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

1. 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;

II) providing a solution of phosgene in a solvent;

III) mixing the solution of the primary amine in a solvent and the solution of phosgene in a solvent to give a liquid reaction mixture having a temperature of 110° C. to 145° C. and a stoichiometric excess of phosgene, based on the amino groups of the primary amine, of 40% to 200% of theory;

IV) running the liquid reaction mixture through a reaction zone and a separation zone arranged downstream in flow direction of the reaction zone to form a gas phase having a pressure of 8.0 bar (abs.) to 50.0 bar (abs.) , where the reaction zone and the separation zone are not heated and not cooled, and where remaining liquid phase and the gas phase are removed from the separation zone separately from one another;

V) expanding the liquid phase withdrawn from the separation zone to thereby partially convert the liquid phase to the gas phase;

VI) running liquid phase that remains after the expansion through an indirectly heated reaction zone, forming a hydrogen chloride- and phosgene-containing gas phase which is removed, and an isocyanate- and solvent-containing liquid phase remaining which is withdrawn from the indirectly heated reaction zone; and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;

wherein: in step V) the liquid phase withdrawn from the separation zone is expanded by an at least two-stage expansion process comprising:

(i) a first stage of expanding the liquid phase withdrawn from the separation zone in a first gas-liquid separation vessel to form a first liquid phase and a first gas phase, and

(ii) a second stage of further expanding the first liquid phase in a second gas-liquid separation vessel to form a second liquid phase and a second gas phase,

wherein the liquid phase that remains after the expansion is the liquid phase obtained in the last stage of the expansion process.

2. The process of claim 1 , in which the solution of the primary amine in a solvent has a proportion by mass of primary amine of 25% to 50%, based on the total mass of the solution of the primary amine in a solvent, and the solution of phosgene in a solvent has a proportion by mass of phosgene of 45% to 90%, based on the total mass of the solution of phosgene in a solvent.

3. The process of claim 1 , in which the mixing of the solution of the primary amine in a solvent and the solution of phosgene in a solvent is carried out in a mixing unit that is not heated and not cooled.

4. The process of claim 1 , in which in which the mixing of the solution of the primary amine in a solvent and the solution of phosgene in a solvent is carried out in a mixing unit comprising one or more dynamic mixers.

5. The process of claim 1 , in which the reaction zone and the separation zone are disposed in a common reactor.

6. The process of claim 5 , in which the reactor is a tubular reactor in an upright arrangement.

7. The process of claim 6 , in which the reaction mixture flows upwardly through the reactor.

8. The process of claim 1 , in which the indirectly heated reaction zone is a part of a shell and tube reactor, wherein the liquid phase remaining after the expansion passes through the tube interior thereof and a heating medium passes through the tube exterior thereof, or wherein the liquid phase remaining after the expansion passes through the tube exterior thereof and a heating medium passes through the tube interior thereof.

9. The process of claim 1 , in which the expanding comprises stages (i) and (ii) only, where the expanding is conducted in such a way that the first gas phase is obtained under a pressure of 10 bar (abs.) to 20 bar (abs.) , and the second gas phase is obtained under a pressure of 1.0 bar (abs.) to 5.0 bar (abs.) .

10. The process of claim 1 , in which the gas phases obtained in steps IV), V) and VI) are worked up to obtain hydrogen chloride and phosgene and optionally solvent.

11. The process of claim 10 , in which the gas phases obtained in steps IV), V) and VI), prior to the workup, are adjusted to a common pressure and combined.

12. The process of claim 11 , in which the gas phase obtained in step IV), the second gas phase obtained in step V) and any third gas phase obtained, and the gas phase obtained in step VI) are adjusted to the pressure of the first gas phase obtained in step V).

13. The process of claim 12 , in which the combined gas phases are condensed and compressed and distilled for separation of hydrogen chloride and phosgene.

14. The process of claim 1 , in which

(i) methylene diphenylene diisocyanate and/or polymethylene polyphenylene polyisocyanate is prepared by reacting methylene diphenylene diamine and/or polymethylene polyphenylene polyamine with phosgene or

(ii) tolylene diisocyanate is prepared by reacting tolylenediamine with phosgene.

15. The process of claim 1 , wherein expanding further comprises:

(iii) in a third stage, further expanding the second liquid phase in a third gas-liquid separation vessel, forming a third liquid phase and a third gas phase.

16. The process of claim 15 , where the expanding is conducted in such a way that the first gas phase is obtained under a pressure of 15 bar (abs.) to 20 bar (abs.) , and the second gas phase is obtained under a pressure of 5.0 bar (abs.) to 10 bar (abs.) , and the third gas phase is obtained under a pressure of 1.0 bar (abs.) to 5.0 bar (abs.) .