Abstract:
The process relates improving terephthalic acid purge filtration rate by controlling % water in filter feed slurry and to the recovery of a metal catalyst from an oxidizer purge stream produced in the synthesis of carboxylic acid, typically terephthalic acid, while utilizing pressure filtration.

Description:
FIELD OF INVENTION 
     This invention relates improving terephthalic acid purge filtration rate by controlling % water in filter feed slurry and to the recovery of a metal catalyst from an oxidizer purge stream produced in the synthesis of carboxylic acid, typically terephthalic acid, while utilizing pressure filtration. 
     BACKGROUND OF THE INVENTION 
     Terephthalic acid is commercially produced by oxidation of paraxylene in the presence of a catalyst, such as, for example, Co, Mn, Br and a solvent. Terephthalic acid used in the production of polyester fibers, films, and resins must be further treated to remove impurities formed as a result of the oxidation of paraxylene. 
     Terephthalic acid (TPA) is an intermediate in the production of polyesters for plastics and fiber applications. Commercial processes for the manufacture of TPA are often based on the heavy-metal catalyzed oxidation of p-xylene, generally with a bromide promoter in an acetic acid solvent. Due to the limited solubility of TPA in acetic acid under practical oxidation conditions, a slurry of TPA crystals is usually formed in the oxidation reactor. Typically, the TPA oxidizer slurry is withdrawn from the reactor and TPA solids are separated from the oxidizer mother liquor using conventional solid-liquid separation techniques. The oxidizer mother liquor, which contains most of the catalyst and promoter used in the process, is recycled to the oxidation reactor. Aside from the catalyst and promoter, the oxidizer mother liquor stream also contains dissolved TPA and many by-products and impurities. These by-products and impurities arise partially from minor impurities present in the p-xylene feed stream. Other impurities arise due to the incomplete oxidation of p-xylene resulting in partially oxidized products. Still other by-products result from competing side reactions formed as a result of the oxidation of p-xylene to terephthalic acid. Patents disclosing the production of terephthalic acid such as U.S. Pat. Nos. 4,158,738 and 3,996,271 are hereby incorporated by reference in their entirety to the extent that they do not contradict statements herein. 
     The TPA solids undergo a solid-liquid separation wherein fresh solvent is utilitized to displace a major portion of the liquid component of the oxidizer mother liquor. Many of the impurities in the oxidizer mother liquor stream that are recycled are relatively inert to further oxidation. Such impurities include, for example, isophthalic acid, phthalic acid and trimellitic acid. Impurities, which may undergo further oxidation are also present, such as, for example, 4-carboxybenzaldehyde, p-toluic acid and p-tolualdehyde. Oxidation inert impurities tend to accumulate in the oxidizer mother liquor upon recycle. The concentration of these inert impurities will increase in the oxidizer mother liquor until an equilibrium is reached whereby the rate of removal of each impurity via the TPA product balances with the rate of formation and the rate of addition to the oxidation process. The normal level of impurities in commercial crude TPA makes it unsuitable for direct use in most polymer applications. 
     Conventionally, crude TPA has been purified either by conversion a dimethyl ester or by dissolution in water with subsequent hydrogenation over standard hydrogenation catalysts. More recently, secondary oxidative treatments have been used to produce polymer-grade TPA. It is desirable to minimize the concentration of impurities in the mother liquor and thereby facilitate subsequent purification of TPA. In some cases, it is not possible to produce a purified, polymer-grade TPA unless some means for removing impurities from the oxidizer mother liquor stream is utilized. 
