KR101034964B1 - Process for the oxidative purification of terephthalic acid - Google Patents

Abstract

The present invention relates to a process for preparing a purified carboxylic acid slurry. The method includes removing impurities from the crystallized product in a solid liquid substitution zone to form a purified carboxylic acid slurry. The purified carboxylic acid slurry is further cooled in the cooling zone, then filtered and dried in the filtration and drying zones. The process produces purified carboxylic acid products with good color and low impurity levels without the use of purification steps such as hydrogenation or filtrate purge.

Description

PROCESS FOR THE OXIDATIVE PURIFICATION OF TEREPHTHALIC ACID}

The present invention relates to a process for purifying crude carboxylic acid slurry. More specifically, the present invention includes the steps of substituting the mother liquor from the crystallized product at elevated temperature and reslurrying the crystallized product to separate in a solid liquid displacement zone to form a purified carboxylic acid slurry. , A purified carboxylic acid slurry is further cooled in the cooling zone, and then filtered and dried in the filtration and drying zone.

Terephthalic acid is commercially prepared by oxidation of para-xylene in the presence of a catalyst such as 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 present due to the oxidation of para-xylene. Typical commercial processes produce crude terephthalic acid as discussed in US Pat. No. 3,584,039, which is incorporated herein by reference, and then dissolve the solid crude terephthalic acid in water at high temperature and pressure, hydrogenate the resulting solution, and prepare the terephthalic acid product. Cool and crystallize out of solution and separate the solid terephthalate product from the liquid.

In many processes the colored impurities from the benzil and fluorenone systems are hydrogenated to a colorless product, leaving the terephthalic acid solid product and wastewater streams in the process. However, the present invention provides an attractive method for preparing a purified carboxylic acid slurry by using a solid liquid substitution zone comprising a solid liquid separator at elevated temperature after oxidation of the crude carboxylic acid slurry product and prior to the final filtration and drying steps.

Summary of the Invention

In one embodiment of the present invention, a method is provided for preparing a purified carboxylic acid product without the process of separating impurities from hydrogenation or oxidation solvents of terephthalic acid as disclosed in US Pat. No. 3,584,039. Another example of a method for separating impurities from an oxidizing solvent is disclosed in US Pat. No. 4,356,319.

In another embodiment of the present invention, a method of preparing a purified carboxylic acid slurry is provided. The method,

(a) removing impurities from the crystallized product in the solid liquid substitution zone to form a purified carboxylic acid slurry, wherein the purified carboxylic acid slurry has a b ^* of less than 3.5 and is formed without a hydrogenation step;

(b) cooling the purified carboxylic acid slurry in a cooling zone to form a cooled purified carboxylic acid slurry; And

(c) filtering and drying the cooled purified carboxylic acid slurry in a filtration and drying zone to remove a portion of the solvent from the cooled carboxylic acid slurry to produce a purified carboxylic acid product.

In another embodiment of the present invention, a method of preparing a purified carboxylic acid slurry is provided. The method,

(a) removing impurities from the crystallized product in the solid liquid substitution zone to form a purified carboxylic acid slurry, wherein the solid liquid substitution zone comprises a solid liquid separator operated at a temperature of about 140 ° C. to about 160 ° C. The solid liquid separator is operated in a continuous mode, the solid liquid separator is operated at a pressure of less than about 70 psia);

(b) cooling the purified carboxylic acid slurry in a cooling zone to form a cooled purified carboxylic acid slurry; And

(c) filtering and drying the cooled purified carboxylic acid slurry in a filtration and drying zone to remove a portion of the solvent from the cooled carboxylic acid slurry to produce a purified carboxylic acid product.

In another embodiment of the present invention, a method of preparing a purified carboxylic acid slurry is provided. The method,

(a) removing impurities from the crude carboxylic acid slurry in an optional solid liquid substitution zone to optionally form a slurry product;

(b) oxidizing the slurry product or the crude carboxylic acid slurry in a staged oxidation zone to form a staged oxidation product;

(c) crystallizing said staged oxidation product in a crystallization zone to form a crystallized product;

(d) removing impurities from the crystallized product in a second solid liquid substitution zone to form a purified carboxylic acid slurry;

(e) cooling said purified carboxylic acid slurry in a cooling zone to form a cooled purified carboxylic acid slurry; And

(f) filtering and drying the cooled purified carboxylic acid slurry in a filtration and drying zone to remove a portion of the solvent from the cooled carboxylic acid slurry to produce a purified carboxylic acid product.

