Multi-stage washing production process and system for p-xylene

Abstract

The invention provides a multi-stage washing production process and system for p-xylene, and the multi-stage washing production process for p-xylene comprises the following steps: carrying out cooling crystallization on a p-xylene-containing raw material under a first temperature condition to obtain first crystal mush; carrying out solid-liquid separation on the first crystal mush to obtain a first crystal and a first filtered mother solution; returning part of the first filtered mother liquor to the first temperature condition for continuous cooling crystallization, and cooling and crystallizing the rest of the first filtered mother liquor at a second temperature condition to obtain second crystal mush; sequentially washing the first crystal by using a raw material and a p-xylene product as washing liquid; and melting the washed first crystal to obtain a first p-xylene product. According to the invention, the washing and production efficiency of p-xylene crystal mush can be improved, and the dosage of eluate is reduced.

Description

Technical Field

[0001] The present application relates to a multi-stage production process and system for paraxylene. Background Art

[0002] Paraxylene (PX) is mainly used as a raw material for the production of purified terephthalic acid (PTA) and dimethyl terephthalate (DMT), while purified terephthalic acid (PTA) is used to manufacture polyester products such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). When the upstream paraxylene (PX) production is insufficient, the downstream terephthalic acid (PTA) factories can only choose to import from neighboring countries. Therefore, the shortage of paraxylene (PX) raw materials has become the biggest constraint on the development of my country's polyester industry.

[0003] At present, p-xylene is mainly obtained by separating mixed xylene, and mixed xylene mainly comes from petroleum aromatics. Limited by the carbon distribution and thermodynamic equilibrium in petroleum aromatics, the p-xylene content in C8 aromatics obtained from different aromatic production processes is very low. Aromatics combined units can be used to convert aromatics, thereby increasing the production of p-xylene. Mixed xylene is mainly composed of p-xylene, m-xylene, o-xylene and ethylbenzene. The boiling points of the components are very different. It is almost impossible to obtain high-purity p-xylene products using distillation methods, which makes the production of high-purity products energy-intensive. However, the melting points of these components are quite different. Generally speaking, the freezing point of p-xylene is about 13.2°C. Compared with other xylenes, the production conditions are milder and the energy consumption is lower. Therefore, the melt crystallization method has obvious technical advantages in separating and purifying p-xylene.

[0004] During the crystallization separation process, the main component paraxylene precipitates from the solution, while other xylenes remain in the solution. High-purity paraxylene can be obtained by completely separating the paraxylene crystals from the xylene solution. However, since the solid-liquid two phases are difficult to separate completely, and the purity of paraxylene is generally required to be >99.7%, many post-processing processes are required to obtain high-purity paraxylene. Washing the crystals during the solid-liquid separation process can effectively remove the residual impurity liquid on the surface of the crystals and improve the purity of the product. The washing effect of the crystal product is related to the crystal temperature, particle size, and the amount of washing liquid. Summary of the invention

[0005] In order to solve the problem of large amount of product washing liquid used due to low washing efficiency in the existing production process when producing paraxylene, the present application proposes a multi-stage washing production process and system for paraxylene, and adopts the following technical solutions:

[0006] In a first aspect, the present application discloses a multi-stage washing production process for paraxylene, comprising:

[0007] Cooling and crystallizing the raw material containing para-xylene under a first temperature condition to obtain a first slurry;

[0008] Performing solid-liquid separation on the first slurry to obtain first crystals and a first filtered mother liquor;

[0009] returning part of the first filtered mother liquor to the first temperature condition for further cooling and crystallization, and cooling and crystallizing the remaining first filtered mother liquor at a second temperature condition to obtain a second crystal slurry;

[0010] Using the raw material and the para-xylene product as washing liquids to wash the first crystals in sequence;

[0011] Melting the washed first crystals to obtain a first p-xylene product;

[0012] Performing solid-liquid separation on the second crystal slurry to obtain second crystals and a second filtered mother liquor;

[0013] returning part of the second filtered mother liquor to the second temperature condition for further cooling and crystallization, and cooling and crystallizing the remaining second filtered mother liquor at a third temperature condition to obtain a third crystal slurry;

[0014] Using the raw material and the para-xylene product as washing liquids to wash the second crystals in sequence;

[0015] Melting the washed second crystals to obtain a second p-xylene product;

[0016] Performing solid-liquid separation on the third slurry to obtain third crystals and a third filtered mother liquor;

[0017] Returning the third crystals to the first temperature condition for further cooling and crystallization, and returning part of the third filtered mother liquor to the third temperature condition for further cooling and crystallization;

[0018] The operating temperatures of the first temperature condition, the second temperature condition and the third temperature condition decrease in sequence.

[0019] Optionally, a C8 mixture with a paraxylene concentration ranging from 92 to 95% is fed as a raw material to the first temperature condition for cooling crystallization.

[0020] Optionally, the operating temperature range of the first temperature condition is 6°C to 10°C, the operating temperature range of the second temperature condition is 2°C to 6°C, and the operating temperature range of the third temperature condition is -10°C to 0°C.

