U.S. patent application number 09/565959 was filed with the patent office on 2002-01-24 for process for recovering toluene diamine from toluene diisocyanate distillation residues.
Invention is credited to Dai, Shenghong, Hock, Kathryn, Kent, Van A., Treybig, Duane S..
Application Number | 20020010369 09/565959 |
Document ID | / |
Family ID | 22454527 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020010369 |
Kind Code |
A1 |
Dai, Shenghong ; et
al. |
January 24, 2002 |
Process for recovering toluene diamine from toluene diisocyanate
distillation residues
Abstract
Residues from the distillation of a toluene diamine phosgenation
mixture are hydrolyzed by mixing them with water and subjecting the
mixture to a temperature of 200-350.degree. C. The hydrolysis is
performed in the presence of a sufficient amount of a base to
maintain the pressure in the reactor to no more than a
predetermined operating level. The product mixture is then
extracted to recover TDA, which can be recycled. This provides an
efficient process whereby nearly quantitative conversion of the
residues to TDA can be achieved.
Inventors: |
Dai, Shenghong; (Lake
Jackson, TX) ; Treybig, Duane S.; (Sugarland, TX)
; Hock, Kathryn; (Lake Jackson, TX) ; Kent, Van
A.; (Lake Jackson, TX) |
Correspondence
Address: |
THE DOW CHEMICAL COMPANY
INTELLECTUAL PROPERTY SECTION
P. O. BOX 1967
MIDLAND
MI
48641-1967
US
|
Family ID: |
22454527 |
Appl. No.: |
09/565959 |
Filed: |
May 5, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60132544 |
May 5, 1999 |
|
|
|
Current U.S.
Class: |
564/415 |
Current CPC
Class: |
C07C 209/52 20130101;
C07C 209/62 20130101; C07C 209/52 20130101; C07C 209/62 20130101;
C07C 211/52 20130101; C07C 211/52 20130101 |
Class at
Publication: |
564/415 |
International
Class: |
C07C 209/00 |
Claims
What is claimed is:
1. A process for treating pumpable TDI distillation residues
comprising a) forming a mixture of the TDI distillation residues
and an excess of water, and b) maintaining said mixture in a closed
reactor at a temperature of from about 200.degree. C. to
350.degree. C. for a period of time sufficient to hydrolyze at
least 95% of the TDI distillation residues to TDA, wherein step b)
is conducted in the presence of an amount of a strong base
sufficient to maintain the pressure in the reactor to a
predetermined level.
2. The process of claim 1 wherein said TDI distillation residues
have a free TDI content of about 12 to about 30% by weight.
3. The process of claim 2 wherein said base is an alkali metal
hydroxide.
4. The process of claim 3, wherein said temperature is from about
230.degree. C. to about 300.degree. C.
5. The process of claim 4, wherein the concentration of the alkali
metal hydroxide in the water is from about 7 to about 10% by
weight.
6. The process of claim 5 wherein said predetermined pressure is no
greater than 110% of the vapor pressure of water at the temperature
of the hydrolysis reaction.
7. The process of claim 5 which is operated as a continuous
process.
8. The process of claim 5 which is operated batch-wise.
9. The process of claim 5 wherein TDA is recovered from the
reaction by extraction with an organic solvent.
10. The process of claim 9 wherein TDA is recovered by separating
it from the organic solvent.
11. A process for treating pumpable TDI distillation residues,
comprising a) forming a mixture of the TDI distillation residues
and an excess of water, and b) maintaining said mixture in a closed
reactor at a temperature of from about 200.degree. C. to
350.degree. C. for a period of time sufficient to hydrolyze at
least 95% of the TDI distillation residues to TDA, c) during step
b), monitoring the pressure in the reactor and d) if the pressure
in the reactor exceeds a predetermined level, adding a sufficient
amount of a base to reduce the pressure to said predetermined
level.
12. The process of claim 11 wherein said TDI distillation residues
have a free TDI content of about 12 to about 30% by weight.
13. The process of claim 12 wherein said base is an alkali metal
hydroxide.
14. The process of claim 13, wherein said temperature is from about
230.degree. C. to about 300.degree. C.
15. The process of claim 14, wherein the concentration of the
alkali metal hydroxide in the water is from about 7 to about 10% by
weight.
