A method for purifying pyromellitic dianhydride

Abstract

The invention discloses a method for purifying pyromellitic dianhydride, comprising: mixing a mixed gas containing pyromellitic dianhydride and an inert gas, and then passing the mixed gas through a membrane contactor into an aqueous solution for hydrolysis reaction, followed by solid-liquid separation to obtain a pre-crystallization liquid; subjecting the pre-crystallization liquid to ultrasonic treatment, cooling crystallization, and solid-liquid separation to obtain pyromellitic acid crystals; and dehydrating the pyromellitic acid crystals to obtain once-purified pyromellitic dianhydride. The method improves the mass and heat transfer efficiency and the hydrolysis rate of pyromellitic dianhydride by passing the mixed gas through a membrane contactor for bubbling. In addition, by controlling the turbidity of the pre-crystallization liquid after ultrasonic treatment, the crystallization efficiency and crystal purity of pyromellitic acid are improved, and a small and uniform crystal product is obtained, thereby preventing the crystals from adhering to the kettle wall and being difficult to collect, and also preventing the entrainment of impurities, and further improving the dehydration efficiency. The obtained pyromellitic dianhydride has a high purity and yield.

Description

Technical Field

[0001] The invention relates to the technical field of purifying pyromellitic dianhydride, and in particular to a method for purifying pyromellitic dianhydride. Background Art

[0002] Pyromellitic dianhydride (abbreviated as anhydride) is an important raw material for the manufacture of special engineering materials polyimide. It can also be used to manufacture curing matting agents for epoxy resins and cross-linking agents for polyester resins. Pyromellitic dianhydride plays an irreplaceable role in the synthesis of polymers and is widely used in many fields such as electrochemistry and photocatalysis.

[0003] Currently, the industrial production of pyromellitic dianhydride primarily relies on gas-phase catalytic oxidation, using durene as the raw material, air as the oxidant, and supported vanadium neodymium oxide as the catalyst. This process produces pyromellitic dianhydride via a catalytic oxidation reaction in a fixed-bed reactor. This process produces low yields and numerous by-products, necessitating purification and refining of the crude product. Hydrolysis purification is a widely used purification and refining method that offers the advantage of high product purity, but also offers a relatively low yield.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The invention provides a method for purifying pyromellitic dianhydride, aiming to improve the purity and yield of pyromellitic dianhydride.

[0006] The present invention is achieved in that:

[0007] A method for purifying pyromellitic dianhydride comprises the following steps:

[0008] (1) mixing a mixed gas containing pyromellitic dianhydride and an inert gas to obtain a mixed gas stream;

[0009] (2) passing the mixed gas stream into an aqueous solution through a membrane contactor for hydrolysis reaction, followed by solid-liquid separation to obtain a pre-crystallization solution;

[0010] (3) subjecting the pre-crystallization liquid to ultrasonic treatment, cooling crystallization, and solid-liquid separation to obtain pyromellitic acid crystals;

[0011] (4) Dehydrating the pyromellitic acid crystals to obtain the primary purified pyromellitic dianhydride.

[0012] In some embodiments, the purified pyromellitic dianhydride is subjected to sublimation to obtain pure pyromellitic dianhydride.

[0013] In some embodiments, ultrasonic treatment refers to ultrasonicating the pre-crystallization solution at a temperature of 90-100° C., stopping the ultrasonication when the turbidity reaches 200-900 NTU, and performing cooling crystallization.

[0014] In some embodiments, the temperature reduction crystallization refers to cooling to 20° C. at a rate of 1 to 3° C. / min and then standing for 8 to 12 hours, and then cooling to 5 to 12° C. and standing for 2 to 4 hours.

[0015] In some embodiments, the mixed gas containing pyromellitic dianhydride is a hot gas flow generated when pyromellitic dianhydride is synthesized by gas-phase oxidation of durene as a raw material; in other embodiments, the mixed gas containing pyromellitic dianhydride is a hot gas flow obtained by heating and sublimating crude pyromellitic dianhydride.

[0016] In some embodiments, the inert gas is at least one of nitrogen, air, and argon.

[0017] In some embodiments, the mixing ratio of the inert gas to the mixed gas containing pyromellitic dianhydride is 70 vt% to 90 vt%: 10 vt% to 30 vt%.

[0018] In some embodiments, the temperature of the mixed gas stream is 130-190°C.

[0019] In some embodiments, the mixing ratio of the mixed gas flow to the aqueous solution is 1 kg: 2-6 L.

