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  Optimized liquid-phase oxidationUSPTO Application #: 20070185347Title: Optimized liquid-phase oxidation Abstract: Disclosed is an optimized process and apparatus for more efficiently and economically carrying out the liquid-phase oxidation of an oxidizable compound. Such liquid-phase oxidation is carried out in a bubble column reactor that provides for a highly efficient reaction at relatively low temperatures. When the oxidized compound is para-xylene and the product from the oxidation reaction is crude terephthalic acid (CTA), such CTA product can be purified and separated by more economical techniques than could be employed if the CTA were formed by a conventional high-temperature oxidation process. (end of abstract) Agent: Michael K. Carrier Eastman Chemical Company - Kingsport, TN, US Inventors: Alan George Wonders, Lee Reynolds Partin, Wayne Scott Strasser, Marcel de Vreede, Puneet Gupta USPTO Applicaton #: 20070185347 - Class: 562414000 (USPTO) Related Patent Categories: Organic Compounds -- Part Of The Class 532-570 Series, Azo Compounds Containing Formaldehyde Reaction Product As The Coupling Component, Carboxylic Acids And Salts Thereof, Aromatic, Formation Of Carboxyl Group By Oxidation, Of Aromatic Compound, Alkyl Side Chain Oxidized, , The Patent Description & Claims data below is from USPTO Patent Application 20070185347. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a divisional of U.S. application Ser. No. 11/154,165 filed Jun. 16, 2005 which claims priority to U.S. Provisional Application Ser. No. 60/606,787, filed Sep. 2, 2004 and 60/631,351, filed Nov. 29, 2004, the disclosures of which are incorporated herein by reference in their entirety. FIELD OF THE INVENTION [0002] This invention relates generally to a process for the liquid-phase, catalytic oxidation of an aromatic compound. One aspect of the invention concerns the partial oxidation of a dialkyl aromatic compound (e.g., para-xylene) to produce a crude aromatic dicarboxylic acid (e.g., crude terephthalic acid), which can thereafter be subjected to purification and separation. Another aspect of the invention concerns an improved bubble column reactor that provides for a more effective and economical liquid-phase oxidation process. BACKGROUND OF THE INVENTION [0003] Liquid-phase oxidation reactions are employed in a variety of existing commercial processes. For example, liquid-phase oxidation is currently used for the oxidation of aldehydes to acids (e.g., propionaldehyde to propionic acid), the oxidation of cyclohexane to adipic acid, and the oxidation of alkyl aromatics to alcohols, acids, or diacids. A particularly significant commercial oxidation process in the latter category (oxidation of alkyl aromatics) is the liquid-phase catalytic partial oxidation of para-xylene to terephthalic acid. Terephthalic acid is an important compound with a variety of applications. The primary use of terephthalic acid is as a feedstock in the production of polyethylene terephthalate (PET). PET is a well-known plastic used in great quantities around the world to make products such as bottles, fibers, and packaging. [0004] In a typical liquid-phase oxidation process, including partial oxidation of para-xylene to terephthalic acid, a liquid-phase feed stream and a gas-phase oxidant stream are introduced into a reactor and form a multi-phase reaction medium in the reactor. The liquid-phase feed stream introduced into the reactor contains at least one oxidizable organic compound (e.g., para-xylene), while the gas-phase oxidant stream contains molecular oxygen. At least a portion of the molecular oxygen introduced into the reactor as a gas dissolves into the liquid phase of the reaction medium to provide oxygen availability for the liquid-phase reaction. If the liquid phase of the multi-phase reaction medium contains an insufficient concentration of molecular oxygen (i.e., if certain portions of the reaction medium are "oxygen-starved"), undesirable side-reactions can generate impurities and/or the intended reactions can be retarded in rate. If the liquid phase of the reaction medium contains too little of the oxidizable compound, the rate of reaction may be undesirably slow. Further, if the liquid phase of the reaction medium contains an excess concentration of the oxidizable compound, additional undesirable side-reactions can generate impurities. [0005] Conventional liquid-phase oxidation reactors are equipped with agitation means for mixing the multi-phase reaction medium contained