MXPA00011782A - Method for preparing purified terephthalic acid and isophthalic acid from mixed xylenes - Google Patents

Abstract

A method and apparatus for preparing purified terephthalic acid and, optionally, isophthalic acid from mixed xylenes. The method of the present invention purifies the oxidation reactor effluent containing a mixture of terephthalic acid and isophthalic acid as well as minor amounts of 4-carboxybenzaldehyde (4-CBA), 3-carboxybenzaldehyde (3-CBA), and toluic acid isomers, to produce purified terephthalic acid and, optionally purified isophthalic acid in an integrated process.

Description

METHOD FOR PREPARING PURE TERTIDAL ACID AND ISOFTALIC ACID FROM MIXED XYLENE RELATED APPLICATIONS This application is a continuation in part of the US Copending Application Serial No. 09 / 074,251, filed May 7, 1988, entitled "Method and Apparatus for Preparing Purified Terephthalic Acid," which is a fraction of the US Copending Application No. 08 / 477,898, filed June 7, 1995, entitled "Method and Apparatus for Preparing Purified Terephthalic Acid," now US Patent No. 5,767,311, and also a continuation in part of US Application Serial No. 08 / 962,030, filed on March 31, 1995. October 1997, entitled "Method and Apparatus for Preparing Purified Terephthalic Acid," which is a continuation in part of US Application Serial No. 08 / 760,890, filed December 6, 1996, entitled "Method and Apparatus for Preparing Purified Terephthalic Acid," which is a continuation in part of the United States Copendent Application Series No. 08 / 477,898, filed on June 7, 1995, entitled Method and Apparatus for Preparing Purified Terephthalic Acid, now US Patent No. 5,767,311, all four are assigned to the same concessionaire with this application, and all of the statements which are incorporated herein by reference for all purposes .

INTRODUCTION The present invention relates to the production of terephthalic and isophthalic acids and, more specifically, to a method and apparatus for preparing purified terephthalic acid and isophthalic acid from mixed xylenes.

BACKGROUND The traditional terephthalic acid (ATF) manufacturing process requires p-xylene of relatively high purity (99.7 +%) to improve product quality and reduce the cost of manufacturing. This is due to the fact that the prior art process uses hydrogenation as the main method for purification of the crude terephthalic acid produced in the oxidation section of such a process. Although the hydrogenation method is very selective to remove most of the impurities, 4-carboxybenzaldehyde (4-CBA), converting it to p-toluidic acid, the method only operates in the presence of a very small amount of 4-CBA (from preference less than 3,000 ppm). Also, conventional ATF manufacturing processes are not able to separate ATF from its isomers, such as isophthalic acid (AIF) and phthalic acid (AF).

