Refining method of pyromellitic dianhydride

Abstract

The invention discloses a refining method of pyromellitic dianhydride, which comprises the following steps: adding a good solvent into a pre-crystallization solution obtained by hydrolysis reaction of a pyromellitic dianhydride crude product to obtain a pyromellitic acid crystal; carrying out flash evaporation dehydration on the pyromellitic acid crystal to obtain primarily purified pyromellitic dianhydride; and carrying out sublimation treatment on the primarily purified pyromellitic dianhydride to obtain a pure pyromellitic dianhydride product. According to the invention, a hydrolysis refining method and a sublimation purification method are combined to purify the pyromellitic dianhydride crude product, and the hydrolysis refining method is improved, so that the purity and yield of the pyromellitic acid crystal are improved, and the pyromellitic acid crystal with fine and uniform particle size is obtained, thereby improving the purity and yield of the pyromellitic dianhydride; furthermore, the pyromellitic acid crystals are dehydrated by adopting a flash evaporator, so that the dispersity of the pyromellitic dianhydride purified once is improved, the sublimation treatment efficiency is improved, and the purity and yield of the pyromellitic dianhydride are further improved.

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] PMDA is an important chemical raw material used in the production of plasticizers, curing agents, coatings, films, etc. The products produced are widely used in aerospace, electronics, military, automobile, industrial building materials and other fields, and have good development prospects.

[0003] At present, the gas phase catalytic oxidation method is mainly used in industry to prepare pyromellitic anhydride, which is mostly prepared by catalytic oxidation reaction in a fixed bed reactor with durene as raw material, air as oxidant, and supported vanadium neodymium oxide as catalyst. The process has low yield and many by-products, so it is necessary to purify and refine the crude product. Hydrolysis refining method is a widely used purification and refining method, which specifically dissolves the crude product of pyromellitic anhydride in water, heats for hydrolysis, filters, and cools and crystallizes to obtain pyromellitic acid, which is then vacuum dehydrated to obtain the finished product of pyromellitic anhydride, but this method is difficult to take into account both high purity and high efficiency.

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

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

[0006] The present invention is achieved in that:

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

[0008] (1) introducing a crude product of pyromellitic dianhydride into a hydrolysis reactor to carry out a hydrolysis reaction, and obtaining a pre-crystallization liquid after solid-liquid separation;

[0009] (2) adding a good solvent to the pre-crystallization liquid, cooling the liquid for crystallization, and performing solid-liquid separation to obtain pyromellitic acid crystals;

[0010] (3) flash-evaporating and dehydrating the pyromellitic acid crystals to obtain primary purified pyromellitic dianhydride;

[0011] (4) subliming the once purified pyromellitic dianhydride to obtain a pure product of pyromellitic dianhydride.

[0012] In some embodiments, the crude product of pyromellitic anhydride is a gas containing pyromellitic anhydride; in some embodiments, the gas containing pyromellitic anhydride is a hot gas flow generated when pyromellitic anhydride is synthesized by gas-phase oxidation of tetramethylbenzene as a raw material; in other embodiments, the gas containing pyromellitic anhydride is a hot gas flow obtained after the crude product of pyromellitic anhydride is heated and sublimated.

[0013] In some embodiments, the crude product of pyromellitic dianhydride is mixed with atomized droplets and then introduced into the hydrolysis reactor; the atomized droplets refer to droplets obtained after water is atomized.

[0014] In some embodiments, the particle size of the atomized droplets is 5 to 50 μm.

[0015] In some embodiments, the flow rate of the atomized droplets is 0.5 to 0.9 L / min.

[0016] In some embodiments, the temperature of the atomized droplets is 40-60°C.

[0017] In some embodiments, the flow rate of the gas containing pyromellitic dianhydride is 100 to 200 g / min;

[0018] In some embodiments, the temperature of the hydrolysis reaction is 90-100° C., the pressure is 0.1-0.3 MPa, and the reaction time is 1-2 h.

[0019] In some embodiments, the good solvent is selected from at least one of acetonitrile, N,N-dimethylformamide, and dimethyl sulfoxide.

[0020] In some embodiments, the amount of good solvent added is 5% to 15% of the volume of the pre-crystallization solution.

[0021] In some embodiments, the end point temperature of the cooling crystallization is 10-20°C, and the cooling rate is controlled at 1-3°C / min.

