A continuous apparatus for producing terephthalic acid diester by oxidation of xylene with air

Abstract

This invention provides a continuous apparatus for the air oxidation of p-xylene to produce diterephthalate, comprising an oxidation unit, a flash evaporation unit, an esterification unit, and a distillation unit connected in series. This apparatus solves the bottleneck problem of process continuity and large-scale industrial continuous production in the process of air oxidation of p-xylene to p-methylbenzoic acid, further oxidation to terephthalic acid, and finally to diterephthalate. It enables the integrated production of diterephthalate compounds from continuous air oxidation, esterification, and separation of p-xylene. Furthermore, the apparatus has a simple structure, reasonable design, and wide applicability, facilitating large-scale application. It can also be used for the integrated continuous production of other organic compounds whose intermediate oxidation products are solid substances.

Description

Technical Field

[0001] This invention relates to an organic chemical reaction apparatus, and more particularly to a continuous apparatus for the air oxidation of p-xylene to produce diterephthalate, belonging to the field of chemical technology. Background Technology

[0002] Diterephthalate (DTB) is the most in-demand organic carboxylic acid compound and an important basic chemical raw material. It is mainly used in the production of polyester resins, polyester fibers, films, magnetic tapes, electroplated films, and beverage bottles. It can also be used in the production of insulating varnishes, adhesives, and printing inks. The raw materials for DTB are p-xylene and alcohol. Industrially, there are two methods for preparing DTB. One method involves the primary oxidation of p-xylene to p-methylbenzoic acid, followed by a secondary oxidation of p-methylbenzoic acid to terephthalic acid. Terephthalic acid then undergoes esterification with an alcohol to form terephthalate diester. This reaction is completed in three steps, including two oxidation steps and one esterification step. Another method uses p-xylene as a raw material. First, p-xylene is oxidized to p-methylbenzoic acid by air oxidation. Then, p-methylbenzoic acid is esterified with an alcohol to obtain p-methylbenzoate ester. This p-methylbenzoate ester is then oxidized to p-carboxybenzoate ester by air oxidation. Finally, an alcohol is added to esterify the p-carboxybenzoate ester to terephthalate diester. This reaction is completed in four steps, including two oxidation steps and two esterification steps. The first method produces terephthalate diester with one less reaction step, requiring less investment and having a simpler process. However, large-scale industrial production faces a bottleneck: both the primary oxidation product, p-methylbenzoic acid, and the secondary oxidation product, terephthalic acid, are solid products, making it impossible to directly continuousize the three-step process and achieve large-scale continuous industrial production of the aforementioned technology. Currently, the industrial process uses a mixed system containing 10% p-xylene, 80% acetic acid, and 10% water for p-xylene oxidation. This 90% solvent solution solves the problem of continuous primary and secondary oxidation processes in the production of terephthalic acid diester from p-xylene, but it still cannot solve the problems of continuous esterification of terephthalic acid and p-xylene oxidation, nor the problem of continuous production. Therefore, a second method has been applied industrially. The second method for producing terephthalic acid diester involves four steps in the same reactor. First, p-xylene is oxidized to p-methylbenzoic acid with an oxidant. Then, an alcohol is added for esterification to produce p-methylbenzoate. Next, an oxidant is added for secondary oxidation to oxidize p-methylbenzoate to p-carboxybenzoate. Finally, an alcohol is added again for secondary esterification to obtain the target product, terephthalic acid diester. This second method solves the problem of continuous four steps in the preparation of terephthalic acid from xylene, but it is still an intermittent industrial production process and cannot achieve large-scale continuous industrial production.

[0003] The reason why existing technology cannot achieve continuous industrial production of xylene to diterephthalate is that existing production equipment cannot solve the bottleneck technical problems of transferring solid intermediate acid and clogging pipelines involved in the production process. It is this technical problem that prevents the continuous production of diterephthalate from xylene. Summary of the Invention

[0004] In view of the shortcomings of existing methods for producing terephthalic acid diester from p-xylene, the purpose of this invention is to provide an integrated continuous production device that can realize the conversion of p-xylene into acid, the conversion of acid into ester, and the separation and purification of ester to obtain terephthalic acid diester. The device has a simple structure, reasonable design, wide applicability, and is conducive to large-scale promotion and application. It is particularly suitable for the integrated continuous production of terephthalic acid diester with solid intermediate oxidation products.

[0005] To achieve the above-mentioned technical objectives, the present invention provides a continuous apparatus for the air oxidation of p-xylene to produce diterephthalate, which is composed of an oxidation unit, a flash evaporation unit, an esterification unit and a distillation unit connected in series.

[0006] The apparatus provided by this invention realizes the integrated and continuous completion of air oxidation, esterification and purification of p-xylene compounds, and finally obtains diterephthalate, which solves the defect of the prior art that requires separate oxidation and esterification processes for the production process from p-xylene to diterephthalate.

