CN110746435B - Method for producing pyromellitic dianhydride by low-temperature liquid-phase continuous oxidation of durene - Google Patents
Method for producing pyromellitic dianhydride by low-temperature liquid-phase continuous oxidation of durene Download PDFInfo
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Abstract
The invention discloses a method for producing pyromellitic dianhydride by continuously oxidizing durene at low temperature and liquid phase, which belongs to the technical field of chemical production, and comprises the steps of firstly, continuously pumping mixed liquid of durene, acetic acid and a cobalt-manganese-bromine-zirconium-guanidine catalyst into an oxidation reaction tower, continuously introducing oxygen-enriched gas into the tower body while feeding liquid, keeping the temperature in the oxidation reaction tower at 200-300 ℃, keeping the pressure in the tower body at 0.5-3MPa, and catalytically reacting the durene with oxygen to generate the pyromellitic acid, wherein the generated pyromellitic acid has low density and floats on the upper layer of the liquid level; pumping out liquid on the upper layer of the liquid level with high pyromellitic acid content for flash evaporation to evaporate light components and obtain crude pyromellitic acid at the bottom of the tank; the pyromellitic acid crude product is recrystallized and centrifugally separated to obtain pyromellitic acid crystal, and then the pyromellitic dianhydride is obtained by dehydration. The bromine consumption in the catalyst is low, so that the corrosion of equipment is reduced; the method has simple process and easy realization, and is beneficial to the industrial production of the pyromellitic dianhydride.
Description
Technical Field
The invention relates to a low-temperature liquid-phase continuous oxidation production process, in particular to a process for preparing pyromellitic acid by adopting low-temperature liquid-phase continuous oxidation of durene, and then preparing pyromellitic dianhydride by dehydrating the pyromellitic acid. Belongs to the technical field of chemical industry.
Background
Durene, known by the chemical name 1, 2, 4, 5-tetramethylbenzene, is a granular/leaf-like crystalline substance with camphor flavor, has a melting point of 80 ℃, is soluble in organic solvents such as ethanol, diethyl ether, acetone, benzene and the like, and has sublimability. Durene is an important organic chemical raw material, and is mainly used for producing pyromellitic acid and pyromellitic anhydride. The pyromellitic acid is obtained after the oxidation of the durene, and then the pyromellitic dianhydride is obtained after further dehydration.
The structure of 1, 2, 4, 5-tetrasubstituted compound of durene makes two ends of its molecule have symmetrical dipole pairs, so that it raises the crystallinity of molecule and possesses higher melting point. Thus, one of the uses of durene and its derivatives is to increase the crystallinity of the polymer. Durene derivatives contain two pairs of adjacent functional groups, the ring system compounds of adjacent functional groups allowing the polymer to crosslink. However, the reactivity of the functional groups of durene derivatives as two linear pairs is unique, allowing the production of chain-like stepwise polymers such that single and double bonds are alternately present. The high temperature resistance of the polymer material can be obviously provided because double bonds must be opened when the bonds are broken. Thus, the durene derivatives can be used well as components of polymers at higher temperatures.
Pyromellitic acid and its derivatives are also industrially important. Pyromellitic acid can separate heavy metals and is used as a scale remover and a preservative. The ortho carboxyl activity of sodium pyromellitate can generate chelate with polyvalent metal ion to be used as assistant of detergent. Pyromellitic acid ester is a good low-fluidity plasticizer and is also a PVC heat stabilizer. Phthalocyanine made from pyromellitic acid derivatives can be used as pigment, oxidation catalyst, and high-performance lubricant.
Pyromellitic dianhydride, whose chemical name is 1, 2, 4, 5-benzoic dianhydride, abbreviated as pyromellitic dianhydride, and abbreviated as PMDA in english. The appearance of the pyromellitic anhydride is white powder or needle-shaped crystal, the melting point is 284-286 ℃, the boiling point is 397-400 ℃, the pyromellitic anhydride is dissolved in dimethylformamide, dimethyl sulfoxide, acetone, butanone, methyl isobutyl ketone and the like, is insoluble in chloroform, diethyl ether, normal hexane, benzene and petroleum ether, and is easily hydrolyzed into pyromellitic acid when meeting water or being exposed in wet air.