[0021] Optionally, 5% to 10% of the first filtered mother liquor is returned to the first temperature condition to continue cooling and crystallization, 5% to 10% of the second filtered mother liquor is returned to the second temperature condition to continue cooling and crystallization, and 5% to 10% of the third filtered mother liquor is returned to the third temperature condition to continue cooling and crystallization.

[0022] Optionally, using part of the first para-xylene product as a washing liquid to wash the first crystal slurry to obtain a first eluate;

[0023] A portion of the second p-xylene product is used as a washing liquid to wash the second crystal slurry to obtain a second eluate.

[0024] Optionally, 10% to 20% of the first p-xylene product is used as a washing liquid to wash the first crystal slurry;

[0025] 10% to 20% of the second p-xylene product is used as a washing liquid to wash the second crystal slurry.

[0026] Optionally, the first eluate and the second eluate are returned to the first temperature condition to continue cooling and crystallization.

[0027] Optionally, the purity of the first p-xylene product and the second p-xylene product is not less than 99.8%.

[0028] In a second aspect, the present application further discloses a multi-stage washing production system for para-xylene, which is used to implement the multi-stage washing production process for para-xylene as described in the first aspect, comprising a primary crystallization device, a secondary crystallization device and a tertiary crystallization device;

[0029] The primary crystallization device comprises a first crystallizer, a first solid-liquid separator and a first melter, the secondary crystallization device comprises a second crystallizer, a second solid-liquid separator and a second melter, and the tertiary crystallization device comprises a third crystallizer and a third solid-liquid separator;

[0030] The first crystallizer outlet is in communication with the first solid-liquid separator inlet, and the first solid-liquid separator outlet is in communication with the first melter inlet;

[0031] The first solid-liquid separator outlet is also connected to the first crystallizer and the second crystallizer;

[0032] The second crystallizer outlet is communicated with the second solid-liquid separator inlet, and the second solid-liquid separator outlet is communicated with the second melter inlet;

[0033] The second solid-liquid separator outlet is connected to the second crystallizer inlet and the third crystallizer inlet;

[0034] The third crystallizer outlet is communicated with the third solid-liquid separator inlet, and the third solid-liquid separator outlet is also communicated with the third crystallizer inlet;

[0035] The first crystallizer is used to cool and crystallize the raw material containing para-xylene under a first temperature condition to obtain a first crystal slurry;

[0036] The first solid-liquid separator is used to separate the first slurry into solid and liquid and wash it with the raw materials and products to obtain first crystals, a first filtered mother liquor and a first eluate;

[0037] The first melter is used to melt the first crystals to obtain a first paraxylene product;

[0038] The second crystallizer is used to cool and crystallize part of the first filtered mother liquor under a second temperature condition to obtain a second crystal slurry;

[0039] The second solid-liquid separator is used to separate the second crystal slurry into solid and liquid and wash it with the raw materials and products to obtain second crystals, second filtered mother liquor and second eluate;

[0040] The second melter is used to melt the second crystals to obtain a second paraxylene product;

[0041] The third crystallizer is used to cool and crystallize part of the second filtered mother liquor under a third temperature condition to obtain a third crystal slurry;

[0042] The third solid-liquid separator is used to filter the third slurry to obtain third crystals and a third filtered mother liquor;

[0043] The outlet of the third solid-liquid separator is also connected to the first crystallizer, so as to return the third crystals to the first temperature condition for further cooling and crystallization.

[0044] Optionally, the first melter outlet is communicated with the first solid-liquid separator inlet, and is used to use part of the first para-xylene product as a washing liquid to wash the first crystal slurry;

[0045] The first solid-liquid separator outlet is connected to the first crystallizer for returning the first eluate to the first temperature condition for further cooling and crystallization.

[0046] Optionally, the second melter outlet is communicated with the second solid-liquid separator inlet, and is used to use part of the second para-xylene product as a washing liquid to wash the second crystal slurry;

[0047] The outlet of the second solid-liquid separator is also connected to the first crystallizer, so as to return the second eluate to the first temperature condition for further cooling and crystallization.

[0048] Based on the above technical solution, the beneficial effects of this application compared with the prior art are as follows:

[0049] The present application discloses a multi-stage washing production process for paraxylene, which adopts step-by-step cooling and crystallization, so that the circulation volume of slurry and crystal is small and the single-pass yield is high. After filtering the slurry, the filtered mother liquor is first discharged from the solid-liquid separator, and then the crystal is washed in the solid-liquid separator, that is, the filtering and separation of the slurry and the washing process of the crystal are integrated in a solid-liquid separator. Part of the raw material directly enters the first crystallizer, and part of the raw material enters the first crystallizer in the form of the first filtered mother liquor. Using raw materials to wash the slurry can increase the crystal temperature and reduce the impurity inclusion caused by the explosive nucleation of the crystal in the product washing stage. At the same time, using raw materials to wash the slurry to discharge some impurities first can improve the product washing efficiency and reduce the amount of product washing liquid, so as to greatly improve production efficiency. The solid-liquid separator separates the filtered mother liquor and the eluate, and processes the relatively high-concentration eluate and the relatively low-concentration filtered mother liquor separately. The filtered mother liquor poor in paraxylene goes to the next process section, and the eluate rich in paraxylene returns to the first crystallizer to stabilize the product purity and output and reduce the loss of paraxylene. The first crystallizer and the second crystallizer can directly produce paraxylene products that meet the requirements, reduce the amount of material circulation, and have higher production efficiency. The third crystallizer does not directly produce products, and its main purpose is to recover paraxylene. The third crystal returns to the first crystallizer and continues to grow in the first crystallizer to increase the overall particle size of the crystal, which is conducive to further improving the subsequent solid-liquid separation and washing efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 1 is a schematic flow chart of a multi-stage washing production process for paraxylene in an embodiment of the present application;

[0051] Figure 2 It is a schematic diagram of a multi-stage washing production system for paraxylene in an embodiment of the present application.

