A multi-stage washing process and system for producing high-concentration para-xylene

Abstract

This application proposes a multi-stage washing process for producing high-concentration para-xylene, comprising: cooling and crystallizing the raw material at a first temperature to obtain a first crystal slurry; washing the first crystal slurry with solid-liquid separation and a portion of the raw material and product to obtain a first crystal, a first filtrate mother liquor, and a first wash liquid; melting the first crystal to obtain a first para-xylene product; sending a portion of the first filtrate mother liquor to a second temperature for cooling and crystallization to obtain a second crystal slurry; repeating the above steps with the second crystal slurry to obtain a second para-xylene product; cooling and crystallizing a portion of the second filtrate mother liquor at a third temperature to obtain a third crystal slurry; and repeating the above steps with the third crystal slurry to obtain a third para-xylene product. This application can solve the problems of large and difficult crystal transfer and large crystal particle recycling in para-xylene production.

Description

Technical Field

[0001] This application relates to a multi-stage washing process and system for producing high-concentration para-xylene. Background Technology

[0002] Paraxylene (PX) is mainly used as a raw material for producing purified terephthalic acid (PTA) and dimethyl terephthalate (DMT). Purified terephthalic acid (PTA) is used to manufacture polyester products such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). When upstream PX production is insufficient, downstream PTA plants have no choice but to import it from neighboring countries. Therefore, the shortage of PX raw materials has become the biggest constraint on the development of my country's polyester industry.

[0003] Currently, para-xylene is mainly obtained by separating mixed xylenes, which primarily originate from petroleum aromatics. Due to limitations imposed by the carbon distribution and thermodynamic equilibrium within petroleum aromatics, the para-xylene content in C8 aromatics obtained from different aromatic production processes is consistently low. However, combined aromatic units can facilitate inter-aromatic conversion, thereby increasing para-xylene production. Mixed xylenes mainly consist of para-xylene, m-xylene, o-xylene, and ethylbenzene. The boiling points of these components are very similar, making high-purity para-xylene products almost impossible to obtain using distillation methods, resulting in high energy consumption for high-purity production. However, the melting points of these components differ significantly. Generally, para-xylene has a freezing point of around 13.2°C, offering milder production conditions and lower energy consumption compared to other xylenes. Therefore, melt crystallization separation and purification of para-xylene offers a clear technological advantage.

[0004] During the crystallization separation process, the main component, p-xylene, precipitates from the solution, while other xylenes remain in the solution. High-purity p-xylene can be obtained simply by completely separating the p-xylene crystals from the xylene solution. However, since the solid-liquid two phases are difficult to completely separate, and the purity of p-xylene is generally required to be >99.7%, many post-processing techniques are needed to obtain high-purity p-xylene. Summary of the Invention

[0005] To address the problems of large and difficult crystal transfer and large crystal particle recycling in existing production processes for para-xylene, this application discloses a multi-stage washing production process and system for high-concentration para-xylene, employing the following technical solution:

[0006] In a first aspect, this application discloses a multi-stage washing process for producing high-concentration p-xylene, comprising:

[0007] The raw material containing para-xylene is cooled and crystallized under a first temperature condition to obtain the first crystal slurry;

[0008] The first crystal slurry is subjected to solid-liquid separation to obtain the first crystal and the first filtrate mother liquor;

[0009] Part of the first filtrate mother liquor is returned to the first temperature condition for further cooling and crystallization, and the remaining first filtrate mother liquor is cooled and crystallized at the second temperature condition to obtain the second crystal slurry.

[0010] The first crystal was washed sequentially with the raw material and the p-xylene product as washing liquid;

[0011] The first crystal, after being washed with raw materials and para-xylene product, is melted to obtain the first para-xylene product;

[0012] The second crystal slurry is subjected to solid-liquid separation to obtain the second crystal and the second filtrate mother liquor.

[0013] Part of the second filtrate mother liquor is returned to the second temperature condition for continued cooling and crystallization, and the remaining second filtrate mother liquor is cooled and crystallized at the third temperature condition to obtain the third crystal slurry;

[0014] The second crystal was washed sequentially with the raw material and the p-xylene product as washing solution;

[0015] The second crystal, after being washed with raw materials and para-xylene product, is melted to obtain the second para-xylene product;

[0016] The third crystal slurry is subjected to solid-liquid separation to obtain the third crystal and the third filtrate mother liquor.

[0017] A portion of the third filtration mother liquor is returned to the third temperature condition for further cooling and crystallization, and the remaining third filtration mother liquor is discharged from the system.

[0018] The raw materials and p-xylene product were used as washing solutions to wash the third crystal in sequence.

[0019] The third crystal, after being washed with raw materials and para-xylene product, is melted to obtain the third para-xylene product;

[0020] The operating temperatures under the first, second, and third temperature conditions decrease sequentially.

