DESC:FIELD
The present disclosure relates to a process for the purification of crude carboxylic acid.
BACKGROUND
Polyesters are polymers formed from a dicarboxylic acid and a diol. Polymer grade or "purified" carboxylic acid is the starting material in the synthesis of polyester resin, which is the principal polymer in the manufacture of polyester fibers, polyester films, resins, and the like. Polyester resin is used for manufacturing commercial material having a variety of applications, depending on how they have been produced and the resulting orientation of the polymer chains.
Various techniques have been explored to purify crude carboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid, and the like. Conventionally, the crude terephthalic acid is subjected to catalytic hydrogenation to convert 4-carboxybenzaldehyde (4-CBA) into p-toluic acid; the latter is then separated and eliminated from the system. Hydrogenation is carried out at very high temperature and pressure conditions which makes the overall process energy inefficient. There are some techniques which use catalysts such as 5% Pd/C to produce purified terephthalic acid from crude terephthalic acid.
Further, some methods disclose reacting crude carboxylic acids with a Lewis base to effect formation of a salt or an adduct of the acid with the base. This adduct is subsequently crystallized and de-adducted to leave behind the purified carboxylic acid. Such processes, however, require longer time periods and have higher complexity levels.

There is, therefore, felt a need for developing a process for the purification of crude carboxylic acids which mitigates the afore-stated drawbacks.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:
An object of the present disclosure is to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
Another object of the present disclosure is to provide a process for the purification of crude carboxylic acid.
Still another object of the present disclosure is to provide a reusable and environment friendly fluid system for the purification of crude carboxylic acid.
Other objects and advantages of the present disclosure will be more apparent from the following description which is not intended to limit the scope of the present disclosure.
SUMMARY
The present disclosure provides a process for the purification of crude carboxylic acid. The process comprises admixing crude carboxylic acid with a fluid system comprising at least one first fluid medium and at least one second fluid medium, in an inert atmosphere at a temperature in the range of 50 °C to 300 °C and at a pressure in the range of 1 bar to 50 bar for a time period in the range of 1 minute to 180 minutes to obtain a resultant mass. The resultant mass comprises partially dissolved crude carboxylic acid in the fluid system. The resultant mass is subjected to selective crystallization at a pre-determined temperature and at a pre-determined pressure to obtain a mixture. The mixture comprises a solid phase comprising the carboxylic acid crystals and a liquid phase comprising the fluid system and the impurities. The carboxylic acid crystals are separated from the mixture to obtain separated crystals, which are washed using a washing medium and the washed crystals are dried to obtain the purified carboxylic acid. Typically, the amount of 4-CBA in the purified carboxylic acid is below 550 ppm.
DETAILED DESCRIPTION
The present disclosure envisages a process for the purification of crude carboxylic acid. The process involves admixing the crude carboxylic acid with a fluid system in an inert atmosphere to obtain a resultant mass, crystallizing carboxylic acid crystals from the resultant mass, separating the carboxylic acid crystals, followed by washing and drying the separated carboxylic acid crystals to obtain the purified carboxylic acid.
Initially, the crude carboxylic acid is admixed in a fluid system comprising at least one first fluid medium and at least one second fluid medium, in an inert atmosphere at a temperature in the range of 50 °C to 300 °C and at a pressure in the range of 1 bar to 50 bar for a time period in the range of 1 minute to 180 minutes to obtain a resultant mass. In one embodiment, the inert atmosphere of the present disclosure is nitrogen atmosphere. Typically, the step of admixing is carried out in a high pressure reactor.
Typically, the crude carboxylic acid can be selected from the group consisting of terephthalic acid, isophthalic acid, orthophthalic acid, and mixtures thereof.
In one embodiment, the crude carboxylic acid is crude terephthalic acid. Crude terephthalic acid typically comprises impurities including intermediates such as para-tolualdehyde, para-toluic acid, 4-carboxybenzaldehyde (4-CBA), and side products or by-products such as isophthalic acid, phthalic acid, meta or ortho-tolualdehyde, meta or ortho-toluic acid, 2 or 3-carboxybenzaldehyde, 3 or 4-bromo methyl benzoic acid, benzoic acid, trimellitic acid, trimesic acid, benzaldehyde, phthalaldehyde, ethylbenzaldehyde, methylstyrene, diphenic acid, 2- biphenyl carboxylic acid, hemi mellitic acid, dimethyl terephthalate, methyl p-toluate, 3-hydroxy 4-methyl benzoic acid, terephthaldehyde, styrene, phenol, toluene, benzene, ethylbenzene, methylethylbenzene, formaldehyde, 1,3-cyclopentadiene, indene, methylnaphthalene, anthracene, phenanthrene, phenylacetylene, methylbiphenyl, diphenylbutane, naphthalene, 4,4-dimethylbibenzyl, and vinylacetylene, and metallic impurities which include, but are not limited to, cobalt, magnesium, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, zirconium, cesium, sodium, and titanium. The word ‘impurity’ used throughout the present disclosure refers to the undesirable components like intermediate(s), by product(s), side product(s), co-product(s), catalyst(s), metals, and the like.