[0052] Description of reference numerals:

[0053] a, raw material; b, first slurry; c, first filtered mother liquor; d, first crystal; e, first para-xylene product; f, first eluate; g, second slurry; h, second filtered mother liquor; i, second crystal; j, second para-xylene product; k, second eluate; m, third slurry; n, third filtered mother liquor; o, third crystal;

[0054] 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. DETAILED DESCRIPTION

[0055] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0056] Exemplary embodiments will be described in detail herein, examples of which are shown in the accompanying drawings. When the following description refers to the drawings, the same numbers in different drawings represent the same or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present disclosure. Instead, they are merely examples of devices and methods consistent with some aspects of the present disclosure as detailed in the appended claims.

[0057] In the description of the present disclosure, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present disclosure and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present disclosure. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0058] In the description of the present disclosure, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present disclosure can be understood according to specific circumstances.

[0059] The inventors found that in the related art, the p-xylene crystals precipitated in the crystallizer need to be further slurried in the crystal slurry tank, and the slurried crystals are washed with products and then filtered through a solid-liquid separator to obtain products with satisfactory purity. In this process, due to the large difference between the crystallizer temperature and the crystal slurry tank temperature, there will be a large temperature difference between the crystal slurry temperature entering the crystal slurry tank and the washing liquid, which makes it easy for the washing liquid to precipitate when it contacts the low-temperature crystal slurry, reducing the washing efficiency and solid-liquid separation efficiency, thereby resulting in a large amount of product washing liquid used and causing waste.

[0060] Based on the above problems, the embodiments of the present application disclose a multi-stage washing production process and system for para-xylene, which solves the problem of large amount of product washing liquid used due to low washing efficiency in the existing production process when producing para-xylene. The following is a detailed description of various specific implementation methods of the multi-stage production system and production method of para-xylene provided in the embodiments of the present disclosure.

[0061] In the first aspect, the present application provides a multi-stage washing production process for paraxylene, referring to Figure 1 ,include:

[0062] S101. Cooling and crystallizing the para-xylene-containing raw material a under a first temperature condition to obtain a first slurry b;

[0063] S102. performing solid-liquid separation on the first slurry b to obtain first crystals d and a first filtered mother liquor c;

[0064] S103. returning part of the first filtered mother liquor c to the first temperature condition for further cooling and crystallization, and cooling and crystallizing the remaining first filtered mother liquor c at the second temperature condition to obtain a second slurry g;

[0065] S104. Using the raw material a and the para-xylene product as washing liquids, washing the first crystal d in sequence;

[0066] S105. Melting the washed first crystal d to obtain a first paraxylene product e;

[0067] S106. performing solid-liquid separation on the second slurry g to obtain second crystals i and a second filtered mother liquor h;

[0068] S107. returning part of the second filtered mother liquor h to the second temperature condition for further cooling and crystallization, and cooling and crystallizing the remaining second filtered mother liquor h at the third temperature condition to obtain a third slurry m;

[0069] S108. Using the raw material a and the paraxylene product as washing liquids, washing the second crystal i in sequence;

[0070] S109. Melting the washed second crystal i to obtain a second para-xylene product j;

[0071] S110. performing solid-liquid separation on the third slurry m to obtain third crystals o and a third filtered mother liquor n;

[0072] S111. Return the third crystal o to the first temperature condition for further cooling and crystallization, and return part of the third filtered mother liquor n to the third temperature condition for further cooling and crystallization.

[0073] It should be noted that the operating temperatures of the first temperature condition, the second temperature condition and the third temperature condition are successively reduced, wherein the operating temperature range of the first temperature condition is 6°C to 10°C, the operating temperature range of the second temperature condition is 2°C to 6°C, and the operating temperature range of the third temperature condition is -10°C to 0°C. The purity of the first paraxylene product e and the second paraxylene product j is not less than 99.8%.

[0074] Specifically, in an optional embodiment, in step S101, a C8 mixture with a paraxylene concentration ranging from 92% to 95% is used as raw material a and cooled and crystallized under a first temperature condition.

[0075] In an optional embodiment, the first slurry b is first separated from the solid and liquid to obtain the first filtered mother liquor c and the first crystal d. The first filtered mother liquor is first discharged, and then washed with the raw material a and the p-xylene product in sequence to obtain the first eluate f. The first eluate f is returned to the first temperature condition for further cooling and crystallization. Using the raw material a for washing can increase the crystal temperature and reduce the impurity inclusion caused by the explosive nucleation in the product washing stage. At the same time, the raw material a washing can first discharge some impurities, reduce the amount of product washing liquid, and thus greatly improve the production efficiency.

