A method for the crystallization separation of m-toluic acid

Abstract

The application discloses a crystallization separation method of m-toluic acid, which comprises the following steps: collecting MX oxidation reaction liquid, cooling to a certain temperature under stirring, and performing IPA crystallization operation; cooling the oxidation liquid filtrate after removing IPA to a certain temperature under stirring, and performing MTA crystallization operation; evaporating and concentrating the IPA filter cake washing liquid and the MTA filter cake washing liquid, distilling the condensate to recover MX, and returning the concentrated liquid as a MTA-rich liquid to the IPA crystallization link or the MTA crystallization link to perform crystallization operation to recover IPA and MTA therein. The process has the characteristics of short operation period, good separation effect and high MTA yield, and can recover IPA as a by-product, thereby avoiding the generation of a large amount of residues.

Description

Technical Field

[0001] This invention belongs to the field of chemical processes, and specifically relates to a method for the crystallization and separation of m-methylbenzoic acid. Background Technology

[0002] m-Toluic acid (MTA) can be used to synthesize herbicides, mosquito repellents, polyesters, coatings, and pharmaceutical chemicals. It can also be used to synthesize fragrances and flavorings, and as an auxiliary packing material in chromatography to study the behavior of nucleotides. MTA is mainly synthesized by the catalytic oxidation of m-xylene (MX). After the reaction, the reaction solution needs to be separated to remove unreacted MX, byproducts, and other impurities to obtain MTA with high purity.

[0003] In industrial production, batch distillation is mainly used to separate MTA, but the corresponding separation process is not perfect, MTA separation is difficult, and the yield is low; the distillation operation cycle is long and the temperature is high, and the feed liquid is prone to coking, resulting in a large amount of residue. Some researchers have also used multi-stage series continuous distillation devices [8] and short-path molecular distillation technology to separate MTA, but they all inevitably have problems such as long distillation time, large amount of residue, high energy consumption, low MTA yield, and serious environmental pollution.

[0004] During the catalytic oxidation process, MX generates a significant amount of isophthalic acid (IPA), which is also the main regenerated impurity component in the reaction solution. IPA possesses strong heat and chemical resistance and can participate in reactions such as dehydration, hydrogenation, and halogenation. It has wide applications in alkyd resin coatings, polyester resins, specialty fibers, and resin plasticization.

[0005] The solubility of MTA in MX varies significantly with temperature, while IPA is insoluble in MX, providing a basis for the crystallization separation of MTA. Crystallization is a complex process involving crystal nucleation and growth, influenced by the material composition and its physicochemical properties, as well as operating conditions such as temperature and stirring. In actual operation, the solution system is made supersaturated with a certain degree of solute by lowering the temperature and evaporating the solvent, thereby precipitating crystal nuclei, which then grow into uniformly sized crystal particles under specific operating conditions.

[0006] Currently, there are no reported processes for separating MTA from MX oxidation liquid using crystallization. Therefore, this invention proposes a novel crystallization process to achieve green and efficient separation of MTA. Summary of the Invention

[0007] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0008] In view of the problems existing in the above and / or prior art, the present invention is proposed.

[0009] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a method for the crystallization and separation of m-methylbenzoic acid.

[0010] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for crystallizing and separating m-methylbenzoic acid, comprising,

[0011] The m-xylene oxidizing solution was cooled and crystallized. The resulting crystal slurry was kept warm and filtered to obtain a wet filter cake of isophthalic acid and a mother liquor of isophthalic acid crystallization.

[0012] The wet filter cake of isophthalic acid was washed with warm m-xylene solution, filtered, and dried to obtain the product isophthalic acid.

[0013] After cooling and crystallizing the isophthalic acid crystallization mother liquor, the obtained crystal slurry was kept warm and filtered to obtain m-methylbenzoic acid wet filter cake and m-methylbenzoic acid crystallization mother liquor, respectively.

[0014] The wet filter cake of m-methylbenzoic acid was washed, filtered, and dried using cold xylene solution to obtain the product m-methylbenzoic acid.

[0015] The mother liquor from the crystallization of m-methylbenzoic acid was vacuum distilled to obtain the light component m-xylene and the heavy component;

[0016] The recombinant components were crystallized at low temperature, filtered and washed, and the filter cake was dried to obtain m-methylbenzoic acid.