Pyromellitic dianhydride is an important chemical raw material, and is used for synthesizing polyimide which is a very important special high-molecular polymer with aromatic diamine. Polyimide is a new material with high temperature resistance, low temperature resistance, radiation resistance, impact resistance and excellent mechanical properties of electrical insulation, is widely applied to the aspects of electronics, electromechanics, aviation industry and the like, and has important roles which cannot be replaced by other engineering plastics in aerospace and electromechanics industries. At present, pyromellitic dianhydride is used as polyimide resins and films, epoxy resin curing agents, surfactants, plasticizers and polyester resin crosslinking agents, coating additives, water treatment agents, low-temperature performance improvers for diesel oil, electrode materials, azo dyes, and high-temperature resistant lubricants and electrophotographic tone improvers, and the like.
The existing method for synthesizing pyromellitic dianhydride mainly comprises the following four methods: (1) durene process, (2) pseudocumene alkylation process, (3) pseudocumene carbonylation process, and (4) 2, 4-xylene chloromethylation process. At present, the industry generally adopts durene as a raw material to produce the pyromellitic dianhydride, and other three methods do not realize large-scale industrialization due to serious environmental pollution, high requirements on equipment, immature new technology and the like.
The durene process is classified into a liquid-phase oxidation process of durene and a gas-phase oxidation process of durene.
The first liquid-phase oxidation method of pyromellitic dianhydride is to oxidize durene raw material in durene liquid phase air to prepare pyromellitic acid, and then dehydrate the pyromellitic acid to obtain the pyromellitic dianhydride.
Patent JP2515296 produces crude pyromellitic acid by a durene liquid-phase oxidation method, and then heats it to partially dehydrate the pyromellitic acid, and the produced crude pyromellitic acid is converted into pyromellitic dianhydride by anhydrization in the presence of an aliphatic anhydride (e.g., acetic anhydride). In this patent, two moles of acetic anhydride are required per mole of pyromellitic acid in anhydrization, and at the same time, by-products are generated during the reaction, and acetic anhydride becomes acetic acid, which needs to be treated, thereby increasing the production cost.
In the patent EP0215431B1, pyromellitic acid obtained by liquid phase oxidation is heat-exchanged with a heat carrier in a trough dryer, and dehydration is carried out under a nitrogen atmosphere to prepare pyromellitic dianhydride. It was found that the absence of pyromellitic acid in water affects the dehydration time, the same conversion rate, and the dehydration time required for pyromellitic acid containing no water is shortened by 4 times as much as that of water.
Patent CN200610171912.0 discloses heating pyromellitic acid in a tank dryer to partially dehydrate and convert it into pyromellitic anhydride, adding acetic anhydride/acetic acid mixture, and further heating the product mixture to completely anhydrize pyromellitic acid, and the final molar yield of pyromellitic dianhydride is 98%.
Patent JP6259280A discloses a method of converting pyromellitic acid into pyromellitic dianhydride by heating pyromellitic formic acid in a tank dryer at a limited temperature, which is disadvantageous in that it is difficult to control the properties of particles and prevent discoloration of pyromellitic dianhydride, whereby discoloration occurs during processing of thermoplastic resins produced using pyromellitic dianhydride as a raw material or an additive. Meanwhile, the properties of the particles can cause the blockage of pipelines, hopper outlets, reactor inlets and the like, thereby influencing the production of the pyromellitic dianhydride.
The above patents all adopt the liquid phase oxidation of pyromellitic acid with toluene to produce pyromellitic dianhydride, and then dehydrate to anhydride. The yield and the purity of the pyromellitic dianhydride synthesized by the process are high, but the process route is long, the requirements on equipment materials are high, and the investment is high.
