Abstract:
The present invention relates to a process by which a carboxylic acid/diol mixture is obtained from a carboxylic acid/solvent slurry without isolation of a substantially dry carboxylic acid solid. More specifically, the present invention relates to a process by which a terephthalic acid/ethylene glycol mixture is obtained from a terephthalic acid/solvent slurry without isolation of a substantially dry terephthalic acid solid.

Description:
FIELD OF INVENTION 
     The present invention relates to a process by which a carboxylic acid/diol mixture is obtained from a carboxylic acid/solvent slurry without isolation of a substantially dry carboxylic acid solid. More specifically, the present invention relates to a process by which a terephthalic acid/diol mixture suitable as a starting material for polyester production is obtained from a terephthalic acid/solvent slurry without isolation of a substantially dry terephthalic acid solid. 
     BACKGROUND OF THE INVENTION 
     Pursuant to the goal of making polyethylene terephthalate(PET) and other polyesters, a great deal of patent literature is dedicated to the describing processes for preparing a carboxylic acid/diol mixture suitable as starting material. In general, these inventions describe specific mixing schemes with a purified terephthalic acid solid and liquid ethylene glycol. Additionally, there is substantial body of literature devoted to producing a purified terephthalic acid in the powder form that is suitable for use in producing PET and other polyesters. 
     The objective of this invention is to describe a process by which the carboxylic acid/diol mixture suitable as a starting material for polyester production is obtained from a carboxylic acid/solvent slurry without isolation of a substantially dry carboxylic acid solid. More specifically, the objective of this invention is to describe a process by which a terephthalic acid/diol mixture suitable as a starting material for polyester production is obtained from a terephthalic acid/solvent slurry without isolation of a substantially dry terephthalic acid solid. 
     Usually, purified terephthalic acid solid is produced in a multi-step process wherein a crude terephthalic acid is produced. The crude terephthalic acid does not have sufficient quality for direct use as starting material in commercial PET. Instead, the crude terephthalic acid is usually refined to purified terephthalic acid solid. 
     Liquid phase oxidation of p-xylene produces crude terephthalic acid. The crude terephthalic acid is dissolved in water and hydrogenated for the purpose of converting 4-carboxybenzaldehyde to p-toluic acid, which is a more water-soluble derivative, and for the purpose of converting characteristically yellow compounds to colorless derivatives. Any 4-carboxybenzaldehyde and p-toluic acid in the final purified terephthalic acid product is particularly detrimental to polymerization processes as they act as a chain terminator during the condensation reaction between terephthalic acid and ethylene glycol in the production of PET. Typical purified terephthalic acid contains on a weight basis less than 25 parts per million (ppm) 4-carboxybenzaldehyde and less than 150 ppm p-toluic acid. 
     A number of other processes have been developed where a terephthalic acid suitable as starting material for commercial PET production without the use of hydrogenation. Typically, these processes usually involve catalyzed oxidation of p-xylene in an acetic acid solvent followed by filtration and drying of the terephthalic acid from the acetic acid solvent. 
     To produce a terephthalic acid/diol mixture acceptable for PET production from a terephthalic acid/solvent slurry poses a substantially different problem than from a terephthalic acid and water mixture. 
     Typically, TPA produced via catalyzed oxidation of p-xylene in an acetic acid solvent produces a terephthalic acid/solvent slurry that contains residual catalyst (e.g cobalt, manganese, and bromine). In a common method of producing a substantially dry TPA solid from a terephthalic acid/solvent slurry, the terephthalic acid/solvent slurry is filtered to separate a substantial amount of the acetic acid liquid from the TPA solids. Residual catalyst is usually separated from the terephthalic acid/solvent slurry by washing (rinsing) the wet cake with catalyst-free acetic acid, water or other solvent. The TPA solid is isolated by drying. 
     In the present invention, a novel process has been discovered resulting in fewer steps than the currently employed processes. The primary utility of the invention is reduction of capital and operating costs associated with the isolation of a terephthalic acid powder. In the conventional approach toward producing terephthalic acid via catalyzed oxidation of p-xylene in an acetic acid solvent, a terephthalic acid/solvent slurry is filtered, washed, then dried to produce a terephthalic acid powder suitable as starting material for PET production. 
