Crystallization washing method and system for p-xylene

Abstract

The invention discloses a p-xylene 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 (k) is used for carrying out solid-liquid separation on crystal mush crystallized by the third-stage crystallizer (g). The invention also discloses a paraxylene crystal washing method operated on the paraxylene crystal washing system. In order to solve the problem that the raw material containing medium and 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 p-xylene product is improved.

Description

Technical Field

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

[0002] p-Xylene (PX) is an important bulk chemical product, and its products are widely used in various industries. During the production process of p-xylene, a large number of by-products of isomers are accompanied. The boiling points of various isomers of xylene are extremely close, and it is difficult to obtain high-purity products through conventional distillation methods. However, the freezing points of p-xylene and other xylenes vary greatly, and crystallization separation and purification of p-xylene have obvious technical advantages. Moreover, the freezing point of PX is 13.2 °C, the production conditions are mild, and the required energy consumption is lower.

[0003] Industrially, suspension crystallization is usually adopted to produce p-xylene on a large scale. The crystal slurry obtained by suspension crystallization needs to be subjected to solid-liquid separation to obtain pure p-xylene products. During the separation process of crystals and liquid, a small amount of liquid remains on the crystal surface, thereby affecting the product purity. Crystal washing can effectively remove the residual impurity liquid on the crystal surface and improve the product purity. However, in the actual production process, there are problems such as uneven distribution of crystal particles and small average particle size, which result in poor solid-liquid separation effect. According to the thermodynamic phase equilibrium relationship of xylene, the concentration of p-xylene in the mixed xylene liquid decreases with the decrease of temperature. Therefore, in order to ensure the overall yield of the process, the crystallization temperature is usually set very low. During the washing process, when low-temperature crystals contact with high-temperature and high-concentration washing liquid, violent burst nucleation will occur, thus reducing the washing effect. Summary of the Invention

[0004] In order to further improve the efficiency of crystallization washing of p-xylene, save operation steps 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, including three-stage suspension crystallizers: a first-stage crystallizer (a), a second-stage crystallizer (d), and a third-stage crystallizer (g); and further including:

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

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

[0008] A third solid-liquid separation device (k), which is used for solid-liquid separation of the crystal slurry crystallized by the third-stage crystallizer (g).

[0009] In the specific implementation manner, the p-xylene crystallization washing system further includes: an enhancing mixer (h) and a fourth solid-liquid separation device (i); the enhancing mixer (h) performs re-suspension crystallization treatment on the crystals crystallized in the secondary crystallizer (d) and the tertiary crystallizer (g), and the fourth solid-liquid separation device (i) performs solid-liquid separation and washing on the crystal slurry mixed by the enhancing mixer (h).

[0010] In another aspect of the present invention, it relates to a p-xylene crystallization washing method, which operates on the above-mentioned p-xylene 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) and the fourth solid-liquid separation device (i) as a washing liquid to wash the crystals, and enters the enhancing mixer (h) to perform suspension treatment on the crystals crystallized in the secondary crystallizer (d) and 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 crystal slurry (2);

[0013] (2) The crystal slurry (2) enters the first solid-liquid separation device (b), and after washing, a first filtrate mother liquor (7), a first eluate (10) and first p-xylene crystals (4) are obtained. The first eluate (10) is returned to the primary crystallizer (a);

[0014] (3) A part of the first filtrate mother liquor (7) is returned to the primary crystallizer (a), and the remaining part enters the secondary crystallizer (d) for cooling crystallization to obtain crystal slurry (12);

[0015] (4) The crystal slurry (12) enters the second solid-liquid separation device (e), and after washing, a second filtrate mother liquor (16), a second eluate (15) and second p-xylene crystals (14) are obtained. The second eluate (15) is returned to the secondary crystallizer (d);

[0016] (5) A part of the second filtrate mother liquor (16) is returned to the secondary crystallizer (d), and the remaining part enters the tertiary crystallizer (g) for cooling crystallization to obtain crystal slurry (19);

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

[0018] (7) The third p-xylene crystals (20) enter the enhancing mixer (h), and the second p-xylene crystals (14) and the first p-xylene crystals (4) are also added to the enhancing mixer (h) to perform re-suspension crystallization treatment on the crystals to obtain crystal slurry (21);

[0019] (7) The second p - xylene crystal (14), the third p - xylene crystal (20) and the raw material containing p - xylene enter the synergistic mixer (h), and after being fully mixed, they enter the fourth solid - liquid separation device (i), and are separated after being washed by the raw material containing p - xylene and part of the p - xylene product.

[0020] In a specific embodiment, the operating temperature of the primary crystallizer (a) is 3 - 11 °C, the operating temperature of the secondary crystallizer (d) is - 5 - 8 °C, the operating temperature of the tertiary crystallizer (g) is - 20 - 4 °C, and the operating temperature of the primary crystallizer (a) is higher than that of the secondary crystallizer (d).

[0021] In a specific embodiment, the operating temperature of the primary crystallizer (a) is 5 - 9 °C, the operating temperature of the secondary crystallizer (d) is - 4 - 7 °C, and the operating temperature of the tertiary crystallizer (g) is - 18 - 6 °C.

[0022] In a specific embodiment, in step (2), the crystal slurry (2) in the first solid - liquid separation device (b) is washed by the raw material containing p - xylene (3) and the p - xylene product (6).

[0023] In a specific embodiment, in step (2), a part of the melted first p - xylene crystal (4) is returned as a washing liquid to the first solid - liquid separation device (b).

[0024] In a specific embodiment, in step (4), after the crystal slurry (12) enters the second solid - liquid separation device (e), it is washed by the raw material containing p - xylene.

[0025] The raw material containing p - xylene is a C8 mixture containing 88 wt% - 95 wt% of p - xylene.

[0026] In view of the problem of difficult washing during the refining process of raw materials containing medium - high concentration (88 wt% - 95 wt%) of p - xylene, the present invention creatively proposes a washing scheme using a raw material containing 88 wt% - 95 wt% of p - xylene (in each embodiment, the concentration of the raw material containing p - xylene for washing is the same as that of the C8 mixture raw material 1 containing p - xylene). In the p - xylene production method of the present invention, the primary crystallizer a, the secondary crystallizer d, and the tertiary crystallizer g are all suspension crystallizations. By optimizing the process parameters of the three - stage suspension crystallization and adopting the washing scheme using the raw material containing p - xylene, a p - xylene product with a purity greater than 99.8 wt% is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 : Schematic flow chart of the suspension crystallization and washing method of p - xylene of the present invention.

