A two-stage production process and system for paraxylene

Abstract

The application provides a p-xylene secondary production process and system, and the p-xylene secondary production process comprises the following steps: cooling and crystallizing raw materials containing p-xylene under a first temperature condition to obtain first crystal slurry; performing solid-liquid separation on the first crystal slurry to obtain first crystals and first filtration mother liquor; sequentially washing the first crystals with the raw materials and p-xylene products as washing liquid; melting the first crystals to obtain first p-xylene products; performing solid-liquid separation on second crystal slurry to obtain second crystals and second filtration mother liquor; returning part of the second filtration mother liquor to the second temperature condition for continuous cooling and crystallization; sequentially washing the second crystals with the raw materials and p-xylene products as washing liquid; and melting the washed second crystals to obtain second p-xylene products. The p-xylene secondary production process can solve the problem of large consumption of washing liquid due to low washing efficiency in the production of p-xylene in the prior art.

Description

Technical Field

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

[0002] Paraxylene is an important chemical product used across various industries. Separating mixed xylenes (commonly known as the C8 fraction) in paraxylene production is a recognized technical challenge. Mixed xylenes typically include paraxylene (PX), metaxylene (MX), o-xylene (OX), and ethylbenzene (EB).

[0003] Since the boiling points of the components of mixed xylene are relatively close, such as the boiling point difference between p-xylene and m-xylene is only 0.6°C, the energy consumption and cost of separating the system by direct distillation are high; however, the freezing points of the components vary greatly. Generally speaking, the freezing point of p-xylene is around 13.2°C. Compared with other xylenes, the production conditions are milder and the energy consumption required is lower. Therefore, the melt crystallization method has obvious technical advantages in separating and purifying p-xylene.

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

[0005] In order to solve the problem of low crystal washing efficiency when processing mixed xylene materials in the existing crystallization production process, the present application discloses a two-stage production process and system for para-xylene, and adopts the following technical solutions:

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

[0007] Cooling and crystallizing the raw material containing paraxylene 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 a portion 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 slurry;

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

[0011] Melting the first crystals after washing with the raw material and the para-xylene product to obtain a first para-xylene product;

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

[0013] returning a portion of the second filtered mother liquor to the second temperature condition to continue cooling and crystallizing, and discharging the remaining second filtered mother liquor from the system;

[0014] washing the second crystals in sequence using the raw material and the p-xylene product as washing liquids;

[0015] Melting the second crystals after washing with the raw material and the p-xylene product to obtain a second p-xylene product;

[0016] An operating temperature of the first temperature condition is greater than an operating temperature of the second temperature condition.

[0017] Optionally, after obtaining the first p-xylene product, a portion of the first p-xylene product is returned to the first crystals in the previous step as a washing liquid to wash the first crystals;

[0018] After obtaining the second para-xylene product, part of the second para-xylene product is returned to the second crystals in the previous step as washing liquid to wash the second crystals.

[0019] Optionally, 10% to 20% of the first p-xylene product is returned as a washing liquid to the first crystals in the previous step to wash the first crystals;

[0020] 10% to 20% of the second p-xylene product is returned to the second crystals of the previous step as washing liquid to wash the second crystal slurry.

[0021] Optionally, the raw material and the paraxylene product are used as washing liquids to wash the first crystals in sequence to obtain a first eluate, and the first eluate is returned to the first temperature condition to continue cooling and crystallizing;

[0022] The second crystals are washed in sequence with the raw material and the paraxylene product as washing liquids to obtain a second eluate, and the second eluate is returned to the first temperature condition for further cooling and crystallization.

[0023] Optionally, the concentration of p-xylene in the raw material is greater than 98%;

[0024] Optionally, the operating temperature range of the first temperature condition is 6°C to 12°C, and the operating temperature range of the second temperature condition is 0°C to 8°C.

[0025] Optionally, 30% to 50% of the first filtered mother liquor is returned to the first temperature condition to continue cooling and crystallizing;

[0026] 60% to 80% of the second filtered mother liquor is returned to the second temperature condition to continue cooling and crystallizing.