[0022] In some embodiments, the average particle size of the pyromellitic acid crystals is 5 to 12 μm.

[0023] In some embodiments, the broadening coefficient of the pyromellitic acid crystals is 1 to 1.5, wherein the broadening coefficient = (Dv90-Dv10) / Dv50, wherein Dv90, Dv50 and Dv10 are the particle sizes corresponding to the cumulative volume percentages reaching 90%, 50% and 10%, respectively.

[0024] In some embodiments, flash dehydration is carried out in a flash evaporator with an inlet air temperature of 220-300°C, an outlet air temperature of 100-150°C, a rotation speed of 10-30 Hz, and a feed frequency of 10-30 Hz.

[0025] 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.

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

[0027] The invention combines a hydrolysis purification method and a sublimation purification method to purify a crude product of pyromellitic dianhydride, and improves the hydrolysis purification method, thereby improving the purity and yield of pyromellitic acid crystals, and obtaining pyromellitic acid crystals with fine and uniform particle sizes, thereby improving the purity and yield of pyromellitic dianhydride; further, the invention uses a flash evaporator to dehydrate the pyromellitic acid crystals, thereby improving the dispersibility of the once purified pyromellitic dianhydride, thereby improving the efficiency of the sublimation treatment, and thus improving the purity and yield of pyromellitic dianhydride.

[0028] The purification method provided by the invention improves the efficiency of the hydrolysis refining method. Through two-step purification, the obtained pyromellitic dianhydride has high purity, and the method is simple and suitable for industrial production. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0030] In the description of the embodiments of the present 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.

[0031] Reference to "embodiment" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiment may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments.

[0032] 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.

[0033] In addition, the character “ / ” in this article generally indicates that the previous and next associated objects are in an “or” relationship.

[0034] 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.

[0035] In the embodiments of the present application, "at least one" means one or more than one.

[0036] Those skilled in the art may understand the specific meanings of the above terms in the embodiments of the present application according to specific circumstances.

[0037] The present invention provides a method for refining pyromellitic dianhydride, comprising the following steps:

[0038] (1) introducing a crude product of pyromellitic dianhydride into a hydrolysis reactor to carry out a hydrolysis reaction, and obtaining a pre-crystallization liquid after solid-liquid separation;

[0039] (2) adding a good solvent to the pre-crystallization liquid, cooling the liquid for crystallization, and performing solid-liquid separation to obtain pyromellitic acid crystals;

[0040] (3) flash-evaporating and dehydrating the pyromellitic acid crystals to obtain primary purified pyromellitic dianhydride;

[0041] (4) subliming the once purified pyromellitic dianhydride to obtain a pure product of pyromellitic dianhydride.

[0042] The invention combines a hydrolysis purification method and a sublimation purification method to purify a crude product of pyromellitic dianhydride, and improves the hydrolysis purification method, thereby improving the purity and yield of pyromellitic acid crystals, and obtaining pyromellitic acid crystals with fine and uniform particle sizes, thereby improving the purity and yield of pyromellitic dianhydride; further, the invention uses a flash evaporator to dehydrate the pyromellitic acid crystals, thereby improving the dispersibility of the once purified pyromellitic dianhydride, thereby improving the efficiency of the sublimation treatment, and thus improving the purity and yield of pyromellitic dianhydride.

[0043] Specifically, the present invention adds a good solvent to the pre-crystallization liquid obtained by hydrolysis, which is conducive to slowing down the crystal growth of pyromellitic acid, promoting its growth into a crystal with high crystallinity and small defects, so that the impurities carried away are few, and the crystal particle size obtained is small and uniform; the crystal particle size is small and uniform, which is conducive to improving the dispersibility of the once-purified pyromellitic acid anhydride obtained by dehydration, improving its contact area with the hot air flow during biochemical treatment, improving its sublimation efficiency, thereby improving the purity and yield of pyromellitic acid anhydride. The flash evaporator is used to dehydrate the pyromellitic acid crystals, further improving the dispersibility of the once-purified pyromellitic acid anhydride, and avoiding crystal agglomeration. In addition, the present invention uses a flash evaporation dryer to dehydrate pyromellitic acid, improves the dehydration speed, and improves the efficiency of the entire process.

