A method for purifying pyromellitic dianhydride

Abstract

The application 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 crude pyromellitic dianhydride to obtain pyromellitic acid crystals; performing flash dehydration on the pyromellitic acid crystals to obtain first-purified pyromellitic dianhydride; and performing sublimation treatment on the first-purified pyromellitic dianhydride to obtain pure pyromellitic dianhydride. The hydrolysis refining method and the sublimation purification method are combined to purify the crude pyromellitic dianhydride, the hydrolysis refining method is improved, the purity and yield of the pyromellitic acid crystals are improved, and the pyromellitic acid crystals with small and uniform particle size are obtained, so that the purity and yield of the pyromellitic dianhydride are improved. Furthermore, the pyromellitic acid crystals are dehydrated by using a flash evaporator, the dispersibility of the first-purified pyromellitic dianhydride 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 present application relates to the technical field of purifying pyromellitic dianhydride, in particular to a refining method of pyromellitic dianhydride. BACKGROUND

[0002] Pyromellitic dianhydride (PMDA) is an important chemical raw material, which is used in the production of plasticizer, curing agent, paint, film, etc. The products produced have wide application in the fields of aerospace, electronics industry, military industry, automobile, industrial building materials, etc. and have good development prospect.

[0003] At present, pyromellitic dianhydride is mainly prepared by gas phase catalytic oxidation method in industry. Most of the method uses durene as raw material, air as oxidant and supported vanadium neodymium oxide as catalyst to prepare pyromellitic dianhydride through catalytic oxidation reaction in a fixed bed reactor. The process has low yield and many by-products, so the crude product needs to be purified and refined. Hydrolysis refining method is a widely used purification and refining method. Specifically, the crude pyromellitic dianhydride product is dissolved in water, heated to hydrolyze, filtered and cooled to crystallize, so as to obtain pyromellitic acid. The pyromellitic acid is then dehydrated under vacuum to obtain pyromellitic dianhydride product. However, the method is difficult to balance high purity and high efficiency.

[0004] In view of this, the present application is proposed. SUMMARY

[0005] The present application provides a refining method of pyromellitic dianhydride, which aims to improve the production efficiency and purity of pyromellitic dianhydride.

[0006] The present application is realized as follows:

[0007] A refining method of pyromellitic dianhydride, comprising the following steps:

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

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

[0010] (3) flash dehydrating the pyromellitic acid crystals to obtain first purified pyromellitic dianhydride;

[0011] (4) sublimating the first purified pyromellitic dianhydride to obtain pyromellitic dianhydride product.

[0012] In some embodiments, the crude product of pyromellitic dianhydride is a gas containing pyromellitic dianhydride; in some embodiments, the gas containing pyromellitic dianhydride is a hot gas stream generated when pyromellitic dianhydride is synthesized by gas phase oxidation using durene as raw material; in other embodiments, the gas containing pyromellitic dianhydride is a hot gas stream obtained by heating and sublimating the crude product of pyromellitic dianhydride.

[0013] In some embodiments, the crude product of pyromellitic dianhydride is mixed with atomized liquid droplets before being introduced into the hydrolysis reactor; the atomized liquid droplets refer to liquid droplets obtained by atomizing water.

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

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

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

[0017] In some embodiments, the flow rate of the gas containing pyromellitic dianhydride is 100-200 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 the good solvent added is 5%-15% of the volume of the liquid before crystallization.

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

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

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

[0024] In some embodiments, the flash dehydration is performed in a flash evaporator, the inlet air temperature is 220-300 °C, the outlet air temperature is 100-150 °C, the rotation speed is 10-30 Hz, and the feeding frequency is 10-30 Hz.

