Energy-saving process and system for separating para-xylene by secondary crystallization

Abstract

This invention provides an energy-saving secondary crystallization process and system for separating para-xylene. The process includes the following steps: S1, feeding a mixed xylene raw material into a primary dynamic layer crystallizer for dynamic crystallization at -20°C to 0°C, centrifuging the crystallized material to obtain primary crystallization mother liquor and primary crystal crystals, and melting the primary crystal crystals to obtain para-xylene product; S2, feeding the primary crystallization mother liquor into a secondary static crystallizer for static crystallization at -60°C to -20°C to obtain secondary crystal crystals and secondary crystallization mother liquor; S3, melting the secondary crystal crystals and mixing them with the mixed xylene raw material from step S1; S4, passing the secondary crystallization mother liquor through a heat exchanger and a heater sequentially, and then collecting it as a by-product or discharging it externally. This process can simultaneously ensure high product purity, high total yield, and low system energy consumption while maintaining stable operation of the cryogenic system.

Description

Technical Field

[0001] This invention relates to the field of chemical separation technology, and in particular to an energy-saving two-stage crystallization process and system for separating paraxylene. Background Technology

[0002] Para-xylene (PX) is an important raw material in the polyester industry, serving as a key intermediate connecting upstream petroleum and downstream polyester products. Its primary use is in the production of purified terephthalic acid (PTA), which is then used to manufacture polyester (PET) fibers and resins. The main production method for para-xylene is the separation of mixed xylenes. Mixed xylenes mainly contain ortho-, para-, and meta-xylenes. Due to the similar boiling points of the isomers, adsorption separation and crystallization separation methods are commonly used industrially. Crystallization, with its lower energy consumption, has attracted considerable attention.

[0003] Existing crystallization patent technologies (such as CN201210553071.5, CN201210553163.3, CN200910057546.X, etc.) mostly employ two- or three-stage traditional suspension melt crystallization processes for multi-stage cooling crystallization. However, these methods generally suffer from the following drawbacks: Limitations of a single crystallization mode. The problem with suspension crystallization is that the crystals are suspended in the liquid phase by stirring or convection. Stirring or convection is needed to enhance the mass and heat transfer process between the solid and liquid phases. The crystallization rate is slow, and the mother liquor is easily trapped, resulting in low product purity.

[0004] Existing patented technologies (such as CN201310512725.4 and CN201310512340.8) employ advanced scraped-wall crystallizers and a two-stage crystallization process. However, under conditions of low paraxylene concentration in the feed, cryogenic treatment at -60°C is necessary to ensure recovery rates. In actual production, due to the presence of moving parts in the crystallizer, material aging and shedding occur at cryogenic temperatures. Numerous problems arise with the lubrication system, motor, system assembly precision, and sealing. Furthermore, the use of the same crystallizer for two-stage crystallization in practice makes it impossible to guarantee stable operation under cryogenic conditions, thus compromising the maintenance of a consistently low PX concentration in the mother liquor.

[0005] Therefore, there is an urgent need for a new crystallization separation process that can combine the advantages of different crystallization equipment, ensure high product purity and high yield, and optimize energy consumption. Summary of the Invention

[0006] The technical problem to be solved by the present invention is that the active equipment in the existing crystallization method for producing para-xylene cannot operate continuously and stably at low temperatures. The present invention provides a secondary crystallization separation process and system for para-xylene that can simultaneously ensure high product purity, high total yield and stable operation of the cryogenic system with low system energy consumption.

[0007] To achieve the above objectives, in a first aspect, this application provides an energy-saving secondary crystallization process for separating p-xylene, comprising the following steps: S1. The mixed xylene raw material is fed into a primary dynamic layer crystallizer for dynamic crystallization at -20℃ to 0℃. The crystallized material is centrifuged to obtain primary crystallization mother liquor and primary crystallization crystals. The primary crystallization crystals are melted to obtain para-xylene product. S2. The primary crystallization mother liquor is fed into a secondary static crystallizer and statically crystallized at -60℃ to -20℃ to obtain secondary crystals and secondary crystallization mother liquor. S3. Melt the secondary crystallized crystals and mix them with the mixed xylene raw materials in step S1; S4. The secondary crystallization mother liquor is passed through a heat exchanger and a heater in sequence and then collected or discharged as a by-product.