     In one embodiment of the present invention, the terephthalic acid/solvent slurry is filtered to produce a terephthalic acid cake with solvent and a TPA solvent mother liquor stream. The terephthalic acid cake with solvent is then washed (rinsed) with water to recover residual metal catalyst material and to produce a water-wet terephthalic acid cake and an TPA solvent/water by-product liquor. The water-wet terephthalic acid cake is then combined with a diol to produce a terephthalic acid/diol mixture suitable as starting material in a commercial PET process. By bypassing conventional processes for isolating a terephthalic acid solid, the equipment necessary produce a terephthalic acid powder is eliminated. 
     Another surprising and seemingly contradictory aspect of the invention is the benefit of addition of water to the acetic acid and ethylene glycol solvents. In general, in conventional processes for producing terephthalic acid, it is necessary to remove water produced in the oxidation process. Typically, use of acetic acid as an oxidation solvent necessitates an additional process step where acetic acid and water are separated. It is seemingly contradictory to produce an acetic acid and water mixture when it can be avoided by drying the terephthalic acid from the acetic acid solvent. 
     Additionally, in processes for producing PET via esterification of TPA with ethylene glycol, water is generated as a reaction by-product. In general, it is necessary to remove the water produced in the esterification process via an additional process step where ethylene glycol and water are separated. It is seemingly contradictory to produce an ethylene glycol and water mixture when it can be avoided by not introducing water with the TPA. 
     However, the one benefit of this invention is based on the premise that ethylene glycol/water and acetic acid/water separation systems normally exist for conventional TPA and PET production processes. In this invention, the savings associated with eliminating the TPA drying is of greater benefit than the incremental increase in ethylene glycol and water separation capacity plus the incremental increase in acetic acid and water separation capacity. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a process by which a carboxylic acid/diol mixture is obtained from a carboxylic acid/solvent slurry without isolation of a substantially dry carboxylic acid solid. More specifically, the present invention relates to a process for the production of a terephthalic acid/ethylene glycol mixture suitable as feedstock for the production of commercial PET. The resulting process has fewer steps than currently employed processes and can be constructed at lower capital cost. Specifically, the present invention incorporates a direct displacement of water with ethylene glycol. Incorporation of the displacement step eliminates the need to isolate a purified terephthalic acid solid thereby eliminating the need for crystallization, solid-liquid separation, and solids handling equipment normally found in commercial purified terephthalic acid processes. 
     It is an object of this invention to provide a process for producing a carboxylic acid/diol mixture from a carboxylic acid/solvent slurry without isolation of a substantially dry carboxylic acid solid. 
     It is another object of this invention to provide a process for producing a terephthalic acid/diol mixture from a terephthalic acid/solvent slurry without isolation of a substantially dry terephthalic acid solid. 
     It is another object of this invention to provide a process for producing a terephthalic acid/ethylene glycol mixture from a terephthalic acid solvent slurry without isolation of a substantially dry terephthalic acid solid. 
     It is another object of this invention to provide a process for producing a terephthalic acid/ethylene glycol mixture without isolation of a substantially dry terephthalic acid solid by removing water from a water-wet terephthalic acid cake through the use of a carboxylic acid/diol mixing zone. 
     In a first embodiment of this invention, a process for producing a carboxylic acid/diol mixture is provided comprising the adding a diol to a water-wet carboxylic acid cake in a carboxylic acid/diol mixing zone to remove a portion of the water to form the carboxylic acid/diol mixture. 
     In another embodiment of this invention, a process for producing a carboxylic acid/diol mixture is provided, the process comprising the following steps: 
     (a) removing in a first solid-liquid separation device impurities from a carboxylic acid/solvent slurry to form a carboxylic acid cake with acetic acid and a solvent mother liquor stream. 
     (b) removing a substantial portion of a solvent in a second solid-liquid separation device from the carboxylic acid cake with acetic acid to form a water-wet carboxylic acid cake and a solvent/water byproduct liquor. 
     (c) adding a diol to the water-wet carboxylic acid cake in a carboxylic acid/diol mixing zone to remove a portion of the water to form the carboxylic acid/diol mixture. 
     In another embodiment of this invention, a process for producing a carboxylic acid/diol mixture is provided, the process comprising the following steps: 
     (a) removing a solvent from a carboxylic acid/solvent slurry in a solid-liquid separation zone; wherein a substantial portion of the solvent in the carboxylic acid/solvent slurry is replaced with water to form a water-wet carboxylic acid cake. 