In another embodiment of the present invention, a method of preparing a purified carboxylic acid slurry is provided. The method,

(a) removing impurities from the crude carboxylic acid slurry in an optional solid liquid substitution zone to optionally form a slurry product;

(b) oxidizing the slurry product or the crude carboxylic acid slurry in a staged oxidation zone to form a staged oxidation product;

(c) removing impurities from said staged oxidation product in a solid liquid substitution zone to form a purified staged oxidation product;

(d) crystallizing the purified staged oxidation product in a crystallization zone to form a purified carboxylic acid slurry;

(e) cooling said purified carboxylic acid slurry in a cooling zone to form a cooled purified carboxylic acid slurry; And

(f) filtering and drying the cooled purified carboxylic acid slurry in a filtration and drying zone to remove a portion of the solvent from the cooled carboxylic acid slurry to produce a purified carboxylic acid product.

These and other objects will be apparent to those skilled in the art after carefully reading the disclosure of the present invention.

1 is a schematic diagram of the process of the present invention for the oxidative purification of carboxylic acids, wherein a liquid substitution zone is used after the crystallization zone.

2 is a schematic diagram of the method of the present invention for the oxidative purification of carboxylic acids wherein a liquid substitution zone is used after the staged oxidation zone.

The present invention provides a process for purifying crude carboxylic acid slurry. The method includes substituting the mother liquor from the crystallized product at elevated temperature and reslurrying the crystallized product in a solid liquid substitution zone to form a purified carboxylic acid slurry.

Crude terephthalic acid is conventionally prepared via liquid phase air oxidation of para-xylene in the presence of a suitable oxidation catalyst. Suitable catalysts include, but are not limited to, one or more selected from cobalt, bromine and manganese compounds soluble in the selected solvent. Suitable solvents include, but are not limited to, aliphatic mono-carboxylic acids, preferably aliphatic mono-carboxylic acids 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, with a ratio of about 5: 1 to about 25: 1, preferably about 8: 1 to about 20: 1 as the ratio of acetic acid: water. Throughout the specification acetic acid will be referred to as a solvent. However, it should be understood that other suitable solvents, such as those disclosed herein, may also be used. Patents that disclose the preparation of terephthalic acid, such as US Pat. Nos. 4,158,738 and 3,996,271, are incorporated herein by reference.

In an embodiment of the present invention, a process for preparing the purified carboxylic acid product 230 is provided in FIG. 1. The method includes the following steps:

(a) removing impurities from the crystallized product 160 in the solid liquid substitution zone 180 to form a purified carboxylic acid slurry 190, wherein the purified carboxylic acid slurry 190 has a b ^{* of} less than 3.5 ^*. Having no hydrogenation step);

(b) cooling the purified carboxylic acid slurry 190 in a cooling zone 200 to form a cooled purified carboxylic acid slurry 210; And

(c) filtering and drying the cooled purified carboxylic acid slurry 210 in a filtration and drying zone 220 to remove a portion of the solvent from the cooled carboxylic acid slurry 210 to produce a purified carboxylic acid product 230. Steps.

Solid liquid substitution zone 180, impurities, crystallized product 160 and purified carboxylic acid slurry 190 are all described herein later.

In another embodiment of the present invention, a method of making a purified terephthalic acid slurry 230 is provided in FIG. 1. The method includes the following steps:

Step (a) optionally includes removing impurities from the crude carboxylic acid slurry 30 in the optional solid liquid substitution zone 40 to form the slurry product 70.

Crude carboxylic acid slurry 30 comprising one or more carboxylic acids, catalysts, one or more solvents, and impurities may be from about 120 ° C. to about 120 ° C. from primary oxidation zone 20 where aromatic feedstock 10, typically para-xylene, is oxidized. Discharge through line 30 at a temperature of about 200 ° C, preferably about 140 ° C to about 170 ° C. Impurities typically comprise one or more of the following compounds: 4-carboxybenzaldehyde, trimellitic acid and 2,6-dicarboxyfluorenone. The solvent typically includes acetic acid, but any of the solvents described above can also be used.

In general, the crude carboxylic acid slurry 30 is prepared by oxidizing the aromatic feedstock 10 in the primary oxidation zone 20. In one embodiment, the aromatic feedstock comprises para-xylene. The primary oxidation zone 20 comprises one or more oxidation reactors and the crude carboxylic acid slurry 30 comprises one or more carboxylic acids. In general, the carboxylic acid is terephthalic acid.

