DESC:FIELD
The present disclosure relates to a process for the purification of crude carboxylic acids.
DEFINITIONS
As used in the present disclosure, the following term is generally intended to have the meaning as set forth below, except to the extent that the context in which they are used indicate otherwise.
The term ‘Crude carboxylic acid’ for the purpose of the present disclosure refers to a crude terephthalic acid which may comprise at least one isophthalic acid, orthophthalic acid, 4-carboxybenzaldehyde, p-toluic acid, benzoic acid, trimellitic acid and metallic impurities such as cobalt, magnesium, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, zirconium, cesium, sodium, and titanium. The 4-carboxybenzaldehyde content in the crude carboxylic acid is greater than 2500 ppm.
BACKGROUND
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.
4-carboxybenzaldehyde (4-CBA) content in carboxylic acid limits molecular mass buildup and chain growth in polymerisation. Also, 4-CBA is long credited with being the major contaminant leading to discolouration of the polyester product. Various techniques have been explored to purify crude carboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid and the like. Conventionally, crude terephthalic acid containing 4-carboxybenzaldehyde impurity is subjected to hydrogenation to convert 4-carboxybenzaldehyde (4-CBA) into p-toluic acid; the latter is then separated and eliminated from the system. There are some techniques which use catalysts such as 5% Pd/C for the purification of crude terepthalic acid. However, the use of such catalysts makes the overall process expensive. Furthermore, some techniques necessitate conditions that make the overall process energy inefficient.
There is, therefore, felt a need to provide a simple, efficient, and economic process for the purification of crude carboxylic acids.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows.
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
An object of the present disclosure is to provide a simple, efficient, and economic process for the purification of crude carboxylic acids.
Another object of the present disclosure is to provide a process for the purification of crude carboxylic acids by reducing the metallic and other impurities.
Another object of the present disclosure is to provide a process for obtaining carboxylic acids with reduced 4-carboxybenzaldehyde content.
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 relates to a process for purifying crude carboxylic acids. The process comprises:
a. Crude carboxylic acid is added to at least one Lewis base at a first predetermined temperature under stirring to obtain a first mixture comprising Lewis base-carboxylic acid adduct.
b. The first mixture is cooled to selectively crystallize the adduct to obtain crystals of the adduct in the first mixture.
c. The crystals are separated from the Lewis base in the first mixture to obtain crystals of the adduct.
d. The crystals are de-adducted by introducing the crystals to at least one de-adducting fluid media at a second predetermined temperature and at a pressure in the range of 1 bar to 30 bar to obtain a second mixture comprising particles of carboxylic acid, the Lewis base, and the de-adducting fluid media.
e. The carboxylic acid is separated from the Lewis base and the de-adducting fluid media in the second mixture, followed by drying the particles of carboxylic acid at a temperature in the range of 50 °C to 80 °C to obtain carboxylic acid having less than 100 ppm of 4-carboxybenzaldehyde.
In one embodiment of the present disclosure, the crystals obtained in step (c) is added to at least one Lewis base at a third predetermined temperature under stirring to obtain a third mixture comprising Lewis base-carboxylic acid adduct; and iterating steps (b) to (e) to obtain carboxylic acid having less than 100 ppm of 4-carboxybenzaldehyde.
The Lewis base can be at least one selected from the group consisting of N-methylpyrrolidone, 1,5-dimethylpyrrolidone, N-methylpiperidone, and N-methylcaprolactum.
The crude carboxylic acid comprises crude terephthalic acid, crude orthophthalic acid, and mixtures thereof.
The de-adducting fluid media can be at least one selected from the group consisting of tetrahydrofuran, substituted tetrahydrofuran, 1,4-dioxane, substituted 1,4-dioxane, water, methanol, ethanol, and isopropanol.
DETAILED DESCRIPTION
Carboxylic acid is the starting material in the synthesis of polyester resin, which 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, containing 4-carboxybenzaldehyde as an impurity. Some of the techniques include hydrogenation of crude terephthalic acid to convert 4-carboxybenzaldehyde (4-CBA) into p-toluic acid; the latter is then separated and eliminated from the system. There are some techniques which use catalysts such as 5% Pd/C for the purification of crude terepthalic acid. However, the use of such catalysts make the overall process expensive. Furthermore, some techniques necessitate conditions that make the overall process energy inefficient.
The present disclosure envisages a process for the purification of crude carboxylic acids to mitigate the drawbacks mentioned herein above.
In accordance with the present disclosure, a process for the purification of crude carboxylic acid is provided. The process for the purification of crude carboxylic acid comprises the following steps.
In the first step, crude carboxylic acid is added to at least one Lewis base at a first predetermined temperature under stirring at a speed in the range of 5 rpm to 800 rpm to obtain a first mixture comprising Lewis base-carboxylic acid adduct.