[0044] In some embodiments, the crude product of pyromellitic anhydride is a gas containing pyromellitic anhydride; in some embodiments, the gas containing pyromellitic anhydride is a hot gas flow generated when pyromellitic anhydride is synthesized by gas-phase oxidation of tetramethylbenzene as a raw material; in other embodiments, the gas containing pyromellitic anhydride is a hot gas flow obtained after the crude product of pyromellitic anhydride is heated and sublimated.

[0045] In some embodiments, the flow rate of the gas containing pyromellitic dianhydride is 100 to 200 g / min.

[0046] The crude product of pyromellitic dianhydride is mixed with the aqueous solution in the form of gas, which is beneficial to improving the mass transfer area and mass transfer efficiency with the aqueous solution, and at the same time improving its dissolution and hydrolysis degree, thereby improving the yield of pyromellitic acid.

[0047] In some embodiments, the crude product of pyromellitic dianhydride is mixed with atomized droplets and then introduced into the hydrolysis reactor; the atomized droplets refer to droplets obtained after water is atomized.

[0048] In some embodiments, the particle size of the atomized droplets is 5 to 50 μm.

[0049] In some embodiments, the flow rate of the atomized droplets is 0.5 to 0.9 L / min.

[0050] In some embodiments, the temperature of the atomized droplets is 40-60°C.

[0051] Furthermore, the vaporized crude pyromellitic anhydride product is first mixed with the atomized liquid droplets and then introduced into the aqueous solution. On the one hand, the vaporized crude pyromellitic anhydride product can be initially cooled to avoid directly entering the hydrolysis reactor to cause boiling; on the other hand, the mass transfer area and mass transfer efficiency of the gas-liquid two phases can be increased to promote the dissolution and hydrolysis of pyromellitic anhydride.

[0052] In some embodiments, the temperature of the hydrolysis reaction is 90-100° C., the pressure is 0.1-0.3 MPa, and the reaction time is 1-2 h.

[0053] In some embodiments, the good solvent is selected from at least one of acetonitrile, N,N-dimethylformamide, and dimethyl sulfoxide.

[0054] In some embodiments, the amount of good solvent added is 5% to 15% of the volume of the pre-crystallization liquid; the amount of good solvent added should not be too much, as too much will cause the crystal growth rate of pyromellitic acid to be too low, resulting in a lower yield; if the amount of good solvent added is too little, it will be difficult to play a role in regulating the grain size and purity of pyromellitic acid.

[0055] In some embodiments, the end point of cooling crystallization is 10-20°C, and the cooling rate is controlled at 1-3°C / min; controlling the cooling rate is beneficial to slowing down the growth of pyromellitic acid crystals and avoiding too wide grain size distribution and too large particle size.

[0056] In some embodiments, the average particle size of the pyromellitic acid crystals is 5 to 12 μm.

[0057] In some embodiments, the broadening coefficient of the pyromellitic acid crystals is 1 to 1.5, wherein the broadening coefficient = (Dv90-Dv10) / Dv50, wherein Dv90, Dv50 and Dv10 are the particle sizes corresponding to the cumulative volume percentages reaching 90%, 50% and 10%, respectively.

[0058] The pyromellitic acid crystals obtained by the preparation method provided by the present invention have the characteristics of an average particle size of 5 to 12 μm and a broadening coefficient of 1 to 1.5. The pyromellitic acid crystals with this morphology have a higher sublimation rate during the sublimation treatment to obtain the once purified pyromellitic acid dianhydride, thereby improving the yield of the pure pyromellitic acid dianhydride.

[0059] In some embodiments, flash dehydration is carried out in a flash evaporator with an inlet air temperature of 220-300°C, an outlet air temperature of 100-150°C, a rotation speed of 10-30 Hz, and a feed frequency of 10-30 Hz.

[0060] 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.

[0061] Example 1

[0062] (1) After being vaporized, durene is mixed with air and introduced into a fixed bed reactor equipped with a catalyst for reaction. The gas containing durene tetracarboxylic anhydride obtained by the reaction and the atomized droplets are introduced from both ends of a condenser for convection mixing. The negative pressure generated by a fan transports the mixed flow to a hydrolysis reactor and introduces it from the bottom. The hydrolysis reactor contains a 60° C. aqueous solution as a bottom liquid.