SUMMARY OF THE INVENTION Unlike the ATF process technique described above, the present invention provides a method and apparatus for preparing purified terephthalic acid and optionally isophthalic acid from mixed xylenes. It is important to note that it can also purify the effluent from the oxidation reactor containing the mixture of terephthalic acid and isophthalic acid, as well as minor amounts of 4-carboxybenzaldehyde (4-CBA), 3-carboxybenzaldehyde (3-CBA), and isomers of tonic acid, to produce a purified terephthalic acid and, optionally, purified isophthalic acid, in an integrated process. These products are useful for the production of fibers, films, plastic bottles and polyester structures with resin, often reinforced by other materials such as glass fibers. In accordance with the present invention, a method and apparatus for producing purified terephthalic acid and, optionally, purified isophthalic acid, from mixed xylenes in an integrated process is provided. In one embodiment, the method of the present invention includes the production of crude mixed acids (including terephthalic acid and isophthalic acid) by the oxidation of the mixed xylenes containing mainly p-xylene and smaller portions of m-xylene and other isomers. The oxidation step produces not only terephthalic acid and isophthalic acid, but also by incomplete oxidation, 4-CBA, 3-CBA, p-toluidic acid, m-toluidic acid and other trace amounts of acid and isomers of the aldehyde. The product resulting from the oxidation step is a liquid dispersion containing unreacted raw materials, solvents, and if used, the products of side reactions, particularly those just mentioned and other materials that are undesirable in the search for purified terephthalic acid and purified isophthalic acid. The reactor effluent is fed to a series of crystallizers that allow the solids to grow by evaporation of the reaction solvent, preferably acetic acid, through pressure reductions. The slurry from the last crystallizer is filtered and washed. The filtered crystals are then dried to remove the solvent to a level of less than 0.25% in the resulting mixed crude acid crystals. The mother liquor solution is fed to the solvent dehydration unit to recover the solvent (acetic acid) from water for recycling to the oxidizer. In another arrangement with the invention, the mixed crude acids of the dryer of the oxidation section are redissolved in a selective crystallization solvent and then the terephthalic acid (ATF) is crystallized from the selective crystallization solvent in one, or preferably, Two stages of crystallization. The arrangement is made to separate the crystallized and progressively purified ATF from the solvent (with or without co-solvents) of the invention. The finally obtained purified ATF filter cake is soaked with water and washed to remove color and the final traces of the selective crystallization solvent of the ATF product. To recover the isophthalic acid (AIF) from the mother liquor of the crystallizer (after the ATF solids are removed by filtration), an anti-solvent is added to cause the complete precipitation of ATF from the mother liquor. The mother liquors substantially free of ATF are concentrated, by evaporation of the selective crystallization solvent and the antisolvent, from the mother liquor, and cooled to promote crystallization of the crude AIF. The crude AIF is then further purified by recrystallization from another selective crystallization solvent. The invention also contemplates steps for recovering and recycling the solvents of the invention from each stage of crystallization and washing and final soaking. The steps are also taken to closely control the release of any undesirable material to the environment. An important aspect of the present invention is the discovery of solvents that are effective in effecting the purification of ATF, as well as AIF, from the crude mixture containing ATF, up to 20% AIF, small amounts of 3-CBA, 4-CBA, m-toluidic acid, p-toluidic acid and others, through the steps of crystallization and separation. These discoveries can be summarized as follows. For the purification of ATF, the selective crystallization solvents useful in the practice of the present invention include those in which: (a) the impurities (including AIF) that are desired to be separated from the ATF are relatively more soluble in the solvent than the ATF at substantially every temperature within the desired range of temperatures at which the solvent containing ATF is to be handled, and (b) ATF is more soluble at high temperature and less soluble at a reduced or lower temperature. It should be understood that the term "selective crystallization solvent" is intended to include any solvent useful in the selective crystallization of ATF, as already described. For the purification of AIF, anti-solvent that must be added to the mother liquor (of the ATF crystallization effluent) must substantially cause the total precipitation (or crystallization) of ATF from the mother liquor and still retain the largest portion of AIF in the mother waters. The mother liquors substantially free of ATF are concentrated by evaporation (or distillation) to crystallize the crude AIF, which is then separated by filtration and redissolved in a second selective crystallization to produce the purified AIF. According to the invention, the mainly preferred selective crystallization solvent for purification of ATF is N-methyl pyrrolidone (NMP), for the reasons discussed below, and for their superior performance. It is non-aqueous, heat-stable, non-toxic (safe for the environment), non-corrosive and commercially available. The ATF can be dissolved in NMP at high temperatures and precipitate or crystallize from NMP at lower temperatures. The main impurities such as 4-CBA, 3-CBA, p-toluidic acid, m-toluidic acid, as well as AIF, have relatively higher solubility in NMP than ATF at all temperatures. Therefore, by reducing the temperature, only ATF tends to crystallize or precipitate from the solution to form purified ATF crystals. Although NMP is the most preferred selective crystallization solvent, it should be understood that, in accordance with the present invention, other selective crystallization solvents preferred for purification of crude ATF may be selected from various polar organic solvents including, but are not intended to to be limited to, N, N-dimethylacetamide, N, N-dimethylformamide, N-formylpiperidine, N-alkyl-2-pyrrolidone (as