[0020] In some embodiments, the membrane pore size in the membrane contactor is 500 to 2000 nm.

[0021] In some embodiments, the hydrolysis reaction temperature is 90-100° C., the reaction pressure is 0.1-0.3 MPa, and the reaction time is 1-2 h.

[0022] In some embodiments, the ultrasonic treatment time is 0.5 to 1 h.

[0023] In some embodiments, the frequency of sonication is 20-200 kHz.

[0024] In some embodiments, the dehydration is performed under vacuum at a temperature of 220-250°C.

[0025] In some embodiments, the sublimation treatment refers to subliming the primary purified pyromellitic dianhydride at 230-260° C. and −0.09 MPa, and the vaporized pyromellitic dianhydride is captured in a receiver.

[0026] The present invention has the following beneficial effects:

[0027] The present invention improves the hydrolysis refining method by using an inert gas with a relatively low temperature to flow concurrently with a mixed gas (crude product) containing pyromellitic dianhydride, preliminarily cooling the mixed gas and increasing its pressure, and then passing the mixed gas through a membrane contactor and introducing it into an aqueous solution. Under the diversion of the membrane contactor, the mixed gas is formed into micron-sized bubbles that are blown into the aqueous solution, effectively enhancing heat exchange efficiency and improving hydrolysis efficiency, thereby improving yield.

[0028] The present invention also improves the crystallization process of pyromellitic acid to improve crystallization efficiency and crystal purity. Ultrasonic treatment of the pre-crystallization liquid can produce a cavitation effect, which is beneficial to increasing the nucleation speed and thus accelerating crystallization, and obtaining small and uniform crystal grain products, thereby helping to avoid the crystal grains sticking to the kettle wall and being difficult to collect, and also helping to avoid the entrainment of impurities, and further helping to improve the dehydration efficiency.

[0029] The purification method provided by the invention improves the efficiency and yield of a hydrolysis purification method; the obtained pyromellitic dianhydride has high purity through two-step purification; the method is simple and suitable for industrial production. DETAILED DESCRIPTION

[0030] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0031] In the description of the embodiments of this application, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0032] Reference herein to an "embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive with other embodiments.

[0033] In the embodiment of the present application, the term "or / and" is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A or / and B can represent three situations: A exists alone, A and B exist at the same time, and B exists alone.

[0034] In addition, the character “ / ” in this article generally indicates that the previous and next related objects are in an “or” relationship.

[0035] In the embodiments of the present application, "multiple" means more than two (including two). Similarly, "multiple groups" means more than two groups (including two groups), and "multi-layer" means more than two layers (including two layers), unless otherwise clearly specified and limited.

[0036] In the embodiments of the present application, “at least one” means one or more than one.

[0037] Those skilled in the art may understand the specific meanings of the above terms in the embodiments of this application based on specific circumstances.

[0038] The present invention provides a method for purifying pyromellitic dianhydride, comprising the following steps:

[0039] (1) mixing a mixed gas containing pyromellitic dianhydride and an inert gas to obtain a mixed gas stream;

[0040] (2) passing the mixed gas stream into an aqueous solution through a membrane contactor for hydrolysis reaction, followed by solid-liquid separation to obtain a pre-crystallization solution;

[0041] (3) subjecting the pre-crystallization liquid to ultrasonic treatment, cooling crystallization, and solid-liquid separation to obtain pyromellitic acid crystals;

[0042] (4) Dehydrating the pyromellitic acid crystals to obtain the primary purified pyromellitic dianhydride.

[0043] The present invention first mixes a mixed gas containing pyromellitic dianhydride at a temperature of 190° C. to 230° C. with an inert gas. On the one hand, the inert gas is used to initially cool the mixed gas to prevent overheating and boiling of the aqueous solution. On the other hand, the inert gas is used to dilute the mixed gas, making it easier to pressurize it so that it passes through a membrane contactor and promote the dissolution of the mixed gas in water.

[0044] The present invention allows a mixed airflow to pass through a membrane contactor and mix with an aqueous solution before entering a hydrolysis reactor. The membrane contactor contains a hollow fiber membrane, and the aqueous solution enters the liquid channel of the membrane contactor through an infusion pump. The mixed airflow enters the gas channel of the membrane contactor controlled by a gas mass flowmeter. In the membrane contactor, the mixed airflow is bubbled through the membrane and mixed with the aqueous solution to obtain a soda-water mixture containing a large number of micron-sized bubbles, which is then introduced into the aqueous solution in the reactor. This method can greatly improve the gas-liquid mass transfer area and mass transfer efficiency, increase the solubility of pyromellitic dianhydride in water, and thus accelerate the hydrolysis reaction. The faster the hydrolysis reaction, the higher the efficiency, and the more conducive it is to forming pyromellitic acid crystals with high purity and high yield.

