Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1999-08-31
2001-10-16
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S517000, C562S607000
Reexamination Certificate
active
06303813
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an improvement in the process for the carbonylation of methanol to produce acetic acid. More specifically, the improved method of the present invention reduces the formation of carbonyl impurities in the carbonylation reaction by way of conducting the reaction with relatively low hydrogen partial pressures in the reactor.
2. The Related Art
Among currently employed processes for synthesizing acetic acid one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al. on Oct. 30, 1973. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or else supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. Generally, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled. Paulik et al. disclose that water may be added to the reaction mixture to exert a beneficial effect upon the reaction rate, and water concentrations between about 14-15 wt % are typically used. This is the so-called “high water” carbonylation process.
An alternative to the “high water” carbonylation process is the “low water” carbonylation process, as described in U.S. Pat. Nos. 5,001,259, 5,026,908, and 5,144,068. Water concentrations below 14 wt % and even below 10 wt % can be used in the “low water” carbonylation process. Employing a low water concentration simplifies downstream processing of the desired carboxylic acid to its glacial form.
One improvement which has been made to the “low water” carbonylation process is disclosed in U.S. Pat. No. 4,994,608, which discloses a carbonylation process utilizing a rhodium catalyst wherein a partial pressure of hydrogen between 4 and 150 psia is maintained in the carbonylation reactor. The presence of the hydrogen is disclosed as having the effect of increasing the rate of carbonylation by keeping the rhodium in its active rhodium (I) form. It is noted in the '608 patent that it is possible to operate a methanol carbonylation process at relatively low levels of hydrogen partial pressure, albeit at relatively low levels of rhodium. See FIG. 1, as well as Table II, col. 14, lines 8 through 32 of the '608 patent.
In the present invention, however, it has been found that while the presence of hydrogen in the carbonylation reaction does in fact increase the carbonylation rate, the rate of formation of undesirable by-products, such as crotonaldehyde, 2-ethyl crotonaldehyde, butyl acetate, and hexyl iodide, also increases. Since hydrogen can often be an impurity in carbon monoxide feedstocks used in methanol carbonylation, the partial pressure of hydrogen should be maintained such that the rate of formation of by-products is limited. It is therefore an object of the present invention to provide a “low water” carbonylation process wherein a partial pressure of hydrogen in the carbonylation reaction is maintained at a level which limits the rate of by-product formation.
It is postulated in an article by Watson, The Cativa™ Process for the Production of Acetic Acid, Chem. Ind. (Dekker) (1998) 75 Catalysis of Organic Reactions, pp. 369-380 that enhanced rhodium catalyzed systems have increased standing levels of rhodium-acyl species which will form free acetaldehydes at a higher rate. The higher rate of acetaldehyde formation can lend to the increased production of permanganate reducing compounds.
The precise chemical pathway within the methanol carbonylation process that leads to the production of crotonaldehyde, 2-ethyl crotonaldehyde and other permanganate reducing compounds is not well understood. One prominent theory for the formation of the crotonaldehyde and 2-ethyl crotonaldehyde impurities in the methanol carbonylation process is that they result from aldol and cross-aldol condensation reactions starting with acetaldehyde. Substantial efforts have been directed to removing acetaldehyde.
Conventional techniques used to remove acetaldehyde and carbonyl impurities have included treatment of acetic acid with oxidizers, ozone, water, methanol, amines, and the like. In addition, each of these techniques may or may not be combined with the distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the product acetic acid. Likewise, it is known to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxyl amine which reacts with the carbonyl compounds to form oximes followed by distillation to separate the purified organic product from the oxime reaction products. However, this method of treating the product acetic acid adds cost to the process.
There is disclosed in U.S. Pat. No. 5,625,095 to Miura et al. and PCT International Application No. PCT/US97/18711, Publication No. WO 98/17619 various methods of removing acetaldehydes and other impurities from a rhodium-catalyzed acetic acid production process. Generally, these methods involve extracting undesirable impurities from process streams to reduce acetaldehyde concentrations in the system.
These processes have achieved a certain level of success in controlling carbonyl impurity concentrations within product acetic acid produced by methanol carbonylation. Nonetheless, even with the use of these prior art removal methods, acetaldehyde and carbonyl impurities that derive from acetaldehyde, particularly, crotonaldehyde and 2-ethyl crotonaldehyde, continue to be a problem in product acetic acid produced by methanol carbonylation. Accordingly, a need remains for a method to control carbonyl impurities in product acetic acid produced by methanol carbonylation, particularly one which can be performed economically without adding to the impurities in the product acetic acid or incorporating additional processing steps. It has been found that reduced levels of hydrogen lead to improved purity profiles.
SUMMARY OF THE INVENTION
There is provided in the present invention an improved process for producing acetic acid by reacting methanol with a carbon monoxide feedstock in a carbonylation reactor holding a reaction medium containing a catalytically effective amount of rhodium which includes maintaining catalyst stability and system productivity by maintaining in said reaction medium during the course of said reaction at least a finite concentration (0.1 wt %) up to less than 14 wt % of water together with (a) a salt soluble in the reaction medium at the reaction temperature in an amount operative to maintain a concentration of ionic iodide in the range of from about 2 to about 20 wt % effective as a catalyst stabilizer and co-promoter, (b) from about 1 to 20 wt % methyl iodide, (c) from about 0.5 to 30 wt % methyl acetate, (d) a partial pressure of hydrogen between about 0.1 and 4 psia at reaction conditions comprising 15 to 40 atmospheres total reaction pressure (absolute), (e) a rhodium concentration of at least 500 ppm by weight based on the weight of rhodium metal in the reaction mixture; and (f) acetic acid. Typically, a hydrogen partial pressure is maintained between about 1 and 4 psia, and sometimes between about 1.5 and 3.5 psia.
The improvement may be practiced wherein said hydrogen partial pressure is maintained by venting or purging gaseous components of the reaction medium. The invention is also embodied where the ratio of hydrogen to carbon monoxide fed to said carbonylation reactor is from 0 mole percent to about 0.5 mole percent, although a concentration of hydrogen in the carbon monoxide fed to the carbonylation reactor from about 0.001 mole percent to about 0.3 mole percent is more typical. A concentration of hydrogen in the carbon monoxide fed to said carbonylation reactor is from about 0.005 mole percent to about 0.0250 mole percent may likewise be employed. It will be appreciated by those of skill in the art that
Agrawal Pramod
Santillan Valerie
Scates Mark O.
Torrence G. Paull
Warner R. Jay
Celanese International Corporation
Deemie Robert W.
Killos Paul J.
Spiering M. Susan
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