The second gas-phase oxidation method of pyromellitic dianhydride uses pyromellitic dianhydride as raw material and gasifies the pyromellitic dianhydride in one step of air catalytic oxidation. Many studies have been reported on the preparation of pyromellitic dianhydride by gas-phase oxidation of durene, and since the preparation of pyromellitic dianhydride by gas-phase oxidation of durene is a complex heterogeneous catalytic process, the product is a mixture of various oxygen-containing compounds, including acids, anhydrides, aldehydes and alcohol compounds, and therefore, a catalyst is generally used as a carrier catalyst. Therefore, the route for producing pyromellitic dianhydride by a pyromellitic gas-phase oxidation method is simple, and the process for producing pyromellitic dianhydride by a gas-phase method is mainly researched on the aspects of purity, yield, selectivity and the like of the product pyromellitic dianhydride in the selection of the catalyst.
Patent US4925957 prepares a series of composite catalysts with vanadium as main component loaded on alpha-Al2O3The above. It was found that for the Mn-Na-V system, the addition of Mo decreased the reaction temperature by 10 ℃. In the case of V-Na-Nb and V-Ti-Na systems, the pyromellitic dianhydride yield is still 100% or more by mass after Mo is added, but the reaction temperature range is narrowed, and if the temperature range is out of the optimum temperature range, the pyromellitic dianhydride yield is greatly reduced.
Patent US6084109 prepares a series of composite catalysts with vanadium and tungsten as main components, which are respectively or jointly loaded on titanium oxide, silicon carbide and tungsten carbide, and a certain amount of Mn, Sb, Bi, P, Cu, Al and the like are added. The best yield of pyromellitic dianhydride prepared by loading V-Mn-Fe (3 valent) on titanium oxide and silicon carbide is 100 percent. Research also finds that the yield of anatase of the crystal structure of the carrier titanium oxide is higher than that of rutile, the temperature difference of reaction temperature is small, and the performance of the catalyst is stable and is not easy to deactivate.
The patent CN98116910.4 is that Mo and Cs are respectively added into a V-Ti-P system and sprayed on alpha-Al2O3Or SiC or a ring-shaped carrier, and carrying out the oxidation reaction at the temperature of 430-450 ℃. Then mixing the crude anhydride with water and activated carbonMixing and heating in certain proportion, and vacuum sublimation to obtain refined pyromellitic dianhydride with purity over 98.5%.
Patent US5225572 produces a series of V2O5-TiO2The composite catalyst as main component is prepared through adding B in 0.5% at 400 deg.c and 350-2O3The best molar yield of the prepared pyromellitic dianhydride is 60.5 percent, and the purity is 95 percent. It was found that as the reaction temperature increased, the molar yield decreased, but the purity increased.
The above patents all adopt a pyromellitic dianhydride gas phase oxidation method to produce pyromellitic dianhydride. However, the gas phase method for producing pyromellitic dianhydride from durene has the disadvantages of high oxidation temperature, more side reactions, and relatively low purity, selectivity, yield and the like of the product. Therefore, the durene gas phase method has simple process, but no public report is found on how to improve the selectivity of the product, namely the pyromellitic anhydride and further improve the yield and the purity of the product, namely the pyromellitic anhydride.
Disclosure of Invention
The invention aims to provide a method for producing pyromellitic dianhydride by low-temperature liquid-phase continuous oxidation of durene, which ensures that the production temperature of the pyromellitic dianhydride is low and the energy consumption is low; reaction byproducts are less, and the product yield is high; the quality is more stable.