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

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

[0027] The present application combines the hydrolysis refining method and the sublimation purification method to purify the crude pyromellitic dianhydride, and improves the hydrolysis refining method, thereby improving the purity and yield of the pyromellitic acid crystal, and obtaining the pyromellitic acid crystal with small and uniform particle size, thereby improving the purity and yield of the pyromellitic dianhydride. Furthermore, the present application uses the flash evaporator to dehydrate the pyromellitic acid crystal, thereby improving the dispersibility of the once purified pyromellitic dianhydride, and improving the efficiency of the sublimation treatment, thereby improving the purity and yield of the pyromellitic dianhydride.

[0028] The purification method provided by the present application improves the efficiency of the hydrolysis refining method, and 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 application more clear, the technical scheme in the embodiments of the present application will be clearly and completely described below. If the specific conditions are not specified in the embodiments, the conventional conditions or the conditions suggested by the manufacturer are used. If the reagents or instruments are not specified by the manufacturer, they are all conventional products that can be purchased in the market.

[0030] In the description of the embodiments of the present application, the technical terms "first", "second" and the like 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 "embodiments" herein means that the specific features, structures or properties described in connection with the embodiments can be included in at least one embodiment of the present application. The appearance of this phrase at various places in the specification does not necessarily refer to the same embodiment, nor is it mutually exclusive or alternative to other embodiments.

[0032] In the embodiments of the present application, the term "or / and" is only a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A or / and B, which can represent the three cases of A alone, A and B together, and B alone.

[0033] In addition, the character " / " herein generally represents that the front and rear associated objects are in an "or" relationship.

[0034] In the embodiments of the present application, the term "a plurality of" means two or more (including two), and the term "a plurality of groups" means two or more groups (including two groups), and the term "a plurality of layers" means two or more layers (including two layers), unless otherwise specified or limited.

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

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

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

[0038] (1) introducing the crude pyromellitic dianhydride into a hydrolysis reactor for hydrolysis reaction, and then performing solid-liquid separation to obtain a pre-crystallization liquid;

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

[0040] (3) performing flash dehydration on the pyromellitic acid crystals to obtain first-purified pyromellitic dianhydride;

[0041] (4) performing sublimation treatment on the first-purified pyromellitic dianhydride to obtain pure pyromellitic dianhydride.

[0042] The present application combines the hydrolysis purification method and the sublimation purification method to purify the crude pyromellitic dianhydride, and improves the hydrolysis purification method, thereby improving the purity and yield of the pyromellitic acid crystals, and obtaining pyromellitic acid crystals with small and uniform particle size, thereby improving the purity and yield of the pyromellitic dianhydride. Furthermore, the present application uses a flash evaporator to dehydrate the pyromellitic acid crystals, thereby improving the dispersibility of the first-purified pyromellitic dianhydride, and improving the sublimation efficiency, thereby improving the purity and yield of the pyromellitic dianhydride.

[0043] Specifically, the present application adds a good solvent to the pre-crystallization liquid obtained by hydrolysis, which is beneficial to slow down the crystal growth of pyromellitic acid, and promote the growth of the crystals into crystals with high crystallinity and small defects, so that the obtained crystals have small and uniform particle size and contain less impurities. The small and uniform particle size of the crystals is beneficial to improve the dispersibility of the first-purified pyromellitic dianhydride obtained by dehydration, increase the contact area between the first-purified pyromellitic dianhydride and the hot gas flow during sublimation, and improve the sublimation efficiency, thereby improving the purity and yield of the pyromellitic dianhydride. The use of a flash evaporator to dehydrate the pyromellitic acid crystals further improves the dispersibility of the first-purified pyromellitic dianhydride, and avoids the agglomeration of the crystals. In addition, the use of a flash dryer to dehydrate the pyromellitic acid improves the dehydration speed and the efficiency of the entire process.

[0044] In some embodiments, the crude product of pyromellitic dianhydride is a gas containing pyromellitic dianhydride; in some embodiments, the gas containing pyromellitic dianhydride is a hot gas stream generated in the gas phase oxidation synthesis of pyromellitic dianhydride using a raw material of durene; in other embodiments, the gas containing pyromellitic dianhydride is a hot gas stream obtained by heating and sublimating the crude product of pyromellitic dianhydride.