[0063] The atomized droplets are obtained by pressurizing and atomizing deionized water through an atomizing gun, the temperature is 50°C±5°C, and the particle size of the atomized droplets is 5 to 50 μm.

[0064] The flow rate of the atomized liquid droplets was 0.7 L / min, and the flow rate of the gas containing pyromellitic dianhydride was 150 g / min.

[0065] (2) Control the temperature in the hydrolysis reactor to 90±2°C and the pressure to 0.2 MPa, stir the reaction for 1 hour, and filter while hot to obtain the pre-crystallization liquid.

[0066] (3) Add acetonitrile to the pre-crystallization solution, and gradually cool it to 10°C at a rate of 3°C / min, let it stand for 12 hours, filter it, and dry it at 80°C to obtain pyromellitic acid crystals.

[0067] The amount of acetonitrile added was 10% of the volume of the pre-crystallization solution.

[0068] (4) sending the pyromellitic acid crystals into a flash dryer for drying to obtain primary purified pyromellitic acid dianhydride.

[0069] Among them, the inlet air temperature is 300℃, the outlet air temperature is 150℃, the rotation speed is 20Hz, and the feeding frequency is 20Hz. Under this condition, it takes 5 to 10 minutes from feeding to discharging.

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

[0071] Example 2

[0072] (1) The difference from Example 1 is that the flow rate of the atomized droplets is 0.9 L / min, and the flow rate of the gas containing pyromellitic dianhydride is 100 g / min.

[0073] (2) Control the temperature in the hydrolysis reactor to 98±2°C and the pressure to 0.3 MPa, stir the reaction for 1 hour, and filter while hot to obtain the pre-crystallization liquid.

[0074] (3) Dimethyl sulfoxide was added to the pre-crystallization solution, and the temperature was gradually lowered to 20°C at a rate of 2°C / min, and the solution was allowed to stand for 12 hours. After filter pressing, the solution was dried at 80°C to obtain pyromellitic acid crystals.

[0075] The amount of acetonitrile added was 5% of the volume of the pre-crystallization liquid.

[0076] (4) sending the pyromellitic acid crystals into a flash dryer for drying to obtain primary purified pyromellitic acid dianhydride.

[0077] Among them, the inlet air temperature is 350℃, the outlet air temperature is 150℃, the rotation speed is 10Hz, and the feeding frequency is 10Hz. Under this condition, it takes 5 to 10 minutes from feeding to discharging.

[0078] (5) The purified pyromellitic anhydride is placed in a sublimation tube, the temperature is set to 260° C., the vacuum degree is -0.09 MPa, and the vaporized pyromellitic anhydride is condensed in a receiver and collected to obtain pure pyromellitic anhydride.

[0079] Example 3

[0080] (1) A crude solid pyromellitic anhydride is vaporized under an inert gas atmosphere (nitrogen) at 300° C., and the obtained gas containing pyromellitic anhydride and atomized droplets are introduced from both ends of a condenser for convection mixing. The mixed flow is transported to a hydrolysis reactor by the negative pressure generated by a fan and introduced from the bottom. The hydrolysis reactor contains a 60° C. aqueous solution as a bottom liquid.

[0081] The atomized droplets are obtained by pressurizing and atomizing deionized water through an atomizing gun, the temperature is 50°C±5°C, and the particle size of the atomized droplets is 5 to 50 μm.

[0082] The flow rate of the atomized liquid droplets was 0.7 L / min, and the flow rate of the gas containing pyromellitic dianhydride was 200 g / min.

[0083] (2) Control the temperature in the hydrolysis reactor to 90±2°C and the pressure to 0.1 MPa, stir the reaction for 2 h, and filter while hot to obtain the pre-crystallization liquid.

[0084] (3) Add N,N-dimethylformamide to the pre-crystallization solution, and gradually cool it to 10°C at a rate of 1°C / min, let it stand for 12 hours, filter it, and dry it at 80°C to obtain pyromellitic acid crystals.

[0085] The amount of acetonitrile added was 15% of the volume of the pre-crystallization liquid.

[0086] (4) sending the pyromellitic acid crystals into a flash dryer for drying to obtain primary purified pyromellitic acid dianhydride.

[0087] Among them, the inlet air temperature is 250℃, the outlet air temperature is 100℃, the rotation speed is 30Hz, and the feeding frequency is 30Hz. Under this condition, it takes 5 to 10 minutes from feeding to discharging.