N-ethylpyrrolidone), N-mercaptoalkyl-2-pyrrolidone (as N-mercaptoethyl-2-) pyrrolidone), N-alkyl-2-thiopyrrolidone (as N-methyl-2-thiopyrrolidone), N-hydroxyalkyl-2-pyrrolidone (as N-hydroxyethyl-2-pyrrolidone), morpholines (such as morpholine and N-formyl morpholine) , carbithols, Ci to C 2 alcohols, ethers, amines, amides and esters, and mixtures thereof. The principal preferred anti-solvent is methanol, although the anti-solvent for the substantially total precipitation of ATF from the mother liquor may also be selected from various polar organic solvents including, but not limited to, methyl, ethyl, ketone, acetone, alcohols of Cx to C? 2, the carbitoles, the esters, the ethers, carboxylic acids of Ci to C? 2, water and mixtures thereof. The preferred preferred selective crystallization solvent for the purification of AIF is methanol, although the solvent may also be selected from the group, but not limited to, methyl ethyl ketone, acetone, Ci to C12 alcohols, carbithols, esters, ethers, C 1 to C 2 carboxylic acids, water and mixtures thereof. To remove the residual solvent (eg, NMP) trapped in the crystals of the final ATF product, the washed ATF crystals are preferably fed to a high temperature ripener where water is used to completely or partially dissolve the ATF crystals. The residual solvent (methanol) trapped in the AIF crystals can be removed by drying to the level of minus 0.25%. In one embodiment, the method of the present invention for the purification of crude terephthalic acid (ATF) from a liquid dispersion produced from the oxidation of mixed xylenes comprises the steps of: (a) dissolving the crude ATF in a selective crystallization solvent at a temperature from about 50 ° C to about 250 ° C to form a solution; (b) crystallizing a purified acid from the solution by reducing the temperature and / or pressure thereof; (c) separating the crystallized purified ATF from the solution, (d) redissolving the purified, separated ATF in a selective crystallization solvent to form a second solution; (e) crystallizing the ATF from the second stage of the second solution by reducing the temperature and sufficient pressure to instantaneously evaporate the ATF solvent from the second solution but without cooling the solution to below 50 ° C; (f) separating the purified ATF from the second stage of the second solution; (g) washing the purified ATF from the separated second stage with water; (h) moistening the purified ATF from the second, washed, separated stage with water at a temperature between about 150 ° C and about 300 ° C; (i) filter and dry the purified ATF from the second stage, macerated with water; (j) adding an antisolvent to the solution filtered in (c) to cause precipitation of substantially all of the ATF; (k) separating the precipitated ATF from the solution in step (j) and combining the precipitated ATF with the original crude ATF to process in step (a); (1) evaporating the solvents from the filtered ATF-free solution in step (k) to cause crystallization of the AIF at a temperature of about 5 ° C and about 100 ° C; (m) separating the crude, crystallized AIF from the solution in step (1); (n) redissolving the crude AIF in a selective crystallization solvent at a temperature from about 50 to 250 ° C to form a second solution; (o) crystallizing the purified AIF from the second solution in step (n) by reducing the temperature and pressure sufficient to evaporate the solvent instantaneously from the AIF of the second solution but without cooling the solution to less than about 50 ° C, and (p) ) separating and drying the purified AIF from the second stage of the second solution. In this embodiment, the dispersion contains at least 0 to 20% isophthalic acid (AIF), and minor amounts of 4-carboxyaldehyde (4-CBA), 3-carboxyaldehyde (3-CBA) and selected impurities of unreacted raw materials, solvents, products of side reactions and / or other unwanted materials. The selective crystallization solvent for purification of ATF is selected from the group consisting of N-methyl pyrrolidone, (NMP), N, N-dimethyl acetamide, N, N-dimethylformamide, N-formyl piperidine, N-alkyl-2-pyrrolidone. (as N-ethylpyrrolidone), N-mercaptoalkyl-2-pyrrolidone, (N-alkyl-2-thiopyrrolidone, such as N-methyl-2-thiopyrrolidone), N-hydroxyalkyl-2-pyrrolidone (as N-hydroxyethyl-2-pyrrolidone ), morpholines (such as morpholine and N-formyl morpholine), carbithols, Ci to C 2 alcohols, ethers, amines, amides and esters and mixtures thereof. The selective crystallization solvent for the purification of ATF of this embodiment is N-methylpyrrolidone or N, N-dimethyl acetamide or N-methylpyrrolidone. The antisolvent for precipitation of ATF from the ATF / AIF solution is selected from the group consisting of methanol, water, methyl ethyl ketone, acetone, Ci to C 2 alcohols, and carbithols, esters, ethers , carboxylic acids of C12 ia, water, and mixtures thereof. The selective crystallization solvent for the recrystallization of AIF is selected from the group of methanol, water, methyl ethyl ketone, acetone, Ci to CX2 alcohols, carbithols, esters, ethers, carboxylic acids of Ci to C? 2, water and mixtures thereof. The antisolvent is preferably in the antisolvent / solution ratio of 0.1 to 10, and more preferably in a ratio of 0.5 to 3, to make ATF precipitation. In another embodiment, the dispersion contains at least 0 to 20% isophthalic acid (AIF) and minor amounts of 4-carboxyaldehyde (4-CBA), 3-carboxyaldehyde (3-CBA) and selected impurities from unreacted raw materials, solvents , products of side reactions and / or other unwanted materials. The selective crystallization solvent for the purification of ATF is selected from the group consisting of N-methyl pyrrolidone (NMP), N, N-dimethyl acetamide, N, N-dimethyl formamide, N-formyl piperidine, N-alkyl-2-pyrrolidone (such as N-ethyl pyrrolidone), N-mercaptoalkyl-2-pyrrolidone (such as N-mercaptoethyl- 2-pyrrolidone), N-alkyl-2-thiopyrrolidone (such as N-methyl-2-thiopyrrolidone), N-hydroxyalkyl-2-pyrrolidone (such as N-hydroxyethyl-2-pyrrolidone), morpholines (such as morpholine, and N-formyl morpholine), carbithols, Ci to C 2 alcohols, ethers, amines, amides, esters, and mixtures thereof. The selective crystallization solvent for the purification of ATF is N-methyl pyrrolidone or N, N-dimethyl acetamide. The following examples illustrate the effectiveness of the selective crystallization solvent in the separation of ATF and AIF, which is the principle and characteristic of this invention.

