Hexamethylene Diisocyanate from HMDA (Phosgenation Process)

The process under analysis comprises three major sections: (1) Phosgene Generation; (2) HMDA Phosgenation and (3) Purification.

Phosgene Generation. Carbon monoxide and chlorine gas are mixed and fed to a vertical shell and tube reactor, where such components react generating phosgene. The reactor is filled with activated carbon, which serve as catalyst. The effluent from the reactors is forwarded to the phosgenation section.

HMDA Phosgenation. Phosgene from the phosgene generation and phosgene recovered downstream are fed to an absorber. In the absorber the phosgene content in the gaseous feed stream is absorbed in cold ortho-dichlorobenzene (ODCB). Part of the overhead from the absorber, mostly containing HCl by-product is fed to a jacketed agitated vessel along with hexamethylenediamine (HMDA). In this reactor, the HMDA and the HCl react forming an HMDA.HCl slurry in ODCB. Such slurry is then subjected to a phosgenation reaction, carried out in a continuous two-step tank-shaped reactor. The remaining part of the overhead from the absorber is fed to a HCl recovery system, where a 32 wt% hydrochloric acid stream is obtained as by-product.

Purification. The product stream from the phosgenation reactors is initially stripped against nitrogen, for the removal of remaining phosgene and HCl content. The product stream is then passed through a solvent recovery step, so that most of the ODCB is separated and recycled. The stream rich in HDI is then passed through a series of distillation columns for the removal of heavy impurities, yielding high-purity HDI.