Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-02-07
2003-12-02
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S608000
Reexamination Certificate
active
06657078
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to processes for making acetic acid; and in particular to a low energy process for making acetic acid by way of carbonylating methanol with carbon monoxide and utilizing at most two distillation columns in the primary purification train
BACKGROUND ART
Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the rhodium catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 of Paulik et al. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium along with a halogen containing catalyst promotor 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. disclosed that water may be added to the reaction mixture to exert a beneficial effect upon the reaction rate. Water concentrations greater than about 14 weight percent 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 disclosed in U.S. Pat. Nos. 5,001,259; 5,026,908; and 5,144,068. Water concentrations below 14 weight percent and even below 10 weight percent 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.
It is desirable in a carbonylation process for making acetic acid to minimize the number of distillation operations in order to minimize energy usage in the process. In this respect there is disclosed in U.S. Pat. No. 5,416,237 to Aubigne et al. a process for the production of acetic acid by carbonylation of methanol in the presence of a rhodium catalyst, methyl iodide, and an iodide salt stabilizer. The improvement according to the '237 patent resides in maintaining a finite concentration of water up to about 10 percent by weight and a methyl acetate concentration of at least 2 percent by weight in the liquid reaction composition and recovering the acetic acid product by passing the liquid reaction composition through a flash zone to produce a vapor fraction which is passed to a single distillation column from which the acetic acid product is removed. The drawback of eliminating distillation stages is that the level of purity of the product suffers. In particular the distillation columns tend to remove high boiling iodides as well as aldehyde contamination products. Both of these impurities impact the commercial desirability of the final product.
Various means for removing iodides are well known in the art. It was discovered by Hilton that macroreticulated, strong acid cationic exchange resins with at least one percent of their active sites converted to the silver or mercury form exhibited remarkable removal efficiency for iodide contaminants in acetic acid or other organic media. The amount of silver or mercury associated with the resin may be from as low as about one percent of the active sites to as high as 100 percent. Preferably about 25 percent to about 75 percent of the active sites were converted to the silver or mercury form and most preferably about 50 percent. The subject process is disclosed in U.S. Pat. No. 4,615,806 for removing various iodides from acetic acid. In particular there is shown in the examples removal of methyl iodide, HI, I
2
and hexyl iodide.
Various embodiments of the basic invention disclosed in U.S. Pat. No. 4,615,806 have subsequently appeared in the literature. There is shown in U.S. Pat. No. 5,139,981 to Kurland a method for removing iodides from liquid carboxylic acid contaminated with a halide impurity by contacting the liquid halide contaminant acid with a silver (I) exchanged macroreticular resin. The halide reacts with the resin bound silver and is removed from the carboxylic acid stream. The invention in the '981 patent more particularly relates to an improved method for producing the silver exchanged macroreticular resins suitable for use in iodide removal from acetic acid.
U.S. Pat. No. 5,227,524 to Jones discloses a process for removing iodides using a particular silver-exchanged macroreticular strong acid ion exchange resin. The resin has from about 4 to about 12 percent cross-linking, a surface area in the proton exchanged form of less than 10 m
2
/g after drying from the water wet state and a surface area of greater than 10 m
2
/g after drying from a wet state in which the water has been replaced by methanol. The resin has at least one percent of its active sites converted to the silver form and preferably from about 30 to about 70 percent of its active sites converted to the silver form.
U.S. Pat. No. 5,801,279 to Miura et al. discloses a method of operating a silver exchanged macroreticular strong acid ion exchange resin bed for removing iodides from a Monsanto type acetic acid stream. The operating method involves operating the bed silver-exchanged resin while elevating the temperatures in stages and contacting the acetic acid and/or acetic anhydride containing the iodide compounds with the resin. Exemplified in the patent is the removal of hexyl iodide from acetic acid at temperatures of from about 25° C. to about 45° C.
So also, other ion exchange resins have been used to remove iodide impurities from acetic acid and/or acetic anhydride. There is disclosed in U.S. Pat. No. 5,220,058 to Fish et al. the use of ion exchange resins having metal exchanged thiol functional groups for removing iodide impurities from acetic acid and/or acetic anhydride. Typically, the thiol functionality of the ion exchange resin has been exchanged with silver, palladium, or mercury.
There is further disclosed in European Publication No. 0 685 445 A1 a process for removing iodide compounds from acetic acid. The process involves contacting an iodide containing acetic acid stream with a polyvinylpyridine at elevated temperatures to remove the iodides. Typically, the acetic acid was fed to the resin bed according to the '445 publication at a temperature of about 100° C.
With ever increasing cost pressures and higher energy prices, there has been ever increasing motivation to simplify chemical manufacturing operations and particularly to reduce the number of manufacturing steps. In this regard, it is noted that in U.S. Pat. No. 5,416,237 to Aubigne et al. there is disclosed a single zone distillation process for making acetic acid. Such process modifications, while desirable in terms of energy costs, tend to place increasing demands on the purification train. In particular, fewer recycles tend to introduce (or fail to remove) a higher level of iodides into the product stream and particularly more iodides of a higher molecular weight. For example, octyl iodide, decyl iodide and dodecyl iodides may all be present in the product stream as well as hexadecyl iodide; all of which are difficult to remove by conventional techniques.
Other impurities in acetic acid made by way of the rhodium catalyzed carbonylation of methanol, notably aldehydes and propionic acid, are likewise known. It is proposed 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 acetaldehyde undergoes reduction by hydrogen in the rhodium-catalyzed system to give ethanol which subsequently yields propionic acid. It is postulated that enhanced rhodium catalyzed systems have increased standing levels of rhodium-acyl species which will form free acetaldehydes at a higher rate.
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 re
Blay George A.
Broussard Jerry A.
Scates Mark O.
Torrence G. Paull
Celanese International Corporation
Mullen James J.
Richter Johann
Zucker Paul A.
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