FORM - 2
THE PATENTS ACT, 1970
(39 of 1970) & THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
PROCESS FOR THE PURIFICATION OF CRUDE CARBOXYLIC ACID
RELIANCE INDUSTRIES LIMITED
an Indian company
of 3rd Floor, Maker Chamber-IV
222, Nariman Point, Mumbai - 400021
Maharashtra, India.
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

FIELD
The present disclosure relates to a process for the purification of crude carboxylic acid.
BACKGROUND
Terephthalic acid is an organic colourless solid having the formula C6H4(COOH)2. Terephthalic acid is a known precursor to polyethylene terephthalate (PET); the latter being extensively used for applications such as preparation of clothing items, plastic bottles and the like.
Commercially, terephthalic acid is produced by the oxidation of para-xylene using air or oxygen rich air as an oxidant in the presence of a catalyst and a promoter and by employing acetic acid as a solvent.
Wet oxidation process of para-xylene results in crude terephthalic acid as the main product, intermediates such as para-tolualdehyde, para-toluic acid, 4-carboxybenzhaldehyde (4-CBA) and numerous side products such as isophthalic acid, orthophthalic acid, meta or ortho-tolualdehyde, meta or ortho-toluic acid, 2 or 3-carboxybenzhaldehyde, 3 or 4-bromo methyl benzoic acid, benzoic acid, trimellitic acid, trimesic acid, benzaldehyde, phthalaldehyde, ethylbenzaldehyde, methylstyrene, diphenic acid, 2- biphenyl carboxylic acid, hemimellitic 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, and 4,4-dimethylbibenzyl, vinylacetylene.
In order to use terephthalic acid as a starting material, for instance, in the preparation of PET, it is recommended that its 4-CBA content is less than a minimum threshold. Presence of large quantities of 4-CBA in terephthalic acid, acts as a chain terminator during the PET polymerization process; thereby hindering the preparation of PET having

desired molecular weight. During the production of terephthalic acid, 4-CBA is typically present between 2000 ppm to 10,000 ppm and para-toluic acid in the range of 150 ppm to 5000 ppm. Therefore, reducing the 4-CBA content is very crucial for further use of terephthalic acid.
Various techniques have been explored to purify crude terephthalic acid. Attempts have also been made to purify side products such as isophthalic acid, orthophthalic acid and the like. Conventionally, crude terephthalic acid is subjected to hydrogenation to convert 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, use of such catalysts make the overall process expensive. Furthermore, some techniques necessitate conditions that are severe; thereby making the overall process energy inefficient.
There is, thus, 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 is adapted to provide, are described herein below:
It is an object of the present disclosure to provide a process for the purification of crude carboxylic acid.
It is another object of the present disclosure to provide a process for the purification of crude terephthalic acid.
It is still another object of the present disclosure to provide a process for the purification of crude isophthalic acid.

It is yet another object of the present disclosure to provide a process for the purification of crude carboxylic acid by reducing the metallic and other impurities to less than or equal to the standard specification; thereby increasing the industrial applicability of the resulting purified acid.
It is still another object of the present disclosure to provide a process for the purification of crude terephthalic acid to obtain purified terephthalic acid having acceptable 4-CBA content.
Other objects and advantages of the present disclosure will be more apparent from the following description and the accompanying drawing 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 comprising - reacting at least one Lewis base with at least one crude carboxylic acid at a temperature ranging from 50 to 200 °C and at a pressure ranging from 1 to 10 bar to obtain Lewis base-carboxylic acid adducts in a dissolved form in a resultant mass; crystallizing at least one Lewis base-carboxylic acid adduct at a temperature lower than the temperature at which the reaction in the afore-stated step is carried out to obtain crystals of the Lewis base-carboxylic acid adduct; separating the crystals of the Lewis base-carboxylic acid adduct from the resultant mass to obtain separated crystals of the Lewis base-carboxylic acid adduct and heating the separated crystals of the Lewis base-carboxylic acid adduct at a temperature ranging from 50 to 250 °C and at a pressure ranging from 0 to 760 mm of Hg to obtain purified carboxylic acid.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 provides an exemplary illustration of the process of the present disclosure for preparing purified carboxylic acid from crude carboxylic acid.

