Method for purifying low content crude pyromellitic dianhydride
By employing ceramic membrane filtration, activated carbon adsorption, and gradient cooling crystallization, the problem of impurity separation in the purification of low-concentration pyromellitic dianhydride has been solved, achieving efficient and low-cost production of high-purity pyromellitic dianhydride, which is suitable for high-end chemical fields such as high-temperature engineering plastics and insulating films.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- WEIFANG HONGRUN NEW MATERIALS CO LTD
- Filing Date
- 2025-10-29
- Publication Date
- 2026-06-12
AI Technical Summary
Existing solvent crystallization methods for purifying pyromellitic dianhydride at low concentrations result in complex impurity systems, consume large amounts of solvent, are cumbersome to operate, are difficult to separate, have high energy consumption, and are hard to meet high purity requirements.
After pretreatment with ceramic membrane filtration and activated carbon adsorption column, the purity and yield of pyromellitic dianhydride were gradually improved by combining composite solvent and gradient cooling crystallization technology through primary, secondary, and tertiary crystallization and vacuum treatment.
This method achieves low-cost and efficient purification of pyromellitic dianhydride, reduces solvent consumption and time costs, meets the high purity requirements for synthesizing polyimide materials, and improves production efficiency and product purity.
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Abstract
Description
Technical Field
[0001] This invention belongs to the field of fine chemical technology, specifically a method for purifying low-content crude pyromellitic dianhydride. Background Technology
[0002] Pyromellitic dianhydride, a key monomer in the synthesis of polyimide materials, is widely used in high-end chemical fields such as high-temperature engineering plastics, insulating films, and advanced composite materials. Currently, the purification of crude pyromellitic dianhydride in industry mainly employs solvent crystallization.
[0003] When the raw material concentration is less than 25%, the impurity system in low-content pyromellitic dianhydride is complex, mainly including three types of impurity components: carboxylic anhydride compounds with different degrees of benzene ring substitution (triphenylene anhydride, phthalic anhydride, etc.), incompletely oxidized intermediate products (pyromellitic acid, methylpyromellitic acid, etc.), and dark organic polymers (tar-like substances). A large amount of different types of solvents are required to wash the impurities in pyromellitic dianhydride clean. The solvent cost increases and the purification time is also extended. It has problems such as cumbersome operation, low efficiency, difficulty in separating the target substance from impurities at low concentrations, and high energy consumption. Summary of the Invention
[0004] The purpose of this invention is to provide a method for purifying crude pyromellitic dianhydride with low content, so as to solve the problems mentioned in the background art.
[0005] A method for purifying low-content crude pyromellitic dianhydride, the specific steps of which are as follows:
[0006] S1, Raw material pretreatment
[0007] The crude pyromellitic dianhydride was selected, first filtered through a ceramic membrane, then treated with an activated carbon adsorption column, and after cyclic enrichment, a pretreated crude product was obtained.
[0008] S2, Primary Crystallization
[0009] After uniformly mixing the pretreated crude product with the composite solvent, the mixture is cooled in a reaction vessel and filtered to remove impurities. The filtrate is then passed into a primary crystallizer, cooled further, and stirred continuously to obtain a mother liquor containing pyromellitic dianhydride crystal nuclei. The composite solvent is prepared by mixing a polar solvent and a non-polar solvent in a mass ratio of (3-5):1.
[0010] S3, secondary crystallization
[0011] The pyromellitic dianhydride mother liquor containing crystal nuclei is passed into a secondary crystallizer for constant temperature crystal growth, and the pyromellitic dianhydride mother liquor containing crystal nuclei is continuously passed through during the crystal growth process.
[0012] S4, Tertiary Crystallization
[0013] After filtering out the pyromellitic dianhydride crystals precipitated in the secondary crystallizer, the crystals were vacuum-treated at -5-0℃ to obtain crude pyromellitic dianhydride crystals.
[0014] S5. Crystal washing and solvent recovery
[0015] The crude pyromellitic dianhydride crystals were washed with the composite solvent, wherein the mass ratio of the crude pyromellitic dianhydride crystals to the composite solvent was 1:(3-5), and the washing time was 10-20 min. After washing, the composite solvent was recovered by vacuum distillation to obtain high-purity pyromellitic dianhydride crystals.