EXAMPLE 1 This example describes the experimental data on the solubility of ATF as well as on AIF in NMP as the selective crystallization solvent at three different temperatures under atmospheric pressure. The experiments were conducted in a laboratory flask that was immersed in a bath at a constant temperature maintained at a certain temperature. The temperature in the liquid phase in the flask was measured by a thermometer. For the measurement of the high temperature, a total reflux condenser was used to recover the solvent lost due to evaporation. During one experiment, small increases in the amount of solids were added to the solvent constantly stirred in the flask until no more solids were left and dissolved and then the solution was considered saturated with the solids at this temperature. The solubility was calculated based on the weight of the solvent and the total weight of the added solids. Table 1 summarizes the solubility of ATF and AIF in NMP at 15, 40, 70 and 160 ° C.

Table 1 Solvent Solubility Solubility (gm solids / 100 gm solvent) 15 ° C 40 ° C 70 ° C 160 ° C ATMP NMP 2.8 8.0 14.0 23.0 AIF NMP 10.7 22.2 46.0 62.0 4-CBA NMP 18.9 ** 27.4 66.0 125.0 * * at 110 ° C ** at 23 ° C Based on the solubility data shown in Table 1, it is illustrated that the ATF can be purified from the mixture of ATF, AIF and 4-CBA (3-CBA) by crystallization, since both AIF and CBA tend to remain in mother liquors because of their greater solubility. The ATF crystals produced from the mother liquor must have a substantially greater portion of ATF relative to other components than those contained in the mother liquor.

Example 2 A solid mixture containing approximately 95% by weight ATF and 5% by weight AIF was added to NMP according to the solubility of ATF in NMP at 160 ° C. The mixture was then transferred to a crystallizer by cooling equipped with a mixer specially designed to minimize the breaking of the crystals, a heating jacket, and steam condenser. The crystallizer was slowly heated to 160 ° C and kept at this temperature for one hour to ensure that all solids were dissolved. The crystallizer was then cooled to 45 ° C in 90 minutes to allow the ATF crystals to grow. The content of the crystallizer was transferred to a jacket filter and quickly filtered while maintaining the temperature at 40 to 45 ° C. An appropriate amount of hot solvent (at 50 to 70 ° C) was used to wash the cake. In some cases, a hot water wash of the cake is carried out at 95 ° C after a hot solvent wash. The washed cake was then dried and analyzed by gas chromatography to determine the composition of the product. Table 2 indicates a compendium of the results.

Table 2 Non-AIF Temperature in the AIF in the Rinsing Conditions filtered test (° C) feed product ÍA 41 4.94% by weight 0.21% by weight 3 times solvent used at 50 ° C 2A 41 5.02% by weight 0.16% in weight 3 times solvent used at 70 ° C 3A 41 5.00% by weight 0.18% by weight 3 times solvent used at 53 ° C IB 41 4.94% by weight 0.15% by weight Same as IA plus 10 times with water at 95 ° C 2B 41 5.02% by weight 0.13% by weight Same as 2A plus 10 times with water 3B 41 5.00% by weight 0.15% by weight Same as 3A plus 10 times with water at 95 ° C The data in Table 2 above demonstrate that the AIF content in ATF was surprisingly reduced (23 to 39 times) depending on the rinsing conditions.

When the crystallization ATF cake was rinsed with solvent at 70 ° C followed by rinsing with water at 95 ° C (test 2B), the content of AIF was effectively reduced 39 times by crystallization in a single step. Following the same procedure, the AIF content in the ATF mixture can be reduced from 5% by weight to 33 parts per million by weight (ppmp) through a second crystallization step.

Example 3 This example gives the experimental data on the solubility of ATF, as well as AIF, in methanol as the selective crystallization at different temperatures, under atmospheric pressure. The experimental apparatus and methods are the same as those mentioned for Example 1 above, with the exception that the vapor pressure is greater than the atmospheric pressure. The solubility was calculated based on the weight of the solvent and the total weight of the solid added. Table 3 summarizes the solubility of ATF and AIF in methanol at different temperatures.

Table 3 Temperature (° C) Solubility (gm of solid / 100 gm of methanol) ATF AIF 10 0.03 25 0.09 1.82 50 0.47 4.00 150 2.90 161 15.00 It is observed from the Table. 3 that the solubility of AIF in methanol is approximately 8 to 20 times higher than that of ATF in the temperature range from 25 to 50 ° C. The solubility of ATF in methanol becomes significant only at higher temperatures, such as 160 to 200 ° C, under pressure.