[0008] Optionally, before melting the primary crystals in step S1, the primary crystals are first pulped to obtain a pulping liquid. The pulping liquid is centrifuged to obtain a pulping slurry and pulping crystals. The pulping slurry is mixed with the mixed xylene raw material in step S1. After the pulping crystals are melted, the para-xylene product is obtained.

[0009] Optionally, after the primary crystals are melted, part of them are melted and sent out as products, while the remaining part is used as washing liquid to wash centrifuges used for centrifugation.

[0010] Optionally, the centrifuge before pulping is washed to obtain a primary centrifugal washing filtrate, which is then mixed with the mixed xylene raw material in step S1.

[0011] Optionally, the centrifuge after pulping is washed to obtain a secondary centrifugal washing filtrate, which is then mixed with the primary crystals before pulping.

[0012] Optionally, the secondary static crystallizer in step S2 adopts an intermittent operation of alternating hot and cold. First, the primary crystallization mother liquor is crystallized by the cooling medium on the tube side, and the secondary crystallization mother liquor is discharged. Then, by replacing the tube side with a heating medium, the crystals outside the tube are melted to obtain molten secondary crystals.

[0013] Optionally, the cooling medium may be one of low-temperature methanol, propylene, or ethylene, and the heating medium may be steam, hot water, or high-temperature methanol.

[0014] To achieve the above objectives, in a second aspect, this application provides an energy-saving two-stage crystallization system for separating para-xylene, comprising a feed tank connected to a mixed xylene feed pipe, a scraper-type falling film crystallizer connected to the outlet of the feed tank, a primary centrifuge connected to the outlet of the scraper-type falling film crystallizer, a pulping tank connected to the material outlet of the primary centrifuge, a secondary centrifuge connected to the material outlet of the pulping tank, a product melting tank connected to the material outlet of the secondary centrifuge, and a secondary static crystallizer connected to the crystallization mother liquor outlet of the primary centrifuge. The mixed xylene feed pipe is connected to a feed heat exchanger, and the crystallization mother liquor of the secondary static crystallizer is connected to the feed heat exchanger to exchange heat with the material in the mixed xylene feed pipe. The molten material of the secondary static crystallizer is connected to the mixed xylene feed pipe or the feed tank, and the crystallization mother liquor outlet of the secondary centrifuge is connected to the mixed xylene feed pipe or the feed tank.

[0015] Optionally, the outlet of the product melting tank is connected to a washing pipeline, which is connected to the primary centrifuge and the secondary centrifuge respectively. The washing filtrate outlet of the primary centrifuge is connected to the mixed xylene feed pipe or the feed tank, and the washing filtrate outlet of the secondary centrifuge is connected to the pulping tank.

[0016] Optionally, the secondary static crystallizer includes a shell, a plurality of heat exchange tubes disposed within the shell, detachable heat / cold medium inlets and detachable heat / cold medium outlets disposed at both ends of the heat exchange tubes, a material inlet and a material outlet connected to the shell, and a heating system or a second refrigeration system connected to the heat exchange tubes.

[0017] Optionally, the scraper-type falling film crystallizer includes a crystallizer body, a stirring shaft rotatably disposed within the crystallizer body, a scraper and blades fixedly disposed on the stirring shaft, a motor for driving the stirring shaft to rotate, a jacket fixedly disposed on the outside of the crystallizer body, and a material distributor disposed within the crystallizer body.

[0018] Optionally, the jacket is connected to the first refrigeration system.