[0021] Optionally, after obtaining the first paraxylene product, a portion of the first paraxylene product is returned as a washing solution to the first crystal in the previous step to wash the first crystal.

[0022] After obtaining the second paraxylene product, a portion of the second paraxylene product is returned as a washing solution to the second crystal in the previous step to wash the second crystal;

[0023] After obtaining the third para-xylene product, a portion of the third para-xylene product is returned as a washing solution to the third crystal from the previous step to wash the third crystal.

[0024] Optionally, 10% to 30% of the first paraxylene product can be returned to the first crystal from the previous step as a washing solution to wash the first crystal.

[0025] 10% to 30% of the second paraxylene product is returned as a washing solution to the second crystal from the previous step to wash the second crystal;

[0026] 10% to 30% of the third paraxylene product is returned as a washing solution to the third crystal from the previous step to wash the third crystal.

[0027] Optionally, the raw material and the para-xylene product are used as washing liquids to wash the first crystals sequentially to obtain the first washing liquid;

[0028] The second crystal was washed sequentially with the raw material and the p-xylene product as washing liquid to obtain the second washing liquid;

[0029] The raw material and paraxylene product are used as washing liquids to wash the third crystal in sequence to obtain a third washing liquid. The first washing liquid, the second washing liquid and the third washing liquid are all returned to the first temperature condition for continued cooling and crystallization.

[0030] Optionally, a C8 mixture with a paraxylene concentration ranging from 95% to 98% can be used as a raw material for cooling and crystallization under a first temperature condition.

[0031] Optionally, the operating temperature range of the first temperature condition is 8℃~12℃, the operating temperature range of the second temperature condition is 4℃~9℃, and the preferred operating temperature range of the third temperature condition is 0℃~6℃.

[0032] Optionally, 5% to 20% of the first filtrate mother liquor is returned to the first temperature condition for continued cooling and crystallization, 5% to 20% of the second filtrate mother liquor is returned to the second temperature condition for continued cooling and crystallization, and 5% to 20% of the third filtrate mother liquor is returned to the third temperature condition for continued cooling and crystallization.

[0033] Secondly, this application also discloses a multi-stage washing production system for high-concentration para-xylene, based on the multi-stage washing production process for high-concentration para-xylene as described in the first aspect, including a primary crystallization unit, a secondary crystallization unit, and a tertiary crystallization unit.

[0034] The primary crystallization device includes a first crystallizer, a first solid-liquid separator, and a first melter; the secondary crystallization device includes a second crystallizer, a second solid-liquid separator, and a second melter; and the tertiary crystallization device includes a third crystallizer, a third solid-liquid separator, and a third melter.

[0035] The operating temperatures of the first crystallizer, the second crystallizer, and the third crystallizer decrease sequentially.

[0036] The first crystallizer is used to cool and crystallize the raw material containing paraxylene under a first temperature condition to obtain a first crystal slurry;

[0037] The outlet of the first crystallizer is connected to the inlet of the first solid-liquid separator, which is used for solid-liquid separation of the first crystal slurry and washing of the first crystal.

[0038] The outlet of the first solid-liquid separator is connected to the inlet of the first melter, which is used to melt the first crystal to obtain the first paraxylene product.

[0039] The outlet of the first solid-liquid separator is also connected to the inlet of the first crystallizer and the inlet of the second crystallizer;

[0040] The outlet of the second crystallizer is connected to the inlet of the second solid-liquid separator. The second crystallizer is used to cool and crystallize a portion of the first filtered mother liquor under a second temperature condition to obtain a second crystal slurry.

[0041] The second solid-liquid separator is used to perform solid-liquid separation on the second crystal slurry and to wash the second crystal;

[0042] The outlet of the second solid-liquid separator is connected to the inlet of the second melter, which is used to melt the second crystal to obtain the second paraxylene product.

[0043] The outlet of the second solid-liquid separator is also connected to the inlet of the second crystallizer and the inlet of the third crystallizer;

[0044] The outlet of the third crystallizer is connected to the inlet of the third solid-liquid separator. The third crystallizer is used to cool and crystallize a portion of the first filtered mother liquor under a third temperature condition to obtain a third crystal slurry.

[0045] The third solid-liquid separator is used to perform solid-liquid separation on the third crystal slurry and to wash the third crystal;

[0046] The outlet of the third solid-liquid separator is connected to the inlet of the third melter, and the third melter is used to melt the third crystal to obtain the third paraxylene product;

[0047] The outlet of the third solid-liquid separator is also connected to the inlet of the third crystallizer.

[0048] Optionally, the outlet of the first melter is connected to the inlet of the first solid-liquid separator to return a portion of the first paraxylene product as a washing liquid to the first crystal in the previous step for washing the first crystal.

[0049] The outlet of the first solid-liquid separator is connected to the inlet of the first crystallizer, and is used to return the first product wash liquid to the first temperature condition for cooling and crystallization.