In an embodiment of the present disclosure, the Lewis base can be at least one selected from the group consisting of N-methylpyrrolidone, 1,5-dimethylpyrrolidone, N-methylpiperidone, and N-methylcaprolactum.
In accordance with one embodiment of the present disclosure, the Lewis base is N-methylpyrrolidone.
In an embodiment of the present disclosure, the crude carboxylic acid comprises crude terephthalic acid and at least one isophthalic acid, orthophthalic acid, 4-carboxybenzaldehyde, p-toluic acid, benzoic acid, trimellitic acid and metallic impurities such as cobalt, magnesium, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, zirconium, cesium, sodium, and titanium.
In accordance with one embodiment of the present disclosure, the crude carboxylic acid is crude terephthalic acid. Typically, the crude terephthalic acid comprises impurities that include 4-carboxybenzaldehyde, p-toluic acid, orthophthalic acid, benzoic acid, trimellitic acid and metallic impurities such as cobalt, magnesium, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, zirconium, cesium, sodium, and titanium.
In an embodiment of the present disclosure, the first predetermined temperature can be in the range of 50 °C to 120 °C.
The weight ratio of the crude carboxylic acid to the Lewis base can be in the range of 1:2 to 1:8.
In an embodiment of the present disclosure, water in the range of 1 % to 10 % (w/w) of the Lewis base, may be added to the Lewis base. The addition of water leads to decrease in the amount of metal content in purified carboxylic acid.
In the second step, the first mixture is cooled to a temperature lower than the temperature at which the first step of mixing is carried out to selectively crystallize the adduct to obtain crystals of the adduct in the first mixture.
In an embodiment of the present disclosure, a chelating agent may be added to the first mixture prior to cooling. The chelating agent is added to remove the metal impurities and to produce purified carboxylic acid. The chelating agent can be at least one selected from the group consisting of ethylenediamine tetraacetate (EDTA), diethylenetriaminepentaacetic acid (DTPA), and nitrilotriacetic acid (NTA).
In the third step, the crystals are separated from the Lewis base in the first mixture by means of filtration to obtain crystals of Lewis base-carboxylic acid adduct.
In accordance with one embodiment of the present disclosure, the crystals of Lewis base-carboxylic acid adduct is N-methylpyrrolidone-terephthalic acid adduct.
In an embodiment of the present disclosure, the crystals of Lewis base-carboxylic acid adduct obtained in the third step may be subjected to washing to remove the impurities using at least one Lewis base selected from the group consisting of N-methylpyrrolidone, 1,5-dimethylpyrrolidone, N-methylpiperidone, and N-methylcaprolactum prior to the fourth step of de-adducting.
In the fourth step, the crystals are de-adducted by introducing the crystals to at least one de-adducting fluid media at a second predetermined temperature and at a pressure in the range of 1 bar to 30 bar under stirring to obtain a second mixture comprising particles of carboxylic acid, the Lewis base, and the de-adducting fluid media.
In an embodiment of the present disclosure, the fourth step further comprises filtering the second mixture to obtain carboxylic acid followed by washing the carboxylic acid using the at least one de-adducting fluid media selected from the group consisting of tetrahydrofuran, substituted tetrahydrofuran, 1,4-dioxane, and substituted 1,4-dioxane at a temperature in the range of 50 °C to 100 °C and at a pressure in the range of 1 bar to 30 bar.
In an embodiment of the present disclosure, the de-adducting fluid media can be at least one selected from the group consisting of tetrahydrofuran, substituted tetrahydrofuran, 1,4-dioxane, substituted 1,4-dioxane, water, methanol, ethanol, and isopropanol. The inclusion of water, methanol, ethanol, and isopropanol in the at least one de-adducting fluid media removes metallic impurities present in crude carboxylic acid.
In a preferred embodiment of the present disclosure, the at least one de-adducting fluid media can be selected from the group consisting of tetrahydrofuran, and substituted tetrahydrofuran.
In an embodiment of the present disclosure, the second predetermined temperature can be in the range of 50 °C to 120 °C.
The weight ratio of the crystal of Lewis base-carboxylic acid adduct to the de-adducting fluid media can be in the range of 1:2 to 1:6.
The de-adducting fluid media serves a dual role in the present process: first as a de-adduction solvent and second to reduce the 4-carboxybenzaldehyde (4-CBA) content in purified carboxylic acid. The de-adducting fluid media is used to bring about de-adduction, which is nothing but the fragmentation, of the crystals of the Lewis base-carboxylic acid adduct that leads to separation of the carboxylic acid from the Lewis base; thereby facilitating the purification of crude carboxylic acid. The de-adducting fluid media also aids in reducing the 4-carboxybenzaldehyde (4-CBA) content in the purified carboxylic acid. This is because 4-carboxybenzaldehyde has a very high solubility in the fluid media and thus the de-adducted purified carboxylic acid becomes substantially free from 4- carboxybenzaldehyde (4-CBA).
In the fifth step, the carboxylic acid is separated from the Lewis base and the de-adducting fluid media in the second mixture by means of filtration, followed by drying said particles of carboxylic acid at a temperature in the range of 50 °C to 80 °C to obtain carboxylic acid having less than 100 ppm of 4-carboxybenzaldehyde.