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

[0046] Mixing the crude product of pyromellitic dianhydride in the form of a gas with an aqueous solution is advantageous in increasing the mass transfer area and mass transfer efficiency, and in increasing the degree of dissolution and hydrolysis, thereby increasing the yield of pyromellitic acid.

[0047] In some embodiments, the crude product of pyromellitic dianhydride is mixed with atomized liquid droplets before being introduced into the hydrolysis reactor; the atomized liquid droplets refer to liquid droplets obtained by atomizing water.

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

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

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

[0051] Further, the gaseous crude product of pyromellitic dianhydride is first mixed with atomized liquid droplets before being introduced into the aqueous solution, which can preliminarily cool the gaseous crude product of pyromellitic dianhydride, avoiding boiling caused by direct introduction into the hydrolysis reactor; on the other hand, it can increase the mass transfer area and mass transfer efficiency of the gas-liquid two-phase, and promote the dissolution and hydrolysis of pyromellitic dianhydride.

[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 at least one selected from the group consisting of acetonitrile, N,N-dimethylformamide and dimethyl sulfoxide.

[0054] In some embodiments, the amount of the good solvent added is 5%-15% of the volume of the liquid before crystallization; the amount of the good solvent added should not be too much, otherwise the growth rate of the pyromellitic acid crystals will be too low, resulting in a reduced yield; if the amount of the good solvent added is too small, it will be difficult to play a role in adjusting the crystal size and purity of pyromellitic acid.

[0055] In some embodiments, the temperature endpoint of the cooling crystallization is 10-20°C, and the cooling rate is controlled to be 1-3°C / min; controlling the cooling rate is conducive to slowing the growth of the pyromellitic acid crystals and avoiding excessively wide grain size distribution and excessively large particle size.

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

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

[0058] The pyromellitic acid crystals obtained by the preparation method provided in the present application have an average particle size of 5-12 μm and a broadening coefficient of 1-1.5. The pyromellitic acid crystals having this morphology have a high sublimation rate during sublimation treatment of the once-purified pyromellitic dianhydride, thereby improving the yield of the purified pyromellitic dianhydride.

[0059] In some embodiments, the flash dehydration is performed in a flash evaporator, the inlet air temperature is 220-300°C, the outlet air temperature is 100-150°C, the rotation speed is 10-30 Hz, and the feeding frequency is 10-30 Hz.

[0060] In some embodiments, the sublimation treatment refers to sublimation of the once-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) The pyromellitic dianhydride-containing gas obtained after the reaction of the vaporized durene with air in the fixed-bed reactor containing the catalyst is mixed with the atomized liquid droplets in a counterflow manner through two ends of a condenser tube, and the mixed flow is transported to the hydrolysis reactor through negative pressure generated by a fan and is introduced from the bottom, and the hydrolysis reactor contains a 60°C aqueous solution as a bottom liquid.

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

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

[0065] (2) The temperature in the hydrolysis reactor is controlled to be 90±2°C, and the pressure is 0.2 MPa, and the reaction is stirred for 1 h, and the pre-crystallization liquid is obtained by hot filtration.

[0066] (3) Add acetonitrile to the pre-crystallization liquid, and gradually reduce the temperature to 10°C at a rate of 3°C / min, stand for 12 h, and then dry at 80°C after pressure filtration to obtain the crystal of pyromellitic acid.

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

[0068] (4) The crystal of pyromellitic acid is sent into a flash dryer to dry to obtain the first-purified pyromellitic dianhydride.

[0069] The inlet air temperature is 300°C, the outlet air temperature is 150°C, the rotation speed is 20 Hz, and the feeding frequency is 20 Hz. Under these conditions, the time from feeding to discharging is 5-10 min.

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

[0071] Example 2

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

[0073] (2) The temperature in the hydrolysis reactor is controlled at 98±2°C, the pressure is 0.3 MPa, and the reaction is stirred for 1 h. The pre-crystallization liquid is obtained by hot filtration.