[0017] As a preferred embodiment of the crystallization and separation method of m-methylbenzoic acid according to the present invention, the oxidizing liquid is cooled, wherein the cooling method is one or more of flash evaporation, indirect heat exchange, and direct water cooling, and the cooling temperature is 20℃~120℃.

[0018] As a preferred embodiment of the crystallization and separation method of m-methylbenzoic acid according to the present invention, wherein: the m-xylene oxidizing liquid is cooled, wherein the cooling process is either a one-step direct cooling to a set temperature or a step-by-step cooling to a set temperature.

[0019] As a preferred embodiment of the crystallization and separation method of m-methylbenzoic acid described in this invention, the prepared crystal slurry is kept warm and filtered to obtain a wet filter cake of isophthalic acid and a mother liquor for crystallization of isophthalic acid, wherein the temperature of the warming is the same as the crystallization temperature.

[0020] As a preferred embodiment of the crystallization and separation method of m-methylbenzoic acid described in this invention, the wet filter cake of isophthalic acid is washed with a warm m-xylene solution, wherein the warm m-xylene solution is fresh industrial grade m-xylene or m-xylene solution, and the temperature is consistent with the temperature of the filter cake layer; the volume ratio of the warm m-xylene solution to the wet filter cake is 2 to 10:1.

[0021] As a preferred embodiment of the crystallization and separation method of m-methylbenzoic acid according to the present invention, the product isophthalic acid is obtained by drying, wherein the drying is vacuum drying or atmospheric pressure drying. When vacuum drying is performed at <10KPa and 60℃~70℃ for 3~5 hours, m-methylbenzoic acid with a purity >95% can be obtained.

[0022] As a preferred embodiment of the crystallization and separation method of m-methylbenzoic acid described in this invention, the step of cooling the mother liquor of isophthalic acid crystallization for crystallization is wherein the crystallization cooling method is one or more of flash evaporation, indirect heat exchange, and direct water cooling, and the cooling temperature is -20℃ to 10℃.

[0023] In a preferred embodiment of the crystallization and separation method for m-methylbenzoic acid described in this invention, the cooling process is either a one-step direct cooling to a set temperature or a step-by-step cooling to a set temperature; the cold m-xylene solution is fresh industrial-grade m-xylene or m-xylene solution, and the temperature of the feed solution is the same as the temperature of the mother liquor.

[0024] The wet filter cake of m-methylbenzoic acid is washed with cold m-xylene solution, and the amount of m-xylene solution used is 1 to 10 times the volume of the wet filter cake.

[0025] The drying process yields m-methylbenzoic acid, wherein the drying is performed by vacuum drying, atmospheric pressure drying, or melt-forming. When melt-forming is used, residual MX is removed under melting conditions, and the resulting product is m-methylbenzoic acid with a purity >99.5%.

[0026] As a preferred embodiment of the crystallization and separation method of m-methylbenzoic acid according to the present invention, the recombinant component is crystallized at low temperature, wherein the crystallization cooling temperature is -20℃ to 10℃.

[0027] As a preferred embodiment of the crystallization and separation method of m-methylbenzoic acid described in this invention, the method further includes: collecting the isophthalic acid filter cake washing liquid and the m-methylbenzoic acid filter cake washing liquid, evaporating and concentrating them, and recovering m-xylene by condensation of the distillate;

[0028] The concentrate is returned to the isophthalic acid crystallization stage or the m-methylbenzoic acid crystallization stage as a feed solution rich in m-methylbenzoic acid, and the crystallization operation is carried out to recover the isophthalic acid and m-methylbenzoic acid therein.

[0029] Beneficial effects of this invention:

[0030] This invention provides a novel crystallization and separation process for m-methylbenzoic acid. This process features a short operation cycle, good separation effect, and high yield of m-methylbenzoic acid. Furthermore, it can recover isophthalic acid as a byproduct, thus avoiding the generation of a large amount of residue. Attached Figure Description

[0031] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0032] Figure 1 This is a schematic diagram of the process flow of the present invention. Detailed Implementation

[0033] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0034] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0035] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0036] The intermediate xylene (MX) oxidation liquid of this invention is derived from the heterogeneous oxidation reaction of intermediate xylene with oxygen in the air under specific pressure and with the action of a noble metal catalyst.