[0019] The present invention provides an energy-saving secondary crystallization process and system for separating p-xylene, which, compared with the prior art, has the following advantages: High product purity: Due to the use of dynamic layer crystallization in the first stage, the crystals can be effectively purified through secondary pulping and separation, directly obtaining ultra-high purity (≥99.8%) paraxylene products.

[0020] High overall yield: The secondary stage uses static crystallization suitable for deep recovery to 'clean up' the paraxylene in the primary mother liquor and return the recovered material to the main system, which significantly improves the overall yield of paraxylene.

[0021] The system boasts excellent energy efficiency: the first stage removes most products at a higher temperature, resulting in a relatively low cooling load; the second stage operates at a lower temperature, but handles a smaller volume. This temperature gradient design optimizes the system's overall energy consumption.

[0022] Good operational stability: The two-stage crystallization has a clear division of labor, with dynamic crystallization responsible for the main product and static crystallization responsible for mother liquor treatment, avoiding the instability of a single crystallizer operating within a wide concentration and temperature range. Attached Figure Description

[0023] Appendix Figure 1 This is a schematic diagram of the process flow of the present invention (Example 1); Appendix Figure 2 This is a schematic diagram of the process flow of the present invention (Example 2); Appendix Figure 3 This is a schematic diagram of a scraping-type falling film crystallizer; Appendix Figure 4 This is a schematic diagram of a two-stage static crystallizer.

[0024] Wherein: A-Feed heat exchanger; B-Feed tank; C-Scraped falling film crystallizer; D-First refrigeration system; E-First-stage centrifuge; F-Pulping tank; G-Second-stage centrifuge; H-Product melting tank; I-Second-stage static crystallizer; J-Heat exchange medium; K-Second refrigeration system; L-Heating system; M-Heater; 1- Mixed xylene feed; 2- Primary crystallization feed; 3- Primary crystallization slurry; 4- Primary crystallization mother liquor; 5- Primary crystallization crystals; 6- Pulping material; 7- Secondary pulverizing slurry; 8- Secondary pulverizing crystals; 9- p-xylene product; 10- Washing liquid; 11- Primary centrifugal washing liquid; 12- Primary centrifugal washing filtrate; 13- Pulping liquid; 14- Secondary centrifugal washing liquid; 15- Secondary centrifugal washing filtrate; 16- Secondary crystallization melt; 17- Secondary crystallization mother liquor; 18- Motor; 19- Agitator shaft; 20- Distributor; 21- Scraper; 22- Jacket; 23- Blade; 24- Shell; 25- Material inlet; 26- Material outlet; 27- Detachable heat / cold medium inlet; 28- Detachable heat / cold medium outlet; 29- Heat exchanger tube. Detailed Implementation

[0025] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to the embodiments.

[0027] The present invention provides an energy-saving secondary crystallization process for separating p-xylene, comprising the following steps.

[0028] Step 1, Primary Dynamic Layer Crystallization: The mixed xylene raw material is fed into the primary dynamic layer crystallizer and crystallized at -20℃ to 0℃ (preferably -5℃ to 0℃). After centrifugation, a second pulping and centrifugation can be performed (this is an optional step and needs to be selected according to the xylene feed concentration) and melting to obtain a para-xylene product with a purity of not less than 99.7% and a primary mother liquor.

[0029] Step 2, Secondary Static Crystallization: The primary mother liquor generated in Step 1 is sent to a secondary static crystallizer and static crystallization is carried out at -60℃ to -20℃ (preferably -60℃ to -30℃). This step is an intermittent operation. The crystallizer goes through the process of crystallization, discharging mother liquor, and melting crystals to obtain a high PX concentration solution and a low PX concentration secondary crystallization mother liquor.

[0030] Step 3, Material Reflux: The secondary high PX concentration solution obtained in Step 2 is returned to the mixed xylene raw material in Step 1, and they enter the primary dynamic layer crystallizer for processing together.

