Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1999-09-08
2001-12-04
Geist, Gary (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S519000, C562S607000
Reexamination Certificate
active
06326515
ABSTRACT:
The present invention relates in general to a carbonylation process for the production of acetic acid and in particular to a process for the production of acetic acid by the carbonylation of methanol and/or a reactive derivative thereof in the presence of a Group VIII noble metal catalyst, methyl iodide as co-catalyst, optionally a promoter, and a finite concentration of water.
Homogeneous liquid phase processes for the production of acetic acid by the Group VIII noble metal catalysed, alkyl halide co-catalysed reaction of carbon monoxide with methanol and/or a reactive derivative thereof are well-known. The process using rhodium as the noble metal catalyst is described in, for example, GB-A-1,233,121; EPA-0384652; and EP-A-0391680. The process using iridium as the noble metal catalyst is described in, for example, GB-A-1234121, U.S. Pat. No. 3,772,380; DE-A-1767150; EP-A061997; EP-A-0618184; EP-A-0618183; EP-A-0657386; and WO-A-95/31426. Carbonylation processes for the production of acetic acid in the presence of either a rhodium or an iridium carbonylation catalyst are operated on a commercial scale at several locations worldwide.
Howard et al in Catalysis Today, 18 (1993), 325-354 describe rhodium and iridium-catalysed carbonylation of methanol to acetic acid. The continuous rhodium-catalysed, homogeneous methanol carbonylation process is said to consist of three basic sections; reaction, purification and off-gas treatment. The reaction section comprises an agitated reactor, operated at elevated temperature and pressure, and a flash vessel. Liquid reaction composition is withdrawn from the reactor and is passed through a flashing valve to a flash tank where the majority of the lighter components of the liquid reaction composition (methyl iodide, methyl acetate and water) together with product acetic acid are vaporised. The vapour fraction is then passed to the purification section whilst the liquid fraction (comprising the rhodium catalyst in acetic acid) is recycled to the reactor (cf FIG. 2 of Howard et al). The purification section is said to comprise a first distillation column (the light ends column), a second distillation column (the drying column) and a third distillation column (the heavy ends column) (cf FIG. 3 of Howard et al). In the light ends column methyl iodide and methyl acetate are removed overhead along with some water and acetic acid. The vapour is condensed and allowed to separate into two phases in a decanter, both phases being returned to the reactor. Wet acetic acid is removed from the light ends column as a sidedraw and is fed to the drying column where water is removed overhead and an essentially dry acetic acid stream is removed from the base of the distillation zone. From FIG. 3 of Howard et al it can be seen that the overhead water stream from the drying column is recycled to the reaction section. Heavy liquid by-products are removed from the base of the heavy ends column with product acetic acid being taken as a sidestream. Simplification of the purification section by elimination of one or more distillation columns thereby economising on capital expenditure and/or operating costs of a plant has been proposed. Thus, for example our EP-A-0849250 (BP Case No. 8644) discloses a process for the production of an acetic acid process stream comprising less than 400 ppm propionic acid and less than 1500 ppm water which process comprises the steps:
(a) feeding methanol and/or a reactive derivative thereof and carbon monoxide to a carbonylation reactor in which there is maintained during the course of the process a liquid reaction composition comprising:
(i) an iridium carbonylation catalyst;
(ii) methyl iodide co-catalyst;
(iii) optionally one or more promoters selected from the group consisting of ruthenium, osmium, rhenium, cadmium, mercury, zinc, gallium, iridium and tungsten;
(iv) a finite amount of water at a concentration of less than about 8% by weight;
(v) methyl acetate;
(vi) acetic acid; and
(vii) propionic acid by-product and its precursors;
(b) withdrawing liquid reaction composition from the carbonylation reactor and introducing at least a part of the withdrawn liquid reaction composition, with or without the addition of heat, to a flash zone to form a vapour fraction comprising water, acetic acid product, propionic acid by-product, methyl acetate, methyl iodide and propionic acid precursors, and a liquid fraction comprising involatile iridium catalyst, involatile optional promoter or promoters, acetic acid and water;
(c) recycling the liquid fraction from the flash zone to the carbonylation reactor;
(d) introducing the vapour fraction from the flash zone into a first distillation zone;
(e) removing from the first distillation zone at a point above the introduction point of the flash zone vapour fraction a light ends recycle stream comprising water, methyl acetate, methyl iodide, acetic acid and propionic acid precursors which stream is recycled in whole or in part to the carbonylation reactor, and
(f) removing from the first distillation zone at a point below the introduction point of the flash zone vapour fraction, a process stream comprising acetic acid product, propionic acid by-product, and less than 1500 ppm water and,
(g) if the process stream removed in step (f) comprises greater than 400 ppm propionic acid introducing said stream into a second distillation column, removing from a point below the introduction point of the stream from (f) propionic acid by-product and from a point above the introduction point of the stream from (f) an acetic acid process stream containing less than 400 ppm propionic acid and less than 1500 ppm water.
In addition to propionic acid impurity, the Group VIII noble metal catalysed, methyl iodide co-catalysed carbonylation of methanol and/or a reactive derivative thereof also produces as impurities higher organic iodides, especially organic iodides in the C
5
-C
7
range, chief amongst which is hexyl iodide. Hexyl iodide forms a constant boiling azeotrope with acetic acid and hence is difficult to remove from acetic acid process streams by distillation. Unless additional non-distillative steps are taken for its removal, such as contact with a silver or mercury loaded cation exchange resin, or other adsorbent, hexyl iodide can therefore be found in significant amounts in the purified acetic acid product. This is undesirable because its presence therein can render the acetic acid unsuitable for use in certain downstream applications. Treatment with an adsorbent, for example, a metal loaded ion exchange resin carries with it an economic penalty. It would therefore be desirable to remove higher organic iodides during the distillative purification of crude acetic acid.
We have found that higher organic iodides, and in particular hexyl iodide, can be removed from their admixture with acetic acid obtained by carbonylation in a distillation column by controlling the water concentration profile in the column such that the concentrations of water on the feed tray in the column and in the head of the column are within defined limits. The excess water (over the levels previously employed) functions to azeotrope out the higher organic iodides and drive them up the column, where they are removeable overhead.
Accordingly the present invention provides a process for removing higher organic iodides, including hexyl iodide, from an acetic acid product obtained by carbonylating methanol and/or a reactive derivative thereof in the presence of a finite concentration of water, a Group VIII noble metal catalyst, methyl iodide as co-catalyst, and optionally a catalyst promoter, which process includes the step of subjecting an aqueous composition comprising acetic acid and at least one higher organic iodide to distillation in a column, or section of a column, separating water overhead from a dry acetic acid fraction, wherein the water concentration on the feed tray of the column, or section of the column, is greater than 8% by weight and/or the water concentration in the head of the column, or section of the column, is greater tha
Clode Kirsten Everald
Watson Derrick John
BP Chemicals Limited
Deemie Robert W.
Geist Gary
Nixon & Vanderhye
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