[0037] Example 1

[0038] (1) 100g of m-xylene MX oxidation solution (IPA 8%, MTA 33%) was naturally cooled to 70℃, and then filtered and washed under negative pressure filtration. 9g of MX was used for washing to obtain 9.05g of filter cake and 99.95g of filtrate. The IPA filter residue was vacuum dried at 60℃ for 4h to obtain 8.05g of IPA with a purity of 98.82% and an IPA yield of 99.44%.

[0039] (2) 99.95g of filtrate was cooled for crystallization at -5℃ and stirred at 300rpm. Filtration and washing were carried out at this temperature under negative pressure. The filter cake was washed with 61g of MX at -5℃ to obtain 30.36g of MTA filter cake and 130.59g of filtrate. The wet filter cake was vacuum dried at 60℃ for 4h to obtain 27.21g of MTA with a purity of 99.83% and an MTA yield of 82.31%.

[0040] (3) Vacuum distillation was performed on 130.59g of filtrate to distill off 113.65g of light component MX and 16.94g of heavy component in the still. The distilled-off MX was reserved for oxidation reaction.

[0041] (4) 16.94 g of the distilled heavy fraction was placed in a cold trap and stirred and cooled for low-temperature crystallization at -5 °C and a stirring speed of 300 rpm. Filtration and washing were performed at -5 °C with 6 g of MX at -5 °C to obtain 3.94 g of filter cake and 19 g of filtrate. The filter cake was dried under the same conditions to obtain 3.54 g of MTA with a purity of 99.13%. The MTA yield was 60.07%, and the total MTA yield was 92.94%.

[0042] (5) The filtrate containing the cobalt catalyst is reserved for use in the oxidation reaction.

[0043] Example 2

[0044] (1) 100g of MX oxidation solution (IPA 8%, MTA 33%) was naturally cooled to 60℃, and then filtered and washed under negative pressure. 9g of MX was used for washing, yielding 9.43g of filter cake and 99.57g of filtrate. The IPA filter residue was vacuum dried at 60℃ for 4 hours, yielding 7.98g of IPA with a purity of 96.73%. The IPA yield was 96.52%.

[0045] (2) 99.57 g of filtrate was cooled for crystallization at -10 °C and stirred at 300 rpm. Filtration and washing were then performed at this temperature using negative pressure filtration. The mixture was washed with 61 g of MX at -10 °C to obtain 31.1 g of MTA filter cake and 129.47 g of filtrate. The wet filter cake was then vacuum dried at 60 °C for 4 h to obtain 26.57 g of MTA with a purity of 99.53%. The MTA yield was 80.14%.

[0046] (3) The 129.47g filtrate was subjected to vacuum distillation to distill off 112.87g of light component MX, while the heavy component in the vessel was 16.6g. The distilled-off MX was reserved for oxidation reaction.

[0047] (4) 16.6 g of the distilled heavy fraction was placed in a cold trap and stirred and cooled for low-temperature crystallization at -10 °C and a stirring speed of 300 rpm. Filtration and washing were performed at -10 °C with 6 g of MX at -10 °C to obtain 3.42 g of filter cake and 19.18 g of filtrate. The filter cake was dried under the same conditions to obtain 3.03 g of MTA with a purity of 99.33%. The MTA yield was 45.95%, and the total MTA yield was 89.26%.

[0048] (5) The filtrate containing the cobalt catalyst is reserved for use in the oxidation reaction.

[0049] Example 3

[0050] (1) 100g of MX oxidation solution (IPA 8%, MTA 33%) was naturally cooled to 100℃, and then filtered and washed under negative pressure. 9g of MX was used for washing, yielding 8.82g of filter cake and 100.18g of filtrate. The IPA filter residue was vacuum dried at 60℃ for 4 hours, yielding 7.85g of IPA with a purity of 96.92%. The IPA yield was 95.13%.

[0051] (2) 100.18 g of filtrate was cooled for crystallization at 15 °C and stirred at 300 rpm. Filtration and washing were then performed at this temperature using negative pressure filtration. The mixture was washed with 61 g of MX at 15 °C, yielding 25.84 g of MTA filter cake and 135.34 g of filtrate. The wet filter cake was then vacuum dried at 60 °C for 4 hours, yielding 23.24 g of MTA with a purity of 99.23%. The MTA yield was 69.88%.