[0050] Optionally, the outlet of the second melter is connected to the inlet of the second solid-liquid separator to return a portion of the second paraxylene product as a washing liquid to the second crystal in the previous step for washing the second crystal;

[0051] The outlet of the second solid-liquid separator is connected to the inlet of the first crystallizer, and is used to return the second product wash liquid to the first temperature condition for cooling and crystallization.

[0052] Optionally, the outlet of the third melter is connected to the inlet of the third solid-liquid separator, and is used to return a portion of the third paraxylene product as washing liquid to the third crystal in the previous step to wash the third crystal;

[0053] The outlet of the third solid-liquid separator is connected to the inlet of the first crystallizer, and is used to return the third product wash liquid to the first temperature condition for cooling and crystallization.

[0054] Based on the above technical solution, the beneficial effects of this application compared with the prior art are as follows:

[0055] This application employs a staged cooling crystallization process. After washing the raw materials and products from the slurry of the three crystallization stages, the final product can be directly produced. This process is suitable for raw materials with a paraxylene concentration range of 95%–98%, and all three crystallization stages can produce products that meet the purity requirements. It features a simple process and high production efficiency. After filtering the crystal slurry, the mother liquor is first discharged from the solid-liquid separator, and then the crystals are washed within the solid-liquid separator. In other words, the filtration and separation of the crystal slurry and the washing of the crystals are all integrated into a single solid-liquid separator, greatly reducing the overall production energy consumption.

[0056] The raw materials are fed into the system in a multi-stage manner. Some raw materials directly enter the first temperature condition, some enter the first crystallizer 1 as the first filtrate mother liquor, some enter the second crystallizer 4 as the second filtrate mother liquor, and some enter the third crystallizer 7 as the third filtrate mother liquor. This reduces the solid content in the first solid-liquid separator 2. The three-stage crystallization design reduces the operating temperature difference between each stage, effectively reducing explosive nucleation and resulting in larger crystal particles. This ensures that the crystals obtained from all three crystallization stages meet the product purity requirements, reduces the overall crystal transfer and circulation volume, and improves production efficiency. The solid-liquid separator separates the filtrate mother liquor and the product wash liquid, treating the relatively high-concentration product wash liquid separately from the relatively low-concentration filtrate mother liquor. The filtrate mother liquor leaning towards paraxylene goes to the next stage, while the product wash liquid rich in paraxylene returns to the first temperature condition, thus stabilizing product purity and yield and reducing paraxylene loss.

[0057] In addition, using raw material washing slurry can raise the crystal temperature, reduce the impurity encapsulation caused by crystal nucleation during the product washing stage, and remove some impurities first, thereby improving product washing efficiency and reducing the amount of product washing liquid used, thus greatly improving production efficiency. Attached Figure Description

[0058] Figure 1 This is a schematic diagram of the multi-stage washing process for high-concentration para-xylene in the embodiments of this application;

[0059] Figure 2 This is a schematic diagram of a multi-stage washing production system for high-concentration para-xylene in an embodiment of this application.

[0060] Explanation of reference numerals in the attached figures:

[0061] a. Raw material; b. First crystal slurry; c. First filtrate mother liquor; d. First crystal; e. First p-xylene product; f. First product eluent; g. Second crystal slurry; h. Second filtrate mother liquor; i. Second crystal; j. Second p-xylene product; k. Second product eluent; m. Third crystal slurry; n. Third filtrate mother liquor; o. Third crystal; p. Third p-xylene product; q. Third product eluent;

[0062] 1. First crystallizer; 2. First solid-liquid separator; 3. First melter; 4. Second crystallizer; 5. Second solid-liquid separator; 6. Second melter; 7. Third crystallizer; 8. Third solid-liquid separator; 9. Third melter. Detailed Implementation

[0063] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0064] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this disclosure. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this disclosure as detailed in the appended claims.

[0065] In the description of this disclosure, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this disclosure and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this disclosure. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0066] In the description of this disclosure, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linkage" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this disclosure based on the specific circumstances.

[0067] The inventors discovered that in related technologies, the purity of the para-xylene product is required when producing para-xylene via melt crystallization. However, in some later stages of cooling crystallization, the crystallization temperature is even lower, resulting in smaller crystal particles that are difficult to filter and wash. Therefore, these crystals are typically re-slurryed in a re-slurry tank for further growth, but this tends to lead to a large circulating volume of materials throughout the process, reducing production efficiency. Therefore, how to ensure product purity and minimize para-xylene loss while minimizing material circulation has long been a problem that those skilled in the art have sought to solve.

[0068] To address the aforementioned issues, this application discloses a multi-stage washing process and system for producing high-concentration para-xylene, solving the problem of excessively large circulating volume of the internal stream during para-xylene production in existing processes. The various specific implementation methods of the multi-stage para-xylene production system and method provided in this disclosure are described in detail below.