[0045] In some embodiments, the purified pyromellitic dianhydride is sublimed to obtain pure pyromellitic dianhydride. The purified pyromellitic dianhydride obtained by the hydrolysis purification method can be further purified by sublimation to further improve the purity of pyromellitic dianhydride.

[0046] In some embodiments, ultrasonic treatment refers to ultrasonicating the pre-crystallization solution at a temperature of 90-100° C., stopping the ultrasonication when the turbidity reaches 200-900 NTU, and performing cooling crystallization.

[0047] The present invention creatively controls the particle size, distribution, and purity of the resulting pyromellitic acid crystals by ultrasonicating the pre-crystallization liquid and utilizing the turbidity of the liquid to control the ultrasonic endpoint. The cavitation effect generated by ultrasonicating the pre-crystallization liquid helps increase the nucleation rate, thereby accelerating crystallization and obtaining fine and uniform crystal products. This helps prevent the crystals from adhering to the reactor wall and becoming difficult to collect, and also helps prevent the entrainment of impurities, further facilitating improved dehydration and sublimation efficiency.

[0048] In the present invention, ultrasonic treatment obtains a large number of crystal nuclei, which belongs to the crystal nucleation stage, and then the growth of the crystal is achieved in the cooling crystallization stage, which belongs to the crystal growth stage. Therefore, the present invention provides cooling the pre-crystallization liquid at a lower cooling rate to make it grow slowly, avoiding the embedding of impurities caused by excessive growth. In some embodiments, cooling crystallization refers to cooling to 20°C at 1-3°C / min and then standing for 8-12h, and then cooling to 5-12°C and standing for 2-4h. Furthermore, cooling to a low temperature of 5-12°C is beneficial to further improve the yield.

[0049] In some embodiments, the mixed gas containing pyromellitic dianhydride is a hot gas flow generated when pyromellitic dianhydride is synthesized by gas-phase oxidation of durene as a raw material; in other embodiments, the mixed gas containing pyromellitic dianhydride is a hot gas flow obtained by heating and subliming crude pyromellitic dianhydride.

[0050] In some embodiments, the inert gas is at least one of nitrogen, air, and argon.

[0051] In some embodiments, the mixing ratio of the inert gas to the mixed gas containing pyromellitic dianhydride is 70% to 90% by volume: 10% to 30% by volume. The mixing ratio of the inert gas and the mixed gas containing pyromellitic dianhydride affects the mass and heat transfer efficiency. If the inert gas content is too low, the dissolution of pyromellitic dianhydride is reduced and the hydrolysis efficiency is lowered; if the inert gas content is too high, production efficiency is reduced and waste is caused.

[0052] In some embodiments, the temperature of the mixed gas stream is 130-190°C.

[0053] In some embodiments, the mixing ratio of the mixed gas flow to the aqueous solution is 1 kg: 2-6 L.

[0054] In some embodiments, the membrane pore size in the membrane contactor is 500 to 2000 nm. The membrane pore size in the membrane contactor affects the size of micron-sized bubbles formed in the mixed gas flow. A larger membrane pore size results in larger bubbles, reducing the gas-liquid contact area. A smaller membrane pore size results in a decreased gas and liquid permeability, lowering efficiency.

[0055] In some embodiments, the hydrolysis reaction temperature is 90-100° C., the reaction pressure is 0.1-0.3 MPa, and the reaction time is 1-2 h.

[0056] In some embodiments, the ultrasonic treatment time is 0.5 to 1 h.

[0057] In some embodiments, the frequency of ultrasonic treatment is 20 to 200 kHz.

[0058] In some embodiments, the dehydration is performed under vacuum at a temperature of 220-250°C.

[0059] In some embodiments, the sublimation treatment refers to sublimating the primary purified pyromellitic dianhydride at 230-260° C. and −0.09 MPa, and the vaporized pyromellitic dianhydride is captured in a receiver.

[0060] The solutions of the present invention are further described below with reference to specific embodiments.

[0061] Example 1

[0062] A method for purifying pyromellitic dianhydride comprises the following steps:

[0063] (1) Duromelene is vaporized and mixed with air and then introduced into a fixed-bed reactor containing a catalyst for reaction. The resulting mixed gas containing pyromellitic dianhydride is mixed with a nitrogen stream to obtain a mixed gas stream having a temperature of 160±2°C.