[0088] (5) The purified pyromellitic anhydride is placed in a sublimation tube, the temperature is set to 230° C., the vacuum degree is -0.09 MPa, and the vaporized pyromellitic anhydride is condensed in a receiver and collected to obtain pure pyromellitic anhydride.

[0089] Example 4

[0090] The difference from Example 3 is that in step (1), the gas containing pyromellitic anhydride obtained by gasifying the solid crude pyromellitic anhydride under an inert gas atmosphere (nitrogen) at 300° C. is directly transported to the hydrolysis reactor and introduced from the bottom, and the hydrolysis reactor contains a 60° C. aqueous solution as the bottom liquid.

[0091] Example 5

[0092] The difference from Example 3 is that the flow rate of the atomized droplets in step (1) is 0.5 L / min.

[0093] Example 6

[0094] The difference from Example 3 is that the flow rate of the atomized droplets in step (1) is 0.9 L / min.

[0095] Example 7

[0096] The difference from Example 1 is that in step (3), N,N-dimethylformamide is used as the good solvent instead of acetonitrile.

[0097] Example 8

[0098] The difference from Example 1 is that in step (3), dimethyl sulfoxide is used instead of acetonitrile as the good solvent.

[0099] Example 9

[0100] The difference from Example 1 is that the amount of acetonitrile added in step (3) is 5% of the volume of the pre-crystallization liquid.

[0101] Example 10

[0102] The difference from Example 1 is that the amount of acetonitrile added in step (3) is 15% of the volume of the pre-crystallization liquid.

[0103] Comparative Example 1

[0104] The difference from Example 1 is that acetonitrile is not added in step (3).

[0105] Comparative Example 2

[0106] The difference from Example 1 is that step (4) uses a conventional vacuum method to dehydrate the pyromellitic acid crystals. Specifically, the pyromellitic acid crystals are placed in an oven at 220° C. and a vacuum degree of -0.09 MPa for dehydration for 12 hours.

[0107] Comparative Example 3

[0108] The difference from Example 1 is that acetonitrile is not added in step (3), and step (4) uses a conventional vacuum method to dehydrate the pyromellitic acid crystals. Specifically, the pyromellitic acid crystals are placed in an oven at 220° C. and a vacuum degree of -0.09 MPa for dehydration for 12 hours.

[0109] Comparative Example 4

[0110] The difference from Example 1 is that the solid crude pyromellitic dianhydride is directly put into the hydrolysis reactor for hydrolysis reaction, and the feed ratio of the solid crude pyromellitic dianhydride to the aqueous solution is 1g:6g.

[0111] Comparative Example 5

[0112] The conventional method for refining pyromellitic dianhydride comprises the following steps:

[0113] (1) The solid crude product of pyromellitic anhydride is directly put into a hydrolysis reactor for hydrolysis reaction, and filtered while hot to obtain a pre-crystallization liquid.

[0114] The feed ratio of the solid crude pyromellitic anhydride to the aqueous solution is 1g:6g, the flow rate of the gas containing pyromellitic anhydride is 150g / min, the temperature in the hydrolysis reactor is controlled to be 90±2°C, the pressure is 0.2MPa, and the reaction is stirred for 1h.

[0115] (2) The pre-crystallization liquid was gradually cooled to 10°C at a rate of 3°C / min, allowed to stand for 12 hours, filtered, and dried at 80°C to obtain pyromellitic acid crystals.

[0116] (3) The pyromellitic acid crystals are placed in an oven at 220° C. and a vacuum degree of −0.09 MPa for dehydration for 12 h to obtain primary purified pyromellitic acid dianhydride.

[0117] (4) The purified pyromellitic anhydride is placed in a sublimation tube, the temperature is set to 240° C., the vacuum degree is -0.09 MPa, and the vaporized pyromellitic anhydride is condensed in a receiver and collected to obtain pure pyromellitic anhydride.

[0118] The physical properties of the products of the embodiment and the comparative example were tested, and the results are shown in Tables 1 and 2. The testing method is as follows:

[0119] (1) Purity: The purity of pure pyromellitic anhydride, crude pyromellitic anhydride, and pyromellitic acid crystals were tested by high performance liquid chromatography (HPLC) under the following test conditions: column temperature 30°C, flow rate 0.5 mL / min, mobile phase methanol when the test object was pyromellitic anhydride, and mobile phase methanol: 0.1 wt% potassium dihydrogen phosphate = 5:1 (v / v) when the test object was pyromellitic acid. The purity was calculated based on the normalized area of ​​the target peak in the spectrum.

