Process for the preparation of isocyanates in the gas phase

A process for the preparation of m-toluene-diisocyanate comprising feeding to a reactor

• (1) a gaseous stream of phosgene and
• (2) a gaseous stream of meta-toluenediamine generated by vaporizing liquid meta-toluenediamine in at least one evaporator in which
• a) the liquid meta-toluenediamine fed to the evaporator contains less than 0.5 wt. %, based on weight of meta-toluenediamine, of toluenediamine residue, and in total less than 0.2 wt. %, based on weight of meta-toluenediamine, of ammonia and cycloaliphatic amines, and
• b) the liquid meta-toluenediamine fed to the evaporator contains less than 20 ppm, based on weight of meta-toluenediamine, of heavy metals, and
• c) a ratio of amount of liquid meta-toluenediamine V in kg present in the evaporator to amount of gas stream M in kg/h leaving the evaporator is less than 2 h, and
• d) the liquid meta-toluenediamine fed to the evaporator is partly vaporized to an extent such that at least 0.1 wt. %, based on weight of meta-toluenediamine, of the liquid meta-toluenediamine is not vaporized, and
• c) the non-vaporized meta-toluenediamine is not included with the gaseous stream fed to the reactor
• where the meta-toluenediamine is phosgenated in the gas phase to form m-toluene diisocyanate.