U.S. patent application number 12/001905 was filed with the patent office on 2008-06-19 for process for the preparation of toluene-diisocyanate.
This patent application is currently assigned to Bayer MaterialScience AG. Invention is credited to Wolfgang Lorenz, Lars Padeken, Bernd Pennemann, Friedhelm Steffens, Lothar Weismantel.
Application Number | 20080146835 12/001905 |
Document ID | / |
Family ID | 39204931 |
Filed Date | 2008-06-19 |
United States Patent
Application |
20080146835 |
Kind Code |
A1 |
Lorenz; Wolfgang ; et
al. |
June 19, 2008 |
Process for the preparation of toluene-diisocyanate
Abstract
The present invention relates to a process for the preparation
of toluene-diisocyanate. In this process, toluenediamine is reacted
with phosgene to give crude toluene-diisocyanate, the crude
toluene-diisocyanate is purified by distillation, the distillation
residue formed during the distillation is hydrolysed at
temperatures of less than 230.degree. C. under absolute pressures
of less than 30 bar, and the toluenediamine formed by this process
is subsequently recycled into the reaction of toluenediamine and
phosgene.
Inventors: |
Lorenz; Wolfgang; (Dormagen,
DE) ; Padeken; Lars; (Dusseldorf, DE) ;
Pennemann; Bernd; (Bergisch Gladbach, DE) ; Steffens;
Friedhelm; (Leverkusen, DE) ; Weismantel; Lothar;
(Bergisch Gladbach, DE) |
Correspondence
Address: |
BAYER MATERIAL SCIENCE LLC
100 BAYER ROAD
PITTSBURGH
PA
15205
US
|
Assignee: |
Bayer MaterialScience AG
|
Family ID: |
39204931 |
Appl. No.: |
12/001905 |
Filed: |
December 13, 2007 |
Current U.S.
Class: |
560/347 |
Current CPC
Class: |
C07C 263/10 20130101;
C07C 209/62 20130101; C07C 263/10 20130101; C07C 265/14 20130101;
C07C 211/51 20130101; C07C 209/62 20130101 |
Class at
Publication: |
560/347 |
International
Class: |
C07C 263/10 20060101
C07C263/10 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 18, 2006 |
DE |
102006060181.5 |
Claims
1. A process for the preparation of toluene-diisocyanate, which
comprises a) reacting toluenediamine with phosgene to form crude
toluene-diisocyanate, b) purifying the crude toluene-diisocyanate
by distillation to form purified toluene-diisocyanate and a mixture
comprising toluene-diisocyanate and the distillation residue of
toluene diisocyanate, c) mixing said mixture comprising
toluene-diisocyanate and the distillation residue of toluene
diisocyanate continuously with water at a temperature of less than
230.degree. C. under an absolute pressure of less than 30 bar, and
allowing said mixture and water to react under said conditions in
one or more tubular reactors connected in series to form
toluenediamine, d) optionally, purifying said toluenediamine formed
in step c), and e) recycling at least a portion of said
toluenediamine into the reaction in step a).
2. The process of claim 1, in which said mixture comprising
toluene-diisocyanate and the distillation residue of
toluene-diisocyanate which is formed in step b) comprises from 10
to 90% by weight of toluene-diisocyanate, based on 100% by weight
of the mixture.
3. The process of claim 1, in which said mixture comprising
toluene-diisocyanate and the distillation residue of
toluene-diisocyanate and said water are mixed in the weight ratio
of from 0.1:1 to 1:1.
4. The process of claim 1, in which said water employed in step c)
additionally comprises one or more bases.
5. The process of claim 4, in which said mixture comprising
toluene-diisocyanate and the distillation residue of
toluene-diisocyanate additionally comprises chlorine in the form of
one or more chlorinated inorganic compounds and/or one or more
organic compounds, and said bases are employed in a stoichiometric
excess with respect to the chlorine.
6. The process of claim 1, in which step c) forms a reaction
mixture comprising water, toluenediamine, hydrolysis residue and,
optionally, chloride salt, and in which said reaction mixture
emerging from the one tubular reactor or from the last of the
tubular reactors connected in series is first expanded, thereby
forming a liquid hydrolysis mixture and a gas mixture.
7. The process of claim 6, additionally comprising distilling said
liquid hydrolysis mixture to yield said toluenediamine.
8. The process of claim 7, in which said distillation is vacuum
distillation.
9. The process of claim 7, additionally comprising distilling said
liquid hydrolysis mixture to first remove the water, and further
distilling to yield said toluenediamine.
10. The process of claim 8, additionally comprising recycling said
water which is removed from the liquid hydrolysis mixture by
distillation into step c).
11. The process of claim 7, in which the distillation of said
liquid hydrolysis mixture additionally forms a mixture comprising
toluenediamine and hydrolysis residue, and feeding said mixture
comprising toluenediamine and hydrolysis residue, together with any
residues containing toluenediamine which are formed in the
catalytic hydrogenation of dinitrotoluene to toluenediamine, and/or
by the working up by distillation, and/or with o-toluenediamine, to
a common thermal utilization.