DETAILED DESCRIPTION
The present disclosure provides a process for the purification of crude carboxylic acid that consists of the steps provided herein below.
Initially, predetermined quantities of crude carboxylic acid and Lewis base are fed to the reaction section (A) via line 1 and 2 respectively. Typically, the crude carboxylic acid comprises crude terephthalic acid and crude isophthalic acid.
The Lewis base of the present disclosure is selected from the group consisting of substituted, unsubstituted, linear, branched, cyclic, polycyclic, heterocyclic, aromatic and heteroaromatic Lewis base and combinations thereof. Furthermore, the Lewis base is selected from the group consisting of linear and branched substituted and non-substituted mono, di and trialkyl amine and phosphine, linear and branched substituted and non-substituted mono and dialkyl sulfide, substituted and non-substituted imidazole, pyrazole, thiazole, isothiazole, azathiozole, oxothiazole, oxazine, oxazoline, oxazaborole, dithiozole, triazole, selenozole, oxahosphole, pyrrole, borole, furan, thiphene, phosphole, pentazole, indole, indoline, oxazole, isothirazole, tetrazole, benzofuran, dibenzofuran, benzothiophene, dibenzothoiphene, thiadiazole, pyridine, pyrimidine, pyrazine, pyridazine, piperazine, piperidine, morpholine, pyran, aniline, phthalazine, quinazoline, quinoxaline, tromethamine, triethanoamine, pyrrolidine l-(2-hydroxyethyl), morpholine 4-(2-hydroxyethyl), L-Lysine, hydrbamine, N-methyl glucamine, ethylene diamine, ethanoamine, 2-dimethylamino ethanol, diethanolamine, deanol, choline, benzathine, benethamine, L-arginine, ammonia and combinations thereof. In one embodiment, the Lewis base used in the present process is 1-methylimidazole.
The temperature in the reaction section (A) is maintained in the range of 50 to 200 °C and the pressure in the range of 1 to 10 bar. When the Lewis base is reacted with crude carboxylic acid at the afore-stated temperature and pressure conditions, Lewis base-carboxylic acid adducts are formed in a dissolved form in a resultant mass. The Lewis

base-carboxylic acid adducts that can be formed include, but are not limited to Lewis base-terephthalic acid adduct and Lewis base-isophthalic acid adduct. The adduct formation takes place due to dipole-dipole interaction between the nitrogen atom of the 1-methyl imidazole and hydrogen atom of carboxylic acid group. An embodiment of the afore-stated adduct - an adduct between the nitrogen atom of the 1-methyl imidazole and hydrogen atom of terephthalic acid is represented herein below in Formula-I:

Typically, the Lewis base and the crude carboxylic acid are charged in a mole ratio varying from 1:1 to 50:1, more particularly from 2:1 to 12:1 and even more from particularly 6:1 to 10:1. In one embodiment, crude carboxylic acid is charged using a solid conveying system. The quantity of crude carboxylic acid to the Lewis base ratio is such that all adducts that are formed of carboxylic acid, its intermediates and impurities remain in soluble form at a predetermined temperature.
The reactors in which formation of the adduct can be carried out are selected from the group consisting of single or series of continuous stirred flow reactors, static mixers, plug flow reactors, fixed bed reactors, fluidized bed reactors and packed bed reactors.
The resultant mass is conveyed via line 3 to the crystallization section (B) where selective crystallization of either of the Lewis base-terephthalic acid adduct or the Lewis base-isophthalic acid adduct is carried out. The adduct is selectively crystallized from the reaction mass by cooling to a temperature lower than the temperature in the reaction section (A). The crystallization section (B) comprises a single or series of crystallizers

selected from the group consisting of Forced Circulation Crystallizers, Draft Tube Baffle Crystallizers and Oslo Crystallizers.
A slurry stream emerging from the crystallization section (B) goes to the filtration section (C) via line 4 to separate either of the afore-stated three Lewis base-carboxylic acid adducts from the excess Lewis base and the other adducts. The filtration section (C) comprises a single or series of filters selected from the group consisting of belt filters, rotary filters and Nutsche filters, operated under positive, negative or atmospheric pressure.
The filtrate is divided into two parts, one part is recycled back to the reaction section (A) via line 14 and the other part goes to reactant recovery section (G) via line 13.
The residue containing the separated Lewis base-carboxylic acid adduct optionally undergoes washing in the washing section (D) via line 5, where the adduct is washed using a first fluid medium coming from line 6. The washing section (D) comprises a reactor and a filtration system and other necessary unit operations and equipment(s). The filtrate from the washing section (D) can either be recycled back to the washing section (D) via line 11 or be sent to the de-adduction section (E) via line 7 or via line 10 to the fluid medium recovery section (F).
The fluid medium used for washing the adduct - the first fluid medium, is selected from the group consisting of methyl acetate, ethyl acetate, acetonitrile, dichloromethane, dichloroethane, 1-methyl imidazole and combinations thereof. Typically, the first fluid medium includes molecular solvents having 1 to 12 carbon atoms. The ratio between the first fluid medium and the adduct varies from 1:1 to 50:1, preferably from 1:1 to 8:1. The step of washing is carried out at a temperature ranging from 50 to 200 °C and at a pressure ranging from 1 to 10 bar.
The washed adduct is then fed to the de-adduction section (E) via line 7 in order to separate the carboxylic acid from the Lewis base; rendering the former in a purified state. The de-adduction section (E) comprises a drying section, scrubbing section and other