     One technique for impurity removal from a recycle stream commonly used in the chemical processing industry is to draw out or “purge” some portion of the recycle stream. Typically, the purge stream is simply disposed of or, if economically justified, subjected to various treatments to remove undesired impurities while recovering valuable components. One example is U.S. Pat. No. 4,939,297 herein incorporated by reference in their entirety to the extent that they do not contradict statements herein. The amount of purge required for control of impurities is process-dependent; however, a purge amount equal to 10-40% of the total oxidizer mother liquor stream is usually sufficient to produce TPA adequate as feedstock for commercial polymer manufacture. In the production of TPA, the percentage purge of the oxidizer mother liquor stream purge necessary to maintain acceptable impurity concentrations, coupled with the economic value of the metal catalyst and solvent components in the oxidizer purge stream, make simple disposal of the oxidizer purge stream economically unattractive. Thus, there is a need for a process that recovers essentially all of the valuable metal catalysts and acetic acid contained in the oxidizer purge stream while removing a major portion of the impurities present in the oxidizer purge stream. The metal catalyst can be recovered in an active form suitable for reuse by direct recycling to the p-xylene oxidation step. 
     A number patents teach a terephthalic acid process comprising a purge process comprising concentration, filtration, followed by extraction. 
     Evaporative concentration of a purge feed comprising acetic acid and water results in a super concentrated purge slurry with lower % weight water content relative to the purge fed because water has a lower boiling point that acetic acid. For example, a purge slurry feed comprising about 94 wt % (weight percent) acetic acid and about 6 wt % water will comprise only about 2.5 wt % water after boiling away about 92 wt % of the purge slurry feed mass. 
     In has been discovered that the filtration rate of the super concentrated purge slurry in a terephthalic acid purge process can vary greatly depending upon the weight water percent in the super concentrated purge slurry. Variability in the super concentrated purge slurry filtration rate is an attribute of the slurry and is related to the particle size distribution of the solids in the slurry. In this disclosure there are embodiments of the invention that disclosure methods for controlling the weight percent water in the super concentrated purge slurry that improves the filtration rate of the super concentrated purge slurry. 
     SUMMARY OF THE INVENTION 
     This invention relates to removal of impurities and the recovery of a metal catalyst from an oxidizer purge stream produced in the synthesis of carboxylic acids, typically terephthalic acid, the process comprising:
         (a) subjecting an oxidizer purge stream formed in a terephthalic acid process comprising a carboxylic acid, a metal catalyst, impurities, water and a solvent comprising acetic acid to evaporation in a 1 st  evaporator zone to produce a 1 st  vapor stream and a concentrated purge stream, and   (b) adding water to the concentrated purge stream in a mixing zone to produce a water rich concentrated purge stream;   (c) subjecting the water rich concentrated purge stream to evaporation in a 2 nd  evaporator zone to produce a 2 nd  solvent rich stream and an intermediate concentrated purge slurry; and   (d) subjecting the intermediate concentrated purge slurry to evaporation in a 3 rd  evaporator zone to produce a 3 rd  solvent rich stream and a super concentrated purge slurry wherein the 3 rd  evaporator zone comprises an evaporator operated at a temperature of about 20° C. to about 70° C.; wherein the super contracted purge slurry has a water content of about 5 wt. % to about 25 wt. %   (e) filtering the super concentrated purge slurry in a solid-liquid separation zone to form a filter cake and mother liquor   (f) washing the filter cake with a wash feed in the solid-liquid separation zone to form a washed filter cake, and wash filtrate; wherein at least 80% of the metal catalyst from the super concentrated purge slurry is recovered through the separation zone into the mother liquor and the wash liquor cumulative; and   (g) optionally dewatering the washed filter cake in the solid-liquid separation zone to form a dewatered filter cake.   (h) combining of water with a mother liquor to recover the metal catalyst and then subjecting an aqueous mixture so formed to extraction with an extraction solvent to produce an extract stream and a raffinate stream comprising a metal catalyst.       

