Process for preparation of high purity methacrylic acid

1. A process for preparation of pure methacrylic acid, comprising:

a) carrying out gas phase oxidation of a C 4 compound to obtain a methacrylic acid-comprising gas phase,

b) condensing the methacrylic acid-comprising gas phase to obtain an aqueous methacrylic acid solution,

c) separating at least a part of the methacrylic acid from the aqueous methacrylic acid solution to obtain at least one crude methacrylic acid-comprising product; and

d) separating at least a part of the methacrylic acid from the at least one crude methacrylic acid-comprising product by a thermal separation process to obtain a pure methacrylic acid;

wherein said thermal separation process is a distillation or rectification;

wherein in d), the methacrylic acid is separated by rectification from at least a part of the at least one crude methacrylic acid-comprising product, wherein the pure methacrylic acid is removed in a side outlet from a column used for the rectification; and

wherein the amount of pure methacrylic acid removed in a defined time interval is 40 to 80% of the amount of the amount of crude methacrylic acid-comprising product introduced into the rectification column in the same time interval.

2. The process according to claim 1 , further comprising in c):

c1α) extracting the methacrylic acid from the aqueous methacrylic acid solution by an organic extraction agent to obtain an aqueous phase and an organic phase,

c1β) separating at least a part of the organic extraction agent from the organic phase by at least one thermal separation process to obtain at least one methacrylic acid-comprising bottom product as the crude methacrylic acid-comprising product.

3. The process according to claim 2 , wherein the organic extraction agent used in c1α) has a boiling point of less than 161° C., measured at atmospheric pressure.

4. The process according to claim 1 , further comprising in c):

c2α) crystallizing at least a part of the methacrylic acid from the aqueous methacrylic acid solution,

c2β) optionally, washing the crystallised methacrylic acid, and

c2g) melting at least a part of the crystallised methacrylic acid to obtain at least one crude methacrylic acid-comprising product.

5. The process according to claim 1 , wherein the at least one crude methacrylic acid-comprising product introduced into d) is obtained by distillation or crystallisation of a composition which comprises at most 95 wt. %, methacrylic acid.

6. The process according to claim 1 , wherein the at least one crude methacrylic acid-comprising product introduced into d) is obtained by distillation or crystallisation of a composition which comprises at least 5 wt. % of a C 4 -C 8 hydrocarbon.

7. The process according to claim 1 , wherein the at least one crude methacrylic acid-comprising product introduced into d) has an APHA number according to DIN ISO 6271 of at least 100.

8. The process according to claim 1 , wherein the rectification in d) is carried out at a bottom pressure in a range from 1 to 100 mbar.

9. The process according to claim 1 , wherein the rectification in d) is carried out at a bottom temperature in a range from 40 to 200° C.

10. The process according to claim 1 , wherein the rectification in d) is carried out at a bottom temperature of less than 90° C.

11. The process according to claim 1 , wherein the rectification column has more than 0 and up to 10 theoretical plates per meter.

12. The process according to claim 1 , wherein the pure methacrylic acid obtained in d) is removed at a height in a range between a lower fourth and an upper fourth of the rectification column.

13. The process according to claim 1 , wherein the at least one crude methacrylic acid-comprising product introduced into d) is subjected to a flow resistance at or after entering into the rectification column.

14. The process according to claim 13 , wherein the at least one crude methacrylic acid-comprising product entering the column impinges on the flow resistance at an angle in the range from 60° to 120°, based on a main flow direction at the flow resistance of the crude methacrylic acid-comprising product entering the column.

15. The process according to claim 1 , wherein the at least one crude methacrylic acid-comprising product obtained in c) is introduced into the bottom of the rectification column.

16. The process according to claim 1 , wherein the at least one crude methacrylic acid-comprising product obtained in c) is introduced into the rectification column from 1 to 5 theoretical floors below the side outlet.

17. The process according to claim 1 , wherein the thermal separation in d) is carried out in the presence of a decolourisation agent.

18. The process according to claim 17 , wherein the decolourisation agent is an aminoguanidinium salt.

19. The process according to claim 18 , wherein the aminoguanidinium salt is aminoguanidinium bicarbonate.

20. The process according to claim 17 , wherein the decolourisation agent is added

A) to the at least one crude methacrylic acid-comprising product obtained in c) before the thermal separation in d), or

B) during the thermal separation in d).

21. The process according to claim 1 , wherein in d), less than 5 wt. %, impurities different from methacrylic acid are precipitated from the at least one crude methacrylic acid-comprising product obtained in c).

22. The process according to claim 1 , wherein at least d) is carried out continuously.

23. The process according to claim 1 , wherein the process is carried out in a device, wherein the device comprises at least the following components in fluid-conducting communication with each other:

a1) a gas phase oxidation unit;

b1) an absorption unit;

c1) a separation unit; and

d1) a purification unit;

wherein the purification unit comprises at least one distillation column and the at least one distillation column comprises at least one side outlet for pure methacrylic acid.

24. A process for preparation of methacrylic acid esters, comprising

A) preparing pure methacrylic acid by a process according to claim 1 , and

B) esterifying the pure methacrylic acid.

25. A process for preparation of a polymethacrylate, comprising

i) preparing pure methacrylic acid by a process according to claim 1 , and

ii) carrying out radical polymerisation of the pure methacrylic acid, optionally in the presence of monomers which are co-polymerisable with methacrylic acid.

26. A process for preparation of a polymethacrylic acid ester, comprising

i) preparing a methacrylic acid ester by a process according to claim 24 , and

ii) carrying out radical polymerisation of the methacrylic acid ester, optionally in the presence of monomers which are co-polymerisable with methacrylic acid esters.

27. The process of claim 1 , wherein said pure methacrylic acid is a methacrylic acid which comprises less than 1 wt. % impurities, based on the total weight of methacrylic acid and impurities.

28. The process of claim 1 , wherein said pure methacrylic acid is a methacrylic acid which comprises less than 0.8 wt. % impurities, based on the total weight of methacrylic acid and impurities.

29. The process of claim 1 , wherein said pure methacrylic acid is a methacrylic acid which comprises less than 0.5 wt. % impurities, based on the total weight of methacrylic acid and impurities.

30. The process of claim 1 , wherein said pure methacrylic acid is a methacrylic acid which comprises less than 0.3 wt. % impurities, based on the total weight of methacrylic acid and impurities.