Process for the production of acetic acid

1. A process for the production of acetic acid which process comprises the steps of (1) carbonylating methanol and/or a reactive derivative thereof in a first carbonylation reaction zone in a liquid reaction composition comprising iridium carbonylation catalyst, ruthenium promoter, methyl iodide co-catalyst, methyl acetate, acetic acid and water;

(2) withdrawing liquid reaction composition together with dissolved and/or entrained carbon monoxide and other gases from said carbonylation reaction zone;

(3) optionally passing said withdrawn liquid reaction composition through one or more further reaction zones to consume at least a portion of the dissolved and/or entrained carbon monoxide;

(4) passing said composition from step (2) and optional step (3) into one or more flash separation stages to form (i) a vapour fraction comprising condensable components and low pressure off-gas, the condensable components comprising acetic acid product and the low pressure off-gas comprising carbon monoxide and other gases dissolved and/or entrained with the withdrawn liquid carbonylation reaction composition and (ii) a liquid fraction comprising iridium carbonylation catalyst, ruthenium promoter and acetic acid solvent:

(5) separating the condensable components from the low pressure off-gas; and

(6) recycling the liquid fraction from the flash separation stage to the first carbonylation reaction zone, wherein the concentration of carbon monoxide in the low pressure off-gas is maintained according to the formula:

Y>mX+C

wherein Y is the molar concentration of carbon monoxide in the low pressure off-gas, X is the concentration in ppm by weight of ruthenium in the liquid reaction composition, m is about 0.012 and C is about −8.7.

2. A process according to claim 1 wherein the liquid reaction composition together with dissolved and/or entrained carbon monoxide withdrawn from the first reaction zone is passed through a second reaction zone.

3. A process according to claim 2 wherein substantially all the liquid reaction composition together with dissolved and/or entrained carbon monoxide withdrawn from the first reaction zone is passed to the second reaction zone.

4. A process according to claim 2 wherein carbon monoxide in addition to that introduced to the second reaction zone as dissolved and/or entrained carbon monoxide is introduced into the second reaction zone.

5. A process according to claim 4 wherein the additional carbon monoxide is co-joined with the first liquid reaction composition prior to introduction to the second reaction zone and/or is fed separately to one or more locations within the second reaction zone.

6. A process according to claim 4 wherein the additional carbon monoxide contains impurities.

7. A process according to claim 4 wherein the additional carbon monoxide comprises high pressure off-gas from the first reaction zone.

8. A process according to claim 4 wherein the additional carbon monoxide comprises a carbon monoxide-containing gas stream.

9. A process according to claim 2 wherein greater than 10% of the dissolved and/or entrained carbon monoxide in the liquid reaction composition withdrawn from the first reaction zone is consumed in the second reaction zone.

10. A process according to claim 2 wherein greater than 25% of the dissolved and/or entrained carbon monoxide in the liquid reaction composition withdrawn from the first reaction zone is consumed in the second reaction zone.

11. A process according to claim 2 wherein greater than 50% of the dissolved and/or entrained carbon monoxide in the liquid reaction composition withdrawn from the first reaction zone is consumed in the second reaction zone.

12. A process according to claim 1 wherein methanol and/or methyl acetate are carbonylated with carbon monoxide in the first reaction zone.

13. A process according to claim 1 wherein the concentration of methyl acetate in the liquid reaction compositions in the first and second reaction zones are independently in the range 1 to 70% by weight.

14. A process according to claim 1 wherein the concentration of water in the liquid reaction compositions in the first and second reaction zones are independently in the range 0.1 to 20% by weight.

15. A process according to claim 1 wherein the concentration of methyl iodide co-catalyst in the liquid reaction compositions in the first and second reaction zones is independently 1 to 20% by weight.

16. A process according to claim 1 wherein the concentration of iridium carbonylation catalyst in the liquid reaction compositions in the first and second reaction zones is independently in the range 100 to 6000 ppm.

17. A process according to claim 1 wherein the ruthenium promoter is present in the liquid reaction compositions in the first and second reaction zones at a molar ratio of each ruthenium promoter: iridium in the range [0.1 to 100]:1.

18. A process according to claim 1 wherein the ruthenium promoter is present in the liquid reaction compositions in the first and second reaction zones at a molar ratio of each ruthenium promoter: iridium in the range [up to 10]:1.

19. A process according to claim 1 wherein the concentration of ruthenium in the liquid reaction compositions in the first and second reaction zones is independently less than 6000 ppm.

20. A process according to claim 1 wherein the concentration of ruthenium in the liquid reaction compositions in the first and second reaction zones is independently 400 to 5000 ppm.

21. A process according to claim 1 wherein the partial pressure of carbon monoxide in the first and second reaction zones is independently in the range 1 to 70 bar.

22. A process according to claim 1 wherein the partial pressure of carbon monoxide in the first and second reaction zones is independently in the range 1 to 15 bar.

23. A process according to claim 1 wherein the concentration of carbon monoxide in the low pressure off-gas is at least 30 mol % and the concentration of ruthenium in the liquid reaction composition is up to 3000 ppm by weight.

24. A process according to claim 23 wherein the concentration of carbon monoxide in the low pressure off-gas is 30 to 40 mol % and the concentration of ruthenium in the liquid reaction composition is up to 3000 ppm by weight.

25. A process according to claim 1 the concentration of carbon monoxide in the low pressure off-gas is at least 50 mol % and the concentration of ruthenium in the liquid reaction composition is up to 6000 ppm by weight.

26. A process according to claim 25 wherein the concentration of carbon monoxide in the low pressure off-gas is 50 to 60 mol % and the concentration of ruthenium in the liquid reaction composition is up to 6000 ppm by weight.

27. A process according to claim 1 wherein the concentration of carbon monoxide in the low pressure off gas is at least 55 mol % and the ruthenium concentration in the liquid reaction composition is up to 5500 ppm by weight.

28. A process according to claim 27 wherein the concentration of carbon monoxide in the low pressure off gas is in the range 55 to 65 mol. % and the ruthenium concentration in the liquid reaction composition is up to 5500 ppm by weight.

29. A process according to claim 1 wherein the process is operated as a continuous process.