[0064] The mixing ratio of nitrogen and the mixed gas containing pyromellitic dianhydride is 80vt%:20vt%.

[0065] (2) The mixed gas flow is passed through a membrane contactor and mixed with a 60°C aqueous solution to form bubbles, and then introduced into the aqueous solution for hydrolysis reaction. Specifically, the aqueous solution enters the liquid channel of the membrane contactor through an infusion pump, and the mixed gas flow is controlled by a gas mass flowmeter to enter the gas channel of the membrane contactor. In the membrane contactor, the mixed gas flow is bubbled through the membrane and then the aqueous solution is mixed to obtain a gas-water mixture containing a large number of micron-sized bubbles; the membrane contactor is placed in an aqueous solution with a temperature of 80±10°C, so that the gas-water mixture is passed into the aqueous solution; after the mixing ratio of the mixed gas flow to the aqueous solution reaches 1kg:4L, the feeding is stopped, the temperature is raised to 90°C, and the pressure is maintained at 0.2MPa for hydrolysis reaction for 1h. After the reaction is completed, it is filtered while hot to obtain a pre-crystallization liquid.

[0066] The membrane contactor is equipped with a hollow fiber membrane with an average pore size of 1 μm, the mixed air flow pressure is 0.4 MPa, and the aqueous solution pressure is 0.5 MPa.

[0067] (3) The pre-crystallization liquid was ultrasonically treated at 90°C until the turbidity of the pre-crystallization liquid was 400-500 NTU (measured by the formazine method using a German Lovibond TB350), then the ultrasonic treatment was stopped and the liquid was cooled to 20°C at 2°C / min and allowed to stand for 10 h. The liquid was then further cooled to 10°C and allowed to stand for 2 h. The solid-liquid separation was performed to obtain pyromellitic acid crystals.

[0068] Among them, the ultrasonic frequency is 100kHz.

[0069] (4) The pyromellitic acid crystals are placed at 220° C. and −0.09 MPa in vacuum for dehydration to obtain a purified pyromellitic dianhydride.

[0070] (5) The purified pyromellitic dianhydride is placed in a sublimation tube, the temperature is set to 240°C, the vacuum degree is -0.09 MPa, and the vaporized pyromellitic dianhydride is condensed in a receiver and collected to obtain pure pyromellitic dianhydride.

[0071] Example 2

[0072] A method for purifying pyromellitic dianhydride comprises the following steps:

[0073] (1) Duromelene is vaporized and mixed with air and then enters a fixed bed reactor equipped with a catalyst for reaction. The resulting mixed gas containing pyromellitic dianhydride is mixed with air to obtain a mixed gas with a temperature of 190±2°C.

[0074] The mixing ratio of air to the mixed gas containing pyromellitic dianhydride is 70vt%:30vt%.

[0075] (2) The mixed gas flow is passed through a membrane contactor and mixed with a 60°C aqueous solution to form bubbles, and then introduced into the aqueous solution for hydrolysis reaction. Specifically, the aqueous solution enters the liquid channel of the membrane contactor through an infusion pump, and the mixed gas flow is controlled by a gas mass flowmeter to enter the gas channel of the membrane contactor. In the membrane contactor, the mixed gas flow is bubbled through the membrane and then the aqueous solution is mixed to obtain a gas-water mixture containing a large number of micron-sized bubbles; the membrane contactor is placed in an aqueous solution with a temperature of 80±10°C, so that the gas-water mixture is passed into the aqueous solution; after the mixing ratio of the mixed gas flow to the aqueous solution reaches 1kg:6L, the feeding is stopped, the temperature is raised to 90°C, and the pressure is maintained at 0.1MPa for hydrolysis reaction for 2h. After the reaction is completed, it is filtered while hot to obtain a pre-crystallization liquid.

[0076] The membrane contactor is equipped with a hollow fiber membrane with an average pore size of 0.5 μm, the mixed air flow pressure is 0.4 MPa, and the aqueous solution pressure is 0.5 MPa.

[0077] (3) The pre-crystallization liquid was ultrasonically treated at 90°C until the turbidity of the pre-crystallization liquid was 200-300 NTU. The ultrasonication was stopped and the liquid was cooled to 20°C at 2°C / min and allowed to stand for 10 h. The temperature was further cooled to 10°C and allowed to stand for 2 h. The solid-liquid separation was performed to obtain pyromellitic acid crystals.

