PROCESS

Abstract

A method of operating a heavy ends column in an acetic acid production unit, said production unit comprising at least a reaction section, a light ends recovery section comprising a light ends distillation column, and a heavy ends column, wherein a stream comprising acetic acid and propionic acid obtained from the light ends recovery section is fed to the heavy ends column through a feed inlet positioned at an intermediate point in the heavy ends column, a product stream comprising essentially acetic acid is withdrawn from the heavy ends column through a sidedraw product outlet position above the feed inlet, and a product stream comprising the propionic acid is withdrawn from the heavy ends column through a heavy product outlet positioned below the feed inlet, wherein the heavy ends column is operated under conditions such that the pressure in the heavy ends column above the feed inlet is lower than the pressure of the stream comprising acetic acid and propionic acid that is fed to the heavy ends column, and wherein the head pressure of the heavy ends column is below 1.0 bara, the number of theoretical separation stages between the feed inlet and the sidedraw product outlet is at least 5, preferably at least 7, more preferably at least 10, and the pressure drop in the part of the heavy ends column between the feed inlet and the sidedraw product outlet is at most 10 mbar per theoretical separation stage.

Claims

1-15. (canceled)

16. A method of operating a heavy ends column in an acetic acid production unit, said production unit comprising at least a reaction section, a light ends recovery section comprising a light ends distillation column, and a heavy ends column, wherein a stream comprising acetic acid and propionic acid obtained from the light ends recovery section is fed to the heavy ends column through a feed inlet positioned at an intermediate point in the heavy ends column, a product stream comprising essentially acetic acid is withdrawn from the heavy ends column through a sidedraw product outlet position above the feed inlet, and a product stream comprising the propionic acid is withdrawn from the heavy ends column through a heavy product outlet positioned below the feed inlet, wherein the heavy ends column is operated under conditions such that the pressure in the heavy ends column above the feed inlet is lower than the pressure of the stream comprising acetic acid and propionic acid that is fed to the heavy ends column, and wherein the head pressure of the heavy ends column is below 1.0 bara, the number of theoretical separation stages between the feed inlet and the sidedraw product outlet is at least 5, preferably at least 7, more preferably at least 10, and the pressure drop in the part of the heavy ends column between the feed inlet and the sidedraw product outlet is at most 10 mbar per theoretical separation stage.

17. A method according to claim 16, wherein the pressure in the heavy ends column at the feed inlet position is below 1.0 bara.

18. A method according to claim 16, wherein the heavy ends column comprises packing in the part of the heavy ends column between the feed inlet and the sidedraw product outlet.

19. A method according to claim 16, wherein the heavy ends column comprises trays in the part of the heavy ends column below the feed inlet.

20. A method according to claim 16, wherein the heavy ends column comprises trays in the part of the heavy ends column above the sidedraw product outlet.

21. A method according to claim 16, wherein the head pressure of the heavy ends column is below 0.9 bara, preferably below 0.8 bara, more preferably below 0.7 bara.

22. A method according to claim 16, wherein the head pressure of the heavy ends column is in the range of from 0.3 to 0.9 bara, preferably in the range of from 0.4 to 0.8 bara, more preferably in the range of from 0.5 to 0.7 bara.

23. A method according to claim 16, wherein the number of theoretical stages between the feed inlet and the sidedraw product outlet is in the range of from 10 to 20, preferably in the range of from 12 to 16.

24. A method according to claim 16, wherein the pressure drop in the part of the heavy ends column between the feed inlet and the sidedraw product outlet is at most 80 mbar, preferably at most 60 mba

25. A method according to claim 16, wherein the heavy ends column comprises trays in the part of the heavy ends column below the feed inlet, and has a pressure drop such that the pressure at the base of the heavy ends column is greater than atmospheric pressure.

26. A method according to claim 16, wherein the light ends recovery section of the acetic acid production unit comprises a light ends distillation column and a drying column.

27. A method according to claim 16, wherein the light ends recovery section of the acetic acid production unit comprises a combined light ends distillation and drying column.

28. A method according to claim 16, wherein the acetic acid production unit comprises a flash zone between the reactor and the light ends recovery section.

29. A process for the separation of acetic acid from a stream comprising acetic acid and propionic acid, wherein in said process the stream comprising acetic acid and propionic acid is fed to a distillation column through a feed inlet positioned at an intermediate point in the distillation column, a product stream comprising essentially acetic acid is withdrawn from the distillation column through a sidedraw product outlet position above the feed inlet, and a product stream comprising the propionic acid is withdrawn from the distillation column through a heavy product outlet positioned below the feed inlet, wherein the distillation column is operated under conditions such that the pressure in the distillation column above the feed inlet is lower than the pressure of the stream comprising acetic acid and propionic acid that is fed to the distillation column, and wherein the distillation column configured such that the head pressure is below 1.0 bara, the number of theoretical separation stages between the feed inlet and the sidedraw product outlet is at least 5, preferably at least 7, more preferably at least 10, and the pressure drop in the part of the distillation column between the feed inlet and the sidedraw product outlet is at most 10 mbar per theoretical separation stage.