[0076] In an optional embodiment, in step S102, for the first filtered mother liquor c, 5% to 10% of the first filtered mother liquor c is returned to the first temperature condition to continue cooling crystallization, which is conducive to promoting the continued growth of crystals in the first temperature condition and improving the subsequent separation efficiency. The remaining part of the first filtered mother liquor c enters the second temperature condition for the next stage of cooling crystallization.

[0077] Further, in step S102, after the first crystal d is melted to obtain the first p-xylene product e, part of the first p-xylene product e is returned to the first crystal d in the previous step as a washing liquid to wash the first crystal d. Among them, the first p-xylene product e returned to the first temperature condition accounts for about 10% to 20%. Using the first p-xylene product e to wash the first crystal slurry b can improve the purity of the obtained first crystal d and reduce other impurities attached to the surface of the first crystal d.

[0078] Correspondingly, in steps S105 to S108, after the second crystal slurry g is filtered in the second solid-liquid separator 5, the second filtered mother liquor h is discharged, and the remaining second crystals i are washed with raw material a and para-xylene product in turn to obtain second eluate k, and the second eluate k is returned to the first temperature condition. 5% to 10% of the second filtered mother liquor h is returned to the second temperature condition, which is conducive to promoting the continued growth of crystals in the second temperature condition and improving the subsequent separation efficiency. The remaining second filtered mother liquor h enters the third temperature condition for the next stage of cooling crystallization. At the same time, similar to the first crystal d, 10% to 20% of the second para-xylene product j is also returned to the second crystal i in the previous step to wash the second crystal i.

[0079] Finally, in steps S109-S110, the third slurry m is filtered to obtain the third filtered mother liquor n and the third crystal o. At this time, the washed third crystal o cannot meet the product concentration, and the third crystal o with a smaller particle size continues to return to the first temperature condition and can continue to grow in the first temperature condition to facilitate the subsequent solid-liquid separation process. At the same time, the low-temperature third crystal o can bring cold energy into the first temperature condition, reducing the energy consumption of the equipment. For the third filtered mother liquor n, 5% to 10% of the third filtered mother liquor n returns to the third temperature condition, and the remaining third filtered mother liquor n is discharged from the system.

[0080] It should be noted that in the actual production process, in the initial state, a pure p-xylene product can be used to wash the crystals, and after the corresponding products are generated, the corresponding p-xylene products are used for washing. For example, in the embodiment of the present application, in the middle and late stages of production, after the corresponding first p-xylene product e and second p-xylene product j are generated, a portion of the first p-xylene product e is used as a washing liquid to wash the first crystal d, and a portion of the second p-xylene product j is used as a washing liquid to wash the second crystal i.

[0081] In a second aspect, the present application discloses a multi-stage washing production system for p-xylene, which is used to implement the multi-stage washing production process for p-xylene in the first aspect, referring to Figure 2 The multi-stage washing production system of paraxylene includes a primary crystallization device, a secondary crystallization device and a tertiary crystallization device. The primary crystallization device includes a first crystallizer 1, a first solid-liquid separator 2 and a first melter 3. The secondary crystallization device includes a second crystallizer 4, a second solid-liquid separator 5 and a second melter 6. The tertiary crystallization device includes a third crystallizer 7 and a third solid-liquid separator 8.

[0082] It should be noted that the temperatures of the first crystallizer 1, the second crystallizer 4 and the third crystallizer 7 are sequentially reduced. Specifically, the temperature range of the first crystallizer 1 is 6°C to 10°C, the temperature range of the second crystallizer 4 is 2°C to 6°C, and the temperature range of the third crystallizer 7 is -10°C to 0°C. Thus, under the premise of meeting the purity of the product, the overall crystallization energy consumption is lowered by the step-by-step cooling and crystallization method.

[0083] It should be further explained that in this embodiment, the first solid-liquid separator 2, the second solid-liquid separator 5 and the third solid-liquid separator 8 are an integrated device that can not only realize the solid-liquid separation of the slurry, but also further wash and filter the separated crystals, which is conducive to reducing the overall energy consumption.

[0084] Among them, the first crystallizer 1 is used to cool and crystallize the raw material a containing paraxylene under the first temperature condition to obtain the first crystal slurry b; the first solid-liquid separator 2 is used to perform solid-liquid separation on the first crystal slurry b and wash the first crystal d; the first melter 3 is used to melt the first crystal d to obtain the first paraxylene product e.

[0085] The second crystallizer 4 is used to cool and crystallize part of the first filtered mother liquor c under the second temperature condition to obtain a second crystal slurry g; the second solid-liquid separator 5 is used to perform solid-liquid separation on the second crystal slurry g and wash the second crystals i; the second melter 6 is used to melt the second crystals i to obtain a second paraxylene product j.

[0086] The third crystallizer 7 is used to cool and crystallize part of the second filtered mother liquor h under a third temperature condition to obtain a third crystal slurry m; the third solid-liquid separator 8 is used to filter the third crystal slurry m to obtain third crystals o and a third filtered mother liquor n, and to wash the third crystals o.