The ratio of the crude carboxylic acid to the fluid system can be in the range of 10:90 to 50:50 by weight. Typically, the first fluid medium comprises at least one ether. The ether can be at least one ether selected from the group consisting of non-substituted and substituted ether having C2-C14 carbon atoms. Typically, the ether can be at least one selected from the group consisting of tetrahydrofuran (THF), 2-methyl tetrahydrofuran, and 1, 4-dioxane. In one embodiment of the present disclosure, the ether is tetrahydrofuran.
The second fluid medium can be selected from the group consisting of water, aliphatic alcohol, aromatic alcohol, ketone, such as acetone, ester, and acid. Typically, the aromatic alcohol can be selected from the group consisting of methanol, ethanol, isopropanol, and benzyl alcohol. In one embodiment, the second fluid medium is water. Inclusion of water in the fluid system decreases the quantity of metallic impurities in the purified carboxylic acid. In an embodiment of the present disclosure, another second fluid medium can be used along with water. The role of the second fluid medium other than water is to keep the water quantity in the fluid system to a minimum which results in substantial energy saving as the cost of heating are minimized, making the process of the present disclosure cost effective. The ratio of the first fluid medium to the second fluid medium can be in the range of 99:1 to 50:50 by volume.
In one embodiment of the present disclosure, the fluid system comprises a combination of tetrahydrofuran, and water. The combination of tetrahydrofuran and water forms a unique fluid system, wherein both THF, and water have hydrogen bonding capability, which helps in removal of 4-CBA and other impurities, hence results in better reduction of 4-CBA as compared to the case when tetrahydrofuran alone is used as the fluid system.
Further, addition of at least one Lewis base in the fluid system can help to remove or decrease the metal content in the purified carboxylic acid. Typically, the Lewis base can be at least one selected from the group consisting of 1-methyl imidazole, N-methyl pyrrolidone, 1, 5-dimethylpyrrolidone, N- methylpiperidone, and N-methylcaprolactam.
As a result of admixing the crude carboxylic acid with the fluid system at the afore-stated temperature, pressure and time conditions, the crude carboxylic acid gets, at least partially, dissolved in the fluid system. 4-CBA, which is one of the impurities in the crude carboxylic acid has a higher solubility than pure carboxylic acid in the fluid system and hence gets solubilized in the fluid system along with the other impurities, thus enabling the crystallization of the pure carboxylic acid.
The resultant mass, thus obtained, is subjected to selective crystallization at a pre-determined temperature and at a pre-determined pressure. The pre-determined temperature and pre-determined pressure is the temperature and pressure below which the step of admixing crude carboxylic acid in the fluid medium is carried out. Typically, the selective crystallization can be carried out at a temperature in the range of 20 oC to 180 oC and at a pressure in the range of 1 bar to 30 bar. At this temperature and pressure the carboxylic acid in the fluid medium gets crystallized to form crystals of the purified carboxylic acid (solid phase) and the fluid system containing impurities, such as 4-CBA and metal moieties to form the liquid phase. The impurities may remain as they are or change to some other form during the step of selective crystallization. In one embodiment, the carboxylic acid crystals are crystals of terephthalic acid.
The carboxylic acid crystals that are obtained are separated from the mixture, by any method of separation commonly known to a person skilled in the art, to obtain separated crystals. In one embodiment, the carboxylic acid crystals are separated by vacuum filtration.
The separated crystals of the purified carboxylic acid are subjected to washing using a washing medium in order to remove impurities adhered to its surface. The washing medium can be at least one selected from the group consisting of water, aliphatic alcohol, aromatic alcohol, ketone, ester, ether, and acid. Typically the washing medium can be at least one selected from the group consisting of tetrahydrofuran, 2-methl tetrahydrofuran, 1, 4-dioxane, and water.
The washed crystals are then subjected to drying to obtain purified carboxylic acid. Typically, the washed crystals can be dried at a temperature in the range of 60 °C to 80 °C.
