Process for purifying methyl acetate mixtures

1. A process for removing acetaldehyde from a mixture of methyl acetate, dimethyl ether and acetaldehyde comprising

(i) feeding a mixture of methyl acetate and acetaldehyde and dimethyl ether to a distillation column;

(ii) distilling the mixture to generate an overhead stream depleted in acetaldehyde as compared to the feed mixture, a base stream depleted in acetaldehyde as compared to the feed mixture and a sidedraw stream enriched in acetaldehyde as compared to the feed mixture;

(iii) withdrawing from the column the sidedraw stream enriched in acetaldehyde at a point above the feed point of the feed mixture to the column; and

wherein the feed mixture to the distillation column is derived from one or more processes for carbonylating dimethyl ether with carbon monoxide in the presence of a zeolite carbonylation catalyst.

2. A process according to claim 1 which comprises a further step (iv) wherein at least a portion of the sidedraw stream withdrawn from the distillation in step (iii) comprises acetaldehyde, dimethyl ether and one or more of methyl acetate, methanol and water and is supplied as feed to a second distillation column and is distilled therein to withdraw from the column a sidedraw stream enriched in acetaldehyde as compared to the feed mixture, a heads stream comprising dimethyl ether and a base stream comprising one or more of methyl acetate, methanol and water.

3. A process according to claim 1 wherein in step (i) the feed mixture comprises methyl acetate, acetaldehyde and >0 to 50 mol % dimethyl ether.

4. A process according to claim 1 wherein in step (i) the feed mixture comprises >100 ppm up to 1 mol % acetaldehyde.

5. A process according to claim 1 wherein in step (i) the feed mixture further comprises one or more of acetic acid, water, carbon oxides and hydrogen.

6. A process according to claim 1 wherein in step (ii) the distillation column is operated at a pressure of 10 barg to 30 barg (1000 to 3000 kPa) and at a heads temperature of 40 to 90° C.

7. A process according to claim 6 wherein the distillation column is operated with a return of liquid reflux to the head of the column at a reflux to heads ratio in the range 1 to 4.

8. A process according to claim 1 wherein the distillation column of step (i) has at least 3 theoretical stages below the feed point of the feed mixture to the column.

9. A process according to claim 1 wherein in step (iii) the sidedraw stream is withdrawn from the column at or near the point of maximum concentration of acetaldehyde.

10. A process according to claim 1 wherein in step (iii) the sidedraw stream is withdrawn from the distillation column as a liquid.

11. A process according to claim 1 wherein in step (ii) the base stream comprises 100 ppm or less of acetaldehyde.

12. A process according to claim 1 wherein in step (i) there is further introduced into the distillation column a mixture of dimethyl ether, methanol, water and methyl formate and wherein the majority of the methyl formate present in the feed is removed as a component of the sidedraw stream.

13. A process according to claim 1 wherein the base stream of step (ii) or a part thereof comprising methyl acetate is utilised as a feedstock in a process for the co-production of acetic acid and dimethyl ether by the hydrolysis of methyl acetate and dehydration of methanol and in the presence of at least one catalyst.

14. A process for the co-production of acetic acid and dimethyl ether comprising:

(a) purifying a mixture of methyl acetate, acetaldehyde and dimethyl ether by:

(i) feeding the mixture of methyl acetate, acetaldehyde and dimethyl ether to a distillation column;

(ii) distilling the feed mixture to generate an overhead stream depleted in acetaldehyde as compared to the feed mixture, a base stream depleted in acetaldehyde as compared to the feed mixture and comprising methyl acetate and a sidedraw stream enriched in acetaldehyde as compared to the feed mixture;

(iii) withdrawing from the column the sidedraw stream enriched in acetaldehyde at a point above the feed point of the feed mixture to the column;

(b) feeding at least part of the base stream together with optional methanol to a reaction zone containing at least one catalyst effective to produce a crude reaction product comprising acetic acid and dimethyl ether;

(c) recovering acetic acid and dimethyl ether from the crude reaction product.

15. A process according to claim 14 which comprises a further step (iv) wherein at least a portion of the sidedraw stream withdrawn from the distillation in step (iii) comprises acetaldehyde, dimethyl ether and one or more of methyl acetate, methanol and water and is supplied as feed to a second distillation column and is distilled therein to withdraw from the column a sidedraw stream enriched in acetaldehyde as compared to the feed mixture, a heads stream comprising dimethyl ether and a base stream comprising one or more of methyl acetate, methanol and water.

16. A process according to claim 14 wherein in step (i) the feed mixture is derived from one or more processes for carbonylating dimethyl ether with carbon monoxide in the presence of a zeolite carbonylation catalyst.

17. A process according to claim 14 wherein in step (b) the at least one catalyst is a solid Brønsted acid catalyst.

18. A process according to claim 14 wherein the crude reaction product of step (b) is treated to recover an acetic-rich stream and a dimethyl ether-rich stream comprising dimethyl ether and one or more of acetaldehyde, methyl formate and formic acid, and at least a part of the dimethyl ether-rich stream is returned as a feed to the distillation column in step (i).

19. A process according to claim 14 wherein in step (b) there is supplied to the reaction zone one or more components selected from methanol, methyl acetate and water.