In one embodiment of the present disclosure, the the crystals obtained in the third step is added to at least one Lewis base at a third predetermined temperature under stirring to obtain a third mixture comprising Lewis base-carboxylic acid adduct; and iterating second step to fifth step to obtain carboxylic acid having less than 100 ppm of 4-carboxybenzaldehyde.
In an embodiment of the present disclosure, the third predetermined temperature can be in the range of 50 °C to 120 °C.
The weight ratio of the crystals of Lewis base-carboxylic acid adduct to the Lewis base is in the range of 1:2 to 1:6.
Furthermore, it is found that the use of tetrahydrofuran as the fluid medium, in place of the conventionally used solvents for de-adduction such as water and methanol shows superior results in terms of reduction in the 4-carboxybenzaldehyde (4-CBA) content indicating superior efficiency of the present process. In accordance with the present disclosure, the amount of 4-carboxybenzaldehyde in the purified carboxylic acid is in the range of 2 ppm to 100 ppm. The combined quantity of 4-carboxybenzaldehyde and p-toluic acid in the purified terephthalic acid is 190 ppm and below. Also, the process for purification of the crude carboxylic acid uses reusable and recyclable chemicals. Hence, the process of purification of carboxylic acid is simple, and economic.
The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.
EXPERIMENT
Example 1: Process for the purification of crude terephthalic acid in accordance with the present disclosure
150 g of crude terephthalic acid was mixed with 1000 g of N-methylpyrrolidone at 70 oC to obtain a first mixture containing N-methylpyrrolidone-terephthalic acid adduct. Further, the first mixture containing N-methylpyrrolidone-terephthalic acid adduct was cooled at a controlled rate to selectively crystallize the N-methylpyrrolidone-terephthalic acid adduct and obtain a cooled first mixture containing crystals of N-methylpyrrolidone-terephthalic acid adduct. Crystal growth was carried out up to 30 oC after which 240 g of the crystals of N-methylpyrrolidone-terephthalic acid adduct were separated from the cooled first mixture by vacuum filtration. These crystals of N-methylpyrrolidone-terephthalic acid adduct were then subjected to de-adduction using 960 g of tetrahydrofuran at 66 oC and at a pressure of 15 bar under stirring to obtain a second mixture containing purified terephthalic acid. The purified terephthalic acid was separated from the second mixture by filtration, followed by drying at 70 °C to obtain dry purified terephthalic acid powder. The 4-carboxybenzaldehyde (4-CBA) content was analysed by mercury polarography method and was found to be reduced to 96 ppm in purified terephthalic acid from its initial content in crude terephthalic acid which was of 2900 ppm. Also, the combined content of 4-carboxybenzaldehyde (4-CBA) and p-toluic acid in purified terephthalic acid was found to be reduced to 105 ppm in purified terephthalic acid from its initial content in crude terephthalic acid which was of 3200 ppm.
Example 2: Process for purification of crude terephthalic acid in accordance with the present disclosure
240 g of the crystals of N-methylpyrrolidone-terephthalic acid adduct obtained in example 1 was mixed with 600 g of N-methylpyrrolidone at 70 oC to obtain a first mixture containing N-methylpyrrolidone-terephthalic acid adduct. Further, the first mixture containing N-methylpyrrolidone-terephthalic acid adduct was cooled at a controlled rate to selectively crystallize the N-methylpyrrolidone-terephthalic acid adduct and obtain a cooled first mixture containing crystals of N-methylpyrrolidone-terephthalic acid adduct. Crystal growth was carried out up to 30 oC after which 200 g of the crystals of N-methylpyrrolidone-terephthalic acid adduct were separated from the cooled first mixture by vacuum filtration. These crystals of N-methylpyrrolidone-terephthalic acid adduct were then subjected to de-adduction using 800 g of tetrahydrofuran at 66 oC and at a pressure of 15 bar under stirring to obtain a second mixture containing purified terephthalic acid. The purified terephthalic acid was separated from the second mixture by filtration, followed by drying at 70 °C to obtain dry purified terephthalic acid powder. The 4-carboxybenzaldehyde (4-CBA) content was analysed by mercury polarography method and was found to be reduced to 10 ppm in purified terephthalic acid from its initial content in crude terephthalic acid which was of 2900 ppm. Also, the combined content of 4-carboxybenzaldehyde (4-CBA) and p-toluic acid in purified terephthalic acid was found to be reduced to 11 ppm in purified terephthalic acid from its initial content in crude terephthalic acid which was of 3200 ppm.
Example 3 (Comparative example): Process for the purification of terephthalic acid using methanol as fluid medium
The process of purification of crude terephthalic acid was similar to example 1 except the fluid medium used for de-adduction. The fluid medium used in example 3 was methanol instead of tetrahydrofuran. The 4-carboxybenzaldehyde (4-CBA) content was analysed by mercury polarography method and was found to be reduced to 215 ppm.
Example 4 (Comparative example): Process for the purification of terephthalic acid using water as fluid medium
The process of purification of crude terephthalic acid was similar to example 1 except the fluid medium used for de-adduction. The fluid medium used in example 4 was water instead of tetrahydrofuran. The 4-carboxybenzaldehyde (4-CBA) content was analysed by mercury polarography method and was found to be reduced to 271 ppm.
Table 1 provides the effect of different fluid medium for de-adduction of Lewis-base-terephthalic acid on of the reduction of 4-carboxybenzaldehyde content.

