Process for producing acetic acid

Abstract

A production process of acetic acid according to the present invention inhibits concentration of hydrogen iodide and improves a liquid-liquid separation of an overhead from a distillation column. Acetic acid is produced by distilling a mixture containing hydrogen iodide, water, acetic acid and methyl acetate in a first distillation column (3) to form an overhead and a side cut stream or bottom stream containing acetic acid, cooling and condensing the overhead in a condenser (C3) to form separated upper and lower phases in a decanter (4). According to this process, a zone having a high water concentration is formed in the distillation column above the feed position of the mixture by feeding a mixture having a water concentration of not less than an effective amount to not more than 5% by weight (e.g., 0.5 to 4.5% by weight) and a methyl acetate concentration of 0.5 to 9% by weight (e.g., 0.5 to 8% by weight) as the mixture to the distillation column and distilling the mixture. In the zone having a high water concentration, hydrogen iodide is allowed to react with methyl acetate to produce methyl iodide and acetic acid.

The invention claimed is: 
     
       1. A process for producing acetic acid, comprising:
 distilling a mixture containing hydrogen iodide, water, methyl iodide, acetic acid, and methyl acetate to form an overhead containing a lower boiling point component, and   condensing the overhead to form separated liquid phases,   wherein the mixture contains an effective amount of water in a concentration of not more than 5% by weight and methyl acetate in a concentration of 0.5 to 9% by weight, and is separated, in the distillation step, into the overhead containing methyl iodide and a side cut stream or bottom stream containing acetic acid.   
     
     
       2. The process according to  claim 1 , wherein the mixture has a methyl acetate concentration of 0.07 to 1.2 mol/L and a water concentration of 0.28 to 2.8 mol/L, and is distilled continuously. 
     
     
       3. The process according to  claim 1 , wherein the mixture contains 0.5 to 4.5% by weight of water and 0.5 to 8% by weight of methyl acetate, and is subjected to the distillation step. 
     
     
       4. The process according to  claim 1 , wherein the mixture further contains dimethyl ether. 
     
     
       5. The process according to  claim 1 , wherein the mixture is fed to a distillation column from an intermediate or lower position of the distillation column in height. 
     
     
       6. The process according to  claim 1 , wherein a zone having a high water concentration is formed inside a distillation column at a position upper than a position at which the mixture is fed to the distillation column,
 in the zone having the high water concentration, hydrogen iodide is allowed to react with methyl acetate for producing methyl iodide and acetic acid, and   the distillation provides the overhead containing the resulting methyl iodide.   
     
     
       7. The process for producing acetic acid according to  claim 1 , wherein
 methanol is allowed to continuously react with carbon monoxide by using a catalyst containing a group 8 metal of the Periodic Table, an ionic iodide, and methyl iodide in the presence of water,   the reaction product is separated into a low-volatile phase component and a volatile phase component by a flash distillation,   the volatile phase component as the mixture is distilled to form the overhead containing methyl iodide and the side cut stream or bottom stream containing acetic acid, and   the overhead is condensed to form an aqueous phase and an organic phase,   and wherein the volatile phase component is distilled while being adjusted to a water concentration of an effective amount and not more than 5% by weight and a methyl acetate concentration of 0.5 to 9% by weight in a distillation atmosphere of the volatile phase component in terms of a condensate or liquid form.   
     
     
       8. The process according to  claim 1 , wherein at least one member selected from the group consisting of methyl acetate, methanol and dimethyl ether, and if necessary water, is added to the volatile phase component as the mixture or a distillation atmosphere thereof as the mixture to adjust the concentrations of water and methyl acetate, and the resulting volatile phase component is distilled. 
     
     
       9. The process according to  claim 1 , wherein a distillation atmosphere of a volatile phase component is formed in the distillation column at a height equal to or upper than a feed site of the volatile phase component. 
     
     
       10. The process according to  claim 1 , wherein the mixture contains 1 to 4.3% by weight of water and 0.8 to 7.5% by weight of methyl acetate, and is subjected to the distillation step. 
     
