Abstract:
Disclosed is an optimized process and apparatus for more efficiently producing and purifying aromatic dicarboxylic acids (e.g., terephthalic acid). The optimized system employs at least one zoned slurry concentrator to enhance purification of the aromatic dicarboxylic acid.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the priority benefit of U.S. Provisional Pat. App. Ser. Nos. 60/606,735 and 60/606,807, both filed Sep. 2, 2004, the entire disclosures of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to the production of aromatic dicarboxylic acids, such as terephthalic acid (TPA). One aspect of the invention concerns a process and apparatus for more efficiently and/or effectively purifying crude aromatic dicarboxylic acids, such as crude terephthalic acid (CTA). Another aspect of the invention concerns a more economical system for producing purified aromatic dicarboxylic acids, such as purified terephthalic acid (PTA).  
       BACKGROUND OF THE INVENTION  
       [0003]     Terephthalic acid (TPA) is one of the basic building blocks in the production of linear polyester resins used in the manufacture of polyester films, packaging materials, and bottles. TPA used in the manufacture of such polyesters resins must meet certain minimum purity requirements.  
         [0004]     The purified condition of TPA refers primarily to the absence of significant concentrations of 4-carboxybenzaldehyde (4-CBA) and para-toluic acid (p-TAc) that are present in significant quantities in the commercially-available crude grades of TPA. Both 4-CBA and p-TAc are partial oxidation products formed in the manufacture of TPA by the catalytic oxidation of para-xylene. The purified form of TPA also refers to the absence of color bodies that impart a characteristic yellow hue to the crude material. The color bodies are aromatic compounds having the structures of benzils, fluorenones, and/or anthraquinones. 4-CBA and p-TAc are particularly detrimental to the polymerization process because they act as chain terminators during the condensation reaction between TPA and ethylene glycol in the production of polyethylene terephthalate (PET).  
         [0005]     In a typical process for producing TPA, a crude slurry is withdrawn from the primary oxidation reactor. The crude slurry contains a liquid mother liquor and solid particles of crude terephthalic acid (CTA). The liquid mother liquor exiting the primary oxidation reactor typically contains a significant amount of impurities. Thus, in many conventional TPA production processes, a substantial portion of the liquid mother liquor exiting the primary oxidation reactor is replaced/displaced with a “clean” replacement solvent prior to purification of the CTA particles. This replacement/displacement of the liquid mother liquor in the crude slurry with a replacement solvent is commonly known as “liquor exchange.” 
         [0006]     Conventional devices employed to perform liquor exchange can be both expensive and unreliable. One common device used to perform liquor exchange is a disc stack centrifuge system. Although effective for replacing the original mother liquor with a replacement solvent, the high-velocity rotating components of such mechanical centrifuge systems cause them to be expensive and unreliable. Thus, a CTA purification system that eliminates the use of one or more mechanical centrifuges would have a lower capital cost and higher reliability than conventional CTA purification systems. Further, if the liquor exchange function typically provided by a mechanical centrifuge could be combined with other functions of a CTA purification system, the overall cost of the system could be reduced while increasing its reliability. Finally, if one or more mechanical centrifuges could be replaced by mechanisms that more effectively replace impurity-laden mother liquor with clean replacement solvent, a purer TPA product could be produced.  
       SUMMARY OF THE INVENTION  
       [0007]     One embodiment of the present invention concerns a process comprising the following steps: (a) oxidizing one or more reactants in a primary oxidation reactor to thereby produce a solid/liquid mixture comprising CTA particles; and (b) subjecting at least a portion of the solid/liquid mixture to oxidative digestion in a zoned slurry concentrator, thereby producing a solids-concentrated mixture containing oxidation-treated TPA particles.  
         [0008]     Another embodiment of the present invention concerns a process comprising the following steps: (a) introducing a feed slurry into a zoned slurry concentrator, wherein the feed slurry comprises solid TPA particles; (b) withdrawing a liquid-concentrated mixture from a liquids outlet of the zoned slurry concentrator; and (c) withdrawing a solids-concentrated mixture from a solids outlet of the zoned slurry concentrator, wherein the ratio of the solids content of the solids-concentrated mixture to the solids content of the liquid-concentrated mixture is at least about 2:1 by weight.  
         [0009]     Still another embodiment of the present invention concerns a process comprising the following steps: (a) oxidizing para-xylene in a primary oxidation reactor to thereby produce an initial solid/liquid mixture containing CTA particles; (b) subjecting at least a portion of the initial solid/liquid mixture to oxidative digestion in an initial digester to thereby produce an initial digested solid/liquid mixture; (c) subjecting at least a portion of the initial digested solid/liquid mixture to oxidative digestion in a zoned slurry concentrator; (d) withdrawing a liquid-concentrated mixture from a liquids outlet of the zoned slurry concentrator, wherein the ratio of the solids content of the initial digested solid/liquid mixture to the solids content of the liquid-concentrated mixture is at least about 1.5:1 by weight; (e) withdrawing a solids-concentrated mixture from a solids outlet of the zoned slurry concentrator; and (f) introducing a dilution liquid into the solids-concentrated mixture to thereby produce a diluted mixture having a solids content less than the solids content of the solids-concentrated mixture.  
         [0010]     Yet another embodiment of the present invention concerns an apparatus for producing and purifying a solid/liquid mixture containing CTA particles. The system includes a primary oxidation reactor for producing the solid/liquid mixture and a zoned slurry concentrator. The zoned slurry concentrator comprises a vessel shell, an upright baffle, a feed inlet, a liquids outlet, and a solids outlet. The vessel shell defines an internal volume. The upright baffle is disposed in the vessel shell and separates at least a portion of the internal volume into a settling zone and an agitated zone. The feed inlet receives at least a portion of the solid/liquid mixture from the primary oxidation reactor and discharges the portion into the agitated zone. The liquids outlet is positioned proximate the settling zone. The solids outlet is positioned below the liquids outlet. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a process flow diagram illustrating a system for the production and purification of terephthalic acid constructed in accordance with a first embodiment of the present invention, particularly illustrating a configuration where the crude slurry from the primary oxidation reactor is subjected to oxidative digestion and the resulting slurry is subjected to combined crystallization and liquor exchange in a zoned slurry concentrator.  
