Method and system for continuously producing p-xylene

Abstract

The invention relates to a method and system for continuously producing p-xylene, the system comprises a raw material unit, a crystallization unit, a recovery unit and a refrigeration unit, the raw material unit comprises a heat exchange equipment group; the crystallization unit comprises raw material mixing equipment, a first crystallization equipment group, first separation equipment and product storage equipment which are connected in sequence; the recovery unit comprises a second crystallization equipment group and second separation equipment which are connected in sequence; the refrigeration unit is respectively connected with the raw material unit, the crystallization unit and the recovery unit. The method is wide in applicability to the concentration range of p-xylene in the raw materials, no process waste liquid is continuously discharged, the blocking phenomenon extremely prone to occurring in the crystallization process is effectively avoided while the quality and yield of the p-xylene product are guaranteed, and the stability and continuity of the p-xylene crystallization process are guaranteed.

Description

Technical Field

[0001] The present disclosure relates to a method and a system for continuously producing p-xylene. Background Art

[0002] Xylene, as one of C8 aromatics, is an important basic raw material in petrochemical industry. Among xylenes, the most used one is p-xylene, which can be oxidized at high temperature to produce terephthalic acid, and this product is the main raw material for polyester. In recent years, the production technology of p-xylene has made great progress, continuously developing towards large-scale production, diversified raw materials, and integrated technology, and at the same time paying more attention to issues such as energy conservation and carbon reduction.

[0003] At present, the industrialization of producing high-purity p-xylene by applying simulated moving bed adsorption separation technology has been mature, and high-purity p-xylene production is achieved by selecting appropriate adsorbents and desorbing agent systems. However, the p-xylene adsorption separation technology is relatively complex and has high requirements for the impurity content of the feed components. In addition to the development and selection of adsorbents and desorbing agents, it also involves a distillation system, with large fuel consumption and high direct carbon emissions.

[0004] The separation of C8 aromatics refers to the separation of C8 aromatic isomers mainly including o-xylene, m-xylene, p-xylene and ethylbenzene. The melting point differences between these isomers are obvious, and the melting point of p-xylene is much higher than that of other C8 aromatics, making it very suitable for the suspension crystallization separation method, and high-purity p-xylene products can be obtained from C8 aromatic mixtures with high impurity content. The process setting range of the p-xylene crystallization temperature depends on the concentration of p-xylene in C8 aromatics. The crystallization process requires a low-temperature heat source. When using the indirect refrigeration method, the refrigeration area is restricted by the size of the crystallizer. Therefore, it is very necessary to improve the refrigeration efficiency and it is advisable to strengthen the adaptability of the production method to the concentration of raw material components as much as possible; during the cooling process, if the temperature difference of the crystallizer is not controlled properly, fine crystals of p-xylene are likely to be generated in the crystallizer and the corresponding pipelines, and the crystal slurry is likely to deposit in the pipelines, resulting in a decrease in heat transfer efficiency, blockage of equipment and pipelines, and in severe cases, the device stops running, affecting the product quality. Summary of the Invention

[0005] The purpose of the present disclosure is to provide a method and a system for continuously producing p-xylene, which can ensure continuous production and improve product quality.

[0006] To achieve the above purpose, in the first aspect of the present disclosure, a continuous production system is provided, which includes a raw material unit, a crystallization unit, a recovery unit and a refrigeration unit.

[0007] The raw material unit includes a heat exchange equipment group.

[0008] The crystallization unit includes a raw material mixing device, a first crystallization equipment group, a first separation device, and a product storage device that are connected in sequence. The raw material mixing device is connected to the heat exchange equipment group. The first separation device has a first liquid phase outlet and a second liquid phase outlet of the first separation device. The first liquid phase outlet of the first separation device is connected to the raw material mixing device through a pipeline;

[0009] The recovery unit includes a second crystallization equipment group and a second separation device that are connected in sequence. The second crystallization equipment group is connected to the second liquid phase outlet of the first separation device through a pipeline. The second separation device is connected to the raw material mixing device;

[0010] The refrigeration unit is respectively connected to the raw material unit, the crystallization unit, and the recovery unit. The refrigeration unit is used to respectively provide a first refrigeration medium to the raw material unit, a second refrigeration medium to the crystallization unit, and a third refrigeration medium to the recovery unit.

[0011] Optionally, the heat exchange equipment group includes an optional first heat exchange module, an optional second heat exchange module, and a third heat exchange module. The raw material mixing device has a first cooling raw material inlet. The third heat exchange module is connected to the first cooling raw material inlet through a pipeline;

[0012] Optionally, the product storage device is connected to the first heat exchange module through a pipeline. The second separation device has a second separation device liquid phase outlet and a second separation device solid phase outlet. The second separation device liquid phase outlet is connected to the second heat exchange module through a pipeline. The second separation device solid phase outlet is connected to the raw material mixing device through a pipeline. A second cooling raw material inlet is provided on the pipeline between the second separation device solid phase outlet and the raw material mixing device. The third heat exchange module is also connected to the second cooling raw material inlet through a pipeline. The refrigeration unit is connected to the third heat exchange module through a pipeline.

[0013] Optionally, the first crystallization equipment group includes a plurality of first crystallizers connected in parallel. Each first crystallizer is connected to the slurry inlet of the first separation device through a corresponding first slurry pump;

[0014] Optionally, a first heat preservation jacket is provided on the outer wall of the first crystallizer. The first heat preservation jacket is used to accommodate the second refrigeration medium. The refrigeration unit is connected to the first heat preservation jacket through a pipeline;

[0015] Optionally, the first separation device has a first liquid phase outlet, a second liquid phase outlet and a solid phase outlet of the first separation device. The first liquid phase outlet of the first separation device is connected to the raw material mixing device through a pipeline. The second liquid phase outlet of the first separation device is connected to the second crystallization device group through a pipeline. The solid phase outlet of the first separation device is connected to the product storage device through a pipeline;

[0016] Optionally, the second crystallization device group includes a plurality of second crystallizers connected in series. Between adjacent two of the second crystallizers and between the last second crystallizer and the slurry inlet of the second separation device, they are connected through corresponding second slurry pumps;

[0017] The outer wall of the second crystallizer is provided with a second heat preservation jacket for accommodating the third refrigerating medium. The refrigerating unit is connected to the second heat preservation jacket through a pipeline;

[0018] Optionally, each of the first heat preservation jacket and the second heat preservation jacket is at least one of an integral jacket, an integral jacket with a deflector, a honeycomb jacket and a profiled steel jacket;

[0019] Optionally, stirrers are respectively arranged in the raw material mixing device, the first crystallizer and the second crystallizer. The stirrer is at least one of a paddle stirrer, a propeller stirrer and a turbine stirrer;

[0020] Optionally, the raw material mixing device is a vertical slurry tank which is provided with a hot flushing port. The bottom of the vertical slurry tank is conical, and the included angle between the bottom conical surface of the vertical slurry tank and the horizontal plane is 45 - 85°;

[0021] Optionally, each of the first crystallizer and the second crystallizer is a kettle - type crystallizer. The kettle - type crystallizer is provided with a hot flushing port. The bottom of the kettle - type crystallizer is conical, and the included angle between the bottom conical surface of the kettle - type crystallizer and the horizontal plane is 45 - 85°.