[0007] As a preferred embodiment, the oxidation unit consists of 3-5 stages of oxidation reactors connected in series. Each stage of the oxidation reactor comprises an oxidizer R, a condenser L, and settling towers SA and SB. The upper part of the oxidizer R is connected to the condenser L via two pipes, the bottom of the oxidizer R is connected to the top of the settling towers SA and SB via a T-junction, the upper part of the oxidizer R is connected to the upper part of the settling towers SA and SB via a T-junction, the lower part of the settling towers SA and SB is connected to the lower part of the next adjacent oxidizer via a pipe, and the lower part of the settling towers SA and SB of the last stage of the oxidation reactor is connected to the flash evaporation unit via a T-junction.

[0008] As a preferred embodiment, the 3-5 stage series oxidation reactor of the oxidation unit includes one primary oxidation reactor, n intermediate oxidation reactors (n=1-3) connected in series, and a final oxidation reactor. The primary oxidation reactor consists of an oxidizer R11, a condenser L110, and two parallel settling towers S1A4 and S1B5. The upper part of the oxidizer R11 is connected to the condenser L1 via two pipes, the bottom of the oxidizer R11 is connected to the top of the settling towers S1A and S1B via a T-junction, and the lower parts of the settling towers S1A and S1B are connected to the lower part of the next adjacent oxidizer via pipes. The intermediate oxidation reactor consists of an oxidizer R11, a condenser L110, and a final oxidation reactor.n 2. Condenser L n 11 and two parallel settling towers S n A 6 and settling tower S n B7 components; Oxidizer R n The upper part of 2 is connected to the condenser L. n The two are connected by two pipes, oxidizer R n The bottom of the settling tower S n A and settling tower S n The top of B is connected via a T-junction pipe to the settling tower S. n A and settling tower S n The lower part of B is connected to the lower part of the next adjacent oxidizer via a pipe. The final oxidation reactor consists of oxidizer R3, condenser L3, and two parallel settling towers S3A and S3B. The upper part of the final oxidizer R3 is connected to the condenser L3 via two pipes. The bottom of the oxidizer R3 is connected to the settling towers S3A and S3B via a three-way pipe. The bottom of the settling towers S3A and S3B is connected to the flash unit via a three-way pipe.

[0009] As a preferred embodiment, in addition to being connected to the oxidizer itself, the condensate outlets of the condensers above all oxidation reactors in the oxidation unit are also connected to the condensate outlets of the condensers above adjacent reactors via T-junctions or four-way pipes. The condenser L1 of the first-stage oxidation reactor R1 or the L3 of the last-stage oxidation reactor R3 is connected to the adjacent intermediate oxidation reactor R... n L n The condenser condensate outlet is connected by a tee pipe, and all other intermediate oxidation reactors R n L n The condensate outlet is connected to the condensate outlets of two adjacent oxidation reactors L by a four-way pipe. The condensate can be selectively returned to any one or all reactors in any proportion by using three-way switches 81, 82 and four-way switch 87.

[0010] The oxidation reactor involved in this invention is a common reactor in the industry, such as a reactor with a stirrer or a bubbling reactor with a gravity settling plate. A common gas distributor can also be installed inside the reactor for the uniform distribution of the oxidizing gas (air or oxygen, etc.), which is beneficial for improving the gas-liquid reaction efficiency. This device, by designing 3-5 stages of reactors in series, can improve the oxidation conversion rate of p-xylene compounds.

[0011] As a preferred embodiment, the flash evaporation unit includes two flash towers, F1 and F2, connected in parallel. Flash towers F1 and F2 are connected to the oxidation unit and the esterification unit via T-junction pipes. The two flash towers, F1 and F2, are in parallel configuration. A T-junction pipe allows for switching between them, controlling the connection between one flash tower and the settling tower of the oxidation unit, and the other flash tower and the esterification reaction tower of the esterification unit. These two can be used alternately. The flash tower connected to the settling tower primarily serves as a storage container for the oxidation reaction liquid (containing solid products), while the flash tower connected to the esterification reaction tower is used to distill off the solvent or unreacted xylene.

[0012] As a preferred embodiment, flash towers F1 and F2 are provided with alcohol feed inlets 61 and 62 and feed and solvent vapor outlets 59 and 60 at their upper parts.

[0013] As a preferred embodiment, the esterification unit includes an esterification reaction tower 15.

[0014] As a preferred embodiment, the distillation unit includes distillation column I16 and distillation column II17 connected in series.

[0015] As a preferred embodiment, the oxidizer has a gas outlet at the top (for exhaust gas), a reaction liquid outlet at the top (for overflow of the reaction liquid), and an oxidation gas inlet (gas oxidant inlet) at the bottom. The first-stage reactor R1 has a para-xylene feedstock inlet at the bottom (for introducing para-xylene feedstock), and the intermediate reactor R... n The inlet of the reaction liquid in the final reactor R3 is located at the bottom of the reactor. During the continuous reaction process, the paraxylene feedstock enters the oxidizer from the bottom of the first-stage oxidizer and overflows from the top. In all subsequent oxidation reactors, the oxidizing liquid from the previous reactor is introduced from the bottom and flows out from the top. For all oxidation reactors, the gas enters from the bottom of the oxidizer and exits from the top into the condenser.

[0016] As a preferred embodiment, the bottom of the settling tower is provided with an acid product outlet (for discharging acid products from the settling tower), the acid product outlet is connected to the next adjacent reactor, and the bottom of the settling tower of the last reactor is connected to the flash evaporation unit.

