Process for the co-production of acetic acid and acetic anhydride

Abstract

Continuous process for co-production of acetic acid and acetic anhydride by contacting carbon monoxide with a liquid reaction composition containing methyl acetate, dimethyl ether or a mixture thereof, a Group VIII metal catalyst, methyl iodide, acetic acid, acetic anhydride, and water in a concentration of 0.1 wt % or less, withdrawing liquid reaction composition from the reaction zone, introducing at least a portion thereof into a flash separation zone, and removing from the flash separation zone a vapor fraction containing acetic anhydride, acetic acid and methyl iodide and a liquid fraction containing acetic anhydride, and Group VIII metal catalyst. The liquid reaction composition and the withdrawn liquid reaction composition introduced into the flash separation zone contains a Group IA and/or Group IIA metal salt and the molar ratio of acetic acid to acetic anhydride in the vapor fraction removed from the flash separation zone is greater than or equal to 1.

Claims

1. A continuous process for the co-production of acetic acid and acetic anhydride which comprises the steps of: (a) contacting in a reaction zone carbon monoxide with a liquid reaction composition comprising methyl acetate or a mixture of dimethyl ether and methyl acetate, a Group VIII metal catalyst, methyl iodide, acetic acid, acetic anhydride, water in a concentration of 0.1 wt % or less; (b) withdrawing liquid reaction composition from the reaction zone and introducing at least a portion of the withdrawn liquid reaction composition into a flash separation zone; and (c) removing from the flash separation zone a vapour fraction comprising acetic anhydride, acetic acid and methyl iodide and a liquid fraction comprising acetic anhydride, and Group VIII metal catalyst; wherein the liquid reaction composition introduced into the flash separation zone comprises at least one metal salt selected from salts of Group IA and Group IIA metals and the molar ratio of acetic acid to acetic anhydride in the vapour fraction removed from the flash separation zone is maintained in the range 1.2 to 3:1; and wherein the process further comprises conducting at least a portion of the acetic acid of the vapour fraction to an esterification process in which the acetic acid is esterified with methanol to form methyl acetate, and conducting at least a portion of the methyl acetate so formed to the reaction zone for reaction in step (a).

2. A process according to claim 1, wherein the molar ratio of acetic acid to acetic anhydride in the vapour fraction removed from the flash separation zone is maintained in the range 1.2 to 2.5:1.

3. A process according to claim 1, wherein the molar ratio of acetic acid to acetic anhydride in the vapour fraction removed from the flash separation zone is maintained in the range 1.2 to 1.5:1.

4. A process according to claim 1, wherein acetic anhydride is maintained in the liquid reaction composition at a concentration in the range of about 5 to about 35% by weight.

5. A process according to claim 1, wherein the metal salt is a Group IA metal salt.

6. A process according to claim 1, wherein the Group IA and Group IIA metal is present in the liquid reaction composition at a total concentration in the range greater than zero to about 14000 ppm.

7. A process according to claim 1, wherein the metal salt is present in the liquid reaction composition in the reaction zone.

8. A process according to claim 1, wherein the acetic acid and acetic anhydride have a flash factor ratio in the range of about 0.2 to about 1.4.

9. A process according to claim 1, wherein the Group VIII metal catalyst is a rhodium catalyst.

10. A process according to claim 1, wherein the liquid reaction composition additionally comprises one or more promoters for the Group VII metal catalyst.

11. A process according to claim 1, wherein the liquid reaction composition comprises acetic anhydride maintained at a concentration of 5 to 35% by weight, acetic acid at a concentration of 20 to 50% by weight, and balance of methyl acetate, methyl iodide, metal salt, Group VIII metal catalyst and optionally one or more promoters for the Group VIII metal catalyst.

12. A process according to claim 1, wherein the flash separation zone is operated at a pressure of zero barg to 10 barg.

13. A process according to claim 1, wherein the flash separation zone is operated at a temperature of from about 50° C. to about 400° C.