Therefore, the method for producing the pyromellitic dianhydride by the low-temperature liquid-phase continuous oxidation of the durene provided by the invention comprises the following steps:
(1) continuously pumping mixed liquid of durene, acetic acid and a cobalt-manganese-bromine-zirconium-guanidine catalyst into an oxidation reaction tower, wherein the cobalt-manganese-bromine-zirconium-guanidine catalyst comprises cobalt acetate, manganese acetate, tetrabromoethane, zirconium acetate and guanidine acetate, and in the mixed liquid, the acetic acid is used as a solvent, and the weight contents of the substances are respectively as follows: 14.8 to 19.4 percent of durene, 80.4 to 84.8 percent of acetic acid, 0.035 to 0.09 percent of cobalt acetate, 0.04 to 0.1 percent of manganese acetate, 0.019 to 0.055 percent of tetrabromoethane, 0.007 to 0.032 percent of zirconium acetate and 0.014 to 0.033 percent of guanidine acetate; oxygen-enriched gas is continuously introduced into the tower body while liquid is fed, the liquid feeding flow is 1200-class 1500 kg/h, and the gas inlet flow is 950-class 1050m3/h；
(2) Keeping the temperature in the oxidation reaction tower at 200-300 ℃, the pressure in the tower body at 0.5-3MPa, carrying out catalytic reaction on durene and oxygen to generate pyromellitic acid, wherein the generated pyromellitic acid has low density and floats on the upper layer of the liquid level;
(3) pumping out the liquid on the upper layer of the liquid level with high pyromellitic acid content, and carrying out flash evaporation in a flash evaporation tank with the temperature of 195-205 ℃ under 0.02-0.08MPa to evaporate pyromellitic acid, acetic acid and other light components, thereby obtaining a crude pyromellitic acid product at the bottom of the tank;
(4) dissolving crude pyromellitic acid in water at 90-95 ℃, decolorizing with active carbon, continuously filtering at constant temperature, continuously recrystallizing at 5-45 ℃, separating with a continuous centrifuge, and removing water to obtain pyromellitic acid crystal; the purity of pyromellitic acid crystal can reach 98-99%, and the conversion rate is 94-98% (by weight);
(5) the pyromellitic acid is dehydrated into anhydride at the temperature of 120 ℃ and the pressure of 0.02-0.06MPa to obtain the pyromellitic dianhydride with the purity of 99.5-99.9 percent (by weight).
The oxygen-enriched gas can be air or a nitrogen-oxygen mixed gas with the oxygen volume content of 21-50%.
Compared with the prior art, the invention has the beneficial effects that:
1. a special catalyst is selected, so that the oxidation reaction has higher selectivity, can be carried out at relatively low temperature, reduces the occurrence of side reaction, obtains high purity products, greatly reduces the consumption of bromine, weakens the corrosivity of a reaction system, and reduces the corrosion of equipment.
2. The invention adopts a continuous liquid phase oxidation method for production, removes light components through flash evaporation, is realized by only adopting vacuum dehydration during anhydride formation, has the advantages of simple process and easy realization, and provides technical support for large-scale industrial production of pyromellitic dianhydride.
Detailed Description
Example 1
A method for producing pyromellitic dianhydride by low-temperature liquid-phase continuous oxidation of durene comprises the following steps:
(1) the method comprises the following steps of continuously pumping mixed liquid of durene, acetic acid and a cobalt-manganese-bromine-zirconium-guanidine catalyst into an oxidation reaction tower, wherein the cobalt-manganese-bromine-zirconium-guanidine catalyst comprises cobalt acetate, manganese acetate, tetrabromoethane, zirconium acetate and guanidine acetate, in the mixed liquid, the acetic acid is used as a solvent, the durene is 223.35 kg, the acetic acid is 1272 kg, the catalyst comprises 1.35 kg of cobalt acetate, 1.5kg of manganese acetate, 0.825 kg of tetrabromoethane, 0.48 kg of zirconium acetate and 0.495 kg of guanidine acetate, and the weight contents of the corresponding substances are respectively as follows: 14.89 percent of durene, 84.8 percent of acetic acid, 0.09 percent of cobalt acetate, 0.1 percent of manganese acetate, 0.055 percent of tetrabromoethane, 0.032 percent of zirconium acetate and 0.033 percent of guanidine acetate; air is continuously introduced into the tower body while liquid is fed, the flow rate of the liquid is 1500 kg/h, and the flow rate of the inlet air is 1050m3/h；