[0022] Optionally, the raw material mixing device is connected to the first crystallization device group through a third slurry pump. A plurality of hot flushing ports are respectively arranged on the inlet pipelines and outlet pipelines of the first slurry pump, the second slurry pump and the third slurry pump. Each hot flushing port is connected to a hot flushing pipeline;

[0023] Optionally, the first separation device and the second separation device are respectively provided with a plurality of hot flushing ports, and each hot flushing port is connected to a hot flushing pipeline.

[0024] Optionally, a first liquid level detection element is provided in the first crystallizer, and a first flow rate detection element and a first flow rate regulating element are provided on the pipeline between the raw material mixing device and each first crystallizer. The first flow rate regulating element is used to adjust the flow rate according to the detection data of the first liquid level detection element and the first flow rate detection element;

[0025] Optionally, a second liquid level detection element is provided in the second crystallizer, and a second flow rate detection element and a second flow rate regulating element are provided on the pipeline between the second liquid phase outlet of the first separation device and each second crystallizer. The second flow rate regulating element is used to adjust the flow rate according to the detection data of the second liquid level detection element and the second flow rate detection element;

[0026] Optionally, a third flow rate detection element and a third flow rate regulating element are provided on the pipeline between the third heat exchange module and the second cooled raw material inlet. The third flow rate regulating element is used to adjust the flow rate according to the detection data of the third flow rate detection element;

[0027] Optionally, pressure regulating elements are respectively provided on the pipelines between the refrigeration unit and the third heat exchange module, the first heat preservation jacket, and the second heat preservation jacket. The pressure regulating elements are respectively used to adjust the operating pressures of the first refrigeration medium, the second refrigeration medium, and the third refrigeration medium.

[0028] In a second aspect of the present disclosure, a method for continuously producing p-xylene using the system described in the first aspect of the present disclosure is provided. The method includes:

[0029] (1) Feeding the mixed xylene raw material into the raw material unit and cooling it in the presence of the first refrigeration medium to obtain cooled raw material; wherein, at least part of the first refrigeration medium comes from the refrigeration unit;

[0030] (2) Feeding the cooled raw material into the raw material mixing device of the crystallization unit for buffering, and then feeding the mixed slurry in the raw material mixing device into the first crystallization equipment group for first crystallization to obtain a first crystallization slurry. Feeding the first crystallization slurry into the first separation device for first separation to obtain a first solid phase and a first liquid phase. Feeding the first solid phase into the product storage device, and returning a first part of the first liquid phase to the raw material mixing device; wherein, the first crystallization is carried out in the presence of the second refrigeration medium, and the second refrigeration medium comes from the refrigeration unit;

[0031] (3) Feed the first liquid phase described in the second part into the second crystallization equipment group of the recovery unit for second crystallization to obtain a second crystallization slurry. Feed the second crystallization slurry into a second separation device for second separation to obtain a second solid phase and a second liquid phase. Feed the second solid phase into the raw material mixing device; wherein, the second crystallization is carried out in the presence of a third refrigeration medium, and the third refrigeration medium comes from the refrigeration unit.

[0032] Optionally, in step (1), the mixed xylene raw material includes at least one of o-xylene, m-xylene, p-xylene, ethylbenzene, toluene, and C9 aromatics. Wherein, based on the total weight of the mixed xylene raw material, the content of p-xylene is 30 to 98% by weight; the temperature of the mixed xylene raw material is 13.5 to 140 °C;

[0033] The temperature of the cooled raw material is 13.5 to 40 °C;

[0034] This method further includes: at least part of the first refrigeration medium comes from the product storage device, and / or, at least part of the first refrigeration medium comes from the second liquid phase;

[0035] Optionally, this method further includes: adjusting the operating pressure of the first refrigeration medium according to the temperature of the cooling.

[0036] Optionally, in step (2), the operating conditions of the first crystallization include: the temperature is -60 °C to 8 °C, and the solid crystal concentration range of the first crystallization slurry is 0 to 60% by weight;

[0037] Optionally, this method further includes: adjusting the slurry flow rate entering the first crystallizer according to the liquid level in the first crystallizer;

[0038] Optionally, this method further includes: adjusting the operating pressure of the second refrigeration medium according to the temperature of the first crystallization.

[0039] Optionally, in step (3), the second crystallization is carried out in a second crystallization equipment group. The second crystallization equipment group includes a plurality of second crystallizers connected in series. The operating conditions of the second crystallization equipment group include: the temperature of the second crystallization slurry is -90 °C to -10 °C, the solid crystal concentration range of the second crystallization slurry is 0 to 60% by weight, and the absolute value of the temperature difference between two adjacent second crystallizers ranges from 1 to 25 °C;

[0040] This method further includes: rinsing at least part of the second solid phase with the cooled raw material and then feeding it into the raw material mixing device;

[0041] Optionally, this method further includes: adjusting the slurry flow rate entering the second crystallizer according to the liquid level in the second crystallizer;

[0042] Optionally, the method further includes: adjusting an operating pressure of the third refrigeration medium according to a temperature of the second crystallization.

[0043] Optionally, the method further includes thermally flushing the slurries obtained from the raw material mixing device, the first crystallization device group, and the second crystallization device group respectively with a thermal flushing medium, and a source of the thermal flushing medium is selected from one or more of the mixed xylene raw material, the cooled raw material, the first liquid phase, and the second liquid phase.

[0044] Through the above technical solution, the mixed xylene raw material is cooled in the raw material unit, and further, the production of high-purity para-xylene products and the recovery of crystallization mother liquor are sequentially achieved in the crystallization unit and the recovery unit, and the cooling unit provides a refrigeration medium for the raw material unit, the crystallization unit, and the recovery unit. The present disclosure has a wide applicability to the concentration range of para-xylene in the raw material, and there is no continuous discharge of process waste liquid. While improving the quality and yield of para-xylene products, the blockage phenomenon that is prone to occur during the crystallization process is effectively avoided, ensuring the stability and continuity of the para-xylene crystallization process.