[0017] As a preferred embodiment, flash distillation towers F1 and F2 are equipped with solvent and p-xylene feedstock outlets at the top (for the p-xylene feedstock after evaporation to be returned to the oxidation unit), an acid product inlet in the middle (connected to the bottom of the settling tower), an alcohol feedstock inlet at the top (for alcohol introduction), and an acid outlet at the bottom (connected to the esterification reaction tower). Unreacted p-xylene feedstock and solvent distilled from flash distillation tower F1 or F2 are recovered from the p-xylene feedstock outlet, and then returned to the oxidation unit for recycling after simple cooling, water washing, and settling separation. The purpose of the alcohol feedstock inlet is to introduce alcohol feedstock for the esterification reaction. However, the device of this invention places the alcohol feedstock inlet in the flash evaporation unit, rather than in the esterification reaction unit. This is mainly to solve the technical problem of the difficulty in transferring the solid product terephthalic acid in continuous production. In the flash evaporation unit, after the xylene feedstock is removed by flash evaporation, the solid product of terephthalic acid will partially crystallize out, resulting in poor fluidity. Introducing the alcohol feedstock can significantly improve the fluidity of the acid, thereby enabling the transfer of the solid product of terephthalic acid without the need for additional solvents, which is beneficial for continuous production.

[0018] The actual production process of the apparatus provided by this invention for producing diterephthalate from p-xylene feedstock is as follows: p-xylene feedstock containing dissolved catalyst and solvent are continuously introduced into the interior of the first oxidizer R1 through the p-xylene feedstock inlet at the bottom of the first oxidizer R1, sequentially filling the primary oxidizer R1 and the intermediate oxidizer R2. n and the final oxidizer R3 (approximately at the height of oxidizer R2 / 3) and all settling towers S1A, S n A, S3A, S1B, S n B, S3B; Connect the primary oxidizer R1 and the intermediate oxidizer R... n The lower part of the final oxidizer R3 is controlled by a three-way pipe to connect with its corresponding settling tower S1A and settling tower S1A. n A and the upper part of the settling tower S3A are connected, connecting the primary oxidizer R1 and the intermediate oxidizer R n The oxidant outlet at the top of the final oxidizer R3 is controlled via a three-way pipe to connect to its corresponding settling tower S1B and settling tower S1B. n B and the upper part of the settling tower S3B are connected, and the settling tower S1B and the settling tower S are connected. n The lower part of B is connected to the next stage oxidation reactor (R) n The upper part of the settling tower S3B is connected to the oxidizing liquid inlet of the oxidizer R1, and the lower part of the settling tower S3B is connected to the flash tower F1; from the oxidizer R1, R n Air or oxygen is introduced into the R3 oxidizing gas inlet, and the temperature is raised to carry out the oxidation reaction. Oxidizers R1 and R2... n The high-temperature reaction exhaust gases from R3 and R3 enter condensers L1 and L2 respectively through the reaction gas outlet. nL3, the coolant in the condenser returns to the oxidizer R1 and R2 via the coolant inlet. n and R3; oxidizer R1, R n The solid products of p-methylbenzoic acid and terephthalic acid generated in R3 settle into settling towers S1A and S1A respectively under gravity from the solid product outlet and inlet. n A, S3A, and simultaneously settling towers S1A and S n A. Paraxylene feedstock in S3A is replaced and enters oxidizers R1 and R2. n It undergoes an oxidation reaction with R3. Settling tower S1A and settling tower S... n When solid products precipitate from settling tower S3A and settling tower A, switch to oxidizer R1 and oxidizer R2 via a three-way valve. n The oxidizer R3 and its corresponding settling towers S1B and S1B are respectively connected to the settling tower S. n B and settling tower S3B are connected. After switching, settling tower S1A and settling tower S... n A and settling tower S3A and oxidizers R1 and R n Disconnect from R3, oxidizers R1 and R n The p-methylbenzoic acid and terephthalic acid solids generated in R3 settle into settling towers S1B and S1B respectively under gravity from the solid product outlet and inlet. n B, S3B, from oxidizer R1, R n The reaction liquid overflowing from the top of R3 flows into settling towers S1A and S2A, which are disconnected from the reactor. n A, S3A: Due to the height difference between the outlet of the preceding settling tower and the inlet of the following reactor, the reaction liquid flowing from the bottom of settling tower S1A enters the intermediate oxidizer R. n The reaction liquid enters R through the inlet. n Secondary oxidation is carried out inside the settling tower S. n The reaction liquid flowing out from the bottom of A enters the final oxidizer R3 through the reaction liquid inlet for tertiary oxidation, and the reaction liquid flowing out from the bottom of settling tower S3A enters flash evaporator F1 for flash evaporation; the reaction liquid from settling towers S1A and S... n For either A or S3A, once there is no solid p-methylbenzoic acid and terephthalic acid left in the settling tower, the oxidation reactor R (R1 or R) will be activated. n Or R3) The solid product flow outlet at the bottom is switched to the corresponding settling tower SA (S1A or S) n Connect A or S3A) and simultaneously connect oxidation reactor R (R1 or R) n Or R3) The overflowing reaction liquid is switched to the corresponding settling tower SB (S1B or S) nConnecting to (B or S3B) allows for the alternating settling of p-toluic acid and terephthalic acid solid products generated in oxidizer R between settling towers A and B. The reaction liquid entering flash tower F1 undergoes sudden decompression, causing the p-xylene feedstock and solvent in F1 to flash into gaseous form. The high-boiling-point terephthalic acid product remains in flash tower F1. By controlling the flash pressure and temperature, p-xylene and solvent are completely separated from the oxidation product terephthalic acid. The p-xylene and solvent separated by flash evaporation are cooled, washed with water, and allowed to settle before being recycled. Once the p-xylene and solvent in flash tower F1 are completely flashed, alcohol is introduced. The terephthalic acid solids in flash tower F1 are carried by the alcohol to the esterification tower for esterification. After all the terephthalic acid in flash tower F1 has been carried to the esterification tower, the addition of alcohol to flash tower F1 is stopped, and the pressure in flash tower F1 is maintained. The pressure of the oxidation reaction system is consistent with that of the 1-flash tower. Flash tower F1 is switched to be connected to the settling tower again. Through the continuous switching between the oxidation unit and flash towers F1 and F2, the terephthalic acid obtained from the oxidation unit is separated from the unreacted p-xylene and solvent in the flash unit and enters the esterification unit. In the esterification unit, terephthalic acid and p-methylbenzoic acid (if any) undergo catalytic esterification with alcohol to produce p-methylbenzoate and diterephthalate. Then, the mixture is introduced into distillation tower I to separate the alcohol and p-methylbenzoate. The alcohol is returned to the flash unit, the p-methylbenzoate is returned to the oxidation unit, and the diterephthalate enters distillation tower II for high-purity diterephthalate separation.