     These embodiments and other embodiments and other embodiments will become more apparent to others with ordinary skill in the art after reading this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an embodiment of the invention wherein a method for removing impurities from an oxidizer purge stream formed in a terephthalic acid process comprising acetic acid, water, a metal catalyst, and oxidation impurities by (a) evaporating a portion of an oxidizer purge stream in a first evaporator zone to produce a concentrated purge stream and a 1 st  vapor stream comprising acetic acid and water (b) adding water in a controlled fashion to the concentrated purge stream in a 1 st  mixing zone to produce a water rich concentrated purge stream (c) evaporating a portion of water rich concentrated purge stream in a 2 nd  evaporator zone to form an intermediate concentrated purge slurry stream and a 2 nd  vapor stream comprising acetic acid and water; (d) evaporating a portion of the intermediate concentrated purge slurry stream in a 3 rd  evaporator zone to form a super concentrated purge slurry stream with a water content ranging from 5.8% to 24.4% and a 3 rd  vapor stream comprising acetic acid and water; (e) separating solids from the super concentrated purge slurry stream and subjecting the solids to a wash feed in a solid-liquid separation zone to form a mother liquor stream, a wash liquor stream, and a washed wet cake stream; (f) mixing in a 2 nd  mixing zone water with the mother liquor stream and optionally a portion of the wash liquor stream to form an aqueous mixture; and (g) adding an extraction solvent to the aqueous mixture in an extraction zone to form an extract stream and a raffinate stream; and (h) feeding the extract stream to a distillation column to form a extraction solvent recycle stream and a sludge stream. 
         FIG. 2  illustrates different embodiments of the invention wherein the filtration rate of the super concentrated purge slurry varies greatly depending on % water in the stream. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In this specification and in the claims, which follow, reference will be made to a number of terms which shall be defined to have the following meanings: 
     As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a pipe reactor includes one or more pipe reactors. 
     Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event or circumstance occurs and instances where it does not. For example, the phrase “optionally heated” means that the material may or may not be heated and that such phrase includes both heated and unheated processes. 
     Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. 
     As used in the specification and the appended claims, the use of “percent” or “%” refers to a weight percent. 
     In one embodiment of this invention, a process to remove oxidation by-product impurities from an oxidizer purge stream  101  is provided as shown in  FIG. 1 . The process comprises the following steps. 
     Step (a) comprises subjecting an oxidizer purge stream  101  to evaporation in a first evaporator zone  120  to produce a 1 st  vapor stream  121  and a concentrated purge stream  122 . 
     The oxidizer purge stream  101  is withdrawn from a carboxylic acid oxidative synthesis process comprising terephthalic acid. One method for generating oxidizer purge stream  101  is to filter terephthalic acid oxidizer slurry and collect a portion of the mother liquor exiting the filter and routing it to the purge process. Yet another method for generating oxidizer purge stream  101  is to conduct a solvent swap on terephthalic acid oxidation slurry displacing a portion of the oxidation mother liquor and routing it to the purge process. The oxidation mother liquor obtained from a terephthalic acid process can be cooled to a temperature ranging from 90° C. to 45° C. and routed it to a clarification pressure filter such as a candle filter to remove any solids present before routing it to the 1 st  evaporator in the purge process. 
     The oxidizer purge stream  101  serves as the feed stream to the present terephthalic acid purge process. The oxidizer purge stream  101  comprises carboxylic acid, water, a solvent, the metal catalyst and impurities. The impurities comprise organic bromides, corrosion metals, p-xylene oxidation by-products, and impurities derived as a result of impurities in the p-xylene. The organic bromides may be used as promoters in the oxidation reaction. Examples of corrosion metals are iron and chromium compounds, which inhibit, reduce or entirely destroy the activity of the metal catalyst. Aside from the catalyst and promoter, the oxidizer mother liquor stream also contains by-products and impurities. These by-products and impurities arise partially from minor impurities present in the p-xylene feed stream. Other impurities arise due to the incomplete oxidation of p-xylene resulting in partially oxidized products. Still other by-products result from competing side reactions in the oxidation of p-xylene to terephthalic acid. Carboxylic acids include aromatic carboxylic acids produced via controlled oxidation of an organic substrate. Such aromatic carboxylic acids include compounds with at least one carboxylic acid group attached to a carbon atom that is part of an aromatic ring, preferably having at least 6 carbon atoms, even more preferably having only carbon atoms. Suitable examples of such aromatic rings include, but are not limited to, benzene, biphenyl, terphenyl, naphthalene, and other carbon-based fused aromatic rings. Examples of suitable carboxylic acids include, but are not limited to, terephthalic acid, benzoic acid, p-toluic acid, isophthalic acid, trimellitic acid, naphthalene dicarboxylic acid, 2,5-diphenyl-terephthalic acid and mixtures thereof. 
     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 acetic acid mixed with water, in a mass ratio of about 5:1 to about 25:1, preferably between about 8:1 and about 20:1, and most preferably between about 11:1 and 20:1. Throughout the specification, acetic acid will be referred to as the solvent. However, it should be appreciated that other suitable solvents, such as those disclosed previously, may also be utilized. 
     In the first step of the present process, the oxidizer purge stream  101  is concentrated by conventional means in a 1 st  evaporator zone  120  comprising an evaporator to produce a 1 st  vapor stream  121  and concentrated purge slurry  122 . The evaporator is operated at atmospheric or slightly super atmospheric conditions, generally from about 1 atmosphere to about 10 atmospheres. The vapor stream  121  comprises a majority of the water and solvent, and the concentrated purge stream  122  comprises the remainder of the water and solvent not removed from the oxidizer purge stream  101 . The evaporation removes about 50 wt % to about 85 wt % of the solvent mass present in the oxidizer purge stream  101 . 
     Step (b) subjecting the concentrated purge stream  122  and stream  131  comprising water to mix zone  130  to produce a water rich concentrated purge stream  132 . The water rich concentrated purge stream  132  is enriched in water in that the weight % water in the water rich concentrated purge stream  132  is greater the concentrated purge stream  122 . Any equipment know in the art for mixing two liquid streams can be utilized including mixing the two streams in a pipe equipped with an internal static mixer. In an embodiment of the invention, the feed rate (mass/time) of stream  131  added in mix zone  130  is manipulated to control the weight percent water contained in the downstream super concentrated purge slurry stream  152  from about 5.0 wt % (weight percent) to about 25.0 wt % water. Another range to control the water content of stream  152  is from about 8.0 wt % water to about 23.0 wt % water. Yet another range to control the water content of stream  152  is from about 11.0 wt % water to about 21.0 wt % water. In still another range to control the water content of stream  152  is from about 13.0 wt % water to about 19.0 wt % water. The conduit between the 1 st  evaporator  120  and mix zone  130  should be maintained at a temperature at or above 90° C. to minimize solids from coming out of solution in the conduit.
 