[0016] As a preferred embodiment of the above technical solution, the specific steps of S1 are as follows:
[0017] Crude pyromellitic dianhydride is selected, containing 10%-15% by mass of pyromellitic dianhydride. The crude pyromellitic dianhydride is first filtered through a ceramic membrane at an operating pressure of 0.3-0.5 MPa and a flow rate of 3-5 m / s. After filtration, it is then treated with an activated carbon adsorption column at a space velocity of 1-2 h⁻¹. -1 The column temperature is 40-50℃, and the product is cyclically enriched until the mass ratio of pyromellitic dianhydride increases to 25%-30%, thus obtaining the pretreated crude product.
[0018] As a preferred embodiment of the above technical solution, the specific steps of S1 are as follows:
[0019] The pretreated crude product and the composite solvent are uniformly mixed at 50-60℃ in a mass ratio of (1-2):1. The mixture is then cooled to 25℃ in a reaction vessel at a rate of 2℃ / min. After filtration to remove impurities, the filtrate is passed into a primary crystallizer and cooled to 15-20℃ at a rate of 0.5℃ / min. During the cooling process, the mixture is continuously stirred at a speed of 150-200 rpm to obtain a mother liquor of pyromellitic dianhydride containing crystal nuclei. The composite solvent is prepared by mixing a polar solvent and a non-polar solvent in a mass ratio of (3-5):1.
[0020] As a preferred embodiment of the above technical solution, the specific steps of S3 are as follows:
[0021] The mother liquor containing the crystal nuclei of pyromellitic dianhydride is introduced into a secondary crystallizer and kept at a constant temperature of 5-10℃ for 2-3 hours to grow crystals. During the crystal growth process, the mother liquor containing the crystal nuclei of pyromellitic dianhydride is continuously introduced to ensure that the pyromellitic dianhydride crystals precipitated in the secondary crystallizer account for 25%-30% of the total mass of the material in the secondary crystallizer.
[0022] As a preferred embodiment of the above technical solution, the specific steps of S5 are as follows:
[0023] The crude pyromellitic dianhydride crystals were washed with the composite solvent, wherein the mass ratio of the crude pyromellitic dianhydride crystals to the composite solvent was 1:(3-5), and the washing time was 10-20 min. After washing, the composite solvent was recovered by vacuum distillation to obtain high-purity pyromellitic dianhydride crystals.
[0024] As a preferred embodiment of the above technical solution, the polar solvent is at least one of N,N-dimethylformamide and dimethyl sulfoxide.
[0025] As a preferred embodiment of the above technical solution, the non-polar solvent is at least one of toluene and cyclohexane.
[0026] As a preferred embodiment of the above technical solution, the pore size of the ceramic membrane is 0.5-1μm.
[0027] As a preferred embodiment of the above technical solution, the activated carbon adsorption column is filled with granular activated carbon with a particle size of 0.5-1.0 mm.
[0028] As a preferred embodiment of the above technical solution, the activated carbon adsorption column is filled with granular activated carbon with a particle size of 0.8 mm.
[0029] Compared with the prior art, the present invention has the following beneficial effects:
[0030] 1. This invention involves uniformly mixing the pretreated crude product with a composite solvent at 50-60℃, allowing both pyromellitic dianhydride and impurities to dissolve in the composite solvent. This is followed by rapid cooling at 2℃ / min. Utilizing the characteristic of low pyromellitic dianhydride content and high impurity content, even if the solubility temperature coefficients of impurities and pyromellitic dianhydride are similar, within a certain temperature range, the amount of impurity crystals precipitated will inevitably be much higher than that of pyromellitic dianhydride crystals. Furthermore, the lower the pyromellitic dianhydride content, the better the crystallization and impurity removal effect, thus achieving the technical effect of low-cost enrichment of pyromellitic dianhydride. Filtration then removes most of the crystallized impurity crystals and a very small portion of the precipitated pyromellitic dianhydride crystals. Simultaneously, because the impurity crystals do not have time for regular growth and agglomerate, further promoting impurity crystal agglomeration and precipitation, the impurity removal effect is improved.