[0045] Other features and advantages of the present disclosure will be described in detail in the subsequent specific implementation section. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The drawings are used to provide a further understanding of the present disclosure, and constitute a part of the specification, and are used to explain the present disclosure together with the following specific implementation manners, but do not constitute a limitation to the present disclosure. In the drawings:

[0047] Figure 1 is a structural schematic block diagram of a continuous production system provided by the present disclosure.

[0048] Figure 2 is a process schematic diagram of a specific implementation manner of a method and a system for continuously producing para-xylene provided by the present disclosure.

[0049] DESCRIPTION OF THE REFERENCE NUMERALS

[0050] 100 - Raw material unit, 101 - First heat exchange module, 102 - Second heat exchange module 102, 103 - Third heat exchange module;

[0051] 200 - Crystallization unit, 201 - Raw material mixing device, 202 - First crystallization device group, 203 - First separation device, 204 - Product storage device, 205 - First slurry pump, 206 - Third slurry pump, 207 - Product pump;

[0052] 300 - Recovery unit, 301 - Second crystallization device group, 302 - Second separation device, 303 - Second slurry pump;

[0053] 400 - Refrigeration unit;

[0054] S1 - Mixed xylene raw material, S2 - Cooled raw material, S3 - First part of the cooled raw material, S4 - Second part of the cooled raw material, S5 - Mixed slurry, S6 - First crystallization slurry, S7 - First solid phase, S8 - First part of the first liquid phase, S9 - Second part of the first liquid phase, S10 - Second crystallization slurry, S11 - Second solid phase, S12 - Second liquid phase, S13 - High-purity product;

[0055] E1 - Solid phase outlet of the first separation device, E2 - First liquid phase outlet of the first separation device, E3 - Second liquid phase outlet of the first separation device;

[0056] F1 - Liquid phase outlet of the second separation device, F2 - Solid phase outlet of the second separation device;

[0057] L1 - First refrigeration medium, L2 - Second refrigeration medium, L3 - Third refrigeration medium. Detailed implementation mode

[0058] The following will describe in detail the specific implementation modes of the present disclosure with reference to the accompanying drawings. It should be understood that the specific implementation modes described herein are only for the purpose of illustrating and explaining the present disclosure, and are not intended to limit the present disclosure.

[0059] In the first aspect of the present disclosure, a continuous production system is provided. Referring to Figure 1 and Figure 2 , the system includes a raw material unit 100, a crystallization unit 200, a recovery unit 300, and a refrigeration unit 400.

[0060] Among them, the raw material unit 100 is used for heat exchange and cooling of the mixed xylene raw material, and includes a heat exchange equipment group. The crystallization unit 200 is used for crystallizing the cooled raw material to obtain a high-purity p-xylene product, and includes a raw material mixing equipment 201, a first crystallization equipment group 202, a first separation equipment 203, and a product storage equipment 204 connected in sequence. The raw material mixing equipment 201 is connected to the heat exchange equipment group, and the first separation equipment 203 is also connected to the raw material mixing equipment 201. The recovery unit 300 is used for recovering the crystallization mother liquor obtained by the crystallization unit 200, and includes a second crystallization equipment group 301 and a second separation equipment 302 connected in sequence. The second crystallization equipment group 301 is connected to the first separation equipment 203, and the second separation equipment 302 is connected to the raw material mixing equipment 201. The refrigeration unit 400 is used for respectively providing a first refrigeration medium to the raw material unit 100, a second refrigeration medium to the crystallization unit 200, and a third refrigeration medium to the recovery unit 300, and is respectively connected to the raw material unit 100, the crystallization unit 200, and the recovery unit 300.

[0061] According to the present disclosure, in the raw material unit 100, the heat exchange equipment group has a raw material inlet and a raw material outlet for the entry and exit of raw materials, as well as a refrigerant inlet and a refrigerant outlet for the entry and exit of the refrigeration medium. The heat exchange equipment group can be flexibly configured according to needs. It can not only cool the raw materials, but also be further used to recover the cold energy of the crystallization unit 200 and the recovery unit 300, realizing the crystallization process while hierarchically recycling and utilizing the cold energy. The system has good applicability to fluctuations in the feed rate.

[0062] In one implementation, the heat exchange equipment group includes an optional first heat exchange module 101, an optional second heat exchange module 102, and a third heat exchange module 103. The first heat exchange module 101, the second heat exchange module 102, and the third heat exchange module 103 can be connected in sequence, that is, the mixed xylene raw material can be cooled in three stages by passing through the first heat exchange module 101, the second heat exchange module 102, and the third heat exchange module 103 according to needs.

[0063] The first heat exchange module 101 can recover the cold energy of the crystallization unit 200. At this time, the product storage device 204 is connected to the refrigerant inlet of the first heat exchange module 101 through a pipeline, that is, the high-purity product obtained from the crystallization unit 100 is used to perform the first-stage cooling on the hot fluid raw material of the first heat exchange module 101.

[0064] The second heat exchange module 102 can recover the cold energy of the recovery unit 300. At this time, the second separation device 302 has a second separation device liquid phase outlet and a second separation device solid phase outlet. The second separation device liquid phase outlet is connected to the refrigerant inlet of the second heat exchange module 102 through a pipeline, that is, the second liquid obtained from the recovery unit 300 is used to perform the second-stage cooling on the hot fluid raw material of the second heat exchange module 102.

[0065] The refrigeration unit 400 is connected to the refrigerant inlet of the third heat exchange module 103 through a pipeline, that is, the first refrigerant of the refrigeration unit 400 is used to perform the third-stage cooling on the hot fluid raw material of the third heat exchange module 103. The cooled raw material obtained from the third heat exchange module 103 enters the crystallization unit 200.

[0066] The first heat exchange module 101, the second heat exchange module 102, and the third heat exchange module 103 can each include heat exchangers, and the heat exchangers can be various common heat exchange equipment. Among them, the number of heat exchangers in the first heat exchange module 101 can be 0 to 5, the number of heat exchangers in the second heat exchange module 102 can be 0 to 5; the number of heat exchangers in the third heat exchange module 103 can be 1 to 5.