[0027] In a second aspect, the present application further discloses a secondary production system of p-xylene, which is used to implement the secondary production process of p-xylene as described in the first aspect, comprising a primary crystallization device and a secondary crystallization device;

[0028] The primary crystallization device includes a first crystallizer, a first solid-liquid separator and a first melter, and the secondary crystallization device includes a second crystallizer, a second solid-liquid separator and a second melter;

[0029] The operating temperature of the second crystallizer is lower than the operating temperature of the first crystallizer;

[0030] 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;

[0031] The first crystallizer outlet is connected to the first solid-liquid separator inlet, and the first solid-liquid separator is used to separate the first slurry;

[0032] The outlet of the first solid-liquid separator is communicated with the inlet of the first melter, and the first melter is used to melt the first crystals to obtain a first paraxylene product;

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

[0034] The first melter outlet is also in communication with the first solid-liquid separator inlet, and is used to use part of the first paraxylene product as a washing liquid to wash the first slurry;

[0035] The outlet of the second crystallizer is connected to the inlet of the second solid-liquid separator, and 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 slurry;

[0036] The second solid-liquid separator is used to separate the second slurry;

[0037] The outlet of the second solid-liquid separator is communicated with the inlet of the second melter, and the second melter is used to melt the second crystals to obtain a second paraxylene product;

[0038] The second solid-liquid separator outlet is also connected to the second crystallizer inlet;

[0039] The second melter outlet is also connected to the second solid-liquid separator inlet, and is used to use part of the second paraxylene product as a washing liquid to wash the second crystal slurry.

[0040] Optionally, the first melter outlet is further connected to the first solid-liquid separator inlet, for returning part of the first paraxylene product as washing liquid to the first crystals in the previous step to wash the first crystals;

[0041] The second melter outlet is also connected to the second solid-liquid separator inlet for returning part of the second paraxylene product as washing liquid to the second crystals of the previous step to wash the second crystals.

[0042] Optionally, the first solid-liquid separator outlet is connected to the first crystallizer inlet, so as to return the first eluate to the first temperature condition for further cooling and crystallization;

[0043] The outlet of the second solid-liquid separator is communicated with the inlet of the first crystallizer, and is used to return the second eluate to the first temperature condition for further cooling and crystallization.

[0044] Based on the above technical solution, the beneficial effects of this application compared with the prior art are as follows:

[0045] The present application discloses a two-stage production process for p-xylene, which is suitable for raw materials with a p-xylene concentration greater than 98%. It adopts step-by-step cooling crystallization. The slurry of the two crystallization stages can directly produce products after washing the raw materials and products. It has the characteristics of simple process and high production efficiency. After filtering the crystal slurry, the present application first discharges the filtered mother liquor out of the solid-liquid separator, and then washes the crystals in the solid-liquid separator, that is, the filtering and separation of the crystal slurry and the washing process of the crystals are integrated in one solid-liquid separator, which can greatly reduce energy consumption while ensuring product yield. The relatively high concentration eluate and the relatively low concentration filtered mother liquor are treated separately, the filtered mother liquor poor in p-xylene goes to the next stage, and the eluate rich in p-xylene returns to the crystallizer to continue cooling crystallization, so as to stabilize the product purity and output and reduce the loss of p-xylene. Before product washing, using raw material to wash the crystal slurry can increase the crystal temperature, reduce the impurity inclusion caused by explosive nucleation in the crystal during the product washing stage, and discharge some impurities in advance, which can improve product washing efficiency and reduce the amount of product washing liquid used, thereby greatly improving production efficiency. Due to the high feed concentration, high recovery rates can be achieved without the need for a low second-stage crystallization temperature. This also allows the second-stage crystallizer to directly wash out qualified products, thereby reducing the circulation volume of in-process logistics and improving production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1Schematic diagram of the secondary production process of paraxylene in the embodiment of the present application;

[0047] Figure 2 Schematic diagram of the secondary production system of paraxylene in the embodiment of the present application.