[0019] The innovative design of the multi-stage series oxidation reactor in the device provided by this invention is as follows: First, the reaction liquid flowing out of all oxidizers R does not enter the next stage oxidation reactor for continuous oxidation. Instead, the reaction liquid outlet of oxidizer R flows through the settling tower SB, which is disconnected from the current oxidizer R, before flowing into the next stage oxidation reactor for continuous oxidation. This solves the problem of the transfer of p-methylbenzoic acid and terephthalic acid solid products generated during the oxidation reaction between the oxidation reactors and prevents pipeline blockage from hindering continuous production. It also avoids the deposition of p-methylbenzoic acid and terephthalic acid solids at the bottom of the reactor. The key to this device is the use of the p-xylene feedstock overflowing from the oxidizer R to improve the flowability of p-methylbenzoic acid and terephthalic acid products in each settling tower, thereby achieving the transfer of solid p-methylbenzoic acid and terephthalic acid products. Secondly, the condensate outlets on all condensers connected to the oxidation reactor are connected via T-junctions or four-way pipes. The condensate can be selectively returned to any or all reactors in any proportion using three-way switches 81 and 82 and four-way switches 87. This design facilitates the adjustment of the residence time distribution of p-xylene in different oxidation reactors and allows control of the ratio and oxidation depth of p-methylbenzoic acid and terephthalic acid within the oxidation reactors, achieving ideal oxidation selectivity. Thirdly, this invention places the alcohol feedstock inlet in the flash evaporation unit, rather than in the esterification reaction unit, solving the technical problem of transferring solid terephthalic acid from the flash evaporation tower to the esterification tower, thus achieving the transfer of solid terephthalic acid products from the flash evaporation tower to the esterification tower, which is beneficial for continuous production.

[0020] Compared with the prior art, the beneficial technical effects of this invention are as follows:

[0021] The apparatus provided by this invention enables the continuous production of diterephthalate using p-xylene as a direct raw material through integrated operations such as air oxidation, esterification, and separation. This overcomes the shortcomings of existing technologies where separate intermittent operation steps are required for processes such as the oxidation and esterification of p-xylene, making it difficult to achieve continuous production.

[0022] The device provided by this invention has a simple structure and reasonable design, and meets the continuous production requirements for preparing the corresponding diterephthalate from p-xylene.

[0023] The device provided by this invention uses a multi-stage series oxidation reactor, which can ensure the oxidation conversion efficiency of p-xylene. Moreover, by designing parallel settling towers, continuous operation from the oxidation unit to the separation unit can be achieved.

[0024] The device provided by this invention can significantly reduce the amount of solvent used, thereby reducing energy consumption and production costs during the production process.

[0025] The device provided by this invention can realize the recovery and reuse of reaction raw materials and intermediate products, reducing the generation of waste liquid.

[0026] The apparatus provided by this invention solves the technical problem of the difficulty in transferring intermediate solid acid products in the continuous production process from paraxylene feedstock to acid and then to ester. This apparatus, on the one hand, connects the reaction liquid outlet of oxidizer R3 to settling towers S1A and S1B, and settling tower S... n A and settling tower S n B. The tops of settling towers S3A and S3B are connected, and the transfer of acid solid products between the oxidation unit and the flash unit is achieved by using p-xylene-type reaction feedstock. On the other hand, the methanol feedstock inlet is designed in the flash unit, and the transfer of solid acid between the flash unit and the esterification unit is achieved by using alcohol feedstock. This realizes a continuous production process of the reaction system containing solid intermediate products, and there is no need to introduce a large amount of solvent into the reaction system.