[0067] In a specific embodiment, the cooled raw materials obtained from the third heat exchange module 103 enter the raw material mixing device 201 in two parts. Specifically, the raw material mixing device 201 has a first cooled raw material inlet, and the raw material outlet of the third heat exchange module 103 is connected to the first cooled raw material inlet through a pipeline; the solid phase outlet of the second separation device 302 of the second separation device is connected to the raw material mixing device 201 through a pipeline, and a second cooled raw material inlet is provided on the pipeline between the solid phase outlet of the second separation device and the raw material mixing device 201, and the raw material outlet of the third heat exchange module 103 is also connected to the second cooled raw material inlet through a pipeline; in this way, a part of the cooled raw materials obtained from the third heat exchange module 103 directly enters the raw material mixing device 201 from the first cooled raw material inlet, and the other part enters the pipeline between the solid phase outlet of the second separation device and the raw material mixing device 201 from the second cooled raw material inlet, thereby assisting the second solid phase obtained from the second separation device 302 to flow, preventing pipeline blockage, and ensuring the continuous production of the system.

[0068] According to the present disclosure, in the crystallization unit 200, the raw material mixing device 201 is used to mix the cooled raw materials from the raw material unit 100, a part of the first liquid phase from the first separation device 203, and the rinsed second solid phase S11 from the second separation device 302 in the recovery unit 300. A stirrer may be provided in the raw material mixing device 201 to improve the uniformity of the mixed slurry, and the type of the stirrer may be paddle (blade) type, propeller type or turbine type, etc. The raw material mixing device 201 may be a vertical slurry tank, and further may be provided with a hot flushing pipe orifice, and the bottom may be conical, and the included angle between the conical surface and the horizontal plane may be 45 to 85°. The raw material mixing device 201 may be connected to the first crystallization device group 202 through the third slurry pump 206, and the third slurry pump 206 is used to pump and transport the mixed slurry to the first crystallization device group 202.

[0069] The first crystallization device group 202 is used to perform first crystallization on the mixed slurry, and may include a plurality of first crystallizers connected in parallel. The mixed slurry from the raw material mixing device 201 may enter the plurality of first crystallizers connected in parallel evenly. The crystallization unit 200 further includes a plurality of first slurry pumps 205, and each first crystallizer is connected to the slurry inlet of the first separation device 203 through the corresponding first slurry pump 205.

[0070] The outer wall of the first crystallizer may be provided with a first thermal insulation jacket for containing a second refrigerating medium from the refrigeration unit 400, and the refrigeration unit 400 is connected to the first thermal insulation jacket through a pipeline. The first thermal insulation jacket may be an integral jacket, an integral jacket with a flow guide plate, a honeycomb jacket or a profiled steel jacket, etc. A stirrer may be provided in the first crystallizer to improve the crystallization efficiency, and the type of the stirrer may be a paddle (blade) type, a propeller type or a turbine type, etc. The first crystallizer may be a kettle-type crystallizer, and may further be provided with a hot flushing port, and the bottom may be conical, and the included angle between the conical surface and the horizontal plane may be 45-85°.

[0071] The number of the first crystallizers may be adjusted according to actual needs. Specifically, the number of the first crystallizers may be 1-20, and the number of the first slurry pumps 205 may be 1-40.

[0072] The first separation device 203 is used for solid-liquid separation of the first crystallization slurry obtained by the first crystallization equipment group 202. In one embodiment, the first separation device 203 has a first liquid phase outlet of the first separation device, a second liquid phase outlet of the first separation device and a solid phase outlet of the first separation device. The first liquid phase outlet of the first separation device is connected to the raw material mixing device 201 through a pipeline, the second liquid phase outlet of the first separation device is connected to the second crystallization equipment group 301 through a pipeline, and the solid phase outlet of the first separation device is connected to the product storage device 204 through a pipeline. In this way, the first solid phase obtained by the first separation device 203 enters the product storage device 204, part of the first liquid phase returns to the raw material mixing device 201, and part of the first liquid phase enters the second crystallization equipment group 301. The first separation device 203 may be various common solid-liquid separation devices, and may further be provided with a plurality of (such as 1-10) hot flushing ports, and each hot flushing port is connected to a hot flushing pipeline.

[0073] The product storage device 204 is used for storing high-purity products (i.e., high-purity p-xylene products). Among them, part of the high-purity products may be used as a refrigerating medium and transported to the refrigeration unit 400 through the product pump 207 according to needs.

[0074] According to the present disclosure, in the recovery unit 300, the second crystallization equipment group 301 is used for second crystallization of part of the first liquid phase obtained by the crystallization unit 200, and may include a plurality of second crystallizers connected in series. The second liquid phase outlet of the first separation device 203 of the first separation device is connected to the slurry inlet of the first second crystallizer among the plurality of second crystallizers connected in series. The recovery unit 300 further includes a plurality of second slurry pumps 303, and adjacent two of the second crystallizers and between the last second crystallizer and the slurry inlet of the second separation device 302 are connected through the corresponding second slurry pumps 303.

[0075] The outer wall of the second crystallizer may be provided with a second thermal insulation jacket for containing a third refrigerating medium from the refrigerating unit 400, and the refrigerating unit 400 is connected to the second thermal insulation jacket through a pipeline. The second thermal insulation jacket may be an integral jacket, an integral jacket with a deflector, a honeycomb jacket, a profiled steel jacket, etc. A stirrer may be provided in the second crystallizer to improve the crystallization efficiency, and the stirrer type may be paddle (blade) type, propeller type, turbine type, etc. The second crystallizer may be a kettle-type crystallizer, further provided with a hot flushing port, and the bottom may be conical, and the included angle between the conical surface and the horizontal plane may be 45 to 85°.

[0076] The number of the second crystallizers can be adjusted according to actual needs. Specifically, the number of the second crystallizers can be 1 to 20, and the number of the second slurry pumps 303 can be 1 to 40.

[0077] The second separation device 302 is used for solid-liquid separation of the second crystallization slurry obtained by the second crystallization equipment group 301. In one embodiment, the second separation device 302 has a second separation device solid phase outlet and a second separation device liquid phase outlet. The second separation device solid phase outlet is connected to the raw material mixing device 201 through a pipeline, and the second separation device liquid phase outlet is connected to the second heat exchange module 102 through a pipeline. The second separation device 302 may be various common solid-liquid separation devices, and further provided with a plurality of (such as 1 to 10) hot flushing ports, and each hot flushing port is connected to a hot flushing pipeline.