16. The process of claim 15 wherein said predetermined pressure is
no greater than 110% of the vapor pressure of water at the
temperature of the hydrolysis reaction.
17. The process of claim 15 which is operated as a continuous
process.
18. The process of claim 15 which is operated batch-wise.
19. The process of claim 15 wherein TDA is recovered from the
reaction by extraction with an organic solvent and TDA is recovered
by separating it from the organic solvent.
20. A process for recovering TDA from TDI Tar comprising the steps
of: (1) a first step of admixing fresh or stabilized TDI Tar with
an aqueous basic solution in one or more vessels or reactors
wherein the admixture is prepared by either: (a) first admixing the
TDI Tar with the aqueous basic solution at a temperature of from
about the melting point of the aqueous basic solution to about
200.degree. C., with sufficient mixing so that the TDI Tar is
converted to ureas, and then heating the admixture of ureas and
aqueous basic solution to a temperature of from about 200.degree.
C. to 350.degree. C.; or (b) admixing the tar with an aqueous basic
solution at a temperature of from about 200.degree. C. to about
350.degree. C.; 2) a second step of maintaining the admixture of
TDI Tar or ureas and aqueous basic solution at a temperature of
from about 200.degree. C. to 350.degree. C. for a period of time
sufficient to hydrolyze the ureas or TDI Tar to TDA wherein the
pressure of the vessel or vessels or reactor or reactors is
maintained at about the vapor pressure of water at the reaction
temperature by (i) adding a sufficient initial quantity of base in
Step (1) to absorb all or most of the carbon dioxide produced
during the hydrolysis and (ii) adding as much additional base as
necessary to maintain the pressure of the vessel or vessels or
reactor or reactors at about the vapor pressure of water; (3) a
third step of extracting TDA by admixing the crude TDA admixture
from Step 2 with an organic solvent to produce a TDA and solvent
extract and an aqueous raffinate; and (4) a fourth step of removing
the solvent from the extract to produce TDA.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims benefit under 35 USC .sctn.120 of
U.S. Provisional Application No. 60/132,544, filed May 5, 1999.
That provisional application is hereby incorporated by reference in
its entirety.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a process for recovering toluene
diamine from toluene diisocyanate process residues. This invention
particularly relates to hydrolysis of TDI distillation residues to
toluene diamine.
[0003] Toluene diisocyanate (TDI) is commercially produced by
phosgenating a toluene diamine (TDA) solution which is then
distilled to produce a substantially pure TDI distillate and a
process residue. The residue is a very complex mixture of high
boiling materials and free TDI, which is commonly referred to as
TDI Tar or TDI distillation residues.
[0004] The TDI distillation residues have no commercial value. As a
result, these residues are usually disposed of. In order to try to
reduce the waste from these residues, various attempts have been
made to hydrolyze them and recover TDA. One such method is
disclosed in U.S. Pat. No. 4,091,009 to Cassata ('009). In
Cassata's process, TDI distillation residues are treated with
aqueous ammonia or an alkaline earth to form a granular solid.
While easier to handle than a tar-like material, the solids of this
process likely have little value and must be further treated or
disposed of in an economically and environmentally responsible
manner.
[0005] Another process for disposing of TDI distillation residues
is to hydrolyze them in the presence of aqueous ammonia as
disclosed in U.S. Pat. No. 4,137,266, also to Cassata. In this
reference, a granular solid such as that produced in '009, is
heated in the presence of aqueous ammonia resulting in a recovery
of about 60 to 70 percent of the solids as TDA. It is not always
desirable to work with gaseous reactants such as ammonia.
[0006] Still another method for dealing with TDI distillation
residues is disclosed in U.S. Pat. No. 4,654,443 to Marks et al.
Marks describes treating TDI distillation residues by contacting
them with a quantity of a polyamine to form a polyurea, and then
hydrolyzing with water. A problem with this approach is that the
additional step of adding a polyamine, even when all of that
polyamine is recovered, requires that the polyamine go through the
entire process, reducing the initial yield of the underlying TDI
process and increasing equipment and energy required for handling
the TDI distillation residues.
[0007] It would be desirable to provide an effective and relatively
inexpensive method by which TDI distillation residues can be
hydrolyzed to recover TDA in good yields.