[0028] Such as Figure 1The method comprises the following steps: adding a C8 mixture containing paraxylene as a raw material 1 to a primary crystallizer a for cooling and crystallization; the obtained crystal slurry 2 enters a first solid-liquid separation device b to obtain paraxylene crystals and a first filtered mother liquor 7; the paraxylene crystals are washed with a raw material 3 containing paraxylene and a paraxylene product 6 to obtain a first eluate 10 and first paraxylene crystals 4; the first eluate 10 is returned to the primary crystallizer a; the first paraxylene crystals 4 are melted in a first melter c, a portion of which is returned to the first solid-liquid separation device b as the paraxylene product 6, and the remaining The remaining part leaves the crystallization system as the para-xylene product 5; part of the first filtered mother liquor 7 is returned to the primary crystallizer a as the first filtered mother liquor 8, and the remaining part enters the secondary crystallizer d as the first filtered mother liquor 9 for cooling and crystallization. The resulting crystal slurry 12 enters the second solid-liquid separation equipment e to obtain para-xylene crystals and the second filtered mother liquor 16. After the para-xylene crystals are washed with the para-xylene-containing raw material 13, the second eluate 15 and the second para-xylene crystals 14 are obtained; the second eluate 15 is returned to the secondary crystallizer d; the second para-xylene crystals 14 are sent to the increasing The crystal slurry 19 enters the third solid-liquid separation device k to obtain the third filtered mother liquor and the third p-xylene crystals 20. Part of the third filtered mother liquor 16 is returned to the secondary crystallizer d as the second filtered mother liquor 17, and the remaining part enters the tertiary crystallizer g as the second filtered mother liquor 18 for cooling and crystallization. The obtained crystal slurry 19 enters the third solid-liquid separation device k to obtain the third filtered mother liquor and the third p-xylene crystals 20. Part 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 21; the third p-xylene crystals 20 and the raw material 23 containing p-xylene enter the synergistic mixer h, and the The mixed crystal slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtered mother liquor. After the p-xylene crystals are washed with a raw material 26 containing p-xylene and a p-xylene product 29, a fourth eluate and fourth p-xylene crystals 27 are obtained. The mixed liquid 25 of the fourth filtered mother liquor and the fourth eluate returns to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in the second melter j, of which part is returned to the fourth solid-liquid separation device i as a p-xylene product 29, and the remaining part leaves the crystallization system as a p-xylene product 28. Specific implementation method

[0029] The inventor conducted experiments with reference to patent CN104557433, using two crystallization stages and a slurry tank, in which the slurry tank uses an external sleeve heat exchanger for temperature control. The crystals of the first-stage crystallization are directly discharged from the product after washing, and the crystals of the second-stage crystallization go to the slurry tank. After heating and aging, the crystals are washed with products and then go to the product tank. This method solves the problem of difficult crystal washing by heating and aging. However, during this operation, the secondary crystals melt and recrystallize, resulting in energy loss. At the same time, due to the large amount of secondary crystallization, its crystals go to the slurry tank, resulting in a large volume of the slurry tank, resulting in high equipment operating costs.

[0030] The inventor conducted experiments with reference to Patent CN106831302, and processed mixed xylene in a manner of one crystallization stage and one aging stage. Among them, the crystallizer is mainly responsible for recovering p-xylene, and the produced p-xylene crystals go to the aging kettle. The p-xylene crystals and part of the raw materials are aged at an elevated temperature in the aging kettle. After the crystal slurry in the aging kettle undergoes solid-liquid separation and product washing, p-xylene products with qualified purity are obtained. The process flow of this method is simple, but due to the large temperature difference in the first-stage crystallization, it is easy to result in relatively small crystal particle sizes. Small-sized crystals are not conducive to solid-liquid separation, and a large amount of product washing filter cake is required, so its energy consumption is relatively high.

[0031] In view of the fact that the existing technologies cannot meet the inventor's expectations, through further research and development, the present invention is made. To solve the problem of difficult crystal washing in the production process of p-xylene, the present invention discloses a crystallization washing method for medium- and high-concentration p-xylene. The method includes three crystallization stages and one synergistic mixing stage. Among them, the crystal slurry produced by the first-stage crystallizer directly produces products after being washed with raw materials and products. The crystals produced by the second-stage and third-stage crystallizers are mixed with raw materials in the synergistic mixer and then produce products after being washed with raw materials and products. The specific implementation process examples are as follows.

[0032] Example 1

[0033] A C8 mixture containing 95 wt% p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 11°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 494 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 156 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -4°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2190 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (374 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 15% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.88 wt% and a yield of 91.03%.

[0034] Example 2

[0035] A C8 mixture containing 95 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 9°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 494 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 156 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 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 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2190 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (374 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 15% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final PX product has a purity of 99.84 wt% and a yield of 92.84%.

[0036] Example 3

[0037] A C8 mixture containing 93 wt% p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 9°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 539 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 16% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 177 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -8°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2258 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (406 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 16% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.86 wt% and a yield of 90.31%.

[0038] Example 4

[0039] The C8 mixture containing 93 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and added to the primary crystallizer a for cooling crystallization at a crystallization temperature of 7°C. The resulting crystal slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and the first filtrate mother liquor 7. After being washed with Feedstock 3 containing p-xylene (flow rate: 539 kg / h) and p-xylene product 6, the p-xylene crystals yield the first eluate 10 and the first p-xylene crystals 4; the first eluate 10 is returned to the primary crystallizer a; the first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 16% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5; 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of 4°C. The resulting crystal slurry 12 enters the second solid-liquid separation device e to obtain p-xylene crystals and the second filtrate mother liquor 16. After being washed with Feedstock 13 containing p-xylene (flow rate: 177 kg / h), the p-xylene crystals yield the second eluate 15 and the second p-xylene crystals 14; the second eluate 15 is returned to the secondary crystallizer d; the second p-xylene crystals 14 go to the synergistic mixer h; 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -10°C. The resulting crystal slurry 19 enters the third solid-liquid separation device k to obtain the third filtrate mother liquor and the third p-xylene crystals 20. 20% 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 21; the third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2258 kg / h) enter the synergistic mixer h. After thorough mixing, the crystal slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtrate mother liquor. After being washed with Feedstock 26 containing p-xylene (406 kg / h) and p-xylene product 29, the p-xylene crystals yield the fourth eluate and the fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in the second melter j. Among them, 16% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.85 wt% and a yield of 91.48%.

[0040] Example 5

[0041] A C8 mixture containing 91 wt% p-xylene is added as Feedstock 1 (flow rate: 1806 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 8°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 582 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 17% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 205 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -12°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2476 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (423 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 17% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.85 wt% and a yield of 90.03%.

[0042] Example 6

[0043] A C8 mixture containing 91 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and added to a primary crystallizer a for cooling crystallization at a crystallization temperature of 6°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 582 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 17% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 205 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -14°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2476 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (423 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 17% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.83 wt% and a yield of 91.13%.

[0044] Example 7

[0045] A C8 mixture containing 90 wt% p-xylene is added as Feed 1 (flow rate: 1806 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 7°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feed 3 containing p-xylene (flow rate: 604 g / h) and p-xylene product 6 to obtain a first eluate 10 and a first p-xylene crystal 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystal 4 is melted in a first melter c. Among them, approximately 18% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feed 13 containing p-xylene (flow rate: 221 kg / h) to obtain a second eluate 15 and a second p-xylene crystal 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystal 14 goes to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -14°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and a third p-xylene crystal 20. 20% 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 21. The third p-xylene crystal 20 and Feed 23 containing p-xylene (flow rate: 2544 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feed 26 containing p-xylene (440 kg / h) and p-xylene product 29 to obtain a fourth eluate and a fourth p-xylene crystal 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystal 27 is melted in a second melter j. Among them, 18% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.83 wt% and a yield of 90.01%.