[0048] Description of reference numerals:

[0049] a, raw material; b, first slurry; c, first filtered mother liquor; d, first crystals; e, first para-xylene product; f, first eluate; g, second slurry; h, second filtered mother liquor; i, second crystals; j, second para-xylene product; k, second eluate;

[0050] 1. First crystallizer; 2. First solid-liquid separator; 3. First melter; 4. Second crystallizer; 5. Second solid-liquid separator; 6. Second melter. DETAILED DESCRIPTION

[0051] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although 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. Rather, 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.

[0052] Exemplary embodiments will be described in detail herein, with examples illustrated in the accompanying drawings. In the following description, when referring to the drawings, identical numerals in different figures represent identical or similar elements, unless otherwise indicated. The embodiments described in the following exemplary embodiments are not intended to represent all possible embodiments consistent with the present disclosure. Rather, they are merely examples of apparatus and methods consistent with certain aspects of the present disclosure, as detailed in the appended claims.

[0053] In the description of this disclosure, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate the description of this disclosure and simplify the description. They do not indicate or imply that the devices or components referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limitations on 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.

[0054] In the description of the present disclosure, it should be explained that, unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connecting" should be understood in a broad sense, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through intermediate medium, or the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present disclosure can be understood according to the specific circumstances.

[0055] The inventors found that in the related art, mixed xylene is treated by adopting one crystallization stage and one aging stage. The crystallizer is mainly responsible for recovering p-xylene, and the p-xylene crystals produced thereby are sent to the aging kettle. The crystal slurry of the aging kettle is subjected to solid-liquid separation and product washing to obtain a p-xylene product with a required purity. This method has a simple process flow, but due to a large temperature difference between the saturation temperature of the raw material and the operating temperature of the crystallizer, small crystal particle size is easily caused, and small particle size crystals are not conducive to solid-liquid separation, a large amount of product washing filter cake is required, thereby increasing the consumption of the product.

[0056] Based on the above problems, the embodiments of the present application disclose a two-stage production process and system of p-xylene, which solves the problem of large consumption of product washing liquid caused by low washing efficiency in the production of p-xylene by the existing production process. The various specific embodiments of the two-stage production system and process of p-xylene provided by the embodiments of the present disclosure will be described in detail below.

[0057] In a first aspect, the embodiments of the present application provide a two-stage production process of p-xylene, referring to Figure 1 , comprising:

[0058] S101. Cooling and crystallizing raw material a containing p-xylene under a first temperature condition to obtain first crystal slurry b;

[0059] S102. Subjecting the first crystal slurry b to solid-liquid separation to obtain first crystals d and first filter mother liquor c;

[0060] S103. Returning part of the first filter mother liquor c to the first temperature condition for continuous cooling and crystallization, and cooling and crystallizing the remaining first filter mother liquor c under a second temperature condition to obtain second crystal slurry g;

[0061] S104. Washing the first crystals d with raw material a and p-xylene product as washing liquid in sequence;

[0062] S105. Melting the first crystals d after washing to obtain first p-xylene product e;

[0063] S106. Subjecting the second crystal slurry g to solid-liquid separation to obtain second crystals i and second filter mother liquor h;

[0064] S107. Returning part of the second filtered mother liquor h to the second temperature condition to continue cooling and crystallizing, and discharging the remaining second filtered mother liquor h from the system;

[0065] S108. Using raw material a and paraxylene product as washing liquid, washing the second crystal i in sequence;

[0066] S109. Melt the washed second crystals i to obtain a second paraxylene product j.

[0067] The operating temperature of the first temperature condition is greater than the operating temperature of the second temperature condition. Specifically, the operating temperature range of the first temperature condition is 6°C to 12°C, and the operating temperature range of the second crystallizer 4 is 0°C to 8°C.

[0068] Specifically, in an optional embodiment, in steps S101 to S107, the concentration of p-xylene in the raw material is greater than 98%. The purity of the first p-xylene product e and the second p-xylene product j is not less than 99.7%. Preferably, the purity of the first p-xylene product e and the second p-xylene product j is not less than 99.8%.