Step (c) subjecting the water rich concentrated purge stream  132  to evaporation in a 2 nd  evaporator zone  140  to produce a 2 nd  vapor stream  141  and an intermediate concentrated purge slurry stream  142 . The conduit between mix zone  130  and the second evaporator  140  should be maintained at a temperature at or above 80° C. to minimize the amount of solids coming out of solution in the conduit. The second evaporator zone  140  comprises at least one evaporator operated at vacuum conditions. The evaporation can be conducted at a temperature from about 20° C. to about 90° C.; another range is from about 30° C. to about 80° C. In an embodiment of the invention, a portion up to 100% of the vapor stream  141  can be condensed and returned to the second evaporator  140 . This allows for residence time in the 2nd evaporator  140  for particle size growth of the solids contained in the 2 nd  evaporator  140  without concentrating the slurry. This also allows for the decoupling of the slurry temperature in the 2 nd  evaporator  140  and the vacuum level in the 2 nd  evaporator.
 
Step (d) subjecting the intermediate concentrated purge slurry stream  142  to evaporation in a 3 rd  evaporator zone  150  to produce a 3 rd  vapor stream  151  and a super concentrated purge slurry stream  152 . The third evaporator zone  150  comprises at least one evaporator operated at vacuum conditions. The evaporation can be conducted at a temperature from about 20° C. to about 70° C.; another range is from about 30° C. to about 60° C. The combination of evaporators  120 ,  140 , and  150  are operated so as to concentrate the oxidizer purge stream  101  to a condition wherein about 75 wt % to about 97 wt % of mass of stream  101  is removed by evaporation. Another range for operation of the combination of evaporators  120 ,  140 , and  150  to concentrate the oxidizer purge stream as represented by stream  101  to a condition wherein about 85 wt % to about 94 wt % of the mass of stream  101  is removed by evaporation. Yet, another range for operation of the combination of evaporators  120 ,  140 , and  150  to concentrate the oxidizer purge stream as represented by stream  101  to a condition wherein about 87 wt % to about 93 wt % of the mass of stream  101  is removed by evaporation.
 