         [0012]      FIG. 2  is a schematic representation of a zoned slurry concentrator having an agitated zone and a settling zone, where a liquid-concentrated mixture is withdrawn near the top of the settling zone and a solids-concentrated mixture is withdrawn a substantial distance below the top of the settling zone.  
         [0013]      FIG. 3  is a schematic representation of an alternative zoned slurry concentrator, where the liquid-concentrated mixture is withdrawn near the top of the settling zone and the solids-concentrated is withdrawn near the bottom of the settling zone.  
         [0014]      FIG. 4  is a process flow diagram illustrating a system for the production and purification of terephthalic acid constructed in accordance with a second embodiment of the present invention, particularly illustrating a configuration where the crude slurry exiting the primary oxidation reactor is subjected to combined crystallization and liquor exchange in a zoned slurry concentrator prior to oxidative digestion, and the slurry exiting the digester is also subjected to combined crystallization and liquor exchange prior to solids recovery.  
         [0015]      FIG. 5  is a process flow diagram illustrating a system for the production and purification of terephthalic acid constructed in accordance with a third embodiment of the present invention, particularly illustrating a configuration where the crude slurry exiting the primary oxidation reactor is subjected to combined oxidative digestion and liquor exchange in a zoned slurry concentrator, followed by final oxidative digestion, crystallization, and solids recovery steps.  
         [0016]      FIG. 6  is a process flow diagram illustrating a system for the production and purification of terephthalic acid constructed in accordance with a fourth embodiment of the present invention, particularly illustrating a configuration where the crude slurry exiting the primary oxidation reactor is subjected to initial oxidative digestion in a conventional reactor, followed by combined oxidative digestion and liquor exchange in a zone slurry concentrator. 
     
    
     DETAILED DESCRIPTION  
       [0017]     FIGS.  1  and 4-6 illustrate embodiments of the present invention where terephthalic acid (TPA) produced in a primary/initial oxidation reactor is purified in a system employing at least one zoned slurry concentrator. Zoned slurry concentrators are described in detail below with reference to  FIGS. 2 and 3 .  
         [0018]     In the embodiment illustrated in  FIG. 1 , a predominately liquid-phase feed stream containing an oxidizable compound (e.g., para-xylene), a solvent (e.g., acetic acid+water), and a catalyst system (e.g., Co+Mn+Br) is introduced into a primary/initial oxidation reactor  10 . A predominately gas-phase oxidant stream containing molecular oxygen is also introduced into primary oxidation reactor  10 . The liquid- and gas-phase feed streams form a multi-phase reaction medium in oxidation reactor  10 . The oxidizable compound undergoes partial oxidation in a liquid phase of the reaction medium contained in reactor  10 .  
         [0019]     Primary oxidation reactor  10  is preferably an agitated reactor. Agitation of the reaction medium in oxidation reactor  10  can be provided by any means known in the art. As used herein, the term “agitation” shall denote work dissipated into the reaction medium causing fluid flow and/or mixing. In one embodiment, primary oxidation reactor  10  is a mechanically-agitated reactor equipped with means for mechanically agitating the reaction medium. As used herein, the term “mechanical agitation” shall denote agitation of the reaction medium caused by physical movement of a rigid or flexible element(s) against or within the reaction medium. For example, mechanical agitation can be provided by rotation, oscillation, and/or vibration of internal stirrers, paddles, vibrators, or acoustical diaphragms located in the reaction medium. In a preferred embodiment of the invention, primary oxidation reactor  10  is a bubble column reactor. As used herein, the term “bubble column reactor” shall denote a reactor for facilitating chemical reactions in a multi-phase reaction medium, wherein agitation of the reaction medium is provided primarily by the upward movement of gas bubbles through the reaction medium. As used herein, the terms “majority,” “primarily,” and “predominately” shall mean more than 50 percent.  
         [0020]     The oxidizable compound present in the liquid-phase feed stream introduced into primary oxidation reactor  10  preferably comprises at least one hydrocarbyl group. More preferably, the oxidizable compound is an aromatic compound. Still more preferably, the oxidizable compound is an aromatic compound with at least one attached hydrocarbyl group or at least one attached substituted hydrocarbyl group or at least one attached heteroatom or at least one attached carboxylic acid function (—COOH). Even more preferably, the oxidizable compound is an aromatic compound with at least one attached hydrocarbyl group or at least one attached substituted hydrocarbyl group with each attached group comprising from 1 to 5 carbon atoms. Yet still more preferably, the oxidizable compound is an aromatic compound having exactly two attached groups with each attached group comprising exactly one carbon atom and consisting of methyl groups and/or substituted methyl groups and/or at most one carboxylic acid group. Even still more preferably, the oxidizable compound is para-xylene, meta-xylene, para-tolualdehyde, meta-tolualdehyde, para-toluic acid, meta-toluic acid, and/or acetaldehyde. Most preferably, the oxidizable compound is para-xylene.  
         [0021]     A “hydrocarbyl group,” as defined herein, is at least one carbon atom that is bonded only to hydrogen atoms or to other carbon atoms. A “substituted hydrocarbyl group,” as defined herein, is at least one carbon atom bonded to at least one heteroatom and to at least one hydrogen atom. “Heteroatoms,” as defined herein, are all atoms other than carbon and hydrogen atoms. Aromatic compounds, as defined herein, comprise an aromatic ring, preferably having at least  6  carbon atoms, even more preferably having only carbon atoms as part of the ring. Suitable examples of such aromatic rings include, but are not limited to, benzene, biphenyl, terphenyl, naphthalene, and other carbon-based fused aromatic rings.  