     
       11. The process according to  claim 1 , wherein the mixture has a hydrogen iodide concentration of 100 to 10000 ppm, and is subjected to a distillation to form the side cut stream having a hydrogen iodide concentration of 1 to 350 ppm. 
     
     
       12. The process according to  claim 1 , wherein the separated liquid phases are a lower phase and an upper phase, the lower phase has a methyl acetate concentration of 1 to 15% by weight, and the upper phase has a methyl acetate concentration of 0.4 to 8% by weight. 
     
     
       13. A method for improving a liquid-liquid separation of a condensate while reducing a concentration of hydrogen iodide in an overhead and a side cut stream, comprising:
 distilling a mixture containing hydrogen iodide, water, methyl iodide, acetic acid, and methyl acetate to form an overhead containing a lower boiling point component, and   condensing the overhead to give a condensate containing separated liquid phases,   wherein the mixture contains an effective amount of water in a concentration of not more than 5% by weight and methyl acetate in a concentration of 0.5 to 9% by weight.   
     
     
       14. The method according to  claim 13 , wherein the concentration of hydrogen iodide in the overhead and the side cut stream is reduced by adjusting a concentration of methyl acetate in the mixture to 0.5 to 8% by weight. 
     
     
       15. The method according to  claim 13 , wherein said method improves the liquid-liquid separation of the condensate, wherein concentrations of methyl iodide and methyl acetate in the lower phase are adjusted to 76 to 98% by weight and 1 to 15% by weight, respectively (with the proviso that the total of components in the lower phase is 100% by weight), and concentrations of water and methyl acetate in the upper phase are adjusted to 50 to 90% by weight and 0.4 to 8% by weight, respectively (with the proviso that the total of components in the upper phase is 100% by weight). 
     
     
       16. A process for producing acetic acid and for improving a liquid-liquid separation of a condensate while reducing a concentration of hydrogen iodide in an overhead and a side cut stream, comprising:
 flash evaporation or flash distillation of a reaction mixture in a flasher, wherein the reaction mixture is separated in the flasher into a volatile phase component and a low-volatile phase component;   distilling a mixture containing hydrogen iodide, water, methyl iodide, acetic acid, and methyl acetate in a first distillation column to form the overhead containing a lower boiling point component, the mixture that is distilled in the first distillation column comprises said volatile phase component;   condensing the overhead from the first distillation column to form the condensate containing separated liquid phases of an aqueous phase and an organic phase;   recycling at least one selected from the aqueous phase and the organic phase to the first distillation column for reflux; and   distilling the side cut stream in a second distillation column;   wherein the mixture comprising the volatile phase component contains an effective amount of water in a concentration of not more than 5% by weight and methyl acetate in a concentration of 0.5 to 9% by weight, and is separated, in the distillation step in the first distillation column, into the overhead containing methyl iodide, the side cut stream, and a bottom stream containing acetic acid;   wherein the overhead is withdrawn from a top of the first distillation column, the bottom stream is withdrawn from a bottom of the first distillation column, and the side cut stream is withdrawn from a side of the first distillation column and is fed to the second distillation column;   wherein the overhead from the first distillation column has a concentration of hydrogen iodide of 7 ppm to 240 ppm;   wherein the side cut stream has a concentration of hydrogen iodide of 1-350 ppm, water of 0.3 to 5% by weight, and methyl acetate of 0.1 to 3% by weight;   wherein the aqueous phase of said condensate comprises: a concentration of water of 50-85% by weight; a concentration of acetic acid of 10-40% by weight; and a concentration of hydrogen iodide of 10-1000 ppm; and   wherein the organic phase of said condensate comprises: a concentration of methyl iodide of 75-98% by weight; a concentration of acetic acid of 1-10% by weight; a concentration of water of not more than 1% by weight; and a concentration of hydrogen iodide of not more than 70 ppm.   
     
     
       17. The process according to claim 16, wherein the mixture has a methyl acetate concentration of 0.07 to 1.2 mol/L and a water concentration of 0.28 to 2.8 mol/L, and is distilled continuously. 
     