     (b) adding a diol to the water-wet carboxylic acid cake in a carboxylic acid/diol mixing zone to remove a portion of the water to form the carboxylic acid/diol mixture. 
     In another embodiment of this invention, a process for producing a terephthalic acid/diol mixture is provided, the process comprising the adding a diol to a water-wet terephthalic acid cake in a terephthalic acid/diol mixing zone to remove a portion of the water to form the terephthalic acid/diol mixture. 
     In another embodiment of this invention, a process for producing a terephthalic acid/diol mixture is provided, the process comprising the following steps: 
     (a) removing in a first solid-liquid separation device impurities from a terephthalic acid/solvent slurry to for a terephthalic acid cake with acetic acid. 
     (b) removing a substantial portion of a solvent in a second solid-liquid separation device to form the terephthalic acid cake with acetic acid to form a water-wet terephthalic acid cake. 
     (c) adding a diol to the water-wet terephthalic acid cake in a terephthalic acid/diol mixing zone to remove a portion of the water to form the terephthalic acid/diol mixture. 
     In another embodiment of this invention, a process for producing a terephthalic acid/diol mixture is provided, the process comprising the following steps: 
     (a) removing a solvent from a terephthalic acid/solvent slurry in a solid-liquid separation zone; wherein a substantial portion of the solvent in the terephthalic acid/solvent slurry is replaced with water to form a water-wet terephthalic acid cake. 
     (b) adding a diol to the water-wet terephthalic acid cake in a terephthalic acid/diol mixing zone to remove a portion of the water to form the terephthalic acid/diol mixture. 
     In another embodiment of this invention, a process to produce a carboxylic acid/diol mixture from a carboxylic acid/solvent slurry is provided without the isolation of a substantially dry carboxylic acid solid. 
     In another embodiment of this invention, a process to produce a terephthalic/diol mixture from a terephthalic acid/solvent slurry is provided, without the isolation of a substantially dry terephthalic acid solid. 
     In another embodiment of this invention a process for producing a terephthalic acid/diol mixture is provided, the process comprising the following steps: 
     (a) removing in a first solid-liquid separation device impurities from a terephthalic acid/solvent slurry to form a terephthalic acid cake with acetic acid; wherein the first solid-liquid separation device is operated at a temperature between about 40° C. to about 155° C. 
     (b) removing a substantial portion of a solvent in a second solid-liquid separation device to form the terephthalic acid cake with acetic acid to form a water-wet terephthalic acid cake; wherein the second solid-liquid separation device is operated at a temperature between about 40 C to about 155. 
     (c) adding a diol to the water-wet terephthalic acid cake in a terephthalic acid/diol mixing zone to remove a portion of the water to form the terephthalic acid/diol mixture; wherein the adding occurs at a temperature between about 40° C. and 290° C.; wherein the diol is ethylene glycol. 
     In another embodiment of this invention, a process for producing a terephthalic acid/diol mixture is provided, the process comprising the following steps in the order named: 
     (a) removing in a first solid-liquid separation device impurities from a terephthalic acid/solvent slurry to form a terephthalic acid cake with acetic acid; wherein the first solid-liquid separation device is operated at a temperature between about 40° C. to about 155° C. 
     (b) removing a substantial portion of a solvent in a second solid-liquid separation device to form the terephthalic acid cake with acetic acid to form a water-wet terephthalic acid cake; wherein the second solid-liquid separation device is operated at a temperature between about 40 C to about 155. 
     (c) adding a diol to the water-wet terephthalic acid cake in a terephthalic acid/diol mixing zone to remove a portion of the water to form the terephthalic acid/diol mixture; wherein the adding occurs at a temperature between about 40° C. and 290° C.; wherein the diol is ethylene glycol. 
     These objects, and other objects, will become more apparent to others with ordinary skill in the art after reading this disclosure. 
    
    
     DESCRIPTION OF THE INVENTION 
     In a first embodiment of this invention shown in  FIG. 1 , a process for producing a carboxylic acid/diol mixture  108  is provided, the process comprises adding a diol  107  to a water-wet carboxylic acid cake  106  in a carboxylic acid/diol mixing zone  130  to remove a portion of the water to form the carboxylic acid/diol mixture  108 . 