[0069] In a first aspect, embodiments of this application provide a multi-stage washing process for producing high-concentration p-xylene, referring to... Figure 1 ,include:

[0070] S101. Raw material a containing p-xylene is cooled and crystallized under a first temperature condition to obtain a first crystal slurry b;

[0071] S102. Perform solid-liquid separation on the first crystal slurry b to obtain the first crystal d and the first filtrate mother liquor c;

[0072] S103. Part of the first filtrate mother liquor c is returned to the first temperature condition for continued cooling and crystallization, and the remaining first filtrate mother liquor c is cooled and crystallized under the second temperature condition to obtain the second crystal slurry g;

[0073] S104. The first crystal d is washed sequentially with raw material a and p-xylene product as washing solution;

[0074] S105. The first crystal d, after being washed with raw material a and para-xylene product, is melted to obtain the first para-xylene product e;

[0075] S106. The second crystal slurry g is subjected to solid-liquid separation to obtain the second crystal i and the second filtrate mother liquor h;

[0076] S107. Return a portion of the second filtration mother liquor h to the second temperature condition for continued cooling and crystallization, and return the remaining second filtration mother liquor h;

[0077] S108. The second crystal i is washed sequentially with raw material a and p-xylene product as washing liquid;

[0078] S109. The second crystal i, after being washed with raw material a and p-xylene product, is melted to obtain the second p-xylene product j;

[0079] S110. The third crystal slurry is subjected to solid-liquid separation to obtain the third crystal and the third filtrate mother liquor;

[0080] S111. A portion of the third filtration mother liquor is returned to the third temperature condition for continued cooling and crystallization, and the remaining third filtration mother liquor is discharged from the system;

[0081] S112. The raw materials and p-xylene product are used as washing solutions to wash the third crystal in sequence;

[0082] S113. The third crystal, after being washed with raw materials and para-xylene product, is melted to obtain the third para-xylene product.

[0083] Among them, the operating temperatures of the first temperature condition, the second temperature condition and the third temperature condition decrease sequentially, and the purity of the first paraxylene product e, the second paraxylene product j and the third paraxylene product o is not less than 99.8%.

[0084] Specifically, in an optional embodiment, in step S101, a C8 mixture with a p-xylene concentration ranging from 95% to 98% is used as raw material a for cooling and crystallization under a first temperature condition, the operating temperature range of the first temperature condition is 8°C to 12°C, the operating temperature range of the second crystallizer 4 is 4°C to 9°C, and the operating temperature of the third temperature condition is 0°C to 6°C.

[0085] In an optional embodiment, the first crystal slurry b undergoes solid-liquid separation to obtain a first filtrate mother liquor c and a first crystal d. After discharging the first filtrate mother liquor c, the first crystal d is then washed sequentially with raw material a and p-xylene product. Washing with raw material a can increase the crystal temperature, reducing impurity encapsulation caused by explosive nucleation during the product washing stage. Simultaneously, washing with raw material a can remove some impurities beforehand, reducing the amount of product washing liquid used, thereby greatly improving production efficiency. Furthermore, the solid-liquid separation of the crystal slurry and the washing of the crystals can be integrated into a single device, significantly reducing energy consumption.

[0086] In an optional embodiment, in step S103, 5% to 20% of the first filtrate c is returned to the first temperature condition, which is beneficial to promote the continued growth of crystals in the first temperature condition and improve the subsequent separation efficiency. The remaining portion of the first filtrate c is then subjected to cooling and crystallization at the second temperature condition.

[0087] Further, in step S105, after melting the first crystal d to obtain the first para-xylene product e, a portion of the first para-xylene product e is returned as a washing liquid to the first crystal d in the previous step to wash the first crystal d, obtaining the first product wash liquid f. The first product wash liquid f is returned to the first temperature condition for further cooling and crystallization. The proportion of the first para-xylene product e returned to the first temperature condition is approximately 10% to 30%. Washing the first crystal slurry b with the first para-xylene product e can improve the purity of the obtained first crystal d and reduce other impurities adhering to the surface of the first crystal d.

[0088] Accordingly, in steps S106 to S109, the second crystal slurry g is first subjected to solid-liquid separation to obtain the second crystal i and the second filtrate mother liquor h. After the second filtrate mother liquor h is discharged, the raw material a and the para-xylene product are used as washing liquids to continue washing the second crystal i to obtain the second washing liquid k. The second product washing liquid k is also returned to the first temperature condition to continue cooling and crystallization.

[0089] In an optional embodiment, in step S107, 5% to 20% of the second filtration mother liquor h is returned to the second temperature condition for continued cooling and crystallization, which is beneficial to promote the continued growth of crystals under the second temperature condition and improve the subsequent separation efficiency. The remaining portion of the second filtration mother liquor h then enters the third temperature condition for the next stage of cooling and crystallization.

