Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1999-07-19
2002-10-01
Geist, Gary (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
Reexamination Certificate
active
06458996
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates 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 and a hydrocarbyl halide co-catalyst.
Processes for producing acetic acid by the Group VIII noble metal catalysed, hydrocarbyl halide co-catalysed carbonylation of alcohols and/or their reactive derivatives are well-known in the art. Representative of such art employing rhodium as the Group VIII noble metal catalyst may be mentioned, for example, U.S. Pat. No. 3,772,380; GB-A-1468940; GB-A-1538783 and EP-A-0087070. Representative of such art using iridium as the Group VIII noble metal catalyst may be mentioned, for example, GB-A-1,234,121; U.S. Pat. No. 3,772,380; DE-A-1767150; EP-A-0616997; EP-A-0618184; EP-A-0618183; and EP-A-0657386.
In continuous liquid phase processes 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 the acetic acid product is recovered from the liquid reaction composition and dried; the remaining components of the reaction composition being recycled to the reactor to maintain their concentration therein.
Howard et al in Catalysis Today, 18(1993), 325-354 describe the 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 a stirred tank 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 Liquid reaction composition is withdrawn from the reactor and is passed through a flashing valve to the flash tank where the majority of the lighter components of the liquid reaction composition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 vapourised. 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 (as shown in
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) (as shown in
FIG. 3
of Howard et al). In the lights 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 typically as a side draw 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 side stream.
In practice the upper (aqueous layer) from the decanter, in whole or in part, is returned to the light ends column as reflux and the lower (organic layer) from the decanter is recycled to the reactor. For operational reasons it is highly desirable that two separable phases are maintained in the decanter. Decanter stability is of paramount importance in the successful operation of the continuous carbonylation process. If the decanter becomes single phase, the resulting composition change tends to increase the water content in the reactor, which in turn has a significant impact on reaction activity for iridium catalysed carbonylation.
EP-A-0768295 describes one method of maintaining two separable phases in the reactor in circumstances such that the concentration of water contained in the carbonylation liquid reaction composition decreases or the concentration of methyl acetate contained in the liquid reaction composition increases. Thus EP-A-0768295 discloses a process for producing acetic acid by reacting continuously at least one selected from methanol, methyl acetate and dimethyl ether with carbon monoxide in the presence of a Group VIII metal-containing catalyst, methyl iodide and water, comprising (a) a step in which a crude reaction liquid is withdrawn from a carbonylation step and introduced into a flash zone, and a catalyst circulating liquid containing a catalyst component which is not evaporated in the flash zone is circulated into a carbonylation reactor, (b) a step in which a vapour fraction evaporated in the flash zone is fed into a first distillation column in the form of a vapour or a liquid, (c) a step in which a low boiling circulating stream comprising water, methyl acetate, methyl iodide and acetic acid is withdrawn from the top of the first distillation column, and (d) a step in which crude acetic acid is withdrawn from the bottom or the side cut near the bottom of the first distillation column, characterised in that a liquid separation state in the decanter at the top of the first distillation column is maintained by adding water to the first distillation column, lowering the cooling temperature at the overhead part of the first distillation column, or reducing the concentration of methyl acetate contained in the liquid fed into the decanter at the top of the first distillation column.
EP-A-0768295 teaches that when two phases do not form in the decanter liquid and the unseparated liquid is recycled to the reactor, by-product carbonyl compounds, such as acetaldehyde, crotonaldehyde and 2-ethylcrotonaldehyde, and organic iodine compounds such as hexyl iodide, build up to an unacceptable level in the product acetic acid.
European patent publication number EP-0573189-A1 describes a process for the production of acetic acid by carbonylation of methanol in the presence of a rhodium carbonylation catalyst. The methyl acetate concentration in the liquid reaction composition is said to be at least 2% by weight, preferably in the range 2% to 15% by weight more preferably in the range 3% to 10% by weight. Whilst in Examples 4 and 5 the combined overhead streams forming the light ends recycles shown were calculated to have 0.96% and 1.33% by weight acetic acid, the methyl acetate concentrations in the reactors were only 3.1% and 7.3% by weight.
DESCRIPTION OF THE INVENTION
We have found that at high methyl acetate concentrations, typically 8% w/w or greater in the liquid reaction composition in the carbonylation reactor, particularly at low levels of water and methyl iodide, which conditions are typically associated with the use of iridium as the carbonylation catalyst, it becomes increasingly difficult to achieve two separable phases in the decanter, which in turn may give rise to product quality problems of the type referred to in EP-A-0768295, and plant capacity problems, largely as a result of hydraulic limitations to both control valves and pumps.
We have found that a solution to the problem of maintaining two liquid phases in a continuously operated decanter is to control the concentration of acetic acid in the overhead fraction fed from the light ends column to the decanter. EP-A-0768295 makes no mention of acetic acid concentration in the overhead fraction and its impact on the maintenance of two phases. In off-line experiments we have found that a typical decanter feed will form a single phase with about 14% w/w or more of acetic acid present. However, in a continuously operated decanter, even lower levels of acetic acid must be achieved (8 wt % or lower) in order to maintain stable operation. This is due to the increasing water content of the organic phase, which depletes the light ends column overheads of water by recycling it directly back to the reactor. This causes the water concentration to fall and the phase separation to become more difficult. A feed-back mechanism then becomes dominant and the decanter becomes single phase.
Accordingly the
BP Chemicals Limited
Deemie Robert W
Geist Gary
Nixon & Vanderhye
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