     In an embodiment of the present invention, the condition of the super concentrated purge slurry  152  can be as a solid-liquid mixture with only enough solvent to provide pump ability. 
     Step (e) comprises filtering a super concentrated purge slurry  152  in a filtration zone  160  to form a filter cake  164  and a mother liquor  163 ; and 
     Step (f) washing the filter cake  164  with a wash feed  166  in a wash zone  161  to form a washed cake  165  and a wash liquor  167 ; and optionally dewatering the washed cake  165  in an optional dewatering zone  162  with a gas feed  168  to form a dewatered cake  169 . In an embodiment of the present invention, the wash stream  166  comprises water. 
     In an embodiment of the invention the filtration zone  160  comprises at least one solid liquid separation device. In another embodiment of the invention, the filtration zone  160  and the wash zone  161  and optionally the dewater zone  162  can be accomplished in one solid liquid separation device or in multiple devices in a solid liquid separation zone  200 . Example of such devices include but are not limited to continuous pressure filters, continuous vacuum filters, batch pressure filters, centrifuges, and like devices. In another embodiment of the invention, the solid liquid separation zone and the wash zone and the optional dewatering zone can be accomplished in one device. Example of such devices include but are not limited to continuous pressure filters, continuous vacuum filters, batch pressure filters, centrifuges, and like devices 
     In an embodiment of the invention the solid liquid separation zone  200  comprises at least one solid liquid separation device. In another embodiment of the invention, the solid liquid separation zone and the wash zone can be accomplished in one solid liquid separation device. Example of such devices include but are not limited to continuous pressure filters, continuous vacuum filters, batch pressure filters, centrifuges, and like devices. In another embodiment of the invention, the solid liquid separation zone and the wash zone and the optional dewatering zone can be accomplished in one device. Example of such devices include but are not limited to continuous pressure filters, continuous vacuum filters, batch pressure filters, centrifuges, and like devices 
     A suitable pressure filter which can be adapted to the requirements of the instant invented process is a BHS-FEST™, rotary drum pressure filter, although other pressure filters which can accomplish the required operation can be used. Examples of other devices that can used in the solid-liquid separation zone  200  include, but are not limited to; vacuum belt filters, filter presses, centrifuges, pressure leaf filters, and cross-flow filters, pressure drum filters and vacuum drum filters. The pressure filter can be operated at temperature and pressure sufficient to obtain at least 80% recovery of the metal catalyst from the solute of the mother liquor  163 . Preferably the pressure filter can be operated at a temperature of about 25° C. to about 80° C., and a pressure of 2 bar to 6 bar gauge. 
     Step (g) comprises mixing in 2 nd  mixing zone  170  a water stream  171  with mother liquor stream  163  and optionally a portion of the wash liquor stream  167  to form an aqueous mixture  172 . In one embodiment of the invention, the mixing zone  170  comprises a conventional mixer. If necessary, the water  171  can be added to the mixing zone  170  in sufficient quantity to dissolve the metal catalyst in the aqueous mixture stream  172 . 
     The water stream  171  is added to in mixing zone  170  in sufficient quantity to dissolve catalyst resulting in an aqueous mixture  172  where in the ratio of acetic acid to water ranges from about 0.7:1 to 1.4:1, preferably from about 0.8:1 to 1.3:1, and most preferably from about 0.9:1 to 1.2:1. It is desirable to keep the aqueous mixture  172  circulating with an external circulation loop. A small amount of extraction solvent  181 , generally about 1 to about 10% by weight, preferably less than 5% by weight may be added to the mixing zone  170  to enhance slurry handling by reducing adherence of solids to the side of vessels. This is represented by the dashed arrow from stream  181  in  FIG. 1 . It is desirable, but not necessary, to subject the aqueous mixture  172 , prior to extraction, to a heat treatment at about 60° C. to about 95° C., another range is about 80° C. about 90° C. for about 0.5 to about 4 hours, preferably about 1 to about 2 hours. By this treatment, organic bromides are reacted to yield inorganic bromides which are preferentially retained in the raffinate stream  183 . The quantity of bromine-containing compounds purged from the system along with the unwanted impurities is thereby minimized. The heat treatment conserves bromides and simplifies disposal of the organic impurities. 