Description
CROSS REFERENCE TO RELATED PATENT APPLICATION
[0001] The present patent application claims the right of priority
under 35 U.S.C. .sctn. 119 (a)-(d) of German Patent Application No.
10 2006 060 181.5, filed Dec. 18, 2006.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a process for the
preparation of toluene-diisocyanate (TDI) in which toluenediamine
(TDA) is reacted with phosgene to give TDI, the resultant TDI is
purified by distillation, and the distillation residue formed
during the distillation is hydrolysed at temperatures of less than
230.degree. C. under absolute pressures of less than 30 bar, and
the resultant TDA from this procedure is subsequently recycled into
the reaction of TDA and phosgene.
[0003] The preparation of TDI by phosgenation of TDA and the
subsequent purification of the crude TDI by distillation are
generally known. All the known processes for the purification of
crude TDI by distillation have the common feature that, in addition
to the desired purified TDI, a distillation residue which must be
further treated is formed by the distillation.
[0004] The known prior art describes various processes for
treatment of the distillation residue which is formed in the
preparation of TDI. In general, the treatment of the residue aims
to maximize the yield of TDI, minimize the amount of residue which
is formed, and as much as possible, provide an appropriate
inexpensive and simple use for the amount of distillation residue
which can no longer be used in the TDI preparation process.
[0005] The following processes are known in principle:
[0006] The mixture of isocyanate product and distillation residue
can, in principle, be burned either continuously or
discontinuously. The process is technically simple and can be
employed for generation of service steam if a facility or
installation for thermal utilization suitable for this purpose
exists in the general vicinity of the isocyanate production
facility of installation, in order to ensure disposal via a
pipeline connection. The great disadvantage of this process,
however, is the loss in yield of product which is caused by
combustion of product isocyanate. Since the TDI-free or
approximately TDI-free distillation residue is solid, a combustion
process such as this requires that some of the TDI product be
present to produce a flowable stream of material to the combustion
facility.
[0007] To minimize the loss in isocyanate yield, a mixture of TDI
and the distillation residue can be transferred into a stirred and
heated container, and mixed with high-boiling hydrocarbons
(preferably bitumen) which are inert under the distillation
conditions, in order to completely distill off the free isocyanate
(or as much as is reasonably possible) that is present in the
residue. The remaining residue can be discharged as a flowable
solid and fed to a combustion facility. Disadvantages of this
process include an additional step and the use of a substance
foreign to the process (e.g. bitumen), and the more involved
handling of the residue product as a solid (as disclosed in EP
0548685 A2).
[0008] A further process for separating off the TDI residue is
characterized by the use of kneader dryers as described in U.S.
Pat. No. 5,446,196. In this process, the heated and stirred
containers described above are replaced by kneader dryers. By
using, for example, bitumen, the residue which remains is obtained
as a flowable solid, as described in the abovementioned example,
which can be employed as a fuel in, for example, cement works. The
advantage of this process over the above described process is an
increase in the yield of TDI, but the higher investment costs
required due to the more involved technique can be regarded as a
disadvantage.
[0009] Processes in which TDI distillation residues are reacted
with reactants other than water in order to obtain, in addition to
the amine employed in the phosgenation, valuable substances which
can also be used industrially, such as, for example, the reaction
of TDI residue with alkanolamine (see U.S. Pat. No. 5,902,459) or
with MDI (see DE-A-4211774, and U.S. Pat. No. 3,694,323), are also
known and described in the patent literature.
[0010] The hydrolysis of isocyanate distillation residues, and
particularly in the preparation of TDI, is a field which has been
addressed for a relatively long time. The hydrolysis of isocyanate
distillation residues is described in, for example, U.S. Pat. No.
3,128,310, U.S. Pat. No. 3,331,876, GB 795,639 and DE 2703313
A1.
[0011] In these processes, liquid or solid TDI distillation residue
is hydrolysed with water under increased pressure at elevated
temperature. During this procedure, some of the residue is
converted into the original amine, in this case TDA, which can be
recycled back into the phosgenation process after appropriate
working up, and therefore, in principle, leads to an increase in
the yield of TDI and a minimization of the residue. In some cases,
bases such as ammonia, the original amine employed and also alkali
metal hydroxide, are employed to accelerate the reaction. The
process can also be conducted in two stages, in this case with the
use of the original amine and water (as described in U.S. Pat. No.
4,654,443). The use of steam in the hydrolysis of solid residue is
also described, with temperatures of up to 400.degree. C. being
claimed (see U.S. Pat. No. 3,225,094). Acid hydrolysis of
distillation residues with subsequent drying and partial
phosgenation to give the desired isocyanate is described in U.S.
Pat. No. 3,636,030. WO 2004/108656 A1 describes the processing of
solid TDI distillation residue, which is pulverized, suspended in
water and reacted with alkali metal hydroxides, or carbonates,
under pressure of 40 to 250 bar at temperatures of 200 to
370.degree. C. The intermediate step of handling of a solid causes
difficulty in a continuous TDI process and therefore seems to be a
disadvantage here.