[0078] Among them, the ultrasonic frequency is 50kHz.

[0079] (4) The pyromellitic acid crystals are placed at 240° C. and −0.09 MPa in vacuum for dehydration to obtain a purified pyromellitic dianhydride.

[0080] (5) The purified pyromellitic dianhydride is placed in a sublimation tube, the temperature is set to 260°C, the vacuum degree is -0.09 MPa, and the vaporized pyromellitic dianhydride is condensed in a receiver and collected to obtain pure pyromellitic dianhydride.

[0081] Example 3

[0082] (1) gasifying a crude solid pyromellitic dianhydride product under a nitrogen atmosphere at 300° C. to obtain a gas containing pyromellitic dianhydride and mixing it with an argon gas flow to obtain a mixed gas flow at a temperature of 130±2° C.;

[0083] The mixing ratio of the argon gas and the mixed gas containing pyromellitic dianhydride is 90vt%:10vt%.

[0084] (2) The mixed air flow is mixed with a 60°C aqueous solution through a membrane contactor to form bubbles, and then introduced into the aqueous solution for hydrolysis reaction. Specifically, the aqueous solution enters the liquid channel of the membrane contactor through an infusion pump, and the mixed air flow is controlled by a gas mass flowmeter to enter the gas channel of the membrane contactor. After the mixed air flow is bubbled through the membrane in the membrane contactor, the aqueous solution is mixed to obtain a steam-water mixture containing a large number of micron-sized bubbles; the membrane contactor is placed in an aqueous solution with a temperature of 80±10°C, so that the steam-water mixture is introduced into the aqueous solution; after the mixing ratio of the mixed air flow to the aqueous solution reaches 1kg:2L, the feeding is stopped, the temperature is raised to 90°C, and the pressure is maintained at 0.3MPa for hydrolysis reaction for 1h. After the reaction is completed, it is filtered while hot to obtain a pre-crystallization liquid.

[0085] The membrane contactor is equipped with a hollow fiber membrane with an average pore size of 2 μm, the mixed air flow pressure is 0.4 MPa, and the aqueous solution pressure is 0.5 MPa.

[0086] (3) The pre-crystallization liquid was ultrasonically treated at 90°C until the turbidity of the pre-crystallization liquid was 800-900 NTU. The ultrasonication was stopped and the liquid was cooled to 20°C at 2°C / min and allowed to stand for 10 h. The liquid was then further cooled to 10°C and allowed to stand for 2 h. The solid-liquid separation was performed to obtain pyromellitic acid crystals.

[0087] Among them, the ultrasonic frequency is 200kHz.

[0088] (4) The pyromellitic acid crystals are placed at 250° C. and −0.09 MPa in vacuum for dehydration to obtain a purified pyromellitic dianhydride.

[0089] (5) The purified pyromellitic dianhydride is placed in a sublimation tube, the temperature is set to 230°C, the vacuum degree is -0.09 MPa, and the vaporized pyromellitic dianhydride is condensed in a receiver and collected to obtain pure pyromellitic dianhydride.

[0090] Example 4

[0091] The difference from Example 1 is that the mixed gas containing pyromellitic dianhydride in step (1) is obtained by heating and subliming a crude solid pyromellitic dianhydride, that is, the crude solid pyromellitic dianhydride is vaporized at 300° C. in a nitrogen atmosphere to obtain a gas containing pyromellitic dianhydride.

[0092] Example 5

[0093] The difference from Example 4 is that in step (1), the mixing ratio of nitrogen and the mixed gas containing pyromellitic dianhydride is 70vt%:30vt%.

[0094] Example 6

[0095] The difference from Example 4 is that in step (1), the mixing ratio of nitrogen and the mixed gas containing pyromellitic dianhydride is 90vt%:10vt%.

[0096] Example 7

[0097] The difference from Example 4 is that the membrane contactor in step (2) is equipped with a hollow fiber membrane with an average pore size of 0.5 μm.

[0098] Example 8

[0099] The difference from Example 4 is that the membrane contactor in step (2) is equipped with a hollow fiber membrane with an average pore size of 2 μm.

[0100] Example 9

[0101] The difference from Example 4 is that in step (3), the pre-crystallization liquid is ultrasonically treated at 90° C. until the turbidity of the pre-crystallization liquid is 200-300 NTU.

[0102] Example 10

[0103] The difference from Example 4 is that in step (3), the pre-crystallization liquid is ultrasonically treated at 90° C. until the turbidity of the pre-crystallization liquid is 800-900 NTU.

