Para-xylene-rich crystal washing method and system

Abstract

The invention discloses a p-xylene-rich crystal washing system which comprises three stages of suspension crystallizers, namely a first-stage crystallizer (a), a second-stage crystallizer (d) and a third-stage crystallizer (g), the first solid-liquid separation equipment (b) is used for carrying out solid-liquid separation and washing on crystal mush crystallized by the first-stage crystallizer (a); the second solid-liquid separation equipment (e) is used for carrying out solid-liquid separation and washing on crystal mush crystallized by the secondary crystallizer (d); and the third solid-liquid separation equipment (h) is used for carrying out solid-liquid separation and washing on crystal mush crystallized by the third-stage crystallizer (g). The invention also discloses a p-xylene-rich crystal washing method which is operated on the p-xylene-rich crystal washing system. Aiming at the problem that the raw material containing high-concentration p-xylene is difficult to wash in the refining process, the invention creatively provides a scheme for washing the raw material containing p-xylene, so that the use amount of a product washing solution is reduced, and the yield of the product is improved.

Description

Technical Field

[0001] The present invention relates to a crystallization washing method and system for p-xylene-rich feedstock. Background Art

[0002] p-Xylene (PX) is an important chemical raw material, mainly used in polyester production. Industrially, PX is mainly obtained by separating C8 aromatics, which in addition to PX also include: m-xylene (MX), o-xylene (OX), and ethylbenzene (EB). The boiling points of the components of C8 aromatics are extremely close, and it is difficult to obtain high-purity products through conventional distillation methods. The freezing points of the components of C8 vary greatly, and the freezing point of PX is 13.2 °C, which is particularly suitable for separating and purifying PX by crystallization.

[0003] According to different operation modes, the crystallization method can be divided into the suspension method and the static method. The static method is easier to obtain high-purity products, but its equipment processing capacity is low. The suspension method has a large processing capacity, and its post-treatment process is more complex and important. During the crystallization process, single crystals are usually pure substances, while impurities exist in the crystallization mother liquor in the form of mixtures. It is impossible to completely separate the crystals and the mother liquor. If the final product purity is to be improved, it can be achieved by reducing the amount of the mother liquor and the impurity content in the mother liquor. The washing process is an important post-treatment section of suspension crystallization, and the washing directly affects the product purity and yield. Summary of the Invention

[0004] In order to at least partially solve the problem of low crystallization washing efficiency under p-xylene-rich feedstock, improve the washing efficiency of p-xylene and reduce energy consumption, the present invention is made.

[0005] As an aspect of the present invention, it relates to a crystallization washing system for p-xylene-rich feedstock, including a three-stage suspension crystallizer: a primary crystallizer (a), a secondary crystallizer (d), and a tertiary crystallizer (g); and further including:

[0006] A first solid-liquid separation device (b), used for solid-liquid separation and washing of the crystal slurry crystallized in the primary crystallizer (a);

[0007] A second solid-liquid separation device (e), used for solid-liquid separation and washing of the crystal slurry crystallized in the secondary crystallizer (d);

[0008] A third solid-liquid separation device (h), used for solid-liquid separation and washing of the crystal slurry crystallized in the tertiary crystallizer (g).

[0009] In the specific implementation manner, the p-xylene-rich crystallization washing system further includes: a synergistic mixer (i) and a fourth solid-liquid separation device (j); the synergistic mixer (i) uses the wash liquid of the first solid-liquid separation device (b) and the wash liquid of the second solid-liquid separation device (e) to perform re-suspension crystallization treatment on the crystals crystallized by the tertiary crystallizer (g), and the fourth solid-liquid separation device (j) performs solid-liquid separation and washing on the crystal slurry mixed by the synergistic mixer (i).

[0010] As another aspect of the present invention, it relates to a p-xylene-rich crystallization washing method, which operates on the above-mentioned p-xylene-rich crystallization washing system. The C8 mixture containing p-xylene enters the primary crystallizer (a) as a raw material for suspension crystallization and enters the first solid-liquid separation device (b), the second solid-liquid separation device (e), the third solid-liquid separation device (h) and the fourth solid-liquid separation device (j) as a washing liquid to wash the crystals; the wash liquid of the first solid-liquid separation device (b), the wash liquid of the second solid-liquid separation device (e), the wash liquid of the fourth solid-liquid separation device (j) and a part of the filtered mother liquor of the fourth solid-liquid separation device (j) enter the synergistic mixer (i) to perform re-suspension crystallization treatment on the crystals crystallized by the tertiary crystallizer (g).

[0011] In the specific implementation manner, the method includes:

[0012] (1) The p-xylene-containing raw material (1) is added to the primary crystallizer (a) for cooling crystallization to obtain a crystal slurry (3);

[0013] (2) The crystal slurry (3) enters the first solid-liquid separation device (b), and after washing, a first filtered mother liquor, a first wash liquid (8) and first p-xylene crystals (4) are obtained;

[0014] (3) A part of the first filtered mother liquor is returned to the primary crystallizer (a), and the remaining part enters the secondary crystallizer (d) for cooling crystallization to obtain a crystal slurry (10);

[0015] (4) The crystal slurry (10) enters the second solid-liquid separation device (e), and after washing, a second filtered mother liquor, a second wash liquid (16) and second p-xylene crystals (12) are obtained;

[0016] (5) A part of the second filtered mother liquor is returned to the secondary crystallizer (d), and the remaining part enters the tertiary crystallizer (g) for cooling crystallization to obtain a crystal slurry (18);

[0017] (6) The crystal slurry (18) enters the third solid-liquid separation device (h) to obtain a third filtered mother liquor, a third wash liquid (20) and third p-xylene crystals (19), a part of the third filtered mother liquor is returned to the tertiary crystallizer (g), and the third wash liquid (20) is returned to the secondary crystallizer (d);

[0018] (7) The first eluate (8), the second eluate (16), the fourth eluate (29) of the fourth solid-liquid separation device (j), and a part of the filtered mother liquor of the fourth solid-liquid separation device (j) enter the synergistic mixer (i) to perform re-suspension crystallization treatment on the crystals crystallized in the third-stage crystallizer (g), and then enter the fourth solid-liquid separation device (j) for solid-liquid separation and washing.

[0019] In a specific embodiment, the operating temperature of the first-stage crystallizer (a) is 3 to 11 °C; the operating temperature of the second-stage crystallizer (d) is 0 to 8 °C; the operating temperature of the third-stage crystallizer (g) is -18 to 5 °C; the operating temperature of the first-stage crystallizer (a) is greater than the operating temperature of the second-stage crystallizer (d).

[0020] In a specific embodiment, the operating temperature of the first-stage crystallizer (a) is 5 to 10 °C, the operating temperature of the second-stage crystallizer (d) is 1 to 7 °C, and the operating temperature of the third-stage crystallizer (g) is -15 °C to 3 °C.

[0021] In a specific embodiment, the crystal slurry (3) in the first solid-liquid separation device (b) is washed with the raw material (2) containing p-xylene and the p-xylene product (6).

[0022] In a specific embodiment, in step (2), a part of the melted first p-xylene crystal (4) is returned to the first solid-liquid separation device (b) as a washing liquid.

[0023] In a specific embodiment, in step (4), after the crystal slurry (10) enters the second solid-liquid separation device (e), it is washed with the raw material containing p-xylene.

[0024] In a specific embodiment, in step (6), after the crystal slurry (18) enters the third solid-liquid separation device (h), it is washed with the raw material containing p-xylene.

[0025] The p-xylene-containing raw material is a C8 mixture containing 92 wt% to 98 wt% of p-xylene.

[0026] Compared with the existing inventions, the method of the present invention has the advantages of smaller total equipment size and lower crystallization energy consumption when processing a feed with a concentration of 92 wt% to 98 wt%. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a process flow chart of the suspension crystallization production of p-xylene according to the present invention patent.

[0028] The C8 mixture rich in p-xylene is added as raw material 1 to the primary crystallizer a for cooling crystallization. The resulting crystal slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and the first p-xylene crystals. After the p-xylene crystals are washed with raw material 2 and product 6, the first eluate 8 and the first p-xylene crystals 4 are obtained. The first p-xylene crystals 4 are melted in the first melter c. Among them, part of the p-xylene product is returned as product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as the p-xylene product 5. Part of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d as the first filtrate mother liquor 9 for cooling crystallization. The resulting crystal slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. After the p-xylene crystals are washed with raw material 11 and product 14, the second eluate 16 and the second p-xylene crystals 12 are obtained. The second p-xylene crystals 12 are melted in the second melter f. Part of the p-xylene product is returned as product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as the p-xylene product 13. Part of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g as the second filtrate mother liquor 17 for cooling crystallization. The resulting crystal slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and p-xylene crystals. After the p-xylene crystals are washed with raw material 21, the third eluate 20 and the third p-xylene crystals 19 are obtained. Part of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter the synergistic mixer i. The fully mixed crystal slurry 24 enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. After the p-xylene crystals are washed with raw material 28 and product 27, the fourth eluate 29 and the fourth p-xylene crystals 25 are obtained. At least part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the remaining part of the fourth filtrate mother liquor and the fourth eluate 29 are returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in the third melter k. Part of the p-xylene product 27 is returned to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as the p-xylene product 26. Detailed implementation mode

[0029] The inventor conducted experiments with reference to Patent US3177265. The primary crystallization process of this process is mainly responsible for recovering p-xylene, and the secondary crystallization process is responsible for purifying p-xylene. Although this method can obtain products that meet the requirements, the energy consumption of the recrystallization process is relatively high, and the economic applicability is poor.