Therefore, if terephthalic acid is used, the crude carboxylic acid slurry 30 will be referred to as crude terephthalic acid slurry. Suitable carboxylic acids, however, include but are not limited to terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid and mixtures thereof. Crude terephthalic acid slurry is commonly synthesized via liquid phase oxidation of para-xylene in the presence of a suitable oxidation catalyst. Suitable catalysts include, but are not limited to, cobalt, manganese and bromine compounds soluble in the selected solvent.

The crude carboxylic acid slurry of conduit 30 is fed to an optional solid liquid substitution zone 40 capable of removing some of the liquid contained in crude carboxylic acid slurry 30 to produce slurry product in conduit 70. Removing a portion of the liquid to produce slurry product in conduit 70 may be accomplished by any means known in the art. Typically, the solid liquid displacement zone 40 comprises a solid liquid separator selected from the group consisting of decanter centrifuges, rotary disk centrifuges, belt filters, rotary vacuum filters, and the like. The crude carboxylic acid slurry in conduit 30 is fed to an optional solid liquid displacement zone 40 that includes a solid liquid separator. The solid liquid separator is operated at a temperature of about 50 ° C. to about 200 ° C., preferably about 140 ° C. to about 170 ° C., and a pressure of about 30 psig to about 200 psig. The residence time can be any residence time suitable for removing some of the solvent and producing slurry product in conduit 70. Although it is understood that the continuous mode is preferred in a commercial process, the optional solid liquid separator in the optional solid liquid substitution zone 40 may be operated in a continuous or batch mode.

Impurities are replaced from optional solid liquid displacement zone 40 in the mother liquor and discharged through line 60. Additional solvent is supplied via line 50 to the optional solid liquid substitution zone 40 to reslurminate the crude carboxylic acid slurry 30 and form the slurry product 70. The mother liquor exits the solid liquid substitution zone 40 via line 60 and includes a solvent, typically acetic acid, a catalyst and a bromine compound. The mother liquor of line 60 may be sent to a process for separating impurities from the oxidizing solvent via a line (not shown), or may be recycled to the catalyst system via a line (not shown). One technique commonly used in the chemical processing industry for the removal of impurities from mother liquors is to vent or “purge” a portion of the recycle stream. Typically, the purge stream may be simply removed or, if economically justified, various treatments may be applied to remove undesirable impurities while recovering the effective components. Examples of impurity removal processes include US Pat. No. 4,939,297 and US Pat. No. 4,356,319, which are incorporated herein by reference.

Step (b) comprises oxidizing the slurry product 70 or the crude carboxylic acid slurry 30 in a staged oxidation zone 80 to form a staged oxidation product 110.

In one embodiment of the present invention, slurry product 70 or crude carboxylic acid slurry 30 is discharged through line 70 to staged oxidation zone 80 to about 190 ° C to about 280 ° C, preferably about 200 ° C. To about 250 ° C., and further oxidized through the air supplied by line 100 to produce staged oxidation product 110.

The staged oxidation zone 80 includes one or more staged oxidation reactor vessels. Crude carboxylic acid slurry 30 or slurry product 70 is fed to a staged oxidation zone 80. The term "staged" means that oxidation occurs in both the staged oxidation zone 80 as well as in the above-described primary oxidation zone 20. For example, the staged oxidation zone 80 may comprise a tandem staged oxidation reactor vessel.

If the carboxylic acid is terephthalic acid, the crude carboxylic acid slurry 30 or slurry product 70 in the staged oxidation zone 80 is about 190 ° C to about 280 ° C, preferably about 200 ° C to about 250 ° C, most preferably about And an oxidation reactor heated to 205 ° C. to about 225 ° C. and further oxidized through a source of air or molecular oxygen supplied by line 100 to form staged oxidation product 110. In general, oxidation in the staged oxidation zone 80 is carried out at a higher temperature than oxidation in the primary oxidation zone 20 to promote impurity removal. The staged oxidation zone 80 may be heated directly with solvent vapor or steam through conduit 90, or indirectly by any known technique in the art. The staged oxidation zone 80 is operated at a sufficient temperature and pressure such that the b ^* color of the staged oxidation product 110 is less than about four. Preferably, the b ^* color of the staged oxidation product 110 in the conduit 110 is less than about 3.5. Most preferably, the b ^* color of the staged oxidation product 110 in the conduit 110 is less than about 3. b ^* Color is one of the three color attributes measured on a spectroscopic reflectance-based instrument. Color can be measured by any device known in the art. Hunter Ultrascan XE equipment is a typical measurement device. Positive readings mean yellowness (or blue absorptivity), while negative readings mean blueness (or yellow absorptivity).