[0012] Multi-stage and therefore technically involved processes, or
the handling of solid residues are also necessary as described in
the processes of U.S. Pat. No. 3,499,035, U.S. Pat. No. 4,091,009
and U.S. Pat. No. 4,137,266. DE 19827086 A1 discloses a hydrolysis
process for recovery of TDA from TDI distillation residue in a
continuous flow, back-mixed reactor in the presence of hydrolysis
product. Back-mixed reactors which are mentioned are stirred tanks,
cascades of stirred tanks, a reaction mixing pump, a pumping
circulation with a static mixer and/or two-component mixing nozzle,
a jet loop reactor or a jet nozzle reactor. The reaction is carried
out under 1 to 50 bar at temperatures of 120 to 250.degree. C. The
amine obtained from the hydrolysis is in turn fed to the
phosgenation. A device and a process, inter alia, for hydrolysing
TDI distillation residue and recycling the toluenediamine recovered
into the phosgenation process are claimed in U.S. Pat. No.
6,630,517. The hydrolysis with pure water is described under a
reactor pressure of 30 to 300 bar at a reaction temperature of 190
to 370.degree. C. Working up of the reaction mixture is carried out
successively by devolatilization (i.e. separating off of the carbon
dioxide formed), dehydration and separation of the product obtained
by the hydrolysis (in this case TDA) by distillation under reduced
pressure. The reaction component comprises one or more tubular
reactors. The hydrolysis of, inter alia, TDI distillation residue
with water in a continuous process is likewise described in a
tubular reactor in U.S. Pat. No. 6,255,529. The reaction conditions
are stated as 100.degree. C. or higher under 50 bar or higher, and
the hydrolysing agent is water.
[0013] However, disadvantages of the processes mentioned above are
the sometimes high expenditure in working up of the residue, the
high loss in yield and the high consumption of energy required,
which inter alia is caused by high pressures and temperatures
during the hydrolysis.
[0014] In view of the prior art, there is a need to provide a
process for the preparation of TDI in the integrated system of
nitration of toluene, preparation of TDA, phosgenation of TDA,
working up of TDI and recycling of chlorine, in which the highest
possible yield of TDI can be achieved and the production of
residual substances which have to be disposed of is minimized.
[0015] It has now been found that this aim can be achieved by
employing a hydrolysis of the TDI distillation residue, and that
the distillation residue of the TDI working up, in a mixture with
TDI, already hydrolyses with water below a reaction pressure of 30
bar at a temperature of less than 230.degree. C., optionally with
the addition of a base, to give TDA in good yields. This TDA can be
fed back to the phosgenation process.
SUMMARY OF THE INVENTION
[0016] It was an object of the present invention to provide a
simple process for the preparation of TDI, in which the expenditure
in the working up of the residue and the energy consumption
required are minimized, while simultaneously minimizing the loss in
yield of TDI and the production of residual substances which have
to be disposed off.
[0017] The invention relates to a process for the preparation of
toluene-diisocyanate, which comprises [0018] a) reacting
toluenediamine with phosgene to form crude toluene-diisocyanate,
[0019] b) purifying the crude toluene-diisocyanate by distillation,
thus forming purified toluene-diisocyanate and a mixture containing
toluene-diisocyanate and the distillation residue of
toluene-diisocyanate, [0020] c) continuously mixing the mixture
containing toluene-diisocyanate and the distillation residue of
toluene-diisocyanate with water, at a temperature of less than
230.degree. C. under an absolute pressure of less than 30 bar, and
allowing said mixture to react under the same conditions (i.e. at a
temperature of less than 230.degree. C. under an absolute pressure
of less than 30 bar), in one or more tubular reactors connected in
series, to form toluenediamine, [0021] d) optionally purifying the
resultant toluenediamine from step c), and [0022] e) recycling at
least a portion of said toluenediamine into the reaction in step
a).
DETAILED DESCRIPTION OF THE INVENTION
[0023] The reaction of TDA with phosgene is known in principle and
is described in, for example, Ullmann's Encyclopedia of Industrial
Chemistry, 5th ed. Vol. A 19 p. 390 et seq., VCH
Verlagsgesellschaft mbH, Weinheim, 1991; and G. Oertel (ed.)
Polyurethane Handbook, 2nd edition, Hanser Verlag, Munich, 1993, p.
60 et seq.; and G. Wegener et al. Applied Catalysis A: General 221
(2001), p. 303-335, Elsevier Science B.V.
[0024] In accordance with the present invention, the reaction of
TDA and phosgene in step a) preferably takes place as follows:
[0025] TDI is prepared by reacting TDA with phosgene in process
step a). The TDA preferably originates from the hydrogenation of
dinitrotoluene (DNT). Process step a) is also called phosgenation
herein. The phosgenation reaction takes place with the formation of
hydrogen chloride as a by-product.