[0030] The inventor conducted experiments with reference to Patent CN101941883 and adopted a method of two-stage crystallization plus a pulping tank to treat mixed xylene. Due to the limitation of the solid content of the crystal slurry in the crystallizer, a large amount of mother liquor circulation is required in both the first-stage crystallizer and the second-stage crystallization to ensure a high yield of the process. At the same time, due to the large difference between the feed temperature and the crystallization temperature, the obtained crystal particles are relatively small. This method reduces the power of the filtration equipment by setting a clear mother liquor overflow port at the upper part of the crystallizer. However, this also easily leads to the loss of fine crystals, thereby reducing the process yield. At the same time, since the temperature of the crystal cake is lower than the temperature of the product washing liquid, it is easy to cause nucleation during the washing process, and the fine crystals will wrap more impurity mother liquor, further reducing the product purity.

[0031] The inventor conducted experiments with reference to Patent CN103772131 and Patent CN103880586. Since the secondary crystals need to stay in the pulping tank once after being formed in the second-stage crystallizer and the amount of secondary crystals is relatively large, the overall volume of the equipment is relatively large. The large temperature difference between the two-stage crystallization also results in relatively small overall crystal particle sizes.

[0032] In view of the fact that the above prior art cannot meet the inventor's expectations, through further research and development, the present invention is made.

[0033] The present invention uses a first-stage crystallizer a, a second-stage crystallizer d, and a synergistic mixer i to produce crystal slurry for the crystallization washing method of para-xylene-rich. After the crystal slurry is filtered and washed by a solid-liquid separation device respectively, high-purity products are obtained. The crystal slurry of the third-stage crystallizer g is filtered and washed by a solid-liquid separation device to obtain para-xylene crystals, which enter the synergistic mixer i to be mixed with the washing liquid. By optimizing the process, high-purity para-xylene products can be obtained with lower energy consumption. The specific implementation examples are as follows.

[0034] Example 1

[0035] A C8 mixture containing 98 wt% of p-xylene is added as Feedstock 1 (flow rate: 3850 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 11°C. The resulting slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 886 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 10% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 8°C. The resulting slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 179 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 20% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of 5°C. The resulting slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 57 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 56 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 15% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.89 wt%, and the yield reaches 91.76%.

[0036] Example 2

[0037] A C8 mixture containing 98 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and added to a primary crystallizer a for cooling crystallization at a crystallization temperature of 9 °C. The resulting crystal slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 886 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 10% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 6 °C. The resulting crystal slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 179 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 20% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of 0 °C. The resulting crystal slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 57 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 56 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. A part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 15% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.82 wt%, and the yield reaches 95.44%.

[0038] Example 3

[0039] A C8 mixture containing 95 wt% of p-xylene is added as raw material 1 (flow rate is 3850 kg / h) into the primary crystallizer a for cooling and crystallization. The crystallization temperature is 7°C. The obtained slurry 3 enters the first solid-liquid separation equipment b to obtain the first filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with raw material 2 (flow rate is 707 kg / h) and product 6 to obtain the first eluate 8 and the first p-xylene crystals 4; the first p-xylene crystals 4 are melted in the first melter c, of which about 15% by weight are returned to the first solid-liquid separation equipment b as the p-xylene product 6, and the remaining part of the p-xylene product 5 leaves the crystallization system; 60% by weight of the first filtered mother liquor The first filtered mother liquor 7 is returned to the primary crystallizer a, and the remaining part is used as the first filtered mother liquor 9 to enter the secondary crystallizer d for cooling and crystallization. The crystallization temperature is 4°C. The obtained crystal slurry 10 enters the second solid-liquid separation equipment e to obtain the second filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with the raw material 11 (with a flow rate of 238 kg / h) and the product 14 to obtain the second eluate 16 and the second p-xylene crystals 12; the second p-xylene crystals 12 are melted in the second melter f, among which about 20% by weight of the p-xylene product is returned to the second solid-liquid separation equipment e as the product 14, and the remaining part leaves the crystallization system as the p-xylene product 13; 40% by weight of the second filtered mother liquor ... The mother liquor is returned to the secondary crystallizer d as the second filtered mother liquor 15, and the remaining part is entered into the tertiary crystallizer g as the second filtered mother liquor 17 for cooling and crystallization. The crystallization temperature is -4°C. The obtained crystal slurry 18 enters the third solid-liquid separation equipment h to obtain the third filtered mother liquor and the third p-xylene crystals. After the p-xylene crystals are washed with the raw material 21 (with a flow rate of 103 kg / h), the third eluate 20 and the third p-xylene crystals 19 are obtained. About 35% by weight of the third filtered mother liquor is returned to the tertiary crystallizer g as the third filtered mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtered mother liquor 23. The third eluate 20 returns to the secondary crystallizer d; the third p-xylene crystals 19, the first washed The liquid 8 and the second eluate 16 enter the synergistic mixer i, and the fully mixed slurry 24 enters the fourth solid-liquid separation equipment j to obtain the fourth filtered mother liquor and p-xylene crystals. After the p-xylene crystals are washed by the raw material 28 (flow rate is 103kg / h) and the product 27, the fourth eluate 29 and the fourth p-xylene crystals 25 are obtained. Part of the fourth filtered mother liquor is returned to the primary crystallizer a as the fourth filtered mother liquor 31, and the fourth eluate returns to the synergistic mixer i; the fourth p-xylene crystals 25 are melted in the third melter k, among which 20% of the weight of the p-xylene product is returned to the fourth solid-liquid separation equipment j as the product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and continuously operated, the system fluctuation is small, and the purity of the obtained product reaches 99.85wt%, and the yield reaches 91.08%.

[0040] Example 4

[0041] A C8 mixture containing 95 wt% p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting crystal slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 707 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 15% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 60% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 2°C. The resulting crystal slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 238 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 20% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 40% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -7°C. The resulting crystal slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 103 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter an enhancement mixer i. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 103 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the enhancement mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.81 wt%, and the yield reaches 92.88%.

[0042] Example 5

[0043] The C8 mixture containing 92 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and added to the first-stage crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting crystal slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 597 kg / h) and Product 6 to obtain the first eluate 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 20% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the first-stage crystallizer a, and the remaining part enters the second-stage crystallizer d for cooling crystallization at a crystallization temperature of 1°C. The resulting crystal slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 320 kg / h) and Product 14 to obtain the second eluate 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 are melted in the second melter f. Among them, approximately 25% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the second-stage crystallizer d, and the remaining part enters the third-stage crystallizer g for cooling crystallization at a crystallization temperature of -13°C. The resulting crystal slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 153 kg / h) to obtain the third eluate 20 and the third p-xylene crystals 19. Approximately 10% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the third-stage crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the second-stage crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter the synergistic mixer i. The fully mixed crystal slurry 24 enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 158 kg / h) and Product 27 to obtain the fourth eluate 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the first-stage crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in the third melter k. Among them, 20% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.88 wt%, and the yield reaches 91.71%.

[0044] Example 6

[0045] The C8 mixture containing 92 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and added to the primary crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. After being washed with Feedstock 2 (flow rate: 597 kg / h) and Product 6, the p-xylene crystals yield the first eluate 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 20% by weight of the p-xylene product is returned to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of 0°C. The resulting slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. After being washed with Feedstock 11 (flow rate: 320 kg / h) and Product 14, the p-xylene crystals yield the second eluate 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 are melted in the second melter f. Among them, approximately 25% by weight of the p-xylene product is returned to the second solid-liquid separation device e as Product 14, and the remaining part leaves the crystallization system as the p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -18°C. The resulting slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. After being washed with Feedstock 21 (flow rate: 153 kg / h), the p-xylene crystals yield the third eluate 20 and the third p-xylene crystals 19. Approximately 10% by weight of the third filtrate mother liquor is returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter the synergistic mixer i. After sufficient mixing, the resulting slurry 24 enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. After being washed with Feedstock 28 (flow rate: 158 kg / h) and Product 27, the p-xylene crystals yield the fourth eluate 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in the third melter k. Among them, 20% by weight of the p-xylene product is returned to the fourth solid-liquid separation device j as Product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.8 wt%, and the yield is 93.72%.

[0046] Example 7

[0047] A C8 mixture containing 96 wt% p-xylene is added as Feedstock 1 (flow rate: 3850 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 9°C. The resulting crystal slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 793 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 12% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 6°C. The resulting crystal slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 202 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 20% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of 0°C. The resulting crystal slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 85 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 83 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.85 wt%, and the yield reaches 90.16%.

[0048] Example 8

[0049] A C8 mixture containing 96 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 7°C. The resulting crystal slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 793 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 12% by weight of the p-xylene is returned as Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 4°C. The resulting crystal slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 202 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 20% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -3°C. The resulting crystal slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 85 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter an enhancing mixer i. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 83 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the enhancing mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.81 wt%, and the yield reaches 92.49%.

[0050] Example 9

[0051] A C8 mixture containing 93 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 6°C. The resulting slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 653 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 17% by weight of the p-xylene is returned as Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 3°C. The resulting slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 269 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 22% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -10°C. The resulting slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 132 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 136 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. A part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.85 wt%, and the yield reaches 91.48%.

[0052] Example 10

[0053] The C8 mixture containing 93 wt% of p-xylene is added as Feedstock 1 (flow rate: 3850 kg / h) to the primary crystallizer a for cooling crystallization at a crystallization temperature of 4°C. The resulting crystal slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 653 kg / h) and Product 6 to obtain the first wash liquor 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 17% by weight of the p-xylene product is returned to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of 1°C. The resulting crystal slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 269 kg / h) and Product 14 to obtain the second wash liquor 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 are melted in the second melter f. Among them, approximately 22% by weight of the p-xylene product is returned to the second solid-liquid separation device e as Product 14, and the remaining part leaves the crystallization system as the p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -14°C. The resulting crystal slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 132 kg / h) to obtain the third wash liquor 20 and the third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third wash liquor 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first wash liquor 8, and the second wash liquor 16 enter the synergistic mixer i. The fully mixed crystal slurry 24 enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 136 kg / h) and Product 27 to obtain the fourth wash liquor 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth wash liquor is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in the third melter k. Among them, 20% by weight of the p-xylene product is returned to the fourth solid-liquid separation device j as Product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.82 wt%, and the yield reaches 93.29%.

