separating off the organic from the aqueous phase (reaction
wastewater), the DPC solution is washed with 0.6% hydrochloric acid
and water. After removing the solvent, 99.9% diphenyl carbonate is
obtained. The reaction wastewater is not combined with the washing
phases here and is freed of solvent residues and catalyst by
stripping with steam. After neutralising with hydrochloric acid and
treating with activated carbon, the reaction wastewater contains
17% NaCl and <2 ppm phenol.
[0080] It can be fed into the sodium chloride electrolytic cell
without any further purification.
[0081] The electrolysis is performed for example in a laboratory
electrolytic cell with an anode area of 0.01 m.sup.2. The current
density was 4 kA/m.sup.2, discharge temperature on the cathode side
88.degree. C. and discharge temperature on the anode side
89.degree. C. An electrolytic cell with a standard anode and
cathode coating from DENORA, Germany, was used. A Nafion 982 WX ion
exchange membrane from DuPont was used. The electrolytic voltage
was 3.02 V. A sodium chloride-containing solution was pumped
through the anode compartment at a mass flow rate of 0.98 kg/h. The
concentration of the solution fed into the anode compartment was 25
wt. % NaCl. A 20 wt. % NaCl solution could be removed from the
anode compartment. 0.121 kg/h of 17 wt. % reaction wastewater from
diphenyl carbonate production and 0.0653 kg/h of solid sodium
chloride were added to the NaCl solution removed from the anode
compartment. The solution was then fed back into the anode
compartment. The water transport through the membrane was 3.5 moles
of water per mole of sodium.
[0082] On the cathode side, a sodium hydroxide solution was pumped
round at a mass flow rate of 1.107 kg/h. The concentration of the
sodium hydroxide solution fed into the cathode side was 30 wt. %
NaOH and the sodium hydroxide solution removed from the cathode
side had a concentration of 32% NaOH. 0.188 kg/h of the 31.9% lye
were removed from the volume flow and the remainder was topped up
with 0.0664 kg/h of water and fed back into the cathode
element.
[0083] 23.3% of the reacted sodium chloride originates from the DPC
reaction wastewater.
Example 2
Addition of Sodium Chloride-Containing Reaction Wastewater to
Sodium Chloride Electrolysis with a Gas Diffusion Electrode
Addition of a 17 Wt. % Sodium Chloride Solution (Reaction
Wastewater) from DPC Production
[0084] The wastewater corresponded to the quality of that according
to Example 1. Since no hydrogen is required for the production of
DPC, the formation of hydrogen during the electrolysis can be
omitted. The electrolysis was therefore operated with gas diffusion
electrodes. The current density was 4 kA/m.sup.2, cathode side
outlet temperature 88.degree. C. and anode side outlet temperature
89.degree. C. An electrolytic cell with a standard anode coating
from DENORA, Germany, was used. A Nafion 982 WX ion exchange
membrane from DuPont was used. The electrolytic voltage was 2.11 V.
The sodium chloride concentration of the solution removed from the
anode compartment was 17 wt. % NaCl. 0.166 kg/h of 17 wt. %
reaction wastewater and 0.0553 kg/h of solid sodium chloride were
added to the NaCl solution removed from the anode compartment. The
solution was then fed back into the anode compartment. The water
transport through the membrane was 4.9 moles of water per mole of
sodium.
[0085] On the cathode side, a sodium hydroxide solution was pumped
round at a mass flow rate of 0.848 kg/h. The concentration of the
sodium hydroxide solution fed into the cathode side was 30 wt. %
NaOH and the sodium hydroxide solution removed from the cathode
side had a concentration of 32 wt. % NaOH. 0.192 kg/h of the 31.2%
lye were removed from the volume flow and the remainder was topped
up with 0.033 kg/h of water and fed back into the cathode
element.
[0086] The proportion of reacted sodium chloride from the DPC
reaction wastewater was 32.4%.