SUMMARY OF THE INVENTION
[0008] In one aspect, this invention is a process for treating
pumpable TDI distillation residues comprising
[0009] a) forming a mixture of the TDI distillation residues and an
excess of water, and
[0010] b) maintaining said mixture in a closed reactor at a
temperature of from about 200.degree. C. to 350.degree. C. for a
period of time sufficient to hydrolyze at least 95% of the TDI
distillation residues to TDA,
[0011] wherein step b) is conducted in the presence of an amount of
a strong base sufficient to maintain the pressure in the reactor to
a predetermined level.
[0012] In another aspect, this invention is a process for treating
pumpable TDI distillation residues, comprising
[0013] a) forming a mixture of the TDI distillation residues and an
excess of water, and
[0014] b) maintaining said mixture in a closed reactor at a
temperature of from about 200.degree. C. to 350.degree. C. for a
period of time sufficient to hydrolyze at least 95% of the TDI
distillation residues to TDA,
[0015] c) during step b), monitoring the pressure in the reactor
and
[0016] d) if the pressure in the reactor exceeds a predetermined
level, adding a sufficient amount of a strong base to reduce the
pressure in the reactor to said predetermined level.
[0017] In a third aspect, this invention is a process for
recovering TDA from TDI Tar comprising the steps of: (1) a first
step (Step 1) of admixing fresh or stabilized TDI Tar with an
aqueous basic solution in one or more vessels or reactors wherein
the admixture is prepared by either: (a) first admixing the TDI Tar
with the aqueous basic solution at a temperature of from about the
melting point of the aqueous basic solution to about 200.degree.
C., with sufficient mixing so that the TDI Tar is converted to
ureas, and then heating the admixture of ureas and aqueous basic
solution to a temperature of from about 200.degree. C. to
350.degree. C.; or (b) admixing the tar with an aqueous basic
solution at a temperature of from about 200.degree. C. to about
350.degree. C.; 2) a second step (Step 2) of maintaining the
admixture of TDI Tar or ureas and aqueous basic solution at a
temperature of from about 200.degree. C. to 350.degree. C. for
period of time sufficient to hydrolyze the ureas or TDI Tar to TDA
wherein the pressure of the vessel or vessels or reactor or
reactors is maintained at about the vapor pressure of water at the
reaction temperature by (i) adding a sufficient initial quantity of
base in Step (1) to absorb all or most of the carbon dioxide
produced during the hydrolysis and (ii) adding as much additional
base as necessary to maintain the pressure of the vessel or vessels
or reactor or reactors at about the vapor pressure of water; (3) a
third step (Step 3) of extracting TDA by admixing the crude TDA
admixture from Step 2 with an organic solvent to produce a TDA and
solvent extract and an aqueous raffinate; and (4) a fourth step
(Step 4) of removing the solvent from the extract to produce
TDA.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] Residues from the distillation of a crude TDA phosgenation
mixture are used as a starting material in this invention. These
residues contain an amount of free toluene diisocyanate, along with
higher boiling by-products of the phosgenation reaction. These
by-products may include, for example, tarry TDI oligomers, ureas,
urethanes, isocyanurates, biurets, allophanates, uretdiones,
carbodiimides, urethane imines and other byproducts. Depending on
how stringent the distillation conditions are (and to some extent
the chlorine content), these by-products can advance in molecular
weight to form an intractable, nonhydrolyzable mass. The starting
material used in this invention is one that remains pumpable, which
in the context of this invention means that the residue is a
pumpable fluid at 160.degree. C. TDI distillation residues
typically remain pumpable if, during the distillation process, the
free TDI content of the residues is not reduced below about 10% by
weight. For purposes of overall efficiency of the TDI distillation
process and subsequent hydrolysis according to this invention, TDI
distillation residues having a free TDI content from about 12-30%
by weight, preferably about 14-20% by weight, most preferably about
17-20%, by weight are preferred.
[0019] The TDI distillation residues are preferably used freshly
from the distillation process, as a certain amount of undesirable
molecular weight advancement occurs upon standing. However, it is
possible to store the distillation residues for a period of time,
especially if they have been stabilized. One process for
stabilizing TDI residues is to pre-treat the fresh residues with
enough of an aqueous basic solution to substantially prevent
molecular weight advancement. Another process for stabilizing TDI
distillation residues is to mix fresh residues with a solvent,
which helps retard molecular weight advancement through simple
dilution. The solvent is preferably one that is easily separated
from the residues before conducting the hydrolysis or else does not
react under the conditions of the hydrolysis reaction. Toluene is a
preferred solvent. Toluene diisocyanate may also be used to dilute
the distillation residues in order to stabilize them, but that is
less preferred.