[0026] The synthesis of isocyanates in general, and of TDI in
particular, is known adequately from the prior art, and as a rule,
phosgene is employed in a stoichiometric excess, based on the
quantity of TDA. The phosgenation in step a) conventionally takes
place in the liquid phase as is disclosed in, for example, DE
3744001 C1 and EP 0314985 A1, which are believed to correspond to
U.S. Pat. No. 5,117,048 and U.S. Pat. No. 4,851,570, respectively,
the disclosures of which are hereby incorporated by reference, with
it being possible for the phosgene and TDA to be dissolved in a
solvent. Preferred solvents are chlorinated aromatic hydrocarbons
such as, for example, chlorobenzene, o-dichlorobenzene,
p-dichlorobenzene, trichlorobenzenes, the corresponding
chlorotoluene or chloroxylenes, chloroethylbenzene,
monochlorodiphenyl, .alpha.- and .beta.-naphthyl chloride, benzoic
acid ethyl ester, phthalic acid dialkyl esters, diisodiethyl
phthalate, toluene and xylenes. Additional examples of suitable
solvents are known and described in the prior art. As is moreover
known from the prior art, e.g. in WO-A-96/16028 which is believed
to correspond to U.S. Pat. No. 5,925,783, the disclosure of which
is hereby incorporated by reference, the isocyanate formed can also
function as a solvent for the phosgene. In another, preferred
embodiment, the phosgenation takes place above the boiling point of
the TDA. Gas phase phosgenation is described in, for example, EP
570 799 A, EP 1555258 A1, EP 1526129 A1 or DE 10161384 A1, which
are believed to correspond to U.S. Pat. No. 5,449,818, U.S. Pat.
No. 6,930,199, U.S. Published Patent Application 2005/0113601 or
U.S. Pat. No. 7,019,164, respectively, the disclosures of which are
hereby incorporated by reference. Advantages of this process over
the otherwise conventional liquid phase phosgenation lie in the
saving in energy due to the minimization of an involved solvent and
phosgene circulation.
[0027] The TDA can be reacted with phosgene in a one-stage or
two-stage, or optionally, a multi-stage reaction. In this context,
both a continuous and a discontinuous operating procedure are
possible.
[0028] If a one-stage phosgenation in the gas phase is chosen, the
reaction is carried out above the boiling temperature of TDA.
Preferably the reaction is carried out within an average contact
time of from 0.05 to 5 seconds, at temperatures of from 200.degree.
C. to 600.degree. C. as is described in DE 10161384 A1, which is
believed to correspond to U.S. Pat. No. 7,019,164, the disclosure
of which is hereby incorporated by reference.
[0029] Temperatures of from 20.degree. C. to 240.degree. C. and
pressures of from 1 bar to approx. 50 bar are conventionally
employed in the phosgenation in the liquid phase as disclosed in
U.S. Pat. No. 3,544,611, the disclosure of which is hereby
incorporated by reference. The phosgenation in the liquid phase can
be carried out in one stage or several stages, with it being
possible for phosgene to be employed in a stoichiometric excess. In
this context, the TDA solution and the phosgene solution are
preferably combined via a static mixing element and then led, for
example, from the bottom upwards through one or more reaction
towers, where the mixture reacts to yield the desired isocyanate.
In addition to reaction towers provided with suitable mixing
elements, reaction containers with a stirring device can also be
employed. Apart from static mixing elements, specific dynamic
mixing elements can also be used. Suitable static and dynamic
mixing elements are known from the prior art.
[0030] As a rule, on an industrial scale, the continuous liquid
phase isocyanate preparation is carried out in two stages. In this
context, in general in the first stage, at maximum temperatures of
220.degree. C., preferably maximum temperatures of 160.degree. C.,
the carbamoyl chloride is formed from the amine and phosgene and
the amine hydrochloride is formed from the amine and the hydrogen
chloride split off. This first stage is highly exothermic. In the
second stage, both the carbamoyl chloride is cleaved to give TDI
and hydrogen chloride, and the amine hydrochloride is converted
into the carbamoyl chloride. The second stage is as a rule carried
out at temperatures of at least 90.degree. C., preferably from
100.degree. C. to 240.degree. C. After the phosgenation in step a),
in which a crude toluene-diisocyanate is formed, the separating off
and purification of the TDI formed in the phosgenation step is
carried out in step b). This is preferably effected by first
separating the reaction mixture of the phosgenation into a liquid
product stream and a gaseous product stream in a manner known to
the person skilled in the art. The liquid product stream, i.e. the
crude toluene-diisocyanate, substantially contains TDI, the solvent
and a small portion of unreacted phosgene. The gaseous product
stream substantially comprises hydrogen chloride gas,
stoichiometrically excess phosgene and minor amounts of solvent and
inert gases, such as, for example, nitrogen and carbon monoxide.
This gaseous product stream is fed to a further working up, where
as a rule solvent, excess phosgene and the hydrogen chloride gas
formed are separated off. The solvent and excess phosgene are fed
back to the reaction for economic reasons. The hydrogen chloride
can be fed to various possible uses such as, for example, an
oxychlorination of ethylene to give ethylene dichloride or a
recycling process which recycles chorine back into the isocyanate
process. These recycling processes include catalytic oxidation of
hydrogen chloride, for example, by the deacon process, electrolysis
of gaseous hydrogen chloride and electrolysis of an aqueous
solution of hydrogen chloride (hydrochloric acid). A process for
catalytic oxidation by the Deacon process is known and described
in, WO-A-04/14845 which is believed to correspond to U.S. Pat. No.