[0069] In an optional embodiment, after separating the first filtered mother liquor c and the first crystals d, in step S104, the first crystals d are first washed with raw material a and then washed with a para-xylene product to obtain a first eluate f. Accordingly, in step S108, the second crystals i are first washed with raw material a and then washed with a para-xylene product to obtain a second eluate k. Both the first eluate f and the second eluate k are returned to the first temperature and continue cooling crystallization. Using raw material a for washing can increase the crystal temperature and reduce the impurity inclusion caused by explosive nucleation during the product washing stage. At the same time, washing with raw material a can initially discharge some impurities, reducing the amount of product washing liquid used, thereby greatly improving production efficiency. Furthermore, the solid-liquid separation of the slurry and the washing of the crystals can be integrated into a single device, greatly reducing energy consumption.

[0070] Furthermore, in an optional embodiment, after melting the first crystals d to obtain the first paraxylene product e, a portion of the first paraxylene product e is returned to the first crystals d from the previous step as a washing liquid to wash the first crystals d. The resulting first eluate f is then returned to the first temperature and subsequently cooled and crystallized. The first paraxylene product e returned to the first temperature accounts for approximately 10% to 20% of the total crystal volume. Using the first paraxylene product e to wash the first crystal slurry b can improve the purity of the resulting first crystals d and reduce impurities adhering to the surface of the first crystals d.

[0071] In an optional embodiment, in step S103, 30% to 50% of the first filtered mother liquor c is returned to the first temperature condition, which is conducive to promoting the continued growth of crystals in the first temperature condition and improving subsequent separation efficiency. The remaining portion of the first filtered mother liquor c is then brought to the second temperature condition for cooling crystallization.

[0072] Accordingly, after the second slurry g undergoes solid-liquid separation to obtain second crystals i and second filtered mother liquor h, 60% to 80% of the second filtered mother liquor h is returned to the second temperature condition for further cooling and crystallization, which promotes the continued growth of crystals at the second temperature condition and improves subsequent separation efficiency. The remaining second filtered mother liquor h is discharged from the crystallization system.

[0073] Correspondingly, in step S109, after the second crystal i is melted to obtain the second para-xylene product j, 10% to 20% of the second para-xylene product j is returned to the second crystal i in the previous step as a washing liquid to wash the second crystal i to obtain a second eluate k. Finally, the second eluate k is returned to the first temperature condition to continue cooling and crystallizing.

[0074] It should be noted that in actual production processes, in the initial state, a pure p-xylene product can be used to wash the crystals. After the corresponding products are generated, the corresponding p-xylene products are then used for washing. For example, in the embodiments 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.

[0075] In the second aspect, the embodiment of the present application discloses a secondary production system for p-xylene, which is used to implement the secondary production process of p-xylene as described in the first aspect, with reference to Figure 2 The multi-stage washing production system of paraxylene includes a primary crystallization device and a secondary crystallization device. The primary crystallization device includes a first crystallizer 1, a first solid-liquid separator 2 and a first melter 3, and the secondary crystallization device includes a second crystallizer 4, a second solid-liquid separator 5 and a second melter 6.

[0076] It should be noted that the operating temperatures of the first crystallizer 1 and the second crystallizer 4 decrease sequentially. Specifically, the temperature range of the first crystallizer 1 is 6°C to 12°C, and the temperature range of the second crystallizer 4 is 0°C to 8°C. Thus, while maintaining product purity, the overall crystallization energy consumption is reduced by using a step-by-step cooling crystallization method.

[0077] It should be noted that in this embodiment, the first solid-liquid separator 2 and the second solid-liquid separator 5 are integrated devices that can not only achieve solid-liquid separation of the slurry, but also perform further washing and filtering operations on the separated crystals, which is beneficial to reducing overall energy consumption.

