Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-06-04
2003-01-07
Ford, John M. (Department: 1624)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S404000, C562S412000, C562S413000
Reexamination Certificate
active
06504051
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to an improved process for the continuous production of aromatic carboxylic acids by the liquid-phase oxidation of alkyl aromatic hydrocarbons with molecular oxygen in the presence of an oxidation catalyst or catalyst system wherein an off-gas derived from the oxidation vessel comprising mainly water vapor and minor amounts of organic components is treated in a pollution control device and the aqueous effluent from the pollution control device is removed from the production system. More particularly, this invention pertains to such oxidation processes carried out in a columnar oxidation reactor provided with means to effectively remove water generated by the process with minimal solvent loss utilizing the energy of oxidation wherein the gaseous effluent stream from the water removal means is fed to a pollution control device and the aqueous effluent from the pollution control device is removed from the production system.
BACKGROUND OF THE INVENTION
The liquid-phase oxidation of an alkyl aromatic hydrocarbon to an aromatic carboxylic acid is a highly exothermic reaction commonly carried out in a vented, intimately-mixed, columnar oxidation reactor. The oxidation process comprises continuously feeding, separately or in admixture, an alkyl aromatic hydrocarbon, fresh and/or recycled solvent or reaction medium, and catalyst components to the reactor to which a molecular oxygen-containing gas also is fed, normally at or near the bottom of the reactor. This process gas rises through the liquid contents of the reactor resulting in vigorous agitation of the reaction mixture and providing intimate contact between the alkyl aromatic hydrocarbon and the process solvent having dissolved therein the catalyst or catalyst components. The aromatic carboxylic acid produced is removed continuously through a lower exit port located at or near the base of the reactor as a solid in the solvent which also contains soluble catalyst components. After separation of the aromatic carboxylic acid product, the solvent is returned to the reactor.
Oxygen-depleted process gas along with a minor amount of solvent decomposition products, is removed through an upper exit port located at or near the top of the reactor. The heat of reaction also is removed through the upper exit port by vaporization of process solvent and water generated by the reaction. The oxygen-depleted process gas and the vaporized process solvent and water comprise the reactor off-gas which is typically condensed by means of one or more condensers to separate the solvent and water for recycling to the reactor. The condensed aqueous solvent may be subjected to a water removal step prior to recycling.
The described production system can be utilized in the manufacture of aromatic carboxylic acids at excellent production rates relative to the volume of the reactor. It is necessary that the production system include a means for the efficient removal of the excess water generated by the reaction since the water concentration must be held at an acceptable level, normally between about 3 and 20 weight percent, preferably between about 3 and 10 weight percent, for the reaction to continue at a reasonable rate. The reaction produces one mole of water per mole of carboxyl moiety produced. In addition, there are other by-product reactions that release water, i.e. the direct oxidation of the alkyl aromatic or direct oxidation of the solvent, and water may be added to the process for other reasons such as scrubbing off-gas for solvent recovery. Typically, water is removed by conventional distillation methods.
Direct distillation of the reactor off-gas to remove water has conventionally been employed utilizing the heat of reaction, as described in British Patent Specification 1,373,230 (Yokota et al.) and U.S. Pat. No. 4,914,230 (Abrams et al.), U.S. Pat. No. 5,723,656 (Abrams), and U.S. Pat. No. 5,463,113 (Yamamoto et al.) However, process limitations exist. Since the amount of distillate reflux determines the purity of the overhead distillate and the heat input to the distillation process determines the amount of reflux that the process can accommodate, the heat of reaction fixes both the amount of reflux and the purity of the overhead distillate. The heat of reaction alone is generally insufficient to obtain a desirable overhead purity that minimizes solvent loss. Therefore, direct distillation generally requires additional heat input. U.S. Pat. No. 5,510,521 (McGehee et al.) discloses an improved process wherein oxidation reactor off-gas is fed directly to the lower section of a removal column. A bottoms liquid of partially de-watered process solvent obtained from the lower section of the water removal column is returned to the upper section of the reactor, usually as a spray above the phase separation of the gas/liquid contents of the reactor. The spray of dewatered process solvent enriches the water content of the reactor off-gas to improve the efficiency of the water removal column without additional heat input beyond that of the heat of reaction.
A problem with the removal of water as liquid using direct distillation is that any such water also contains minor amounts of solvent organic by-products, requiring that the stream be treated as wastewater for removal of solvent and organic by-products due to the aforementioned oxidation reactions before release to the environment. The processes described by Yokota et al., McGehee et al., Abrams et al., Abrams, and Yamamoto et al. all remove water as liquid and thus require wastewater treatment for removal of solvent.
Thus, a need exists for a method to remove water from a carboxylic acid production process by a means which does not require wastewater treatment for removal of solvent and organic by-products before release to the environment. Providing such a water removal means would be especially advantageous if it improved process efficiency by recovering additional power using a power recovery device on oxygen-depleted process gas and water vapor streams.
BRIEF SUMMARY OF THE INVENTION
The present invention provides for the removal and treatment of water of reaction as a vapor in a pollution control device, decreasing or eliminating the need for wastewater treatment of the stream for removal of solvent and organic by-products before release to the environment. These and other advantages are afforded by carrying out the oxidation of an alkyl aromatic hydrocarbon in a columnar reactor wherein the reactor off-gas is fed directly into a water removal column. A portion of the overhead aqueous vapors from the water removal column is removed from the top of the water removal column as a vapor distillate, with the remaining overhead aqueous vapors being refluxed to the fractionating zone of the water removal column. The combined vapor distillate and oxygen-depleted process gas are fed to a pollution control device for the destruction of solvent and organic by-products before exiting the process. Feeding a portion of the overhead aqueous vapor from the water column directly to the pollution control device reduces the required cooling utility duty and required heat transfer area to condense the water from all of the aqueous vapor stream removed from the water column.
Our invention thus provides a process for the continuous production of an aromatic carboxylic acid in a pressurized oxidation reactor by liquid-phase, exothermic oxidation of an alkyl aromatic hydrocarbon with an oxygen-containing gas in the presence of an oxidation catalyst and aqueous, C
2
-C
6
aliphatic, monocarboxylic acid solvent which comprises the steps of:
(1) continuously feeding to a reactor alkyl aromatic hydrocarbon, aqueous, monocarboxylic acid solvent having oxidation catalyst dissolved therein, and an oxygen containing gas;
(2) continuously removing from the lower portion of the reactor product-containing liquid comprising aromatic polycarboxylic acid and the aqueous, monocarboxylic acid solvent having the oxidation catalyst dissolved therein;
(3) continuously removing from the upper po
de Vreede Marcel
Lin Robert
Miller, Jr. Harold David
Eastman Chemical Company
Ford John M.
Graves, Jr. Bernard J.
Harding Karen A.
Tucker Zachary C.
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