6,916,953, the disclosure of which is hereby incorporated by
reference, and a process for gas phase electrolysis of hydrogen
chloride is known and described in WO-A-97/24320, which is believed
to correspond to U.S. Pat. No. 6,010,612, the disclosure of which
is hereby incorporated by reference. An overview of electrochemical
recycling processes is given in the article "Chlorine Regeneration
from Anhydrous Hydrogen" by Dennie Turin Mah, published in "12th
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.".
[0031] The liquid product stream, i.e. the crude
toluene-diisocyanate, is then (in general) fed to a multi-stage
working up by distillation in step b), with the still dissolved
phosgene and the solvent being separated off successively.
[0032] The distillation of the crude toluene-diisocyanate in step
b) can be carried out by generally known methods such as those
described in, for example, in EP-A-1413571 which is believed to
correspond to U.S. Pat. No. 7,108,770, the disclosure of which is
hereby incorporated by reference, and US 2003/0230476 A1, the
disclosure of which is hereby incorporated by reference. This
distillation preferably occurs by one of the three variants
described herein:
Variant 1:
[0033] Variant 1 is described in principle in Chem Systems' PERP
Report for TDI/MDI (Chem Systems, Process Evaluation Research
Planning TDI/MDI 98/99 S8, Tarrytown, N.Y., USA: Chem Systems 1999,
pp 27-32), the disclosure of which is hereby incorporated by
reference. In this variant, after distillation to separate off
phosgene has occurred, the liquid reaction mixture still has a
solvent content of greater than 50% by weight, and preferably 55 to
65% by weight. This mixture is fed to separating off the solvent,
with a solvent-TDI mixture being distilled off in a solvent
distillation column in a pre-evaporator, and a liquid bottom
product of the pre-evaporator being fed to a further processing,
the so-called working up of the residue. This liquid stream
contains, in addition to 2 to 10% by weight of the solvent, approx.
5 to 25% by weight of the distillation residue. Solvent is
distilled off in the solvent distillation column and fed back to
the process. This distillation can be carried out in one or two
stages as described in U.S. Pat. No. 6,803,438, the disclosure of
which is hereby incorporated by reference. The bottom product of
this solvent distillation still contains, in addition to TDI, from
15 to 25% by weight of solvent content. This stream is fed into a
so-called intermediate column, in which residual solvent is
distilled off, and the solvent-free bottom product is fed to a
final column, which is operated under reduced pressure and delivers
the purified marketable isocyanate TDI as the distillate. A
residue-containing part stream from the bottom of the pure column
is likewise fed to the separating off of residue. Alternatively,
the tasks of the fine and pure distillation columns can be combined
here as described in U.S. Published Patent Application 2003/0230476
A1, the disclosure of which is hereby incorporated by reference, in
a dividing wall column, a stream of low-boiling components and
solvent, a fraction of pure TDI and a product stream, as the bottom
product, containing TDI and higher-boiling components being formed.
The last product stream mentioned is in turn fed to a working up of
the distillation residue.
Variant 2:
[0034] In contrast to variant 1, in this embodiment, after
phsosgene has been separated off by distillation, the liquid
reaction mixture still contain a solvent content of less than 50%
by weight. This mixture is fed to a pre-evaporator, from which a
solvent-isocyanate mixture having a solvent content of less than
50% by weight is distilled off in a distillation column, preferably
over the head. This distillation column corresponds to the fine
column in Variant 1. The liquid bottom product from the
pre-evaporator is fed to a further processing, i.e. the so-called
working up of the residue. This liquid stream contains, in addition
to 2 to 10% by weight of solvent, approx. 5 to 20% by weight of the
distillation residue. The solvent-free bottom product of the
intermediate column is fed into the final column, which is operated
under reduced pressure and delivers the purified marketable
isocyanate TDI as the distillate. A residue-containing part stream
from the bottom of the final column is likewise fed to the
separating off of the residue. Alternatively, the tasks of these
intermediate and final distillation columns can be combined here,
as described in EP 1413571 A1 which is believed to correspond to
U.S. Pat. No. 7,108,770, the disclosure of which is hereby
incorporated by reference, in a dividing wall column, a stream of
low-boiling components and solvent, a fraction of pure TDI and a
product stream, as the bottom product, containing TDI and
higher-boiling components being obtained. The last product stream
mentioned is in turn fed to a working up of the distillation
residue.
Variant 3:
[0035] Variant 3 comprises the distillation sequences described in
variants 2 and 1 as set forth above, but without the particular
pre-evaporator mentioned, which feeds a liquid bottom product
comprising approx. 5 to 20% by weight of distillation residue to a
working up of the residue. In this case, the content of
distillation residue in the distillation sequences described is
co-fed via the liquid streams of material to the particular last
TDI purification column. This process is likewise known in
principle, as described in EP 1717223 A2, which is believed to
correspond to U.S. Published Patent Application 2007/0015934, the
disclosure of which is hereby incorporated by reference. In this
case, the distillation residue (i.e. mixture containing
toluene-diisocyanate and the distillation residue) is discharged
completely to the working up of residue from the last distillation
column.