[0087] In an alternative embodiment, referring to Figure 2 The outlet of the first crystallizer 1 is connected to the inlet of the first solid-liquid separator 2, so as to feed the first crystal slurry b into the first solid-liquid separator 2 for filtration. The outlet of the first solid-liquid separator 2 is connected to the inlet of the first melter 3, so as to feed the first crystal d into the first melter 3 for melting to obtain the first paraxylene product e.

[0088] In an alternative embodiment, referring 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 part of the first filtered mother liquor c to the first crystallizer 1, and send the remaining part to the second crystallizer 4 for cooling and crystallization.

[0089] In an alternative embodiment, referring to Figure 2The outlet of the first melter 3 is connected to the inlet of the first solid-liquid separator 2, so as to return part of the first paraxylene product e to the first solid-liquid separator 2 for washing the first crystal d. The outlet of the first solid-liquid separator 2 is also connected to the first crystallizer 1, so as to return the first eluate f to the first crystallizer 1 for further cooling and crystallization.

[0090] In an alternative embodiment, referring to Figure 2 The outlet of the second crystallizer 4 is connected to the inlet of the second solid-liquid separator 5, so as to feed the second crystal slurry g into the second solid-liquid separator 5 for filtration. The outlet of the second solid-liquid separator 5 is connected to the inlet of the second melter 6, so as to feed the second crystal i into the second melter 6 for melting to obtain the second paraxylene product j.

[0091] In an optional embodiment, the outlet of the second solid-liquid separator 5 is also connected to the inlet of the second crystallizer 4 and the inlet of the third crystallizer 7, so as 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.

[0092] In an alternative embodiment, referring to Figure 2 The outlet of the second melter 6 is connected to the inlet of the second solid-liquid separator 5, so as to return part of the second para-xylene product j to the second solid-liquid separator 5 for washing the second crystals i. The outlet of the second solid-liquid separator 5 is also connected to the first crystallizer 1, so as to return the second eluate k to the first crystallizer 1.

[0093] In an alternative embodiment, referring 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 third crystallizer 7. The outlet of the third solid-liquid separator 8 is connected to the first crystallizer 1, so as to return the third crystal o to the first crystallizer 1.

[0094] In summary, in the multi-stage production process and system of paraxylene, step-by-step cooling and crystallization are adopted, so that the circulation amount of slurry and crystal is small and the single-pass yield is high. After filtering the slurry, the filtered mother liquor is first discharged from the solid-liquid separator, and then the crystal is washed in the solid-liquid separator, that is, the filtering and separation of the slurry and the washing process of the crystal are integrated in a solid-liquid separator. Part of the raw material a directly enters the first crystallizer 1 for cooling and crystallization, and part of the raw material a enters the first crystallizer 1 in the form of the first filtered mother liquor c. The feed in shares helps to reduce the solid content in the first crystallizer 1, so that the feed concentration range applicable to the whole process is wider and insensitive to feed concentration fluctuations. In addition, using raw material a to wash the slurry can increase the crystal temperature, reduce the impurity inclusion caused by the explosive nucleation of the crystal in the product washing stage, and use raw material a to discharge some impurities first, which can improve the product washing efficiency and reduce the amount of product washing liquid, so as to greatly improve production efficiency. The mother liquor and the eluate are divided, and the eluate with a relatively high concentration is treated separately from the mother liquor with a relatively low concentration. The mother liquor with a poor concentration of paraxylene is sent to the next stage, and the eluate with a rich concentration of paraxylene is returned to the first crystallizer 1 to stabilize the purity and output of the product and reduce the loss of paraxylene. The first crystallizer 1 and the second crystallizer 4 can directly produce paraxylene products that meet the requirements, reduce the amount of material circulation, and have higher production efficiency. The third crystallizer 7 does not directly produce products, and its main purpose is to recover paraxylene. The third crystal o is returned to the first crystallizer 1 and continues to grow in the first crystallizer 1 to increase the overall particle size of the crystal, which is conducive to improving the subsequent solid-liquid separation and washing efficiency. At the same time, the low-temperature third crystal o brings cold energy into the first crystallizer 1, reducing the energy consumption of related equipment.

[0095] Below, the embodiments of the present application are described in detail through corresponding embodiments and comparative examples:

[0096] Example 1

[0097] A C8 mixture with a paraxylene content of 92% is added as raw material a (flow rate of 1672 kg / h) to the first crystallizer 1 for cooling and crystallization. The crystallization temperature is 7.5°C. The obtained first crystal slurry b enters the first solid-liquid separator 2 to obtain a first filtered mother liquor c and a first crystal d. The first filtered mother liquor c is discharged from the first solid-liquid separator 2, wherein 10% by weight of the first filtered mother liquor c is returned to the first crystallizer 1, and the remaining part enters the second crystallizer 4 for cooling and crystallization. The first crystal d is washed in the first solid-liquid separator 2 with raw material a (flow rate of 193 kg / h) and paraxylene product (flow rate of 193 kg / h) to obtain a first eluate f, which is returned to the first crystallizer 1 for recrystallization. The washed first crystal d is melted in the first melter 3, and 15% by weight of the first paraxylene product e is returned to the first solid-liquid separator 2 as a washing liquid to wash the first crystal d. The remaining first paraxylene product e leaves the crystallization system as a product.