[0046] Example 8

[0047] The C8 mixture containing 90 wt% of p-xylene is added as Feedstock 1 (flow rate: 1806 kg / h) to the first-stage crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting crystal slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and the first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 604 g / h) and p-xylene product 6 to obtain the first eluate 10 and the first p-xylene crystals 4. The first eluate 10 is returned to the first-stage crystallizer a. The first p-xylene crystals 4 are melted in the first melter c. Among them, about 18% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters the second solid-liquid separation device e to obtain p-xylene crystals and the second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 221 kg / h) to obtain the second eluate 15 and the second p-xylene crystals 14. The second eluate 15 is returned to the second-stage crystallizer d. The second p-xylene crystals 14 go to the synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the second-stage crystallizer d, and the remaining part enters the third-stage crystallizer g for cooling crystallization at a crystallization temperature of -17°C. The resulting crystal slurry 19 enters the third solid-liquid separation device k to obtain the third filtrate mother liquor and the third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2544 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (440 kg / h) and p-xylene product 29 to obtain the fourth eluate and the fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the first-stage crystallizer a. The fourth p-xylene crystals 27 are melted in the second melter j. Among them, 18% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.82 wt% and a yield of 91.5%.

[0048] Example 9

[0049] The C8 mixture containing 88 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into the primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting crystal slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and the first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 843 g / h) and p-xylene product 6 to obtain the first eluate 10 and the first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of -3°C. The resulting crystal slurry 12 enters the second solid-liquid separation device e to obtain p-xylene crystals and the second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 338 kg / h) to obtain the second eluate 15 and the second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to the synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -18°C. The resulting crystal slurry 19 enters the third solid-liquid separation device k to obtain the third filtrate mother liquor and the third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2620 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (522 kg / h) and p-xylene product 29 to obtain the fourth eluate and the fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in the second melter j. Among them, 20% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.84 wt% and a yield of 90.04%.

[0050] Example 10

[0051] An 88 wt% p-xylene-containing C8 mixture is added as Feedstock 1 (flow rate: 1806 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 843 g / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4; the first eluate 10 is returned to the primary crystallizer a; the first p-xylene crystals 4 are melted in a first melter c, of which approximately 20% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5; 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of -5°C. The resulting slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 338 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14; the second eluate 15 is returned to the secondary crystallizer d; the second p-xylene crystals 14 go to a synergistic mixer h; 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -20°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21; the third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2620 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (522 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in a second melter j, of which 20% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.80 wt% and a yield of 91.00%.

[0052] Example 11

[0053] A C8 mixture containing 95 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 9°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 494 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 crystal slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 156 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -4°C. The resulting crystal slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2190 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (374 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 15% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.87 wt% and a yield of 91.05%.

[0054] Example 12

[0055] A C8 mixture containing 95 wt% p-xylene is added as Feed 1 (flow rate: 1806 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 9°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feed 3 containing p-xylene (flow rate: 494 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 7°C. The resulting slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feed 13 containing p-xylene (flow rate: 156 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -4°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feed 23 containing p-xylene (flow rate: 2190 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feed 26 containing p-xylene (374 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 15% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.86 wt% and a yield of 91.06%.

[0056] Example 13

[0057] A C8 mixture containing 95 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 9°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 494 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 7°C. The resulting crystal slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 156 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -6°C. The resulting crystal slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2190 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (374 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed solution 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 15% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.85 wt% and a yield of 92.3%.

[0058] Example 14

[0059] The C8 mixture containing 88 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into the primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting crystal slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and the first filtrate mother liquor 7. After being washed with Feedstock 3 containing p-xylene (flow rate: 843 g / h) and p-xylene product 6, the p-xylene crystals yield the first eluate 10 and the first p-xylene crystals 4; the first eluate 10 is returned to the primary crystallizer a; 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 leaves the crystallization system as p-xylene product 5; 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of -4°C. The resulting crystal slurry 12 enters the second solid-liquid separation device e to obtain p-xylene crystals and the second filtrate mother liquor 16. After being washed with Feedstock 13 containing p-xylene (flow rate: 338 kg / h), the p-xylene crystals yield the second eluate 15 and the second p-xylene crystals 14; the second eluate 15 is returned to the secondary crystallizer d; the second p-xylene crystals 14 go to the synergistic mixer h; 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -18°C. The resulting crystal slurry 19 enters the third solid-liquid separation device k to obtain the third filtrate mother liquor and the third p-xylene crystals 20. 20% 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 21; the third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2620 kg / h) enter the synergistic mixer h. After thorough mixing, the crystal slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtrate mother liquor. After being washed with Feedstock 26 containing p-xylene (522 kg / h) and p-xylene product 29, the p-xylene crystals yield the fourth eluate and the fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in the second melter j. Among them, 20% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.83 wt% and a yield of 90.05%.

[0060] Example 15

[0061] The C8 mixture containing 88 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into the primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and the first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 843 g / h) and the p-xylene product 6 to obtain the first eluate 10 and the first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 20% by weight is returned as the p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as the p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of -4°C. The resulting slurry 12 enters the second solid-liquid separation device e to obtain p-xylene crystals and the second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 338 kg / h) to obtain the second eluate 15 and the second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to the synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -20°C. The resulting slurry 19 enters the third solid-liquid separation device k to obtain the third filtrate mother liquor and the third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2620 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (522 kg / h) and the p-xylene product 29 to obtain the fourth eluate and the fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in the second melter j. Among them, 20% by weight is returned as the p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as the p-xylene product 28. The purity of the finally obtained PX product is 99.82 wt%, and the yield is 90.99%.

[0062] Example 16

[0063] The C8 mixture containing 88 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and added to the primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting crystal slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and the first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 843 g / h) and p-xylene product 6 to obtain the first eluate 10 and the first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of -5°C. The resulting crystal slurry 12 enters the second solid-liquid separation device e to obtain p-xylene crystals and the second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 338 kg / h) to obtain the second eluate 15 and the second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to the synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -20°C. The resulting crystal slurry 19 enters the third solid-liquid separation device k to obtain the third filtrate mother liquor and the third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2620 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (522 kg / h) and p-xylene product 29 to obtain the fourth eluate and the fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in the second melter j. Among them, 20% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.81 wt% and a yield of 90.99%.

[0064] Comparative Example 1

[0065] A C8 mixture containing 95 wt% p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 12°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 494 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 9°C. The resulting crystal slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 156 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -2°C. The resulting crystal slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2190 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (374 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed solution 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 15% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The purity of the finally obtained PX product is 99.92 wt%, and the yield is 89.39%.

[0066] Comparative Example 2

[0067] A C8 mixture containing 95 wt% p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 7°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 494 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first mother liquor 7 is returned as the first mother liquor 8 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 slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 156 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second mother liquor 16 is returned as the second mother liquor 17 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 19 enters a third solid-liquid separation device k to obtain a third mother liquor and third p-xylene crystals 20. 20% of the third mother liquor is returned as the third mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third mother liquor 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2190 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (374 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed solution 25 of the fourth mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 15% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.79 wt% and a yield of 93.78%.