therein. Agitation of the reaction medium is supplied in an effort to promote dissolution of molecular oxygen into the liquid phase of the reaction medium, maintain relatively uniform concentrations of dissolved oxygen in the liquid phase of the reaction medium, and maintain relatively uniform concentrations of the oxidizable organic compound in the liquid phase of the reaction medium. [0006] Agitation of the reaction medium undergoing liquid-phase oxidation is frequently provided by mechanical agitation means in vessels such as, for example, continuous stirred tank reactors (CSTRs). Although CSTRs can provide thorough mixing of the reaction medium, CSTRs have a number of drawbacks. For example, CSTRs have a relatively high capital cost due to their requirement for expensive motors, fluid-sealed bearings and drive shafts, and/or complex stirring mechanisms. Further, the rotating and/or oscillating mechanical components of conventional CSTRs require regular maintenance. The labor and shutdown time associated with such maintenance adds to the operating cost of CSTRs. However, even with regular maintenance, the mechanical agitation systems employed in CSTRs are prone to mechanical failure and may require replacement over relatively short periods of time. [0007] Bubble column reactors provide an attractive alternative to CSTRs and other mechanically agitated oxidation reactors. Bubble column reactors provide agitation of the reaction medium without requiring expensive and unreliable mechanical equipment. Bubble column reactors typically include an elongated upright reaction zone within which the reaction medium is contained. Agitation of the reaction medium in the reaction zone is provided primarily by the natural buoyancy of gas bubbles rising through the liquid phase of the reaction medium. This natural-buoyancy agitation provided in bubble column reactors reduces capital and maintenance costs relative to mechanically agitated reactors. Further, the substantial absence of moving mechanical parts associated with bubble column reactors provides an oxidation system that is less prone to mechanical failure than mechanically agitated reactors. [0008] When liquid-phase partial oxidation of para-xylene is carried out in a conventional oxidation reactor (CSTR or bubble column), the product withdrawn from the reactor is typically a slurry comprising crude terephthalic acid (CTA) and a mother liquor. CTA contains relatively high levels of impurities (e.g., 4-carboxybenzaldehyde, para-toluic acid, fluorenones, and other color bodies) that render it unsuitable as a feedstock for the production of PET. Thus, the CTA produced in conventional oxidation reactors is typically subjected to a purification process that converts the CTA into purified terephthalic acid (PTA) suitable for making PET. [0009] One typical purification process for converting CTA to PTA includes the following steps: (1) replacing the mother liquor of the CTA-containing slurry with water, (2) heating the CTA/water slurry to dissolve the CTA in water, (3) catalytically hydrogenating the CTA/water solution to convert impurities to more desirable and/or easily-separable compounds, (4) precipitating the resulting PTA from the hydrogenated solution via multiple crystallization steps, and (5) separating the crystallized PTA from the remaining liquids. Although effective, this type of conventional purification process can be very expensive. Individual factors contributing to the high cost of conventional CTA purification methods include, for example, the heat energy required to promote dissolution of the CTA in water, the catalyst required for hydrogenation, the hydrogen stream required for hydrogenation, the yield loss caused by hydrogenation of some terephthalic acid, and the multiple vessels required for multi-step crystallization. Thus, it would be desirable to provide a CTA product that could be purified without requiring heat-promoted dissolution in water, hydrogenation, and/or multi-step crystallization. OBJECTS OF THE INVENTION [0010] It is, therefore, an object of the present invention to provide a more effective and economical liquid-phase oxidation reactor and process. [0011] Another object of the invention is to provide a more effective and economical reactor and process for the liquid-phase catalytic partial oxidation of para-xylene to terephthalic acid. [0012] Still another object of the invention is to provide a bubble column reactor that facilitates improved liquid-phase oxidation reactions with reduced formation of impurities. [0013] Yet another object