[0031] Step 4, Product Processing: The secondary crystallization mother liquor produced in Step 2 mainly contains o-xylene and m-xylene, which can be collected or discharged as by-products.

[0032] To prevent the scraper springs, blades, and other parts from falling off in step 1 and jamming the pump impeller, a filter is installed on the pipeline transporting slurry containing crystals.

[0033] To ensure stable heat transfer during the low-temperature crystallization and high-temperature melting processes in step 2, methanol is introduced as the heat exchange medium for the secondary crystallization. Low-temperature methanol is obtained through a refrigerant at the low temperature, or ethylene or propylene can be used as the refrigerant depending on the crystallization temperature. At the high temperature, steam, hot water, or higher-temperature methanol are used. This ensures that the heat exchange medium in the secondary crystallizer remains constant, resulting in minimal variation in the heat transfer coefficient.

[0034] To prevent blockages in pipelines transporting materials containing crystals, 60°C PX product solution is used for regular or emergency flushing of easily clogged pipelines and equipment. Electric heat tracing is also installed at easily clogged areas.

[0035] To prevent excessive moisture content in the xylene entering the system, which could affect the stable operation of subsequent crystallization equipment, a dewatering device is designed into the feed. Additionally, the crystallizer, secondary pulping tank, and other components are regularly defrosted and thawed.

[0036] About the system An energy-saving two-stage crystallization system for separating para-xylene includes a feed tank connected to a mixed xylene feed pipe, a scraper-type falling film crystallizer connected to the outlet of the feed tank, a primary centrifuge connected to the outlet of the scraper-type falling film crystallizer, a pulping tank connected to the material outlet of the primary centrifuge, a secondary centrifuge connected to the material outlet of the pulping tank, a product melting tank connected to the material outlet of the secondary centrifuge, and a secondary static crystallizer connected to the crystallization mother liquor outlet of the primary centrifuge. The mixed xylene feed pipe is connected to a feed heat exchanger. The crystallization mother liquor of the secondary static crystallizer is connected to the feed heat exchanger to exchange heat with the material in the mixed xylene feed pipe. The molten material of the secondary static crystallizer is connected to the mixed xylene feed pipe or the feed tank. The crystallization mother liquor outlet of the secondary centrifuge is connected to the mixed xylene feed pipe or the feed tank.

[0037] The outlet of the product melting tank is connected to a washing pipeline, which is connected to the primary centrifuge and the secondary centrifuge respectively. The washing filtrate outlet of the primary centrifuge is connected to the mixed xylene feed pipe or the feed tank, and the washing filtrate outlet of the secondary centrifuge is connected to the pulping tank.

[0038] like Figure 4 As shown, the secondary static crystallizer includes a shell 24, a plurality of heat exchange tubes 29 disposed within the shell 24, detachable heat / coolant inlets 27 and detachable heat / coolant outlets 28 disposed at both ends of the heat exchange tubes 29, a material inlet 25 and a material outlet 26 connected to the shell 24, and a heating system or a second refrigeration system connected to the heat exchange tubes 29.

[0039] like Figure 3 As shown, the scraping-type falling film crystallizer includes a crystallizer body, a stirring shaft 19 rotatably disposed within the crystallizer body, a scraper 21 and blades 23 fixedly disposed on the stirring shaft 19, a motor 18 driving the stirring shaft 19 to rotate, a jacket 22 fixedly disposed on the outside of the crystallizer body, and a feeder 20 disposed within the crystallizer body. The jacket 22 is connected to the first refrigeration system.

[0040] Primary crystallization is the core refining step for paraxylene separation. A scraped-surface falling film crystallizer is used, which is equipped with a raw material inlet, a product outlet, and an internal scraper.

