Process for the production of acetic acid

1. A process for producing acetic acid which comprises:

(a) carbonylating methanol in the presence of a catalyst in a reaction zone to obtain a reaction zone to obtain a reaction mixture (A) comprising the acetic acid, hydrogen iodide, methyl iodide, water and the catalyst;

(b) withdrawing at least a part of the reaction mixture (A) from the reaction zone;

(c) introducing the withdrawn part of the reaction mixture (A) into a flash zone;

(d) contacting the withdrawn part of the reaction mixture (A) in the flash zone with an alkylimidazolium iodide to form a secondary mixture (B);

(e) separating the secondary mixture (B) in the flash zone to obtain a vapor stream (B V ) comprising acetic acid, water and methyl iodide, and a liquid stream (B L ) comprising the catalyst, the alkylimidazolium iodide and hydrogen iodide; and

(f) recycling the liquid stream (B L ) to the reaction zone;

wherein the reaction mixture (A) is brought into contact with the alkylimidazolium iodide in the flash zone

1) by introducing to the flash zone separately from the withdrawn reaction mixture (A) an extraneous alkylimidazolium iodide; or

2) by introducing to the flash zone separately from the withdrawn reaction mixture (A) an alkylimidazole and forming the alkylimidazolium iodide in situ by reacting the alkylimidazole with the hydrogen iodide and/or the methyl iodide.

2. The process of claim 1 , wherein the withdrawn reaction mixture (A) is brought into contact with the alkylimidazolium iodide in the flash zone by introducing the extraneous alkylimidazolium iodide to the flash zone.

3. The process of claim 2 , wherein the extraneous alkylimidazolium iodide is a compound of formula (I)

wherein x is 1, 2, 3 or 4, each R independently is C 1 -C 6 -alkyl, and X − is an iodine anion.

4. The process of claim 2 , wherein the extraneous alkylimidazolium iodide is a compound of formula (Ia)

wherein

R 1 is hydrogen or C 1 -C 6 -alkyl;

R 2 , R 3 , R 4 and R 5 , each independently, is hydrogen or C 1 -C 2 -alkyl; and

X − is an iodine anion;

and wherein at least one of R 1 , R 2 , R 3 , R 4 and R 5 is different from hydrogen, and at least one of R 1 , R 2 , R 3 , R 4 and R 5 denotes hydrogen.

5. The process of claim 4 , wherein the extraneous alkylimidazolium iodide is a hydrogen iodide or C 1 -C 6 -alkyl iodide salt of at least one alkylimidazole selected from the group consisting of 1-methylimidazole, 1-ethylimidazole, 1-propylimidazole, 1-isopropylimidazole, 1-(1-butyl)imidazole, 1-(2-butyl)imidazole, 1-isobutylimidazole, 1-tert-butylimidazole, 3-methylimidazole, 3-ethylimidazole, 4-methylimidazole, 4-ethylimidazole, 1,4-dimethylimidazole, 1,4-diethylimidazole, 1-ethyl-4-methylimidazole, 4-ethyl-1-methylimidazole, 2-methyl-1-propylimidazole, 4-methyl-1-propylimidazole, 5-methyl-1-propylimidazole, 2,4-dimethyl-1-propylimidazole, 2,5-dimethyl-1-propylimidazole, 1-isopropyl-2-methylimidazole, 1-isopropyl-4-methylimidazole, 1-isopropyl-5-methylimidazole, 2,4-dimethyl-1-isopropylimidazole, 2,5-dimethyl-1-isopropylimidazole, 1-(1-butyl)-2-methylimidazole, 1-(1-butyl)-4-methylimidazole, 1-(1-butyl)-5-methylimidazole, 1-(1-butyl)-2,4-dimethylimidazole, 1-(1-butyl)-2,5-dimethylimidazole, 1-(2-butyl)-2-methylimidazole, 1-(2-butyl)-4-methylimidazole, 1-(2-butyl)-5-methylimidazole, 1-(2-butyl)-2,4-dimethylimidazole, 1-(2-butyl)-2,5-dimethylimidazole, 1-isobutyl-2-methylimidazole, 1-isobutyl-4-methylimidazole, 1-isobutyl-5-methylimidazole, 1-isobutyl-2,4-dimethylimidazole, 1-isobutyl-2,5-dimethylimidazole, 1-tert-butyl-2-methylimidazole, 1-tert-butyl-4-methylimidazole, 1-tert-butyl-5-methylimidazole, 1-tert-butyl-2,4-dimethylimidazole, and 1-tert-butyl-2,5-dimethylimidazole.

6. The process of claim 2 , wherein the extraneous alkylimidazolium iodide is introduced to the flash zone separately and independently from any recycle stream which is recovered downstream of the flash zone.

