Process for preparing an isocyanate by partly adiabatically operated phosgenation of the corresponding amine

A process for preparing an isocyanate by reacting a primary amine with phosgene, comprising:

• I) providing a solution of the primary amine in a solvent and adjusting the temperature of the solution of the primary amine by indirect heat transfer in a heat exchanger operated with a heat carrier medium;
• II) providing a solution of phosgene in a solvent and adjusting the temperature of the solution of phosgene by indirect heat transfer in a heat exchanger operated with a heat carrier medium;
• III) mixing the solution of the primary amine and the solution of phosgene in a mixing unit to give a reaction mixture having a stoichiometric excess of phosgene based on the amino groups of the primary amine in the range from 40% to 200% of theory;
• IV) running the reaction mixture through a reaction zone and through a separation zone that is arranged downstream in the flow direction of the reaction zone to form a gas phase under a pressure of 8.0 bar(abs.) to 50.0 bar(abs.), wherein the reaction zone and the separation zone are not heated and are not cooled, and wherein the gas phase and remaining liquid phase from the separation zone are removed separately from one another;
• V) expanding the liquid phase withdrawn from the separation zone with partial conversion of the liquid phase to the gas phase;
• VI) running the liquid phase that remains after the expansion through an indirectly heated reaction zone, thereby forming:
• (a) a hydrogen chloride and phosgene-containing gas phase which is removed, and
• (b) an isocyanate- and solvent-containing liquid phase which is withdrawn from the indirectly heated reaction zone;
• and
• VII) working up the isocyanate- and solvent-containing liquid phase to recover the solvent and obtain the isocyanate;
• where the process comprises adjusting the temperature in the reaction zone and the temperature in the separation zone by fixing a target temperature of 110° C. to 145° C. for the temperature of the reaction mixture and using the actual temperature of the reaction mixture, measured continuously or at intervals, for closed-loop control of at least one of:
• (a) the temperature of the solution of the primary amine to a temperature of 30° C. to 130° C. by means of the heat exchanger used in step I);
• and
• (b) the temperature of the solution of phosgene to a temperature of −20° C. to 120° C. by means of the heat exchanger used in step II).