[0104] Example 11

[0105] The difference from Example 4 is that the ultrasonic frequency in step (3) is 50 kHz.

[0106] Example 12

[0107] The difference from Example 4 is that the ultrasonic frequency in step (3) is 200 kHz.

[0108] Comparative Example 1

[0109] The difference from Example 4 is that step (1) is omitted; in step (2), the crude solid pyromellitic dianhydride is directly added to an aqueous solution at 80±10°C, and then heated and pressurized to carry out a hydrolysis reaction, the feed ratio of the crude solid pyromellitic dianhydride to the aqueous solution is 1kg:4Lg, and other conditions remain unchanged; steps (3) to (5) remain unchanged.

[0110] Comparative Example 2

[0111] The difference from Example 4 is that step (2) does not use a membrane contactor, that is, the mixed gas flow is introduced into an aqueous solution at a temperature of 80±10° C. at a rate of 1 L / min.

[0112] Comparative Example 3

[0113] The difference from Example 4 is that the gas containing pyromellitic dianhydride is directly introduced into the membrane contactor without being mixed with the inert gas, and the gas flow pressure is 0.4 MPa.

[0114] Comparative Example 4

[0115] The difference from Example 4 is that in step (3), ultrasonic treatment is not performed, that is, the pre-crystallization liquid is cooled to 20°C at 2°C / min and allowed to stand for 10 hours, and then further cooled to 10°C and allowed to stand for 2 hours, and solid-liquid separation is performed to obtain pyromellitic acid crystals.

[0116] Comparative Example 5

[0117] The difference from Example 4 is that in step (3), the pre-crystallization liquid is ultrasonically treated at 90° C. until the turbidity of the pre-crystallization liquid is 1500 NTU.

[0118] Test Case

[0119] The purity and yield (if any) of the pyromellitic acid crystals and pure pyromellitic dianhydride prepared in Test Examples 1 to 12 and Comparative Examples 1 to 5, as well as the particle size and distribution (if any) of the pyromellitic acid crystals were statistically analyzed and shown in Table 1.

[0120] Table 1

[0121]

[0122] The test method is as follows:

[0123] (1) Purity: The purity of pure pyromellitic dianhydride, crude pyromellitic dianhydride, and pyromellitic acid crystals was determined by high performance liquid chromatography (HPLC) under the following conditions: column temperature 30°C, flow rate 0.5 mL / min, and a mobile phase of methanol for pyromellitic dianhydride and methanol:0.1 wt% potassium dihydrogen phosphate (5:1) (v / v) for pyromellitic acid. Purity was calculated based on the normalized area of ​​the target peak in the spectrum.

[0124] (2) Yield:

[0125] ① in, are the yield and purity of crude pyromellitic dianhydride respectively; are the mass and purity of pyromellitic acid crystals; M PMDA 、M PMA are the relative molecular masses of pyromellitic dianhydride and pyromellitic acid, respectively.

[0126] ② in, are the yield and purity of pure pyromellitic dianhydride respectively; are the mass and purity of pyromellitic acid crystals; M PMDA 、M PMA are the relative molecular masses of pyromellitic dianhydride and pyromellitic acid, respectively.

[0127] (3) Particle size and its distribution: The particle size of pyromellitic acid crystals was tested in accordance with GB / T 19077.1. The average particle size refers to the number average particle size of the pyromellitic acid crystals. The particle size widening coefficient (Span) = (Dv90-Dv10) / Dv50, where Dv90, Dv50, and Dv10 are the particle sizes corresponding to the cumulative volume percentages reaching 90%, 50%, and 10%, respectively. The particle size widening coefficient can be used to evaluate the particle size distribution width of the pyromellitic acid crystals.

[0128] As can be seen from the data in Table 1, the method provided by the present invention can obtain pyromellitic acid crystals with high purity and yield, and the pyromellitic dianhydride obtained by further dehydration also has high purity and yield.

[0129] As can be seen from Examples 4 to 6, the volume ratio of the inert gas to the mixed gas containing pyromellitic dianhydride has an impact on the purity and yield of the resulting pyromellitic acid crystals, but has no significant effect on the particle size and distribution of the pyromellitic acid crystals. A greater proportion of the inert gas in the mixed gas results in higher gas-liquid heat and mass transfer efficiency, higher hydrolysis efficiency of the pyromellitic dianhydride in the mixed gas, and therefore higher purity and yield of the pyromellitic acid crystals.