[0027] The device provided by this invention can also be used for continuous production in chemical processes that generate solid intermediates or products. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the apparatus used for the air oxidation of paraxylene feedstock to produce diterephthalate.

[0029] Wherein, 1 is the primary oxidizer R1, 2 is the intermediate oxidizer Rn, 3 is the final oxidizer R3, 4 is the settling tower S1A, 5 is the settling tower S1B, and 6 is the settling tower S... n A, 7 are settling towers S n B, 8 are settling tower S3A, 9 are settling tower S3B, 10 are gas cooler L1, and 11 are gas cooler L n12 is gas cooler L3, 13 is flash tower F1, 14 is flash tower F2, 15 is esterification reaction tower, 16 is distillation tower I, 17 is distillation tower II, 18 is back pressure valve, 19-21 are gas inlets, 22 is p-xylene and solvent inlet, 23-24 are oxidation liquid inlet, 25-27 are oxidation liquid outlet, 28-30 are reaction gas outlet, 31-33 are mixed acid flow outlet of oxidation products p-methylbenzoic acid and terephthalic acid, 34-37 are mixed acid flow inlet of p-methylbenzoic acid and terephthalic acid, 38-39 are terephthalic acid, 40-45 are oxidation liquid outlet, 46-51 are oxidation liquid inlet of final reactor, 51-54 are... Condensate inlet, 55-56 oxidizing liquid inlet, 57-58 alcohol and terephthalic acid mixture outlet, 59-60 solvent and p-xylene vapor outlet, 61-62 alcohol raw material inlet, 63 alcohol and terephthalic acid mixture inlet, 64 esterification liquid outlet, 65 esterification liquid inlet, 66 alcohol vapor outlet, 67 non-volatile non-esterification oxidation component vapor outlet, 68 p-methylbenzoate and high-boiling-point liquid outlet, 69 p-methylbenzoate and high-boiling-point liquid inlet, 70 terephthalate diester outlet, 71 non-ester high-boiling-point liquid outlet, 72-86 three-way connection point, 87-88 four-way connection point. Detailed Implementation

[0030] The following specific embodiments are intended to provide a detailed description of the present invention in conjunction with the accompanying drawings, but the scope of protection of the present invention is not limited to the specific embodiments.

[0031] The present invention provides an apparatus for producing diterephthalate from p-xylene feedstock, specifically as follows: Figure 1 As shown. The main body of the device consists of an oxidation unit, a flash evaporation unit, an esterification unit, and a distillation unit connected in series. The oxidation unit is used to oxidize p-xylene feedstock into terephthalic acid by air. It consists of a primary oxidation reactor, an intermediate oxidation reactor, and a final oxidation reactor connected in series. The primary oxidation reactor consists of an oxidizer R11, a condenser L110, and two parallel settling towers S1A4 and S1B5. The oxidizer R1 is connected to the settling towers S1A and S1B via a three-way pipe. The oxidizer R1 can be switched to connect to the settling tower S1A or the settling tower S1B. The intermediate oxidation reactor consists of an oxidizer R11, an intermediate oxidation reactor, and a final oxidation reactor. n 2. Condenser L n 11 and two parallel settling towers S n A 6 and settling tower S n B7 is composed of oxidizer R. n With settling tower S n A and settling tower S n B is connected to the oxidizer R via a three-way pipe. n With settling tower Sn A or oxidizer R n With settling tower S n B can be switched and connected; the final oxidation reactor consists of oxidizer R3 3, condenser L3 12, and two parallel settling towers S3A 8 and S3B 9. Oxidizer R3 and settling towers S3A and S3B are connected by a three-way pipe, and oxidizer R3 and settling tower S3A or oxidizer R3 and settling tower S3B can be switched and connected; the reaction liquid outlet 25 of oxidizer R1 is connected to oxidizer R n The oxidizing liquid inlet 23 is connected to the oxidizer R. n The reaction liquid outlet 26 is connected to the oxidation liquid inlet 24 of oxidizer R3, and the reaction liquid outlet 27 of oxidizer R3 is connected to settling towers S1A and S1B, and settling tower S n A and settling tower S n B. The final reaction liquid inlets 46-51 at the top of settling towers S3A and S3B are connected via T-junctions, which can control the connection between oxidizer R3 and settling towers S1A and S2B. n A and settling tower S3A or settling tower S1B, settling tower S n B is connected to settling tower S3B. The bottom of each settling tower is connected to the flash evaporation unit via a pipe. The flash evaporation unit is mainly used to separate acid products from unreacted p-xylene feedstock, recover the p-xylene feedstock, and realize its recycling. The flash evaporation unit includes flash towers F1 13 and F2 14 connected in parallel. Flash towers F1 and F2 are connected to the oxidation unit and the esterification unit via T-junction pipes. The T-junction can switch the connection between flash tower F1 and the oxidation unit and flash tower F2 and the esterification unit, or switch the connection between flash tower F2 and the oxidation unit and flash tower F1 and the esterification unit. The bottom of the settling tower of the oxidation unit is connected to the acid product inlet 55 in the middle of flash towers F1 and F2 via a T-junction pipe. Flash towers F1 and F2 have a p-xylene feedstock outlet 59 at the top and an acid outlet 57 and an alcohol feedstock inlet 61 at the bottom. The acid outlet at the bottom of the flash distillation tower is connected to the esterification unit, which is primarily an esterification reaction tower filled with conventional esterification catalysts, mainly used for the esterification reaction of acids and alcohols. The esterification unit is connected to the distillation unit, which includes two distillation columns, I16 and II17, connected in series. Distillation column I is used to separate the esterification reaction products from the alcohol feedstock, while distillation column II is used for the distillation separation of various terephthalic acid diesters.

