Patent ID: 7915444

Claim:
A process for preparing a diisocyanate, the process comprising reacting a diamine with a stoichiometric excess of phosgene in a reaction zone, to form the diisocyanate, wherein the diamine, diisocyanate and phosgene are in a gaseous state in the reaction zone, in the reaction zone, a gaseous reaction mixture comprising unreacted excess phosgene, hydrogen chloride gas (HCl), and the diisocyanate is formed from the reaction of the diamine with some of the phosgene, the gaseous reaction mixture is transferred out of the reaction zone, the unreacted excess phosgene, and the hydrogen chloride gas (HCl) are separated from diisocyanate to form a precursor recirculation stream comprising the separated excess phosgene and hydrogen chloride gas, but not the diisocyanate, some of the hydrogen chloride gas is separated off from the precursor recirculation stream to form a recirculation stream comprising the separated excess phosgene and the remaining hydrogen chloride gas, but not the diisocyanate, wherein the mass fraction of the remaining hydrogen chloride gas in the recirculation stream is less than 15% by weight and at least 0.001% by weight, and the recirculation stream is at least partly recirculated to the reaction zone.