(2) Keeping the temperature in the oxidation reaction tower at 280-300 ℃, the pressure in the tower body at 2.5-3MPa, carrying out catalytic reaction on durene and oxygen to generate pyromellitic acid, wherein the generated pyromellitic acid has low density and floats on the upper layer of the liquid level;
(3) pumping out the liquid on the upper layer of the liquid level with high pyromellitic acid content, and carrying out flash evaporation in a flash evaporation tank with the pressure of 0.06-0.08MPa and the temperature of 195 plus-205 ℃ to evaporate pyromellitic acid, acetic acid and other light components, thereby obtaining a crude pyromellitic acid product at the bottom of the tank;
(4) dissolving crude pyromellitic acid in water at 90-95 ℃, decolorizing with active carbon, continuously filtering at constant temperature, continuously recrystallizing at 35-45 ℃, separating with a continuous centrifuge, and removing water to obtain pyromellitic acid crystal; the purity of pyromellitic acid crystal can reach 99 percent, and the conversion rate is 95 percent;
(5) the pyromellitic acid is dehydrated into anhydride under the environment of the temperature of 200-220 ℃ and the pressure of 0.04-0.06MPa to obtain the pyromellitic dianhydride, the purity can reach 99.9 percent, and the yield is 96 percent.
Example 2
A method for producing pyromellitic dianhydride by low-temperature liquid-phase continuous oxidation of durene comprises the following steps:
(1) continuously feeding mixed solution of durene, acetic acid and cobalt-manganese-bromine-zirconium-guanidine catalyst into oxidationIn the reaction tower, the cobalt-manganese-bromine-zirconium-guanidine catalyst comprises cobalt acetate, manganese acetate, tetrabromoethane, zirconium acetate and guanidine acetate, in the mixed solution, acetic acid is used as a solvent, 291 kg of durene is used, 1206 kg of acetic acid is used, 0.525 kg of cobalt acetate, 1.5kg of manganese acetate, 0.375 kg of tetrabromoethane, 0.105 kg of zirconium acetate and 0.495 kg of guanidine acetate are used as catalysts, and the weight contents of the corresponding substances are respectively as follows: 19.4 percent of durene, 80.4 percent of acetic acid, 0.035 percent of cobalt acetate, 0.1 percent of manganese acetate, 0.025 percent of tetrabromoethane, 0.007 percent of zirconium acetate and 0.033 percent of guanidine acetate; air is continuously introduced into the tower body while liquid is fed, the flow rate of the liquid is 1200 kg/h, and the flow rate of the inlet air is 1050m3/h；
(2) Keeping the temperature in the oxidation reaction tower at 200-220 ℃, the pressure in the tower body at 0.5-0.6MPa, carrying out catalytic reaction on durene and oxygen to generate pyromellitic acid, wherein the generated pyromellitic acid has low density and floats on the upper layer of the liquid level;
(3) pumping out the liquid on the upper layer of the liquid level with high pyromellitic acid content, and carrying out flash evaporation in a flash evaporation tank with the temperature of 195-205 ℃ under 0.02-0.03MPa to evaporate pyromellitic acid, acetic acid and other light components, thereby obtaining a crude pyromellitic acid product at the bottom of the tank;
(4) dissolving the crude pyromellitic acid in water at 90-95 ℃, decolorizing with active carbon, continuously filtering at constant temperature, continuously recrystallizing at 5-8 ℃, separating with a continuous centrifuge, and removing water to obtain pyromellitic acid crystal; the purity of pyromellitic acid crystal can reach 98 percent, and the conversion rate is 98 percent;
(5) the pyromellitic acid is dehydrated into anhydride at the temperature of 120 ℃ and the pressure of 0.02-0.03MPa to obtain the pyromellitic dianhydride, the purity can reach 99.5 percent, and the yield is 95 percent.