[0020] The TDI distillation residues are mixed with an excess of
water, and said mixture is subjected to a temperature of from about
200.degree. C. to 350.degree. C. for a period of time sufficient to
hydrolyze at least 95% of the TDI distillation residues to TDA.
Preferably, the conversion of residues to TDA is at least 98%, more
preferably at least about 99% and most preferably at least 99.8%.
An excess of water is used. Preferably at least about 5, more
preferably at least about 6 parts of water are used per part by
weight residues. The maximum amount of water is not critical, but
excessive amounts of water do not provide any advantages and
require larger and thus more expensive equipment. It is preferred
to use less than about 20 parts water, and more preferred to use
less than about 10 parts water, per part by weight residues.
[0021] If desired, the TDI distillation residues may be mixed with
water at some temperature below 200.degree. C., and the resulting
mixture heated to 200.degree. C. to 350.degree. C. If the residues
are mixed with water in this way, a preferred temperature is from
about ambient to about 135.degree. C., and a more preferred
temperature is from about 35.degree. C. to about 75.degree. C. When
the mixture is made at such lower temperatures, isocyanate groups
in the residues will typically react with water to form ureas,
which typically form a particulate dispersed in the aqueous phase.
The mixture of residues and water is preferably agitated so that
the urea forms fine particles suspended in excess water. The urea
particles preferably are formed having an average diameter of less
than 0.1 mm, more preferably less than 0.05 mm, and even more
preferably less than 0.01 mm, in order to facilitate more rapid
hydrolysis to TDA. The resulting suspension is then heated to a
temperature of from about 200.degree. C. to about 350.degree. C.
for a time sufficient to hydrolyze the particles to TDA.
Preferably, the temperature of the hydrolysis reaction is above the
melting temperature of the urea particles, i.e., above about
230.degree. C. A more preferred temperature range is from about
230.degree. C. to about 300.degree. C., and a most preferred
temperature range is from about 240.degree. C. to about 270.degree.
C.
[0022] Alternatively, the residues can be mixed with water at the
hydrolysis temperature, i.e., from about 200.degree. C. to about
350.degree. C., preferably from about 230.degree. C. to about
300.degree. C., and most preferably from about 240.degree. C. to
about 270.degree. C.
[0023] The time required to hydrolyze the residues to TDA will vary
as a function of the temperature and molecular weight of the TDI
distillation residues being hydrolyzed. The lower the temperature
used and the higher the molecular weight of the TDI distillation
residues, the longer the time needed to fully hydrolyze the
residues to TDA. Typically, hydrolysis of 95% or more of the
residues to TDA is accomplished within about 1 hour. It is more
typically accomplished within about 15 minutes, and often within 10
minutes, at the most preferred temperature of about 240 to about
270.degree. C.
[0024] The hydrolysis is performed in the presence of a strong
base. The strong base is one that reacts with carbon dioxide to
form a material that is liquid or solid at the temperature of the
hydrolysis reaction. It is preferably one that accelerates
hydrolysis reaction, as well. Preferred bases are alkali metal and
alkaline earth hydroxides. Alkali metal hydroxides are more
preferred, and sodium hydroxide and potassium hydroxide are most
preferred. The amount of base that is used is sufficient to
maintain the pressure in the hydrolysis reactor(s) to a
predetermined level. Thus, in this invention, the addition of base
is a important and sometimes a primary or even sole means of
controlling process pressures to within a desirable range.
Accordingly, this invention reduces or eliminates the need to vent
the reactor in order to maintain pressure control. As a result, the
loss of reactants such as TDA and water that often accompanies
reactor venting can be reduced or eliminated. In addition, because
reactor pressures are reduced, it is not necessary to construct the
reactor to accommodate the higher pressures. As a result, less
expensive reactors and associated equipment can be used.