[0074] (3) Add dimethyl sulfoxide to the pre-crystallization liquid, and gradually reduce the temperature to 20°C at a rate of 2°C / min, stand for 12 h, and then dry at 80°C after pressure filtration to obtain the crystal of pyromellitic acid.

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

[0076] (4) The crystal of pyromellitic acid is sent into a flash dryer to dry to obtain the first-purified pyromellitic dianhydride.

[0077] The inlet air temperature is 350°C, the outlet air temperature is 150°C, the rotation speed is 10 Hz, and the feeding frequency is 10 Hz. Under these conditions, the time from feeding to discharging is 5-10 min.

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

[0079] Example 3

[0080] (1) The gas containing pyromellitic dianhydride obtained by vaporizing the solid pyromellitic dianhydride crude product under an inert gas atmosphere (nitrogen) at 300°C is mixed with the atomized liquid droplets introduced from both ends of the condenser tube in a countercurrent manner, and the mixed flow is transported to the hydrolysis reactor from the bottom by negative pressure generated by a fan, and a 60°C aqueous solution is introduced into the hydrolysis reactor as a bottom liquid.

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

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

[0083] (2) The temperature in the hydrolysis reactor is controlled at 90 ± 2°C, the pressure is 0.1 MPa, and the reaction is stirred for 2 h, and the pre-crystallization liquid is obtained by hot filtration.

[0084] (3) N,N-dimethylformamide is added to the pre-crystallization liquid, and the temperature is gradually lowered to 10°C at a rate of 1°C / min, and the mixture is allowed to stand for 12 h, and then filtered under pressure and dried at 80°C to obtain pyromellitic acid crystals.

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

[0086] (4) The pyromellitic acid crystals are sent to a flash dryer for drying to obtain the first purified pyromellitic dianhydride.

[0087] The inlet air temperature is 250°C, the outlet air temperature is 100°C, the rotation speed is 30 Hz, and the feeding frequency is 30 Hz, and under these conditions, the time from feeding to discharging is 5-10 min.

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

[0089] Example 4

[0090] The difference between Example 3 and Example 4 is that in step (1), the gas containing pyromellitic dianhydride obtained by vaporizing the solid pyromellitic dianhydride crude product under an inert gas atmosphere (nitrogen) at 300°C is directly transported to the hydrolysis reactor and introduced from the bottom, and a 60°C aqueous solution is introduced into the hydrolysis reactor as a 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 N,N-dimethylformamide is used instead of acetonitrile as a good solvent in step (3).

[0097] Example 8

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

[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 liquid before crystallization.

[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 liquid before crystallization.

[0103] Comparative Example 1

[0104] The difference from Example 1 is that no acetonitrile is added in step (3).

[0105] Comparative Example 2

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

[0107] Comparative Example 3

[0108] The difference from Example 1 is that no acetonitrile is added in step (3), and the conventional vacuum method is used to dehydrate the pyromellitic acid crystals in step (4), specifically: the pyromellitic acid crystals are sent into an oven at 220°C and a vacuum degree of -0.09 MPa for dehydration for 12 h.

[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 feeding ratio of the solid crude pyromellitic dianhydride to the aqueous solution is 1 g:6 g.

[0111] Comparative Example 5

[0112] The purification of pyromellitic dianhydride using conventional methods includes the following steps:

[0113] (1) The crude solid pyromellitic dianhydride was directly added to the hydrolysis reactor for hydrolysis reaction, and the liquid before crystallization was obtained by hot filtration.

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

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

[0116] (3) The pyromellitic acid crystals were placed in an oven at 220°C and a vacuum of -0.09MPa for 12 hours to dehydrate, thus obtaining a first-purified pyromellitic acid dianhydride.