Table 1. Effect of using different de-adduction fluid medium
Example Fluid medium for de-adduction 4-carboxybenzaldehyde in ppm after purification
Example 1 Tetrahydrofuran 96
Example 2 Tetrahydrofuran 10
Example 3 Methanol 215
Example 4 Water 271

It is clearly seen from the table 1 that the use of tetrahydrofuran for de-adduction of the crystals of N-methylpyrrolidone-terephthalic acid adduct results in more pronounced reduction in the 4-carboxybenzaldehyde (4-CBA) content in purified terephthalic acid as compared to the reduction of 4-carboxybenzaldehyde content in purified terephthalic acid when methanol or water are used as the de-adduction fluid medium.

TECHNICAL ADVANCES AND ECONOMICAL SIGNIFICANCE
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 that:
- provides purified carboxylic acid with reduced 4-carboxybenzaldehyde content;
- is simple, economical, and efficient.
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 disclosure to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the components and component parts of the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other changes in the preferred embodiment as well as other embodiments 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:
a. adding crude carboxylic acid to at least one Lewis base at a first predetermined temperature under stirring to obtain a first mixture comprising Lewis base-carboxylic acid adduct;
b. cooling said first mixture to selectively crystallize said adduct to obtain crystals of said adduct in said first mixture;
c. separating said crystals from said Lewis base in said first mixture to obtain crystals of said adduct;
d. de-adducting said crystals by introducing said crystals to at least one de-adducting fluid media at a second predetermined temperature and at a pressure in the range of 1 bar to 30 bar to obtain a second mixture comprising particles of carboxylic acid, said Lewis base, and said de-adducting fluid media; and
e. separating said carboxylic acid from said Lewis base and said de-adducting fluid media in said second mixture, followed by drying said particles of carboxylic acid at a temperature in the range of 50 °C to 80 °C to obtain carboxylic acid having less than 100 ppm of 4-carboxybenzaldehyde.

