Process for preparing cyanohydrins and their use in the preparation of alkyl esters of methacrylic acid

1. A process for preparing acetone cyanohydrin, comprising:

reacting a reaction mixture comprising acetone and hydrocyanic acid in the presence of a catalyst in a loop reactor to form acetone cyanohydrin as the reaction product,

said loop reactor comprising

at least one cooling apparatus in which the reaction mixture is cooled,

at least one pump,

at least one mixing apparatus in which the reaction mixture is mixed, and

at least one feed orifice through which hydrocyanic acid, acetone and catalyst are fed, the catalyst being fed in downstream of the feed of acetone or hydrocyanic acid or both.

2. The process according to claim 1 , wherein the catalyst is fed in at a point in the loop reactor which is closer to an entry orifice of the reaction mixture into said at least one mixing apparatus than to a feed orifice through which hydrocyanic acid is fed or a feed orifice through which acetone is fed or both feed orifices.

3. The process according to claim 1 , wherein the catalyst is fed in at a point in the loop reactor at which the path length of the reaction mixture before entry into an entry orifice of the reaction mixture into said at least one mixing apparatus is at most 30% of the path length that the reaction mixture travels after the feeding of hydrocyanic acid and acetone before entry into an entry orifice of the reaction mixture into said at least one mixing apparatus.

4. The process according to claim 1 , wherein the residence time of the catalyst in the reaction mixture between feed and mixing in said at least one mixing apparatus is at most 10 s.

5. The process according to claim 1 , wherein the catalyst is fed in continuously or intermittently.

6. The process according to claim 1 , wherein the temperature of the reaction mixture in the loop reactor is 11 to 70° C.

7. The process according to claim 1 , wherein the catalyst used is an inorganic or an organic base.

8. The process according to claim 1 , wherein the catalyst used is an organic amine.

9. The process according to claim 1 , wherein the reaction product is withdrawn continuously or intermittently from the loop reactor, the reaction product being cooled to a temperature of 0 to 10° C. in one or more heat exchangers after leaving the loop reactor.

10. The process according to claim 9 , wherein the cooled reaction product is admixed with an acid.

11. The process according to claim 10 , wherein the reaction product is admixed with sulphuric acid.

12. The process according to claim 10 , wherein sulphuric acid with a water content of 0.1 to 8% by weight is used.

13. The process according to claim 1 , wherein a sample of the reaction mixture is withdrawn from the loop reactor at least one point and the sample is analyzed spectroscopically.

14. The process according to claim 13 , wherein the sampling is effected continuously or intermittently and the sample is fed back to the loop reactor after the analysis.

15. The process according to claim 1 , wherein the reaction is performed at a pH of about 6.5 to about 8.

16. A process for preparing acetone cyanohydrin, comprising:

reacting a reaction mixture comprising acetone and hydrocyanic acid in the presence of a catalyst in a loop reactor to form acetone cyanohydrin as the reaction product,

said loop reactor comprising

at least one cooling apparatus in which the reaction mixture is cooled,

at least one pump,

at least one mixing apparatus in which the reaction mixture is mixed, and in each case

at least one feed orifice through which hydrocyanic acid, acetone and catalyst are fed,

withdrawing said reaction product continuously or intermittently from the loop reactor, and

cooling the reaction product, after leaving the loop reactor, to a temperature of 0 to 10° C. in one or more heat exchangers and admixing said reaction product with sulphuric acid with a water content of 0.1 to 8% by weight.

17. The process according to claim 16 , wherein the catalyst is fed in downstream from the feed of acetone or hydrocyanic acid or both.

18. The process according to claim 16 , wherein the catalyst is fed in at a point in the loop reactor which is closer to an entry orifice of the reaction mixture into said at least one mixing apparatus than to a feed orifice through which hydrocyanic acid is fed or a feed orifice through which acetone is fed or both feed orifices.

19. The process according to claim 16 , wherein the catalyst is fed in at a point in the loop reactor at which the path length of the reaction mixture before entry into an entry orifice of the reaction mixture into said at least one mixing apparatus is at most 30% of the path length that the reaction mixture travels after the feeding of hydrocyanic acid and acetone before entry into an entry orifice of the reaction mixture into said at least one mixing apparatus.

20. The process according to claim 16 , wherein the residence time of the catalyst in the reaction mixture between feed and mixing in said at least one mixing apparatus is at most 10 s.

21. The process according to claim 16 , wherein the catalyst is fed in continuously or intermittently.

22. The process according to claim 16 , wherein the temperature of the reaction mixture in the loop reactor is 11 to 70° C.

23. The process according to claim 16 , wherein the catalyst used is an inorganic base or an organic base.

24. The process according to claim 16 , wherein the catalyst used is an organic amine.

25. The process according to claim 16 , wherein a sample of the reaction mixture is withdrawn from the loop reactor at least one point and the sample is analyzed spectroscopically.

26. The process according to claim 25 , wherein the sampling is effected continuously or intermittently and the sample is fed back to the loop reactor after the analysis.

27. The process according to claim 16 , wherein the reaction is performed at a pH of about 6.5 to about 8.