[0041] After the material enters from the top of the crystallizer, it forms a uniform liquid film along the inner wall of the tube under the action of the shell-side cooling medium (low-temperature refrigerant). At the same time, the built-in rotating scraper removes the paraxylene crystals formed on the inner wall of the tube in real time, avoiding the decrease in heat transfer efficiency caused by crystal adhesion. This design makes the liquid film residence time much shorter than that of traditional suspension crystallization, which can significantly reduce the entrainment and inclusion of impurities in the mother liquor.

[0042] This device has the following features: (1) Due to the dynamic heat and mass transfer process of the liquid film, the separation efficiency and yield are greatly improved, and continuous operation is possible with high production intensity.

[0043] (2) The scraper speed is adjustable, the liquid film residence time is short, and the crystal purity is high.

[0044] (3) The process is easy to scale up. When producing large quantities, the same crystallizers can be connected in parallel for production, and there is no scale-up effect.

[0045] Secondary crystallization is a key recovery step for improving the yield of paraxylene. Deep treatment of the mother liquor from primary crystallization increases the overall system yield. The equipment uses a detachable static crystallizer, operated intermittently by two or more units. It is equipped with a raw material inlet, a product outlet, and internal heat exchange piping.

[0046] The secondary crystallization feed comes from the primary mother liquor. After entering, the PX in the primary mother liquor is separated into crystals by the cooling medium (low-temperature refrigerant) on the pipe side. After crystallization, the secondary mother liquor is first discharged, and then the heating medium is replaced on the pipe side to melt the crystals outside the pipe. Finally, the molten "crude para-xylene material" is pumped back to the crystallization feed tank to achieve material circulation; while the remaining secondary mother liquor is sent out of the boundary area as a by-product.

[0047] This device has the following features: (1) The intermittent operation mode is flexible and the cycle time can be adjusted according to the concentration of paraxylene in the feed; (2) The detachable structure reduces equipment maintenance costs; (3) There are no moving parts inside, so there are no problems with expansion, sealing, or fit during low-temperature operation, and the operation is stable and reliable; (4) Low feed requirements, can adapt to low concentration and high impurity feed.

[0048] Secondary pulping and centrifugal washing are used for low-concentration PX feed, where the crystal purity is low after the first centrifugation. Secondary pulping and centrifugal washing further purify the PX.

[0049] The crystals discharged from the first centrifugation first enter the pulping tank and are pulped with pure para-xylene. The crystals are dispersed by stirring and the impurities attached to the surface are dissolved. The pulp is then sent to a centrifuge for secondary centrifugation and washing. After separation, the crystal purity is ≥99.7%, and it is sent to the finished product tank for melting. The mother liquor from the secondary centrifugation is recycled.

[0050] Example 1: like Figure 1 As shown, the raw materials are: flow rate 20.48 t / h, composition: p-xylene 83.46 wt% (optional range 80~85%), m-xylene 11.18 wt%, o-xylene 1.92 wt%.

[0051] Process conditions: Primary dynamic crystallization: Mixed xylene feed 1 (para-xylene concentration 80~85%) is cooled by feed heat exchanger A and then temporarily stored in feed tank B. The primary crystallization feed 2 at the bottom of feed tank B is sent to primary crystallizer C. Primary crystallizer C is connected to the first refrigeration system D and falls film crystallization is carried out at 0℃ and 110 kPa A. Primary crystallization slurry 3 flows into primary centrifuge E by gravity. The separated primary crystal crystals 5 are washed with product para-xylene and then melted in product melting tank H (operating temperature about 25℃). Part of the melted para-xylene is sent out as para-xylene product 9, and part is recycled as washing liquid 10 and heat source. Secondary static crystallization: The primary crystallization mother liquor 4 (PX content 60~70%) is fed into the secondary static crystallizer I (A / B alternating intermittent operation). First, the second refrigeration system K is used to slowly cool the liquid to -30℃ and maintain this temperature for approximately 1 hour to promote crystal growth. Methanol is used as the heat exchange medium J. Then, the heating system L is switched to raise the temperature to ~20℃ to melt the crystals. Coarse crystals (PX concentration similar to the raw material) are collected. After melting, the resulting secondary crystallization melt 16 is used as reflux material and returned to the feed tank B to mix with fresh mixed xylene feed 1. The secondary crystallization mother liquor 17, after passing through the feed heat exchanger A and heater M, is heated to room temperature and used as a by-product.