The additional air or molecular oxygen is conduit in the amount necessary to oxidize a substantial portion of the partially oxidized product, such as 4-carboxybenzaldehyde (4-CBA), in the crude carboxylic acid slurry 30 or slurry product 70 to the corresponding carboxylic acid. Via 100 may be fed to the staged oxidation zone 80. Generally, at least 70% by weight of 4-CBA is converted to terephthalic acid in the staged oxidation zone 80. Preferably at least 80% by weight of 4-CBA is converted to terephthalic acid in the staged oxidation zone 80. Significant concentrations of 4-carboxybenzaldehyde and p-toluic acid in the terephthalic acid product are particularly harmful in the polymerization process because they act as chain terminators during the condensation reaction between terephthalic acid and ethylene glycol in the production of polyethylene terephthalate (PET). . Typical terephthalic acid products contain less than about 250 ppm 4-carboxybenzaldehyde and less than about 150 ppm p-toluic acid by weight.

As the terephthalic acid particles dissolve and recrystallize in the staged oxidation zone 80, impurities in the crude carboxylic acid slurry 30 or slurry product 70 go into solution. Exhaust gas from the staged oxidation zone 80 is discharged via line 105 and fed to a recovery system where solvent is removed from the exhaust gas comprising volatile organic compounds (VOC). VOC and methyl bromide can be treated in a catalytic oxidation unit, for example by incineration. Staged oxidation product 110 in staged oxidation zone 80 exits line 110.

Step (c) includes crystallizing the staged oxidation product 110 in the crystallization zone 120 to form the crystallized product 160. Generally, crystallization zone 120 includes one or more crystallizers. Vapor product from crystallization zone 120 exits line 130, condenses in condenser zone 150 including one or more condensers, and returns to crystallization zone 120 through conduit 140. Optionally, liquid 130 in conduit 140 or vapor 130 in condenser section 150 may be recycled, vented, or sent to an energy recovery device. In addition, the crystallization off gas 170 in the condenser zone 150 may be removed via line 170 and directed to a recovery system where solvent is removed, wherein the crystallization off gas comprising VOCs and contaminants is for example catalytic. It can be processed by incineration in the oxidation unit.

If the carboxylic acid is terephthalic acid, the staged oxidation product 110 from the staged oxidation zone 80 is withdrawn via line 110 and fed to a crystallization zone 120 comprising one or more crystallizers to from about 110 ° C. to about Cooling to a temperature of 190 ° C., preferably from about 140 ° C. to about 180 ° C., most preferably from about 150 ° C. to about 170 ° C., yields crystallized product 160. The b ^* color of the crystallized product in conduit 160 is less than four. Preferably the b ^* color of the crystallized product in conduit 160 is less than 3.5. Most preferably, the b ^* color of the crystallized product in conduit 160 is less than three.

Crystallized product 160 in crystallization zone 120 is discharged via line 160. Typically, crystallized product 160 is then fed directly to the vessel and cooled to form cooled crystallized product. If the carboxylic acid is terephthalic acid, the cooled crystallized product is typically cooled in a vessel to a temperature of about 90 ° C. or less before being introduced into the process of recovering terephthalic acid as a dry powder or wet cake.

In some processes prior to the present invention, crystallized product 160 was fed directly to a flash tank. Crystallized product 160 was typically cooled in a flash tank below about 90 ° C. and fed to a filtration and drying system. However, as described above, the present invention provides an attractive process for preparing a purified carboxylic acid slurry by using a solid liquid substitution zone comprising a solid liquid separator at elevated temperature after oxidation of the crude carboxylic acid slurry product and prior to the final filtration and drying steps. To provide. The results of Examples 1 and 2 discussed later clearly show the significant purity improvement achieved when the solid liquid substitution zone of the present invention is used.

Step (d) involves removing impurities from the crystallized product in solid liquid substitution zone 180 to form a purified terephthalic acid slurry.