[0025] Preferably, the amount of base that is used is sufficient to
maintain the pressure in the reactor to no more than about 125% of
the vapor pressure of water at the hydrolysis temperature. More
preferably, the pressure is maintained at no more than 110% of the
vapor pressure of water at the hydrolysis temperature. Even more
preferably, the pressure is maintained at no more than about 105%
of that of water and most preferably the pressure is maintained at
no more than the vapor pressure of water at the hydrolysis
temperature. Suitable operating pressures are from about 600 psi
[4137 kPa], preferably from about 700 psi [4826 kPa], more
preferably from 710 psi [4895 kPa], most preferably from about 720
psi [4964 kPa], to about 1000 psi [6895 kPa], preferably to about
900 psi [6205 kPa], more preferably to about 850 psi [5861 kPa],
most preferably to about 800 [5516 kPa].
[0026] Although the invention is not to be limited by any theory,
it is believed that the base limits pressure in the reactor by
reacting with carbon dioxide to form water-soluble carbonate and
bicarbonate salts. Carbon dioxide is generated by the hydrolysis of
TDI Tar and this carbon dioxide, in a closed system, would
significantly increase the pressure in the reactor. By reacting
with the base, the gaseous carbon dioxide is thus eliminated from
the headspace of the reactor, and thus does not contribute to the
pressure inside of the reactor. Therefore, it is preferred to
provide at least enough of the base to react with substantially all
of the carbon dioxide that is generated in the hydrolysis
reaction.
[0027] The amount of base required per a given quantity of residues
will vary with the precise composition of the residues. In the
operation of an industrial-scale TDI distillation unit, the
composition of the residues tends to change over time. Thus, the
precise amount of carbon dioxide that will be generated by
hydrolyzing the residues may vary somewhat. In general, at least
two equivalents of alkali metal hydroxide are used per NCO
equivalent contained in the residues. Particularly suitable amounts
of base are from about 2, preferably from about 7.5, more
preferably from about 10 to about 50, preferably 30, more
preferably to about 20, most preferably about 15 moles of base per
kilogram of residues.
[0028] In addition to tying up carbon dioxide, the base,
particularly alkali metal hydroxide, is believed to react with
chlorine atoms in the residue to form water-soluble salts. Alkali
metal hydroxides also provide an additional benefit of increasing
the density of the aqueous phase, thereby simplifying the
subsequent separation of the TDA from the water.
[0029] Thus, the amount of base is preferably selected together
with the amount of water so the concentration of the base in the
water is from about 6, preferably from about 6.5, most preferably
from about 7 to about 10, preferably to about 9, and to about 8
percent by weight. When used at these concentrations, the salts
produced in the hydrolysis reaction tend to stay in aqueous
solution even if the solution is cooled substantially, for example,
to 100.degree. C., preferably to 800.degree. C. or below, during
subsequent filtering and/or extraction steps.
[0030] The base may all be added when the residues and water are
first mixed together. Alternatively, the base may be fed into the
mixture in two or more increments during the course of the
hydrolysis reaction. If the residues and water are first mixed at
temperatures below about 200.degree. C., it is preferred that at
least enough base be present in the initial mixture to react with
the chlorine atoms in the residues. More preferably, most or all of
the base that is required to control the reactor pressure is
present when the residues are first mixed with the water.
[0031] In a highly preferred process, the pressure inside the
reactor is monitored as the hydrolysis proceeds. If the pressure
exceeds a predetermined level, base is added until the pressure is
reduced to below that predetermined level.
[0032] Once the hydrolysis step is completed, the TDA is
advantageously recovered from the reaction mixture. This is
conveniently done by extracting the reaction mixture with an
organic solvent to remove the TDA from the water. Preferably, the
reaction mixture is cooled to below the boiling temperature (at
atmospheric pressure) of the aqueous phase and organic solvent (but
above the saturation point for the dissolved salts and by-products)
before conducting the extraction. A preferred temperature is below
about 100.degree. C. a more preferred temperature is from about 70
to about 90.degree. C. If any solids are present, they may be
removed by any convenient solid-liquid separation technique, such
as filtration or centrifugation, before the extraction is
performed. Similarly, if some of the TDA has phase separated, it
may be decanted or otherwise removed from the aqueous phase before
the extraction is done. This allows the extraction to be performed
with smaller volumes of materials, thereby reducing the capacity
requirements of the extraction equipment.