[0098] About 90% by weight of the first filtered mother liquor c enters the second crystallizer 4 for cooling and crystallization, and the crystallization temperature is 3°C. The obtained second crystal slurry g enters the second solid-liquid separator 5 for solid-liquid separation to obtain the second filtered mother liquor h and the second crystal i. The second filtered mother liquor h, which accounts for 10% by weight, returns to the second crystallizer 4, and the remaining part enters the third crystallizer 7 for cooling and crystallization. The second crystal i is washed with raw material a (flow rate of 135 kg / h) and para-xylene product in the second solid-liquid separator 5 to obtain the second eluate k, which is returned to the first crystallizer 1 for recrystallization. The washed second crystal i is melted in the second melter 6, and the second para-xylene product j, which accounts for about 20%, is returned to the second solid-liquid separator 5 as a washing liquid to wash the second crystal i. The remaining second para-xylene product j leaves the crystallization system as a product.

[0099] About 90% by weight of the second filtered mother liquor h enters the third crystallizer 7 for cooling and crystallization at a crystallization temperature of -5°C, and the obtained third crystal slurry m enters the third solid-liquid separator 8 for solid-liquid separation to obtain the third filtered mother liquor n and the third crystal o. About 5% by weight of the third filtered mother liquor n is returned to the third crystallizer 7, the rest is discharged from the crystallization system, and all the third crystals o are returned to the first crystallizer 1.

[0100] Example 2

[0101] A C8 mixture with a paraxylene content of 95% is added as raw material a (flow rate of 1500kg / h) to the first crystallizer 1 for cooling and crystallization. The crystallization temperature is 9°C. The obtained first crystal slurry b enters the first solid-liquid separator 2 to obtain a first filtered mother liquor c and a first crystal d. The first filtered mother liquor c is discharged from the first solid-liquid separator 2, wherein 30% by weight of the first filtered mother liquor c is returned to the first crystallizer 1, and the remaining part enters the second crystallizer 4 for cooling and crystallization. The first crystal d is washed in the first solid-liquid separator 2 with raw material a (flow rate of 107kg / h) and paraxylene product (flow rate of 107kg / h) to obtain a first eluate f, which is returned to the first crystallizer 1 for recrystallization. The washed first crystal d is melted in the first melter 3, and 15% by weight of the first paraxylene product e is returned to the first solid-liquid separator 2 as a washing liquid to wash the first crystal d. The remaining first paraxylene product e leaves the crystallization system as a product.

[0102] About 90% by weight of the first filtered mother liquor c enters the second crystallizer 4 for cooling and crystallization, and the crystallization temperature is 6°C. The obtained second crystal slurry g enters the second solid-liquid separator 5 for solid-liquid separation to obtain the second filtered mother liquor h and the second crystal i. The second filtered mother liquor h, which accounts for 10% by weight, returns to the second crystallizer 4, and the remaining part enters the third crystallizer 7 for cooling and crystallization. The second crystal i is washed with raw material a (flow rate of 107 kg / h) and para-xylene product in the second solid-liquid separator 5 to obtain the second eluate k, which is returned to the first crystallizer 1 for recrystallization. The washed second crystal i is melted in the second melter 6, and about 20% of the second para-xylene product j is returned to the second solid-liquid separator 5 as a washing liquid to wash the second crystal i. The remaining second para-xylene product j leaves the crystallization system as a product.

[0103] About 90% by weight of the second filtered mother liquor h enters the third crystallizer 7 for cooling and crystallization, and the crystallization temperature is -5°C. The obtained third crystal slurry m enters the third solid-liquid separator 8, and the solid-liquid separation obtains the third filtered mother liquor n and the third crystal o. About 30% by weight of the third filtered mother liquor n is returned to the third crystallizer 7, and the rest is discharged from the crystallization system, and all the third crystals o are returned to the first crystallizer 1.

[0104] Comparative Example 1

[0105] A C8 mixture with a paraxylene content of 90% is added as raw material a (flow rate of 1500kg / h) to the first crystallizer 1 for cooling and crystallization. The crystallization temperature is 6°C. The obtained first crystal slurry b enters the first solid-liquid separator 2 to obtain a first filtered mother liquor c and a first crystal d. The first filtered mother liquor c is discharged from the first solid-liquid separator 2, wherein the first filtered mother liquor c enters the second crystallizer 4 for cooling and crystallization. The first crystal d is washed with raw material a (flow rate of 299kg / h) and paraxylene product (flow rate of 299kg / h) in the first solid-liquid separator 2 in turn to obtain a first eluate f, and the first eluate f is returned to the first crystallizer 1 for recrystallization. The washed first crystal d is melted in the first melter 3, and 25% by weight of the first paraxylene product e is returned to the first solid-liquid separator 2 as a washing liquid to wash the first crystal d. The remaining first paraxylene product e leaves the crystallization system as a product.