[0036] All the known processes for purification of crude TDI by
distillation in step b) have the common feature, however, that, in
addition to the desired purified TDI from the distillation, a
mixture containing toluene-diisocyanate and distillation residue is
obtained, and that this mixture must be further treated.
[0037] The hydrolysis of the distillation residue in step c) is
carried out by a procedure in which the mixture containing
toluene-diisocyanate and the distillation residue is mixed
continuously with water at a temperature of less than 230.degree.
C. under an absolute pressure of less than 30 mbar (preferably in a
static or dynamic mixing unit, or by means of nozzles, perforated
diaphragms, etc.) and is reacted at a temperature of less than
230.degree. C. under an absolute pressure of less than 30 bar, in
one or more tubular reactors which are connected in series,
toluenediamine is obtained. In this context, the tubular reactors
employed are operated without back-mixing and a uniform dwell time
distribution is thus ensured.
[0038] The hydrolysis of the distillation residue is preferably
carried out as follows:
[0039] The pre-evaporator bottom product formed in step b) as
described in working up variants 1 and 2 above, preferably
contains, in addition to TDI and the distillation residue of TDI,
from 2.0 to 10% by weight of solvent. This stream is preferably fed
over a distillation column with a circulatory evaporator in order
to distill off the residual solvent, which can be fed back to the
preparation of TDI. The TDI-residue mixture which remains is
preferably passed over a further heat exchanger, preferably a
falling film evaporator, in order to adjust the desired ratio of
amounts of TDI and distillation residue.
[0040] If the mixture to be hydrolysed, which contains TDI and the
distillation residue of TDI, from the distillation according to
variant 3 is removed as a liquid stream from the last distillation
column, the additional separating off of residual solvent as
described above is not necessary. The mixture containing
toluene-diisocyanate and the distillation residue of TDI is then
preferably conveyed directly by means of forced delivery pumps into
the hydrolysis reactor, i.e. one or more tubular reactors connected
in series.
[0041] Regardless of the distillation process employed in step b)
and likewise regardless of the separating off of the solvent which
may be necessary, the mixture which contains toluene-diisocyanate
and the distillation residue of TDI, and which is introduced into
the hydrolysis reactor in step c) preferably contains from 10 to
90% by weight, and preferably from 40 to 60% by weight of
toluene-diisocyanate, based on 100% by weight of the mixture. The
mixture which contains toluene-diisocyanate and the distillation
residue of TDI, and which is introduced into the hydrolysis reactor
in step c) likewise preferably contains less than 1% by weight of
solvent, and preferably from 0.001 to 0.5% by weight of
solvent.
[0042] At the same time, water (preferably after being preheated in
a heat exchanger) is conveyed into the hydrolysis reactor.
Preferably, in this context, the mixture containing
toluene-diisocyanate and the distillation residue of TDI, and the
water are employed in the weight ratio of from 0.1:1 to 1:1. The
water preferably employed is fresh water, water which has been
distilled off from step c), and/or water of reaction which has been
formed in the hydrogenation of dinitrotoluene (DNT) and has been
distilled off as described in, for example, EP 1484312 A 1 which is
believed to correspond to U.S. Pat. No. 7,122,701, the disclosure
of which is hereby incorporated by reference. It is also possible
to employ steam. Usable synergies manifest themselves here in the
integrated system of nitration of toluene, preparation of TDA,
phosgenation of TDA, working up of TDI and recycling of chlorine,
in that purified water of reaction from the hydrogenation of DNT as
disclosed in EP-A-236839 which is believed to correspond to U.S.
Pat. No. 4,720,326, the disclosure of which is hereby incorporated
by reference, can be employed for the hydrolysis.
[0043] It is preferred that the water employed in step c)
additionally comprises one or more bases. Bases which are
preferably employed are aqueous sodium or potassium hydroxide
solution, meta-toluenediamine, ortho-toluenediamine,
amine-containing residue from a toluenediamine pure distillation,
ammonia or secondary compounds formed in the hydrogenation of
dinitrotoluene such as, for example, toluidines,
aminomethylcyclo-hexanes or diaminomethylcyclohexanes. The
distillation residue conventionally contains small amounts of
chlorinated inorganic compounds and/or organic compounds having
concentrations of preferably less than 1.5% by weight of chlorine,
based on 100% by weight of the mixture. These secondary components
are substantially hydrogen chloride and side chain-chlorinated and
nucleus-chlorinated isocyanates. In addition, traces of compounds
such as aromatic isocyanide dichlorides can also occur. The origin
of the chlorine in these side reactions is to be found in traces of
chlorine in the phosgene employed or in side reactions of
phosgene.
[0044] Preferably, the bases fed in with the water are employed in
a stoichiometric excess with respect to the chlorine contained in
the chlorinated inorganic compounds and/or organic compounds which
are contained in the mixture containing toluene-diisocyanate and
the distillation residue of TDI. On the one hand, the base serves
to bond the chloride, and on the other hand the base acts as a
hydrolysis catalyst, and thus allows relatively mild reaction
conditions, which are to be classified as favorable with respect to
energy consumption and apparatus requirements, particularly when
compared with examples in the literature which have been carried
out in the absence of bases such as described, for example, in U.S.