[0090] Similarly, 10% to 30% of the second paraxylene product j is returned as a washing solution to the second crystal i in the previous step to wash the second crystal i.

[0091] Finally, in steps S110 to S113, the third crystal slurry m undergoes solid-liquid separation and washing with raw material a and a portion of product p to obtain the third crystal o, the third filtrate mother liquor n, and the third wash liquid q. 5% to 20% of the third filtrate mother liquor n is returned to the third temperature condition for further cooling and crystallization, while the remaining third filtrate mother liquor n is discharged from the system. The third wash liquid q is also returned to the first temperature condition for further cooling and crystallization.

[0092] Meanwhile, since the concentration of para-xylene in raw material a is high, the third crystal o obtained during the third-stage cooling crystallization process can meet the product purity requirements. After melting the third crystal o to obtain the third para-xylene product p, 10% to 30% of the third para-xylene product p is used as a washing solution to wash the third crystal o.

[0093] It should be noted that in the actual production process, in the initial state, pure p-xylene product can be used to wash the crystals. After the corresponding product is generated, the generated p-xylene product is used for washing. As in the embodiment of this application, in the later stages of production, after the corresponding first p-xylene product and second p-xylene product are generated, the first p-xylene product is used as the washing liquid to wash the first crystal, and the second p-xylene product is used as the washing liquid to wash the second crystal.

[0094] Secondly, embodiments of this application disclose a multi-stage washing production system for high-concentration p-xylene, used to implement the multi-stage washing production process for high-concentration p-xylene in the first aspect, with reference to... Figure 2The multi-stage washing production system for paraxylene includes a primary crystallization unit, a secondary crystallization unit, and a tertiary crystallization unit. The primary crystallization unit includes a first crystallizer 1, a first solid-liquid separator 2, and a first melter 3. The secondary crystallization unit includes a second crystallizer 4, a second solid-liquid separator 5, and a second melter 6. The tertiary crystallization unit includes a third crystallizer 7, a third solid-liquid separator 8, and a third melter 9.

[0095] It should be noted that the operating temperatures of the first crystallizer 1, the second crystallizer 4, and the third crystallizer 7 decrease sequentially. Specifically, the temperature range of the first crystallizer 1 is 8℃~12℃, the temperature range of the second crystallizer 4 is 4℃~9℃, and the temperature range of the third crystallizer 7 is 0℃~6℃. Thus, while meeting product purity requirements, the overall crystallization energy consumption is lowered through a step-down cooling crystallization method.

[0096] It should be noted that in this embodiment, the first solid-liquid separator and the second solid-liquid separator are an integrated device that can not only separate the solid and liquid components of the crystal slurry, but also perform further washing and filtration operations on the separated crystals. This helps to reduce overall energy consumption.

[0097] Specifically, refer to Figure 2 The outlet of the first crystallizer 1 is connected to the inlet of the first solid-liquid separator 2, and the outlet of the first solid-liquid separator 2 is further connected to the inlet of the first melter 3. The first crystallizer 1 is used to cool and crystallize the raw material a containing para-xylene under a first temperature condition to obtain a first crystal slurry b. The first solid-liquid separator 2 is used to separate the first crystal slurry b into solid and liquid components and to wash the first crystal d. The first melter 3 is used to melt the first crystal d to obtain a first para-xylene product e.

[0098] In an alternative embodiment, refer to Figure 2 The outlet of the first solid-liquid separator 2 is also connected to the inlet of the first crystallizer 1 and the inlet of the second crystallizer 4, so as to return a portion of the first filtered mother liquor c to the first crystallizer 1 and send the remaining portion to the second crystallizer 4 for cooling and crystallization.

[0099] In an alternative embodiment, refer to Figure 2 The outlet of the first melter 3 is also connected to the inlet of the first solid-liquid separator 2, for returning a portion of the first paraxylene product e to the first solid-liquid separator 2 to wash the first crystal slurry b, obtaining the first product wash liquid f. The outlet of the first solid-liquid separator 2 is also connected to the inlet of the first crystallizer 1, for returning the first product wash liquid f to the first crystallizer 1.

[0100] Furthermore, in an optional embodiment, refer to Figure 2The outlet of the second crystallizer 4 is connected to the inlet of the second solid-liquid separator 5, and the outlet of the second solid-liquid separator 5 is connected to the inlet of the second melter 6. The second crystallizer 4 is used to cool and crystallize a portion of the first filtered mother liquor c under a second temperature condition to obtain the second crystal slurry g; the second solid-liquid separator 5 is used to separate the second crystal slurry g into solid and liquid components, and to wash the second crystal i.

[0101] In an alternative embodiment, refer to Figure 2 The outlet of the second solid-liquid separator 5 is also connected to the second crystallizer 4 and the third crystallizer 7, which is used to return a portion of the second filtered mother liquor h to the second crystallizer 4 and send the remaining portion to the third crystallizer 7 for cooling and crystallization.