[0068] Comparative Example 3

[0069] A C8 mixture containing 93 wt% p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 11°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 539 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 16% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 7°C. The resulting crystal slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 177 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 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 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2258 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (406 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 16% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The purity of the finally obtained PX product is 99.87 wt%, and the yield is 89.62%.

[0070] Comparative Example 4

[0071] A C8 mixture containing 93 wt% p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 6°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 539 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 16% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 177 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -11°C. The resulting crystal slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2258 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (406 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 16% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.77 wt% and a yield of 92.07%.

[0072] Comparative Example 5

[0073] A C8 mixture containing 91 wt% of p-xylene is added as Feedstock 1 (flow rate: 1806 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 10°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 582 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c, of which approximately 17% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 205 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -11°C. The resulting crystal slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2476 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (423 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed solution 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j, of which 17% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final PX product has a purity of 99.86 wt% and a yield of 89.41%.

[0074] Comparative Example 6

[0075] A C8 mixture containing 91 wt% of p-xylene is added as Feed 1 (flow rate: 1806 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 4°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feed 3 containing p-xylene (flow rate: 582 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 17% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feed 13 containing p-xylene (flow rate: 205 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 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 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feed 23 containing p-xylene (flow rate: 2476 kg / h) enter the synergistic mixer h. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feed 26 containing p-xylene (423 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 17% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.76 wt% and a yield of 92.09%.

[0076] Comparative Example 7

[0077] A C8 mixture containing 90 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and added to a primary crystallizer a for cooling crystallization at a crystallization temperature of 9°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 604 g / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 18% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 221 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -11°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2544 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (440 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 18% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final PX product has a purity of 99.85 wt% and a yield of 88.1%.

[0078] Comparative Example 8

[0079] A C8 mixture containing 90 wt% p-xylene is added as Feedstock 1 (flow rate: 1806 kg / h) to a primary crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 604 g / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 18% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first mother liquor 7 is returned as the first mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 221 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. 30% of the second mother liquor 16 is returned as the second mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -20°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third mother liquor and third p-xylene crystals 20. 20% of the third mother liquor is returned as the third mother liquor 22 to the tertiary crystallizer g, and the remaining part is discharged from the crystallization system as the third mother liquor 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2544 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (440 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed solution 25 of the fourth mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 18% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.75 wt% and a yield of 92.72%.

[0080] Comparative Example 9

[0081] The C8 mixture containing 88 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and added to the primary crystallizer a for cooling crystallization at a crystallization temperature of 6°C. The resulting slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and the first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 843 g / h) and p-xylene product 6 to obtain the first eluate 10 and the first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of -1°C. The resulting slurry 12 enters the second solid-liquid separation device e to obtain p-xylene crystals and the second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 338 kg / h) to obtain the second eluate 15 and the second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to the synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -16°C. The resulting slurry 19 enters the third solid-liquid separation device k to obtain the third filtrate mother liquor and the third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2620 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (522 kg / h) and p-xylene product 29 to obtain the fourth eluate and the fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in the second melter j. Among them, 20% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The purity of the finally obtained PX product is 99.84 wt%, and the yield is 88.96%.

[0082] Comparative Example 10

[0083] The C8 mixture containing 88 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and added to the primary crystallizer a for cooling crystallization at a crystallization temperature of 1°C. The resulting slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and the first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 843 g / h) and p-xylene product 6 to obtain the first eluate 10 and the first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters the secondary crystallizer d for cooling crystallization at a crystallization temperature of -6°C. The resulting slurry 12 enters the second solid-liquid separation device e to obtain p-xylene crystals and the second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 338 kg / h) to obtain the second eluate 15 and the second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to the synergistic mixer h. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters the tertiary crystallizer g for cooling crystallization at a crystallization temperature of -21°C. The resulting slurry 19 enters the third solid-liquid separation device k to obtain the third filtrate mother liquor and the third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2620 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (522 kg / h) and p-xylene product 29 to obtain the fourth eluate and the fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in the second melter j. Among them, 20% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The purity of the finally obtained PX product is 99.75 wt%, and the yield is 91.44%.

[0084] Comparative Example 11

[0085] A C8 mixture containing 95 wt% of p-xylene is added as raw material 1 (flow rate is 1806 kg / h) to a primary crystallizer a for cooling and crystallization at a crystallization temperature of 11°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtered mother liquor 7. The p-xylene crystals are washed with a p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c, wherein about 21% by weight are used as The p-xylene product 6 returns to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as the p-xylene product 5; 40% of the first filtered mother liquor 7 returns to the primary crystallizer a as the first filtered mother liquor 8, and the remaining part enters the secondary crystallizer d as the first filtered mother liquor 9 for cooling and crystallization. The crystallization temperature is 8°C, and the resulting crystal slurry 12 enters the second solid-liquid separation device e to obtain the second filtered mother liquor 16 and the second p-xylene crystals 14; the second p-xylene crystals 14 go to the synergistic mixer h; 30% of the second filtered mother liquor 16 is used as the second The filtered mother liquor 17 is returned to the secondary crystallizer d, and the remaining part is used as the second filtered mother liquor 18 to enter the tertiary crystallizer g for cooling and crystallization. The crystallization temperature is -4°C. The obtained crystal slurry 19 enters the third solid-liquid separation equipment k to obtain the third filtered mother liquor and the third p-xylene crystals 20. 20% 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 21; the third p-xylene crystals 20 and the raw material 23 containing p-xylene (with a flow rate of 2190 kg / h) enter the synergistic The fully mixed crystal slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtered mother liquor. After the p-xylene crystals are washed with the p-xylene product 29, the fourth eluate and the fourth p-xylene crystals 27 are obtained. The mixed liquid 25 of the fourth filtered mother liquor and the fourth eluate returns to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in the second melter j, of which 21% by weight is returned to the fourth solid-liquid separation device i as the p-xylene product 29, and the remaining part leaves the crystallization system as the p-xylene product 28. The final PX product purity is 99.8wt%, and the yield is 80.4%.