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

[0118] The physical properties of the products in the examples and comparative examples were tested, and the results are shown in Tables 1 and 2. The test methods are as follows:

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

[0120] (2) Yield:

[0121] ① in, The yield and purity of crude pyromellitic dianhydride are respectively. These represent the mass and purity of pyromellitic acid crystals, respectively; M PMDA M PMA These are the relative molecular masses of pyromellitic dianhydride and pyromellitic acid, respectively.

[0122] ② in, respectively are the yield and purity of pure pyromellitic dianhydride; respectively are the mass and purity of pyromellitic acid crystal; M PMDA , M PMA respectively are the relative molecular mass of pyromellitic dianhydride and pyromellitic acid.

[0123] When the gas containing pyromellitic dianhydride is the hot gas stream obtained after heating and sublimation of the crude pyromellitic dianhydride, the total section pyromellitic dianhydride yield can also be calculated,

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

[0125] Table 1

[0126]

[0127]

[0128] Table 2

[0129]

[0130] From the data in Table 1, it can be seen that the purity and yield of the pyromellitic dianhydride pure product obtained in the examples of the present application are higher than those of the comparative examples.

[0131] Comparing Example 3~4 and Comparative Example 4, it can be seen that the way of entering the aqueous solution during the hydrolysis of pyromellitic dianhydride has a great influence on the yield and purity of the obtained pyromellitic acid. Specifically, the way of Example 3 is better than that of Example 4, and the way of Example 4 is better than that of Comparative Example 4. After the crude pyromellitic dianhydride is gasified and mixed with the aqueous solution, the mass transfer area and efficiency between the aqueous solution can be improved, and the degree of dissolution and hydrolysis is also improved, thereby improving the yield of pyromellitic acid. In Comparative Example 4, the crude pyromellitic dianhydride is directly mixed with the aqueous solution, and due to the decrease in mass transfer efficiency, the hydrolysis efficiency of pyromellitic dianhydride decreases, and the number of crystal nuclei formed is small, so the obtained pyromellitic acid crystal has large and uneven particle size, which is easy to carry impurities, so the purity and yield are low. In Example 3, the crude pyromellitic dianhydride is first mixed with the atomized liquid droplets and then introduced into the aqueous solution, which is more conducive to forming a large number of active sites (hydrolysis reaction active sites and crystallization precipitation active sites), so the yield and purity are better than those of the other two. Further, from Examples 3, 5, and 6, it can be seen that adjusting the flow of atomized liquid droplets has an effect on the purity and yield of the obtained pyromellitic acid crystal. Although Examples 3~6 and Comparative Example 4 have the same subsequent treatment of the obtained pyromellitic acid crystal, they have different second-stage pyromellitic dianhydride yields, indicating that the purity of the pyromellitic acid crystal and its morphology have an effect on the subsequent purification effect, specifically, the higher the purity of the pyromellitic acid crystal, the higher the second-stage pyromellitic dianhydride yield; the smaller the particle size of the pyromellitic acid crystal and the smaller the width coefficient, the higher the second-stage pyromellitic dianhydride yield.

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

[0133] Examples 1, 9, and 10 use different amounts of good solvents. From the results, the more good solvents, the more conducive to improving the purity of pyromellitic acid, reducing its average particle size and width coefficient, thereby facilitating the improvement of the second-stage pyromellitic dianhydride yield and purity.

[0134] The comparative example 2 uses conventional vacuum drying dehydration, so the yield and purity of the second-stage pyromellitic dianhydride are lower than those of the example 1, because the conventional vacuum drying dehydration causes the obtained pyromellitic dianhydride powder to agglomerate, thereby reducing the sublimation efficiency and being not conducive to separation from impurities. The comparative example 3 does not add a good solvent during the crystallization of pyromellitic acid, and uses conventional vacuum drying dehydration, so the yield and purity of the second-stage pyromellitic dianhydride are further reduced compared with the comparative example 1. The comparative example 5 uses conventional hydrolysis purification and sublimation purification methods to purify the pyromellitic dianhydride, and the yield and purity of the second-stage pyromellitic dianhydride are the lowest.