necessary unit operations and equipment(s). The drying section comprises one or a series of dryers selected from the group consisting of fluidized bed, atmospheric tray, vacuum tray, agitated bed, direct rotary, indirect rotary, spray, spouted bed, vibrated bed, drum, belt, plate, vacuum, disc, paddle, column, filter, ring, jet-zone, microwave, freeze and solar dryer. Relative motion between the drying medium and solid to be dried can be concurrent, counter current and mixed flow. The dryer can be a batch, a continuous and a combination of both mode with heat input type including convection, conduction, radiation, combination of heat transfer modes, intermittent or continuous, adiabatic or non-adiabatic or any combination thereof at atmospheric pressure or vacuum with single or multiple stages. Batch and continuous dryers include layer and dispersion type dryers that employ different types of drying media that are selected from the group consisting of air, nitrogen, superheated steam, flue gases and other gases which do not react with the adduct or the purified carboxylic acid.
Vapors and gases coming out from the drying section consist of Lewis base which optionally undergoes the step of scrubbing in the scrubbing section, after which it is sent to the Lewis base recovery section (G) via line 9. Purified carboxylic acid is collected from line 8 and the Lewis base is recycled back via lines 14 and 15 to the reaction section (A). The Lewis base coming from the fluid medium recovery section (F), intermediate recovery section (H) and de-adduction section (E) goes to the reactant recovery section (G) via lines 16,17 and 9 respectively.
The adduct of intermediates with the Lewis base is charged to the intermediate recovery section (H) via line 18. The recovered intermediates are recycled back to the reaction section (A) via line 19.
The de-adduction of the carboxylic acid from the Lewis base is carried out at a temperature of 50 to 250 °C and at a pressure ranging from 0 to 760 mm of Hg. By varying the pressure parameter, the temperature of adduct breaking can be varied. In one preferred embodiment, the step of de-adduction is carried out under vacuum.

In one optional embodiment, the step of de-adduction is carried out by heating as provided herein above, followed by treating the heated separated crystals of the Lewis base-carboxylic acid adduct with at least one second fluid medium selected from the group consisting of methanol, ethanol, propanol, benzyl alcohol, water and combinations thereof to obtain purified carboxylic acid to enhance the de-adduction.
In accordance with the present disclosure, the adduct formation is carried out in single or multiple stages to get the desired purity and specification of terephthalic acid/ isophthalic acid. In one embodiment of the present disclosure, the adduct formed is re-dissolved in fresh or recycled Lewis base in single or multiple stages to achieve the desired purity and the desired specifications of terephthalic acid. After dissolution, the adduct is separated from the excess Lewis base by cooling or anti-solvent crystallization or combinations thereof. In accordance with the present disclosure the Lewis base, the first fluid medium, the gases and the chemicals used in the process are capable of being recycled and reused.
It is significant to note that the process of the present disclosure effects the removal of impurities such as carboxybenzaldehyde and toluic acid from the carboxylic acid and metal catalysts.
The present disclosure is further illustrated herein below with the help of the following experiments. The experiments used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of embodiments herein. These laboratory scale experiments can be suitably scaled up to industrial/commercial scale.

Example 1: Process for the purification of terephthalic acid in accordance with the present disclosure
49.8 grams of crude terephthalic acid is mixed with 147.78 grams of 1-methyl imidazole in 1:6 mole ratio at temperature 105 °C. After cooling to 27-30 °C, solid adduct was obtained which was filtered out and washed with methyl acetate. A white colored adduct formation was observed. Pure terephthalic acid was regenerated by heating in an oven at atmospheric conditions to break adduct of terephthalic acid and 1 -methyl imidazole at 100 °C. 4-CBA content reduced to 174 ppm from its initial content in crude terephthalic acid of 2900 ppm.
Example 2: Process for the purification of terephthalic acid in accordance with the present disclosure
The process steps of Example 1 were repeated except for the step of regeneration of terephthalic acid, which was carried out in an indirect heating rotary dryer at 120 °C under 700 mmHg absolute vacuum. The 4-CBA content reduced to 50 ppm from its initial content in crude terephthalic acid of 2900 ppm.
Example 3: Process for the purification of isophthalic acid in accordance with the present disclosure
25 grams of crude isophthalic acid were mixed with 75 grams of 1-methyl imidazole in 1:6 mole ratio at temperature 105 °C. After cooling to 27-30 °C, solid adduct was obtained which was filtered out and washed with methyl acetate. A white colored adduct formation was observed. Pure isophthalic acid was regenerated by heating in an oven at atmospheric conditions to break adduct of isophthalic acid and 1-methyl imidazole at 100 °C. 3-CBA content in separated isophthalic acid reduced to 800 ppm from its initial content in crude isophthalic acid of 3200 ppm.