When the crystallized product 160 in the crystallization zone 120 is processed in the solid liquid substitution zone 180, the purified carboxylic acid slurry 190 will contain significantly less impurities, which may result in the color of the product made from the carboxylic acid. It was surprisingly found to improve. The solid liquid displacement zone 180 includes a solid liquid separator, including but not limited to decantation centrifuges, rotating disk pack centrifuges, and other suitable solid liquid separation devices. In embodiments where the carboxylic acid is terephthalic acid, the level of impurities in the purified carboxylic acid slurry is reduced by 60%. Terephthalic acid of this purity by using a second solid liquid substitution zone 180 of the present invention without the use of a process for separating impurities from an oxidizing solvent as disclosed in US Pat. No. 4,939,297 or hydrogenation as disclosed in US Pat. No. 3,584,039. It is entirely unexpected that these can be prepared (both of which are incorporated herein by reference).

Thus, in one embodiment of the present invention, crystallized product 160 exits crystallization zone 120 via line 160 and is fed to a solid liquid displacement zone 180 comprising a solid liquid separator to provide conduit ( 190) to a purified carboxylic acid slurry. The solid liquid substitution zone 180 includes a solid liquid separator. In one embodiment of the invention, the solid liquid separator may be operated at a temperature of about 50 ° C. to about 200 ° C., and other ranges in which the solid liquid separator may be operated are 110 ° C. to 200 ° C., preferably about 120 ° C. To about 180 ° C., more preferably about 140 ° C. to about 160 ° C., and the pressure is a pressure above the flash point for the selected solvent. In the case where the solvent is acetic acid, the pressure is generally less than 200 psia. Although it is understood that a continuous mode is preferred in a commercial process, the solid liquid separator in the second solid liquid substitution zone 180 may be operated in a continuous or batch mode.

Impurities may be displaced from the solid liquid displacement zone 180 in the mother liquor stream and discharged through line 185. Additional solvent is supplied via line 183 to the solid liquid substitution zone 180 to resludge the crystallized product and form a purified carboxylic acid slurry. The purified carboxylic acid slurry is withdrawn from substitution zone 180 via line 190. Optionally, the purified carboxylic acid slurry of conduit 190 is then fed to a flash tank and flash cooled to less than about 90 ° C.

Step (e) includes cooling the purified carboxylic acid slurry in the cooling zone 200 to form a cooled purified carboxylic acid slurry 210.

The purified carboxylic acid slurry 190 exits the solid liquid substitution zone 180 via line 190. The purified carboxylic acid slurry 190 is fed to the cooling zone 200 and cooled to a temperature below about 90 ° C. Cooling of the purified carboxylic acid slurry can be accomplished by any means known in the art, and typically the cooling zone 200 comprises a flash tank.

Step (f) comprises filtering and drying the cooled purified carboxylic acid slurry 210 in the filtration and drying zone 220 to remove some of the solvent from the cooled carboxylic acid slurry 210 to purify the purified carboxylic acid product 230. It includes preparing a.

The cooled purified carboxylic acid slurry 190 exits the cooling zone 200 and is fed to the filtration and drying zone 220. A portion of the solvent, residual catalyst and impurities are separated and the purified carboxylic acid product is drawn off via line 230.

Filtration and drying zone 220 includes a filter and a dryer suitable for recovering the solid carboxylic acid. Filtration can be accomplished by any means known in the art. For example, a rotary vacuum filter can be used for the filtration to produce the filter cake. The filter cake can go through an initial dehydration step, then washed with an acid wash to remove residual catalyst and dewater again before being sent to the dryer. Drying of the filter cake may be accomplished by any means known in the art that can evaporate at least 10% of the volatiles remaining in the filter cake to produce the carboxylic acid product. For example, a single shaft porcupine processor dryer may be used.

In another aspect of the invention, the solid liquid substitution zone 180 may be located after the staged oxidation zone 80 as shown in FIG. Although the process zones are located in different orders, the function of the zones is the same as described above. Impurities are displaced from the feed stream in the solid liquid displacement zone 180 via line 185. The feed stream directed to the solid liquid displacement zone 180 is a staged oxidation product 110. If the feed stream directed to the solid liquid displacement zone 180 is a staged oxidation product 110, purified staged oxidation product 165 is prepared. Impurities may be displaced from the solid liquid displacement zone 180 in the mother liquor stream and discharged through line 185.