[0106] The first filtered mother liquor c enters the second crystallizer 4 for cooling and crystallization, and the crystallization temperature is 0°C. The obtained second crystal slurry g enters the second solid-liquid separator 5 for solid-liquid separation to obtain the second filtered mother liquor h and the second crystal i. The second filtered mother liquor h enters the third crystallizer 7 for cooling and crystallization. The second crystal i is washed with raw material a (flow rate is 168 kg / h) and para-xylene product in the second solid-liquid separator 5 to obtain the second eluate k, and the second eluate k is returned to the first crystallizer 1 for recrystallization. The washed second crystal i is melted in the second melter 6, and the second para-xylene product j, which accounts for about 35%, is returned to the second solid-liquid separator 5 as a washing liquid to wash the second crystal i, and the remaining second para-xylene product j leaves the crystallization system as a product.

[0107] The second filtered mother liquor h enters the third crystallizer 7 for cooling and crystallization, and the crystallization temperature is -10°C. The obtained third crystal slurry m enters the third solid-liquid separator 8, and the solid-liquid separation obtains the third filtered mother liquor n and the third crystal o. About 5% of the weight of the third filtered mother liquor n is returned to the third crystallizer 7, and the rest is discharged from the crystallization system, and all the third crystals o are returned to the first crystallizer 1.

[0108] Comparative Example 2

[0109] A C8 mixture with a paraxylene content of 98% is added as raw material a (flow rate of 1500kg / h) to the first crystallizer 1 for cooling and crystallization. The crystallization temperature is 10°C. The obtained first crystal slurry b enters the first solid-liquid separator 2 to obtain a first filtered mother liquor c and a first crystal d. The first filtered mother liquor c is discharged from the first solid-liquid separator 2, wherein 30% by weight of the first filtered mother liquor c is returned to the first crystallizer 1, and the remaining part enters the second crystallizer 4 for cooling and crystallization. The first crystal d is washed with raw material a (flow rate of 218kg / h) in the first solid-liquid separator 2 in turn to obtain a first eluate f, and the first eluate f is returned to the first crystallizer 1 for recrystallization. The washed first crystal d is melted in the first melter 3 to obtain a first paraxylene product e, and the first paraxylene product e leaves the crystallization system as a product.

[0110] About 70% by weight of the first filtered mother liquor c enters the second crystallizer 4 for cooling and crystallization, and the crystallization temperature is 7°C. The obtained second crystal slurry g enters the second solid-liquid separator 5 for solid-liquid separation to obtain the second filtered mother liquor h and the second crystal i. The second filtered mother liquor h enters the third crystallizer 7 for cooling and crystallization. The second crystal i is washed with the raw material a (flow rate of 56 kg / h) and the para-xylene product (flow rate of 56 kg / h) in the second solid-liquid separator 5 to obtain the second eluate k, which is returned to the first crystallizer 1 for recrystallization. The washed second crystal i is melted in the second melter 6, and about 15% of the second para-xylene product j is returned to the second solid-liquid separator 5 as a washing liquid to wash the second crystal i. The remaining second para-xylene product j leaves the crystallization system as a product.

[0111] The second filtered mother liquor h enters the third crystallizer 7 for cooling and crystallization, and the crystallization temperature is 2°C. The obtained third crystal slurry m enters the third solid-liquid separator 8, and the solid-liquid separation obtains the third filtered mother liquor n and the third crystal o. About 20% of the weight of the third filtered mother liquor n is returned to the third crystallizer 7, and the rest is discharged from the crystallization system, and all the third crystals o are returned to the first crystallizer 1.

[0112] According to the above comparative examples, the crystallization temperature, washing ratio, yield, etc. of the comparative examples with feed concentrations lower than (such as comparative example 1) and higher than (such as comparative example 2) the specified concentration range are greatly deteriorated. It can be seen that the multi-stage crystallization system with washing is a strong nonlinear system. The treatment of feeds of different concentrations cannot be efficiently solved by optimization within the conventional conditions. It is more critical to change the nonlinear process structure, which requires sophisticated design and optimization and cannot be simply expected.

[0113] The above description includes examples of one or more embodiments. Of course, it is impossible to describe all possible combinations of components or methods for the purpose of describing the above embodiments, but it should be recognized by those skilled in the art that the various embodiments may be further combined and arranged. Therefore, the embodiments described herein are intended to cover all such changes, modifications and variations that fall within the scope of protection of the appended claims. In addition, with respect to the term "comprising" used in the specification or claims, the word is covered in a manner similar to the term "including", as explained by "including" used as a transitional word in the claims. In addition, any term "or" used in the specification of the claims is intended to mean "non-exclusive or".

Claims

1. A multi-stage washing production process for paraxylene, characterized in that: include: Cooling and crystallizing the raw material containing para-xylene under a first temperature condition to obtain a first slurry; Performing solid-liquid separation on the first slurry to obtain first crystals and a first filtered mother liquor; returning part of the first filtered mother liquor to the first temperature condition for further cooling and crystallization, and cooling and crystallizing the remaining first filtered mother liquor at a second temperature condition to obtain a second crystal slurry; Using the raw material and the para-xylene product as washing liquids to wash the first crystals in sequence; Melting the washed first crystals to obtain a first p-xylene product; Performing solid-liquid separation on the second crystal slurry to obtain second crystals and a second filtered mother liquor; returning part of the second filtered mother liquor to the second temperature condition for further cooling and crystallization, and cooling and crystallizing the remaining second filtered mother liquor at a third temperature condition to obtain a third crystal slurry; Using the raw material and the para-xylene product as washing liquids to wash the second crystals in sequence; Melting the washed second crystals to obtain a second p-xylene product; Performing solid-liquid separation on the third slurry to obtain third crystals and a third filtered mother liquor; Returning the third crystals to the first temperature condition for further cooling and crystallization, and returning part of the third filtered mother liquor to the third temperature condition for further cooling and crystallization; The operating temperatures of the first temperature condition, the second temperature condition and the third temperature condition decrease in sequence.