Comparative Example 1: Process for the purification of terephthalic acid using N-methyl pyrrolidone
49.8 grams of terephthalic acid was mixed with 178.44 grams of N-methyl pyrrolidone in 1:6 mole ratio at a temperature of 105 °C. As no clear solution was observed at this temperature, the temperature was further raised to get a clear solution which was obtained at 140 °C. However, at this temperature the color of N-methyl pyrrolidone turned dark brown from colourless. After cooling to 27-30 °C, a solid adduct was obtained which was filtered out and washed with N-methyl pyrrolidone to remove/reduce the colour of adduct which was brown after the reaction. Pure terephthalic acid was regenerated by heating in an oven at 100 °C and at atmospheric conditions to break the adduct of terephthalic acid and N-methyl pyrrolidone. The 4-CBA content was found to be reduced to 1300 ppm from its initial content in crude terephthalic acid of 2900 ppm.
Comparative Example 2: Process for the purification of isophthalic acid using N-methyl pyrrolidone
A similar experiment to that of comparative example 1 was carried out except that terephthalic acid was replaced by isophthalic acid.
TECHNICAL ADVANCEMENTS AND ECONOMIC SIGNIFICANCE
The present disclosure provides:
• A process for the generation of purified terephthalic acid with the required specification of 4-CBA.
• A process for the generation of purified isophthalic acid .
• A process for the purification of crude terephthalic acid and isophthalic acid using reusable and recyclable chemicals.
The exemplary embodiments herein quantify the benefits arising out of this disclosure and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the description. Descriptions of well-known

components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
Any discussion of documents, acts, materials, devices, articles and 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.
While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications 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 modifications in the nature of the disclosure or the preferred embodiments 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.

We Claim:
1. A process for the purification of crude carboxylic acid; said process comprising the following steps: i. reacting at least one Lewis base with at least one crude carboxylic acid at a
temperature ranging from 50 to 200 °C and at a pressure ranging from 1 to
10 bar to obtain Lewis base-carboxylic acid adducts in a dissolved form in
a resultant mass; ii. crystallizing at least one Lewis base-carboxylic acid adduct at a
temperature lower than the temperature at which the reaction in step (a) is
carried out to obtain crystals of the Lewis base-carboxylic acid adduct; iii. separating said crystals of the Lewis base-carboxylic acid adduct from the
resultant mass to obtain separated crystals of the Lewis base-carboxylic
acid adduct; and iv. heating said separated crystals of the Lewis base-carboxylic acid adduct at a
temperature ranging from 50 to 250 °C and at a pressure ranging from 0 to
760 mm of Hg to obtain purified carboxylic acid.
2. The process as claimed in claim 1, wherein said crude carboxylic acid comprises crude terephthalic acid, crude isophthalic acid and combinations thereof.
3. The process as claimed in claim 1, wherein said Lewis base is at least one selected from the group consisting of substituted, unsubstituted, linear, branched, cyclic, polycyclic, heterocyclic, aromatic and heteroaromatic Lewis base.

4. The process as claimed in claim 1, wherein said Lewis base is 1-methylimidazole.
5. The process as claimed in claim 1, wherein said Lewis base and said crude carboxylic acid are taken in a mole ratio varying from 1:1 to 50:1.
6. The process as claimed in claim 1, wherein said at least one carboxylic acid adduct of step (b) is Lewis base-terephthalic acid adduct.
7. The process as claimed in claim 1, wherein said at least one carboxylic acid adduct of step (b) is Lewis base-isophthalic acid adduct.
8. The process as claimed in claim 1, further comprises washing said crystals of the Lewis base-carboxylic acid adduct with at least one first fluid medium prior to the step of heating.
9. The process as claimed in claim 8, wherein said first fluid medium recovered after the step of washing is recycled for the purification of crude carboxylic acid.
10. The process as claimed in claim 8, wherein said first fluid medium is selected from the group consisting of methyl acetate, ethyl acetate, acetonitrile, dichloromethane, dichloroethane, 1-methyl imidazole and combinations thereof.
11. The process as claimed in claim 1, wherein said step of heating further comprises treating the heated separated crystals of the Lewis base-carboxylic acid adduct with at least one second fluid medium selected from the group consisting of methanol, ethanol, propanol, benzyl alcohol, water and combinations thereof to obtain purified carboxylic acid.

12. The process as claimed in claim 1, wherein the Lewis base remaining after the step of separation of the crystals of the Lewis base-carboxylic acid adduct is subjected to scrubbing, followed by recovery and recycling.
13.Purified terephthalic acid prepared by the process as claimed in claim 1 having 4-carboxybenzaldehyde (4-CBA) content less than 200 ppm.