[0078] In a specific embodiment, a plurality of hot flushing ports are respectively provided on the inlet pipelines and outlet pipelines of the first slurry pump 205, the second slurry pump 303 and the third slurry pump 206. For example, 1 to 30 hot flushing ports can be evenly spaced on each pipeline section, and each hot flushing port is connected to a hot flushing pipeline. The hot flushing pipeline is used to transport the hot flushing medium, and hot flushing the slurry in the pipeline with hot flushing media of various different temperatures and concentrations is beneficial to avoiding blockages at different positions and reducing the interference with the stable production state.

[0079] In one embodiment, a first liquid level detection element may be provided in the first crystallizer, and a first flow rate detection element and a first flow rate adjustment element may be provided on the pipeline between the raw material mixing device 201 and each of the first crystallizers. The first flow rate adjustment element is used to adjust the flow rate according to the detection data of the first liquid level detection element and the first flow rate detection element, so as to control the solid content of the crystal-containing slurry, and avoid clogging of equipment and pipelines while ensuring product purity and yield. Specifically, a liquid level display and a liquid level transmitter instrument are arranged at the top of the first crystallizer, and a flow rate detection element, a flow rate transmitter and a flow rate display control instrument are arranged on the pipeline between the third slurry pump 206 and the first crystallizer to adjust the slurry flow rate entering the first crystallizer according to the liquid level of the first crystallizer.

[0080] In one embodiment, a second liquid level detection element may be provided in the second crystallizer, and a second flow rate detection element and a second flow rate adjustment element may be provided on the pipeline between the second liquid phase outlet of the first separation device and each of the second crystallizers. The second flow rate adjustment element is used to adjust the flow rate according to the detection data of the second liquid level detection element and the second flow rate detection element, so as to control the solid content of the crystal-containing slurry, and avoid clogging of equipment and pipelines while ensuring product purity and yield. Specifically, a liquid level display and a liquid level transmitter instrument are arranged at the top of the second crystallizer, and a flow rate detection element, a flow rate transmitter and a flow rate display control instrument are arranged on the pipeline between the second liquid phase outlet of the first separation device and the second crystallizer to adjust the slurry flow rate entering the second crystallizer according to the liquid level of the second crystallizer.

[0081] In one embodiment, a third flow rate detection element and a third flow rate adjustment element may be provided on the pipeline between the third heat exchange module 103 and the second cooling raw material inlet. The third flow rate adjustment element is used to adjust the flow rate according to the detection data of the third liquid level detection element. Specifically, a flow rate detection element, a flow rate transmitter and a flow rate display control instrument are arranged on the pipeline between the third heat exchange module 103 and the second cooling raw material inlet to control the flow rate of the second part of the cooling raw material used to wash the second solid phase.

[0082] In a specific embodiment, pressure regulating elements are respectively provided on the pipelines between the refrigeration unit 400 and the third heat exchange module 103, the first thermal insulation jacket, and the second thermal insulation jacket. The pressure regulating elements are respectively used to regulate the operating pressures of the first refrigeration medium, the second refrigeration medium, and the third refrigeration medium. Specifically, pressure transmitting and pressure display control instruments are provided on the pipeline between the refrigeration unit 400 and the third heat exchange module 103, and the operating pressure of the first refrigeration medium is set according to the cooling capacity required by the third heat exchange module 103; pressure transmitting and pressure display control instruments are provided on the pipeline between the refrigeration unit 400 and the first thermal insulation jacket, and the operating pressure of the second refrigeration medium is set according to the cooling capacity required by each first crystallizer; pressure transmitting and pressure display control instruments are provided on the pipeline between the refrigeration unit 400 and the second thermal insulation jacket, and the operating pressure of the third refrigeration medium is set according to the cooling capacity required by each second crystallizer. In this way, it is beneficial to improve the refrigeration efficiency and heat transfer effect, and effectively avoid local supercooling of the crystallizer.

[0083] In a second aspect of the present disclosure, there is provided a method for continuously producing p-xylene using the system described in the first aspect of the present disclosure. The method includes:

[0084] (1) Feeding a mixed xylene raw material into a raw material unit and cooling it in the presence of a first refrigeration medium to obtain a cooled raw material; wherein, at least part of the first refrigeration medium comes from the refrigeration unit;

[0085] (2) Feeding the cooled raw material into a raw material mixing device in the crystallization unit for buffering, and then feeding the mixed slurry in the raw material mixing device into a first crystallization device group for first crystallization to obtain a first crystallization slurry. Feeding the first crystallization slurry into a first separation device for first separation to obtain a first solid phase and a first liquid phase. Feeding the first solid phase into a product storage device, and returning a first part of the first liquid phase to the raw material mixing device; wherein, the first crystallization is carried out in the presence of a second refrigeration medium, and the second refrigeration medium comes from the refrigeration unit;

[0086] (3) Feeding a second part of the first liquid phase into a second crystallization device group in a recovery unit for second crystallization to obtain a second crystallization slurry. Feeding the second crystallization slurry into a second separation device for second separation to obtain a second solid phase and a second liquid phase. Feeding the second solid phase into the raw material mixing device; wherein, the second crystallization is carried out in the presence of a third refrigeration medium, and the third refrigeration medium comes from the refrigeration unit.

[0087] According to the present disclosure, in step (1), the mixed xylene raw material includes at least one of o-xylene, m-xylene, p-xylene, ethylbenzene, toluene, and C9 aromatics, wherein the content of p-xylene is 30 to 98% by weight based on the total weight of the mixed xylene raw material. The present disclosure has a wide applicability to the concentration range of p-xylene in the mixed xylene raw material, has low requirements on the impurity content, can match the refrigeration medium of different operating conditions according to the concentration of p-xylene, and has good adaptability to the concentration of raw material components.

[0088] The first refrigeration medium may be liquid CO 2 , ethylene glycol aqueous solution, ethylene or propylene. Further, the method further comprises: adjusting the operating pressure of the first refrigerant according to the cooling temperature; specifically, setting a pressure transmitter and pressure display control instrument on the pipeline between the refrigeration unit 400 and the third heat exchange module 103, and setting the operating pressure of the first refrigerant according to the cooling capacity required by the third heat exchange module 103. The temperature of the mixed xylene raw material can be adjusted in a wide range, for example, it can be 13.5 to 140°C, and the temperature of the cooling raw material can be 13.5 to 40°C.