         [0022]     The amount of oxidizable compound present in the liquid-phase feed stream introduced into primary oxidation reactor  10  is preferably in the range of from about 2 to about 40 weight percent, more preferably in the range of from about 4 to about 20 weight percent, and most preferably in the range of from 6 to 15 weight percent.  
         [0023]     The solvent present in the liquid-phase feed stream introduced into primary oxidation reactor  10  preferably comprises an acid component and a water component. The solvent is preferably present in the liquid-phase feed stream at a concentration in the range of from about 60 to about 98 weight percent, more preferably in the range of from about 80 to about 96 weight percent, and most preferably in the range of from 85 to 94 weight percent. The acid component of the solvent is preferably an organic low molecular weight monocarboxylic acid having 1-6 carbon atoms, more preferably 2 carbon atoms. Most preferably, the acid component of the solvent is acetic acid. Preferably, the acid component makes up at least about 75 weight percent of the solvent, more preferably at least about 80 weight percent of the solvent, and most preferably in the range of from 85 to 98 weight percent of the solvent, with the balance being water.  
         [0024]     The liquid-phase feed stream introduced into primary oxidation reactor  10  can also include a catalyst system. The catalyst system is preferably a homogeneous, liquid-phase catalyst system capable of promoting partial oxidation of the oxidizable compound. More preferably, the catalyst system comprises at least one multivalent transition metal. Still more preferably, the multivalent transition metal comprises cobalt. Even more preferably, the catalyst system comprises cobalt and bromine. Most preferably, the catalyst system comprises cobalt, bromine, and manganese.  
         [0025]     When cobalt is present in the catalyst system, it is preferred for the amount of cobalt present in the liquid-phase feed stream to be such that the concentration of cobalt in the liquid phase of the reaction medium is maintained in the range of from about 300 to about 6,000 parts per million by weight (ppmw), more preferably in the range of from about 700 to about 4,200 ppmw, and most preferably in the range of from 1,200 to 3,000 ppmw. When bromine is present in the catalyst system, it is preferred for the amount of bromine present in the liquid-phase feed stream to be such that the concentration of bromine in the liquid phase of the reaction medium is maintained in the range of from about 300 to about 5,000 ppmw, more preferably in the range of from about 600 to about 4,000 ppmw, and most preferably in the range of from 900 to 3,000 ppmw. When manganese is present in the catalyst system, it is preferred for the amount of manganese present in the liquid-phase feed stream to be such that the concentration of manganese in the liquid phase of the reaction medium is maintained in the range of from about 20 to about 1,000 ppmw, more preferably in the range of from about 40 to about 500 ppmw, most preferably in the range of from 50 to 200 ppmw.  
         [0026]     The weight ratio of cobalt to bromine (Co:Br) in the catalyst system introduced into primary oxidation reactor  10  is preferably in the range of from about 0.25:1 to about 4:1, more preferably in the range of from about 0.5:1 to about 3:1, and most preferably in the range of from 0.75:1 to 2:1. The weight ratio of cobalt to manganese (Co:Mn) in the catalyst system introduced is preferably in the range of from about 0.3:1 to about 40:1, more preferably in the range of from about 5:1 to about 30:1, and most preferably in the range of from 10:1 to 25:1.  
         [0027]     During oxidation, it is preferred for the oxidizable compound (e.g., para-xylene) to be continuously introduced into primary oxidation reactor  10  at a rate of at least about 5,000 kilograms per hour, more preferably at a rate in the range of from about 10,000 to about 80,000 kilograms per hour, and most preferably in the range of from 20,000 to 50,000 kilograms per hour. During oxidation, it is preferred for the ratio of the mass flow rate of the solvent to the mass flow rate of the oxidizable compound entering oxidation reactor  10  to be maintained in the range of from about 2:1 to about 50:1, more preferably in the range of from about 5:1 to about 40:1, and most preferably in the range of from 7.5:1 to 25:1.  
         [0028]     The predominately gas-phase oxidant stream introduced into primary oxidation reactor  10  preferably comprises in the range of from about 5 to about 40 mole percent molecular oxygen, more preferably in the range of from about 15 to about 30 mole percent molecular oxygen, and most preferably in the range of from 18 to 24 mole percent molecular oxygen. It is preferred for the balance of the oxidant stream to be comprised primarily of a gas or gases, such as nitrogen, that are inert to oxidation. More preferably, the oxidant stream consists essentially of molecular oxygen and nitrogen. Most preferably, the oxidant stream is dry air that comprises about 21 mole percent molecular oxygen and about 78 to about 81 mole percent nitrogen. In an alternative embodiment of the present invention, the oxidant stream can comprise substantially pure oxygen.  
         [0029]     During liquid-phase oxidation in primary oxidation reactor  10 , it is preferred for the oxidant stream to be introduced into reactor  10  in an amount that provides molecular oxygen somewhat exceeding the stoichiometric oxygen demand. Thus, it is preferred that the ratio of the mass flow rate of the oxidant stream (e.g., air) to the mass flow rate of the oxidizable compound (e.g., para-xylene) entering reactor  10  is maintained in the range of from about 0.5:1 to about 20:1, more preferably in the range of from about 1:1 to about 10:1, and most preferably in the range of from 2:1 to 6:1.  
         [0030]     The liquid-phase oxidation reaction carried out in reactor  10  is preferably a precipitating reaction that generates solids. More preferably, the liquid-phase oxidation carried out in reactor  10  causes at least about 10 weight percent of the oxidizable compound (e.g., para-xylene) introduced into oxidation reactor  10  to form solids (e.g., CTA particles) in the reaction medium. Still more preferably, the liquid-phase oxidation causes at least about 50 weight percent of the oxidizable compound to form solids in the reaction medium. Most preferably, the liquid-phase oxidation causes at least 90 weight percent of the oxidizable compound to form solids in the reaction medium. It is preferred for the solids content of the reaction medium to be maintained in the range of from about 5 to about 40 weight percent, still more preferably in the range of from about 10 to about 35 weight percent, and most preferably in the range of from 15 to 30 weight percent. As used herein, the term “solids content” shall denote the weight percent solids in a multi-phase mixture.  