Example 3
A method for producing pyromellitic dianhydride by low-temperature liquid-phase continuous oxidation of durene comprises the following steps:
(1) continuously pumping mixed liquid of durene, acetic acid and a cobalt-manganese-bromine-zirconium-guanidine catalyst into an oxidation reaction tower, wherein the cobalt-manganese-bromine-zirconium-guanidine catalyst comprises cobalt acetate, manganese acetate, tetrabromoethane, zirconium acetate and guanidine acetate,the solvent of acetic acid is 236.25 kg durene, 1260.75kg acetic acid, the catalyst is cobalt acetate 1.05 kg, manganese acetate 1.005 kg, tetrabromoethane 0.285 kg, zirconium acetate 0.45 kg and guanidine acetate 0.21 kg, the weight contents of the corresponding substances are respectively: 15.75 percent of durene, 84.05 percent of acetic acid, 0.07 percent of cobalt acetate, 0.067 percent of manganese acetate, 0.019 percent of tetrabromoethane, 0.03 percent of zirconium acetate and 0.014 percent of guanidine acetate; feeding oxygen-enriched gas into the tower body while feeding liquid, wherein the oxygen-enriched gas is nitrogen-oxygen mixed gas with the oxygen volume content of 21%; the inlet flow is 1250 kg/h and the inlet flow is 950m3/h；
(2) The temperature in the oxidation reaction tower is kept at 250 ℃, the pressure in the tower body is 1.5MPa, the durene and oxygen catalytically react to generate the pyromellitic acid, and the generated pyromellitic acid has low density and floats on the upper layer of the liquid level;
(3) pumping out the liquid on the upper layer of the liquid level with high pyromellitic acid content, and carrying out flash evaporation in a flash evaporation tank at the temperature of 200 ℃ under the pressure of 0.05MPa to evaporate durene, acetic acid and other light components, thereby obtaining a crude pyromellitic acid product at the bottom of the tank;
(4) dissolving a crude pyromellitic acid product in water at the temperature of 90-95 ℃, decoloring by using activated carbon, continuously filtering at constant temperature, continuously recrystallizing at the crystallization temperature of 25 ℃, separating by using a continuous centrifuge, and removing water to obtain a pyromellitic acid crystal; the purity of pyromellitic acid crystal can reach 98 percent, and the conversion rate is 97 percent;
(5) the pyromellitic acid is dehydrated into anhydride under the environment of 185 ℃ and 0.4MPa to obtain the pyromellitic dianhydride, the purity can reach 99.9 percent, and the yield is 97 percent.
Example 4
A method for producing pyromellitic dianhydride by low-temperature liquid-phase continuous oxidation of durene comprises the following steps:
(1) continuously pumping mixed liquid of durene, acetic acid and a cobalt-manganese-bromine-zirconium-guanidine catalyst into an oxidation reaction tower, wherein the cobalt-manganese-bromine-zirconium-guanidine catalyst comprises cobalt acetate, manganese acetate, tetrabromoethane, zirconium acetate and guanidine acetate, the mixed liquid contains the acetic acid as a solvent, the durene is 260.4 kg, the acetic acid is 1237.5kg, and the catalyst is acetic acid0.6 kg of cobalt, 0.6 kg of manganese acetate, 0.3 kg of tetrabromoethane, 0.3 kg of zirconium acetate and 0.3 kg of guanidine acetate, wherein the weight contents of the corresponding substances are respectively as follows: 17.36 percent of durene, 82.5 percent of acetic acid, 0.04 percent of cobalt acetate, 0.04 percent of manganese acetate, 0.02 percent of tetrabromoethane, 0.02 percent of zirconium acetate and 0.02 percent of guanidine acetate; feeding oxygen-enriched gas into the tower body while feeding liquid, wherein the oxygen-enriched gas is nitrogen-oxygen mixed gas with the oxygen volume content of 50%; the liquid inlet flow is 1400 kg/h, and the air inlet flow is 1000m3/h；
(2) Keeping the temperature in the oxidation reaction tower at 200-300 ℃, the pressure in the tower body at 2.2MPa, carrying out catalytic reaction on durene and oxygen to generate pyromellitic acid, wherein the generated pyromellitic acid has low density and floats on the upper layer of the liquid level;
(3) pumping out the liquid on the upper layer of the liquid level with high pyromellitic acid content, and carrying out flash evaporation in a flash tank with the temperature of 195 ℃ under 0.03-0.05MPa to evaporate pyromellitic acid, acetic acid and other light components, thus obtaining a crude pyromellitic acid product at the bottom of the tank;
(4) dissolving crude pyromellitic acid in water at 90-95 ℃, decolorizing with active carbon, continuously filtering at constant temperature, continuously recrystallizing at 5-25 ℃, separating with a continuous centrifuge, and removing water to obtain pyromellitic acid crystal; the purity of pyromellitic acid crystal can reach 98.5 percent, and the conversion rate is 98 percent;
(5) the pyromellitic acid is dehydrated into anhydride at the temperature of 170 ℃ and 180 ℃ and under the pressure of 0.05-0.06MPa to obtain the pyromellitic dianhydride with the purity of 99.5 percent and the yield of 96.5 percent.