[0054] Example 11

[0055] The C8 mixture containing 98 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and added to the primary crystallizer a for cooling crystallization at a crystallization temperature of 10°C. The resulting slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. After being washed with Feedstock 2 (flow rate: 886 kg / h) and Product 6, the p-xylene crystals yield the first eluate 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 10% by weight of the p-xylene is returned as Product 6 to the first solid-liquid separation device b, and the remaining p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of 8°C. The resulting slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. After being washed with Feedstock 11 (flow rate: 179 kg / h) and Product 14, the p-xylene crystals yield the second eluate 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 are melted in the second melter f. Among them, approximately 20% by weight of the p-xylene is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of 4°C. The resulting slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. After being washed with Feedstock 21 (flow rate: 57 kg / h), the p-xylene crystals yield the third eluate 20 and the third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter the synergistic mixer i. After being fully mixed, the resulting slurry 24 enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. After being washed with Feedstock 28 (flow rate: 56 kg / h) and Product 27, the p-xylene crystals yield the fourth eluate 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in the third melter k. Among them, 15% by weight of the p-xylene is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.88 wt%, and the yield reaches 92.74%.

[0056] Example 12

[0057] The C8 mixture containing 98 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and added to the primary crystallizer a for cooling crystallization at a crystallization temperature of 10°C. The resulting crystal slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. After being washed with Feedstock 2 (flow rate: 886 kg / h) and Product 6, the p-xylene crystals yield the first eluate 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 10% by weight of the p-xylene is returned as Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of 7°C. The resulting crystal slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. After being washed with Feedstock 11 (flow rate: 179 kg / h) and Product 14, the p-xylene crystals yield the second eluate 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 are melted in the second melter f. Among them, approximately 20% by weight of the p-xylene is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as the p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of 4°C. The resulting crystal slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. After being washed with Feedstock 21 (flow rate: 57 kg / h), the p-xylene crystals yield the third eluate 20 and the third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter the synergistic mixer i. After sufficient mixing, the crystal slurry 24 enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. After being washed with Feedstock 28 (flow rate: 56 kg / h) and Product 27, the p-xylene crystals yield the fourth eluate 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in the third melter k. Among them, 15% by weight of the p-xylene is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.86 wt%, and the yield reaches 92.81%.

[0058] Example 13

[0059] The C8 mixture containing 98 wt% of p-xylene is added as Feedstock 1 (flow rate: 3850 kg / h) to the primary crystallizer a for cooling crystallization at a crystallization temperature of 10°C. The resulting slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 886 kg / h) and Product 6 to obtain the first eluate 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 10% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene Product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of 7°C. The resulting slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 179 kg / h) and Product 14 to obtain the second eluate 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 are melted in the second melter f. Among them, approximately 20% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene Product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of 3°C. The resulting slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 57 kg / h) to obtain the third eluate 20 and the third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter the synergistic mixer i. The fully mixed slurry 24 enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 56 kg / h) and Product 27 to obtain the fourth eluate 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in the third melter k. Among them, 15% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene Product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.84 wt%, and the yield reaches 93.70%.

[0060] Example 14

[0061] A C8 mixture containing 92 wt% of p-xylene is added as Feed 1 (flow rate: 3850 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feed 2 (flow rate: 597 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 20% by weight of the p-xylene product is returned to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 1°C. The resulting slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feed 11 (flow rate: 320 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 25% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as the p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -15°C. The resulting slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feed 21 (flow rate: 153 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 10% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feed 28 (flow rate: 158 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. A part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.85 wt%, and the yield reaches 92.62%.

[0062] Example 15

[0063] A C8 mixture containing 92 wt% of p-xylene is added as raw material 1 (flow rate is 3850 kg / h) into the primary crystallizer a for cooling and crystallization. The crystallization temperature is 5°C. The obtained slurry 3 enters the first solid-liquid separation equipment b to obtain the first filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with raw material 2 (flow rate is 597 kg / h) and product 6 to obtain the first eluate 8 and the first p-xylene crystals 4; the first p-xylene crystals 4 are melted in the first melter c, of which about 20% by weight are returned to the first solid-liquid separation equipment b as the p-xylene product 6, and the remaining part of the p-xylene product 5 leaves the crystallization system; 30% by weight of the first filtered mother liquor The first filtered mother liquor 7 is returned to the primary crystallizer a, and the remaining part is used as the first filtered mother liquor 9 to enter the secondary crystallizer d for cooling and crystallization. The crystallization temperature is 1°C. The obtained crystal slurry 10 enters the second solid-liquid separation equipment e to obtain the second filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with the raw material 11 (with a flow rate of 320 kg / h) and the product 14 to obtain the second eluate 16 and the second p-xylene crystals 12; the second p-xylene crystals 12 are melted in the second melter f, among which the p-xylene product accounting for about 25% by weight is returned to the second solid-liquid separation equipment e as the product 14, and the remaining part leaves the crystallization system as the p-xylene product 13; 20% by weight of the second filtered mother liquor ... The mother liquor is returned to the secondary crystallizer d as the second filtered mother liquor 15, and the remaining part is entered into the tertiary crystallizer g as the second filtered mother liquor 17 for cooling and crystallization. The crystallization temperature is -16°C. The obtained crystal slurry 18 enters the third solid-liquid separation equipment h to obtain the third filtered mother liquor and the third p-xylene crystals. After the p-xylene crystals are washed with the raw material 21 (with a flow rate of 153 kg / h), the third eluate 20 and the third p-xylene crystals 19 are obtained. About 10% by weight of the third filtered mother liquor is returned to the tertiary crystallizer g as the third filtered mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtered mother liquor 23. The third eluate 20 returns to the secondary crystallizer d; the third p-xylene crystals 19, the first washed The liquid 8 and the second eluate 16 enter the synergistic mixer i, and the fully mixed slurry 24 enters the fourth solid-liquid separation equipment j to obtain the fourth filtered mother liquor and p-xylene crystals. After the p-xylene crystals are washed by the raw material 28 (flow rate is 158kg / h) and the product 27, the fourth eluate 29 and the fourth p-xylene crystals 25 are obtained. Part of the fourth filtered mother liquor is returned to the primary crystallizer a as the fourth filtered mother liquor 31, and the fourth eluate returns to the synergistic mixer i; the fourth p-xylene crystals 25 are melted in the third melter k, among which 20% of the p-xylene product by weight is returned to the fourth solid-liquid separation equipment j as the product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and continuously operated, the system fluctuation is small, and the purity of the obtained product reaches 99.83wt%, and the yield reaches 93.02%.

[0064] Example 16

[0065] The C8 mixture containing 92 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and fed into the primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting crystal slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 597 kg / h) and Product 6 to obtain the first eluate 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 20% by weight of the p-xylene product is returned to the first solid-liquid separation device b as Product 6, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of 0°C. The resulting crystal slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 320 kg / h) and Product 14 to obtain the second eluate 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 are melted in the second melter f. Among them, approximately 25% by weight of the p-xylene product is returned to the second solid-liquid separation device e as Product 14, and the remaining part leaves the crystallization system as the p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -17°C. The resulting crystal slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 153 kg / h) to obtain the third eluate 20 and the third p-xylene crystals 19. Approximately 10% by weight of the third filtrate mother liquor is returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter the synergistic mixer i. The fully mixed crystal slurry 24 enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 158 kg / h) and Product 27 to obtain the fourth eluate 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in the third melter k. Among them, 20% by weight of the p-xylene product is returned to the fourth solid-liquid separation device j as Product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.82 wt%, and the yield reaches 93.38%.

[0066] Comparative Example 1

[0067] A C8 mixture containing 98 wt% of p-xylene is added as raw material 1 (flow rate is 3850 kg / h) into the primary crystallizer a for cooling and crystallization. The crystallization temperature is 12°C. The obtained slurry 3 enters the first solid-liquid separation equipment b to obtain the first filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with raw material 2 (flow rate is 886 kg / h) and product 6 to obtain the first eluate 8 and the first p-xylene crystals 4; the first p-xylene crystals 4 are melted in the first melter c, of which about 10% by weight are returned to the first solid-liquid separation equipment b as the p-xylene product 6, and the remaining p-xylene product 5 leaves the crystallization system; 30% by weight of the first filtered mother liquor is ... The first crystallizer a returns the first filtered mother liquor 7, and the remaining part enters the second crystallizer d as the first filtered mother liquor 9 for cooling and crystallization. The crystallization temperature is 9°C. The obtained crystal slurry 10 enters the second solid-liquid separation equipment e to obtain the second filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with the raw material 11 (with a flow rate of 179 kg / h) and the product 14 to obtain the second eluate 16 and the second p-xylene crystals 12; the second p-xylene crystals 12 are melted in the second melter f, wherein about 20% by weight of the p-xylene product is returned to the second solid-liquid separation equipment e as the product 14, and the remaining part leaves the crystallization system as the p-xylene product 13; 20% by weight of the second The filtered mother liquor is returned to the secondary crystallizer d as the second filtered mother liquor 15, and the remaining part is entered into the tertiary crystallizer g as the second filtered mother liquor 17 for cooling and crystallization. The crystallization temperature is 6°C. The obtained crystal slurry 18 enters the third solid-liquid separation equipment h to obtain the third filtered mother liquor and the third p-xylene crystals. After the p-xylene crystals are washed with the raw material 21 (with a flow rate of 57 kg / h), the third eluate 20 and the third p-xylene crystals 19 are obtained. About 35% by weight of the third filtered mother liquor is returned to the tertiary crystallizer g as the third filtered mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtered mother liquor 23. The third eluate 20 returns to the secondary crystallizer d; the third p-xylene crystals 19, the first eluate 20, and the second filtered mother liquor 17 are discharged from the crystallization system. The liquid 8 and the second eluate 16 enter the synergistic mixer i, and the fully mixed slurry 24 enters the fourth solid-liquid separation equipment j to obtain the fourth filtered mother liquor and p-xylene crystals. After the p-xylene crystals are washed by the raw material 28 (flow rate is 56kg / h) and the product 27, the fourth eluate 29 and the fourth p-xylene crystals 25 are obtained. Part of the fourth filtered mother liquor is returned to the primary crystallizer a as the fourth filtered mother liquor 31, and the fourth eluate returns to the synergistic mixer i; the fourth p-xylene crystals 25 are melted in the third melter k, among which 15% of the weight of the p-xylene product is returned to the fourth solid-liquid separation equipment j as the product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and continuously operated, the system fluctuation is small, and the purity of the obtained product reaches 99.92wt%, and the yield reaches 89.94%.