     Step (h) comprises contacting an extraction solvent  181  with the aqueous mixture  172  in an extraction zone  180  to form an extract stream  182  and the raffinate stream  183 . 
     The aqueous mixture  172  is fed to an extraction zone  180  wherein the aqueous mixture  172  and the extraction solvent  181  are contacted in the extraction zone  180 . The aqueous mixture  172  and the extraction solvent  181  are mixed to form an extract stream  182  comprising solvent, water, organic impurities, and extraction solvent which forms a lighter phase, and the raffinate stream  183  comprising a metal catalyst, corrosion metals, and water. The extract stream  182  is withdrawn as an overhead stream and the raffinate stream  183  is withdrawn from the bottom of extractor in extraction zone  180 . In this invention, one embodiment of the extraction zone  180  is a single stage extractor. 
     The extraction solvent  181  used in the extractor should be substantially water-immiscible to minimize the amount of organic solvent dissolved in the aqueous fraction. Additionally, the extraction solvent  181  is preferably an azeotropic agent which serves to assist solvent recovery from the organic extract. Solvents which have proven to be particularly useful are C 1  to C 6  alkyl acetates, particularly n-propyl acetate, isopropyl acetate, isobutyl acetate, sec-butyl acetate, ethyl acetate and n-butyl acetate, although other substantially water-immiscible organic solvents having an appropriate density and a sufficiently low boiling point may also be used, such as p-xylene. N-propyl acetate and isopropyl acetate are particularly preferred due to their relatively low water miscibility and excellent azeotropic behavior. 
     The extraction can be effected using solvent ratios from about 1-4 parts by weight extraction solvent per part aqueous mixture. Although the extraction can be operated at ambient temperature and pressure, heating the solvent and extractor to about 30° C. to about 70° C., another range of about 40° C. to about 60° C. can be used. Although the extract stream  109  comprises small amounts of the metal catalyst and corrosion metals, essentially all of the metal catalyst and the majority of the remaining corrosion metals are contained in the heavier phase, raffinate stream  183 . 
     Step (i) comprises separating the extract stream  182  in a separation zone  190  to form a high boiling point organic impurities stream  192  and a recovered extraction solvent stream  191 . 
     The extract stream  182  comprises organic solvent and organic impurities. The extract stream  182  can further comprises acetic acid and water, often in minor amounts. The extract stream  182  may be distilled in a separation zone  190  comprising conventional distillation equipment. Convention distillation equipment includes, for example, a distillation column. 
     Most of the organic impurities are extracted by the organic solvent in the extraction zone,  180 . This occurs because the organic impurities show a high degree of solubility for the organic solvent and to a lesser extent for acetic acid. By distilling the lighter phase from the extractor, the organic solvent is evaporated allowing the organic impurities to concentrate in the column underflow. 
     The recovered extraction solvent stream  191  may be recycled to the extractor in the extraction zone  180 . The high-boiling organic impurities stream  192  are removed as sludge from the base of the distillation column for disposal. 
     In an embodiment of the invention, evaporator zones  120 ,  140 , and  150  are operated in a continuous fashion as opposed to batch operation. In an embodiment of the invention, all unit operations (zones  120  to  190 ) are operated in a continuous fashion as opposed to a batch operation. 
     EXAMPLES 
     This invention can be further illustrated by the following examples of other embodiments thereof, although it will be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated. 
     The data for Examples 1 through 9 is outlined in Table 1 and  FIG. 2  was generated in a laboratory. An objective of these examples is to illustrate the relationship between the % water in the super concentrated purge slurry stream  152  and the filtration rate of the super concentrated purge slurry stream  152  in a pressure filter. It is also an objective of these examples to illustrate how to generate super concentrated purge slurry stream  152  with a range of water contents. The % water in the super concentrated purge slurry stream  152  ranges from 2.5% to 25% and the filtration rate of the super concentrated purge slurry stream  152  ranges from 30 kg filtrate/minute square meter to 1110 kg filtrate/minute square meter respectively. 