[0086] Comparative Example 12

[0087] A C8 mixture containing 90 wt% of p-xylene is added as raw material 1 (flow rate is 1806 kg / h) into a primary crystallizer a for cooling and crystallization at a crystallization temperature of 5°C. The resulting slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and a first filtered mother liquor 7. The p-xylene crystals are washed with a p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c, wherein about 26% by weight of the p-xylene crystals are used as p-xylene. The xylene product 6 returns to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as the para-xylene product 5; 40% of the first filtered mother liquor 7 returns to the primary crystallizer a as the first filtered mother liquor 8, and the remaining part enters the secondary crystallizer d as the first filtered mother liquor 9 for cooling and crystallization. The crystallization temperature is 1°C, and the resulting crystal slurry 12 enters the second solid-liquid separation device e to obtain the second filtered mother liquor 16 and the second para-xylene crystals 14; the second para-xylene crystals 14 go to the synergistic mixer h; 30% of the second filtered mother liquor 16 is used as the second The filtered mother liquor 17 is returned to the secondary crystallizer d, and the remaining part is used as the second filtered mother liquor 18 to enter the tertiary crystallizer g for cooling and crystallization. The crystallization temperature is -17°C. The obtained crystal slurry 19 enters the third solid-liquid separation equipment k to obtain the third filtered mother liquor and the third p-xylene crystals 20. 20% 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 21; the third p-xylene crystals 20 and the raw material containing p-xylene 23 (flow rate is 2544 g / h) enter the synergistic The fully mixed crystal slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtered mother liquor. After the p-xylene crystals are washed with the p-xylene product 29, the fourth eluate and the fourth p-xylene crystals 27 are obtained. The mixed liquid 25 of the fourth filtered mother liquor and the fourth eluate returns to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in the second melter j, of which 26% by weight is returned to the fourth solid-liquid separation device i as the p-xylene product 29, and the remaining part leaves the crystallization system as the p-xylene product 28. The final PX product purity is 99.76wt%, and the yield is 78.6%.

[0088] Comparative Example 13

[0089] An 88 wt% p-xylene-containing C8 mixture is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. After being washed with p-xylene product 6, the p-xylene crystals yield a first eluate 10 and first p-xylene crystals 4; the first eluate 10 is returned to the primary crystallizer a; the first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 29% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5; 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the primary crystallizer a, and the remaining part enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of -5°C. The resulting crystal slurry 12 enters a second solid-liquid separation device e to obtain a second filtrate mother liquor 16 and second p-xylene crystals 14; the second p-xylene crystals 14 go to a synergistic mixer h; 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -20°C. The resulting crystal slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21; the third p-xylene crystals 20 and p-xylene-containing feedstock 23 (flow rate: 2620 g / h) enter the synergistic mixer h. After thorough mixing, the crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. After being washed with p-xylene product 29, the p-xylene crystals yield a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in a second melter j. Among them, 29% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.73 wt% and a yield of 76.2%.

[0090] Comparative Example 14

[0091] A C8 mixture containing 95 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 11°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 494 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 crystal slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 156 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a slurring kettle. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -4°C. The resulting crystal slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 enter the slurring kettle. After the temperature is raised for slurring, the crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (374 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 19% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The purity of the finally obtained PX product is 99.87 wt%, and the yield is 72.5%.

[0092] Comparative Example 15

[0093] A C8 mixture containing 90 wt% p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 604 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 18% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 221 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a slurring kettle. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 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 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 enter the slurring kettle. After the temperature is raised for slurring, the slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (440 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 23% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final PX product has a purity of 99.83 wt% and a yield of 70.1%.

[0094] Comparative Example 16

[0095] The C8 mixture containing 88 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and added to the first crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting crystal slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and the first filtrate mother liquor 7. After being washed with Feedstock 3 containing p-xylene (flow rate: 843 kg / h) and p-xylene product 6, the p-xylene crystals yield the first eluate 10 and the first p-xylene crystals 4; the first eluate 10 is returned to the first crystallizer a; 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 leaves the crystallization system as p-xylene product 5; 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 to the first crystallizer a, and the remaining part enters the second crystallizer d as the first filtrate mother liquor 9 for cooling crystallization at a crystallization temperature of -5°C. The resulting crystal slurry 12 enters the second solid-liquid separation device e to obtain p-xylene crystals and the second filtrate mother liquor 16. After being washed with Feedstock 13 containing p-xylene (flow rate: 338 kg / h), the p-xylene crystals yield the second eluate 15 and the second p-xylene crystals 14; the second eluate 15 is returned to the second crystallizer d; the second p-xylene crystals 14 go to the slurring kettle; 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the second crystallizer d, and the remaining part enters the third crystallizer g as the second filtrate mother liquor 18 for cooling crystallization at a crystallization temperature of -20°C. The resulting crystal slurry 19 enters the third solid-liquid separation device k to obtain the third filtrate mother liquor and the third p-xylene crystals 20. 20% of the third filtrate mother liquor is returned as the third filtrate mother liquor 22 to the third crystallizer g, and the remaining part is discharged from the crystallization system as the third filtrate mother liquor 21; the third p-xylene crystals 20 enter the slurring kettle. After the temperature is raised for slurring, the crystal slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtrate mother liquor. After being washed with Feedstock 26 containing p-xylene (522 kg / h) and p-xylene product 29, the p-xylene crystals yield the fourth eluate and the fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the first crystallizer a; the fourth p-xylene crystals 27 are melted in the second melter j. Among them, 25% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The purity of the finally obtained PX product is 99.80 wt%, and the yield is 68.4%.

[0096] Comparative Example 17

[0097] A C8 mixture containing 95 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 11°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 494 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4; the first eluate 10 is returned to the primary crystallizer a; 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 leaves the crystallization system as p-xylene product 5; the first filtrate mother liquor 7 enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 8°C. The resulting slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 156 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14; the second eluate 15 is returned to the secondary crystallizer d; the second p-xylene crystals 14 go to a synergistic mixer h; the second filtrate mother liquor 16 enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -4°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. The third filtrate mother liquor is discharged from the crystallization system; the third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2190 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (374 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in a second melter j. Among them, 15% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.78 wt% and a yield of 87.5%.

[0098] Comparative Example 18

[0099] A C8 mixture containing 90 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 5°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 604 g / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 18% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. The first filtrate mother liquor 7 enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of 1°C. The resulting slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 221 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. The second filtrate mother liquor 16 enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -17°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. The third filtrate mother liquor is discharged from the crystallization system. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2544 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (440 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed solution 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 18% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.75 wt% and a yield of 88.1%.

[0100] Comparative Example 19

[0101] An 88 wt% p-xylene-containing C8 mixture is used as Feedstock 1 (flow rate: 1,806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 3°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 843 g / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. The first filtrate mother liquor 7 enters a secondary crystallizer d for cooling crystallization at a crystallization temperature of -5°C. The resulting slurry 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 338 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 go to a synergistic mixer h. The second filtrate mother liquor 16 enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -20°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. The third filtrate mother liquor is discharged from the crystallization system. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 2,620 kg / h) enter the synergistic mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (522 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed solution 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 20% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.71 wt% and a yield of 88.8%.