[0052] (3) Vacuum distillation was performed on 135.34g of filtrate to distill off 117.76g of light component MX and 17.58g of heavy component in the still. The distilled-off MX was reserved for oxidation reaction.

[0053] (4) 17.58 g of the distilled heavy fraction was placed in a cold trap and stirred and cooled for low-temperature crystallization at 15 °C and a stirring speed of 300 rpm. Filtration and washing were performed at 15 °C with 6 g of MX at 15 °C, yielding 4.78 g of filter cake and 18.80 g of filtrate. The filter cake was dried under the same conditions as above, yielding 3.68 g of MTA with a purity of 98.62%. The MTA yield was 36.51%, and the total MTA yield was 80.88%.

[0054] (5) The filtrate containing the cobalt catalyst is reserved for use in the oxidation reaction.

[0055] Example 4

[0056] (1) 1000g of MX oxidation solution (IPA 5%, MTA 35%) was flash-evaporated and cooled to 70℃, then filtered and washed with 70g of MX at room temperature to obtain 56g of filter cake and 1014g of filtrate (including washings). The IPA filter cake was vacuum dried at 80℃ for 3h to obtain 49.95g of IPA with a purity of 99.12%. The IPA yield was 99.02%.

[0057] (2) The 1014g filtrate was cooled in steps and crystallized under stirring. The first step involved cooling at a rate of 3℃ / min to 30℃ and holding for 30min; the second step involved cooling at a rate of 1.5℃ / min to -5℃ and holding for 1h. Filtration and washing were performed at -5℃, using 650g of MX solution at -5℃ to continuously wash the MTA filter cake. After filtration, 322g of MTA filter cake and 1342g of filtrate were obtained. The wet filter cake was melt-distilled and condensed to obtain MX condensate, simultaneously yielding 290g of solid MTA with a purity of 99.72%. The MTA yield was 82.63%.

[0058] (3) Vacuum distillation was performed on 1342g of filtrate to distill off 1180g of light component MX, and 162g of heavy component remained in the vessel. The distilled-off MX was reserved for oxidation reaction.

[0059] (4) 162g of the distilled heavy fraction was placed in a cold trap and stirred and cooled for low-temperature crystallization at -5℃ and a stirring speed of 300rpm. Filtration and washing were performed at -10℃ with 200g of MX at -5℃, yielding 56g of filter cake and 306g of filtrate. The filter cake was melt-distilled to obtain 49g of MTA with a purity of 99.02%. The MTA yield was 79.79%, and the overall MTA yield was 96.49%.

[0060] (5) The filtrate containing the cobalt catalyst is reserved for use in the oxidation reaction.

[0061] Example 5

[0062] (1) 3000 kg of MX oxidation solution (IPA 6%, MTA 39%) was cooled to 90 °C in a stirred tank, and then filtered and washed with 200 kg of MX at room temperature to obtain 201 kg of filter cake and 2999 kg of filtrate (including washings). The IPA filter cake was vacuum dried at 65 °C for 5 h to obtain IPA with a purity of 99.10% (mass 179 kg). The IPA yield was 98.55%.

[0063] (2) 2999 kg of filtrate was cooled in a stirred tank in stages for crystallization. The first stage involved cooling at a rate of 2 °C / min to 45 °C and holding for 60 min; the second stage involved cooling at a rate of 1 °C / min to -5 °C and holding for 2 h. Filtration and washing were performed at -5 °C. The MTA filter cake was washed with 1000 kg of MX solution at -5 °C, and after filtration, 1200 kg of MTA filter cake and 2799 kg of filtrate were obtained. The wet filter cake was melt-distilled, condensed to obtain MX condensate, and simultaneously cooled to form sheets, yielding 1080 kg of solid MTA with a purity of 99.51%. The MTA yield was 91.86%.

[0064] (3) The filtrate containing the cobalt catalyst is returned to the oxidation process for reuse.