         [0031]     During oxidation in oxidation reactor  10 , the multi-phase reaction medium is preferably maintained at an elevated temperature in the range of from about 125 to about 200° C., more preferably in the range of from about 150 to about 180° C., and most preferably in the range of from 155 to 165° C. The overhead pressure in oxidation reactor  10  is preferably maintained in the range of from about 1 to about 20 bar gauge (barg), more preferably in the range of from about 2 to about 12 barg, and most preferably in the range of from 4 to 8 barg.  
         [0032]     As illustrated in  FIG. 1 , a crude product slurry is withdrawn from an outlet of primary oxidation reactor  10  via line  12 . The solid phase of the crude product slurry in line  12  is preferably formed primarily of solid particles of crude terephthalic acid (CTA). The liquid phase of the crude product slurry in line  12  is a liquid mother liquor comprising at least a portion of the solvent, the catalyst system, and minor amounts of dissolved TPA. The solids content of the crude product slurry in line  12  is preferably the same as the solids content of the reaction medium in primary oxidation reactor  10 , discussed above. The crude product slurry in line  12  typically has a combined 4-carboxybenzaldehyde (4-CBA) and para-toluic acid (pTAc) content in a range of from about 100 to about 500 parts per million by weight (ppmw) based on the weight of the solids.  
         [0033]     In one embodiment of the present invention, the crude product slurry in line  12  is transported directly (i.e., without intermediate processing steps) to a digester  18  for purification by oxidative digestion. In an alternative embodiment, an optional liquor exchange system  14  is employed to remove at least a portion of the liquid mother liquor from the crude product slurry and replace the removed mother liquor (i.e., filtrate) with a replacement solvent prior introduction into digester  18 . The replacement solvent typically comprises acetic acid and/or water. The primary purpose of liquor exchange  14  is to provide cleaner solvent media for subsequent oxidative digestion of the CTA solids.  
         [0034]     Optional liquor exchange system  14  can employ a variety of different apparatuses to remove and replace at least a portion of the crude mother liquor. For example, liquor exchange system  14  can employ a disc stack centrifuge to separate at least a portion of the mother liquor from the CTA solids and replace the removed mother liquor with a clean replacement solvent. Alternatively, liquor exchange can be accomplished using any suitable solid/liquid separator (e.g., a decanter centrifuge, a rotary disk centrifuge, a belt filter, or a rotary vacuum filter) and then diluting the resulting solids (typically a wet cake) with a clean replacement solvent. In one embodiment of the present invention, at least about 25 weight percent of the mother liquor in the crude product slurry is replaced with a replacement solvent, more preferably at least about 50 weight percent of the mother liquor is replaced with a replacement solvent, and most preferably in the range of from 75 to 99 weight percent of the mother liquor is replaced with a replacement solvent. When liquor exchange system  14  is employed, the resulting liquor-exchanged slurry is passed through line  16  to digester  18 .  
         [0035]     In digester  18 , the crude product slurry or liquor-exchanged slurry is subjected to purification by oxidative digestion. As used herein, the term “oxidative digestion” denotes a process step or steps where a feed is subjected to oxidation. Digester  18  can be one or more reactors or zones. Preferably, digester  18  comprises one or more mechanically-agitated reactors. A secondary oxidant stream, which can have the same composition as the gas-phase oxidant stream fed to primary oxidation reactor  10 , is introduced into digester  18  to provide the molecular oxygen required for oxidative digestion. Additional oxidation catalyst can also be added if necessary. As mentioned above, the crude product slurry or liquor-exchange slurry introduced into digester  18  contains significant quantities of impurities such as, for example, 4-CBA and p-TAc. The oxidative digestion in digester  18  preferably causes oxidation of a substantial portion of the 4-CBA and p-TAc to TPA.  
         [0036]     The slurry fed to digester  18  typically has a 4-CBA content of at least about 100 parts per million based on the weight of the solids in the feed slurry (ppmw fs ), more typically in the range of from about 200 to about 10,000 ppmw fs  and most typically in the range of from 800 to 5,000 ppmw fs . The slurry fed to digester  18  typically has a p-TAc content of at least about 250 ppmw fs , more typically in the range of from about 300 to about 5,000 ppmw fs  and most typically in the range of from 400 to 1,500 ppmw fs . The slurry product exiting digester  18  preferably has a 4-CBA content of less than about 150 parts per million based on the weight of the solids in the product slurry (ppmw ps ), more preferably less than about 100 ppmw ps , and most preferably less than 50 ppmw ps . The slurry product exiting digester  18  preferably has a p-TAc content of less than about 300 ppmw ps , more preferably less than about 200 ppmw ps  and most preferably less than 150 ppmw ps . Preferably, oxidative digestion in digester  18  causes the product slurry exiting digester  18  to have 4-CBA and/or p-TAc content that is at least about 50 percent less than the 4-CBA and/or p-TAc content of the slurry fed to digester  18 , more preferably at least about 85 percent less, and most preferably at least 95 percent less. By way of illustration, if the 4-CBA content of the slurry fed to digester  18  is 200 ppmw fs  and the 4-CBA content of the product slurry exiting digester  18  is 100 ppmw ps , then the 4-CBA content of the product slurry is 50 percent less than the 4-CBA content of the feed slurry.  
         [0037]     The temperature at which oxidative digestion is carried out in digester  18  is preferably at least about 110° C. greater than the temperature of oxidation in primary oxidation reactor  10 , more preferably in the range of from about 20 to about 80° C. greater, and most preferably in the range of from 30 to 50° C. greater. The additional heat required for the operation of digester  18  may be provided by supplying a vaporized solvent to digester  18  and allowing the vaporized solvent to condense therein. The oxidative digestion temperature in digester  18  is preferably maintained in the range of from about 180 to about 240° C., more preferably in the range of from about 190 to about 220° C., and most preferably in the range of from 200 to 210° C. The oxidative digestion pressure in digester  18  is preferably maintained in the range of from about 100 to about 350 pounds per square inch gauge (psig), more preferably in the range of from about 175 to about 275 psig, and most preferably in the range of from 185 to 225 psig.  