of the invention is to provide a more effective and economical system for producing pure terephthalic acid (PTA) via liquid-phase oxidation of para-xylene to produce crude terephthalic acid (CTA) and subsequently, purifying the CTA to PTA. [0014] A further object of the invention is to provide a bubble column reactor for oxidizing para-xylene and producing a CTA product capable of being purified without requiring heat-promoted dissolution of the CTA in water, hydrogenation of the dissolved CTA, and/or multi-step crystallization of the hydrogenated PTA. [0015] It should be noted that the scope of the present invention, as defined in the appended claims, is not limited to processes or apparatuses capable of realizing all of the objects listed above. Rather, the scope of the claimed invention may encompass a variety of systems that do not accomplish all or any of the above-listed objects. Additional objects and advantages of the present invention will be readily apparent to one skilled in the art upon reviewing the following detailed description and associated drawings. SUMMARY OF THE INVENTION [0016] One embodiment of the present invention concerns a process comprising the following steps: (a) introducing an oxidant stream comprising molecular oxygen into a reaction zone of a bubble column reactor; (b) introducing a feed stream comprising an oxidizable compound into the reaction zone, wherein the feed stream is introduced into the reaction zone in a manner such that when the reaction zone is theoretically partitioned into 4 vertical quadrants of equal volume by a pair of intersecting vertical planes, not more than about 80 weight percent of the oxidizable compound enters the reaction zone in a common one of the vertical quadrants; and (c) oxidizing at least a portion of the oxidizable compound in a liquid phase of a multi-phase reaction medium contained in the reaction zone. [0017] Another embodiment of the present invention concerns a process comprising the following steps: (a) introducing an oxidant stream comprising para-xylene into a reaction zone of a bubble column reactor; (b) introducing a feed stream comprising an oxidizable compound into the reaction zone via a plurality of feed openings, wherein the reaction zone has a maximum diameter (D), wherein at least two of the feed openings are spaced from one another by at least about 0.5 D; and (c) oxidizing at least a portion of the oxidizable compound in a liquid phase of a multi-phase reaction medium contained in the reaction zone to thereby form crude terephthalic acid particles. [0018] Still another embodiment of the present invention concerns a process for producing terephthalic acid comprising the following steps: (a) introducing an oxidant stream comprising molecular oxygen into a reaction zone of a bubble column reactor; (b) introducing a feed stream comprising para-xylene into the reaction zone, wherein the para-xylene enters the reaction zone in a manner such that when the reaction zone is theoretically partitioned into 4 vertical quadrants of equal volume by a pair of intersecting vertical planes, not more than about 80 weight percent of the para-xylene enters the reaction zone in a single one of the vertical quadrants; (c) oxidizing at least a portion of the para-xylene in a liquid phase of the reaction medium contained in the reaction zone to thereby form crude terephthalic acid; and (d) oxidizing at least a portion of the crude terephthalic acid in a secondary oxidation reactor to thereby form purer terephthalic acid. [0019] Yet another embodiment of the present invention concerns a bubble column reactor for reacting a predominately liquid-phase stream and a predominately gas-phase stream. The bubble column reactor includes a vessel shell, one or more gas openings, and one or more liquid openings. The vessel shell defines an elongated reaction zone extending along a normally-upright central shell axis. The reaction zone presents axially-spaced normally-upper and normally-lower ends. The one or more gas openings discharge the gas-phase stream into the reaction zone. The one or more liquid openings introduce the liquid-phase stream into the reaction zone. When the reaction zone is theoretically partitioned into 4 vertical quadrants of equal volume by a pair of intersecting vertical planes, not more than about 80 percent of the cumulative open area defined by all of the liquid openings is attributable to liquid openings located in a common one of the vertical quadrants. Continue reading... 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