[0102] In an alternative embodiment, refer to Figure 2 The outlet of the second melter 6 is connected to the inlet of the second solid-liquid separator 5, and is used to return a portion of the second paraxylene product j to the second solid-liquid separator 5 to wash the second crystal slurry g, obtaining the second product wash liquid k. The outlet of the second solid-liquid separator 5 is connected to the first crystallizer 1, and is used to return the second product wash liquid k to the first temperature condition.

[0103] In an alternative embodiment, refer to Figure 2 The outlet of the third crystallizer 7 is connected to the inlet of the third solid-liquid separator 8, and the outlet of the third solid-liquid separator 8 is connected to the inlet of the third melter 9. The third crystallizer 7 is used to cool and crystallize a portion of the second filtrate mother liquor h under a third temperature condition to obtain a third crystal slurry m; the third solid-liquid separator 8 is used to separate the third crystal slurry m into solid and liquid components and to wash the third crystal o; the third melter 9 is used to melt the third crystal o to obtain the third para-xylene product p.

[0104] In an alternative embodiment, refer to Figure 2 The outlet of the third solid-liquid separator 8 is also connected to the third crystallizer 7, which is used to return part of the third filtered mother liquor n to the third temperature condition for continued cooling and crystallization.

[0105] In an alternative embodiment, refer to Figure 2 The outlet of the third melter 9 is also connected to the inlet of the third solid-liquid separator 8, which is used to return a portion of the third paraxylene product p to the third solid-liquid separator 8 to wash the third crystal slurry m, obtaining the third product wash liquid q. The outlet of the third solid-liquid separator 8 is also connected to the first crystallizer 1, which is used to return the third product wash liquid q to the first temperature condition for continued cooling and crystallization.

[0106] In summary, this embodiment employs a staged cooling crystallization process. After washing the raw materials and products from the slurry of the three crystallization stages, the product can be directly produced. This process is suitable for raw materials with a paraxylene concentration range of 95%–98%, and all three crystallization stages can produce products that meet the purity requirements. It features a simple process and high production efficiency. After filtering the crystal slurry, the filtrate is first discharged from the solid-liquid separator, and then the crystals are washed in the solid-liquid separator. This integrates the filtration and separation of the crystal slurry and the washing of the crystals into a single solid-liquid separator, significantly reducing overall production energy consumption. The raw materials enter the system in a multi-stage feeding manner. Some raw materials directly enter the first temperature condition, some enter the first crystallizer 1 as the first filtrate mother liquor, some enter the second crystallizer 4 as the second filtrate mother liquor, and some enter the third crystallizer 7 as the third filtrate mother liquor. This reduces the solid content in the first solid-liquid separator 2, ensuring that the crystals obtained from all three crystallization stages meet the product purity requirements, reducing the overall crystal transfer and circulation volume, and improving production efficiency. The solid-liquid separator separates the filtrate mother liquor and the product wash liquor, treating the relatively high-concentration product wash liquor separately from the relatively low-concentration filtrate mother liquor. The filtrate mother liquor, which is lean towards paraxylene, goes to the next stage, while the product wash liquor, which is rich in paraxylene, returns to the initial temperature condition. This ensures stable product purity and yield, and reduces paraxylene loss. Furthermore, using raw material to wash the crystal slurry can raise the crystal temperature, reducing the risk of impurity encapsulation caused by explosive nucleation during the product washing stage. It also allows for the preliminary removal of some impurities, improving product washing efficiency and reducing the amount of product washing liquid used, thus significantly increasing production efficiency.

[0107] The following is a detailed description of the multi-stage washing production process and system for high-concentration p-xylene in this application, through a specific embodiment:

[0108] A C8 mixture with a p-xylene content of 95% is added as raw material a (flow rate of 1560 kg / h) to the first crystallizer 1 for cooling and crystallization at a temperature of 9°C. The resulting first crystal slurry b enters the first solid-liquid separator 2 for separation, yielding the first filtrate mother liquor c and the first crystal d. 20% by weight of the first filtrate mother liquor c is returned to the first crystallizer 1, and the remainder enters the second crystallizer 4 for cooling and crystallization. After washing with raw material a (flow rate of 240 kg / h) and the p-xylene product (flow rate of 240 kg / h), the first wash liquid f is obtained and returned to the first crystallizer 1 for recrystallization. The washed first crystal d is melted in the first melter 3 to obtain the first p-xylene product e. 15% by weight of the first p-xylene product e is returned to the first solid-liquid separator 2 as washing liquid to wash the first crystal d, and the remaining first p-xylene product e leaves the crystallization system as the product.