[0089] In one embodiment, the method further includes: the first refrigerant medium is at least partially from the product storage device, that is, used in the first heat exchange module 101 described above; and / or, the first refrigerant medium is at least partially from the second liquid phase, that is, used in the second heat exchange module 102 described above. The amount of the first refrigerant medium used in the first heat exchange module 101 and the second heat exchange module 102 can be adjusted according to the raw material flow rate, temperature and the temperature of the corresponding first refrigerant medium.

[0090] In one embodiment, the method further includes: using at least part of the cooling raw material to flush the second solid phase and then feeding it into the raw material mixing device. Specifically, among the cooling raw materials obtained by the third heat exchange module 103, the first part of the cooling raw material directly enters the raw material mixing device 201 from the first cooling raw material inlet, and the second part of the cooling raw material enters the pipeline between the solid phase outlet of the second separation device and the raw material mixing device 201 from the second cooling raw material inlet, and the second solid phase obtained by the second separation device 302 is flushed and then fed into the raw material mixing device 201. The ratio of the first part of the cooling raw material to the second part of the cooling raw material can be adjusted as needed. Specifically, the volume ratio of the first part of the cooling raw material to the second part of the cooling raw material can be 0.05-0.95, preferably 0.25-0.8.

[0091] In step (2), the solid crystal concentration range of the mixed slurry in the raw material mixing equipment is 5-60% by weight. The operating conditions of the first crystallization may include: the temperature is -60°C to 8°C, and the solid crystal concentration range of the first crystallization slurry in the first crystallizer is 0-60% by weight. The second refrigerating medium may be one or more of liquid CO 2 , aqueous ethylene glycol solution, propylene or ethylene. Further, the method further includes: adjusting the operating pressure of the second refrigerating medium according to the temperature of the first crystallization; specifically, a pressure transmitter and a pressure display control instrument are arranged on the pipeline between the refrigeration unit 400 and the first heat preservation jacket, and the operating pressure of the second refrigerating medium is set according to the cooling capacity required by each first crystallizer. In one embodiment, the method further includes: adjusting the slurry flow rate entering the first crystallizer according to the liquid level in the first crystallizer, so as to avoid equipment and pipeline blockage while ensuring product purity and yield.

[0092] In step (3), the operating conditions of the second crystallization equipment group may include: the temperature of the second crystallization slurry is -90°C to -10°C, the solid crystal concentration range of the second crystallization slurry in the second crystallizer is 0-60% by weight. In the embodiment where the second crystallization equipment group includes a plurality of second crystallizers connected in series, the absolute value of the temperature difference between two adjacent second crystallizers ranges from 1-25°C. The third refrigerating medium may be one or more of liquid CO 2 , ethylene or propylene. Further, the method further includes: adjusting the operating pressure of the third refrigerating medium according to the temperature of the second crystallization; specifically, a pressure transmitter and a pressure display control instrument are arranged on the pipeline between the refrigeration unit 400 and the second heat preservation jacket, and the operating pressure of the third refrigerating medium is set according to the cooling capacity required by each second crystallizer.

[0093] In one embodiment, the method further includes: adjusting the slurry flow rate entering the second crystallizer according to the liquid level in the second crystallizer, so as to avoid equipment and pipeline blockage while ensuring product purity and yield.

[0094] In one embodiment, the method further includes thermally flushing the slurries obtained from the raw material mixing equipment, the first crystallization equipment group, and the second crystallization equipment group with a thermal flushing medium. The thermal flushing medium may be various slurries from within the system. Specifically, the source of the thermal flushing medium is selected from one or several of the mixed xylene raw material, the cooled raw material, the first liquid phase, the second liquid phase, and the high-purity product.

[0095] The present disclosure realizes strong adaptability to the concentration of p-xylene in the feed components, fractionally recovers and utilizes cold energy during the crystallization process, continuously discharges no process waste liquid, improves the yield while enhancing the quality of the p-xylene product, effectively avoids the clogging phenomenon that is extremely likely to occur during the crystallization process, and ensures the stability and continuity of the p-xylene crystallization process.

[0096] The following further illustrates the present disclosure through examples.

[0097] Example 1

[0098] A mixed xylene raw material from the xylene fractionation unit is used, with a p-xylene content of 75% by weight and a temperature of 40°C. The operation process of this example is as Figure 2 shown.

[0099] In the system of this example, the first heat exchange module 101 includes 2 heat exchangers, the second heat exchange module 102 includes 1 heat exchanger, the third heat exchange module 103 includes 1 heat exchanger, and the first refrigeration medium L1 from the refrigeration unit in the third heat exchange module 103 is propylene. The first crystallization equipment group 202 adopts a form of two first crystallizers in parallel (each crystallizer is correspondingly equipped with a slurry pump), and uses jacket refrigeration. The jacket is an integral jacket, and the second refrigeration medium L2 (propylene) from the refrigeration unit is accommodated in the jacket. The second crystallization equipment group 301 consists of 4 second crystallizers in series (each crystallizer is correspondingly equipped with a slurry pump) and uses jacket refrigeration. The jacket is an integral jacket, and the third refrigeration medium L3 (ethylene) from the refrigeration unit is accommodated in the jacket. Level display and level transmission instruments are provided at the tops of the crystallizers in the first crystallization equipment group and the second crystallization equipment group. Flow detection elements, flow transmitters, and flow display and control instruments are provided on the pipeline between the third slurry pump 206 and the first crystallizer, and on the pipelines between the second liquid phase outlet E3 of the first separation equipment and each second crystallizer. Flow detection elements, flow transmitters, and flow display and control instruments are provided on the pipeline between the third heat exchange module 103 and the second cooling raw material inlet. Pressure transmitters and pressure display and control instruments are respectively provided on the pipelines between the refrigeration unit and the third heat exchange module 103, the first insulation jacket, and the second insulation jacket. Three hot flushing ports are respectively provided on the inlet pipelines and outlet pipelines of the first slurry pump 205, the second slurry pump 303, and the third slurry pump 206, and each of the hot flushing ports is connected to a hot flushing pipeline. The hot flushing medium in the hot flushing pipeline is the heat-exchanged hot second liquid phase from the second heat exchange module 102. Paddle stirrers are respectively provided in the raw material mixing equipment 201, the first crystallizer, and the second crystallizer. The raw material mixing equipment 201 is a vertical slurry tank with multiple hot flushing ports, the bottom is conical, and the angle between the bottom conical surface and the horizontal plane is 45°. The first crystallizer and the second crystallizer are respectively kettle-type crystallizers with multiple hot flushing ports, the bottom is conical, and the angle between the bottom conical surface and the horizontal plane is 45°.