[0120] (2) Yield:

[0121] 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.

[0122] 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.

[0123] When the gas containing pyromellitic anhydride is a hot gas stream obtained by heating and sublimating the crude pyromellitic anhydride, the yield of the whole section pyromellitic anhydride can also be calculated.

[0124] (3) Particle size and its distribution: The particle size of pyromellitic acid is tested in accordance with GB / T 19077.1. The average particle size refers to the number average particle size of pyromellitic acid crystals; particle size broadening coefficient = (Dv90-Dv10) / Dv50, where Dv90, Dv50 and Dv10 are the particle sizes corresponding to the cumulative volume percentages of 90%, 50% and 10%, respectively. The particle size broadening coefficient can be used to evaluate the particle size distribution width of pyromellitic acid crystals.

[0125] Table 1

[0126]

[0127]

[0128] Table 2

[0129]

[0130] It can be seen from the data in Table 1 that the purity and yield of the pure product of pyromellitic dianhydride obtained in the embodiment of the present invention are higher than those in the comparative example.

[0131] By comparing Examples 3 to 4 with Comparative Example 4, it can be seen that the way in which pyromellitic anhydride enters the aqueous solution during hydrolysis has a great influence on the yield and purity of the obtained pyromellitic acid. Specifically, the method of Example 3 is better than the method of Example 4, and the method of Example 4 is better than the method of Comparative Example 4. After the crude pyromellitic anhydride is gasified and mixed with the aqueous solution, the mass transfer area and mass transfer efficiency with the aqueous solution can be increased, while the degree of dissolution and hydrolysis can be increased, thereby increasing the yield of pyromellitic acid. In Comparative Example 4, the crude pyromellitic anhydride is directly mixed with the aqueous solution. Due to the decrease in mass transfer efficiency, the hydrolysis efficiency of pyromellitic anhydride decreases, and the number of crystal nuclei formed is small. Therefore, the crystal size of the obtained pyromellitic acid is large and uneven, which leads to easy entrainment of impurities, so the purity and yield are low. Embodiment 3After the crude product of pyromellitic anhydride is gasified, it is first mixed with atomized droplets and then passed into the aqueous solution, which is more conducive to forming a large number of active sites (hydrolysis reaction active sites and crystallization active sites), so the yield and purity are better than the three. Further, from the perspective of embodiments 3, 5, and 6, the flow rate of the atomized droplets is regulated, and the purity and yield of the obtained pyromellitic acid crystals are affected. Although the subsequent treatment of the obtained pyromellitic acid crystals is consistent with that of the comparative example 4, there are different second-stage pyromellitic anhydride yields, indicating that the purity of the pyromellitic acid crystals and their morphology have an impact on their subsequent purification effect, specifically, the pyromellitic acid crystals with higher purity have a higher second-stage pyromellitic anhydride yield; the smaller the particle size of the pyromellitic acid crystals, the smaller the widening coefficient, and the higher the second-stage pyromellitic anhydride yield.

[0132] Examples 1, 7, and 8 use different good solvents. Comparative Example 1 does not add a good solvent during the crystallization stage of pyromellitic acid. From the results, the addition of a good solvent is beneficial to improving the purity of pyromellitic acid, reducing its particle size and particle size distribution width (broadening coefficient), thereby helping to improve the yield and purity of the second-stage pyromellitic dianhydride; the reason is that the good solvent increases the solubility of pyromellitic acid, reduces its supersaturation, thereby slowing down crystal growth, which is conducive to the growth of crystals with high crystallinity, few defects and few impurities.

[0133] Examples 1, 9 and 10 use different amounts of good solvent added. From the results, the more good solvent is added, the more beneficial it is to improve the purity of pyromellitic acid, reduce its average particle size and broadening coefficient, and thus help to improve the yield and purity of the second-stage pyromellitic dianhydride.