[0102] Comparative Example 20

[0103] The C8 mixture containing 98 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into the primary crystallizer a for cooling crystallization at a crystallization temperature of 11°C. The resulting crystal slurry 2 enters the first solid-liquid separation device b to obtain p-xylene crystals and the first filtrate mother liquor 7. After being washed with Feedstock 3 containing p-xylene (flow rate: 461 kg / h) and p-xylene product 6, the p-xylene crystals yield the first eluate 10 and the first p-xylene crystals 4; the first eluate 10 is returned to the primary crystallizer a; the first p-xylene crystals 4 are melted in the first melter c. Among them, approximately 8% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5; 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 crystal slurry 12 enters the second solid-liquid separation device e to obtain p-xylene crystals and the second filtrate mother liquor 16. After being washed with Feedstock 13 containing p-xylene (flow rate: 98 kg / h), the p-xylene crystals yield the second eluate 15 and the second p-xylene crystals 14; the second eluate 15 is returned to the secondary crystallizer d; the second p-xylene crystals 14 leave the crystallization system; 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 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 19 enters the third solid-liquid separation device k to obtain the third filtrate mother liquor and the third p-xylene crystals 20. 20% 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 21; the third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 177 kg / h) enter the synergistic mixer h. After sufficient mixing, the crystal slurry 24 enters the fourth solid-liquid separation device i to obtain p-xylene crystals and the fourth filtrate mother liquor. After being washed with Feedstock 26 containing p-xylene (249 kg / h) and p-xylene product 29, the p-xylene crystals yield the fourth eluate and the fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in the second melter j. Among them, 8% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The purity of the finally obtained PX product is 99.92 wt%, and the yield is 96.58%.

[0104] Comparative Example 21

[0105] A C8 mixture containing 98 wt% of p-xylene is added as raw material 1 (flow rate is 1806 kg / h) into a primary crystallizer a for cooling and crystallization at a crystallization temperature of 10°C. The resulting slurry 2 enters the first solid-liquid separation equipment b to obtain p-xylene crystals and a first filtered mother liquor 7. The p-xylene crystals are washed with a raw material 3 containing p-xylene (flow rate is 461 kg / h) and a p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c, of which about 8% by weight are used as p-xylene products. The product 6 is returned to the first solid-liquid separation equipment b, and the remaining part leaves the crystallization system as the para-xylene product 5; 40% of the first filtered mother liquor 7 is returned to the primary crystallizer a as the first filtered mother liquor 8, and the remaining part enters the secondary crystallizer d as the first filtered mother liquor 9 for cooling crystallization, and the crystallization temperature is 7°C. The obtained crystal slurry 12 enters the second solid-liquid separation equipment e to obtain para-xylene crystals and the second filtered mother liquor 16. After the para-xylene crystals are washed with the para-xylene-containing raw material 13 (flow rate is 98 kg / h), the second eluate 15 and the second para-xylene crystals 14 are obtained; the second eluate 15 is returned to the secondary crystallizer d ; The second para-xylene crystals 14 leave the crystallization system; 30% of the second filtered mother liquor 16 is returned to the secondary crystallizer d as the second filtered mother liquor 17, and the remaining part is entered into the tertiary crystallizer g as the second filtered mother liquor 18 for cooling and crystallization, and the crystallization temperature is -6°C. The resulting slurry 19 enters the third solid-liquid separation equipment k to obtain the third filtered mother liquor and the third para-xylene crystals 20. 20% 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 21; the third para-xylene crystals 20 and the para-xylene-containing raw material 23 (flow rate is 177kg / h) enters the synergistic mixer h, and the fully mixed crystal slurry 24 enters the fourth solid-liquid separation equipment i to obtain p-xylene crystals and the fourth filtered mother liquor. After the p-xylene crystals are washed with the p-xylene-containing raw material 26 (249kg / h) and the p-xylene product 29, the fourth eluate and the fourth p-xylene crystals 27 are obtained. The mixed liquid 25 of the fourth filtered mother liquor and the fourth eluate returns to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in the second melter j, of which 8% by weight is returned to the fourth solid-liquid separation equipment i as the p-xylene product 29, and the remaining part leaves the crystallization system as the p-xylene product 28. The purity of the final PX product is 99.91wt%, and the yield is 97.07%.

[0106] Comparative Example 22

[0107] A C8 mixture containing 98 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 9°C. The resulting crystal slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 461 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c. Among them, approximately 8% by weight is returned as p-xylene product 6 to the first solid-liquid separation device b, and the remaining part leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 98 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 leave the crystallization system. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 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 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 177 kg / h) enter an enhancing mixer h. The fully mixed crystal slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (249 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 8% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The purity of the finally obtained PX product is 99.89%, and the yield is 97.29%.

[0108] Comparative Example 23

[0109] A C8 mixture containing 97 wt% of p-xylene is used as Feedstock 1 (flow rate: 1806 kg / h) and fed into a primary crystallizer a for cooling crystallization at a crystallization temperature of 11°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtrate mother liquor 7. The p-xylene crystals are washed with Feedstock 3 containing p-xylene (flow rate: 482 kg / h) and p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. 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 leaves the crystallization system as p-xylene product 5. 40% of the first filtrate mother liquor 7 is returned as the first filtrate mother liquor 8 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 12 enters a second solid-liquid separation device e to obtain p-xylene crystals and a second filtrate mother liquor 16. The p-xylene crystals are washed with Feedstock 13 containing p-xylene (flow rate: 125 kg / h) to obtain a second eluate 15 and second p-xylene crystals 14. The second eluate 15 is returned to the secondary crystallizer d. The second p-xylene crystals 14 leave the crystallization system. 30% of the second filtrate mother liquor 16 is returned as the second filtrate mother liquor 17 to the secondary crystallizer d, and the remaining part enters a tertiary crystallizer g for cooling crystallization at a crystallization temperature of -4°C. The resulting slurry 19 enters a third solid-liquid separation device k to obtain a third filtrate mother liquor and third p-xylene crystals 20. 20% 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 21. The third p-xylene crystals 20 and Feedstock 23 containing p-xylene (flow rate: 209 kg / h) enter an enhanced mixer h. The fully mixed slurry 24 enters a fourth solid-liquid separation device i to obtain p-xylene crystals and a fourth filtrate mother liquor. The p-xylene crystals are washed with Feedstock 26 containing p-xylene (256 kg / h) and p-xylene product 29 to obtain a fourth eluate and fourth p-xylene crystals 27. The mixed liquid 25 of the fourth filtrate mother liquor and the fourth eluate is returned to the primary crystallizer a. The fourth p-xylene crystals 27 are melted in a second melter j. Among them, 10% by weight is returned as p-xylene product 29 to the fourth solid-liquid separation device i, and the remaining part leaves the crystallization system as p-xylene product 28. The final obtained PX product has a purity of 99.9 wt% and a yield of 94.78%.