[0052] result: Testing revealed that the purity of the p-xylene obtained from the first-stage product was ≥99.8%. The PX content in the secondary crystallization mother liquor was reduced to ≤30%; Calculations show that the total yield of PX in the raw materials using this process is ≥92%.

[0053] Example 2: like Figure 2As shown, the first-stage dynamic crystallization process is as follows: Mixed xylene feed 1 (para-xylene concentration 55~60%) is cooled and then temporarily stored in feed tank B before being sent to the first-stage crystallizer C for falling film crystallization at -20℃ and 110 kPa A; the first-stage crystallization slurry 3 flows into the first-stage centrifuge E by gravity, and the separated first-stage crystal crystals 5 are washed with product para-xylene and then enter the pulping tank F for secondary pulping at -7℃ and 110 kPa A; the pulp 6 flows into the second-stage centrifuge G by gravity, and then melts to extract high-purity PX crystals. The crystal suspension enters the centrifuge for separation by gravity. The separated secondary pulping slurry 7 is returned to the feed tank B. The separated secondary pulping crystals 8 are washed with product p-xylene and then melted in the product melting tank H (operating temperature about 25°C). Part of the melted p-xylene is sent out as p-xylene product 9, and part is recycled as washing liquid 10 and heat source. The washing liquid 10 includes primary centrifugal washing liquid 11 used to wash the primary centrifuge E. The primary centrifugal washing filtrate 12 after washing is returned to the feed tank B. The secondary centrifugal washing liquid 14 used to wash the secondary centrifuge G is returned to the pulping tank F.

[0054] Secondary static crystallization: Primary crystallization mother liquor 4 (PX content 30~40%) is fed into multiple intermittently operated secondary static crystallizers I. First, a cooling medium is circulated to slowly lower the temperature to -50℃ and maintain this temperature for approximately 1 hour to grow crystals. Then, a heating medium is switched to raise the temperature to ~20℃ to melt the crystals. Coarse crystals (PX concentration similar to the raw material) are collected. The resulting secondary crystallization melt 16 is used as reflux material and returned to feed tank B to mix with fresh mixed xylene feed 1. Secondary crystallization mother liquor 15 is heated to room temperature after passing through feed heat exchanger A and heater M, and then used as a byproduct.

[0055] result: Testing revealed that the purity of the p-xylene obtained from the first-stage product was ≥99.8%. The PX content in the secondary crystallization mother liquor was reduced to ≤15%; Calculations show that the total yield of PX in the raw materials using this process is ≥87%.

[0056] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. An energy-saving secondary crystallization process for separating p-xylene, characterized in that, Includes the following steps: S1. The mixed xylene raw material is fed into a primary dynamic layer crystallizer for dynamic crystallization at -20℃ to 0℃. The crystallized material is centrifuged to obtain primary crystallization mother liquor and primary crystallization crystals. The primary crystallization crystals are melted to obtain para-xylene product. S2. The primary crystallization mother liquor is fed into a secondary static crystallizer and statically crystallized at -60℃ to -20℃ to obtain secondary crystals and secondary crystallization mother liquor. S3. Melt the secondary crystallized crystals and mix them with the mixed xylene raw materials in step S1; S4. The secondary crystallization mother liquor is passed through a heat exchanger and a heater in sequence and then collected or discharged as a by-product.

2. The energy-saving secondary crystallization process for separating p-xylene as described in claim 1, characterized in that: Before melting the primary crystals in step S1, the primary crystals are first pulped to obtain a pulping liquid. The pulping liquid is centrifuged to obtain a pulping slurry and pulping crystals. The pulping slurry is mixed with the mixed xylene raw material in step S1. After the pulping crystals are melted, the para-xylene product is obtained.