[0068] Comparative Example 2

[0069] A C8 mixture containing 98 wt% of p-xylene is added as raw material 1 (flow rate is 3850 kg / h) into the primary crystallizer a for cooling and crystallization. The crystallization temperature is 8°C. The obtained slurry 3 enters the first solid-liquid separation equipment b to obtain the first filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with raw material 2 (flow rate is 886 kg / h) and product 6 to obtain the first eluate 8 and the first p-xylene crystals 4; the first p-xylene crystals 4 are melted in the first melter c, of which about 10% by weight are returned to the first solid-liquid separation equipment b as the p-xylene product 6, and the remaining part of the p-xylene product 5 leaves the crystallization system; 30% by weight of the first filtered mother liquor The first filtered mother liquor 7 is returned to the primary crystallizer a, and the remaining part is used as the first filtered mother liquor 9 to enter the secondary crystallizer d for cooling and crystallization. The crystallization temperature is 5°C. The obtained crystal slurry 10 enters the second solid-liquid separation equipment e to obtain the second filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with the raw material 11 (with a flow rate of 179 kg / h) and the product 14 to obtain the second eluate 16 and the second p-xylene crystals 12; the second p-xylene crystals 12 are melted in the second melter f, among which the p-xylene product accounting for about 20% by weight is returned to the second solid-liquid separation equipment e as the product 14, and the remaining part leaves the crystallization system as the p-xylene product 13; the 20% by weight of the second filter mother liquor 11 is used as the product 14. The mother liquor is returned to the secondary crystallizer d as the second mother liquor 15, and the remaining part is entered into the tertiary crystallizer g as the second mother liquor 17 for cooling and crystallization. The crystallization temperature is -1°C. The obtained crystal slurry 18 enters the third solid-liquid separation equipment h to obtain the third mother liquor and the third p-xylene crystals. After the p-xylene crystals are washed with the raw material 21 (with a flow rate of 57 kg / h), the third eluate 20 and the third p-xylene crystals 19 are obtained. About 35% by weight of the third mother liquor is returned to the tertiary crystallizer g as the third mother liquor 22, and the remaining part is discharged from the crystallization system as the third mother liquor 23. The third eluate 20 returns to the secondary crystallizer d; the third p-xylene crystals 19, the first eluate 20, and the second mother liquor 17 are discharged from the crystallization system. The liquid 8 and the second eluate 16 enter the synergistic mixer i, and the fully mixed crystal slurry 24 enters the fourth solid-liquid separation equipment j to obtain the fourth filtered mother liquor and p-xylene crystals. After the p-xylene crystals are washed by the raw material 28 (flow rate is 56 kg / h) and the product 27, the fourth eluate 29 and the fourth p-xylene crystals 25 are obtained. Part of the fourth filtered mother liquor is returned to the primary crystallizer a as the fourth filtered mother liquor 31, and the fourth eluate returns to the synergistic mixer i; the fourth p-xylene crystals 25 are melted in the third melter k, wherein 15% by weight of the p-xylene product is returned to the fourth solid-liquid separation equipment j as the product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and continuously operated, the system fluctuation is small, and the purity of the obtained product reaches 99.78wt%, and the yield reaches 95.92%.

[0070] Comparative Example 3

[0071] The C8 mixture containing 95 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and added to the primary crystallizer a for cooling crystallization at a crystallization temperature of 9°C. The resulting slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 707 kg / h) and Product 6 to obtain the first eluate 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 15% by weight of the p-xylene product is returned to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 60% by weight of the first filtrate mother liquor is returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of 5°C. The resulting slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 238 kg / h) and Product 14 to obtain the second eluate 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 are melted in the second melter f. Among them, approximately 20% by weight of the p-xylene product is returned to the second solid-liquid separation device e as Product 14, and the remaining part leaves the crystallization system as the p-xylene product 13. 40% by weight of the second filtrate mother liquor is returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -2°C. The resulting slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 103 kg / h) to obtain the third eluate 20 and the third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter the synergistic mixer i. The fully mixed slurry 24 enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 103 kg / h) and Product 27 to obtain the fourth eluate 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in the third melter k. Among them, 20% by weight of the p-xylene product is returned to the fourth solid-liquid separation device j as Product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.9 wt%, and the yield reaches 89.43%.

[0072] Comparative Example 4

[0073] A C8 mixture containing 95 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 4°C. The resulting crystal slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 707 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 15% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 60% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 1°C. The resulting crystal slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 238 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 20% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 40% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -10°C. The resulting crystal slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 103 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 103 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.77 wt%, and the yield reaches 94.18%.

[0074] Comparative Example 5

[0075] A C8 mixture containing 92 wt% of p-xylene is added as Feed 1 (flow rate: 3850 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 6°C. The resulting slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feed 2 (flow rate: 597 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 20% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 2°C. The resulting slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feed 11 (flow rate: 320 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 25% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -9°C. The resulting slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feed 21 (flow rate: 153 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 10% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feed 28 (flow rate: 158 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.89 wt%, and the yield reaches 89.49%.

[0076] Comparative Example 6

[0077] A C8 mixture containing 92 wt% of p-xylene was used as Feedstock 1 (flow rate: 3850 kg / h) and added to a primary crystallizer a for cooling crystallization at a crystallization temperature of 2°C. The resulting slurry 3 entered a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 2 (flow rate: 597 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 were melted in a first melter c. Among them, approximately 20% by weight was returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 left the crystallization system. 30% by weight of the first filtrate mother liquor was returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part entered a secondary crystallizer d for cooling crystallization at a crystallization temperature of -1°C. The resulting slurry 10 entered a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 11 (flow rate: 320 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 were melted in a second melter f. Among them, approximately 25% by weight of the p-xylene product was returned as Product 14 to the second solid-liquid separation device e, and the remaining part left the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor was returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part entered a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -20°C. The resulting slurry 18 entered a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals were washed with Feedstock 21 (flow rate: 153 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 10% by weight of the third filtrate mother liquor was returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part was discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 was returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 entered a synergistic mixer i. The fully mixed slurry 24 entered a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 28 (flow rate: 158 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor was returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate was returned to the synergistic mixer i. The fourth p-xylene crystals 25 were melted in a third melter k. Among them, 20% by weight of the p-xylene product was returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part left the crystallization system as p-xylene product 26. The implementation process was stable and continuously operated, with small system fluctuations. The purity of the obtained product reached 99.67 wt%, and the yield reached 94.39%.

[0078] Comparative Example 7

[0079] The C8 mixture containing 98 wt% p-xylene was used as Feedstock 1 (flow rate: 3850 kg / h) and added to the first primary crystallizer a for cooling crystallization at a crystallization temperature of 11°C. The resulting slurry 3 entered the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 2 (flow rate: 886 kg / h) and Product 6 to obtain the first eluate 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 were melted in the first melter c. Among them, approximately 12% by weight was returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part left the crystallization system as p-xylene Product 5. 30% by weight of the first filtrate mother liquor was returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part entered the secondary crystallizer d for cooling crystallization at a crystallization temperature of 8°C. The resulting slurry 10 entered the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 11 (flow rate: 179 kg / h) and Product 14 to obtain the second eluate 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 were melted in the second melter f. Among them, approximately 22% by weight of the p-xylene product was returned as p-xylene Product 14 to the second solid-liquid separation device e, and the remaining part left the crystallization system as p-xylene 13. 20% by weight of the second filtrate mother liquor was returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part entered the tertiary crystallizer g for cooling crystallization at a crystallization temperature of 5°C. The resulting slurry 18 entered the third solid-liquid separation device h to obtain the third filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 21 (flow rate: 57 kg / h) to obtain the third eluate 20 and the third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor 22 was returned to the tertiary crystallizer g, and the remaining part was discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 was returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 entered the synergistic mixer i. The fully mixed slurry 24 entered the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Product 27 to obtain the fourth eluate 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor was returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate was returned to the synergistic mixer i. The fourth p-xylene crystals 25 were melted in the third melter k. Among them, 18% by weight of the p-xylene was returned as p-xylene Product 27 as the washing liquid to the fourth solid-liquid separation device j, and the remaining part left the crystallization system as p-xylene Product 26. The purity of the obtained product was 99.77 wt%, and the yield reached 92.15%.

[0080] Comparative Example 8

[0081] A C8 mixture containing 95 wt% of p-xylene is added as Feedstock 1 (flow rate: 3850 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 7°C. The resulting slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 707 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 17% by weight of the p-xylene product is returned to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 60% by weight of the first filtrate mother liquor is returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 4°C. The resulting slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 238 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 22% by weight of the p-xylene product is returned to the second solid-liquid separation device e as Product 14, and the remaining part leaves the crystallization system as p-xylene product 13. 40% by weight of the second filtrate mother liquor is returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -7°C. The resulting slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 103 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 21% by weight of the p-xylene product is returned to the fourth solid-liquid separation device j as Product 27, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.72 wt%, and the yield reaches 92.99%.