Mother liquor stream 185 includes carboxylic acids, water, solvents, suitable oxidation catalysts and bromine compounds and corrosive metals. Bromine compounds are used as promoters in oxidation reactions. Examples of corrosive metals are iron and chromium compounds that inhibit, reduce or totally destroy the activity of a suitable oxidation catalyst. Suitable carboxylic acids are selected from the group consisting of terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid and mixtures thereof. Additional solvent is supplied via line 183 to the solid liquid substitution zone 180 to resludge the crystallized product and form a purified terephthalic acid slurry. The purified carboxylic acid slurry exits the solid liquid substitution zone 180 via line 190. Optionally, the purified carboxylic acid slurry of conduit 190 may then be fed to a flash tank and flash cooled to less than about 90 ° C.

It should be understood that the process zones described above may be used in any other logical order. It should also be understood that if the process zones are rearranged, the process conditions may change.

In other embodiments of the present invention, each embodiment optionally comprises an additional step comprising decolorization of the carboxylic acid or esterified carboxylic acid via hydrotreatment.

Decolorization of the purified carboxylic acid slurry or esterified carboxylic acid can be accomplished by any means known in the art, which is not limited to hydrogenation. However, in one embodiment of the present invention, for example, decolorization is carried out by reacting a carboxylic acid, for example with esterification with ethylene glycol, with molecular hydrogen in the presence of a catalyst in the reactor zone to produce a decolorized carboxylic acid solution or decolorized ester product. Can be achieved by doing so. In the reactor zone, there is no particular limitation on the shape or structure of the reactor, and it is made in an arrangement for supplying hydrogen to enable intimate contact of the catalyst with carboxylic acid or ester product in the reactor zone. Typically, the catalyst is usually a single Group VIII metal or a combination of Group VIII metals. Preferably, the catalyst is selected from the group consisting of palladium, ruthenium, rhodium and combinations thereof. The reactor zone includes a hydrogenation reactor that operates at a temperature and pressure sufficient to hydrogenate a portion of the characteristic yellow compound into a colorless derivative.

Although the invention may be further illustrated by the following preferred embodiments of the invention, these examples are included for illustrative purposes only and are not intended to limit the scope of the invention unless specifically indicated otherwise. I will understand.

Example 1 (comparative example)

Para-xylene was oxidized at 160 ° C. using Co, Mn, Br catalyst system to produce a crude terephthalic acid slurry with 30 to 35% solids. The crude terephthalic acid slurry was crystallized, purified using a process in which zone 180 was omitted in the process shown in FIG. 1, and the crystallized product from crystallization zone 120 was transferred directly to the flash tank. The product was removed after filtration and drying and analyzed for 4-CBA, TMA, 2,6-DCF,% transmission and b ^* . b ^* is one of the three color attributes measured on spectral reflectance-based instruments. Hunter Ultrascan XE equipment is a typical measurement device. Positive readings mean yellowness (or blue absorptivity), while negative readings mean blueness (or yellow absorptivity).

The concentrations of 4-CBA, TMA, 2,6-DCF in terephthalic acid were analyzed by liquid chromatography. To determine the% transmission, a 10% terephthalic acid product solution in 2M KOH was measured using an ultraviolet visible spectrometer at 340 nm. B ^* of terephthalic acid was measured using the reflectance color method at 340 nm. The results are shown in Table 1.

Example 2 (Invention Example)

The crystallized product from crystallization zone 120 was fed to a solid liquid displacement zone 180 containing a rotating disk centrifuge at 155 ° C., and the solvent wash was fed to a rotating disk centrifuge at 145 ° C. 1 was repeated. Purified terephthalic acid product was collected and analyzed as in Example 1. The results are shown in Table 1.

The results of Examples 1 and 2 clearly show the significant purity improvement achieved when the solid liquid substitution zone of the present invention is used. The amount of impurities present in the purified terephthalic acid product produced by the process of the present invention was reduced by about 50 to about 60%. The percent transmission of the purified terephthalic acid product directly affects the color of the resulting polyethylene terephthalate (PET). Preferred PTA (purified terephthalic acid) is white (referred to as having a low color). Higher% transmission means lower color in PTA. The human eye can detect 0.5 differences in b ^* . Thus, the difference of 1.2 between the non-hydrogenation process (Example 1) showing b ^* greater than about 4 and the process of the present invention (Example 2) showing b ^* less than about 3 was found in the yellowness of the terephthalic acid slurry. It means a significant, noticeable reduction. The degree of improvement in all categories measured given by simple centrifugation in the solid liquid substitution zone 180 is particularly surprising. In the past, a similar level of purity could only be achieved by the use of a hydrogenation plant and many capital investments, including many stages and equipment parts.

