Phosgene free route to Methyl Diphenyl Diisocynate (MDI): A technical and economical evaluation

The objective is to design an alternative production process for Methyl Diphenyl Diisocyanate (MDI) in which no toxic phosgene is used. The plant should use 4,4'-Methylenedianiline (MDA) as a raw material. Seven routes found in the open literature were investigated and compared on process conditions (yield, conversion, and thermodynamics), technical and economical feasibility, and safety. Three routes remained which all involved the reaction of MDA to a carbamate intermediate and the successive thermal decomposition of this intermediate to MDI and side-products. For these routes, a systematic design procedure was applied. The design for the promising route using urea and 3-methyl-1-butanol was completed to mass and heat balances, heat integration and a preliminary design of the reactors. Uncertainties in the design were indicated. In the process design it was demonstrated that a non-phosgene route for MDI is technically feasible and economical attractive for the design of a new plant (ROI 33%, estimated margin from 20% to 50%).