The present invention is not limited to the above-mentioned embodiments, and based on the technical solutions disclosed in the present invention, those skilled in the art can make some substitutions and modifications to some technical features without creative efforts according to the disclosed technical contents, and these substitutions and modifications are all within the protection scope of the present invention.
Claims (3)
1. A method for producing pyromellitic dianhydride by low-temperature liquid-phase continuous oxidation of durene is characterized by comprising the following steps:
(1) will be sym-tetramethylThe method comprises the following steps of continuously pumping mixed liquid of benzene, acetic acid and a cobalt-manganese-bromine-zirconium-guanidine catalyst into an oxidation reaction tower, wherein the cobalt-manganese-bromine-zirconium-guanidine catalyst comprises cobalt acetate, manganese acetate, tetrabromoethane, zirconium acetate and guanidine acetate, and in the mixed liquid, the acetic acid is used as a solvent, and the weight contents of the substances are as follows: 14.8 to 19.4 percent of durene, 80.4 to 84.8 percent of acetic acid, 0.035 to 0.09 percent of cobalt acetate, 0.04 to 0.1 percent of manganese acetate, 0.019 to 0.055 percent of tetrabromoethane, 0.007 to 0.032 percent of zirconium acetate and 0.014 to 0.033 percent of guanidine acetate; oxygen-enriched gas is continuously introduced into the tower body while liquid is fed, the liquid feeding flow is 1200-class 1500 kg/h, and the gas inlet flow is 950-class 1050m3/h；
(2) Keeping the temperature in the oxidation reaction tower at 200-300 ℃, the pressure in the tower body at 0.5-3MPa, carrying out catalytic reaction on durene and oxygen to generate pyromellitic acid, wherein the generated pyromellitic acid has low density and floats on the upper layer of the liquid level;
(3) pumping out the liquid on the upper layer of the liquid level with high pyromellitic acid content, and carrying out flash evaporation in a flash evaporation tank with the temperature of 195-205 ℃ under 0.02-0.08MPa to evaporate pyromellitic acid, acetic acid and other light components, thereby obtaining a crude pyromellitic acid product at the bottom of the tank;
(4) dissolving crude pyromellitic acid in water at 90-95 ℃, decolorizing with active carbon, continuously filtering at constant temperature, continuously recrystallizing at 5-45 ℃, separating with a continuous centrifuge, and removing water to obtain pyromellitic acid crystal;
(5) the pyromellitic acid is dehydrated into anhydride at the temperature of 120 ℃ and the temperature of 220 ℃ and under the pressure of 0.02-0.06MPa to obtain the pyromellitic dianhydride.
2. The method for producing pyromellitic dianhydride according to claim 1, wherein the oxygen-rich gas is air.
3. The method for producing pyromellitic dianhydride by low-temperature liquid-phase continuous oxidation of durene according to claim 1, wherein the oxygen-rich gas is a mixed gas of nitrogen and oxygen having an oxygen content of 21-50% by volume.
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