[0032] The apparatus provided by this invention is used for the air oxidation of p-xylene feedstock to produce diterephthalate. The process is as follows: the primary oxidizer R11 and the intermediate oxidizer R... n 2. Oxidizer R3; 3. Settling tower S1A; 4. Settling tower S1B; 5. Settling tower S...n A 6, Settlement Tower S n B 7, Settling tower S3A 8, Settling tower S3B 9 are filled with p-xylene feedstock containing dissolved catalyst; First-stage oxidizer R1 1, intermediate oxidizer R n 2 and oxidizer R3 are connected to their respective settling towers S1A and S4 via a three-way pipe. n A6 and the upper part of the settling tower S3A8 are connected, connecting the primary oxidizer R11 and the intermediate oxidizer R n The oxidant outlets 25, 26, and 27 at the top of oxidizer R3 are controlled by three-way pipes to their corresponding settling towers S1B and S1B, respectively. n B and the upper part of the settling tower S3B are connected at 46, 48, and 50. Settling tower S1B and settling tower S n The lower parts of B, 41 and 43, are connected to the next stage oxidation reactor (R). n The oxidizing liquid inlets 23 and 24 at the bottom of R3B are connected to the oxidizing liquid inlet 55 at the bottom of the settling tower S3B and the flash tower F1; the oxidizing gas inlet 19 at the bottom of the oxidizer R1 and the oxidizer R n Oxidizing gas inlet 20, oxidizer R3, oxidizing gas inlet 21 to oxidizer R1, R n Air or oxygen is introduced into R3, and the temperature is raised to carry out the oxidation reaction. Oxidizers R1 and R2... n The high-temperature reaction tail gases from R3 enter condensers L1 and L2 respectively through reaction gas outlets 28, 29, and 30. n The coolant in the condenser returns to the oxidizer R1 and R2 via coolant inlets 52, 53 and 54. n and R3; oxidizer R1, R n The solid acid products generated in R3, under the influence of gravity, flow out of solid product outlets 31, 32, and 33 respectively through settling towers S1A and S1B. n Solid products from A and S3A flow into settling towers S1A and S3A via inlets 34, 36, and 38, settling into the settling towers. n A, S3A, and simultaneously settling towers S1A and S n A. Paraxylene feedstock in S3A is replaced and enters oxidizers R1 and R2. n It undergoes an oxidation reaction with R3, until the sedimentation towers S1A and S1A are settled. n When solid products precipitate from settling tower S3A and settling tower A, switch to oxidizer R1 and oxidizer R2 via a three-way valve. n The oxidizer R3 and its corresponding settling towers S1B and S1B are respectively connected to the settling tower S. n B and settling tower S3B are connected. After switching, settling tower S1A and settling tower S... n A and settling tower S3A and oxidizers R1 and R nDisconnect from R3, oxidizers R1 and R n The p-methylbenzoic acid and terephthalic acid solids generated in R3, under the influence of gravity, flowed from solid product outlets 31, 32, and 33 through settling towers S1B and S1B, respectively. n Solid products from B and S3B flow into settling towers S1B and S3B via inlets 35, 37, and 39, settling into the settling towers. n B and S3B thus achieve alternating settling of the solid acid products generated in the oxidizer R between settling towers A and B. During the continuous reaction process, with the oxidizer R connected to settling tower SB and disconnected from settling tower SA, the p-xylene feedstock containing the catalyst and solvent are continuously introduced into the first oxidizer R1 from the feedstock inlet 22 at the bottom. The p-xylene feedstock undergoes its first oxidation in oxidizer R1, and the resulting solid acid product settles into settling tower S1B under gravity from the solid product outlet 32 ​​through the solid product inlet 35. The reaction liquid overflows from the reaction liquid outlet 25 at the top of oxidizer R1 and enters settling tower S1A through the inlet 46 at the top of settling tower S1A, carrying the solid p-methylbenzoic acid from settling tower S1A, and then flows from the intermediate oxidizer R... n The reaction liquid enters R through inlet 23. n Secondary oxidation is carried out inside, oxidizer R n The solid acid product generated in the process flows out of the solid product outlet 32 ​​and into the settling tower S under the action of gravity. n The solid products of B settle into the settling tower S at inlet 37. n B, simultaneously settling tower S n The para-xylene feedstock in B is replaced and enters oxidizer R. n Oxidation occurs, and the reaction solution flows from oxidizer R. n The overflow from the upper reaction liquid outlet 26 is discharged through the settling tower S. n The upper inlet 48 leads into the settling tower S. n A, with sinking tower S n Solid p-methylbenzoic acid and terephthalic acid in A, from settling tower S n The reaction liquid outlet 42 at the bottom of A enters the final oxidizer R3 through the reaction liquid inlet 24 for final oxidation. The resulting solid product, terephthalic acid, settles into the settling tower S3B under gravity through the reaction liquid outlet 33 and the solid product inlet 39. The reaction liquid is discharged from the reaction liquid outlet 27 at the bottom of oxidizer R3 and enters the settling tower S3A through the final reactor oxidation liquid inlet 50. This carries the terephthalic acid product from the settling tower S3A into the flash tower F1 through the oxidation liquid outlet 44 and the oxidation liquid inlet 55 for stirring and flash evaporation. The reaction liquid is then discharged from the settling towers S1A and S2A. nFor either A or S3A, once there is no solid p-methylbenzoic acid and terephthalic acid left in the settling tower, the oxidation reactor R (R1 or R) will be activated. n Or R3) The solid product flow outlet at the bottom is switched to the corresponding settling tower SA (S1A or S) n Connect A or S3A) and simultaneously connect oxidation reactor R (R1 or R) n Or R3) The overflowing reaction liquid is switched to the corresponding settling tower SB (S1B or S) n Connecting to (B or S3B) allows for the alternating settling of p-toluic acid and terephthalic acid solid products generated in oxidizer R between settling towers A and B. In flash tower F1, due to sudden depressurization, the low-boiling-point p-xylene feedstock and solvent flash into gas, while the high-boiling-point acid products remain in flash tower F1. Controlling the flash pressure and temperature of the flash tower ensures complete separation of p-xylene and oxidation products. The p-xylene and solvent separated by flash evaporation are cooled, washed with water, and allowed to settle before being recycled. After the p-xylene and solvent in flash tower F1 are completely flashed, alcohol is introduced. The terephthalic acid solids in flash tower F1 are carried by the alcohol to the esterification tower for esterification. Once all the terephthalic acid in flash tower F1 has been carried to the esterification tower, alcohol addition to flash tower F1 is stopped, and pressure is maintained in flash tower F1 to continue the oxidation reaction. With the system consistent, flash tower F1 is switched back to be connected to the settling tower. Through continuous switching between the oxidation unit and flash towers F1 and F2, the terephthalic acid obtained from the oxidation unit is ensured to be separated from unreacted p-xylene and solvent in the flash unit and enter the esterification unit. In the esterification unit, terephthalic acid and p-methylbenzoic acid (if present) undergo catalytic esterification with alcohol to produce p-methylbenzoate and diterephthalate. Then, the mixture is introduced into distillation tower I to separate the alcohol and p-methylbenzoate. The alcohol is returned to the flash unit, the p-methylbenzoate is returned to the oxidation unit, and the diterephthalate enters distillation tower II for high-purity diterephthalate separation.