[0082] Comparative Example 9

[0083] A C8 mixture containing 92 wt% of p-xylene is added as raw material 1 (flow rate is 3850 kg / h) into a primary crystallizer a for cooling and crystallization. The crystallization temperature is 3°C. The resulting slurry 3 enters the first solid-liquid separation equipment b to obtain a first filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with raw material 2 (flow rate is 597 kg / h) and product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c, of which about 22% by weight are returned to the first solid-liquid separation equipment b as p-xylene product 6, and the remaining p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtered mother liquor The first filtered mother liquor 7 is returned to the primary crystallizer a, and the remaining part is used as the first filtered mother liquor 9 to enter the secondary crystallizer d for cooling and crystallization. The crystallization temperature is 0°C. The obtained crystal slurry 10 enters the second solid-liquid separation equipment e to obtain the second filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with the raw material 11 (flow rate is 320 kg / h) and the product 14 to obtain the second eluate 16 and the second p-xylene crystals 12; the second p-xylene crystals 12 are melted in the second melter f, among which the p-xylene product accounting for about 27% by weight is returned to the second solid-liquid separation equipment e as the product 14, and the remaining part leaves the crystallization system as the p-xylene product 13; 20% by weight The amount of the second filtered mother liquor is returned to the secondary crystallizer d as the second filtered mother liquor 15, and the remaining part is entered into the tertiary crystallizer g as the second filtered mother liquor 17 for cooling and crystallization, and the crystallization temperature is -18°C. The obtained crystal slurry 18 enters the third solid-liquid separation equipment h to obtain the third filtered mother liquor and the third p-xylene crystals. After the p-xylene crystals are washed with the raw material 21 (with a flow rate of 153 kg / h), the third eluate 20 and the third p-xylene crystals 19 are obtained. About 10% by weight of the third filtered mother liquor is returned to the tertiary crystallizer g as the third filtered mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtered mother liquor 23. The third eluate 20 returns to the secondary crystallizer d; the third The p-xylene crystals 19, the first eluate 8 and the second eluate 16 enter the synergistic mixer i, and the fully mixed slurry 24 enters the fourth solid-liquid separation equipment j to obtain the fourth filtered mother liquor and p-xylene crystals. After the p-xylene crystals are washed by the product 27, the fourth eluate 29 and the fourth p-xylene crystals 25 are obtained. Part of the fourth filtered mother liquor is returned to the primary crystallizer a as the fourth filtered mother liquor 31, and the fourth eluate returns to the synergistic mixer i; the fourth p-xylene crystals 25 are melted in the third melter k, wherein 24% of the weight of the p-xylene product is returned to the fourth solid-liquid separation equipment j as the product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and continuously operated, the system fluctuation is small, and the purity of the obtained product reaches 99.69wt%, and the yield reaches 82.3393.80%.

[0084] Comparative Example 10

[0085] The C8 mixture containing 98 wt% of p-xylene is added as Feedstock 1 (flow rate: 3850 kg / h) to the primary crystallizer a for cooling crystallization at a crystallization temperature of 11°C. The resulting slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 886 kg / h) and Product 6 to obtain the first wash liquor 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 10% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of 8°C. The resulting slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 179 kg / h) and Product 14 to obtain the second wash liquor 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 are melted in the second melter f. Among them, approximately 20% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of 5°C. The resulting slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 57 kg / h) to obtain the third wash liquor 20 and the third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third wash liquor 20 is returned to the secondary crystallizer d; the first wash liquor 8 enters the primary crystallizer a, and the second wash liquor 16 enters the secondary crystallizer d. The third p-xylene crystals 19 enter the slurrying kettle. After the temperature is raised for slurrying, it enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 56 kg / h) and Product 27 to obtain the fourth wash liquor 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth wash liquor is returned to the synergistic mixer i; the fourth p-xylene crystals 25 are melted in the third melter k. Among them, 15% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.75 wt%, and the yield reaches 85.23%.

[0086] Comparative Example 11

[0087] A C8 mixture containing 95 wt% p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 7°C. The resulting crystal slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 707 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 15% by weight of the p-xylene is returned as Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 60% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 4°C. The resulting crystal slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 238 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 20% by weight of the p-xylene is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 40% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -7°C. The resulting crystal slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 103 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d; the first eluate 8 enters the primary crystallizer a, and the second eluate 16 enters the secondary crystallizer d. The third p-xylene crystals 19 enter a slurring kettle. After the temperature is raised for slurring, they enter a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 103 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to a synergistic mixer i; the fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.69 wt%, and the yield reaches 84.03%.

[0088] Comparative Example 12

[0089] A C8 mixture containing 92 wt% of p-xylene was used as Feedstock 1 (flow rate: 3850 kg / h) and added to a primary crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting crystal slurry 3 entered a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 2 (flow rate: 597 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 were melted in a first melter c. Among them, approximately 20% by weight of the p-xylene product was returned to the first solid-liquid separation device b as Product 6, and the remaining part of the p-xylene product 5 left the crystallization system. 30% by weight of the first filtrate mother liquor was returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part entered a secondary crystallizer d for cooling crystallization at a crystallization temperature of 0°C. The resulting crystal slurry 10 entered a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 11 (flow rate: 320 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 were melted in a second melter f. Among them, approximately 25% by weight of the p-xylene product was returned to the second solid-liquid separation device e as Product 14, and the remaining part left the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor was returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part entered a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -18°C. The resulting crystal slurry 18 entered a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals were washed with Feedstock 21 (flow rate: 153 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 10% by weight of the third filtrate mother liquor was returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part was discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 was returned to the secondary crystallizer d; the first eluate 8 entered the primary crystallizer a, and the second eluate 16 entered the secondary crystallizer d. The third p-xylene crystals 19 entered a slurring kettle. After the temperature was raised for slurring, they entered a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 28 (flow rate: 158 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. A part of the fourth filtrate mother liquor was returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate was returned to a synergistic mixer i; the fourth p-xylene crystals 25 were melted in a third melter k. Among them, 20% by weight of the p-xylene product was returned to the fourth solid-liquid separation device j as Product 27, and the remaining part left the crystallization system as p-xylene product 26. The implementation process was stable and continuously operated with small system fluctuations. The purity of the obtained product reached 99.65 wt%, and the yield reached 82.83%.

[0090] Comparative Example 13

[0091] A C8 mixture containing 98 wt% of p-xylene is added as raw material 1 (flow rate is 3850 kg / h) to a primary crystallizer a for cooling and crystallization at a crystallization temperature of 11°C. The resulting slurry 3 enters the first solid-liquid separation equipment b to obtain a first filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with raw material 2 (flow rate is 886 kg / h) and product 6 to obtain a first eluate 8 and first p-xylene crystals 4; the first p-xylene crystals 4 are melted in a first melter c, of which about 10% by weight is returned to the first solid-liquid separation equipment b as p-xylene product 6, and the remaining The remaining part of the p-xylene product 5 leaves the crystallization system; the first filtered mother liquor 9 enters the secondary crystallizer d for cooling and crystallization, the crystallization temperature is 8°C, the obtained crystal slurry 10 enters the second solid-liquid separation equipment e, and a second filtered mother liquor and p-xylene crystals are obtained. The p-xylene crystals are washed with the raw material 11 (flow rate is 179 kg / h) and the product 14 to obtain a second eluate 16 and second p-xylene crystals 12; the second p-xylene crystals 12 are melted in the second melter f, wherein about 20% by weight of the p-xylene product is returned to the second solid-liquid separation equipment e as the product 14, and the remaining part is returned as the p-xylene Benzene product 13 leaves the crystallization system; the second filtered mother liquor 17 enters the tertiary crystallizer g for cooling and crystallization, the crystallization temperature is 5°C, the obtained crystal slurry 18 enters the third solid-liquid separation equipment h, and the third filtered mother liquor and the third p-xylene crystals are obtained. After the p-xylene crystals are washed with the raw material 21 (the flow rate is 57 kg / h), the third eluate 20 and the third p-xylene crystals 19 are obtained. The third filtered mother liquor 23 is discharged from the crystallization system, and the third eluate 20 returns to the secondary crystallizer d; the third p-xylene crystals 19, the first eluate 8 and the second eluate 16 enter the synergistic mixer i, and the fully mixed crystals The slurry 24 enters the fourth solid-liquid separation equipment j to obtain the fourth filtered mother liquor and para-xylene crystals. After the para-xylene crystals are washed by the raw material 28 (flow rate is 56kg / h) and the product 27, the fourth eluate 29 and the fourth para-xylene crystals 25 are obtained. Part of the fourth filtered mother liquor is returned to the primary crystallizer a as the fourth filtered mother liquor 31, and the fourth eluate is returned to the synergistic mixer i; the fourth para-xylene crystals 25 are melted in the third melter k, among which 15% by weight of the para-xylene product is returned to the fourth solid-liquid separation equipment j as the product 27, and the remaining part leaves the crystallization system as the para-xylene product 26. The implementation process is stable and continuously operated, the system fluctuation is small, and the purity of the obtained product reaches 99.71wt%, and the yield reaches 82.39%.

[0092] Comparative Example 14

[0093] A C8 mixture containing 95 wt% of p-xylene is added as Feedstock 1 (flow rate: 3850 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 7°C. The resulting slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 707 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 15% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. The first filtrate mother liquor 9 enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 4°C. The resulting slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 238 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 20% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. The second filtrate mother liquor 17 enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -7°C. The resulting slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 103 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. The third filtrate mother liquor 23 is discharged from the crystallization system, and the third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 103 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.68 wt%, and the yield reaches 83.05%.