[0065] Comparative Example 1

[0066] (1) 100g of m-xylene MX oxidation solution (IPA 8%, MTA 33%) was naturally cooled to 20℃, and then filtered and washed under negative pressure filtration. 19g of MX was used for washing to obtain 18.95g of filter cake and 100.05g of filtrate. The IPA filter residue was vacuum dried at 60℃ for 4h to obtain 17.1g of IPA with a purity of 46.23% and an IPA yield of 98.8%.

[0067] (2) 100.05g of filtrate was cooled for crystallization at -20℃ and stirred at 300rpm. Filtration and washing were carried out at this temperature under negative pressure. The filter cake was washed with 25g of MX at -20℃ to obtain 24.87g of MTA filter cake and 100.18g of filtrate. The wet filter cake was vacuum dried at 60℃ for 4h to obtain 22.35g of MTA with a purity of 86.48% and an MTA yield of 58.57%.

[0068] (3) Vacuum distillation was performed on 100.18g of filtrate to distill off 88.53g of light component MX, leaving 11.65g of heavy component in the vessel. The distilled-off MX was reserved for oxidation reaction.

[0069] (4) 11.65 g of the distilled heavy fraction was placed in a cold trap and stirred and cooled for low-temperature crystallization at -20 °C and a stirring speed of 300 rpm. Filtration and washing were performed at -20 °C with 3 g of MX at -20 °C, yielding 2.86 g of filter cake and 11.79 g of filtrate. The filter cake was dried under the same conditions to obtain 2.5 g of MTA with a purity of 88.95%. The MTA yield was 16.27%, and the total MTA yield was 65.31%.

[0070] (5) The filtrate containing the cobalt catalyst is reserved for use in the oxidation reaction.

[0071] Comparative Example 2

[0072] (1) 100g of m-xylene MX oxidation solution (IPA 8%, MTA 33%) was naturally cooled to 30℃, and then filtered and washed under negative pressure filtration. 14g of MX was used for washing to obtain 13.96g of filter cake and 100.04g of filtrate. The IPA filter residue was vacuum dried at 60℃ for 4h to obtain 12.5g of IPA with a purity of 61.65% and an IPA yield of 96.33%.

[0073] (2) 100.04g of filtrate was cooled and crystallized at -20℃ with a stirring speed of 300rpm. Filtration and washing were carried out at this temperature by vacuum filtration and washing with 30g of MX at -20℃ to obtain 29.13g of MTA filter cake and 100.91g of filtrate. The wet filter cake was vacuum dried at 60℃ for 4h to obtain 26.2g of MTA with a purity of 83.87% and an MTA yield of 66.59%.

[0074] (3) Vacuum distillation was performed on 100.91g of filtrate to distill off 86.89g of light component MX, leaving 14.02g of heavy component in the vessel. The distilled-off MX was reserved for oxidation reaction.

[0075] (4) 14.02 g of the distilled heavy fraction was placed in a cold trap and stirred and cooled for low-temperature crystallization at -20 °C and a stirring speed of 300 rpm. Filtration and washing were performed at -20 °C with 4 g of MX at -20 °C to obtain 4.15 g of filter cake and 13.87 g of filtrate. The filter cake was dried under the same conditions as above to obtain 3.7 g of MTA with a purity of 87.15%. The MTA yield was 29.24%, and the total MTA yield was 80.44%.

[0076] (5) The filtrate containing the cobalt catalyst is reserved for use in the oxidation reaction.

[0077] In the synthesis of MTA, MX serves as both a raw material and a solvent in the reaction system. Currently, distillation is used for separation in MTA preparation. However, the high temperatures during distillation can cause esterification, condensation, polymerization, and even carbonization reactions in MTA, m-methylbenzyl alcohol, m-methylbenzaldehyde, and isophthalic acid, resulting in a large amount of residue in the reactor (approximately 40% of the product). MTA has relatively high solubility in MX, and separating it from byproducts through crystallization requires addressing not only product quality issues but also the recycling of the mother liquor. The presence of a large amount of residue with unknown components usually indicates a complex and substantial composition of the reactor liquid, which directly impacts the selection of the MTA production process route, given the high purity requirements of the product. Based on the analysis of the reaction mechanism, residue components, and experimental research, this invention boldly proposes a crystallization separation process and designs a mother liquor recycling route, achieving success and proposing a crystallization separation process for m-methylbenzoic acid.