2. The process as claimed in claim 1, wherein said crystals obtained in step (c) is added to at least one Lewis base at a third predetermined temperature under stirring to obtain a third mixture comprising Lewis base-carboxylic acid adduct; and iterating steps (b) to (e) to obtain carboxylic acid having less than 100 ppm of 4-carboxybenzaldehyde.

3. The process as claimed in claim 1, wherein in step (a) water, in the range of 1 % to 10 % (w/w) of said Lewis base, is added to said at least one Lewis base prior to mixing with crude carboxylic acid.
4. The process as claimed in claim 1, wherein in step (b) at least one chelating agent is added to said first mixture prior to cooling.

5. The process as claimed in claim 4, wherein said at least one chelating agent is selected from the group consisting of ethylenediamine tetraacetate (EDTA), diethylenetriaminepentaacetic acid (DTPA), and nitrilotriacetic acid (NTA).

6. The process as claimed in claim 1, wherein said crystals of Lewis base-carboxylic acid adduct obtained in step (c) are subjected to washing using at least one Lewis base prior to de-adduction.

7. The process as claimed in claim 1 or claim 2, wherein said Lewis base is at least one selected from the group consisting of N-methylpyrrolidone, 1,5-dimethylpyrrolidone, N-methylpiperidone, and N-methylcaprolactum.

8. The process as claimed in claim 1, wherein said first predetermined temperature, second predetermined temperature, and third predetermined temperature are independently in the range of 50 °C to 120 °C.

9. The process as claimed in claim 1, wherein said step (d) further comprises filtering said second mixture to obtain carboxylic acid followed by washing said carboxylic acid using said at least one de-adducting fluid media at a temperature in the range of 50 °C to 100 °C and at a pressure in the range of 1 bar to 30 bar.

10. The process as claimed in claim 1 or claim 9, wherein said at least one de-adducting fluid media is selected from the group consisting of tetrahydrofuran, substituted tetrahydrofuran, 1,4-dioxane, substituted 1,4-dioxane, water, methanol, ethanol, and isopropanol.

11. The process as claimed in claim 1 or claim 9, wherein said at least one de-adducting fluid media is selected from the group consisting of tetrahydrofuran, and substituted tetrahydrofuran.

12. The process as claimed in claim 1, wherein the weight ratio of said crude carboxylic acid to said Lewis base is in the range of 1:2 to 1:8.

13. The process as claimed in claim 1, wherein in step (d) the weight ratio of said crystals of Lewis base-carboxylic acid adduct to said de-adducting fluid media is in the range of 1:2 to 1:6.

14. The process as claimed in claim 2, wherein the weight ratio of said crystals of Lewis base-carboxylic acid adduct to said Lewis base is in the range of 1:2 to 1:6.

15. The process as claimed in claim 1, wherein the separating step (c) and the separating step (e) is carried out by means of filtration.