Toluene Diisocyanate from Toluene (Liquid-Phase Phosgenation)

The process under analysis comprises three major sections: (1) Toluene Nitration; (2) DNT Reduction; and (3) TDA Phosgenation.

Toluene Nitration. The nitration of toluene occurs in two steps. In the first step Toluene reacts with nitric acid (HNO3), in the presence of sulfuric acid, to form three mononitrotoluene isomers ortho-, meta- and para-mononitrotoluenes. In the second step, nitric acid reacts with mononitrotoluene isomers in the presence of sulfuric acid to form dinitrotoluene (DNT), as indicated below. The acid concentration is controlled to prevent the formation of trinitrotoluenes. A mix of isomers result from the second step nitration, but the mixture essentially contains about 80/20 wt% of 2,4-DNT and 2,6-DNT. The effluent is sent to a caustic wash to neutralize remaining acids.

DNT Reduction. In the reduction stage, DNT is hydrogenated in the presence of hydrogen and a Raney Nickel catalyst. Methanol is used as a solvent to keep the catalyst in suspension in the reaction medium. No isomerization takes places, so the DNT isomers are converted to their corresponding diamines. After a few purification steps 80/20 TDA is directed to next section while catalysts and reagents are recycled.

TDA Phosgenation. The final stage, the phosgenation of TDA, also takes place in two steps. First, TDA reacts with a molecule of phosgene to yield toluene dicarbamyl dichloride (TDD). Then, toluene dicarbamyl dichloride is decomposed into toluene diisocyanate and hydrochloric acid. The outlet stream from the reaction zone is subsequently quenched, and most of the unreacted phosgene, hydrogen chloride and other light by-products are separated as a gaseous phase. After a few purification steps 80/20 TDI is recovered as the final product and hydrogen chloride is absorbed into water to be sold as a 32 wt% HCl solution.