[0100] The feed flow rate of the mixed xylene raw material S1 is 110,000 kg / h. It enters the raw material unit and successively passes through the first heat exchange module 101, the second heat exchange module 102, and the third heat exchange module 103 to obtain the cooled raw material S2 cooled to 14.5 °C. The pressure of the first refrigerating medium transported by the cooling unit to the third heat exchange module 103 is 0.75 MPaA.

[0101] The first part of the cooled raw material S3 directly enters the raw material mixing equipment 201 of the crystallization unit and is mixed and stirred together with the first part of the first liquid phase S8 separated from the first separation equipment 203 and the second solid phase S11 separated from the second separation equipment 302 rinsed by the second part of the cooled raw material S4 (the volume ratio of the first part of the cooled raw material S3 to the second part of the cooled raw material S4 is 0.3). After being evenly mixed, the solid crystal concentration in the mixed slurry S5 obtained is 25% by weight and is sent to the first crystallization equipment group 202 by the third slurry pump 206. The pressure of the second refrigerating medium transported by the cooling unit to the first crystallization equipment group 202 is 0.3 MpaA. The operating temperature of the first crystallization equipment group 202 is -5 °C, and the solid crystal concentration in the obtained first crystallization slurry S6 does not exceed 45% by weight. It is transported to the first separation equipment 203 by the first slurry pump 205 for the first separation to obtain the first solid phase and the first liquid phase. The first solid phase S7 is sent from the solid phase outlet E1 of the first separation equipment to the product storage equipment 204, and the second part of the first liquid phase S9 is transported to the recovery unit for recovery through the second liquid phase outlet E3 of the first separation equipment.

[0102] The pressure of the third refrigerating medium L3 (ethylene) transported by the cooling unit to the second crystallization equipment group 301 is 1.1 MPaA. The operating temperatures of the 4 second crystallizers in the second crystallization equipment group 301 in the recovery unit are -15 °C, -25 °C, -35 °C, and -45 °C respectively. The solid crystal concentration in each crystallizer does not exceed 60% by weight. The second crystallization slurry S10 from the last crystallizer is transported to the second separation equipment 302 by the second slurry pump 303 for the second separation to obtain the second solid phase and the second liquid phase. The second solid phase S11 is output from the solid phase outlet F2 of the second separation equipment and returns to the raw material mixing equipment 201 under the flushing action of the second part of the cooled raw material S4. Part of the second liquid phase S12 is transported to the second heat exchange module 102 of the raw material unit through the liquid phase outlet F1 of the second separation equipment.

[0103] The high-purity product S13 in the product storage equipment 204 is melted and sent to the first heat exchange module 101 of the raw material unit, and high-purity p-xylene products are produced through heat exchange. The purity of the p-xylene product is ≥99.8% by weight, and the yield is 99.8%.

[0104] As can be seen from the results of the above embodiments, the use of the system and method of the present disclosure is conducive to improving the quality and yield of p-xylene products, while effectively avoiding the clogging phenomenon that is extremely likely to occur during the crystallization process, and ensuring the stability and continuity of the p-xylene crystallization process.

[0105] The preferred embodiments of the present disclosure have been described in detail above in conjunction with the accompanying drawings. However, the present disclosure is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present disclosure, various simple modifications can be made to the technical solutions of the present disclosure, and these simple modifications all fall within the protection scope of the present disclosure.

[0106] In addition, it should be noted that, in the case of no contradiction, the various specific technical features described in the above specific embodiments can be combined in any suitable manner. To avoid unnecessary repetition, the present disclosure will not separately describe various possible combination methods.

[0107] In addition, any combination can be made between various different embodiments of the present disclosure, as long as it does not violate the idea of the present disclosure, it should also be regarded as the content disclosed by the present disclosure.

Claims

1. A continuous production system, characterized in that, the system includes a raw material unit, a crystallization unit, a recovery unit and a refrigeration unit, the raw material unit includes a heat exchange equipment group; the crystallization unit includes a raw material mixing equipment, a first crystallization equipment group, a first separation equipment and a product storage equipment connected in sequence, the raw material mixing equipment is connected to the heat exchange equipment group, the first separation equipment has a first liquid phase outlet of the first separation equipment and a second liquid phase outlet of the first separation equipment, and the first liquid phase outlet of the first separation equipment is connected to the raw material mixing equipment through a pipeline; the recovery unit includes a second crystallization equipment group and a second separation equipment connected in sequence, the second crystallization equipment group is connected to the second liquid phase outlet of the first separation equipment through a pipeline, and the second separation equipment is connected to the raw material mixing equipment; the refrigeration unit is respectively connected to the raw material unit, the crystallization unit and the recovery unit, and the refrigeration unit is used to respectively provide a first refrigeration medium to the raw material unit, a second refrigeration medium to the crystallization unit, and a third refrigeration medium to the recovery unit.

2. The system according to claim 1, wherein, the heat exchange equipment group includes an optional first heat exchange module, an optional second heat exchange module and a third heat exchange module, the raw material mixing equipment has a first cooling raw material inlet, and the third heat exchange module is connected to the first cooling raw material inlet through a pipeline; optionally, the product storage equipment is connected to the first heat exchange module through a pipeline; the second separation equipment has a second liquid phase outlet of the second separation equipment and a second solid phase outlet of the second separation equipment, the second liquid phase outlet of the second separation equipment is connected to the second heat exchange module through a pipeline, the second solid phase outlet of the second separation equipment is connected to the raw material mixing equipment through a pipeline, and a second cooling raw material inlet is provided on the pipeline between the second solid phase outlet of the second separation equipment and the raw material mixing equipment, and the third heat exchange module is also connected to the second cooling raw material inlet through a pipeline; the refrigeration unit is connected to the third heat exchange module through a pipeline.