     The carboxylic acid/diol mixing zone  130 , the diol  107 , the carboxylic acid/diol mixture  108  and the water-wet carboxylic acid cake  106  is described subsequently in a second embodiment of this invention. 
     In the second embodiment of this invention shown in  FIG. 1 , a process for producing a carboxylic acid/diol mixture  108  is provided. The process comprises the following steps. 
     Step (1) comprises removing in a first solid-liquid separation device  110  impurities from a carboxylic acid/solvent slurry  101  to form a carboxylic acid cake with solvent  103  and a solvent mother liquor stream  102 . Conduit  101  contains a carboxylic acid/solvent slurry comprising a carboxylic acid, impurities and a solvent. The impurities comprises residual catalyst (typically but not limited to cobalt, manganese, or bromine). Suitable solvents include, but are not limited to, aliphatic mono-carboxylic acids, preferably containing 2 to 6 carbon atoms, or benzoic acid and mixtures thereof and mixtures of these compounds with water. Preferably, the solvent is comprised of mainly acetic acid and/or some water. The ratio of acetic acid to water can range from 50:50 to 99:1 acetic acid to water by mass, more preferably in the range of 85:15 to 95:5, and most preferably in the range of 90:10 to 95:5. Suitable carboxylic acids include by are not limited to terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, trimellitic acid, and mixtures thereof. 
     The carboxylic acid/solvent slurry  101  is in the range of 10–40% by weight carboxylic acid. Preferably the carboxylic acid/solvent slurry  101  is in the range of 25–35% by weight carboxylic acid. Most preferably, the carboxylic acid/solvent slurry  101  is in the range of 30–35% by weight carboxylic acid. The carboxylic acid/solvent slurry in conduit  101  is then introduced into a first solid-liquid separation device,  110 , wherein a substantial portion of the solvent mother liquor is recovered in conduit  102 . The solvent mother liquor  102  comprises a substantial portion of the solvent. 
     The first solid-liquid separation device  110  comprises any device capable of efficiently separating solids and liquids. The first solid-liquid separation device  110  can typically be comprised of, but not limited to, the following types of devices: centrifuges, cyclones, rotary drum filters, belt filters, press filters, etc. The first solid-liquid separation device  110  can operate within a temperature range of from approximately 40° C. to 155° C. Preferably the first solid-liquid separation device  110  can operate within a temperature range of from approximately 80° C. to 150° C. Most preferably the first solid-liquid separation device  110  can operate within a temperature range of from approximately 90° C. to 150° C. A carboxylic acid cake with solvent  103 , is produced wherein the moisture composition of the carboxylic acid cake with solvent  103  is in the range of 0.5–30% by weight moisture, preferably in the range of 1–20% moisture, most preferably in the range of 1–5% moisture. Optionally, the residual solvent can be removed by a gas displacement step to minimize solvent contamination with wash. 
     Step (2) comprises removing a substantial portion of a solvent in a second solid-liquid separation device  120  from the carboxylic acid cake with solvent  103  to form a water-wet carboxylic acid cake  106  and a solvent/water byproduct liquor  105 . 
     The carboxylic acid cake with solvent  103 , is then subjected to a wash or “rinsing” with water or substantially water with residual amounts of solvent in the solid-liquid separation device,  120 , wherein a substantial portion of the initial solvent is replaced with water to form a water-wet carboxylic acid cake  106 . The water-wet carboxylic acid cake  106 , is preferably in the range of about 0.5–30% moisture, more preferably in the range of 1–20% moisture, and most preferably in the range of 1–5% moisture. The residual moisture of the water-wet carboxylic acid cake  106 , should contain less than about 2% solvent on a mass basis. Additionally, the water-wet carboxylic acid cake should contain less than 1% of any metals (e.g. cobalt, manganese, etc . . . ), typically used as catalysts in p-xylene oxidation, in the slurry mixture in conduit  101 , should remain in the water-wet carboxylic acid cake  106 . 
     Wash water is introduced into the second solid-liquid separation device  120  via conduit  104 . The wash water should be, on a continuous basis, comprise a mass feed rate in ratio with the solids in  103  in the range of about 0.1:1 to 1.5:1, preferably in the range of 0.1:1 to 0.6:1, most preferably in the range of 0.2:1 to 0.4:1. There are no limitations on the temperature or pressure of the wash water including the use of vaporized water, steam, or a combination of water and steam, as wash. 