Example 4 From this example, it was found that the solubility of AIF in methanol is substantially greater than that of ATF at room temperature (25 ° C to 50 ° C). Accordingly, the experiments were performed to determine if the ATF precipitation of the ATF solution, AIF, a lower amount of 4-CBA could be effected by adding the appropriate amount of methanol to the solution. This solution can be the mother liquor of the ATF crystallizer after the ATF crystals are separated with a filter. The mother liquor may have the following composition: 100 grams of NMP, 20 grams of ATF, 10 grams of AIF, and a minor amount of 4-CBA (and 3-CBA). This example shows that the addition of methanol to the mother liquor can essentially cause the total precipitation of ATF, but only lower precipitation of AIF. To a mixture of 100 grams of NMP, 4 grams of ATF, and 1.5 grams of AIF, approximately 210 grams of methanol were added to the mixture at room temperature. The total mixture was stirred for approximately 90 minutes to allow the solids to crystallize and precipitate from the mixture. The crystals were filtered, washed and dried for analysis. It was found that 47.5% of ATF in the mother liquor was recovered, and the crystals contained approximately 99.0% by weight ATF and 1.0% by weight of AIF. To increase ATF recovery, the mother liquor was concentrated by removing one part of NMP and the mixture containing 100 grams of NMP, 20 grams of ATF, and 10 grams of AIF. Approximately 260 grams of methanol were added to the mixture to cause the crystallization of ATF from the mixture at room temperature. After the addition of methanol, the mixture was stirred for 90 minutes before filtering the ATF crystals from the slurry. It was found that up to 97.5% of ATF was recovered from the mother liquor, and the ATF crystals containing 97.3% by weight of ATF and 2.7% by weight of AIF. The data indicate that it is possible to reach 100% of ATF recovered by separating more NMP from the mother liquor (higher concentration), or by adding more methanol to the mother liquor or the combination of both. In common mother liquors, the content of 4-CBA should be around 0.01 grams per 100 grams of NMP (0.01%). Since the amount of 4-CBA in the mother liquor is very small and the solubility of 4-CBA in NMP is very high around room temperature (shown in Table 1), the addition of methanol should cause the precipitation of 4-CBA of mother liquors. After 100% ATF is recovered from the mother liquor and recycled by the addition of methanol, ATF-free mother liquors can be traditionally processed to recover AIF. The details of the process schema are represented in Figure 1 and are described later in the next section.

DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic flow diagram for a plant for practicing a preferred embodiment of the invention to produce both ATF and AIF; and Figure 2 is a schematic flow diagram for a plant for practicing another preferred embodiment of the invention for production of ATF only.