[0134] Comparative Example 2 adopts conventional vacuum drying and dehydration, so the yield and purity of the second section of pyromellitic dianhydride are lower than those of Example 1, because the conventional vacuum drying and dehydration causes the obtained pyromellitic dianhydride powder to agglomerate, thereby reducing its sublimation efficiency, which is not conducive to separation from impurities. Comparative Example 3 does not add a good solvent during the crystallization of pyromellitic acid, and adopts conventional vacuum drying for dehydration, so the yield and purity of the second section of pyromellitic dianhydride are further reduced than those of Comparative Example 1. Comparative Example 5 adopts conventional hydrolysis purification and sublimation purification methods to purify pyromellitic dianhydride, and the yield and purity of the second section of pyromellitic dianhydride are the smallest.

[0135] In summary, the preparation method provided by the present invention is conducive to improving the yield and purity of pyromellitic acid dianhydride. Specifically, the improved hydrolysis purification method improves the yield and purity of pyromellitic acid, and obtains crystals with fine and uniform particle size, which is conducive to the subsequent improvement of the sublimation of pyromellitic acid dianhydride, thereby improving the yield and purity of the whole segment of pyromellitic acid dianhydride; the improved dehydration method is conducive to improving the dispersibility of the once purified pyromellitic acid dianhydride, thereby facilitating the improvement of the sublimation of pyromellitic acid dianhydride, thereby improving the yield and purity of the whole segment of pyromellitic acid dianhydride.

[0136] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for refining pyromellitic dianhydride, characterized in that: The steps include: (1) introducing a crude product of pyromellitic dianhydride into a hydrolysis reactor to carry out a hydrolysis reaction, and obtaining a pre-crystallization liquid after solid-liquid separation; (2) adding a good solvent to the pre-crystallization liquid, cooling the liquid for crystallization, and performing solid-liquid separation to obtain pyromellitic acid crystals; (3) flash-evaporating and dehydrating the pyromellitic acid crystals to obtain primary purified pyromellitic dianhydride; (4) subliming the once purified pyromellitic dianhydride to obtain a pure product of pyromellitic dianhydride.

2. A method for refining pyromellitic dianhydride according to claim 1, characterized in that, The crude product of pyromellitic anhydride is a gas containing pyromellitic anhydride; the gas containing pyromellitic anhydride is a hot gas flow generated when pyromellitic anhydride is synthesized by gas-phase oxidation of durene as a raw material, or / and, the gas containing pyromellitic anhydride is a hot gas flow obtained after the crude product of pyromellitic anhydride is heated and sublimated.

3. A method for refining pyromellitic dianhydride according to claim 2, characterized in that, The crude pyromellitic dianhydride product is mixed with atomized droplets and then introduced into the hydrolysis reactor; the atomized droplets refer to droplets obtained after atomizing water.

4. A method for refining pyromellitic dianhydride according to claim 3, characterized in that: The particle size of the atomized droplets is 5 to 50 μm; Or / and, the flow rate of the atomized droplets is 0.5 to 0.9 L / min; Or / and, the temperature of the atomized droplets is 40-60°C.

5. A method for refining pyromellitic dianhydride according to any one of claims 2 to 4, characterized in that: The flow rate of the gas containing pyromellitic anhydride is 100 to 200 g / min; Or / and, the temperature of the hydrolysis reaction is 90-100° C., the pressure is 0.1-0.3 MPa, and the reaction time is 1-2 h.

6. A method for refining pyromellitic dianhydride according to any one of claims 1 to 5, characterized in that: The good solvent is selected from at least one of acetonitrile, N,N-dimethylformamide and dimethyl sulfoxide.

7. A method for refining pyromellitic dianhydride according to any one of claim 6, characterized in that: The added amount of the good solvent is 5% to 15% of the volume of the pre-crystallization liquid.

8. A method for refining pyromellitic dianhydride according to any one of claims 1 to 3, characterized in that: The temperature end point of the cooling crystallization is 10-20°C, and the cooling rate is controlled to be 1-3°C / min.

9. A method for refining pyromellitic dianhydride according to any one of claims 1 to 3, 6 to 7, characterized in that: The flash dehydration is carried out in a flash evaporator, with an air inlet temperature of 220-300° C., an air outlet temperature of 100-150° C., a rotation speed of 10-30 Hz, and a feed frequency of 10-30 Hz.

10. A method for refining pyromellitic dianhydride according to any one of claims 1 to 3, characterized in that: 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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