Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Reexamination Certificate
2000-05-05
2002-08-06
Davis, Brian J. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C564S415000, C564S437000
Reexamination Certificate
active
06429336
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
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.
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.
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.
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.
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
In one aspect, this invention is 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° C. to 350° 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.
In another aspect, this invention is 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° C. to 350° 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 strong base to reduce the pressure in the reactor to said predetermined level.
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° 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° C. to 350° C.; or (b) admixing the tar with an aqueous basic solution at a temperature of from about 200° C. to about 350° 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° C. to 350° 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 (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
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° 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.
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.
The TDI distillation residues are mixed with an excess of water, and said mixture is subjected to a temperature of from about 200° C. to 350° 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
Dai Shenghong A.
Hock Kathryn
Kent Van A.
Treybig Duane S.
Davis Brian J.
The Dow Chemical Company
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