[0094] Comparative Example 15

[0095] A C8 mixture containing 92 wt% of p-xylene is added as Feedstock 1 (flow rate: 3,850 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 597 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 20% by weight of the p-xylene product is returned to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. The first filtrate mother liquor 9 enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 0°C. The resulting slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 320 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 25% by weight of the p-xylene product is returned to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. The second filtrate mother liquor 17 enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -18°C. The resulting slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 153 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. The third filtrate mother liquor 23 is discharged from the crystallization system, and the third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 158 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned to the fourth solid-liquid separation device j as Product 27, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with relatively small system fluctuations. The purity of the obtained product reaches 99.65 wt%, and the yield reaches 83.85%.

[0096] Comparative Example 16

[0097] A C8 mixture containing 96 wt% p-xylene is added as Feedstock 1 (flow rate: 3850 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 10°C. The resulting slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 793 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 12% by weight of the p-xylene product is returned to the first solid-liquid separation device b as Product 6, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 8°C. The resulting slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 202 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 20% by weight of the p-xylene product is returned to the second solid-liquid separation device e as Product 14, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of 3°C. The resulting slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 85 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter an intensifying mixer i. The fully mixed slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 83 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. A part of the fourth filtrate mother liquor is returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate is returned to the intensifying mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned to the fourth solid-liquid separation device j as Product 27, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with relatively small system fluctuations. The purity of the obtained product reaches 99.89 wt%, and the yield reaches 86.43%.

[0098] Comparative Example 17

[0099] A C8 mixture containing 96 wt% p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting crystal slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 793 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 12% by weight of the p-xylene product is returned to the first solid-liquid separation device b as Product 6, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 3°C. The resulting crystal slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 202 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 20% by weight of the p-xylene product is returned to the second solid-liquid separation device e as Product 14, and the remaining part leaves the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -5°C. The resulting crystal slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 85 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 83 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 20% by weight of the p-xylene product is returned to the fourth solid-liquid separation device j as Product 27, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.78 wt%, and the yield reaches 93.61%.

[0100] Comparative Example 18

[0101] A C8 mixture containing 93 wt% of p-xylene is added as Feed 1 (flow rate: 3850 kg / h) to the primary crystallizer a for cooling crystallization at a crystallization temperature of 8°C. The resulting crystal slurry 3 enters the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feed 2 (flow rate: 653 kg / h) and Product 6 to obtain the first wash liquor 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 17% by weight of the p-xylene product is returned to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of 4°C. The resulting crystal slurry 10 enters the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feed 11 (flow rate: 269 kg / h) and Product 14 to obtain the second wash liquor 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 are melted in the second melter f. Among them, approximately 22% by weight of the p-xylene product is returned to the second solid-liquid separation device e as Product 14, and the remaining part leaves the crystallization system as the p-xylene product 13. 20% by weight of the second filtrate mother liquor is returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -7°C. The resulting crystal slurry 18 enters the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. The p-xylene crystals are washed with Feed 21 (flow rate: 132 kg / h) to obtain the third wash liquor 20 and the third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third wash liquor 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first wash liquor 8, and the second wash liquor 16 enter the synergistic mixer i. The fully mixed crystal slurry 24 enters the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feed 28 (flow rate: 136 kg / h) and Product 27 to obtain the fourth wash liquor 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth wash liquor is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in the third melter k. Among them, 20% by weight of the p-xylene product is returned to the fourth solid-liquid separation device j as Product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.89 wt%, and the yield reaches 89.59%.

[0102] Comparative Example 19

[0103] A C8 mixture containing 93 wt% of p-xylene was used as Feedstock 1 (flow rate: 3,850 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting crystal slurry 3 was fed into a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 2 (flow rate: 653 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 were melted in a first melter c. Among them, approximately 17% by weight of the p-xylene product was returned to the first solid-liquid separation device b as Product 6, and the remaining p-xylene product 5 left the crystallization system. 30% by weight of the first filtrate mother liquor was returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part entered a secondary crystallizer d for cooling crystallization at a crystallization temperature of 0°C. The resulting crystal slurry 10 was fed into a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 11 (flow rate: 269 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 were melted in a second melter f. Among them, approximately 22% by weight of the p-xylene product was returned to the second solid-liquid separation device e as Product 14, and the remaining part left the crystallization system as p-xylene product 13. 20% by weight of the second filtrate mother liquor was returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part entered a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -16°C. The resulting crystal slurry 18 was fed into a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals were washed with Feedstock 21 (flow rate: 132 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor was returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part was discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 was returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 entered a synergistic mixer i. The fully mixed crystal slurry 24 was fed into a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 28 (flow rate: 136 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. A part of the fourth filtrate mother liquor was returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate was returned to the synergistic mixer i. The fourth p-xylene crystals 25 were melted in a third melter k. Among them, 20% by weight of the p-xylene product was returned to the fourth solid-liquid separation device j as Product 27, and the remaining part left the crystallization system as p-xylene product 26. The implementation process was stable and continuously operated with small system fluctuations. The purity of the obtained product reached 99.73 wt%, and the yield reached 94.05%.

[0104] Comparative Example 20

[0105] A C8 mixture containing 91 wt% of p-xylene is added as raw material 1 (flow rate is 3850 kg / h) into the primary crystallizer a for cooling and crystallization. The crystallization temperature is 5°C. The obtained slurry 3 enters the first solid-liquid separation equipment b to obtain the first filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with raw material 2 (flow rate is 623 kg / h) and product 6 to obtain the first eluate 8 and the first p-xylene crystals 4; the first p-xylene crystals 4 are melted in the first melter c, of which about 20% by weight are returned to the first solid-liquid separation equipment b as the p-xylene product 6, and the remaining part of the p-xylene product 5 leaves the crystallization system; 30% by weight of the first filtered mother liquor The first filtered mother liquor 7 is returned to the primary crystallizer a, and the remaining part is used as the first filtered mother liquor 9 to enter the secondary crystallizer d for cooling and crystallization. The crystallization temperature is 1°C. The obtained crystal slurry 10 enters the second solid-liquid separation equipment e to obtain the second filtered mother liquor and p-xylene crystals. The p-xylene crystals are washed with the raw material 11 (with a flow rate of 378 kg / h) and the product 14 to obtain the second eluate 16 and the second p-xylene crystals 12; the second p-xylene crystals 12 are melted in the second melter f, among which the p-xylene product accounting for about 27% by weight is returned to the second solid-liquid separation equipment e as the product 14, and the remaining part leaves the crystallization system as the p-xylene product 13; 30% by weight of the second filtered mother liquor is returned to the second solid-liquid separation equipment e as the product 14, and the remaining part leaves the crystallization system as the p-xylene product 13; The mother liquor is returned to the secondary crystallizer d as the second filtered mother liquor 15, and the remaining part is entered into the tertiary crystallizer g as the second filtered mother liquor 17 for cooling and crystallization. The crystallization temperature is -16°C. The obtained crystal slurry 18 enters the third solid-liquid separation equipment h to obtain the third filtered mother liquor and the third p-xylene crystals. After the p-xylene crystals are washed with the raw material 21 (with a flow rate of 167 kg / h), the third eluate 20 and the third p-xylene crystals 19 are obtained. About 35% by weight of the third filtered mother liquor is returned to the tertiary crystallizer g as the third filtered mother liquor 22, and the remaining part is discharged from the crystallization system as the third filtered mother liquor 23. The third eluate 20 returns to the secondary crystallizer d; the third p-xylene crystals 19, the first washed The liquid 8 and the second eluate 16 enter the synergistic mixer i, and the fully mixed slurry 24 enters the fourth solid-liquid separation equipment j to obtain the fourth filtered mother liquor and p-xylene crystals. After the p-xylene crystals are washed by the raw material 28 (flow rate is 175kg / h) and the product 27, the fourth eluate 29 and the fourth p-xylene crystals 25 are obtained. Part of the fourth filtered mother liquor is returned to the primary crystallizer a as the fourth filtered mother liquor 31, and the fourth eluate returns to the synergistic mixer i; the fourth p-xylene crystals 25 are melted in the third melter k, wherein 23% of the weight of the p-xylene product is returned to the fourth solid-liquid separation equipment j as the product 27, and the remaining part leaves the crystallization system as the p-xylene product 26. The implementation process is stable and continuously operated, the system fluctuation is small, and the purity of the obtained product reaches 99.68wt%, and the yield reaches 69.89%.

[0106] Comparative Example 21

[0107] A C8 mixture containing 91 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 4°C. The resulting slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 623 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 20% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 1°C. The resulting slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 378 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 27% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 30% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -17°C. The resulting slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 167 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 175 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. A part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 23% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.67 wt%, and the yield reaches 70.01%.

[0108] Comparative Example 22

[0109] A C8 mixture containing 91 wt% of p-xylene is added as Feedstock 1 (flow rate: 3850 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 623 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 20% by weight is returned as p-xylene Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 0°C. The resulting slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 378 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 27% by weight of the p-xylene product is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 30% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -18 - 20°C. The resulting slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 167 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 175 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 23% by weight of the p-xylene product is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.66 wt%, and the yield reaches 70.28%.

[0110] Comparative Example 23

[0111] A C8 mixture containing 90 wt% of p-xylene is used as Feedstock 1 (flow rate: 3850 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting crystal slurry 3 enters a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 2 (flow rate: 667 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 24% by weight of the p-xylene is returned as Product 6 to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 leaves the crystallization system. 30% by weight of the first filtrate mother liquor is returned as the first filtrate mother liquor 7 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 1°C. The resulting crystal slurry 10 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 11 (flow rate: 431 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 are melted in a second melter f. Among them, approximately 30% by weight of the p-xylene is returned as Product 14 to the second solid-liquid separation device e, and the remaining part leaves the crystallization system as p-xylene product 13. 30% by weight of the second filtrate mother liquor is returned as the second filtrate mother liquor 15 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -16°C. The resulting crystal slurry 18 enters a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals are washed with Feedstock 21 (flow rate: 228 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 is returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 enter a synergistic mixer i. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals are washed with Feedstock 28 (flow rate: 230 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. A part of the fourth filtrate mother liquor is returned as the fourth filtrate mother liquor 31 to the primary crystallizer a, and the fourth eluate is returned to the synergistic mixer i. The fourth p-xylene crystals 25 are melted in a third melter k. Among them, 26% by weight of the p-xylene is returned as Product 27 to the fourth solid-liquid separation device j, and the remaining part leaves the crystallization system as p-xylene product 26. The implementation process is stable and operates continuously with small system fluctuations. The purity of the obtained product reaches 99.65 wt%, and the yield reaches 70.32%.