[0078] This invention discloses a crystallization and separation process for m-methylbenzoic acid (MX), comprising the following steps: collecting the MX oxidation reaction solution, cooling it to a certain temperature under stirring, and performing IPA crystallization; cooling the filtrate from the MX oxidation solution after IPA removal to a certain temperature under stirring, and performing MTA crystallization; evaporating and concentrating the collected IPA filter cake washing liquid and MTA filter cake washing liquid, condensing the distillate to recover MX, and returning the concentrated liquid as a feed solution rich in MTA to the IPA crystallization stage or the MTA crystallization stage for crystallization to recover IPA and MTA. This process features a short operation cycle, good separation effect, and high MTA yield, and can recover IPA as a byproduct, avoiding the generation of a large amount of residue.

[0079] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the present invention.

Claims

1. A method for crystallizing and separating m-methylbenzoic acid, characterized in that: include, The m-xylene oxidizing solution is cooled and crystallized. The resulting crystal slurry is kept warm and filtered to obtain isophthalic acid wet filter cake and isophthalic acid crystallization mother liquor. The cooling method is one or more of flash evaporation, indirect heat exchange, and direct water cooling. The cooling temperature is 20℃~120℃, and the holding temperature is the same as the crystallization temperature. Isophthalic acid wet filter cake is washed, filtered, and dried with a warm m-xylene solution to obtain the product isophthalic acid. The warm m-xylene solution is fresh industrial grade m-xylene, and the temperature is the same as the temperature of the filter cake layer. The volume ratio of the warm m-xylene solution to the wet filter cake is 2~10:

1. After cooling and crystallizing the isophthalic acid crystallization mother liquor, the obtained crystal slurry is kept warm and filtered to obtain m-methylbenzoic acid wet filter cake and m-methylbenzoic acid crystallization mother liquor, respectively. The crystallization cooling method is one or more of flash evaporation, wall heat exchange, and direct water cooling, and the cooling temperature is -20℃~10℃. The wet filter cake of m-methylbenzoic acid was washed, filtered, and dried using a cold xylene solution to obtain the product m-methylbenzoic acid. The cold xylene solution was fresh industrial grade m-xylene, and the temperature was the same as that of the mother liquor. The mother liquor from the crystallization of m-methylbenzoic acid was vacuum distilled to obtain the light component m-xylene and the heavy component; The recombinant components were crystallized at low temperature, filtered and washed, and the filter cake was dried to obtain m-methylbenzoic acid. The crystallization cooling temperature was -20℃ to 10℃.

2. The crystallization and separation method of m-methylbenzoic acid as described in claim 1, characterized in that: The cooling of the m-xylene oxide liquid can be performed in two ways: either by directly cooling to a set temperature in one step or by cooling to a set temperature in steps.

3. The crystallization and separation method of m-methylbenzoic acid as described in claim 1, characterized in that: The drying process yields isophthalic acid, wherein the drying is performed under vacuum or at normal pressure. Under vacuum drying conditions of <10 kPa and 60~70℃ for 3~5 hours, isophthalic acid with a purity >95% can be obtained.

4. The crystallization and separation method of m-methylbenzoic acid as described in claim 1, characterized in that: The wet filter cake of m-methylbenzoic acid is washed with cold m-xylene solution, and the amount of m-xylene solution used is 1 to 10 times the volume of the wet filter cake. The drying process yields m-methylbenzoic acid, wherein the drying is performed by vacuum drying, atmospheric pressure drying, or melt-forming. When melt-forming is used, residual m-xylene is removed under melting conditions, and the resulting flakes contain m-methylbenzoic acid with a purity >99.5%.

5. The crystallization and separation method of m-methylbenzoic acid as described in claim 1, characterized in that: It also includes collecting isophthalic acid filter cake washing liquid and m-methylbenzoic acid filter cake washing liquid, evaporating and concentrating them, and recovering m-xylene by condensation of the distillate; The concentrate is returned to the isophthalic acid crystallization stage or the m-methylbenzoic acid crystallization stage as a feed solution rich in m-methylbenzoic acid, and the crystallization operation is carried out to recover the isophthalic acid and m-methylbenzoic acid therein.

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