14. A process according to claim 1, wherein the reaction zone is maintained at a temperature of from 150 to 220° C. and at a total pressure of from 10 to 100 bara.

15. A process according to claim 1, wherein the molar ratio of acetic acid to acetic anhydride in the vapour fraction removed from the flash separation zone is maintained in the range 1.2 to 2.8:1.

16. A process according to claim 1, wherein the metal salt is a lithium salt.

17. A process according to claim 1, wherein a portion of the acetic acid in the vapour fraction is passed through an acetic acid purification system to maintain continuous operation of said acetic acid purification system.

18. A process according to claim 1, wherein the Group IA and Group IIA metal is present in the liquid reaction composition at a total concentration in the range of about 1000 to about 6000 ppm.

19. A process according to claim 1, further comprising increasing the temperature of the liquid reaction composition withdrawn from the reaction zone before it is introduced into the flash separation zone.

20. A process according to claim 1, wherein the acetic acid and acetic anhydride have a flash factor ratio in the range of about 0.3 to about 0.75.

21. A process according to claim 1, further comprising adding metal salt to the liquid reaction composition after withdrawing the liquid reaction composition from the reaction zone but before introducing the liquid reaction composition to the flash separation zone.

22. A process according to claim 1, wherein the metal salt is a lithium salt; the liquid reaction composition comprises acetic anhydride maintained at a concentration of 5 to 35% by weight, acetic acid at a concentration of 20 to 50% by weight, and balance of methyl acetate, methyl iodide, lithium salt, Group VIII metal catalyst and optionally one or more promoters for the Group VIII metal catalyst; and the acetic acid and acetic anhydride have a flash factor ratio in the range of about 0.3 to about 0.75.

Description

REFERENCE EXAMPLE

(1) In this Reference Example a liquid reaction composition comprising acetic anhydride and acetic acid was produced by contacting in a reaction zone methyl acetate, methanol, rhodium catalyst, methyl iodide and an imidazolium salt with carbon monoxide under substantially anhydrous conditions at a temperature of about 190° C. and a pressure of about 35 barg

EXAMPLES 1 TO 3

(2) In these Examples, liquid reaction compositions were produced in accordance with the Reference Example except that varying amounts of lithium acetate were added to the reaction zone. Liquid reaction composition was withdrawn from the reaction zone and introduced to a flash separation zone operated at about 135° C. and at a pressure of about 2 barg. A vapour fraction comprising acetic anhydride and acetic acid was removed as an overhead from the flash separation zone and a liquid fraction comprising acetic anhydride, acetic acid and catalyst components was removed therefrom as a base stream.

(3) The mass of acetic acid and acetic anhydride present in the flash vapour and liquid fractions and in the withdrawn liquid reaction composition were determined by gas chromatography. The flow rates of the flash vapour and liquid fractions and the withdrawn liquid reaction composition were determined using an orifice meter.

(4) The capability of the metal salt to enhance the separation of acetic anhydride relative to acetic acid in the flash separation zone was determined from the ratio of the flash factors of acetic acid to acetic anhydride. The results obtained are given in Table 1 below.

(5) TABLE-US-00001 TABLE 1 Li Flash factor Example (ppm) ratio Reference 0 1.45 1 1000 1.14 2 3000 0.87 3 4000 0.75

(6) As can be seen from Table 1, the results demonstrate that the addition of the metal salt decreases the flash factor ratio of acetic acid to acetic anhydride and thus more acetic anhydride relative to acetic acid may be separated in the flash separation zone.

EXAMPLE 4

(7) A liquid reaction composition comprising methyl acetate, acetic acid, methyl iodide, acetic anhydride, Group VIII metal catalyst, catalyst promoter and optionally lithium iodide was continuously fed to a commercial scale reactor. Carbon monoxide gas contacted the reaction composition in the reactor by means of a sparging device to produce acetic anhydride and acetic acid. The liquid reaction mixture was continuously withdrawn from the reactor and passed via a flashing valve to a flash zone operated at a reduced pressure compared to the reactor to form an overhead vapour fraction comprising acetic acid and acetic anhydride and a liquid fraction comprising acetic acid and the catalyst components. The amounts of acetic acid, acetic anhydride produced (tpd (tonnes per day)) in the presence and absence of the metal salt are shown in Table 2.