3. The energy-saving secondary crystallization process for separating p-xylene as described in claim 2, characterized in that: After the primary crystals are melted, part of them are melted and sent out as products, while the remaining part is used as washing liquid to wash centrifuges used for centrifugation.

4. The energy-saving secondary crystallization process for separating p-xylene as described in claim 3, characterized in that: The centrifuge before pulping is washed to obtain the first centrifuge washing filtrate, which is then mixed with the mixed xylene raw material in step S1.

5. The energy-saving secondary crystallization process for separating p-xylene as described in claim 4, characterized in that: The centrifuge after pulping is washed to obtain a secondary centrifugal washing filtrate, which is then mixed with the primary crystals before pulping.

6. The energy-saving secondary crystallization process for separating p-xylene as described in claim 3, characterized in that: In step S2, the secondary static crystallizer adopts an intermittent operation of alternating hot and cold. First, the primary crystallization mother liquor is crystallized by the cooling medium on the tube side, and the secondary crystallization mother liquor is discharged. Then, by replacing the tube side with the heating medium, the crystal outside the tube is melted to obtain the molten secondary crystal.

7. The energy-saving secondary crystallization process for separating p-xylene as described in claim 6, characterized in that: The cooling medium is one of low-temperature methanol, propylene or ethylene, and the heating medium is steam, hot water or high-temperature methanol.

8. An energy-saving secondary crystallization system for separating p-xylene, characterized in that: The system includes a feed tank connected to a mixed xylene feed pipe, a scraper-type falling film crystallizer connected to the outlet of the feed tank, a primary centrifuge connected to the outlet of the scraper-type falling film crystallizer, a pulping tank connected to the material outlet of the primary centrifuge, a secondary centrifuge connected to the material outlet of the pulping tank, a product melting tank connected to the material outlet of the secondary centrifuge, and a secondary static crystallizer connected to the crystallization mother liquor outlet of the primary centrifuge. The mixed xylene feed pipe is connected to a feed heat exchanger. The crystallization mother liquor of the secondary static crystallizer is connected to the feed heat exchanger to exchange heat with the material in the mixed xylene feed pipe. The molten material of the secondary static crystallizer is connected to the mixed xylene feed pipe or the feed tank. The crystallization mother liquor outlet of the secondary centrifuge is connected to the mixed xylene feed pipe or the feed tank.

9. The energy-saving secondary crystallization system for separating p-xylene as described in claim 8, characterized in that: The outlet of the product melting tank is connected to a washing pipeline, which is connected to the primary centrifuge and the secondary centrifuge respectively. The washing filtrate outlet of the primary centrifuge is connected to the mixed xylene feed pipe or the feed tank, and the washing filtrate outlet of the secondary centrifuge is connected to the pulping tank.

10. The energy-saving secondary crystallization system for separating p-xylene as described in claim 8, characterized in that: The secondary static crystallizer includes a shell, a plurality of heat exchange tubes disposed within the shell, detachable heat / coolant inlets and detachable heat / coolant outlets disposed at both ends of the heat exchange tubes, a material inlet and a material outlet connected to the shell, and a heating system or a second refrigeration system connected to the heat exchange tubes.

11. The energy-saving secondary crystallization system for separating p-xylene as described in claim 8, characterized in that: The scraping-type falling film crystallizer includes a crystallizer body, a stirring shaft rotatably disposed within the crystallizer body, scrapers and blades fixedly disposed on the stirring shaft, a motor for driving the stirring shaft to rotate, a jacket fixedly disposed on the outside of the crystallizer body, and a material distributor disposed within the crystallizer body.

12. The energy-saving secondary crystallization system for separating p-xylene as described in claim 11, characterized in that: The jacket is connected to the first refrigeration system.

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