[0110] Comparative Example 24

[0111] A C8 mixture containing 97 wt% of p-xylene is added as raw material 1 (flow rate is 1806 kg / h) to a primary crystallizer a for cooling and crystallization at a crystallization temperature of 10°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtered mother liquor 7. The p-xylene crystals are washed with a raw material 3 containing p-xylene (flow rate is 482 kg / h) and a p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c, wherein about 10% by weight is used as a p-xylene product. The product 6 is returned to the first solid-liquid separation equipment b, and the remaining part leaves the crystallization system as the para-xylene product 5; 40% of the first filtered mother liquor 7 is returned to the primary crystallizer a as the first filtered mother liquor 8, and the remaining part enters the secondary crystallizer d as the first filtered mother liquor 9 for cooling crystallization, and the crystallization temperature is 7°C. The obtained crystal slurry 12 enters the second solid-liquid separation equipment e to obtain para-xylene crystals and the second filtered mother liquor 16. After the para-xylene crystals are washed with the para-xylene-containing raw material 13 (flow rate is 125 kg / h), the second eluate 15 and the second para-xylene crystals 14 are obtained; the second eluate 15 is returned to the secondary crystallizer d ; The second para-xylene crystals 14 leave the crystallization system; 30% of the second filtered mother liquor 16 is returned to the secondary crystallizer d as the second filtered mother liquor 17, and the remaining part is entered into the tertiary crystallizer g as the second filtered mother liquor 18 for cooling and crystallization, and the crystallization temperature is -6°C. The resulting slurry 19 enters the third solid-liquid separation equipment k to obtain the third filtered mother liquor and the third para-xylene crystals 20. 20% 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 21; the third para-xylene crystals 20 and the para-xylene-containing raw material 23 (flow rate is 209kg / h) enters the synergistic mixer h, and the fully mixed crystal slurry 24 enters the fourth solid-liquid separation equipment i to obtain p-xylene crystals and the fourth filtered mother liquor. After the p-xylene crystals are washed with the p-xylene-containing raw material 26 (256kg / h) and the p-xylene product 29, the fourth eluate and the fourth p-xylene crystals 27 are obtained. The mixed liquid 25 of the fourth filtered mother liquor and the fourth eluate returns to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in the second melter j, of which 10% by weight is returned to the fourth solid-liquid separation equipment i as the p-xylene product 29, and the remaining part leaves the crystallization system as the p-xylene product 28. The purity of the final PX product is 99.89wt%, and the yield is 95.51%.

[0112] Comparative Example 25

[0113] A C8 mixture containing 97 wt% of p-xylene is added as raw material 1 (flow rate is 1806 kg / h) to a primary crystallizer a for cooling and crystallization at a crystallization temperature of 9°C. The resulting slurry 2 enters a first solid-liquid separation device b to obtain p-xylene crystals and a first filtered mother liquor 7. The p-xylene crystals are washed with a raw material 3 containing p-xylene (flow rate is 482 kg / h) and a p-xylene product 6 to obtain a first eluate 10 and first p-xylene crystals 4. The first eluate 10 is returned to the primary crystallizer a. The first p-xylene crystals 4 are melted in a first melter c, wherein about 10% by weight is used as a p-xylene product. The product 6 is returned to the first solid-liquid separation equipment b, and the remaining part leaves the crystallization system as the para-xylene product 5; 40% of the first filtered mother liquor 7 is returned to the primary crystallizer a as the first filtered mother liquor 8, and the remaining part enters the secondary crystallizer d as the first filtered mother liquor 9 for cooling crystallization, and the crystallization temperature is 6°C. The obtained crystal slurry 12 enters the second solid-liquid separation equipment e to obtain para-xylene crystals and the second filtered mother liquor 16. After the para-xylene crystals are washed with the para-xylene-containing raw material 13 (flow rate is 125 kg / h), the second eluate 15 and the second para-xylene crystals 14 are obtained; the second eluate 15 is returned to the secondary crystallizer d ; The second para-xylene crystals 14 leave the crystallization system; 30% of the second filtered mother liquor 16 is returned to the secondary crystallizer d as the second filtered mother liquor 17, and the remaining part is entered into the tertiary crystallizer g as the second filtered mother liquor 18 for cooling and crystallization, and the crystallization temperature is -7°C. The resulting slurry 19 enters the third solid-liquid separation equipment k to obtain the third filtered mother liquor and the third para-xylene crystals 20, 20% 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 21; the third para-xylene crystals 20 and the para-xylene-containing raw material 23 (flow rate is 209kg / h) enters the synergistic mixer h, and the fully mixed crystal slurry 24 enters the fourth solid-liquid separation equipment i to obtain p-xylene crystals and the fourth filtered mother liquor. After the p-xylene crystals are washed with the p-xylene-containing raw material 26 (256kg / h) and the p-xylene product 29, the fourth eluate and the fourth p-xylene crystals 27 are obtained. The mixed liquid 25 of the fourth filtered mother liquor and the fourth eluate returns to the primary crystallizer a; the fourth p-xylene crystals 27 are melted in the second melter j, of which 10% by weight is returned to the fourth solid-liquid separation equipment i as the p-xylene product 29, and the remaining part leaves the crystallization system as the p-xylene product 28. The purity of the final PX product is 99.87wt%, and the yield is 95.84%.

[0114] All embodiments of the present invention have obtained products with a purity greater than 99.8 wt%, and the yield of p-xylene products 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 third-stage crystallizer g is higher than -4 °C, the yield of p-xylene products 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 third-stage crystallizer g is lower than -7 °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 the inclusion of more impurities. Therefore, for a raw material containing 95 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 third-stage crystallizer g is -7 °C to -4 °C.

[0115] Similarly, by comparing Examples 3-4 with Comparative Examples 3-4, 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 third-stage crystallizer g is higher than -8 °C, the yield of p-xylene products 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 third-stage crystallizer g is lower than -10 °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 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 third-stage crystallizer g is -10 °C to -8 °C.

[0116] By comparing Examples 5-6 with Comparative Examples 5-6, it can be seen that when the operating temperature of the first-stage crystallizer a is higher than 8 °C, the operating temperature of the second-stage crystallizer d is higher than 4 °C, and the operating temperature of the third-stage crystallizer g is higher than -12 °C, the yield of p-xylene products is lower than 90 wt%; when the operating temperature of the first-stage crystallizer a is lower than 6 °C, the operating temperature of the second-stage crystallizer d is lower than 3 °C, and the operating temperature of the third-stage crystallizer g is lower than -14 °C, the purity of the product is lower than 99.8 wt%. Therefore, for a raw material containing 91 wt% of p-xylene, the operating temperature of the first-stage crystallizer a is 6 °C to 8 °C, the operating temperature of the second-stage crystallizer d is 3 °C to 4 °C, and the operating temperature of the third-stage crystallizer g is -14 °C to -12 °C.

[0117] Comparing Examples 7-8 with Comparative Examples 7-8, it can be seen that when the operating temperature of the first-stage crystallizer a is higher than 7°C, the operating temperature of the second-stage crystallizer d is higher than 3°C, and the operating temperature of the third-stage crystallizer g is higher than -14°C, the yield of p-xylene product is lower than 90 wt%; when the operating temperature of the first-stage crystallizer a is lower than 5°C, the operating temperature of the second-stage crystallizer d is lower than 1°C, and the operating temperature of the third-stage crystallizer g is lower than -17°C, the purity of the product is lower than 99.8 wt%. Therefore, for the raw material containing 90 wt% of p-xylene, the operating temperature of the first-stage crystallizer a is 5°C to 7°C, the operating temperature of the second-stage crystallizer d is 1°C to 3°C, and the operating temperature of the third-stage crystallizer g is -17°C to -14°C.