[0031] After impurity removal, the concentration of pyromellitic dianhydride in the filtrate increases. At this point, the solution is introduced into a primary crystallizer and cooled to 15-20℃ at a rate of 0.5℃ / min. During the cooling process, the solution is continuously stirred. 15-20℃ is the inflection point range of the solubility of pyromellitic dianhydride in the composite solvent. At this temperature, the solubility of pyromellitic dianhydride decreases rapidly with decreasing temperature. Crystal nuclei can be gradually precipitated by slow cooling. Continuous stirring during the cooling process can break up the agglomeration of crystal nuclei, ensuring that the concentration of impurities around each crystal nucleus is uniform and avoiding a decrease in crystal purity caused by local impurity enrichment.
[0032] 2. This invention controls the amount of pyromellitic dianhydride crystals precipitated in the secondary crystallizer to be 25%-30% of the total mass of the material in the secondary crystallizer, which can effectively improve the purity of pyromellitic dianhydride crystals. If the concentration is too high, the crystals will squeeze and break each other, which will increase the adsorption of impurities. If the concentration is too low, the crystal growth rate will be slow and the crystal yield per unit volume of liquid will be low, which will reduce production efficiency.
[0033] In addition, crystal purity is positively correlated with particle size. Small crystals have a large specific surface area and more impurities adsorbed on the surface, making subsequent washing more difficult. On the other hand, constant temperature crystal growth at 5-10℃ can extend the time, allowing small crystal nuclei to gradually grow into large crystals, while making the internal structure of the crystal more compact and reducing the amount of impurities remaining in the pores.
[0034] 3. After the pyromellitic dianhydride crystals precipitated in the secondary crystallizer are filtered out, a small amount of composite solvent is still attached to the surface. The solvent contains unprecipitated impurities. If they are directly sent to the washing process, the solvent will carry the impurities with it. The -5℃ low temperature can reduce the solubility of impurities in the solvent and prevent impurities from being re-adsorbed onto the crystal surface. The vacuum can accelerate the evaporation of the solvent on the crystal surface and at the same time desorb the solvent in the crystal pores. Detailed Implementation
[0035] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention.
[0036] Example 1
[0037] A method for purifying low-content crude pyromellitic dianhydride, the specific steps of which are as follows:
[0038] S1, Raw material pretreatment
[0039] Crude pyromellitic dianhydride was selected, containing 10% pyromellitic dianhydride by mass. The crude pyromellitic dianhydride was first filtered through a ceramic membrane with a pore size of 0.5 μm at an operating pressure of 0.3 MPa and a surface flow rate of 3 m / s. After filtration, it was treated with an activated carbon adsorption column packed with 0.5 mm granular activated carbon at a column space velocity of 1 h⁻¹. -1 The column temperature was 40℃, and the cyclic enrichment was carried out until the mass ratio of pyromellitic dianhydride increased to 25%, and the pretreated crude product was obtained.
[0040] S2, Primary Crystallization
[0041] A composite solvent was prepared by mixing N,N-dimethylformamide and toluene at a mass ratio of 3:1.
[0042] The pretreated crude product and the composite solvent were mixed uniformly at 50°C at a mass ratio of 1:1. The mixture was then cooled to 25°C at a rate of 2°C / min in a reaction vessel. After filtration to remove impurities, the filtrate was passed into a primary crystallizer and cooled to 15°C at a rate of 0.5°C / min. During the cooling process, the mixture was continuously stirred at a speed of 150 rpm to obtain a mother liquor containing pyromellitic dianhydride crystal nuclei.
[0043] S3, secondary crystallization
[0044] The mother liquor containing pyromellitic dianhydride crystal nuclei was fed into a secondary crystallizer and kept at a constant temperature of 5°C for 2 hours. During the crystallization process, the mother liquor containing pyromellitic dianhydride crystal nuclei was continuously fed in to ensure that the pyromellitic dianhydride crystals precipitated in the secondary crystallizer accounted for 25% of the total mass of the material in the secondary crystallizer.