[0112] Comparative Example 24

[0113] The C8 mixture containing 90 wt% of p-xylene was used as Feedstock 1 (flow rate: 3850 kg / h) and added to the primary crystallizer a for cooling crystallization at a crystallization temperature of 4°C. The resulting crystal slurry 3 entered the first solid-liquid separation device b to obtain the first filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 2 (flow rate: 667 kg / h) and Product 6 to obtain the first eluate 8 and the first p-xylene crystals 4. The first p-xylene crystals 4 were melted in the first melter c. Among them, approximately 24% by weight of the p-xylene product was returned to the first solid-liquid separation device b as Product 6, and the remaining part of the p-xylene product 5 left the crystallization system. 30% by weight of the first filtrate mother liquor was returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part entered the secondary crystallizer d for cooling crystallization at a crystallization temperature of 1°C. The resulting crystal slurry 10 entered the second solid-liquid separation device e to obtain the second filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 11 (flow rate: 431 kg / h) and Product 14 to obtain the second eluate 16 and the second p-xylene crystals 12. The second p-xylene crystals 12 were melted in the second melter f. Among them, approximately 30% by weight of the p-xylene product was returned to the second solid-liquid separation device e as Product 14, and the remaining part left the crystallization system as the p-xylene product 13. 30% by weight of the second filtrate mother liquor was returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part entered the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -17°C. The resulting crystal slurry 18 entered the third solid-liquid separation device h to obtain the third filtrate mother liquor and the third p-xylene crystals. The p-xylene crystals were washed with Feedstock 21 (flow rate: 228 kg / h) to obtain the third eluate 20 and the third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor was returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part was discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 was returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 entered the synergistic mixer i. The fully mixed crystal slurry 24 entered the fourth solid-liquid separation device j to obtain the fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 28 (flow rate: 230 kg / h) and Product 27 to obtain the fourth eluate 29 and the fourth p-xylene crystals 25. Part of the fourth filtrate mother liquor was returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate was returned to the synergistic mixer i. The fourth p-xylene crystals 25 were melted in the third melter k. Among them, 26% by weight of the p-xylene product was returned to the fourth solid-liquid separation device j as Product 27, and the remaining part left the crystallization system as the p-xylene product 26. The implementation process was stable and continuously operated with small system fluctuations. The purity of the obtained product reached 99.52 wt%, and the yield reached 70.56%.

[0114] Comparative Example 25

[0115] A C8 mixture containing 90 wt% of p-xylene was used as Feedstock 1 (flow rate: 3850 kg / h) and added to a primary crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting crystal slurry 3 entered a first solid-liquid separation device b to obtain a first filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 2 (flow rate: 667 kg / h) and Product 6 to obtain a first eluate 8 and first p-xylene crystals 4. The first p-xylene crystals 4 were melted in a first melter c. Among them, approximately 24% by weight of the p-xylene product was returned to the first solid-liquid separation device b, and the remaining part of the p-xylene product 5 left the crystallization system. 30% by weight of the first filtrate mother liquor was returned to the primary crystallizer a as the first filtrate mother liquor 7, and the remaining part entered a secondary crystallizer d for cooling crystallization at a crystallization temperature of 0°C. The resulting crystal slurry 10 entered a second solid-liquid separation device e to obtain a second filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 11 (flow rate: 431 kg / h) and Product 14 to obtain a second eluate 16 and second p-xylene crystals 12. The second p-xylene crystals 12 were melted in a second melter f. Among them, approximately 30% by weight of the p-xylene product was returned to the second solid-liquid separation device e as Product 14, and the remaining part left the crystallization system as p-xylene product 13. 30% by weight of the second filtrate mother liquor was returned to the secondary crystallizer d as the second filtrate mother liquor 15, and the remaining part entered a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -18°C. The resulting crystal slurry 18 entered a third solid-liquid separation device h to obtain a third filtrate mother liquor and third p-xylene crystals. The p-xylene crystals were washed with Feedstock 21 (flow rate: 228 kg / h) to obtain a third eluate 20 and third p-xylene crystals 19. Approximately 35% by weight of the third filtrate mother liquor was returned to the tertiary crystallizer g as the third filtrate mother liquor 22, and the remaining part was discharged from the crystallization system as the third filtrate mother liquor 23. The third eluate 20 was returned to the secondary crystallizer d. The third p-xylene crystals 19, the first eluate 8, and the second eluate 16 entered a synergistic mixer i. The fully mixed crystal slurry 24 entered a fourth solid-liquid separation device j to obtain a fourth filtrate mother liquor and p-xylene crystals. The p-xylene crystals were washed with Feedstock 28 (flow rate: 230 kg / h) and Product 27 to obtain a fourth eluate 29 and fourth p-xylene crystals 25. A part of the fourth filtrate mother liquor was returned to the primary crystallizer a as the fourth filtrate mother liquor 31, and the fourth eluate was returned to the synergistic mixer i. The fourth p-xylene crystals 25 were melted in a third melter k. Among them, 26% by weight of the p-xylene product was returned to the fourth solid-liquid separation device j as Product 27, and the remaining part left the crystallization system as p-xylene product 26. The implementation process was stable and continuously operated with small system fluctuations. The purity of the obtained product reached 99.48 wt%, and the yield reached 71.33%.

[0116] The high-efficiency crystallization washing method for p-xylene-rich of the present invention solves the problem in the existing production process that due to the high yield, the tertiary crystallization temperature is low, which in turn leads to difficult crystal washing. The method includes three crystallization stages and one synergistic mixing stage. Among them, the crystal slurries produced by the first-stage crystallization, the second-stage crystallization and the synergistic mixer i are directly washed with the raw materials (in each embodiment, the concentration of the p-xylene-containing raw material for washing is the same as that of the C8 mixture raw material 1 containing p-xylene) and the product, and a p-xylene product with a purity greater than 99.8 wt% is directly produced. The tertiary crystallization is mainly responsible for recovering p-xylene crystals to increase the process yield, and the tertiary crystals are suspended and washed in the synergistic mixer i by the washings from the first and second stages.

[0117] In all embodiments of the present invention, products with a purity greater than 99.8 wt% are obtained, and the yield of the p-xylene product is greater than 90%. By comparing Examples 1-2 with Comparative Examples 1-2, it can be seen that when the operating temperature of the first-stage crystallizer a is higher than 11 °C, the operating temperature of the second-stage crystallizer d is higher than 8 °C, and the operating temperature of the tertiary crystallizer g is higher than 5 °C, the yield of the p-xylene product is lower than 90 wt%; when the operating temperature of the first-stage crystallizer a is lower than 9 °C, the operating temperature of the second-stage crystallizer d is lower than 6 °C, and the operating temperature of the tertiary crystallizer g is lower than 0 °C, the purity of the product is lower than 99.8 wt%. Because lowering the crystallization temperature will crystallize out more crystals and increase the yield, but at the same time, it will reduce the crystal purity due to more impurities being occluded. Therefore, for a raw material containing 98 wt% of p-xylene, the operating temperature of the first-stage crystallizer a is 9 °C to 11 °C, the operating temperature of the second-stage crystallizer d is 6 °C to 8 °C, and the operating temperature of the tertiary crystallizer g is 0 °C to 5 °C.

[0118] By comparing Examples 7-8 with Comparative Examples 16-17, it can be seen that when the operating temperature of the first-stage crystallizer a is higher than 9 °C, the operating temperature of the second-stage crystallizer d is higher than 6 °C, and the operating temperature of the tertiary crystallizer g is higher than 0 °C, the yield of the p-xylene product is lower than 90 wt%; when the operating temperature of the first-stage crystallizer a is lower than 7 °C, the operating temperature of the second-stage crystallizer d is lower than 4 °C, and the operating temperature of the tertiary crystallizer g is lower than -3 °C, the purity of the product is lower than 99.8 wt%. Therefore, for a raw material containing 96 wt% of p-xylene, the operating temperature of the first-stage crystallizer a is 7 °C to 9 °C, the operating temperature of the second-stage crystallizer d is 4 °C to 6 °C, and the operating temperature of the tertiary crystallizer g is -3 °C to 0 °C.

[0119] Similarly, by comparing Examples 3-4 with Comparative Examples 3-4, it can be seen that when the operating temperature of the primary crystallizer a is higher than 7 °C, the operating temperature of the secondary crystallizer d is higher than 4 °C, and the operating temperature of the tertiary crystallizer g is higher than -4 °C, the yield of the p-xylene product is lower than 90 wt%; when the operating temperature of the primary crystallizer a is lower than 5 °C, the operating temperature of the secondary crystallizer d is lower than 2 °C, and the operating temperature of the tertiary crystallizer g is lower than -7 °C, the purity of the product is lower than 99.8 wt%. Therefore, for a raw material containing 95 wt% of p-xylene, the operating temperature of the primary crystallizer a is 5 °C to 7 °C, the operating temperature of the secondary crystallizer d is 2 °C to 4 °C, and the operating temperature of the tertiary crystallizer g is -7 °C to -4 °C.