[0109] Approximately 80% by weight of the first filtration mother liquor c enters the second crystallizer 4 for cooling and crystallization at a temperature of 6°C. The resulting second crystal slurry g enters the second solid-liquid separator 5, where solid-liquid separation yields the second filtration mother liquor h and the second crystal i. 5% by weight of the second filtration mother liquor h is returned to the second crystallizer 4, and the remaining portion enters the third crystallizer 7 for cooling and crystallization. The second crystal i is washed in the second solid-liquid separator 5 by raw material a (flow rate of 108 kg / h) and para-xylene product (flow rate of 240 kg / h) to obtain the second wash liquid k. The second product wash liquid k is returned to the first crystallizer 1 for recrystallization. The washed second crystal i is melted in the second melter 6 to obtain the second para-xylene product j. 20% of the second para-xylene product j is returned to the second solid-liquid separator 5 as washing liquid to wash the second crystal i, and the remaining portion leaves the crystallization system as the product.

[0110] Approximately 95% by weight of the second filtration mother liquor h enters the third crystallizer 7 for cooling and crystallization at 0°C. The resulting third crystal slurry m enters the third solid-liquid separator 8, where solid-liquid separation yields the third filtration mother liquor n and the third crystal o. 10% by weight of the third filtration mother liquor n is returned to the third crystallizer 7, and the remainder is discharged from the crystallization system. The third crystal o is washed in the third solid-liquid separator 8 by raw material a (flow rate of 92 kg / h) and para-xylene product (flow rate of 92 kg / h) to obtain the third product wash liquid q. The third product wash liquid q is returned to the first crystallizer 1 for recrystallization. The washed third crystal o is melted in the third melter 9 to obtain the second para-xylene product o. 30% of the second para-xylene product o is returned to the third solid-liquid separator 8 as washing liquid to wash the third crystal o, and the remainder is discharged from the crystallization system as product.

[0111] The foregoing description includes examples of one or more embodiments. It is certainly impossible to describe all possible combinations of components or methods in order to describe the above embodiments, but those skilled in the art will recognize that further combinations and arrangements of the various embodiments are possible. Therefore, the embodiments described herein are intended to cover all such changes, modifications, and variations that fall within the scope of the appended claims. Furthermore, the term "comprising" as used in the specification or claims is interpreted in a manner similar to the term "including," just as "comprising" is interpreted as a conjunction in the claims. Additionally, the use of any term "or" in the specification of the claims is intended to mean "non-exclusive or."

Claims

1. A multi-stage washing process for producing high-concentration p-xylene, characterized in that: include: The raw material containing para-xylene is cooled and crystallized under a first temperature condition to obtain the first crystal slurry; The first crystal slurry is subjected to solid-liquid separation to obtain the first crystal and the first filtrate mother liquor; Part of the first filtrate mother liquor is returned to the first temperature condition for further cooling and crystallization, and the remaining first filtrate mother liquor is cooled and crystallized at the second temperature condition to obtain the second crystal slurry. The first crystal was washed sequentially with the raw material and the p-xylene product as washing liquid; The first crystal after washing is melted to obtain the first paraxylene product; The second crystal slurry is subjected to solid-liquid separation to obtain the second crystal and the second filtrate mother liquor. Part of the second filtrate mother liquor is returned to the second temperature condition for continued cooling and crystallization, and the remaining second filtrate mother liquor is cooled and crystallized at the third temperature condition to obtain the third crystal slurry; The second crystal was washed sequentially with the raw material and the p-xylene product as washing solution; The washed second crystal was melted to obtain the second paraxylene product; The third crystal slurry is subjected to solid-liquid separation to obtain the third crystal and the third filtrate mother liquor. A portion of the third filtration mother liquor is returned to the third temperature condition for further cooling and crystallization, and the remaining third filtration mother liquor is discharged from the system. The raw materials and p-xylene product were used as washing solutions to wash the third crystal in sequence. The washed third crystal is melted to obtain the third para-xylene product; The operating temperatures of the first, second, and third temperature conditions decrease sequentially. The operating temperature range of the first temperature condition is 8℃~12℃, the operating temperature range of the second temperature condition is 4℃~9℃, and the operating temperature range of the third temperature condition is 0℃~6℃.

2. The multi-stage washing process for high-concentration p-xylene according to claim 1, characterized in that: After obtaining the first paraxylene product, a portion of the first paraxylene product is returned as a washing solution to the first crystal in the previous step to wash the first crystal. After obtaining the second paraxylene product, a portion of the second paraxylene product is returned as a washing solution to the second crystal in the previous step to wash the second crystal; After obtaining the third para-xylene product, a portion of the third para-xylene product is returned as a washing solution to the third crystal from the previous step to wash the third crystal.

3. The multi-stage washing process for high-concentration p-xylene according to claim 2, characterized in that: 10% to 30% of the first paraxylene product is returned to the first crystal from the previous step as a washing solution to wash the first crystal. 10% to 30% of the second paraxylene product is returned to the second crystal from the previous step as a washing solution to wash the second crystal; 10% to 30% of the third paraxylene product is returned as a washing solution to the third crystal from the previous step to wash the third crystal.