[0045] S4, Tertiary Crystallization
[0046] After filtering out the pyromellitic dianhydride crystals precipitated in the secondary crystallizer, the crystals were vacuum treated at -5℃ to obtain crude pyromellitic dianhydride crystals.
[0047] S5. Crystal washing and solvent recovery
[0048] The crude pyromellitic dianhydride crystals were washed with a composite solvent at a mass ratio of 1:3 and a washing time of 10 minutes. After washing, the composite solvent was recovered by vacuum distillation to obtain high-purity pyromellitic dianhydride crystals.
[0049] Example 2
[0050] A method for purifying low-content crude pyromellitic dianhydride, the specific steps of which are as follows:
[0051] S1, Raw material pretreatment
[0052] Crude pyromellitic dianhydride was selected, containing 15% pyromellitic dianhydride by mass. The crude pyromellitic dianhydride was first filtered through a ceramic membrane at an operating pressure of 0.5 MPa and a surface flow rate of 5 m / s. After filtration, it was further treated using an activated carbon adsorption column at a space velocity of 2 h⁻¹. -1 The column temperature is 50℃, and the cyclic enrichment is carried out until the mass ratio of pyromellitic dianhydride is increased to 30%, and the pretreated crude product is obtained.
[0053] S2, Primary Crystallization
[0054] A composite solvent was prepared by mixing N,N-dimethylformamide and toluene at a mass ratio of 5:1.
[0055] The pretreated crude product and the composite solvent were mixed uniformly at 60°C at a mass ratio of 2:1. The mixture was then cooled to 25°C at a rate of 2°C / min in a reaction vessel. After filtration to remove impurities, the filtrate was passed into a primary crystallizer and cooled to 20°C at a rate of 0.5°C / min. During the cooling process, the mixture was continuously stirred at a speed of 200 rpm to obtain a mother liquor containing pyromellitic dianhydride crystal nuclei.
[0056] S3, secondary crystallization
[0057] The mother liquor containing pyromellitic dianhydride crystal nuclei was fed into a secondary crystallizer and kept at a constant temperature of 10°C for 3 hours. During the crystallization process, the mother liquor containing pyromellitic dianhydride crystal nuclei was continuously fed in to ensure that the pyromellitic dianhydride crystals precipitated in the secondary crystallizer accounted for 30% of the total mass of the material in the secondary crystallizer.
[0058] S4, Tertiary Crystallization
[0059] After filtering out the pyromellitic dianhydride crystals precipitated in the secondary crystallizer, the crystals were vacuum treated at 0°C to obtain crude pyromellitic dianhydride crystals.
[0060] S5. Crystal washing and solvent recovery
[0061] The crude pyromellitic dianhydride crystals were washed with a composite solvent at a mass ratio of 1:5 and a washing time of 20 minutes. After washing, the composite solvent was recovered by vacuum distillation to obtain high-purity pyromellitic dianhydride crystals.
[0062] Example 3
[0063] A method for purifying low-content crude pyromellitic dianhydride, the specific steps of which are as follows:
[0064] S1, Raw material pretreatment
[0065] Crude pyromellitic dianhydride was selected, containing 12% pyromellitic dianhydride by mass. The crude pyromellitic dianhydride was first filtered through a ceramic membrane at an operating pressure of 0.4 MPa and a surface flow rate of 4 m / s. After filtration, it was further treated using an activated carbon adsorption column at a space velocity of 1.5 h⁻¹. -1 The column temperature was 45℃, and the cyclic enrichment was carried out until the mass ratio of pyromellitic dianhydride increased to 28%, and the pretreated crude product was obtained.
[0066] S2, Primary Crystallization
[0067] A composite solvent was prepared by mixing N,N-dimethylformamide and toluene at a mass ratio of 4:1.
