Process for the purification of mixtures of toluenediisocyanate incorporating a dividing-wall distillation column

1. A process for the production of toluenediisocyanate comprising the steps of:

a) reacting toluene diamine with phosgene resulting in a crude distillation feed,

b) separating the phosgene from the crude distillation feed from step a) when the crude distillation feed from step a) comprises 2% by weight or more of phosgene, resulting in a crude distillation feed comprising less than 2% by weight phosgene,

c) separating the crude distillation feed comprising toluene-diisocyanate, an organic solvent and less than 2% by weight phosgene in a dividing-wall distillation column into at least four product fractions P 1 –P 4 , whereby

P 1 is a phosgene enriched low-boiler product,

P 2 is a solvent-enriched product,

P 3 is a high boiler enriched bottoms and

P 4 is a toluenediisocyanate product stream.

2. The process of claim 1 wherein the phosgene enriched low-boiler product P 1 comprises 20–50% by weight phosgene and other low boilers, 20–49% by weight solvent, and noncondensible gases.

3. The process of claim 1 wherein the solvent-enriched product fraction P 2 comprises solvent with a isocyanate concentration of less than 100 ppm by weight and a phosgene concentration less than 100 ppm by weight.

4. The process of claim 1 , wherein the high boiler enriched bottoms fraction P 3 comprises toluenediisocyanate and 0.5–15% by weight high-boilers.

5. The process of claim 1 , wherein the toluenediisocyanate product stream fraction P 4 has a toluenediisocyanate concentration of at least 99.5% by weight and comprises less than 200 ppm by weight of solvent and/or chlorinated aromatic hydrocarbons, less than 100 ppm by weight hydrolyzable chlorine (HC) and less than 40 ppm by weight acid.

6. The process of claim 1 , wherein the solvent is at least one of o-dichloro-benzene, p-dichlorobenzene, chlorobenzene, toluene, benzene, nitrobenzene, anisole, or xylene.

7. The process of claim 1 , wherein after the removal of phosgene in step b) the solvent is partially removed from the crude distillation feed before feeding the crude distillation feed to the dividing-wall distillation column according to step c).

8. The process of claim 7 , wherein the solvent is partially removed to produce an isocyanate-enriched product containing from 20 to 80% by weight toluenediisocyanate.

9. The process of claim 7 , wherein the crude distillation feed from step b) is pre-evaporated before the solvent is partially removed.