[0033] The solvent used for the extraction of TDA is an organic
solvent that (1) is immiscible in water, (2) is a better solvent
for TDA than water is and (2) is a poorer solvent than water for
the inorganic salts and other byproducts contained in the reaction
mixture. Particularly suitable as solvents for the process of the
present invention are aromatic hydrocarbons and ethers, chlorinated
aromatic hydrocarbons and some ketones. Preferably, the solvent is
toluene, dichlorobenzene, chlorobenzene, anisole, and mixtures
thereof. Most preferably the organic solvent is anisole.
[0034] The TDA is advantageously recovered from the extracting
solvent. The method of doing this is not critical. One means of
separating TDA from the solvent is recrystallization. A preferred
method is distillation. The organic solvent removed from the
product TDA can be recycled. If the extracting solvent/TDA mixture
contains significant quantities of dissolved salts, it may be
washed with water to remove them. The wash water can also be
recycled into the process. For example, the wash water can be used
to adjust the salt concentration of the aqueous phase of the
reaction mixture before the extraction step.
[0035] If desired, the TDA may be purified further after it is
recovered. Purification in this manner provides a further safeguard
against contaminating TDA feed stocks when the recovered TDA is
recycled into a TDI production process. A convenient purification
method is distillation. Any residue from this step (or other steps)
can be discarded.
[0036] The process of the present invention can be done batch-wise
or continuously. In either approach, the equipment used is selected
to accommodate the pressures and environments of the process. For
example, all vessels, reactors, exchangers and other equipment that
is exposed to the aqueous basic solution of the process of the
present invention is selected to be appropriate for being in
contact with that solution under the reaction conditions of the
process. One of ordinary skill in the art of conducting commercial
scale chemical reactions such as the hydrolysis of TDI Tar to TDA
will know how to select, size, assemble and operate the apparatus
necessary to practice the process of the present invention.
Particularly suitable equipment for the hydrolysis reaction
includes continuous stirred tank reactors, tubular reactors,
stirred tank reactors, and the like. For purposes of the present
invention, the terms vessel and reactor are interchangeable and may
include mixing by stirring or other forms of agitation even when
the process of the present invention does not expressly require
mixing. Also for purposes of the present invention, it is
anticipated that the steps of process of the present invention can
be performed in more that one vessel per step. For example, the
hydrolysis step could be done in a series of 2 or more vessels.
[0037] The following examples are provided to illustrate the
present invention. The examples are not intended to limit the scope
of the present invention and should not be so interpreted. Amounts
are in weight parts or weight percentages unless otherwise
indicated.
EXAMPLE 1
[0038] 51.3 ml/minute of a 7.5 percent aqueous solution of sodium
hydroxide is fed into a 600 ml PARR* reactor equipped with an
agitator (*PARR is a trade designation of Parr Instrument Company).
Simultaneously, 7.6 ml/minute of a mixture TDI distillation
residues containing about 14 percent TDI is fed into the 600 ml
PARR reactor. The initial temperature of the sodium hydroxide
solution is 180.degree. C. and the initial temperature of the TDI
Tar is 80.degree. C. The PARR reactor is maintained at a
temperature of about 265.degree. C. and a pressure of about 750
psig [5171 kPa], or about 104% of the vapor pressure of water at
that temperature. The residence time in the 600 ml PARR reactor is
about 7 minutes after which the effluent from the 600 ml reactor
flows into a second PARR Reactor having a capacity of 1 L and also
held at a temperature of about 265.degree. C. and a pressure of
about 750 psig [5171 kPa]. The 1 L reactor is level controlled so
that the reaction mixture has a residence time of 7 minutes in the
second reactor. The effluent from this reactor is cooled to
80.degree. C. and then extracted with anisole. The TDA/anisole
extract is then distilled to remove the anisole. The product TDA is
itself then distilled to insure purity.
[0039] The expected yield from these distillation residues,
assuming 100% hydrolysis (i.e., assuming the residues contain no
non-hydrolyzable species), is 63.5 grams per 100 grams of TDI
distillation residues. 63 grams of TDA are recovered which
corresponds to 99.2% conversion of distillation residues to
TDA.
EXAMPLE 2
[0040] The process of Example 1 was substantially reproduced using
a different batch of TDI distillation residues. In this case, the
expected yield is about 64 grams per 100 grams residues. 64 grams
of TDA are recovered from 100 grams of the residues, which
corresponds to essentially a 100% conversion of distillation
residues to TDA.
* * * * *