[0068] The pretreated crude product and the composite solvent were mixed uniformly at 55°C at a mass ratio of 1.5:1. The mixture was then cooled to 25°C at a rate of 2°C / min in a reaction vessel. After filtration to remove impurities, the filtrate was passed into a primary crystallizer and cooled to 18°C at a rate of 0.5°C / min. During the cooling process, the mixture was continuously stirred at a speed of 180 rpm to obtain a mother liquor containing pyromellitic dianhydride crystal nuclei.
[0069] S3, secondary crystallization
[0070] The mother liquor containing pyromellitic dianhydride crystal nuclei was fed into a secondary crystallizer and kept at a constant temperature of 8°C for 2.5 hours. During the crystallization process, the mother liquor containing pyromellitic dianhydride crystal nuclei was continuously fed in to ensure that the pyromellitic dianhydride crystals precipitated in the secondary crystallizer accounted for 28% of the total mass of the material in the secondary crystallizer.
[0071] S4, Tertiary Crystallization
[0072] After filtering out the pyromellitic dianhydride crystals precipitated in the secondary crystallizer, the crystals were vacuum treated at -3℃ to obtain crude pyromellitic dianhydride crystals.
[0073] S5. Crystal washing and solvent recovery
[0074] The crude pyromellitic dianhydride crystals were washed with a composite solvent at a mass ratio of 1:4 and a washing time of 15 minutes. After washing, the composite solvent was recovered by vacuum distillation to obtain high-purity pyromellitic dianhydride crystals.
[0075] Comparative Example 1
[0076] The difference between this comparative example and Example 3 is that the crude pyromellitic dianhydride containing 12% pyromellitic dianhydride does not undergo the raw material pretreatment operation S1, but directly undergoes the three-step crystallization operation S2-S4. All other methods and steps are the same.
[0077] Comparative Example 2
[0078] The difference between this comparative example and Example 3 is that the pretreated crude product obtained in S1 does not undergo the primary crystallization operation in S2, but directly undergoes the secondary crystallization operation in S3. All other methods and steps are the same.
[0079] Comparative Example 3
[0080] The difference between this comparative example and Example 3 is that in S3, the mother liquor containing pyromellitic dianhydride with crystal nuclei is introduced into a secondary crystallizer and kept at a constant temperature of 8°C for crystal growth. During the crystal growth process, the mother liquor containing pyromellitic dianhydride with crystal nuclei is no longer introduced. The crystal growth continues until the pyromellitic dianhydride crystals are completely precipitated. All other steps are the same.
[0081] Blank control group
[0082] The crude pyromellitic dianhydride containing 12% by mass was selected, and the crude pyromellitic dianhydride was purified by recrystallization with acetone using the existing solvent crystallization method.
[0083] The seven methods provided by the three examples, three comparative examples and one blank control group above were used to purify the crude pyromellitic dianhydride with low content. The various indicators of the purified pyromellitic dianhydride are shown in Table 1.
[0084] Table 1
[0085]
[0086] As shown in Table 1, when using the process of Comparative Example 1, the crude pyromellitic dianhydride containing 12% pyromellitic dianhydride is directly subjected to a three-step crystallization operation. In actual operation, due to the presence of particulate impurities and macromolecular organic matter in the crude pyromellitic dianhydride, the purification of pyromellitic dianhydride cannot be achieved. This can lead to blockage of equipment pipelines, contamination of the inner wall of the crystallizer, and in severe cases, even damage to the equipment.
[0087] Using the process of Comparative Example 2, since the enrichment and purification operation of S2 was not carried out, the impurity content was much higher than that of pyromellitic dianhydride. Directly using cooling to induce crystallization resulted in the simultaneous precipitation of a large number of low-temperature supersaturated impurity crystals and low-temperature supersaturated pyromellitic dianhydride crystals. The purity of pyromellitic dianhydride was only 65.2%, which could not meet the requirement that the purity be >99% when used as a raw material for the synthesis of polyimide materials.
[0088] Using the process of Comparative Example 3, the mother liquor containing pyromellitic dianhydride with crystal nuclei was passed into a secondary crystallizer and kept at a constant temperature of 8°C for crystal growth. During the crystal growth process, the mother liquor containing pyromellitic dianhydride with crystal nuclei was no longer passed in. The crystal growth continued until the pyromellitic dianhydride crystals were completely precipitated, which led to a longer precipitation time and an increase in the amount of impurity crystals precipitated. Ultimately, the purity of pyromellitic dianhydride was 92.5%, which also failed to meet the requirement that the purity should be >99% when used as a raw material for synthesizing polyimide materials.