     For each laboratory experiment, oxidizer purge feed  101  was obtained from a commercial plant comprising about 6% water and 94% acetic acid. 75% of the mass of oxidizer purge feed was removed by evaporation at 120° C. in a 1 st  evaporator zone resulting in a concentrated purge stream  122  comprising about 4.6% water. 
     In Experiments 1 and 2, the concentrated purge stream  122  was subjected to additional evaporative concentration at a final temperature of at about 55° C. resulting in a super concentrated purge slurry  152  comprising water of about 2.5% and a total evaporative loss of about 92% of the original oxidizer purge feed. The resulting super concentrated purge slurry was filtered in a lab scale pressure filter operated at 3 bar pressure gauge with filtration area of 20 cm 2 , washed with water, and dewatered with N 2 . The filtration rate was calculated by dividing the total liquid mass (mother liquor+wash liquor) by the filtration area of 20 cm 2  and the sum of the filtration time and wash time. For example, the filtration rate for Experiment 1 is =(507 grams/20 cm 2 /(345 sec+165 sec)=0.0497 grams/cm 2  second or 29.8 kg/meter 2  minute. 
     In Experiments 3 through 9, 120° C. concentrated purge stream  122  was mixed continuously in a mix zone with varying amounts of ambient temperature water and then subjected to additional evaporative concentration at a final temperature of about 55° C. resulting in super concentrated purge stream  152  comprising water ranging from about 8 weight percent (wt %) to 25 wt %. About 92 wt % of the original mass of the oxidizer purge feed is lost during evaporative concentration. The resulting super concentrated purge stream  152  were filtered in a lab scale pressure filter operated at 3 bar pressure gauge with filtration area of 20 cm 2 , washed with water, and dewatered with N 2 . 
     It is instructive to compare Experiments 1 and 5 to see how the addition of water to the concentrated purge stream  122  has a significant impact on the filtration rate of the respective resulting super concentrated purge stream  152 . In both examples, about 92% of the oxidizer purge feed is boiled off. In Example 1, no water is added to the concentrated purge stream  122  resulting in a super concentrated purge stream  152  feed to the filter containing 2.5% water and a filtration rate of about 30 kg/meter 2  minute. In example 5, sufficient water is added to the concentrated purge stream  122  such that the % water in the downstream super concentrated purge stream  152  fed to the filter contains 14% water, and the filtration rate of this super concentrated purge stream  152  slurry is 1,059 kg/meter 2  minute. It is clear that by adding water to the concentrated purge stream  122  in Example 5, the filtration rate of the resulting downstream super concentrated purge stream  152  increases from 30 kg/meter 2  minute to 1,059 kg/meter 2  minute. This is a 3,530% increase in filtration rate. 
     The addition of water as taught above is not simply a dilution effect but impacts the crystal growth of the solids during evaporative concentration resulting in fewer small particles in the slurry fed to the filter. It is well known in the art that the presence of small particles, particularly at or below 10 microns has a negative impact on a slurries filtration rate. Water addition to the concentrated purge slurry  122  as taught above has a positive impact on the filtration rate of the downstream super concentrated purge slurry fed to the filter by decreasing the weight % of particles smaller than 10 microns in the super concentrated purge slurry. If water is added to the super concentrated purge slurry  152  just prior to filtration, no material improvement in filtration rate is realized. The addition of water must occur at a point in the purge process prior to the final evaporative concentration zone. For example, the water addition step can occur after the 1 st  evaporative concentration zone in a purge process with two evaporative concentration zones. The water addition can also occur prior to the 1 st  evaporator zone but is not desirable due to the higher energy costs associated with evaporative concentration prior to the solid-liquid separation zone  200 . 
     It is clear from the data plotted in  FIG. 2  below that it is desirable to control the % water contained in the super concentrated purge slurry stream feed  152  to the filter from 5.8% to 24.4% water. A more preferable range to control the water content of stream  152  is from 7.9% water to 22.8% water. A still more preferable range to control the water content of stream  152  is from 10.5% water to 20.4% water. The most preferred range to control the water content of stream  152  is from 12.5% water to 18.9% water. It is also clear that the physical location in the purge process where water is added to achieve this improvement in stream  152  filtration rate will be located prior to the last evaporation zone in the purge process. 
     