Typically, the amount of 4-CBA in the purified carboxylic acid is below 550 ppm.
The process of the present disclosure also includes the step of recovering and recycling the fluid system, the impurities and/ or intermediates. The intermediates can be at least one selected from the group consisting of p-toluic acid, 4-carboxybenzaldehyde, 3-carboxybenzaldehyde, 2-carboxybenzaldehyde, m-toluic acid and o-toluic acid. The metal impurities can be selected from the group consisting of cobalt, magnesium, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, zirconium, cesium, and titanium. The fluid system can be recovered by known processes, such as distillation, membrane separation and the like.
The present disclosure is further described in light of the following laboratory scale experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. These laboratory scale experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial/commercial scale.
Experimental Details
Experiment-1: Process for the purification of crude terephthalic acid in accordance with the present disclosure
30 grams of crude terephthalic acid, 196 ml of tetrahydrofuran and 4 ml of water were admixed in a high pressure reactor system, while maintaining the pressure at 1 bar. This reactor was flushed with nitrogen to create an inert atmosphere of nitrogen. The reactor temperature was raised to 200 °C by means of an electrical heating jacket and the reaction pressure was increased to 26 bar. The reaction mixture was maintained at this temperature for 15 minutes after which it was cooled down to 50 °C and the pressure was reduced to 1 bar, along with stirring to obtain crystals of purified terephthalic acid. The purified terephthalic acid (PTA) crystals (solid) were separated from the mother liquor by means of vacuum filtration. The separated solid was washed with 50 ml of tetrahydrofuran at 50 °C and filtered. Then PTA was dried at 70 °C for 30 minutes and analyzed for 4-CBA content using dropping mercury polarography. The 4-CBA content in PTA was reduced to 405 ppm from its original content of 3118 ppm in the crude terephthalic acid (CTA). A 4-CBA reduction of 87 % was observed. The yield of the purified PTA was found to be 99.96 %.

Experiment-2: Process for the purification of crude terephthalic acid in accordance with the present disclosure
60 grams of crude terephthalic acid, 140 ml of tetrahydrofuran and 60 ml of water were admixed in a high pressure reactor system, while maintaining the pressure at 1 bar. This reactor was flushed with nitrogen to create an inert atmosphere of nitrogen. The reactor temperature was raised to 200 °C by means of an electrical heating jacket and the pressure was increased to 27 bar. The reaction mixture was maintained at this temperature for 15 minutes after which it was cooled down to 50 °C and the pressure was reduced to 1 bar, along with stirring to obtain crystals of purified terephthalic acid. The purified terephthalic acid (PTA) crystals (solid) were separated from the mother liquor by means of vacuum filtration. The separated solid was washed with 50 ml of tetrahydrofuran at 50 °C and filtered. Then PTA was dried at 70 °C for 30 minutes and analyzed for 4-CBA content using dropping mercury polarography. The 4-CBA content in PTA was reduced to 528 ppm from its original content of 3118 ppm in the crude terephthalic acid (CTA). A 4-CBA reduction of 83 % was observed. The yield of the purified PTA was found to be 99.95 %
Experiment-3: Process for the purification of crude terephthalic acid in accordance with the present disclosure
30 grams of crude terephthalic acid, 194 ml of tetrahydrofuran, 2.30 g 1-methyl imidazole and 3.7 ml of water were admixed in a high pressure reactor system. This reactor was flushed with nitrogen to create an inert atmosphere of nitrogen. The reactor temperature was raised to 200 °C by means of an electrical heating jacket, and the pressure was increased to 25.5 bar. The reaction mixture was maintained at this temperature for 15 minutes after which it was cooled down to 50 °C, and the pressure was reduced to 1 bar, along with stirring to obtain crystals of purified terephthalic acid. The purified terephthalic acid (PTA) crystals (solid) were separated from the mother liquor by means of vacuum filtration. The separated solid was washed with 50 ml of tetrahydrofuran at 50 °C and filtered. Then PTA was dried at 70 °C for 30 minutes and analyzed for 4-CBA content using dropping mercury polarography. The 4-CBA content in PTA was reduced to 472 ppm from its original content of 3118 ppm in the crude terephthalic acid (CTA). A 4-CBA reduction of 84.86 % was observed. The yield of the purified PTA was found to be 99.40 %.
