Process for preparing isocyanates

1. A continuous process for preparing an isocyanate by reacting the corresponding amine with phosgene in an inert solvent in a reaction section comprising:

(a) a mixing zone for mixing amine, phosgene and inert solvent

and

(b) a reaction zone arranged downstream of the mixing zone;

at a target temperature, wherein the process comprises

(A) starting up continuous production,

(B) continuous production and

(C) shutting down continuous production

wherein (A), (B) and (C) are run successively, and wherein,

in step (A),

(I) the mixing zone and the reaction zone are at first at least partly charged

(i) with inert solvent only, then heated up to the target temperature and then additionally charged with phosgene but not with amine;

or

(ii) with inert solvent and phosgene without the amine and then heated up to the target temperature;

(II) only after step (A)(I) is the reaction zone supplied continuously with the amine and also further phosgene and further inert solvent via the mixing zone;

and, in step (C), the continuous production is shut down by first ending the supply of the amine only, while continuous supply of phosgene and inert solvent still continues.

2. The process of claim 1 , in which an additional delay device is present between the mixing zone and reaction zone.

3. The process of claim 1 , in which the reaction zone has a downstream apparatus for cleaving carbamoyl chloride.

4. The process of claim 1 , in which, after step (C), step (D) comprising displacing the phosgene from the reaction section is run, which is conducted by at first ending the supply of phosgene only, while continuous supply of inert solvent still continues.

5. The process of claim 1 , in which the target temperature has a value of 80° C. to 130° C.

6. The process of claim 1 , in which, in step (A)(I), a proportion by mass of phosgene of 0.5% to 20%, based on the total mass of phosgene and inert solvent, is established.

7. The process of claim 1 , wherein, at the end of step (A)(I), at least 50% by volume of the internal volume of the reaction zone available for the reaction of the amine with phosgene in the inert solvent is charged with the mixture of amine, phosgene and inert solvent.

8. The process of claim 1 , in which, in step (C), the supply of phosgene and inert solvent is still continued for a period of time that is chosen such that the internal volume of the reaction zone available for the reaction of the amine with phosgene in the inert solvent is run through 0.1 time to 10 times.

9. The process of claim 4 , in which, in step (D), after the supply of phosgene has ended, inert solvent is still supplied continuously for a period of time that is chosen such that the internal volume of the reaction zone available for the reaction of the amine with phosgene in the inert solvent is run through 0.1 time to 10 times.

10. The process of claim 1 , wherein, in the mixing zone, after complete displacement of the mixture of amine, phosgene and inert solvent initially charged in step A (I), a molar ratio of phosgene to primary amino groups of 1.1:1 to 30:1 is established.

11. The process of claim 1 , in which the amine is selected from the group consisting of methylenediphenyldiamine, polymethylenepolyphenylpolyamine, a mixture of methylenediphenyldiamine and polymethylenepolyphenylpolyamine, tolylenediamine, xylylenediamine, hexamethylenediamine, isophoronediamine and naphthyldiamine.

12. The process of claim 11 , in which the amine is methylenediphenyldiamine or a mixture of methylenediphenyldiamnine and polymethylenepolyphenylpolyamine or tolylenediamine.

13. The process of claim 11 , in which the amine is methylenediphenyldiamine or a mixture of methylenediphenyldiamine and polymethylenepolyphenylpolyamine.

14. The process of claim 11 , in which the amine is tolylenediamine.

15. The process of claim 1 , in which the inert solvent is selected from the group consisting of monochlorobenzene, dichlorobenzene, dioxane, toluene, xylene, methylene chloride, perchloroethylene, trichlorofluoromethane and butyl acetate.

16. The process of claim 5 , in which the target temperature is from 95° C. to 115° C.

17. The process of claim 6 , in which, in step (A)(I), a proportion by mass of phosgene of 1% to 10%, based on the total mass of phosgene and inert solvent, is established.

18. The process of claim 7 , wherein, at the end of step (A)(I), at least 80% by volume of the internal volume of the reaction zone available for the reaction of the amine with phosgene in the inert solvent is charged with the mixture of amine, phosgene and inert solvent.

19. The process of claim 7 , wherein, at the end of step (A)(I), at least 99% by volume of the internal volume of the reaction zone available for the reaction of the amine with phosgene in the inert solvent is charged with the mixture of amine, phosgene and inert solvent.

20. The process of claim 7 , wherein, at the end of step (A)(I), 100% by volume of the internal volume of the reaction zone available for the reaction of the amine with phosgene in the inert solvent is charged with the mixture of amine, phosgene and inert solvent.

21. The process of claim 8 , wherein the internal volume of the reaction zone available for the reaction of the amine with phosgene in the inert solvent is nm through 1 time to 5 times by phosgene and inert solvent.

22. The process of claim 8 , wherein the phosgene and inert solvent is in the form of a solution of phosgene in the inert solvent.

23. The process of claim 9 , wherein the internal volume of the reaction zone available for the reaction of the amine with phosgene in the inert solvent is run through 1 time to 5 times by inert solvent.

24. The process of claim 10 , wherein, in the mixing zone, after complete displacement of the mixture of amine, phosgene and inert solvent initially charged in step A (I), a molar ratio of phosgene to primary amino groups of 1.25:1 to 3:1, is established.