[0130] As can be seen from Examples 4, 7, and 8, the average pore size of the membrane contactor has an impact on the purity and yield of the obtained pyromellitic acid crystals, but has no impact on the particle size and distribution of the pyromellitic acid crystals. Specifically, the purity of the pyromellitic acid crystals first increases and then decreases with the average pore size of the membrane, and the yield decreases with the increase. The reason is that when the pore size is too large, the bubble volume obtained is too large, the contact area with the liquid phase is reduced, and the mass transfer and heat transfer efficiency are reduced, thereby reducing the hydrolysis rate; when the pore size is too small, pyromellitic dianhydride is prone to desublimation and precipitation, blocking the membrane pores, thereby reducing efficiency, reducing the hydrolysis rate, and reducing the yield. The lower the hydrolysis efficiency of pyromellitic dianhydride, the lower the purity of the pyromellitic acid crystals obtained after crystallization.

[0131] As can be seen from Examples 4, 9, and 10, the turbidity of the liquid before the crystallization after ultrasound control during the ultrasonic treatment can be controlled to the purity, particle size, and distribution of the pyromellitic acid crystals obtained. Turbidity reflects the degree of obstruction that occurs when suspended matter in water is transparent to light. In the present invention, the liquid before the crystallization after ultrasound contains a large amount of tiny nuclei, which, as suspended particles, makes the liquid before the crystallization gradually turbid, and its content and particle size have an impact on turbidity. The present invention utilizes the cavitation effect produced by ultrasound to improve the nucleation speed of the liquid before the crystallization, so that it forms a large amount of nuclei in a relatively short period of time, and along with the extension of the ultrasonic time and the control of the ultrasonic frequency, the liquid before the crystallization after ultrasound can have different turbidities; further, the crystal growth stage of subsequent cooling is carried out by the liquid before the crystallization after the ultrasonic treatment, so that the crystallization after the growth has different particle sizes and distribution. Specifically, before the ultrasonic crystallization, the turbidity of liquid is larger, shows that it contains the larger crystalline particles of quantity or / and particle size, these crystalline particles are owing to being grown under ultrasonic, so particle size is comparatively uniform, and along with the increase of turbidity, the saturation degree of pyromellitic acid in liquid phase declines, therefore the crystal growth stage of follow-up cooling has less impact on the morphology of crystal, therefore the pyromellitic acid crystals that embodiment 10 obtains are distributed narrower, and particle size is smaller; Before the ultrasonic crystallization of embodiment 9, the turbidity of liquid is lower, shows that the amount that pyromellitic acid separates out in the follow-up cooling stage is more, and the stability of cooling stage is low compared to ultrasonic process, so the pyromellitic acid crystals that obtain are distributed wider, and average particle size is larger. In addition, before the ultrasonic crystallization, the turbidity of liquid is higher, and the purity of the pyromellitic acid crystals obtained is also higher, shows that ultrasonic is conducive to reducing impurity and enters crystal lattice, reduces the defect of crystal.

[0132] As can be seen from Examples 4, 11, and 12, the ultrasonic frequency has an impact on the purity, yield, particle size, and distribution of pyromellitic acid crystals. The higher the ultrasonic frequency, the higher the purity, the higher the yield, the smaller the particle size, and the narrower the distribution. When obtaining a pre-crystallization solution after ultrasound with the same turbidity, if a higher ultrasonic frequency is used, the amount of crystals obtained is large but the particle size is small; if a lower ultrasonic frequency is used, the amount of crystals obtained is small but the particle size is large. Therefore, the higher the ultrasonic frequency, the more crystal nuclei there are, and the particle size of the pyromellitic acid crystals finally obtained is smaller; in addition, since crystallization is a process of continuous dissolution-precipitation, when the number of crystal nuclei is large, the more precipitation sites there are, the more conducive it is to improving the yield. Generally, the lower the particle size of the pyromellitic acid crystals, the lower the impurity content. The reason is that the impurity structure is similar to that of pyromellitic acid and is co-crystallized and embedded in the target product lattice in the form of a eutectic. The small grain size can reduce the probability of impurity embedding.