Pat. No. 6,255,529 B1, the disclosure of which is hereby
incorporated by reference.
[0045] The dwell time of the reaction in step c) is preferably up
to 50 minutes, and more preferably from 2 to 40 minutes.
[0046] A reaction mixture containing water, toluenediamine,
hydrolysis residue and where appropriate, for example, sodium salts
such as sodium chloride and sodium carbonate, is preferably formed
in the reaction in step c). Preferably, the reaction mixture
emerging from the one tubular reactor or from the last of the
tubular reactors connected in series employed in step c) is first
expanded, after the service water has been preheated in a heat
exchanged led in countercurrent, thus forming a liquid hydrolysis
mixture and a gas mixture. The reaction gases formed can be fed to
a suitable disposal, for example, a waste gas combustion
installation, or can be released into the atmosphere after
appropriate purification such as, for example, in active charcoal
absorption towers.
[0047] The isolation of TDA by distillation from the hydrolysis
mixture in step d) is preferably preceded by the separating off of
water by distillation. However, the separating off of water and the
isolation of TDA from the hydrolysis mixture can also be carried
out in a common distillation step.
[0048] The distillation in step d) can be carried out in a separate
sequence of distillation steps in which water is preferably first
separated off, and then TDA is then isolated by distillation.
However, the distillation in step d) can also be carried out in a
distillation which is in any case present such as, for example in a
distillation unit present in the preparation of TDA.
[0049] In a preferred embodiment, in step d) the excess water is
first distilled off from the liquid hydrolysis mixture obtained.
For this, the hydrolysis mixture is preferably fed to a dewatering
column with a circulatory evaporator, and optionally, one or more
post-evaporators for residual dewatering. Residual carbon dioxide
which still remains is separated off over the head in this column.
The water distilled off is condensed and partly introduced back
into the column as a reflux, in order to retain any entrained TDA.
The water reflux is led through a container which serves as a
separating bottle in order to separate out any low-boiling amine
components formed such as, for example, diaminomethylcyclohexane.
Depending on the hydrolysis reaction procedure, the amine can be
employed again as the base, or it can be combined together with the
low-boiling components separated off in the TDA dewatering of the
TDA operation in the isocyanate integrated system, for the purpose
of disposal. The low-boiling components obtained in the dewatering
stage of the hydrolysis can optionally be combined with other
amine-containing residues in the isocyanate integrated system such
as, for example, with the TDA residue from the removal of the TDA
residue, for the purpose of utilization.
[0050] The purified stream of water which is generated in the
dewatering column can be removed as waste water or can be at least
partly recycled as hydrolysis water into the hydrolysis in step
c).
[0051] In step d), toluenediamine is obtained by distillation from
the hydrolysis mixture, which has preferably been dewatered
beforehand, and is at least partly recycled into the reaction in
step a). The distillation is preferably carried out in vacuo, i.e.
under reduced pressure.
[0052] Preferably, a mixture containing from 70 to 90% by weight of
TDA and from 30 to 10% by weight of the hydrolysis residue, based
on 100% by weight of the mixture, is then conveyed at a temperature
of from 180 to 220.degree. C. into a distillation column (i.e. a
TDA purification column), which preferably serves to purify the TDA
obtained from the hydrogenation of DNT by distillation. The
distillation column is preferably operated under an absolute
pressure of less than 80 mbar. The distillation column can be
operated with an external circulatory evaporator, which is supplied
with high-pressure steam. The TDA is distilled off over the column
head and is at least partly introduced on to the column again as a
reflux. The other part is fed to a TDA storage. In all cases, the
TDA which formed by hydrolysis in step c) is at least partially
subsequently recycled into the phosgenation of TDA in step a).
[0053] The hydrolysis residue which results from the distillation
in step d) conventionally still contains from 5 to 20% by weight of
TDA, based on 100% by weight of the mixture, and is preferably
removed from the bottom of the TDA purification column. This
hydrolysis residue is then preferably fed to a disposal, which
preferably serves for thermal utilization within the isocyanate
integrated system of nitration of toluene, preparation of TDA,
phosgenation of TDA, working up of TDI and recycling of chlorine.
Preferably, the residue components obtained in the context of the
preparation of TDA and TDI, which are substantially based on liquid
amine components, such as, for example, TDI hydrolysis residue or
residue of the pure distillation of TDA, or secondary compounds
formed in the hydrogenation of dinitrotoluene, such as, for
example, toluidines, aminomethylcyclo-hexanes or
diaminomethylcyclohexanes, and any ortho-TDA which has not been
used, are fed together to a thermal utilization (e.g. by combustion
and subsequent generation of steam or heating up of substance
streams). Processes for the catalytic hydrogenation of
dinitrotoluene to give TDA and subsequent purification thereof by
distillation are known to the person skilled in the art and
described in, for example, EP-A-223 035 and EP-A-1 602 640, which
are believed to correspond to U.S. Pat. No. 4,792,626 and U.S.
Published Patent Application 2005/0263385, respectively, the
disclosures of which are hereby incorporated by reference. Therein
lies an advantage of this hydrolysis process, since it thereby
makes it possible that only the handling of one overall residue is
necessary in the total production chain of TDA and TDI, which means
a considerable simplification of the process in practice.