         [0038]     In one embodiment of the present invention, digester  18  includes two digestion reactors/zones—an initial digester and a final digester. When digester  18  includes an initial digester and a final digester, it is preferred for the final digester to be operated at a lower temperature and pressure than the initial digester. Preferably, the operating temperature of the final digester is at least about 2° C. lower than the operating temperature of the initial digester. Most preferably, the operating temperature of the final digester is in the range of from about 5 to about 15° C. lower than the operating temperature of the initial digester. Preferably, the operating pressure of the final digester is at least about 5 psi lower than the operating pressure of the initial digester. Most preferably, the operating pressure of the final digester is in the range of from about 10 to about 50 psig lower than the operating temperature of the initial digester. Preferably, the operating temperature of the initial digester is in the range of from about 195 to about 225° C., more preferably in the range of from 205 to 215° C., and most preferably about 210° C. Preferably, the operating pressure of the initial digester is in the range of from about 215 to 235 psig, most preferably about 225 psig. Preferably, the operating temperature of the final digester is in the range of from about 190 to about 220° C., more preferably in the range of from 200 to 210° C., and most preferably about 205° C. Preferably, the operating pressure of the final digester is in the range of from about 190 to 210 psig, most preferably about 200 psig.  
         [0039]     Referring again to  FIG. 1 , an oxidation-treated slurry is discharged from digester  18  via line  20 . The solid phase of the oxidation-treated slurry is formed primarily of purified terephthalic acid (PTA) particles, while the liquid phase is formed of an oxidation-treated mother liquor. The solids content of the oxidation-treated slurry in line  20  is preferably in the range of from about 1 to about 50 percent by weight, more preferably in the range of from about 5 to about 40 percent by weight, and most preferably in the range of from 20 to 30 percent by weight. The oxidation-treated slurry in line  20  is introduced into a zoned slurry concentrator  100 .  
         [0040]     As used herein, the term “zoned slurry concentrator” means a single vessel having at least one internal upright baffle that divides at least a portion of the vessel&#39;s internal volume into an agitated zone and a settling zone; where the agitated zone communicates with the settling zone proximate a lower end/opening of the lowest baffle; and where the vessel defines a liquids outlet for removing a liquid-concentrated mixture from the settling zone at an elevation above the lower end/opening of the lowest baffle and a solids outlet for removing a solids-concentrated mixture from the vessel at an elevation below the liquids outlet.  
         [0041]      FIG. 2  illustrates a preferred zoned slurry concentrator  100  suitable for use in the present invention. Zoned slurry concentrator  100  comprises a vessel shell  102  that defines and internal volume  104 . Vessel shell  102  preferably has at least one substantially vertical sidewall that has a generally cylindrical and/or frustoconical configuration. Zoned slurry concentrator  100  also comprises at least one upright baffle  106  that separates at least a portion of internal volume  104  into an agitated zone  108  and a settling zone  110 . Upright baffle  106  can be formed of a single member, or a plurality of baffle members. Upright baffle  106  preferably includes at least one generally cylindrical and/or frustoconical baffle spaced inwardly from the upright sidewalls of vessel shell  102 . Zoned slurry concentrator  100  also comprises an upright draft tube  112  at least partly disposed in agitated zone  108 . Draft tube  112  is preferably a substantially hollow, generally cylindrical or frustoconical tube spaced inwardly from upright baffle  106 . An upwardly moving section  114  of agitated zone  108  is defined within draft tube  112 , and a downwardly moving section  116  of agitated zone  108  is defined between draft tube  112  and vessel shell  102 .  
         [0042]     In operation, a feed slurry in line  118  is introduced into zoned slurry concentrator  100  via a feed inlet  120 . The feed slurry enters a lower opening  122  of draft tube  112  and is forced upwardly by a rotating propeller  124 . Propeller  124  is powered by a motor  126 . Motor  126  and propeller  124  are connected by a drive shaft  128  that extends through vessel shell  102  and into agitated zone  108  of internal volume  104 . The feed slurry moves upwardly though draft tube  112  and then exits an upper opening  130  of draft tube  112 . The level of slurry in vessel shell  102  is maintained so that the upper surface  131  of the slurry is above upper opening  130  of draft tube  112 . As the slurry exits upper opening  130  of draft tube  112 , it circulates into a downward flow pattern in downwardly-moving section  116  of agitated zone  108 . At least a portion of downwardly-moving section  116  of agitated zone  108  is defined between draft tube  112  and upright baffle  106 . An agitated zone outlet opening  133  is defined between baffle  106  and draft tube  112  proximate the bottom of upright baffle  106 . The slurry exits agitated zone  108  through agitated zone outlet opening  133 . The after exiting agitated zone  108  the slurry enter settling zone  110  via a settling zone opening  135 . Settling zone opening  135  is defined between vessel shell  102  and baffle  106  proximate the bottom of upright baffle  106 .  
         [0043]     Upright baffle  106  is operable to substantially isolate settling zone  110  from the turbulence generated by rotating propeller in agitated zone  108 . In settling zone  110 , the liquid phase of the slurry moves upwardly in a relatively non-turbulent manner at a velocity less than the velocity necessary to suspend the solid phase in the upwardly moving liquid phase. This relatively slow, non-turbulent, upward flow of the liquid phase in settling zone  110  permits the downward gravitational force acting on the solid phase particles to overcome the upward liquid flow forces acting on the solid phase particles, thereby cause solid/liquid disengagement in settling zone  110 . This solid/liquid disengagement in settling zone  110  produces a liquid-concentrated mixture with low solids content from the top of settling zone  110 . The liquid-concentrated mixture is withdrawn from internal volume  104  via liquids outlet  134  and is transported away from zoned slurry concentrator  100  in line  136 . The solid particles that travel downward through settling zone  110  exit settling zone  110  via settling zone opening  135 . These solids accumulate near the bottom of internal volume  104  of vessel shell  102 , thereby forming a solids-concentrated mixture with high solids content. The solids-concentrated mixture is withdrawn from internal volume  104  via solids outlet  138  and is transported away from zoned slurry concentrator  100  in line  140 . Vapor introduced into or formed in zoned slurry concentrator  100  is discharged through a vapor outlet  142  and transported away from zoned slurry concentrator  100  in line  144 .  