[0033] In the above process, the connection selection of three-way switch 72-74 realizes the connection switching between reactor R and settling towers SA and SB; the connection selection of three-way switch 78-80 realizes the connection switching between the reaction liquid flowing out of the final oxidation reactor and settling towers SA and SB; the connection selection of three-way switch 75-77 realizes the connection switching between the effluent from settling towers SA and SB and flash evaporator F; the connection selection of three-way switch 83 realizes the connection switching between flash evaporator F1 and flash evaporator F2 and the effluent from the settling tower; and the connection selection of three-way switch 84 realizes the connection switching between flash evaporator F1 and flash evaporator F2 and the esterification tower.

[0034] Application Implementation

[0035] The following examples illustrate the production of dimethyl terephthalate from p-xylene using the apparatus provided by the present invention.

[0036] Example 1

[0037] 1) A mixture of 30% p-xylene, 70% chlorobenzene, and 30 PPM tetraphenylporphyrin cobalt was fed into a three-stage oxidation reaction tower. The oxidation reaction conditions were: temperature 165℃, total reaction time 3.5 hours, and air pressure 14 atm. Solid terephthalic acid and a very small amount of p-methylbenzoic acid products accumulated in the settling tower. The liquid escaping from the R3 oxidizer was chlorobenzene and a small amount of p-xylene. The p-xylene conversion rate was 95%.

[0038] 2) Terephthalic acid, a very small amount of p-methylbenzoic acid and chlorobenzene escaping from the R3 oxidizer are mixed and flow into the flash tower. The conditions of the flash tower are controlled as follows: temperature is 120℃ and pressure is 0.5 atm. The evaporated chlorobenzene and xylene are cooled and recovered and returned to the solvent tank for recycling. The remaining mixture in the flash tower is terephthalic acid and a very small amount of p-methylbenzoic acid.

[0039] 3) The mixture in the flash evaporator is mixed and dissolved with methanol and then enters the esterification reaction tower for esterification reaction. The esterification reaction conditions are: the molar ratio of methanol to mixed acid is 6:1 (measured by the molar amount of carboxyl groups in the mixed acid), and the reaction is carried out at 75°C for 8 hours in the presence of a solid acid catalyst. The esterification conversion rate of the mixed acid is 98%.