DESCRIPTION OF THE PREFERRED MODALITIES One of the preferred embodiments of this invention for the production of both purified ATF and AIF is depicted in Figure 1. Now, with reference to FIGURE 1, the crude ATF of the oxidation section of a reactor ( not shown) containing approximately 95% ATF, 5% AIF and minor amounts of other impurities (4-CBA, 3-CBA, p-toluidic acid, m-toluidic acid, etc.), is fed to the first dissolver 100 through line 1 to mix it with the mother liquors of filter II 105 (through line 7) and filter slurry IV 113 (through line 19). The temperature in the first dissolver 100 is maintained at 160 to 180 ° C to completely dissolve the solids and to evaporate substantially all of the methanol carried by the line 19. The saturated solution of the first dissolver 100 is then continuously fed to the First Crystallizer by Cooling 101 through line 2 to generate crystals of the ATF salt at 30 to 50 ° C. The slurry containing salt crystals of ATF leaves the first crystallizer by cooling 101 through line 3 to the filter I 102 where the raw crystal cake separates and feeds the Second Dissolver 103 through line 4. In the Second Dissolver 103, the cake is redissolved in clean NMP recycled through line 35 of the solvent recovery system. Again, the temperature in the second Dissolver 103 is maintained at 160 to 180 ° C to completely dissolve the ATF salt crystals. The saturated solution of the second Dissolver 103 is fed continuously through the line 5 to a second instantaneous crystallizer 104 where the temperature is maintained at a minimum of 60 ° C to prevent the formation of salt crystals of ATF. The degree of reduction of temperature in the crystallizer is controlled by the amount of NMP evaporated from the crystallizer through the reduction of the pressure. The evaporated NMP is recycled to the first dissolver 100 through the line 36. The slurry of the second crystallizer 104 is fed to the filter II 105 through the line 6 where the solid ATF cake is recovered and sent to the washing of the cake 106 , while the mother liquor is recycled the first Dissolver 100 through the line 7. In the washing unit of the cake 106, the residual NMP in the cake is separated by backwashing with water and the washed cake is fed through from line 3 to a ripener 107 to separate the final traces of NMP in the ATF solids by maceration with water at room temperature between 160 to 280 ° C. The NMP free cake is filtered on filter III 108 and dried on Dryer I 109 to produce the final ATF product. The mother liquor of the Filter I 102 is transferred through line 15 to a Precipitator 112. Where it also passes through the oxidizer 111 which is useful in the practice of the related invention published and claimed in the US Copending Solvent Series No. 09 / 098,060, entitled "Method for reducing carboxybenzaldehyde isomers in terephthalic acid or isophthalic acid", owned by the assignee of the present application, the disclosure of which is incorporated herein by reference for all purposes. The methanol is added to the precipitator through line 16 to cause complete precipitation (or crystallization) of ATF and a small amount of precipitation of AIF from the mother liquor. The slurry from precipitator 112 is fed to filter IV 113 through line 18 to remove the largest portion of the mother liquor from the slurry before being recycled to First Dissolver 100 through line 19. The mother liquor in Filter IV 113 is sent to Evaporator 114 to separate NMP and methanol by evaporation through heat, as well as vacuum, so that the concentrated mother liquor becomes a saturated solution of AIF, which is fed to a First Crystallizer of AIF 115 to crystallize AIF to a temperature between 30 to 50 ° C by cooling or instantaneous evaporation. The vaporized NMP and methanol from the evaporator 114 is fed to a Distillation Column 110 to produce NMP from the residues and methanol from the top of the column. The methanol stream is recycled to a Precipitator 112 through line 16, while the NMP stream is fed to a Second Dissolver 103 through line 35. The slurry from the First AIF 115 crystallizer is transferred to a V 116 filter to produce a raw AIF cake and the mother liquor. The mother liquor is sent to a Precipitator 112 through line 17, but a portion of stream 17 is purged through line 37 to prevent the accumulation of impurities and colored bodies. Filter cake V 116 is then transferred through line 25 to Dissolver AIF 117 where the crude AIF cake is dissolved with methanol at a suitable temperature and pressure. The saturated solution of AIF is filtered in the filter VI 118 to separate the insoluble traces to purge through line 28. The solids-free solution is fed through line 29 to the Second Crystalizer of AIF 119 to produce AIF crystals by instantaneous evaporation of methanol from the crystallizer by reduction of pressure. The slurry from the second AIF 119 crystallizer is transferred through line 30 to a VII 120 filter to recover and wash the purified AIF crystals for final drying in Dryer II 121 to produce the final AIF product, while the mother liquor of Filter VII 120 is recycled to evaporator 114 through line 31. Another preferred embodiment of this invention for producing purified ATF is illustrated in Figure 2. The crude ATF (containing approximately 90 to 99% ATF and 1 to 10 % AIF) is fed to a Dissolver I 200 through line 201 to mix it with the mother water M / L-2 of Filter II 206 (recycled through line 220) and the recycled cake of Filter V 215 (recycled through line 221). The temperature in the dissolver is maintained at 140 to 200 ° C to dissolve substantially all solids. The saturated solution is then fed through line 222 to First Crystallizer 202 where the temperature is reduced to 30 to 60 ° C by cooling or evaporating solvent (with reduced pressure) to allow the growth of ATF crystals. The first crystallizer slurry 202 is transferred continuously or in batches through the line 223 to the filter II 203 to recover the solid cake. It is required to wash the depleted or saturated NMP in the filter I 203 to displace the mother water from the cake before being transferred through line 224 to the Dissolver II 204, where the cake is mixed with the evaporated NMP from line 225 and the evaporated NMP of lines 226 and 227. Again, the temperature in Dissolver II 204 is maintained at 140 to 200 ° C to dissolve substantially all solids. The saturated solution is fed through line 228 to a Second Crystallizer 205 where the temperature is reduced from 30 to 60 ° C by cooling or evaporation of the solvent (with pressure reduction) to allow the growth of purified ATF crystals. Again, the slurry from the Second Crystallizer 205 is fed through line 229 to a Filter II 206 to recover the cake, which is then transferred through line 230 to a countercurrent contactor 207 to be washed with water to separate the mass of free MPN of the cake. The solids washed with water are sent through line 231 to Macerator 208 to remove the trace amount of trapped NMP from the purified ATF solids by partial or total dissolution of the solids in the macerator 208 at a temperature of 150 to 280. ° C. The NMP free solids are sent through line 233 to Filter III 209 where the water is separated through line 232 and the ATF cake is sent through line 234 to be dried in a dryer to produce the final product purified ATF. The mother liquor M / L-1 of the Filter I 203 is sent through line 235 to the Evaporator I 210 to separate a substantial amount of NMP. The concentrated solution is transferred through line 236 to Crystallizer III 211 to cause growth of low purity ATF crystals. The crystals of the IV filter 212 are then recovered and recycled to the Dissolver I 200 through line 237. The mother liquor M / L-3 of the Filter IV 212 is transferred via line 238 to the Evaporator II 213, then to crystallizer IV 214 and filter V 215 to recover residual low purity ATF by recycling to Dissolver I 200 by line 239. Final mother water M / L-4 of Filter V 215 containing mainly AIF, NMP, and a smaller amount of ATF passes through line 240 and will be treated to recover additional NMP by mixing with water before discarding.