         [0044]     In a preferred embodiment of the present invention, the feed slurry introduced into zoned slurry concentrator  100  via feed inlet  120  has a solids content in the range of from about 1 to about 50 weight percent, more preferably in the range of from about 5 to about 40 weight percent, and most preferably in the range of from 20 to 30 weight percent. It is preferred for the solids in the feed slurry introduced into zoned slurry concentrator  100  to be formed of at least about 75 weight percent TPA, more preferably at least about 90 weight percent TPA, and most preferably at least 95 weight percent TPA.  
         [0045]     The solids-concentrated mixture exiting zoned slurry concentrator  100  via solids outlet  138  preferably has a solids content of more than about 60 weight percent, more preferably more than about 80 weight percent, and most preferably more than 95 weight percent. The liquid-concentrated mixture exiting zoned slurry concentrator  100  via liquids outlet  134  preferably has a solids content of less than about 40 weight percent, more preferably less than about 20 weight percent, and most preferably less than 5 weight percent.  
         [0046]     Preferably, the solids content of the liquid-concentrated mixture exiting zoned slurry concentrator  100  via liquids outlet  134  is at least about 50 percent less than the solids content of the feed slurry, more preferably at least about 75 percent less than the solids content of the feed slurry, and most preferably at least 90 percent less than the solids content of the feed slurry. For example, if the feed slurry has a solids content of 30 weight percent and the liquid-concentrated mixture has a solids content of 15 weight percent, then the liquid-concentrated mixture has a solids content that is 50 percent less than the solids content of the feed slurry. Preferably, the solids content of the solids-concentrated mixture exiting zoned slurry concentrator  100  via solids outlet  138  is at least about 50 percent greater than the solids content of the feed slurry, more preferably at least about 100 percent greater than the solids content of the feed slurry, and most preferably at least 200 percent greater than the solids content of the feed slurry. For example, if the feed slurry has a solids content of 30 weight percent and the solids-concentrated mixture has a solids content of 45 weight percent, then the solids-concentrated mixture has a solids content that is 50 percent greater than the solids content of the feed slurry.  
         [0047]     The ratio of the solids content of the solids-concentrated mixture exiting zoned slurry concentrator  100  via solids outlet  138  to the liquid-concentrated mixture exiting zoned slurry concentrator  100  via liquids outlet  134  is preferably at least about 2:1, more preferably at least about 10:1, and most preferably at least 50:1. The ratio of the solids content of the solids-concentrated mixture exiting zoned slurry concentrator  100  via solids outlet  138  to the feed slurry entering zoned slurry concentrator  100  via feed inlet  120  is preferably at least about 1.5:1, more preferably at least about 5:1, and most preferably at least 25:1. The ratio of the solids content of the feed slurry entering zoned slurry concentrator  100  via feed inlet  120  to the liquid-concentrated mixture exiting zoned slurry concentrator  100  via liquids outlet  134  is preferably at least about 1.5:1, more preferably at least about 5:1, and most preferably at least 25:1.  
         [0048]     In one embodiment of the present invention, the solids-concentrated mixture is diluted with a dilution liquid. This dilution of the solids-concentrated mixture can be accomplished by introducing the dilution liquid into zoned slurry concentrator  100  at a location near solids outlet  138 . Preferably, dilution is accomplished by introducing the dilution liquid into the solids-concentrated mixture after the solids-concentrated mixture has been removed from zoned slurry concentrator  100 . The dilution liquid can function as a “clean” replacement solvent for the solvent removed from zoned slurry concentrator  100  with the liquid-concentrated mixture. The dilution liquid is preferably a solvent for TPA. Most preferably, the dilution liquid comprises acetic acid and/or water.  
         [0049]     Dilution of the solids-concentrated mixture with the dilution liquid forms a diluted mixture. Preferably, the dilution liquid is added to the solids-concentrated mixture in an amount such that the solids content of the diluted mixture is at least about 25 percent less than the solids content of the solids-concentrated mixture, more preferably at least about 50 percent less of the solids content of the solids-concentrated mixture, and most preferably in the range of from 60 to 90 percent less than the solids content of the solids-concentrated mixture. Preferably, the solids content of the diluted mixture is in the range of from about 1 to about 50 weight percent, more preferably in the range of from about 5 to about 40 weight percent, and most preferably in the range of from 20 to 30 weight percent.  
         [0050]      FIG. 3  illustrates an alternative embodiment of zoned slurry concentrator  100 . Common components of the zoned slurry concentrators illustrated in  FIGS. 2 and 3  are identified with common reference numerals. One difference between the zoned slurry concentrators of  FIGS. 2 and 3  is that upright baffle  106  of the zoned slurry concentrator illustrated in  FIG. 3  includes one or more circulation openings  150  between the upper and lower ends of upright baffle  106 . Another difference between the zoned slurry concentrators of FIGS.  2  and  3  is that solids outlet  138  of the zoned slurry concentrator  100  illustrated in  FIG. 3  withdraws the solids-concentrated mixture at a higher point than solids outlet  138  of  FIG. 2 .  
         [0051]      FIGS. 2 and 3  illustrate specific embodiments of zoned slurry concentrators suitable for use in the present invention. It should be noted, however, that devices having significantly different configurations than those illustrated in  FIGS. 2 and 3  may also fall within the definition of “zoned slurry concentrator,” provided herein. One example of a commercially-available zoned slurry concentrator suitable for use in the present invention is a “Draft Tube Baffle (DTB) Crystallizer,” available from Swenson Technology, Inc., Monee, Ill.  