Comparative Experiment-4: Process for the purification of crude terephthalic acid
60 grams of crude terephthalic acid and 200 ml of tetrahydrofuran were admixed in a high pressure reactor system, while maintaining the pressure at 1 bar. This reactor was flushed with nitrogen to create an inert atmosphere of nitrogen and then pressurized to 18 bar nitrogen pressure. The reactor temperature was raised to 200 °C by means of an electrical heating jacket and the pressure was increased to 24 bar. The reaction mixture was maintained at this temperature for 15 minutes after which it was cooled down to 50 °C and the pressure was reduced to 1 bar, along with stirring to obtain crystals of purified terephthalic acid. The purified terephthalic acid (PTA) crystals (solid) were separated from the mother liquor by means of vacuum filtration. The separated solid was washed with 50 ml of tetrahydrofuran at 50 °C. Then PTA was dried at 70 °C for 30 minutes and analyzed for 4-CBA content using dropping mercury polarography. The 4-CBA content in PTA was reduced to 1900 ppm from its original content of 3118 ppm in crude terephthalic acid (CTA). A 4-CBA reduction of only 39 % was observed. The yield of the purified PTA was found to be 99.81 %.
It is seen from the above experiments that the process of the present disclosure using a combination of THF and water results in enhanced reduction of 4-CBA from the crude terephthalic acid as compared to when only THF is used as the fluid medium.

TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of a process for the purification of crude carboxylic acid which:
- is energy efficient;
- is environment friendly;
- significantly reduces the generation of effluents; and
- uses recyclable chemicals.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results. While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Variations or modifications to the formulation of this invention, within the scope of the invention, may occur to those skilled in the art upon reviewing the disclosure herein. Such variations or modifications are well within the spirit of this invention.
The numerical values given for various physical parameters, dimensions and quantities are only approximate values and it is envisaged that the values higher than the numerical value assigned to the physical parameters, dimensions and quantities fall within the scope of the invention unless there is a statement in the specification to the contrary.
While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

,CLAIMS:WE CLAIM:
1. A process for the purification of crude carboxylic acid; said process comprising the following steps:
a. admixing crude carboxylic acid with a fluid system comprising at least one first fluid medium and at least one second fluid medium, in an inert atmosphere at a temperature in the range of 50 oC to 300 oC and at a pressure in the range of 1 bar to 50 bar for a time period in the range of 1 minute to 180 minutes to obtain a resultant mass comprising at least partially dissolved crude carboxylic acid in said fluid system;
b. subjecting said resultant mass to selective crystallization at a pre-determined temperature and at a pre-determined pressure to obtain a mixture comprising a solid phase comprising carboxylic acid crystals and a liquid phase comprising said fluid system and impurities;
c. separating said carboxylic acid crystals from said mixture to obtain separated crystals;
d. washing said separated crystals using at least one washing medium to obtain washed crystals; and
e. drying said washed crystals to obtain purified carboxylic acid.
2. The process as claimed in claim 1, wherein the amount of 4-CBA in said purified carboxylic acid is below 550 ppm.
3. The process as claimed in claim 1, wherein said carboxylic acid is selected from the group consisting of terephthalic acid, isophthalic acid, orthophthalic acid and mixtures thereof.
4. The process as claimed in claim 1, wherein said first fluid medium is at least one ether selected from the group consisting of non-substituted and substituted C2-C14 ether and said second fluid medium is at least one selected from the group consisting of water, aliphatic alcohol, aromatic alcohol, ketone, ester, and acid.
5. The process as claimed in claim 4, wherein said ether is at least one selected from the group consisting of tetrahydrofuran, 2-methyl tetrahydrofuran and 1,4-dioxane.
6. The process as claimed in claim 4, wherein the ratio of said first fluid medium to said second fluid medium is in the range of 99:1 to 50:50 by volume.
7. The process as claimed in claim 1, wherein said fluid system further comprises at least one Lewis base selected from the group consisting of 1-methyl imidazole, N-methyl pyrrolidone, 1,5-dimethylpyrrolidone, N- methylpiperidone and N-methylcaprolactam.
8. The process as claimed in claim 1, wherein the ratio of said crude carboxylic acid to said fluid system is in the range of 10:90 to 50:50 by weight.
9. The process as claimed in claim 1, wherein said pre-determined temperature and pre-determined pressure in step (b) is the temperature and pressure less than the temperature and pressure at which step (a) is carried out.
10. The process as claimed in claim 1, wherein said washing medium is at least one selected from the group consisting of water, aliphatic alcohol, aromatic alcohol, ketone, ester, ether and acid.
11. The process as claimed in claim 1 further comprises the step of recovering and recycling the fluid system and the impurities.