     The second solid-liquid separation device  120  can typically be comprised of, but not limited to, the following types of devices: centrifuges, cyclones, rotary drum filters, belt filters, press filters, etc. The second solid-liquid separation device  120  can operate within a temperature range of from approximately 40° C. to 155° C. Preferably, the second solid-liquid separation device  120  can operate within a temperature range of from approximately 80° C. to 150° C. Most preferably, the second solid-liquid separation device  120  can operate within a temperature range of from about 90° C. to 150° C. 
     Optionally, the solvent/water byproduct liquor from the second solid-liquid separation device  105 , is segregated from the solvent mother liquor stream produce by the first solid-liquid separation device  102 . 
     Step (3) comprises adding a diol  107  to the water-wet carboxylic acid cake  106  in a carboxylic acid/diol mixing zone  130  to remove a portion from the water-wet carboxylic acid cake  106  of the water to form the carboxylic acid/diol mixture  108 . 
     Finally, the water-wet carboxylic acid cake  106 , which is now substantially free of solvent is combined with a diol  107  in a carboxylic acid mixing zone  130 , to form a carboxylic acid/diol mixture  108  suitable for PET production and other polyesters in device  130 . Conduit  109  is used to remove the portion of water from the water-wet carboxylic acid cake  106 . There are no special limitations on the device  130  with the exception that it must provide intimate contact between the water-wet carboxylic acid cake  106 , and the diol  107  to produce a the carboxylic acid/diol mixture  108 . Examples of such devices include, but are not limited to the following: an agitated vessel, static mixer, screw conveyor, PET esterification reactor(s), etc. (Note: a solid eductor could be used to introduce the water-wet carboxylic acid cake into device  130 ). Nor is there any specific limitation on the temperature range at which device  130  can operate. However, it is preferable that the temperature of device  130  does not exceed approximately 280° C., temperatures normally found within PET esterification reactors. 
     The diol in conduit  107  is introduced in such a manner as to displace the water as the dominant slurrying liquid. This can be accomplished by introducing a diol via conduit  107  as a saturated liquid at a temperature which is sufficient to vaporized the water. Preferably, the diol in conduit  107  is introduced as a saturated or superheated vapor. The diol in conduit  107  is selected from the group consisting of ethylene glycol, diethylene glycol, n-butylene glycol, i-butylene glycol, n-propylene glycol, 1,4 butanediol, cyclohexanedimethanol, and mixtures thereof. Preferably, the diol in conduit  107  is ethylene glycol. Note that within the system shown in  FIG. 1 , a substantially dry carboxylic acid solid is not formed. The primary economic advantage in not forming a carboxylic acid dry solid is the elimination of solids handling equipment (e.g. convey systems, silos, etc . . . ). 
     In a third embodiment of this invention show in  FIG. 1 , a process for producing a carboxylic acid/diol mixture  108 , the process comprising the following steps. 
     Step (1) comprises removing a solvent from a carboxylic acid/solvent slurry  101  in a solid-liquid separation zone  140 ; wherein a substantial portion of the solvent in the carboxylic acid/solvent slurry  101  is replaced with water to form a water-wet carboxylic acid cake  106 . 
     In Step (1) the second solid-liquid separation device  120  and the first solid-liquid separation device  110  in the second embodiment of this invention can be combined to form a single device capable of performing both solid-liquid separations. This is shown schematically in  FIG. 1  showing a solid-liquid displacement zone  140  by a dashed outline box around devices  110  and  120 . The removal of the solvent from a carboxylic/solvent slurry  101  in a solid-liquid separation zone  140  to form a water-wet carboxylic acid cake can be accomplished by any means know in the art. The solid-liquid separation zone  140  comprises any device capable of performing both operations of the first solid-liquid separation device  110  and the second solid-liquid separation device  120  described in the second embodiment of this invention. The device in the solid-liquid separation zone  140  can typically be comprises of but not limited to, the following type of devices centrifuges, cyclones, filters, and such or combination thereof. 
     Step (2) comprises adding a diol  107  to the water-wet carboxylic acid cake  106  in a carboxylic acid/diol mixing zone  130  to remove a portion of the water to form the carboxylic acid/diol mixture  108 . 
     Step (2) is identical to step (3) described in the second embodiment of this invention. 
