pH Control in Phosgene Removal in TDI Production

TDI is produced through three main reactions: nitration, hydrogenation and phosgenation. The effluent from the third reactor is refined via distillation but not before remaining phosgene and hydrogen chloride are removed in a scrubber. As the reaction of phosgene with water is very slow, typically a caustic soda solution is chosen as a scrubbing agent as the reaction is instantaneous and complete.

When using continuous scrubbers for phosgene removal, the pH control set point is usually pH 10. Maintaining this value is very important as too high a pH will reduce the solubility of sodium carbonate and sodium chloride which may cause scaling and clogging of downstream units. Too low a pH reduces phosgene absorption which is very dangerous due to the extreme toxicity of phosgene.

Learn more by downloading the Application Note.