[0120] By comparing Examples 9-10 with Comparative Examples 18-19, it can be seen that when the operating temperature of the primary crystallizer a is higher than 6 °C, the operating temperature of the secondary crystallizer d is higher than 3 °C, and the operating temperature of the tertiary crystallizer g is higher than -10 °C, the yield of the p-xylene product is lower than 90 wt%; when the operating temperature of the primary crystallizer a is lower than 4 °C, the operating temperature of the secondary crystallizer d is lower than 1 °C, and the operating temperature of the tertiary crystallizer g is lower than -14 °C, the purity of the product is lower than 99.8 wt%. Therefore, for a raw material containing 93 wt% of p-xylene, the operating temperature of the primary crystallizer a is 4 °C to 6 °C, the operating temperature of the secondary crystallizer d is 1 °C to 3 °C, and the operating temperature of the tertiary crystallizer g is -14 °C to -10 °C.

[0121] By comparing Examples 5-6 with Comparative Examples 5-6, it can be seen that when the operating temperature of the primary crystallizer a is higher than 5 °C, the operating temperature of the secondary crystallizer d is higher than 1 °C, and the operating temperature of the tertiary crystallizer g is higher than -13 °C, the yield of the p-xylene product is lower than 90 wt%; when the operating temperature of the primary crystallizer a is lower than 3 °C, the operating temperature of the secondary crystallizer d is lower than 0 °C, and the operating temperature of the tertiary crystallizer g is lower than -18 °C, the purity of the product is lower than 99.8 wt%. Therefore, for a raw material containing 92 wt% of p-xylene, the operating temperature of the primary crystallizer a is 3 °C to 5 °C, the operating temperature of the secondary crystallizer d is 0 °C to 1 °C, and the operating temperature of the tertiary crystallizer g is -18 °C to -13 °C.

[0122] Based on the above analysis, in order to obtain a p-xylene product with a purity greater than 99.8 wt% and a yield greater than 90%, it is preferred that the operating temperature of the primary crystallizer a is 3 °C to 11 °C, the operating temperature of the secondary crystallizer d is 0 °C to 8 °C, and the operating temperature of the tertiary crystallizer g is -18 °C to 5 °C.

[0123] By comparing Example 1 with Examples 11 - 13, and Example 6 with Examples 14 - 16 respectively, the product yields of Examples 11 - 13 are higher than that of Example 1, and the product purities of Examples 14 - 16 are higher than that of Example 6. Therefore, in order to increase the product yield while ensuring the qualified product purity for raw materials with relatively high concentrations, and to increase the product purity while ensuring the qualified product yield for raw materials with relatively low concentrations, the operating temperature of the first crystallizer a is further preferably 5°C to 10°C, the operating temperature of the second crystallizer d is 1°C to 7°C, and the operating temperature of the third crystallizer g is - 15°C to 3°C.

[0124] Comparing Examples 1 - 10 with Comparative Examples 20 - 25, it can be seen that due to the relatively low feed concentration, the product concentration of the secondary crystallization is low, and qualified products cannot be obtained through washing. At the same time, the product yield is also low. Therefore, this process is not suitable for treating p - xylene raw materials with low concentrations (less than 92 wt%).

[0125] In terms of increasing the product purity and yield, as described in Examples 2, 3, 6 and Comparative Examples 7 - 9, by introducing raw materials into the fourth solid - liquid separation device j for washing the products, the consumption of the product washing liquid is reduced from 18%, 21%, 24% to 15%, 15%, 20% respectively. The invention uses raw material washing to increase the crystal temperature, reduce the inclusion of impurities caused by burst nucleation during the product washing stage. At the same time, raw material washing can discharge some impurities first, reducing the consumption of the product washing liquid. This method can greatly improve production efficiency. By separating the filtered mother liquor and the eluate, separating the relatively high - concentration eluate from the relatively low - concentration filtered mother liquor for separate treatment, the treatment amount of the low - concentration filtered mother liquor can be effectively reduced. The high - concentration eluate can directly obtain p - xylene products that meet the requirements, thus saving energy consumption.

[0126] From Examples 2, 3, 6 and Comparative Examples 10 - 12, it can be seen that the problem that p - xylene with qualified purity cannot be directly washed out due to the relatively low operating temperature of the synergistic mixer i is solved. The synergistic mixer i operates adiabatically, mixing the low - temperature crystals from the third crystallizer g with the eluate, and the system reaches self - balance internally. When the low - temperature crystal particles are mixed with the high - temperature and high - concentration eluate, the crystal particles grow further while the temperature rises. This process can not only reduce production energy consumption, but also dissolve fine crystals, thus being beneficial to improving the washing efficiency. The first crystallizer a, the second crystallizer d and the synergistic mixer i can all directly produce p - xylene products with a purity greater than 99.8 wt%, reducing the recycle amount of the logistics in the process and having higher production efficiency.

[0127] As can be seen from Examples 2, 3, 6 and Comparative Examples 13-15, by returning a part of the filtered mother liquor of primary crystallization, secondary crystallization, and tertiary crystallization to the primary crystallizer a, secondary crystallizer d, and tertiary crystallizer g, the product purity of p-xylene and the product yield of p-xylene are improved.

Claims

1. A paraxylene-rich crystallization washing system, characterized in that: The invention comprises three-stage suspension crystallizers: a primary crystallizer (a), a secondary crystallizer (d) and a tertiary crystallizer (g); and further comprises: a first solid-liquid separation device (b), used for solid-liquid separation and washing of the slurry crystallized in the primary crystallizer (a); A second solid-liquid separation device (e) is used for solid-liquid separation and washing of the slurry crystallized in the secondary crystallizer (d); The third solid-liquid separation equipment (h) is used for solid-liquid separation and washing of the slurry crystallized in the third-stage crystallizer (g).

2. The paraxylene-rich crystallization washing system according to claim 1, characterized in that: Also includes: A synergistic mixer (i) and a fourth solid-liquid separation device (j); the synergistic mixer (i) utilizes the eluate of the first solid-liquid separation device (b) and the eluate of the second solid-liquid separation device (e) to re-suspend and crystallize the crystals crystallized in the tertiary crystallizer (g), and the fourth solid-liquid separation device (j) performs solid-liquid separation and washing on the crystal slurry mixed in the synergistic mixer (i).

3. A method for washing a crystallization product rich in paraxylene, characterized in that: The method is operated on the paraxylene-rich crystallization and washing system of claim 2, and the C8 mixture containing paraxylene enters the primary crystallizer (a) as a raw material for suspension crystallization, and enters the first solid-liquid separation equipment (b), the second solid-liquid separation equipment (e), the third solid-liquid separation equipment (h) and the fourth solid-liquid separation equipment (j) as a washing liquid to wash the crystals; the eluate of the first solid-liquid separation equipment (b), the eluate of the second solid-liquid separation equipment (e), the eluate of the fourth solid-liquid separation equipment (j) and part of the filtered mother liquor of the fourth solid-liquid separation equipment (j) enter the synergistic mixer (i) to re-suspend and crystallize the crystals crystallized in the tertiary crystallizer (g).

4. The method according to claim 3, characterized in that include: (1) adding a para-xylene-containing raw material (1) to a primary crystallizer (a) for cooling and crystallization to obtain a crystal slurry (3); (2) the crystal slurry (3) enters the first solid-liquid separation device (b), and after washing, obtains a first filtered mother liquor, a first eluate (8) and a first para-xylene crystal (4); (3) a portion of the first filtered mother liquor is returned to the primary crystallizer (a), and the remaining portion enters the secondary crystallizer (d) for cooling and crystallization to obtain a crystal slurry (10); (4) the crystal slurry (10) enters the second solid-liquid separation device (e), and after washing, a second filtered mother liquor, a second eluate (16) and a second para-xylene crystal (12) are obtained; (5) a portion of the second filtered mother liquor is returned to the secondary crystallizer (d), and the remaining portion enters the tertiary crystallizer (g) for cooling and crystallization to obtain a crystal slurry (18); (6) The crystal slurry (18) enters the third solid-liquid separation device (h) to obtain a third filtered mother liquor, a third eluate (20) and a third para-xylene crystal (19), part of the third filtered mother liquor returns to the third crystallizer (g), and the third eluate (20) returns to the secondary crystallizer (d); (7) The first eluate (8), the second eluate (16), the fourth eluate (29) of the fourth solid-liquid separation device (j) and part of the filtered mother liquor of the fourth solid-liquid separation device (j) enter the synergistic mixer (i) to re-suspend and crystallize the crystals crystallized in the tertiary crystallizer (g), and then enter the fourth solid-liquid separation device (j) for solid-liquid separation and washing.

5. The method according to claim 4, characterized in that The operating temperature of the primary crystallizer (a) is 3-11°C; the operating temperature of the secondary crystallizer (d) is 0-8°C; the operating temperature of the tertiary crystallizer (g) is -18-5°C; the operating temperature of the primary crystallizer (a) is greater than the operating temperature of the secondary crystallizer (d).

6. The method according to claim 5, characterized in that The operating temperature of the primary crystallizer (a) is 5-10°C, the operating temperature of the secondary crystallizer (d) is 1-7°C, and the operating temperature of the tertiary crystallizer (g) is -15°C to 3°C.

7. The method of claim 4, characterized in that In step (2), the crystal slurry (3) is washed by the paraxylene-containing raw material (2) and the paraxylene product (6) in the first solid-liquid separation equipment (b).

8. The method of claim 4, characterized in that In step (2), a portion of the first paraxylene crystals (4) after melting is returned to the first solid-liquid separation equipment (b) as washing liquid.

9. The method of claim 4, characterized in that In step (4), the slurry (10) enters the second solid-liquid separation device (e) and is washed with a raw material containing paraxylene.

10. The method of claim 4, characterized in that In step (6), the slurry (18) enters the third solid-liquid separation equipment (h) and is washed with a raw material containing paraxylene.

11. The method according to any one of claims 4 to 10, characterized in that: The para-xylene-containing raw material is a C8 mixture containing 92 wt% to 98 wt% of para-xylene.

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