[0118] Comparing Examples 9-10 with Comparative Examples 9-10, it can be seen that when the operating temperature of the first-stage crystallizer a is higher than 5°C, the operating temperature of the second-stage crystallizer d is higher than -3°C, and the operating temperature of the third-stage crystallizer g is higher than -18°C, the yield of p-xylene product is lower than 90 wt%; when the operating temperature of the first-stage crystallizer a is lower than 3°C, the operating temperature of the second-stage crystallizer d is lower than -5°C, and the operating temperature of the third-stage crystallizer g is lower than -20°C, the purity of the product is lower than 99.8 wt%. Therefore, for the raw material containing 88 wt% of p-xylene, the operating temperature of the first-stage crystallizer a is 3°C to 5°C, the operating temperature of the second-stage crystallizer d is -5°C to -3°C, and the operating temperature of the third-stage crystallizer g is -20°C to -18°C.

[0119] 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%, when the raw material concentration is 88 wt% to 95 wt%, the operating temperature of the first-stage crystallizer a is 3°C to 11°C, the operating temperature of the second-stage crystallizer d is -5°C to 8°C, and the operating temperature of the third-stage crystallizer g is -20°C to -4°C.

[0120] Comparing Example 1 with Examples 11-13 and Example 10 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 10. Therefore, in order to increase the product yield of relatively high-concentration raw materials on the basis of ensuring qualified product purity, and increase the product purity of relatively low-concentration raw materials on the basis of ensuring qualified product yield, it is further preferred that the operating temperature of the first-stage crystallizer a is 5°C to 9°C, the operating temperature of the second-stage crystallizer d is -4°C to 7°C, and the operating temperature of the third-stage crystallizer g is -18°C to -6°C.

[0121] Regarding washing, as can be seen from the comparison between Examples 1, 8, 10 and Comparative Examples 11, 12, 13 respectively, the yields of Examples 1, 8, 10 using raw materials containing p-xylene to wash the product are higher than those of the comparative examples that do not use raw materials to wash the product, and the p-xylene product purity is also higher. Because the raw material washing liquid can replace the mother liquor remaining on the crystal surface, reducing the amount of product washing liquid used, thus increasing the yield of p-xylene product. At the same time, due to the low operating temperature of each stage of the crystallizer, the temperature of the crystals directly produced by the crystallizer is also low. Using raw materials for washing can increase the crystal temperature, reduce the inclusion of impurities caused by burst nucleation during the product washing stage, and thus increase the purity of the p-xylene product. When directly washing with the product, due to its too high purity, when it contacts the cold crystals, significant cooling crystallization will occur in the gaps of the crystals, blocking the pores, resulting in the inability to effectively discharge impurities instead. Therefore, the present invention adopts a raw material washing scheme containing p-xylene to strengthen the washing process.

[0122] As can be seen from the comparison between Examples 1, 8, 10 and Comparative Examples 14, 15, 16 respectively, when no raw material enters the synergistic mixer h, the synergistic mixer h is equivalent to a slurring kettle, and the slurring and temperature increase result in a large loss of crystals, leading to a decrease in the yield of p-xylene product. Therefore, the present invention adopts a scheme in which raw materials enter the synergistic mixer h to suspend the crystals of the second and third stages of crystallization.

[0123] As can be seen from the comparison between Examples 1, 8, 10 and Comparative Examples 17, 18, 19 respectively, in the comparative examples, the first, second, and third filtered mother liquors are not respectively returned to the first-stage crystallizer a, the second-stage crystallizer d, and the third-stage crystallizer g, resulting in a decrease in the purity and yield of the product in the comparative examples compared to the examples. Therefore, the present invention preferably returns at least part of the first, second, and third filtered mother liquors to the first-stage crystallizer a, the second-stage crystallizer d, and the third-stage crystallizer g respectively.

[0124] By analyzing Comparative Examples 20 - 25, it can be obtained that the p-xylene crystals obtained from the raw materials of high-concentration p-xylene (greater than 95 wt%) in the second-stage crystallizer d can directly produce qualified products after being washed with raw materials, without the need to enter the synergistic mixer h and subsequent processes for further purification (re-entering the synergistic mixer will only reduce the processing yield). Therefore, if the process of the present invention processes raw materials of high-concentration p-xylene (greater than 95 wt%), there will be obvious redundancy in the process steps and waste of resources.

Claims

1. A crystallization washing system for paraxylene, 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 device (k) is used to perform solid-liquid separation on the slurry crystallized in the third-stage crystallizer (g).

2. The crystallization washing system of paraxylene according to claim 1, characterized in that: Also includes: A synergistic mixer (h) and a fourth solid-liquid separation device (i); the synergistic mixer (h) performs re-suspended crystallization treatment on the crystals crystallized in the secondary crystallizer (d) and the tertiary crystallizer (g), and the fourth solid-liquid separation device (i) performs solid-liquid separation and washing on the crystal slurry mixed in the synergistic mixer (h).

3. A method for washing the crystallization of p-xylene, characterized in that: The method is operated on the crystallization and washing system of paraxylene as described in any one of claims 1 or 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) and the fourth solid-liquid separation equipment (i) as a washing liquid to wash the crystals, and enters the synergistic mixer (h) to suspend the crystals crystallized in the secondary crystallizer (d) and the tertiary crystallizer (g).

4. The method according to claim 3, characterized in that include: (1) adding a para-xylene-containing raw material (1) into a primary crystallizer (a) for cooling and crystallization to obtain a crystal slurry (2); (2) The crystal slurry (2) enters the first solid-liquid separation device (b), and after washing, a first filtered mother liquor (7), a first eluate (10) and a first para-xylene crystal (4) are obtained, and the first eluate (10) is returned to the primary crystallizer (a); (3) a portion of the first filtered mother liquor (7) 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 (12); (4) the crystal slurry (12) enters the second solid-liquid separation device (e), and after washing, a second filtered mother liquor (16), a second eluate (15) and a second para-xylene crystal (14) are obtained, and the second eluate (15) returns to the secondary crystallizer (d); (5) A portion of the second filtered mother liquor (16) 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 (19); (6) The crystal slurry (19) enters the third solid-liquid separation device (k) to obtain a third filtered mother liquor and third para-xylene crystals (20), and part of the third filtered mother liquor returns to the third-stage crystallizer (g); (7) The second p-xylene crystals (14), the third p-xylene crystals (20) and the raw material containing p-xylene enter the synergistic mixer (h), are fully mixed and then enter the fourth solid-liquid separation equipment (i), and are separated after the raw material containing p-xylene and part of the p-xylene product are washed.

5. The method according to claim 4, characterized in that The operating temperature of the primary crystallizer (a) is 3 to 11°C, the operating temperature of the secondary crystallizer (d) is -5 to 8°C, the operating temperature of the tertiary crystallizer (g) is -20 to -4°C, and the operating temperature of the primary crystallizer (a) is greater than that 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 to 9°C, the operating temperature of the secondary crystallizer (d) is -4 to 7°C, and the operating temperature of the tertiary crystallizer (g) is -18 to -6°C.

7. The method of claim 4, characterized in that In step (2), the slurry (2) is washed by a paraxylene-containing raw material (3) and a paraxylene product (6) in the first solid-liquid separation device (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 (12) enters the second solid-liquid separation device (e) and is washed with a raw material containing paraxylene.

10. The method according to any one of claims 4 to 9, characterized in that: The para-xylene-containing raw material is a C8 mixture containing 88 wt% to 95 wt% of para-xylene.

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