4. The multi-stage washing process for high-concentration p-xylene according to claim 2, characterized in that: The first crystal was washed sequentially with the raw material and the p-xylene product as washing liquid to obtain the first washing liquid; The second crystal was washed sequentially with the raw material and the p-xylene product as washing liquid to obtain the second washing liquid; The raw material and paraxylene product are used as washing liquids to wash the third crystal in sequence to obtain a third washing liquid. The first washing liquid, the second washing liquid and the third washing liquid are all returned to the first temperature condition for continued cooling and crystallization.

5. The multi-stage washing process for high-concentration p-xylene according to claim 1, characterized in that: A C8 mixture with a paraxylene concentration ranging from 95% to 98% was used as raw material and cooled and crystallized under the first temperature condition.

6. The multi-stage washing process for producing high-concentration p-xylene according to claim 1, characterized in that: 5% to 20% of the first filtrate mother liquor is returned to the first temperature condition for continued cooling and crystallization, 5% to 20% of the second filtrate mother liquor is returned to the second temperature condition for continued cooling and crystallization, and 5% to 20% of the third filtrate mother liquor is returned to the third temperature condition for continued cooling and crystallization.

7. A multi-stage washing production system for high-concentration p-xylene, used to implement the multi-stage washing production process for high-concentration p-xylene according to any one of claims 1 to 6, characterized in that: It includes a primary crystallization unit, a secondary crystallization unit, and a tertiary crystallization unit; The primary crystallization device includes a first crystallizer, a first solid-liquid separator, and a first melter; the secondary crystallization device includes a second crystallizer, a second solid-liquid separator, and a second melter; and the tertiary crystallization device includes a third crystallizer, a third solid-liquid separator, and a third melter. The operating temperatures of the first crystallizer, the second crystallizer, and the third crystallizer decrease sequentially. The first crystallizer is used to cool and crystallize the raw material containing paraxylene under a first temperature condition to obtain a first crystal slurry; The outlet of the first crystallizer is connected to the inlet of the first solid-liquid separator, which is used for solid-liquid separation of the first crystal slurry and washing of the first crystal. The outlet of the first solid-liquid separator is connected to the inlet of the first melter, which is used to melt the first crystal to obtain the first paraxylene product. The outlet of the first solid-liquid separator is also connected to the inlet of the first crystallizer and the inlet of the second crystallizer; The outlet of the second crystallizer is connected to the inlet of the second solid-liquid separator. The second crystallizer is used to cool and crystallize a portion of the first filtered mother liquor under a second temperature condition to obtain a second crystal slurry. The second solid-liquid separator is used to perform solid-liquid separation on the second crystal slurry and to wash the second crystal; The outlet of the second solid-liquid separator is connected to the inlet of the second melter, which is used to melt the second crystal to obtain the second paraxylene product. The outlet of the second solid-liquid separator is also connected to the inlet of the second crystallizer and the inlet of the third crystallizer; The outlet of the third crystallizer is connected to the inlet of the third solid-liquid separator. The third crystallizer is used to cool and crystallize a portion of the second filtered mother liquor under a third temperature condition to obtain a third crystal slurry. The third solid-liquid separator is used to perform solid-liquid separation on the third crystal slurry and to wash the third crystal; The outlet of the third solid-liquid separator is connected to the inlet of the third melter, and the third melter is used to melt the third crystal to obtain the third paraxylene product; The outlet of the third solid-liquid separator is also connected to the inlet of the third crystallizer.

8. The multi-stage washing production system for high-concentration p-xylene according to claim 7, characterized in that: The outlet of the first melter is connected to the inlet of the first solid-liquid separator, and is used to return a portion of the first paraxylene product as a washing liquid to the first crystal in the previous step to wash the first crystal. The outlet of the first solid-liquid separator is connected to the inlet of the first crystallizer, and is used to return the first product wash liquid to the first temperature condition for cooling and crystallization.

9. The multi-stage washing production system for high-concentration p-xylene according to claim 7, characterized in that: The outlet of the second melter is connected to the inlet of the second solid-liquid separator, and is used to return a portion of the second paraxylene product as a washing liquid to the second crystal in the previous step to wash the second crystal; The outlet of the second solid-liquid separator is connected to the inlet of the first crystallizer, and is used to return the second product wash liquid to the first temperature condition for cooling and crystallization.

10. The multi-stage washing production system for high-concentration p-xylene according to claim 7, characterized in that: The outlet of the third melter is connected to the inlet of the third solid-liquid separator, and is used to return a portion of the third paraxylene product as a washing liquid to the third crystal in the previous step to wash the third crystal. The outlet of the third solid-liquid separator is connected to the inlet of the first crystallizer, and is used to return the third product wash liquid to the first temperature condition for cooling and crystallization.

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