[0133] Compared to Comparative Example 1, Example 4 has a gas-liquid mixing and uses a membrane contactor, so the hydrolysis rate of phthalic anhydride is faster than the solid-liquid mixing of Comparative Example 1, so the purity and yield of the phthalic acid obtained are both higher. The membrane contactor can greatly increase the mass transfer area of ​​gas and liquid and improve the hydrolysis efficiency. Therefore, Comparative Example 2 does not use a membrane contactor to obtain a lower purity and yield of pyromellitic acid; Comparative Example 3 is not mixed with an inert gas before entering the membrane contactor relative to Example 1, so that the concentration of phthalic anhydride is too high when mixed with the membrane contactor, which is easy to cause membrane clogging and reduces mass transfer efficiency. Both of these lead to a decrease in hydrolysis efficiency, and the purity and yield of the phthalic acid crystals obtained are both lower. Comparative Example 4 is not ultrasonically treated, so the yield is lower, the crystal particle size is larger, and the impurity content is high. In Comparative Example 5, the turbidity of the pre-crystallization liquid after ultrasound was too high, resulting in a decrease in yield and purity and a wider particle size. The reason is that when the ultrasound time is too long, the turbidity of the pre-crystallization liquid after ultrasound is too high, that is, ultrasound treatment is continuously performed during the grain growth stage, and ultrasound accelerates the movement of molecules, thereby increasing the probability of grain collision and causing agglomeration.

[0134] The yields of pyromellitic dianhydride reported in Table 1 are calculated based on pyromellitic acid crystals as the raw material. Therefore, in this embodiment, the smaller the average particle size of the pyromellitic acid crystals, the higher the purity, and the narrower the distribution, the higher the purity and yield of the resulting pyromellitic dianhydride. This is because the smaller the average particle size of the pyromellitic acid crystals and the narrower the distribution, the higher the sublimation efficiency, which is more conducive to impurity separation.

[0135] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for purifying pyromellitic dianhydride, characterized in that: The following steps are involved: (1) mixing a mixed gas containing pyromellitic dianhydride with other gases to obtain a mixed gas flow; the other gases are at least one of nitrogen, air, and argon; (2) The mixed gas flow is mixed with the aqueous solution through a membrane contactor and then enters a hydrolysis reactor for hydrolysis reaction, followed by solid-liquid separation to obtain a pre-crystallization liquid; the mixing ratio of the mixed gas flow to the aqueous solution is 1 kg: 2-6 L; the membrane pore size in the membrane contactor is 500-2000 nm; the temperature of the hydrolysis reaction is 90-100° C., the reaction pressure is 0.1-0.3 MPa, and the reaction time is 1-2 h; (3) subjecting the pre-crystallization liquid to ultrasonic treatment, cooling crystallization, and solid-liquid separation to obtain pyromellitic acid crystals; (4) dehydrating the pyromellitic acid crystals to obtain primary purified pyromellitic dianhydride; The ultrasonic treatment refers to ultrasonicating the pre-crystallization solution at a temperature of 90-100° C., and stopping the ultrasonication when the turbidity reaches 200-900 NTU.

2. The method for purification of pyromellitic dianhydride according to claim 1, wherein The once purified pyromellitic dianhydride is sublimed to obtain pure pyromellitic dianhydride.

3. The method for purification of pyromellitic dianhydride according to claim 1 or 2, wherein The cooling crystallization refers to cooling to 20°C at a rate of 1-3°C / min and then standing for 8-12 hours, and then cooling to 5-12°C and standing for 2-4 hours.

4. The method for purification of pyromellitic dianhydride according to claim 1, wherein The mixed gas containing pyromellitic dianhydride is a hot gas stream generated when pyromellitic dianhydride is synthesized by gas-phase oxidation of durene as a raw material, or the mixed gas containing pyromellitic dianhydride is a hot gas stream obtained by heating and subliming crude pyromellitic dianhydride.

5. The method for purification of the arbitrary pyromellitic dianhydride according to claim 1, wherein The mixing ratio of the other gases to the mixed gas containing pyromellitic dianhydride is 70vt%~90vt%:10vt%~30vt%; And / or, the temperature of the mixed airflow is 130-190°C.

6. The method for purifying the arbitrary pyromellitic dianhydride according to claim 1, wherein The ultrasonic treatment time is 0.5~1h; And / or, the frequency of the ultrasonic treatment is 20-200 kHz.

7. The method for purifying the arbitrary pyromellitic dianhydride according to claim 1, wherein The dehydration is carried out under vacuum at a temperature of 220-250°C.

8. The method for purification of pyromellitic dianhydride according to claim 2, wherein The sublimation treatment refers to sublimating the primary purified pyromellitic dianhydride at 230-260° C. and −0.09 MPa, and the vaporized pyromellitic dianhydride is captured in a receiver.

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