         [0052]     In the embodiment illustrated in  FIG. 1 , feed line  20  is equivalent to feed line  118  of  FIG. 2 , liquids line  24  is equivalent to liquids line  136  of  FIG. 2 , and solids line  26  is equivalent to solids line  140  of  FIG. 2 . The system shown in  FIG. 1  employs zoned slurry concentrator  100  as a post-digestion crystallizer. Post-digestion crystallization is preferably carried out at a temperature and pressure lower than the temperature and pressure of the final oxidative digestion performed in digester  18 . Preferably, the post-digestion crystallization temperature is at least about 10° C. less than the final digestion temperature, more preferably in the range of from about 15 to about 65° C. less than the final digestion temperature, and most preferably in the range of from 25 to 55° C. less than the final digestion temperature. Preferably, the post-digestion crystallization temperature is maintained in the range of from about 130 to about 200° C., more preferably in the range of from about 150 to about 180° C., and most preferably in the range of from 160 to 170° C. Preferably the post-digestion crystallization pressure is maintained in the range of from about 10 to about 150 psig, more preferably in the range of from about 50 to about 120 psig, and most preferably about 80 psig.  
         [0053]     Referring again to  FIG. 1 , the solids-concentrated mixture exiting zoned slurry concentrator  100  via line  26  can be diluted with a dilution liquid supplied via line  30 . As discussed above, in one embodiment of the present invention, the dilution liquid is combined with the solids-concentrated mixture in an external dilution zone located outside of zoned slurry concentrator  100 . In another embodiment of the invention, the dilution liquid is combined with the solids-concentrated mixture in an internal dilution zone located inside zoned slurry concentrator  100 . The amount and composition of the dilution liquid introduced into the solids-concentrated mixture is discussed above with reference to  FIG. 2 .  
         [0054]     The diluted mixture resulting from dilution of the solids-concentrated mixture with the dilution liquid is transported in line  32  to a solids recovery system  34  for recovery of solid PTA particles. Solids recovery system  34  preferably includes at least one solid/liquid separator and at least one drier. The solid/liquid separator employed as part of solids recovery system  34  can be any conventional solid/liquid separator such as, for example, a decanter centrifuge, a rotary disk centrifuge, a belt filter, or a rotary vacuum filter. The solids separated in the solid/liquid separator can then be dried using any suitable drier known in the art. The recovered, dried PTA solids are discharged from solids recovery system  34  via line  36 .  
         [0055]      FIG. 4  illustrates an embodiment of the present invention where two zoned slurry concentrators  100   a  and  100   b  are employed to aid in the purification of the crude oxidation product from primary oxidation reactor  210 . In the embodiment of  FIG. 4 , primary oxidation reactor  210 , digester  218 , and solids recovery system  234  are operated in substantially the same manner as discussed above with reference to primary oxidation reactor  10 , digester  18 , and solids recovery system  34  of  FIG. 1 . However, the system of  FIG. 4  employs zoned slurry concentrator  100   a  to perform liquor exchange and/or crystallization between primary oxidation reactor  210  and digester  218 . Further, the system of  FIG. 4  employs zoned slurry concentrator  100   b  to perform crystallization and/or liquor exchange between digester  218  and solids recovery system  234 .  
         [0056]     Zoned slurry concentrator  100   a  receives the crude product slurry directly from primary oxidation reactor  210  via line  212 . In zoned slurry concentrator  100   a , the crude product slurry is separated into a liquid-concentrated mixture exiting via line  250  and a solids-concentrated mixture exiting via line  252 . Further, vapor exits zoned slurry concentrator  100   a  via line  254 . In the embodiment illustrated in  FIG. 4 , line  212  is equivalent to line  118  of  FIG. 2 , line  250  is equivalent to line  136  of  FIG. 2 , line  252  is equivalent to line  140  of  FIG. 2 , and line  254  is equivalent to line  144  of  FIG. 2 . Zoned slurry concentrator  100   a  of  FIG. 4  preferably operates in substantially the same manner described above with reference to the zoned slurry concentrators of  FIGS. 2 and 3 .  
         [0057]     The solids-concentrated mixture exiting zone slurry concentrator  100   a  via line  252  is diluted with a dilution liquid in line  256 . This dilution liquid is preferably a clean replacement solvent that replaces the crude mother liquor removed from zoned slurry concentrator  100   a  via line  250 . The amount and composition of dilution liquid used to dilute the solids-concentrated mixture is described above with reference to  FIG. 2 . The resulting diluted mixture is introduced into digester  218  via line  258  for purification by oxidative digestion.  
         [0058]     The oxidation-treated slurry exits digester  218  via line  220  and is introduced into zoned slurry concentrator  100   b . In zoned slurry concentrator  100   b , the oxidation-treated slurry is separated into a liquid-concentrated mixture exiting via line  260  and a solids-concentrated mixture exiting via line  262 . Further, vapor exits zoned slurry concentrator  100   b  via line  263 . In the embodiment illustrated in  FIG. 4 , line  220  is equivalent to line  118  of  FIG. 2 , line  260  is equivalent to line  136  of  FIG. 2 , line  262  is equivalent to line  140  of  FIG. 2 , and line  263  is equivalent to line  144  of  FIG. 2 . Zoned slurry concentrator  100   b  of  FIG. 4  preferably operates in substantially the same manner described above with reference to the zoned slurry concentrators of  FIGS. 2 and 3 .  
         [0059]     The solids-concentrated mixture exiting zone slurry concentrator  100   b  via line  162  is diluted with a dilution liquid in line  264 . This dilution liquid is preferably a clean replacement solvent that replaces the oxidation-treated mother liquor removed from zoned slurry concentrator  100   b  via line  260 . The amount and composition of dilution liquid used to dilute the solids-concentrated mixture is described above with reference to  FIG. 2 . The resulting diluted mixture is introduced into solids recovery system  234  via line  266  for recovery of PTA solids.  