7. The process of claim 2 , wherein the extraneous alkylimidazolium iodide is introduced to the flash zone by adding the extraneous alkylimidazolium iodide to a recycle stream recovered downstream of the flash zone and introducing the recycle stream comprising the added alkylimidazolium iodide to the flash zone.

8. The process of claim 1 , wherein the withdrawn reaction mixture (A) is brought into contact with the alkylimidazolium iodide in the flash zone by introducing the alkylimidazole to the flash zone and forming the alkylimidazolium iodide in situ.

9. The process of claim 8 , wherein the alkylimidazole is a compound of formula (II)

wherein x is 1, 2 or 3, and each R independently is C 1 -C 6 -alkyl.

10. The process of claim 8 , wherein the alkylimidazole is a compound of formula (IIa)

wherein

R 1 is hydrogen or C 1 -C 6 -alkyl;

R 2 , R 4 and R 5 , each independently, is hydrogen, or C 1 -C 2 -alkyl;

and wherein at least one of R 1 , R 2 , R 4 and R 5 is different from hydrogen, and at least one of R 1 , R 2 , R 4 and R 5 is hydrogen.

11. The process of claim 10 , wherein the alkylimidazole is 1-methylimidazole, 1-ethylimidazole, 1-propylimidazole, 1-isopropylimidazole, 1-(1-butyl)imidazole, 1-(2-butyl)imidazole, 1-isobutylimidazole, 1-tert-butylimidazole, 3-methylimidazole, 3-ethylimidazole, 4-methylimidazole, 4-ethylimidazole, 1,4-dimethylimidazole, 1,4-diethylimidazole, 1-ethyl-4-methylimidazole, 4-ethyl-1-methylimidazole, 2-methyl-1-propylimidazole, 4-methyl-1-propylimidazole, 5-methyl-1-propylimidazole, 2,4-dimethyl-1-propylimidazole, 2,5-dimethyl-1-propylimidazole, 1-isopropyl-2-methylimidazole, 1-isopropyl-4-methylimidazole, 1-isopropyl-5-methylimidazole, 2,4-dimethyl-1-isopropylimidazole, 2,5-dimethyl-1-isopropylimidazole, 1-(1-butyl)-2-methylimidazole, 1-(1-butyl)-4-methylimidazole, 1-(1-butyl)-5-methylimidazole, 1-(1-butyl)-2,4-dimethylimidazole, 1-(1-butyl)-2,5-dimethylimidazole, 1-(2-butyl)-2-methylimidazole, 1-(2-butyl)-4-methylimidazole, 1-(2-butyl)-5-methylimidazole, 1-(2-butyl)-2,4-dimethylimidazole, 1-(2-butyl)-2,5-dimethylimidazole, 1-isobutyl-2-methylimidazole, 1-isobutyl-4-methylimidazole, 1-isobutyl-5-methylimidazole, 1-isobutyl-2,4-dimethylimidazole, 1-isobutyl-2,5-dimethylimidazole, 1-tert-butyl-2-methylimidazole, 1-tert-butyl-4-methylimidazole, 1-tert-butyl-5-methylimidazole, 1-tert-butyl-2,4-dimethylimidazole, and 1-tert-butyl-2,5-dimethylimidazole.

12. The process of claim 8 , wherein the alkylimidazole is introduced to the flash zone separately and independently from any recycle stream which is recovered downstream of the flash zone.

13. The process of claim 8 , wherein the alkylimidazole is introduced to the flash zone by adding the alkylimidazole to a recycle stream recovered downstream of the flash zone and introducing the recycle stream comprising the added alkylimidazole or, where applicable, the corresponding alkylimidazolium iodide, to the flash zone.

14. The process of claim 1 , wherein the reaction mixture (A) does not comprise an alkylimidazolium iodide other than the alkylimidazolium iodide of the secondary mixture (B) which has been recycled to the reaction zone.

15. The process of claim 1 , wherein the catalyst is a rhodium catalyst.

16. The process of claim 1 , wherein the reaction mixture (A) comprises at least one catalyst stabilizer selected from the group consisting of phosphine oxides and iodides of a metal of Group 1 or 2 of the Periodic Table of the Elements.

17. The process of claim 16 , wherein the stabilizer is triphenylphosphine oxide and/or lithium iodide.

18. The process of claim 1 , wherein the reaction mixture (A) does not comprise a catalyst stabilizer selected from the group consisting of phosphine oxides and iodides of metals of Group 1 or 2 of the Periodic Table of the Elements.

19. The process of claim 1 , wherein the reaction mixture (A) comprises water in a concentration of from about 0.2% to about 10% by weight.