30. A process for the production of acetic acid in an acetic acid production unit comprising a reaction section, a light ends recovery section comprising a light ends column, and a heavy ends column, wherein said process comprises the steps: (a) in the reaction section, carbonylating methanol and/or a reactive derivative thereof with carbon monoxide in a liquid reaction composition comprising a Group VIII metal carbonylation catalyst, a methyl iodide cocatalyst, acetic acid, water, methyl acetate, propionic by-product, and optionally a promoter; (b) withdrawing at least a portion of the liquid reaction composition from the reaction section and introducing the withdrawn liquid reaction composition into a flash zone to produce a vapour fraction comprising water, acetic acid, methyl acetate, methyl iodide, and propionic acid by-product, and a liquid fraction comprising the catalyst; (c) recycling the liquid fraction from the flash zone to the reaction section; (d) feeding the vapour fraction obtained from the flash zone to the light ends column of the light ends recovery section; (e) removing a stream comprising acetic acid and propionic acid from the light ends column; (f) optionally drying the stream comprising acetic acid and propionic acid removed from the light ends column in a separate drying column; and (g) feeding the stream comprising acetic acid and propionic acid to the heavy ends column through a feed inlet positioned at an intermediate point in the heavy ends column, withdrawing a product stream comprising essentially acetic acid through a sidedraw product outlet position above the feed inlet, and withdrawing a product stream comprising the propionic acid through a heavy product outlet positioned below the feed inlet, wherein the heavy ends column is operated under conditions such that the pressure in the heavy ends column above the feed inlet is lower than the pressure of the stream comprising acetic acid and propionic acid that is fed to the heavy ends column, and wherein the head pressure of the heavy ends column is below 1.0 bara, the number of theoretical separation stages between the feed inlet and the sidedraw product outlet is at least 5, preferably at least 7, more preferably at least 10, and the pressure drop in the part of the heavy ends column between the feed inlet and the sidedraw product outlet is at most 10 mbar per theoretical separation stage.

Description

EXAMPLE

[0113] The operation of heavy end columns according to the present invention and heavy end columns falling outside of the present invention were simulated using an ASPEN PLUS (Trademark) (version 7.3) computer model. The heavy ends column in the simulation comprised a feed inlet positioned at an intermediate point in the heavy ends column, a sidedraw product outlet position above the feed inlet, and a heavy product outlet positioned below the feed inlet, and was separated in to three sections: a top section defined as the part of the column above the sidedraw product outlet; a middle section defined as the part of the column between the sidedraw product outlet and the feed inlet; and, a bottom section defined as the part of the column below the feed inlet. For each section of the column, two different types of column internals, trays and packing, were simulated. The details of the parameters used in each of the three sections of the heavy end column simulated are provided in Table 1.

TABLE-US-00001 TABLE 1 Number of theoretical Pressure Drop (mBar) Column Section separation stages Tray Packing Top 3 40 6 Middle 12 270 25 Bottom 18 500 55

[0114] The simulated operation of the heavy ends column was performed using a fixed feed inlet composition and a fixed sidedraw product outlet composition as defined in Table 2 below.

TABLE-US-00002 TABLE 2 % w/w Inlet % w/w Product Methanol 0.003 0.002 Methyl Acetate 0.002 0.001 Acetic Acid 99.795 99.863 Water 0.100 0.104 Propionic Acid 0.100 0.030 Total 100.000 100.000

[0115] The operation of all eight possible permutations of the heavy ends column was simulated using a fixed inlet temperature of 156 C., a fixed inlet pressure of 6 Bara, a fixed head pressure of 0.565 Bara, and a sub cooled temperature set point of 70 C. in the condenser. The results of the simulation are provided in Table 3 below.

TABLE-US-00003 TABLE 3 Column Reboiler Condenser Internals Duty Duty Molar Feed Base Base (Top/Middle/ (MW/tn of (MW/tn of Reflux Pressure Pressure Temperature Bottom)* product) product) Ratio (bara) (bara) ( C.) T/T/T 0.185 0.231 33.39 0.875 1.375 129.3 T/T/P 0.183 0.229 33.11 0.875 0.93 116.3 P/T/T 0.183 0.230 33.26 0.841 1.341 128.5 P/T/P 0.181 0.228 32.98 0.841 0.896 115.1 T/P/T 0.172 0.217 31.42 0.63 1.13 122.7 T/P/P 0.170 0.216 31.14 0.63 0.685 106.7 P/P/T 0.170 0.216 31.24 0.596 1.096 121.6 P/P/P 0.168 0.214 30.95 0.596 0.651 105.1 *T = Tray; P = Packing

[0116] FIG. 1 plots the reboiler duty in terms MW per ton of acetic acid product and also plots the feed pressure for all of the permutations of the heavy ends column simulated.

[0117] FIG. 2 plots the reboiler duty in terms MW per ton of acetic acid product and also plots the column base temperature for all of the permutations of the heavy ends column simulated.

[0118] As can be clearly seen from the results, the reboiler duty required when the middle section of the heavy ends column comprises packing is significantly reduced compared to when the middle section of the heavy ends column comprises trays. The feed pressure also followed the general trend seen in reboiler duty across all permutations (i.e. as feed pressure is lowered, the reboiler duty decreases).

[0119] When packing was used in the bottom section, the largest reduction in column base temperature was observed, however, the use of packing in the bottom section had a significantly less significant effect on reducing reboiler duty than the use of a lower feed pressure as occurs in the heavy end columns comprising packing in the middle section.