Claims (22)

1. A method for purifying crude terephthalic acid (ATF) from a liquid dispersion produced by the oxidation of mixed xylenes, comprising: (a) dissolving the crude ATF in a selective crystallization solvent at a temperature from about 50 ° C to about 250 ° C to form a solution; (b) crystallizing the purified acid from the solution by reducing the temperature and / or pressure thereof; (c) separating the purified, crystallized ATF from the solution; (d) redissolving the purified, separated ATF in a selective crystallization solvent to form a second solution; (e) crystallizing the purified ATF in a second stage of the second solution by reducing the temperature and pressure sufficient to instantaneously evaporate the ATF solvent from the second solution but without cooling the solution to less than 50 ° C; (f) separating the purified ATF from the second stage from the second solution; (g) washing the purified ATF separated from the second stage with water; (h) macerating the purified ATF, separated from the second step, washing, with water at a temperature between about 150 ° C and about 300 ° C; (i) filter and dry the purified ATF from the second stage, macerated with water; (j) adding an anti-solvent to the solution filtered in (c) to cause precipitation of substantially all of the ATF; and (k) separating the precipitated ATF from the solution in step (j) and combining the precipitated ATF with the original crude ATF for processing in (a); (1) evaporate the solvents from the ATF-free solution, filtered in (k) to cause crystallization of AIF at a temperature of about 5 ° C and about 100 ° C; (m) separating the crude, crystallized AIF, from the solution in (1); (n) redissolving crude AIF in a selective crystallization solvent at a temperature from about 50 ° C to 250 ° C to form a second solution; (o) crystallize the purified AIF from the second solution in (n) by reducing the temperature and pressure enough to evaporate the solvent instantly AIF of the second solution, but without cooling the solution to less than about 50 ° C, and (p) separating and drying the purified AIF from the second stage of the second solution. The method according to claim 1 in which the dispersion contains at least 0 to 20% isophthalic acid (AIF), and minor amounts of 4-carboxyaldehyde (4-CBA), 3-carboxyaldehyde (3-CBA) and selected impurities of unreacted raw materials, solvents, products of side reactions and / or other unwanted materials. 3. The method according to claim 1 wherein the selective crystallization solvent for purification of ATF is selected from the group consisting of N-methyl pyrrolidone, (NMP), N, N-dimethyl acetamide, N, N-dimethylformamide, N-formyl piperidine, N-alkyl-2-pyrrolidone, N-mercaptoalkyl-2-pyrrolidone, N-alkyl-2-thiopyrrolidone, N-hydroxyalkyl-2-pyrrolidone, morpholines, carbithols, alcohols of Ci to C? 2 , ethers, amines, amides and esters, and mixtures of these. 4. The method according to claim 3, wherein the selective crystallization solvent for purification of ATF is N-methyl pyrrolidone, or N, N-dimethyl acetamide. 5. The method according to claim 4, wherein the selective crystallization solvent for purification of ATF is N-methyl pyrrolidone. The method according to claim 1, wherein the anti-solvent for ATF precipitation from the ATF / AIF solution is selected from the group consisting of methanol, water, methyl ethyl ketone, acetone, Ci to C12 alcohols, carbodols, esters, ethers, carboxylic acids of Ci to C 2, water, and mixtures thereof. The method according to claim 6, wherein the anti-solvent for precipitation of ATF from the ATF / AIF solution is methanol or water. The method according to claim 7, wherein the anti-solvent for the precipitation of ATF from the ATF / AIF solution is methanol. 9. The method according to claim 1, wherein the selective crystallization solvent for the recrystallization of AIF is selected from the group of methanol, water, methyl ethyl ketone, acetone, Ci to C 2 alcohols, carbithols, esters, ethers, carboxylic acids of Ci to C 12 , water, and mixtures thereof. The method according to claim 9, wherein the selective crystallization solvent for the recrystallization of purified AIF is methanol or water. The method according to claim 1, wherein the antisolvent is in an antisolvent / solution ratio of 0.1 to 10 to cause precipitation of ATF. The method according to claim 11 wherein the anti-solvent / solution ratio is preferably in the range of 0.5 to 3. 