3. The system according to claim 1, wherein, the first crystallization equipment group includes a plurality of first crystallizers connected in parallel, and each first crystallizer is connected to the slurry inlet of the first separation equipment through a corresponding first slurry pump; optionally, a first heat preservation jacket is provided on the outer wall of the first crystallizer, and the first heat preservation jacket is used to accommodate the second refrigeration medium, and the refrigeration unit is connected to the first heat preservation jacket through a pipeline; optionally, the first separation equipment also has a first solid phase outlet of the first separation equipment, and the first solid phase outlet of the first separation equipment is connected to the product storage equipment through a pipeline; optionally, the second crystallization equipment group includes a plurality of second crystallizers connected in series, and adjacent two of the second crystallizers and between the last second crystallizer and the slurry inlet of the second separation equipment are connected through corresponding second slurry pumps; optionally, a second heat preservation jacket is provided on the outer wall of the second crystallizer, and the second heat preservation jacket is used to accommodate the third refrigeration medium, and the refrigeration unit is connected to the second heat preservation jacket through a pipeline; Optionally, each of the first heat-insulating jacket and the second heat-insulating jacket is at least one of an integral jacket, an integral jacket with a flow guide plate, a honeycomb jacket, and a profiled steel jacket; Optionally, stirrers are respectively arranged in the raw material mixing device, the first crystallizer, and the second crystallizer, and each stirrer is at least one of a paddle stirrer, a propeller stirrer, and a turbine stirrer; Optionally, the raw material mixing device is a vertical slurry tank, the vertical slurry tank is provided with a hot flushing port, the bottom of the vertical slurry tank is conical, and the included angle between the bottom conical surface of the vertical slurry tank and the horizontal plane is 45-85°; Optionally, each of the first crystallizer and the second crystallizer is a kettle-type crystallizer; the kettle-type crystallizer is provided with a hot flushing port, the bottom of the kettle-type crystallizer is conical, and the included angle between the bottom conical surface of the kettle-type crystallizer and the horizontal plane is 45-85°.

4. The system according to claim 3, wherein, the raw material mixing device is connected to the first crystallization equipment group through a third slurry pump; a plurality of hot flushing ports are respectively arranged on the inlet pipelines and outlet pipelines of the first slurry pump, the second slurry pump, and the third slurry pump, and each hot flushing port is connected to a hot flushing pipeline; Optionally, each of the first separation device and the second separation device is provided with a plurality of hot flushing ports, and each hot flushing port is connected to a hot flushing pipeline.

5. The system according to claim 3, wherein, a first liquid level detection element is arranged in the first crystallizer, a first flow detection element and a first flow regulating element are arranged on the pipeline between the raw material mixing device and each first crystallizer, and the first flow regulating element is used for regulating the flow according to the detection data of the first liquid level detection element and the first flow detection element; Optionally, a second liquid level detection element is arranged in the second crystallizer, a second flow detection element and a second flow regulating element are arranged on the pipeline between the second liquid phase outlet of the first separation device and each second crystallizer, and the second flow regulating element is used for regulating the flow according to the detection data of the second liquid level detection element and the second flow detection element; Optionally, a third flow detection element and a third flow regulating element are arranged on the pipeline between the third heat exchange module and the second cooling raw material inlet, and the third flow regulating element is used for regulating the flow according to the detection data of the third flow detection element; Optionally, pressure regulating elements are respectively arranged on the pipelines between the refrigeration unit and the third heat exchange module, the first heat-insulating jacket, and the second heat-insulating jacket, and the pressure regulating elements are respectively used for regulating the operating pressures of the first refrigeration medium, the second refrigeration medium, and the third refrigeration medium.

6. A method for continuously producing p-xylene by using the system according to any one of claims 1-5, characterized in that, the method comprises: (1) Feeding a mixed xylene raw material into a raw material unit and cooling it in the presence of a first refrigeration medium to obtain a cooled raw material; wherein, at least part of the first refrigeration medium comes from a refrigeration unit; (2) Feed the cooling raw materials into the raw material mixing equipment of the crystallization unit for buffering, and then feed the mixed slurry in the raw material mixing equipment into the first crystallization equipment group for the first crystallization to obtain a first crystallization slurry. Feed the first crystallization slurry into the first separation equipment for the first separation to obtain a first solid phase and a first liquid phase. Feed the first solid phase into the product storage equipment, and return a first part of the first liquid phase to the raw material mixing equipment; wherein, the first crystallization is carried out in the presence of a second refrigerating medium, and the second refrigerating medium comes from the refrigeration unit; (3) Feed a second part of the first liquid phase into the second crystallization equipment group of the recovery unit for the second crystallization to obtain a second crystallization slurry. Feed the second crystallization slurry into the second separation equipment for the second separation to obtain a second solid phase and a second liquid phase. Feed the second solid phase into the raw material mixing equipment; wherein, the second crystallization is carried out in the presence of a third refrigerating medium, and the third refrigerating medium comes from the refrigeration unit.

7. The method according to claim 6, wherein, In step (1), the mixed xylene raw materials include at least one of o-xylene, m-xylene, p-xylene, ethylbenzene, toluene, and C9 aromatics. Among them, based on the total weight of the mixed xylene raw materials, the content of p-xylene is 30 to 98% by weight; the temperature of the mixed xylene raw materials is 13.5 to 140 °C; The temperature of the cooling raw materials is 13.5 to 40 °C; The method further includes: at least part of the first refrigerating medium comes from the product storage equipment, and / or, at least part of the first refrigerating medium comes from the second liquid phase; Optionally, the method further includes: adjusting the operating pressure of the first refrigerating medium according to the temperature of the cooling.

8. The method according to claim 6, wherein, In step (2), the operating conditions of the first crystallization include: the temperature is -60 °C to 8 °C, and the solid crystal concentration range of the first crystallization slurry is 0 to 60% by weight; Optionally, the method further includes: adjusting the slurry flow rate entering the first crystallizer according to the liquid level in the first crystallizer; Optionally, the method further includes: adjusting the operating pressure of the second refrigerating medium according to the temperature of the first crystallization.

9. The method according to claim 6, wherein, In step (3), the second crystallization is carried out in the second crystallization equipment group, and the second crystallization equipment group includes a plurality of second crystallizers connected in series. The operating conditions of the second crystallization equipment group include: the temperature of the second crystallization slurry is -90 °C to -10 °C, the solid crystal concentration range of the second crystallization slurry is 0 to 60% by weight, and the absolute value of the temperature difference between two adjacent second crystallizers ranges from 1 to 25 °C; The method further includes: flushing at least part of the second solid phase with the cooling raw materials and then feeding it into the raw material mixing equipment; Optionally, the method further includes: adjusting the slurry flow rate entering the second crystallizer according to the liquid level in the second crystallizer; Optionally, the method further includes: adjusting the operating pressure of the third refrigerating medium according to the temperature of the second crystallization.

10. The method according to claim 6, wherein, the method further includes thermally flushing the slurries obtained from the raw material mixing device, the first crystallization device group, and the second crystallization device group respectively with a thermal flushing medium, and the source of the thermal flushing medium is selected from one or more of the mixed xylene raw material, the cooled raw material, the first liquid phase, the second liquid phase, and the high-purity product.

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