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Experimental Filtration Data for Super Concentrated Purge Slurry (SCPS) 
               
             
          
           
               
                   
                 Total Liquid Flux Data 
               
             
          
           
               
                   
                 % Water 
                 Filtration Zone Data 
                 Wash Zone Data 
                 Mother Liquor Combined 
               
             
          
           
               
                   
                 in SCPS 
                 SCPS 
                   
                 Mother 
                   
                 Wash 
                 With Wash Filtrate 
               
             
          
           
               
                   
                 Feed to 
                 Feed 
                 Filt. 
                 Liquor 
                 Wash 
                 Wash 
                 Filtrate 
                   
                   
                 kG 
               
               
                 Expt. 
                 Filter 
                 Mass 
                 Time 
                 Mass 
                 Mass 
                 Time 
                 Mass 
                 Mass 
                 grams 
                 min. 
               
               
                 # 
                 (% W) 
                 (g) 
                 (sec) 
                 (g) 
                 (g) 
                 (sec) 
                 (g) 
                 (g) 
                 sec. 
                 M 2   
               
               
                   
               
             
          
           
               
                 1 
                 2.5%  
                 400.3 
                 345.0 
                 350.0 
                 150 
                 165 
                 157.7 
                 507.7 
                 1.00 
                 30 
               
               
                 2 
                 2.5%  
                 400.5 
                 30.0 
                 103.4 
                 180 
                 654 
                 429.9 
                 533.3 
                 0.78 
                 23 
               
               
                 3 
                 7.7%  
                 400.8 
                 16.0 
                 306.4 
                 76.00 
                 9.1 
                 132.6 
                 439.0 
                 17.49 
                 525 
               
               
                 4 
                 11% 
                 400.2 
                 13.0 
                 321.8 
                 98.03 
                 6.1 
                 150.4 
                 472.2 
                 24.75 
                 742 
               
               
                 5 
                 14% 
                 400.3 
                 8.3 
                 309.3 
                 98.00 
                 5.2 
                 167.4 
                 476.7 
                 35.31 
                 1059 
               
               
                 6 
                 14% 
                 400.1 
                 8.6 
                 303.1 
                 98.00 
                 5.7 
                 173.4 
                 476.5 
                 33.32 
                 1000 
               
               
                 7 
                 18% 
                 400.2 
                 8.0 
                 313.1 
                 140 
                 5.9 
                 201.4 
                 514.5 
                 37.01 
                 1110 
               
               
                 8 
                 20% 
                 400.4 
                 18.3 
                 351.2 
                 65.00 
                 5.2 
                 101.5 
                 452.7 
                 19.26 
                 578 
               
               
                 9 
                 25% 
                 400.0 
                 27.0 
                 323.7 
                 116.7 
                 13.6 
                 168.2 
                 491.9 
                 12.12 
                 363