Example 3
Addition of Sodium Chloride-Containing Reaction Wastewater to
Sodium Chloride Electrolysis with a Gas Diffusion Electrode
Addition of a 17 Wt. % Sodium Chloride Solution (Reaction
Wastewater) from DPC Production
[0087] The wastewater corresponded to the quality of that according
to Example 1. Since no hydrogen is required for the production of
DPC, the formation of hydrogen during the electrolysis can be
omitted. The electrolysis was therefore operated with gas diffusion
electrodes. The current density was 4 kA/m.sup.2, cathode side
outlet temperature 88.degree. C. and anode side outlet temperature
89.degree. C. An electrolytic cell with a standard anode coating
from DENORA, Germany, was used. A Nafion 2030 ion exchange membrane
from DuPont was used. The electrolytic voltage was 1.96 V. The
sodium chloride concentration of the solution removed from the
anode compartment was 15 wt. % NaCl. 0.178 kg/h of 17 wt. %
reaction wastewater and 0.0553 kg/h of solid sodium chloride were
added to the NaCl solution removed from the anode compartment. The
solution was then fed back into the anode compartment. The water
transport through the membrane was 5.26 moles of water per mole of
sodium.
[0088] On the cathode side, a sodium hydroxide solution was pumped
round at a mass flow rate of 0.295 kg/h. The concentration of the
sodium hydroxide solution fed into the cathode side was 30 wt. %
NaOH and the sodium hydroxide solution removed from the cathode
side had a concentration of 32 wt. % NaOH. 0.188 kg/h of the 32%
lye were removed from the volume flow and the remainder was topped
up with 0.0184 kg/h of water and fed back into the cathode
element.
[0089] The proportion of reacted sodium chloride from the DPC
reaction wastewater was 34.4%.
Example 4
Recycling of Wash Phases from the DPC Working-Up in the DPC
Production
Addition of a Waste Water Phase from the Acid Wash to the DPC
Production
[0090] The procedure was carried out as in Example 1, with the
difference that, after separating the organic phase from the
aqueous phase (reaction waste water), the DPC solution was washed
with 0.6 wt.-% hydrochloric acid (acid wash), and then once more
with water (neutral wash). The acidic wash phase from the DPC
working-up was adjusted to pH 10 with NaOH and was then freed from
solvent residues and catalysts by extraction with methylene
chloride or by stripping with steam. After the phase separation an
aqueous phase was obtained containing 1.5 wt.-% NaCl, which can be
re-used as a partial replacement of the water for the preparation
of the 14.5 wt.-% NaOH solution for the DPC production.
Example 5
Recycling of Wash Phases from the DPC Working-Up to the DPC
Production
Addition of a Neutral Wash Phase to the DPC Production
[0091] The procedure was carried out as in Example 4. The neutral
wash phase from the DPC working-up can be re-used without further
treatment as a partial replacement of the water for the preparation
of the 14.5 wt.-% NaOH solution for the DPC production.
Example 6
Recycling of Wash Phases from the DPC Working-Up to the DPC
Production
Addition of the Purified Waste Water Phases from the Acid and
Neutral Wash to the DPC Production
[0092] The procedure was carried out as in Example 4. The acid and
neutral wash phase from the DPC working-up were combined, adjusted
to pH 10 with NaOH, and then freed from solvent residues and
catalysts by extraction with methylene chloride or by stripping
with steam. After phase separation an aqueous phase was obtained
containing about 1 wt.-% NaCl, which can be re-used as a partial
replacement of the water for the preparation of the 14.5 wt.-% NaOH
solution for the DPC production.
[0093] It will be appreciated by those skilled in the art that
changes could be made to the embodiments described above without
departing from the broad inventive concept thereof. It is
understood, therefore, that this invention is not limited to the
particular embodiments disclosed, but it is intended to cover
modifications within the spirit and scope of the present invention
as defined by the appended claims.
* * * * *