         [0060]      FIG. 5  illustrates an embodiment of the present invention where zoned slurry concentrator  100  is employed as the initial digester in a process that includes purification by two-step oxidative digestion. In the embodiment of  FIG. 5 , primary oxidation reactor  310  and solids recovery system  334  operate in substantially the same manner as discussed above with reference to primary oxidation reactor  10  and solids recovery system  34  of  FIG. 1 . However, the system of  FIG. 5  employs zoned slurry concentrator  100  to perform initial oxidative digestion, liquor exchange, and/or crystallization of the crude product slurry exiting primary oxidation reactor  310  via line  350 . Further, the system of  FIG. 5  employs a final digester  368  for providing further oxidative digestion of the solids exiting zoned slurry concentrator  100 . Finally, the system of  FIG. 5  employs a crystallization system  372  downstream of final digester  368  and upstream of solids recovery system  334 .  
         [0061]     Zoned slurry concentrator  100  receives the crude product slurry directly from primary oxidation reactor  310 . In order for oxidative digestion to be carried out in zoned slurry concentrator  100 , an oxidant stream containing molecular oxygen is introduced into zoned slurry concentrator  100  via line  354 . Alternatively, the oxidant stream can be introduced into the crude product slurry upstream of zoned slurry concentrator  100  via line  356 , thereby providing an oxygen-enriched crude slurry in line  352 . As illustrated in  FIGS. 2 and 3 , when the oxidant stream is introduced into zoned slurry concentrator  100 , an additional oxidant inlet  160  is provided in zoned slurry concentrator  100  for receiving the oxidant stream. Oxidant inlet  160  preferably discharges the oxidant stream at a location just below the propeller  124 . In the embodiment illustrated in  FIG. 5 , line  354  is equivalent to line  162  of  FIGS. 2 and 3 , while line  356  is equivalent to line  164  of  FIGS. 2 and 3 .  
         [0062]     Initial oxidative digestion is carried out in zoned slurry concentrator  100  in a manner and under conditions that are substantially similar to the initial oxidative digestion described above. The initial digested solids-concentrated mixture exiting zoned slurry concentrator  100  via line  362  is diluted with a dilution liquid in line  364  to thereby produce a diluted mixture introduced into final digester  368  via line  366 . Final oxidative digestion is carried out in final digester  368  in the manner previously described. The final digested slurry exiting final digester  368  is introduced into crystallization system  372  via line  370 . In crystallization system  372 , solids precipitate due to reduced temperature and pressure—typically in a series of crystallization zones. Typically, the temperature in crystallization system  372  is maintained in the range of from about 30 to about 190° C. The crystallized slurry produced in crystallization system  372  is transported to solids recovery system  334  via line  374  for recovery of the final PTA product.  
         [0063]      FIG. 6  illustrates an embodiment of the present invention where zoned slurry concentrator  100  is employed as the final digester in a process that includes purification by two-step oxidative digestion. In the embodiment of  FIG. 6 , primary oxidation reactor  410 , liquor exchange system  414 , and solids recovery system  434  operate in substantially the same manner as discussed above with reference to primary oxidation reactor  10 , liquor exchange system  14 , and solids recovery system  34  of  FIG. 1 . However, the system of  FIG. 6  employs zoned slurry concentrator  100  to perform final oxidative digestion, liquor exchange, and/or crystallization of the initial digested slurry exiting initial digester  450  via line  452 .  
         [0064]     The crude product slurry or liquor-exchanged slurry produced from primary oxidation reactor  410  or liquor exchange system  414  (depending on whether or not liquor exchange system  414  is employed) is introduced into initial digester  450  for initial oxidative digestion. The oxidative digestion in initial digester  450  is carried out in accordance with the previous description. The resulting initial digested slurry exits initial digester  450  via line  452  and is introduced into zoned slurry concentrator  100  for final oxidative digestion.  
         [0065]     In order for oxidative digestion to be carried out in zoned slurry concentrator  100 , an oxidant stream containing molecular oxygen is introduced into zoned slurry concentrator  100  via line  456 . Alternatively, the oxidant stream can be introduced into the initial digested slurry upstream of zoned slurry concentrator  100  via line  458 , thereby providing an oxygen-enriched slurry in line  454 . As illustrated in  FIGS. 2 and 3 , when the oxidant stream is introduced into zoned slurry concentrator  100 , an additional oxidant inlet  160  is provided in zoned slurry concentrator  100  for receiving the oxidant stream. In the embodiment illustrated in  FIG. 6 , line  456  is equivalent to line  162  of  FIGS. 2 and 3 , while line  458  is equivalent to line  164  of  FIGS. 2 and 3 .  
         [0066]     Final oxidative digestion is carried out in zoned slurry concentrator  100  in a manner and under conditions that are substantially similar to the final oxidative digestion described above. The final digested solids-concentrated mixture exiting zoned slurry concentrator  100  via line  464  is diluted with a dilution liquid in line  486  to thereby produce a diluted mixture. The diluted mixture is transported to solids recovery system  434  via line  470  for recovery of the final PTA product.  
         [0067]     The inventors note that for all numerical ranges provided herein, the upper and lower ends of the ranges can be independent of one another. For example, a numerical range of 10 to 100 means greater than 10 and/or less than 100. Thus, a range of 10 to 100 provides support for a claim limitation of greater than 10 (without the upper bound), a claim limitation of less than 100 (without the lower bound), as well as the full 10 to 100 range (with both upper and lower bounds).  
         [0068]     The inventors also note that, as used herein, “coupled in communication” denotes a direct or indirect connection that permits the flow of solids and/or liquids. For example, the outlet of primary oxidation reactor  10  ( FIG. 1 ) is “coupled in communication” with the inlet of zoned slurry concentrator  100 , even though there is intermediate equipment (e.g., digester  18 ) located therebetween.  
         [0069]     The invention has been described in detail with particular reference to preferred embodiments thereof, but will be understood that variations and modification can be affected within the spirit and scope of the invention.