     In a fourth embodiment of this invention is provided in  FIG. 2 , comprises a process for producing a terephthalic acid/diol mixture  208  is provided, the process comprises adding a diol  207  to a water-wet terephthalic acid cake  206  in a terephthalic acid/diol mixing zone  230  to remove a portion of the water to form the terephthalic acid/diol mixture  208 . 
     The terephthalic acid/diol mixing zone  230 , the diol  207 , the terephthalic acid/diol mixture  208  and the water-wet terephthalic acid cake  206  are described subsequently in a fifth embodiment of this invention. 
     In the fifth embodiment of this invention shown in  FIG. 2 , a process for producing a terephthalic acid/diol mixture  208  is provided. The process comprising the following steps. 
     Step (1) comprises removing in a first solid-liquid separation device  210  impurities from a terephthalic acid/solvent slurry  201  to form a terephthalic acid cake with solvent  203  and a TPA solvent mother liquor stream  202 . 
     Conduit  201  contains a terephthalic acid/solvent slurry comprising a terephthalic acid, impurities and a solvent. The impurities comprises residual catalyst (typically but not limited to cobalt, manganese, or bromine). Suitable solvents include, but are not limited to, aliphatic mono-carboxylic acids, preferably containing 2 to 6 carbon atoms, or benzoic acid and mixtures thereof and mixtures of these compounds with water. Preferably, the solvent is comprised of mainly acetic acid and/or some water. The ratio of acetic acid to water can range from 50:50 to 99:1 acetic acid to water by mass, more preferably in the range of 85:15 to 95:5, and most preferably in the range of 90:10 to 95:5. The terephthalic acid/solvent slurry  201  is in the range of about 10–40% by weight terephthalic acid. Preferably the terephthalic acid/solvent slurry  201  is in the range of 25–35% by weight terephthalic acid. Most preferably, the terephthalic acid/solvent slurry  201  is in the range of 30–35% by weight terephthalic acid. The terephthalic acid/solvent slurry in conduit  201  is then introduced into a first solid-liquid separation device  210 , wherein a substantial portion of the solvent mother liquor is recovered in conduit  202 . The solvent mother liquor  202  comprises a substantial portion of the solvent. 
     The first solid-liquid separation device  210  is any device capable of efficiently separating solids and liquids. The first solid-liquid separation device  210  can typically be comprised of, but not limited to, the following types of devices: centrifuges, cyclones, rotary drum filters, belt filters, press filters, etc. The first solid-liquid separation device  210  can operate within a temperature range of from about 40° C. to 155° C. Preferably the first solid-liquid device  210  can operate within a temperature range of from approximately 80° C. to 150° C. Most preferably the first solid-liquid separation device  210  can operate within a temperature range of from about 90° C. to 150° C. A terephthalic acid cake with solvent  203 , is produced wherein the moisture composition of the terephthalic acid cake with solvent is in the range of 0.5–30% moisture, preferably in the range of 1–10% moisture, most preferably in the range of 1–5% moisture. Optionally, the residual solvent can be removed by a gas displacement step to minimize solvent contamination with wash. 
     Step (2) comprises removing a substantial portion of a solvent in a second solid-liquid separation device  220  from the terephthalic acid cake with solvent  203  to form a water-wet terephthalic acid cake  206  and a TPA solvent/water byproduct liquor  205 . 
     The terephthalic acid cake with solvent  203 , is then subjected to a wash or “rinsing” with water in the second solid-liquid separation device,  220 , wherein a substantial portion of the initial solvent is replaced with water to form a water-wet terephthalic acid cake  206 . The water-wet terephthalic acid cake  206 , is preferably in the range of 0.5–30% moisture, more preferably in the range of about 1–20% moisture, and most preferably in the range of 1–5% moisture. The residual moisture of the water-wet terephthalic acid cake  206 , should contain less than about 2% solvent on a mass basis. Additionally, the water-wet terephthalic acid cake should contain less than 1% of any metals (e.g. cobalt, manganese, etc . . . ), typically used as catalysts in p-xylene oxidation, in the slurry mixture in conduit  201 , should remain in the water-wet terephthalic acid cake  206 . 
     Wash water is introduced into device  220  via conduit  204 . The wash water should be, on a continuous basis, comprise a mass feed rate in ratio with the solids in  203  in the range of about 0.1:1 to 1.5:1, preferably in the range of 0.1:1 to 0.6:1, most preferably in the range of 0.2:1 to 0.4:1. There are no limitations on the temperature or pressure of the wash water including the use of vaporized water, steam, or a combination of water and steam as wash. 