[0089] Although the purity of the pyromellitic dianhydride crystals obtained using the blank control group process meets the raw material requirements for the synthesis of polyimide materials and the yield is relatively high, the final solvent recovery rate is only 9.4% because it requires repeated recrystallization with different solvents for various impurities. The raw material cost is much higher than that of this invention. At the same time, repeated recrystallization is time-consuming, and the time cost is also much higher than that of this invention.
[0090] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.
Claims
1. A method for purifying low-content crude pyromellitic dianhydride, characterized in that, The specific steps are as follows: S1, Raw material pretreatment Crude pyromellitic dianhydride is selected, containing 10%-15% by mass of pyromellitic dianhydride. The crude pyromellitic dianhydride is first filtered through a ceramic membrane at an operating pressure of 0.3-0.5 MPa and a flow rate of 3-5 m / s. After filtration, it is then treated with an activated carbon adsorption column at a space velocity of 1-2 h⁻¹. -1 The column temperature is 40-50℃, and the mixture is cyclically enriched until the mass ratio of pyromellitic dianhydride is increased to 25%-30% to obtain the pretreated crude product. S2, Primary Crystallization The pretreated crude product and the composite solvent are uniformly mixed at 50-60℃ in a mass ratio of (1-2):
1. The mixture is then cooled to 25℃ in a reaction vessel at a rate of 2℃ / min. After filtration to remove impurities, the filtrate is passed into a primary crystallizer and cooled to 15-20℃ at a rate of 0.5℃ / min. During the cooling process, the mixture is continuously stirred at a speed of 150-200 rpm to obtain a mother liquor of pyromellitic dianhydride containing crystal nuclei. The composite solvent is prepared by mixing a polar solvent and a non-polar solvent in a mass ratio of (3-5):
1. The polar solvent is N,N-dimethylformamide, and the non-polar solvent is toluene; S3, secondary crystallization The pyromellitic dianhydride mother liquor containing crystal nuclei is passed into a secondary crystallizer and kept at a constant temperature of 5-10℃ for 2-3 hours to grow crystals. During the crystal growth process, the pyromellitic dianhydride mother liquor containing crystal nuclei is continuously passed through to ensure that the pyromellitic dianhydride crystals precipitated in the secondary crystallizer account for 25%-30% of the total mass of the material in the secondary crystallizer. S4, Tertiary Crystallization After filtering out the pyromellitic dianhydride crystals precipitated in the secondary crystallizer, the crystals were vacuum-treated at -5-0℃ to obtain crude pyromellitic dianhydride crystals. S5. Crystal washing and solvent recovery The crude pyromellitic dianhydride crystals were washed with the composite solvent, and the composite solvent was recovered by vacuum distillation after washing to obtain high-purity pyromellitic dianhydride crystals.
2. The purification method for low-content crude pyromellitic dianhydride according to claim 1, characterized in that, The specific steps for S5 are as follows: The crude pyromellitic dianhydride crystals were washed with the composite solvent, wherein the mass ratio of the crude pyromellitic dianhydride crystals to the composite solvent was 1:(3-5), and the washing time was 10-20 min. After washing, the composite solvent was recovered by vacuum distillation to obtain high-purity pyromellitic dianhydride crystals.
3. A method for purifying low-content crude pyromellitic dianhydride according to claim 1 or 2, characterized in that, The ceramic membrane has a pore size of 0.5-1 μm.
4. A method for purifying low-content crude pyromellitic dianhydride according to claim 1 or 2, characterized in that, The activated carbon adsorption column is filled with granular activated carbon with a particle size of 0.5-1.0 mm.
5. The purification method for low-content crude pyromellitic dianhydride according to claim 4, characterized in that, The activated carbon adsorption column is filled with granular activated carbon with a particle size of 0.8 mm.