     
       18. The process according to claim 16, wherein the mixture contains 0.5 to 4.5% by weight of water and 0.5 to 8% by weight of methyl acetate, and is subjected to the distillation step. 
     
     
       19. The process according to claim 16, wherein the mixture further contains dimethyl ether. 
     
     
       20. The process according to claim 16, wherein the mixture is fed to a distillation column from an intermediate or lower position of the distillation column in height. 
     
     
       21. The process according to claim 16, wherein a zone having a high water concentration is formed inside the first distillation column at a position upper than a position at which the mixture containing the volatile phase component is fed to the first distillation column, wherein
 in the zone having the high water concentration, hydrogen iodide is allowed to react with methyl acetate for producing methyl iodide and acetic acid,   the distillation provides the overhead containing the resulting methyl iodide, and   the material of the first distillation column comprises a nickel-based alloy.   
     
     
       22. The process for producing acetic acid according to claim 16, wherein
 methanol is allowed to continuously react with carbon monoxide by using a catalyst containing a group 8 metal of the Periodic Table, an ionic iodide, and methyl iodide in the presence of water,   the reaction product is separated into a low-volatile phase component and a volatile phase component by a flash distillation,   the volatile phase component as the mixture is distilled to form the overhead containing methyl iodide and the side cut stream or bottom stream containing acetic acid, and   the overhead is condensed to form an aqueous phase and an organic phase,   and wherein the volatile phase component is distilled while being adjusted to a water concentration of an effective amount and not more than 5% by weight and a methyl acetate concentration of 0.5 to 9% by weight in a distillation atmosphere of the volatile phase component in terms of a condensate or liquid form.   
     
     
       23. The process according to claim 16, wherein at least one member selected from the group consisting of methyl acetate, methanol and dimethyl ether, and if necessary water, is added to the volatile phase component as the mixture or a distillation atmosphere thereof as the mixture to adjust the concentrations of water and methyl acetate, and the resulting volatile phase component is distilled. 
     
     
       24. The process according to claim 16, wherein a distillation atmosphere of a volatile phase component is formed in the distillation column at a height equal to or upper than a feed site of the volatile phase component. 
     
     
       25. The process according to claim 16, wherein the mixture contains 1 to 4.3% by weight of water and 0.8 to 7.5% by weight of methyl acetate, and is subjected to the distillation step. 
     
     
       26. The process according to claim 16, wherein the mixture has a hydrogen iodide concentration of 100 to 10000 ppm, and is subjected to a distillation to form the side cut stream having a hydrogen iodide concentration of 1 to 350 ppm. 
     
     
       27. The process according to claim 16, wherein the organic phase has a methyl acetate concentration of 1 to 15% by weight, and the aqueous phase has a methyl acetate concentration of 0.4 to 8% by weight. 
     
     
       28. The process according to claim 16, wherein the side cut stream is withdrawn from a site above the position of the feed line connected to the first distillation column. 
     
     
       29. The process according to claim 16, wherein at least one portion of the aqueous phase and/or the organic phase is fed to the impurity-removing system to remove the impurity. 
     
     
       30. The process according to claim 16, wherein the reflux site of the condensate in the first distillation column is located or positioned above a zone having a high hydrogen iodide concentration. 
     
     
       31. The process according to claim 16, wherein a noncondensed component obtained when the overhead from the first distillation column is condensed is fed to a scrubber system to collect a useful component. 
     
     
       32. The process according to claim 16, wherein the mixture comprising the volatile phase component contains an effective amount of water in a concentration of not more than 5% by weight, methyl acetate in a concentration of 0.5 to 9% by weight and acetic acid in a concentration of 35 to 75% by weight. 
     
     
       33. The process according to claim 16, wherein the aqueous phase of the condensate comprises a concentration of hydrogen iodide of 20 to 710 ppm, and wherein the organic phase of the condensate comprises a concentration of hydrogen iodide of not more than 50 ppm.