     The second solid-liquid separation device  220  can typically be comprised of, but not limited to, the following types of devices: centrifuges, cyclones, rotary drum filters, belt filters, press filters, etc. The second solid-liquid separation device  220  can operate within a temperature range of from about 40° C. to 155° C. Preferably, the second solid-liquid separation device  220  can operate within a temperature range of from about 80° C. to 150° C. Most preferably, the second solid-liquid separation device  220  can operate within a temperature range of from about 90° C. to 150° C. 
     Optionally, the solvent/water byproduct liquor from the second solid-liquid separation device  205 , is segregated from the solvent mother liquor stream produce by the first solid-liquid separation device  202 . 
     Step (3) comprises adding a diol  207  to the water-wet terephthalic acid cake  206  in a terephthalic acid/diol mixing zone  230  to remove a portion from the water-wet terephthalic acid cake  206  of the water to form the terephthalic acid/diol mixture  208 . 
     Finally, the water-wet terephthalic acid cake  206 , which is now substantially free of solvent is combined with a diol  207 , to form a terephthalic acid/diol mixture suitable for PET production and other polyesters in device  230 . Conduit  209  is used to remove the portion of water from the water-wet carboxylic acid cake  206 . There are no special limitations on the device  230  with the exception that it must provide intimate contact between the water-wet terephthalic acid cake  206 , and the diol  207  to produce a the terephthalic acid/diol mixture  208 . Examples of such devices include, but are not limited to the following: an agitated vessel, static mixer, screw conveyor, PET esterification reactor(s), etc . . . (Note: a solid eductor could be used to introduce the water-wet terephthalic acid cake into device  230 ). Nor is there any specific limitation on the temperature range at which device  230  can operate. However, it is preferable that the temperature of device  230  does not exceed approximately 280° C., temperatures normally found within PET esterification reactors. 
     The diol in conduit  207  is introduced in such a manner as to displace the water as the dominant slurrying liquid. This can be accomplished by introducing a diol via conduit  207  as a saturated liquid at a temperature which is sufficient to vaporized the water. Preferably, the diol in conduit  207  is introduced as a saturated or superheated vapor. The diol in conduit  207  is selected from the group consisting of ethylene glycol, diethylene glycol, n-butylene glycol, i-butylene glycol, n-propylene glycol, 1,4 butanediol, cyclohexanedimethanol, and mixtures thereof. Preferably, the diol in conduit  207  is ethylene glycol. Note that within the system shown in  FIG. 1 , a substantially dry terephthalic acid solid is not formed. The primary economic advantage in not forming a terephthalic acid dry solid is the elimination of solids handling equipment (e.g. convey systems, silos, etc . . . ). 
     In a sixth embodiment of this invention show in  FIG. 2 , a process for producing a terephthalic acid/diol mixture  208  is provided. The process comprising the following steps: 
     Step (1) removing a solvent from a terephthalic acid/solvent slurry  201  in a solid-liquid separation zone  240 ; wherein a substantial portion of the solvent in the terephthalic acid/solvent slurry  201  is replaced with water to form a water-wet terephthalic acid cake  206 . 
     In Step (1) the second solid-liquid separation device  220  and the first solid-liquid separation device  210  in the second embodiment of this invention can be combined to form a single device capable of performing both solid-liquid separations. This is shown schematically in  FIG. 2  showing a solid-liquid displacement zone  240  by a dashed outline box around devices  210  and  220 . The removal of the solvent from a carboxylic/solvent slurry  201  in a solid-liquid separation zone  240  to form a water-wet terephthalic acid cake can be accomplished by any means know in the art. The solid-liquid separation zone  240  comprises any device capable of performing both operations of the first solid-liquid separation device  210  and the second solid-liquid separation device  220  described in the second embodiment of this invention. The device in the solid-liquid separation zone  240  can typically be comprises of but not limited to, the following type of devices centrifuges, cyclones, filters, and such or combination thereof. 
     Step (2) comprises adding a diol  207  to the water-wet terephthalic acid cake  206  in a terephthalic acid/diol mixing zone  230  to remove a portion of the water to form the terephthalic acid/diol mixture  208 . 
     Step (2) is identical to Step (3) described in the fifth embodiment of this invention.