While the invention has been described with particular reference to preferred embodiments of the invention, it will be understood that variations and modifications may be practiced within the spirit and scope of the invention.

Claims (27)

delete delete delete delete delete delete delete delete delete delete delete delete delete delete (a) removing impurities from the crude carboxylic acid slurry in a solid liquid displacement zone to form a slurry product; (b) oxidizing the slurry product or the crude carboxylic acid slurry in a staged oxidation zone to form a staged oxidation product; (c) crystallizing said staged oxidation product in a crystallization zone to form a crystallized product; (d) removing impurities from the crystallized product in a solid liquid substitution zone to form a purified carboxylic acid slurry; (e) cooling said purified carboxylic acid slurry in a cooling zone to form a cooled purified carboxylic acid slurry; And (f) filtering and drying the cooled purified carboxylic acid slurry in a filtration and drying zone to remove a portion of the solvent from the cooled carboxylic acid slurry to produce a purified carboxylic acid product. Method for producing a purified carboxylic acid product comprising a. (a) removing impurities from the crude carboxylic acid slurry in a solid liquid substitution zone to form a slurry product; (b) oxidizing the slurry product or the crude carboxylic acid slurry in a staged oxidation zone to form a staged oxidation product; (c) removing impurities from said staged oxidation product in a solid liquid substitution zone to form a purified staged oxidation product; (d) crystallizing the purified staged oxidation product in a crystallization zone to form a purified carboxylic acid slurry; (e) cooling said purified carboxylic acid slurry in a cooling zone to form a cooled purified carboxylic acid slurry; And (f) filtering and drying the cooled purified carboxylic acid slurry in a filtration and drying zone to remove a portion of the solvent from the cooled carboxylic acid slurry to produce a purified carboxylic acid product. Method for producing a purified carboxylic acid product comprising a. 17. The method according to claim 15 or 16, Wherein said solid liquid displacement zone comprises a solid liquid separator operated at a temperature of from 110 ° C. to 200 ° C. 17. The method according to claim 15 or 16, A crude carboxylic acid slurry comprising terephthalic acid, catalyst, acetic acid and impurities is withdrawn from the primary oxidation zone at a temperature of 110 ° C to 200 ° C. 17. The method according to claim 15 or 16, Wherein the solid liquid displacement zone comprises a solid liquid separator selected from the group consisting of a belt filter, a rotary vacuum filter and a rotating disk pack centrifuge. delete 17. The method according to claim 15 or 16, Wherein the purified carboxylic acid slurry has a b ^* of less than 3.5. delete The method of claim 15, (a) removing impurities from the crude carboxylic acid slurry in a solid liquid substitution zone to form a slurry product, wherein the crude carboxylic acid slurry comprises terephthalic acid, catalyst, acetic acid and impurities, which is paraxyl in the primary oxidation zone From the oxidation of lene at a temperature between 140 ° C. and 170 ° C.); (b) oxidizing the slurry product in a staged oxidation zone to form a staged oxidation product, wherein the oxidation is carried out at a temperature of 190 ° C. to 280 ° C .; Is carried out at higher temperatures); (c) crystallizing said staged oxidation product in a crystallization zone to form a crystallized product; And (d) removing impurities from the crystallized product in a solid liquid substitution zone to form a purified carboxylic acid slurry, wherein the solid liquid substitution zone comprises a solid liquid separator operated at a temperature of 110 ° C. to 200 ° C. ); (e) cooling said purified carboxylic acid slurry in a cooling zone to form a cooled purified carboxylic acid slurry; And (f) filtering and drying the cooled purified carboxylic acid slurry in a filtration and drying zone to remove a portion of the solvent from the cooled carboxylic acid slurry to produce a purified carboxylic acid product. How to include. The method according to claim 15, 16 or 23, Bleaching said purified carboxylic acid slurry or esterified carboxylic acid in a reactor zone. The method of claim 24, Wherein said decolorizing step is carried out by reacting said crude carboxylic acid solution with hydrogen in the presence of a catalyst in a reactor zone to produce a decolorized carboxylic acid solution. The method according to claim 15, 16 or 23, Wherein the solid liquid displacement zone comprises a solid liquid separator operated at a temperature of from 50 ° C. to 200 ° C. The method of claim 26, The solid liquid displacement zone is operated at a pressure of less than 70 psia.

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