[0040] 4) The esterification reaction product is introduced into the first distillation column: Excess methanol is first recovered by distillation at a pressure of 84 kPa and a temperature of 48°C. The methanol is returned to the esterification reaction process for recycling. Then, methyl p-methylbenzoate is recovered by distillation at a pressure of 33 kPa and a temperature of 120°C. The methyl p-methylbenzoate is returned to the primary oxidation reactor for further oxidation.

[0041] 5) The remaining esterification product is introduced into the second distillation column for vacuum distillation. The number of trays is 25. The distillation process is as follows: pressure is 1.5 kPa, bottom temperature is 125℃. 99% dimethyl terephthalate is collected from the top of the distillation vessel. The residue flows out from the bottom of the distillation vessel and enters the waste treatment system.

[0042] Example 2

[0043] 1) A mixture of 35% p-xylene, 70% biphenyl, 30 PPM tetraphenylporphyrin iron, and 25 PPM cobalt acetate was fed into a five-stage oxidation reaction tower. The oxidation reaction conditions were: temperature 170℃, total reaction time 2.5 hours, and air pressure 15 atm. Solid terephthalic acid and a very small amount of p-methylbenzoic acid products accumulated in the settling tower. The liquid escaping from the R5 oxidizer was biphenyl and a small amount of p-xylene, with a p-xylene conversion rate of 98%.

[0044] 2) Terephthalic acid, a very small amount of p-methylbenzoic acid and biphenyl escaping from the R3 oxidizer are mixed and flow into the flash tower. The conditions of the flash tower are controlled as follows: temperature is 130℃ and pressure is 0.5 atm. The evaporated biphenyl and xylene are cooled and recovered and returned to the solvent tank for recycling. The remaining mixture in the flash tower is terephthalic acid and a very small amount of p-methylbenzoic acid.

[0045] 3) The mixture in the flash evaporator is mixed and dissolved with methanol and then enters the esterification reaction tower for esterification reaction. The esterification reaction conditions are: the molar ratio of methanol to mixed acid is 5:1 (measured by the molar amount of carboxyl groups in the mixed acid), and the reaction is carried out at 85°C for 6 hours in the presence of a solid acid catalyst. The esterification conversion rate of the mixed acid is 98%.

[0046] 4) The esterification reaction product is introduced into the first distillation column: Excess methanol is first recovered by distillation at a pressure of 84 kPa and a temperature of 48°C. The methanol is returned to the esterification reaction process for recycling. Then, methyl p-methylbenzoate is recovered by distillation at a pressure of 33 kPa and a temperature of 120°C. The methyl p-methylbenzoate is returned to the primary oxidation reactor for further oxidation.

[0047] 5) The remaining esterification product is introduced into the second distillation column for vacuum distillation. The number of trays is 25. The distillation process is as follows: pressure is 1.5 kPa, bottom temperature is 125℃. 99% dimethyl terephthalate is collected from the top of the distillation vessel. The residue flows out from the bottom of the distillation vessel and enters the waste treatment system.

Claims

1. A continuous apparatus for the air oxidation of p-xylene to produce diterephthalate, characterized in that: The reactor is composed of an oxidation unit, a flash evaporation unit, an esterification unit, and a distillation unit connected in series. The oxidation unit consists of 3-5 stages of oxidation reactors connected in series. Each stage of the oxidation reactor comprises an oxidizer R, a condenser L, a settling tower SA, and a settling tower SB. The upper part of the oxidizer R is connected to the condenser L via two pipes. The bottom of the oxidizer R is connected to the top of the settling towers SA and SB via a three-way pipe. The upper part of the oxidizer R is connected to the upper part of the settling towers SA and SB via a three-way pipe. The lower part of the settling towers SA and SB is connected to the next adjacent oxidation unit. The lower part of the incubator is connected by a pipe, and the lower parts of the settling towers SA and SB of the last-stage oxidation reactor are connected to the flash evaporation unit by a T-junction pipe; the flash evaporation unit includes two flash evaporation towers F1 and F2 connected in parallel with stirring. Flash evaporation towers F1 and F2 are connected to the oxidation unit and to the esterification unit by T-junction pipes, respectively. The upper part of flash evaporation towers F1 and F2 is provided with alcohol feed inlet; the esterification unit includes an esterification reaction tower; the distillation unit includes two distillation towers I and II connected in series.

2. The continuous apparatus for producing diterephthalate by air oxidation of p-xylene according to claim 1, characterized in that: In addition to being connected to the oxidizer itself, the condensate outlets of the condensers at the top of all oxidation reactors are also connected to the condensate outlets of the condensers at the top of adjacent reactors via T-junctions or four-way pipes. The condenser L1 of the first-stage oxidation reactor R1 or the condenser L5 of the last-stage oxidation reactor R5 is connected to the adjacent intermediate oxidation reactor R... n L n The condenser condensate outlet is connected by a tee pipe, and all other intermediate oxidation reactors R n condenser L n The condensate outlet is connected to the condensate outlets of two adjacent oxidation reactors L by a four-way pipe. The condensate can be selectively returned to any one or all reactors in any proportion by using a three-way switch and a four-way switch.

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