[0054] The advantages of the process according to the invention can
be summarized as follows:
[0055] The production of waste products from the preparation of TDI
is minimized.
[0056] The process according to the invention is simple and
economical due to the working up of the TDI distillation residue in
one or more simple tubular reactors which can be operated at low
temperatures and pressure, and which therefore can also be operated
advantageously in terms of safety.
[0057] Compared with other processes of the prior art which are
employed industrially, on the one hand, the handling of solid on
the solid distillation residue is omitted, and at the same time,
the loss of product by combustion which must be accepted when
handling solid is avoided.
[0058] Due to the preferred use of bases, the introduction of
chlorine into the product is avoided. The use of bases, in
principle, makes it possible to separate off residue-containing
streams from various stages of the working up of TDI, so that an
optimum yield is also rendered possible in this respect.
[0059] The following prophetic example further illustrates details
for the process of this invention. The invention, which is set
forth in the foregoing disclosure, is not to be limited either in
spirit or scope by this example. Those skilled in the art will
readily understand that known variations of the conditions of the
following procedure(s) can be used. Unless otherwise noted, all
temperatures are degrees Celsius.
EXAMPLES
[0060] TDI which is prepared by the phosgenation of TDA in
o-dichlorobenzene (ODB) as the solvent is purified by distillation.
For this, a liquid phase is obtained from a pre-evaporator stage of
the solvent distillation, and this is combined with the bottom
product of a TDI purification column, the so-called fine column.
The two streams have the following composition:
[0061] Bottom product of the pre-evaporator: consists of m-TDI, TDI
distillation residue, and o-dichlorobenzene.
[0062] Bottom product of the distillation column: consists of
m-TDI, and TDI distillation residue.
[0063] The two streams are introduced at the head of a stripping
column equipped with a steam-operated circulatory evaporator. In
the column, residual ODB is separated off over the head and is fed
back to the phosgenation of TDA. This stream of vapors which is
separated off is composed of ODB and m-TDI. The bottom product
removed from this column is solvent-free and is composed of m-TDI
and TDI distillation residue. This stream is then transferred into
a falling film evaporator which is operated under a vacuum and
which further concentrates the residue in this mixture. TDI which
is distilled off is fed back to the process, and the bottom product
of the falling film evaporator which contains a mixture of TDI and
TDI distillation residue is pumped to the residue hydrolysis
reaction. A buffer container is available, into which the mixture
containing TDI and the distillation residue can be placed briefly
in the event of an interruption in operation, and then fed back to
the hydrolysis process. Alternatively, the mixture of TDI and TDI
distillation residue can be fed to a thermal utilization.
[0064] The mixture containing TDI and TDI distillation residue is
pumped continuously by means of a piston membrane pump at
145.degree. C. under 25 bar into the hydrolysis reactor. The
hydrolysis reactor is a tubular reactor through which the mixture
flows from the bottom upward to the top. The hydrolysis medium
(water) is pumped into the reactor intake by means of a second
piston pump and is combined with the mixture containing TDI and TDI
distillation residue by means of a static mixing unit. The aqueous
phase, which is likewise fed continuously under 25 bar at
210.degree. C., represents a larger stream of material than the
stream of the organic phase. The aqueous phase additionally
contains sodium hydroxide in dissolved form. The hydrolysis
reaction occurs under a regulated pressure of 25 bar, and a
two-phase mixture which contains carbon dioxide, the hydrolysis
product, TDA, water and a mixture of sodium chloride and sodium
carbonate, which escapes from the reactor after expansion once the
hydrolysis reaction is complete. The expansion is preceded by a
heat exchanger, which is used to preheat the water fed into the
reactor. The expansion takes place from 25 bar to approximately
atmospheric pressure, and the reaction mixture is passed into an
expansion container upstream of the dewatering column.
[0065] From this expansion container, a gaseous and a liquid stream
are fed into the dewatering column, with gaseous product and the
reaction mixture in liquid form entering the column under the given
pressure and temperature conditions of 106.degree. C./1,400 mbar
absolute. The gaseous content is composed of water and carbon
dioxide, while the liquid stream comprises water and TDA. The
remainder of the liquid stream comprises the hydrolysis product and
the inorganic contents previously mentioned.
[0066] Carbon dioxide and water are separated off from the bottom
product of TDA and hydrolysis product. The bottom product from the
dewatering column, after passing through the circulatory
evaporator, contains water, TDA and hydrolysis product with
inorganic contents, under normal pressure at a temperature of
200.degree. C. Residual water is removed with a downstream
steam-operated tubular heat exchanger, and the mixture which
remains is led into a vacuum column, in which separation of the
hydrolysis product from TDA is carried out under an absolute
pressure of 80 mbar at a column bottom temperature of 273.degree.
C.
[0067] Purified TDA is distilled off from the vacuum column and is
fed back to the phosgenation. The hydrolysis product, as well as
the bottom product which has a residual TDA content, can be fed to
a thermal utilization.
[0068] 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.
* * * * *