13. A method for purification of terephthalic acid (TFA) from a liquid dispersion produced by the oxidation of Mixed xylenes consisting of: (a) dissolving the crude ATF in a selective crystallization solvent at a temperature from about 50 ° C to about 250 ° C to form a solution; (b) crystallizing the purified acid from the solution by reducing the temperature and / or pressure thereof; (c) separating the purified, crystallized ATF from the solution; (d) redissolving the purified, separated ATF in a selective crystallization solvent to form a second solution; (e) crystallizing the purified ATF from the second stage of the second solution by reducing the temperature and pressure sufficient to instantaneously evaporate the ATF solvent from the second solution, but without cooling the solution to less than 50 ° C; (f) separating the purified ATF from the second stage from the second solution; (g) washing the purified ATF from the second separated stage with water; (h) macerating the purified ATF from the second, separate step, washed with water, at a temperature between about 150 ° C and about 300 ° C; (i) filtering and drying the purified ATF from the second stage macerated with water; (j) concentrating the filtered solution in (c) by evaporation and cooling the concentrated solution to cause crystallization of ATF and lower portion of AIF; (k) separating the precipitated ATF and minor portion of AIF from the solution in (j) and recycling the solid mixture for processing in (a); (1) concentrate the filtered solution in (k) by a second evaporation and cool the concentrated solution to cause more ATF crystallization and lower portion of AIF; (m) separating the crystallized ATF and minor portion of AIF from the solution in (1) and recycling the solid mixture for processing in (a); and (n) transfer the filtered solution in (m) to the waste treatment plant or for further processing. The method according to claim 13 wherein the dispersion contains at least 0 to 20% isophthalic acid (AIF), and minor amounts of 4-carboxyaldehyde (4-CBA), 3-carboxyaldehyde (3-CBA) and selected impurities of unreacted raw materials, solvents, products of side reactions and / or other unwanted materials. 15. The method according to claim 13 wherein the selective crystallization solvent for purification of ATF is selected from the group consisting of: N-methyl pyrrolidone (NMP), N, N-dimethyl acetamide, N, N-dimethylformamide, N-formyl piperidine, N-alkyl-2-pyrrolidone, N-mercaptoalkyl-2-pyrrolidone, N-alkyl-2-thiopyrrolidone, N-hydroxyalkyl-2-pyrrolidone, morpholines, carbithols, alcohols of Ci to Ci2, ethers, amines, amides, and esters and mixtures thereof. 16. The method according to claim 15, wherein the selective crystallization solvent for purification of ATF is N-methyl pyrrolidone or N, N-dimethyl acetamide. 17. The method according to claim 16, wherein the selective crystallization solvent for purification of ATF is N-methyl pyrrolidone. 18. A method for purification of crude terephthalic acid (ATF) and isophthalic acid (AIF) from a liquid dispersion produced by the oxidation of mixed xylenes, which consists of: (a) dissolving the crude ATF in a selective crystallization solvent at a temperature from about 50 ° C to about 250 ° C to form a solution; (b) crystallizing the purified acid from the solution by reducing the temperature and / or pressure thereof, and separating the crystallized purified ATF from the solution; (c) repeating the dissolution and crystallization of the ATF, if necessary, to obtain a purified ATF of desired purity; (d) adding an antisolvent to the solution from which the crystallized purified ATF was separated to cause precipitation of substantially all of the remaining ATF in the solution; (e) separating the precipitated ATF; (f) evaporating the solvents from the obtained solution by adding an anti-solvent and separating the precipitated ATF to cause crystallization of AIF, and purifying and recovering the AIF thus purified. 19. The method according to claim 18, and further comprising dissolving and crystallizing the purified AIF to obtain AIF of desired purity. The method according to claim 18 wherein the dispersion contains at least 0 to 20% isophthalic acid (AIF), and minor amounts of 4-carboxyaldehyde (4-CBA), 3-carboxyaldehyde (3-CBA) and selected impurities of unreacted raw materials, solvents, products of side reactions and / or other unwanted materials. 21. The method according to claim 18 wherein the selective crystallization solvent for purification of ATF is selected from the group consisting of N-methyl pyrrolidone (NMP), N, N-dimethyl acetamide, N, N-dimethylformamide, N -formyl piperidine, N-alkyl-2-pyrrolidone, N-mercaptoalkyl-2-pyrrolidone, N-alkyl-2-thiopyrrolidone, N-hydroxyalkyl-2-pyrrolidone, morpholines, carbithols, alcohols of Ci to Ci2, ethers, amines , the amides, and the esters and, mixtures thereof. 2

2. The method according to claim 18 wherein the anti-solvent for the precipitation of ATF from the ATF / AIF solution is selected from the group consisting of methanol, water, methyl ethyl ketone, acetone, Ci to C alcohols. 2, carbithols, esters, ethers, carboxylic acids Ci to Ci 2, water, and mixtures thereof.