(8) TABLE-US-00002 TABLE 2 Molar ratio of % increase Li Acetic Acid Acetic Anhydride acetic acid:acetic in acetic (ppm) (tpd) (tpd) anhydride anhydride 0 285 258 1.88 — 1500 346 335 1.25 29

(9) The results of Table 2 clearly demonstrate that the present invention provides for an increased production rate of acetic anhydride relative to acetic acid without necessitating an increase in the concentration of acetic anhydride in the reactor.

EXAMPLE 5

(10) This Example illustrates the concentration of lithium required, whilst maintaining acetic anhydride at different concentrations in a reactor, to maintain a molar ratio of acetic acid to acetic anhydride of 1 in the flash vapour fraction. Liquid reaction compositions comprising acetic anhydride (5-35% by weight), acetic acid (20-50% by weight) and balance (45% by weight) of methyl acetate, methyl iodide, lithium salt and Group VIII metal catalyst components were continuously contacted with carbon monoxide under substantially anhydrous conditions in the reactor operated at a temperature of about 190° C. and at a pressure of about 36 barg to co-produce acetic anhydride and acetic acid. The reaction compositions were supplied via a flashing valve to a flash vessel operated at a temperature of about 135° C. and at pressure of about 2 barg and separated to produce a vapour fraction comprising acetic anhydride and acetic acid and a liquid fraction comprising acetic acid, lithium and catalyst components.

(11) TABLE-US-00003 TABLE 3 Molar Ratio = 1 Acetic Anhydride Li Flash Factor (% by weight in reactor) (ppm) Ratio 5 13500 0.08 11 12347 0.15 15 11096 0.22 21 8858 0.36 25 7076 0.49 31 3946 0.76 35 1706 1.03

(12) As can be seen from Table 3, for a given standing concentration of acetic anhydride in the reactor, the addition of a metal salt and a molar ratio of acetic acid to acetic anhydride in the flash vapour of 1 increases the amount of acetic anhydride produced relative to acetic acid, as indicated by the reduced flash factor ratios.

EXAMPLE 6

(13) Example 5 was repeated utilising a combination of metal salt and molar ratios of acetic acid to acetic anhydride of 1.2, 1.8, 2.4 and 3. The results are shown in Tables 4 to 7 respectively. Tables 4 to 7 indicate, for a given standing concentration of acetic anhydride in the reactor, the amount of metal salt required to increase the amount of acetic anhydride produced relative to acetic acid, as indicated by the reduced flash factor ratios, at specified molar ratios of acetic acid to acetic anhydride in the flash vapour.

(14) TABLE-US-00004 TABLE 4 Molar Ratio = 1.2 Acetic Anhydride Li Flash Factor (% by weight in reactor) (ppm) Ratio 5 13129 0.10 11 11839 0.18 15 10378 0.26 21 7813 0.44 25 5835 0.58 31 2574 0.91 35 595 1.24

(15) TABLE-US-00005 TABLE 5 Molar ratio = 1.8 Acetic Anhydride Li Flash Factor (% by weight in reactor) (ppm) Ratio 5 12220 0.15 11 10377 0.26 15 8364 0.40 21 5068 0.65 25 2818 0.88 31 154 1.36

(16) TABLE-US-00006 TABLE 6 Molar Ratio = 2.4 Acetic Anhydride Li Flash Factor (% by weight in reactor) (ppm) Ratio 5 11342 0.20 11 9012 0.35 15 6566 0.52 21 2906 0.87 25 860 1.18 31 280 1.82

(17) TABLE-US-00007 TABLE 7 Molar Ratio = 3 Acetic Anhydride Li Flash Factor (% by weight in reactor) (ppm) Ratio 5 10496 0.26 11 7741 0.44 15 4982 0.66 21 1326 1.08 23 465 1.26