2. The multi-stage washing production process of paraxylene according to claim 1, characterized in that: A C8 mixture with a paraxylene concentration ranging from 92% to 95% is fed as a raw material into the first temperature condition for cooling crystallization.

3. The multi-stage washing production process of paraxylene according to claim 1, characterized in that: The operating temperature range of the first temperature condition is 6°C to 10°C, the operating temperature range of the second temperature condition is 2°C to 6°C, and the operating temperature range of the third temperature condition is -10°C to 0°C.

4. The multi-stage washing production process of paraxylene according to claim 1, characterized in that: 5% to 30% of the first filtered mother liquor is returned to the first temperature condition to continue cooling and crystallizing, 5% to 30% of the second filtered mother liquor is returned to the second temperature condition to continue cooling and crystallizing, and 5% to 30% of the third filtered mother liquor is returned to the third temperature condition to continue cooling and crystallizing.

5. The multi-stage washing production process of paraxylene according to claim 1, characterized in that: After obtaining the first p-xylene product, returning part of the first p-xylene product as washing liquid to the first crystals in the previous step to wash the first crystals and obtain a first eluate; After obtaining the second p-xylene product, part of the second p-xylene product is returned to the second crystals of the previous step as washing liquid to wash the second crystals and obtain a second eluate.

6. The multi-stage washing production process of paraxylene according to claim 5, characterized in that: Using 10% to 20% of the first p-xylene product as a washing liquid to wash the first crystals; 10% to 20% of the second p-xylene product is used as a washing liquid to wash the second crystals.

7. The multi-stage washing production process of paraxylene according to claim 5, characterized in that: The first eluate and the second eluate are returned to the first temperature condition to continue cooling and crystallization.

8. The multi-stage washing production process of paraxylene according to claim 1, characterized in that: The purity of the first p-xylene product and the second p-xylene product is not less than 99.8%.

9. A multi-stage washing production system for paraxylene, used for implementing the multi-stage washing production process for paraxylene according to any one of claims 1 to 8, characterized in that: It includes a primary crystallization device, a secondary crystallization device and a tertiary crystallization device; The primary crystallization device comprises a first crystallizer, a first solid-liquid separator and a first melter, the secondary crystallization device comprises a second crystallizer, a second solid-liquid separator and a second melter, and the tertiary crystallization device comprises a third crystallizer and a third solid-liquid separator; The first crystallizer outlet is in communication with the first solid-liquid separator inlet, and the first solid-liquid separator outlet is in communication with the first melter inlet; The first solid-liquid separator outlet is also connected to the first crystallizer and the second crystallizer; The second crystallizer outlet is communicated with the second solid-liquid separator inlet, and the second solid-liquid separator outlet is communicated with the second melter inlet; The second solid-liquid separator outlet is connected to the second crystallizer inlet and the third crystallizer inlet; The third crystallizer outlet is communicated with the third solid-liquid separator inlet, and the third solid-liquid separator outlet is also communicated with the third crystallizer inlet; The first crystallizer is used to cool and crystallize the raw material containing para-xylene under a first temperature condition to obtain a first crystal slurry; The first solid-liquid separator is used to perform solid-liquid separation on the first crystal slurry and wash the first crystals; The first melter is used to melt the first crystals to obtain a first paraxylene product; The second crystallizer is used to cool and crystallize part of the first filtered mother liquor under a second temperature condition to obtain a second crystal slurry; The second solid-liquid separator is used for performing solid-liquid separation on the second crystal slurry and washing the second crystals; The second melter is used to melt the second crystals to obtain a second paraxylene product; The third crystallizer is used to cool and crystallize part 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 slurry to obtain third crystals and a third filtered mother liquor; The outlet of the third solid-liquid separator is also connected to the first crystallizer, so as to return the third crystals to the first temperature condition for further cooling and crystallization.

10. The multi-stage washing production system of paraxylene according to claim 9, characterized in that: The first melter outlet is in communication with the first solid-liquid separator inlet, and is used to return part of the first para-xylene product as washing liquid to the first crystals in the previous step to wash the first crystals; The outlet of the first solid-liquid separator is connected to the first crystallizer, and is used to return the first eluate to the first temperature condition for further cooling and crystallization.

11. The multi-stage washing production system of paraxylene according to claim 9, characterized in that: The second melter outlet is in communication with the second solid-liquid separator inlet, and is used to return part of the second para-xylene product as washing liquid to the second crystals in the previous step to wash the second crystals; The outlet of the second solid-liquid separator is also connected to the first crystallizer, so as to return the second eluate to the first temperature condition for further cooling and crystallization.

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