Process for the distillation of isocyanates

In a process involving at least one liquid ring compressor for generating a vacuum in an apparatus for distillation of a crude isocyanate which was produced by phosgenation of the corresponding amine, that the improvement comprising

• choosing an operating liquid selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene and mixtures thereof for the at least one liquid ring compressor,
• applying a pressure p on a suction side of the at least one liquid ring compressor of 10 to 200 mbar(a),
• setting an operating temperature of the at least one liquid ring compressor at −17° C. to +15° C. and
• wherein the operating liquid at an exit from the at least one liquid ring compressor has an AC value of less than 35 000 ppm, the AC value referring to acidity determined according to ASTM D 5629 for AC values <100 ppm and according to ASTM D 6099 for AC values >100 ppm, wherein the operating liquid replaces the isocyanate as matrix.

A process for operating a liquid ring compressor for providing the vacuum for distillative purification of an isocyanate obtained by phosgenation of the corresponding amine in a distillation apparatus to obtain the corresponding isocyanate, wherein

• an operating liquid is selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene and mixtures thereof for the at least one liquid ring compressor,
• a pressure p on a suction side of the at least one liquid ring compressor is 10 to 200 mbar(a),
• an operating temperature of the at least one liquid ring compressor is −17° C. to +15° C. and
• the operating liquid at an exit from the at least one liquid ring compressor has an AC value of less than 35 000 ppm, the AC value referring to acidity determined according to ASTM D 5629 for AC values <100 ppm and according to ASTM D 6099 for AC values >100 ppm, wherein the operating liquid replaces the isocyanate as matrix.