[0135] In summary, the preparation method provided by the present application is beneficial to improve the yield and purity of pyromellitic dianhydride. Specifically, the improved hydrolysis purification method improves the yield and purity of pyromellitic acid, and the obtained crystals have fine and uniform particle sizes, which is conducive to subsequent improvement of the sublimation of pyromellitic dianhydride, thereby improving the yield and purity of pyromellitic dianhydride in the whole stage. The improved dehydration method is beneficial to improve the dispersibility of the first-stage purified pyromellitic dianhydride, thereby being conducive to improvement of the sublimation of pyromellitic dianhydride, thereby improving the yield and purity of pyromellitic dianhydride in the whole stage.

[0136] The above only describes the preferred embodiments of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for purifying pyromellitic dianhydride, characterized by, It comprises the following steps: (1) introducing the crude pyromellitic dianhydride gas into a hydrolysis reactor for hydrolysis reaction, and then separating the solid and liquid to obtain a pre-crystallization liquid; the crude pyromellitic dianhydride gas is mixed with atomized liquid droplets before being introduced into the hydrolysis reactor; (2) adding a good solvent into the pre-crystallization liquid, and then cooling and crystallizing, and then separating the solid and liquid to obtain pyromellitic acid crystals; the average particle size of the pyromellitic acid crystals is 5-12 μm; the broadening coefficient of the pyromellitic acid crystals is 1-1.5, and the broadening coefficient=(Dv90-Dv10) / Dv50, wherein Dv90, Dv50 and Dv10 are the particle sizes corresponding to the cumulative volume percentages of 90%, 50% and 10% respectively; the good solvent is at least one selected from acetonitrile, N,N-dimethylformamide and dimethyl sulfoxide; the crude pyromellitic dianhydride gas is a hot gas stream generated during the gas-phase oxidation synthesis of pyromellitic dianhydride using durene as a raw material, or / and, the crude pyromellitic dianhydride gas is a hot gas stream obtained after the crude pyromellitic dianhydride is heated and sublimed; (3) flash dehydrating the pyromellitic acid crystals to obtain first-purified pyromellitic dianhydride; the flash dehydration is carried out in a flash machine, the inlet air temperature is 220-300 ℃, the outlet air temperature is 100-150 ℃, the rotating speed is 10-30 Hz, and the feeding frequency is 10-30 Hz; (4) sublimating the first-purified pyromellitic dianhydride to obtain pure pyromellitic dianhydride.

2. The method of purifying pyromellitic dianhydride according to claim 1, characterized by, the atomized liquid droplets are liquid droplets obtained by atomizing water.

3. The method of purifying pyromellitic dianhydride according to claim 2, characterized by, the particle size of the atomized liquid droplets is 5-50 μm; 4. The method of purifying pyromellitic dianhydride according to claim 3, characterized by, or / and, the flow rate of the atomized liquid droplets is 0.5-0.9 L / min; or / and, the temperature of the atomized liquid droplets is 40-60 ℃. the flow rate of the crude pyromellitic dianhydride gas is 100-200 g / min; 5. The method for purifying pyromellitic dianhydride according to any one of claims 2 to 4, characterized by, or / and, the temperature of the hydrolysis reaction is 90-100 ℃, the pressure is 0.1-0.3 MPa, and the reaction time is 1-2 h. the addition amount of the good solvent is 5%-15% of the volume of the pre-crystallization liquid.

6. The method of purifying pyromellitic dianhydride according to any one of claims 1, wherein the terminal temperature of the cooling and crystallization is 10-20 ℃, and the cooling speed is controlled to be 1-3 ℃ / min.

7. The method of purifying pyromellitic dianhydride according to any one of claims 1 to 3, characterized by, the sublimation treatment refers to sublimating the first-purified pyromellitic dianhydride at 230-260 ℃ and -0.09 MPa, and the vaporized pyromellitic dianhydride is captured in a receiver.

8. The method of purifying pyromellitic dianhydride according to any one of claims 1 to 3, characterized by, ​

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