[0078] Specifically, refer to Figure 2 The outlet of the first crystallizer 1 is connected to the inlet of the first solid-liquid separator 2, which is further connected to the inlet of the first melter 3. The first crystallizer 1 is used to cool and crystallize the para-xylene-containing feedstock a under a first temperature condition to obtain a first slurry b; the first solid-liquid separator 2 is used to perform solid-liquid separation on the first slurry b and wash first crystals d; and the first melter 3 is used to melt the first crystals d to obtain a first para-xylene product e.

[0079] 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 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.

[0080] In an alternative embodiment, referring 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 crystals d in the first solid-liquid separator 2 to obtain a first eluate 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 eluate f to the first crystallizer 1 for continued cooling and crystallization.

[0081] Further, in an optional embodiment, referring to Figure 2 The outlet of the second crystallizer 4 is connected to the inlet of the second solid-liquid separator 5, which is in turn 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 the second temperature condition to obtain a second slurry g; the second solid-liquid separator 5 is used to perform solid-liquid separation in the second slurry g and to wash the second crystals i.

[0082] In an alternative embodiment, referring to Figure 2 The outlet of the second solid-liquid separator 5 is also connected to the inlet of the second crystallizer 4, so as to return a portion of the second filtered mother liquor h to the second crystallizer 4, and discharge the remaining portion from the crystallization system.

[0083] In an alternative embodiment, referring to Figure 2The outlet of the second melter 6 is connected to the inlet of the second solid-liquid separator 5, for returning a portion of the second para-xylene product j to the second solid-liquid separator 5 for washing the second crystals i to obtain a second eluate k. The outlet of the second solid-liquid separator 5 is connected to the first crystallizer 1, for returning the second eluate k to the first crystallizer 1 for continued cooling and crystallization.

[0084] In summary, the secondary production process and system of p-xylene in the embodiment of the present application adopts step-by-step cooling crystallization. The slurry of the two crystallization levels can directly produce products after washing the raw materials and products. The secondary crystallization washing process is more suitable for raw materials with a p-xylene concentration greater than 98%, and has the characteristics of simple process and high production efficiency. After filtering the slurry, the filtered mother liquor is first discharged from the solid-liquid separator, and then the crystals are washed in the solid-liquid separator, that is, the filtering and separation of the slurry and the washing process of the crystals are integrated in a solid-liquid separator, which can greatly reduce energy consumption while ensuring product yield. The relatively high concentration eluate and the relatively low concentration filtered mother liquor are treated separately, the filtered mother liquor poor in p-xylene goes to the next section, and the eluate rich in p-xylene returns to the crystallizer to continue cooling crystallization, so as to stabilize the product purity and output and reduce the loss of p-xylene. Before washing the product, using raw materials to wash the crystal slurry can increase the crystal temperature, reduce the impurity inclusion caused by explosive nucleation in the crystal during the product washing stage, and discharge some impurities first, which can improve the product washing efficiency and reduce the amount of product washing liquid used, thereby greatly improving production efficiency.

[0085] Below, the secondary production process and system of paraxylene in the embodiment of the present application are described in detail through a specific embodiment:

[0086] A C8 mixture containing 98% paraxylene as feedstock a (at a flow rate of 4000 kg / h) was fed into the first crystallizer 1 for cooling crystallization at a crystallization temperature of 11°C. The resulting first slurry b entered the first solid-liquid separator 2 for solid-liquid separation, yielding first crystals d and a first filtered mother liquor c. 40% by weight of the first filtered mother liquor c was returned to the first crystallizer 1 for further cooling crystallization, while the remainder entered the second crystallizer 4 for further cooling crystallization. The first crystals d were washed sequentially in the first solid-liquid separator 2 with feedstock a (at a flow rate of 523 kg / h) and a paraxylene product (at a flow rate of 523 kg / h). The resulting eluate f was then filtered to yield a first eluate f, which was returned to the first crystallizer 1 for further recrystallization. After washing with feedstock 1 and the paraxylene product, the first crystals d were melted in the first melter 3. 15% by weight of the first paraxylene product e was returned to the first solid-liquid separator 2 as a wash liquid to wash the first crystals d within the first solid-liquid separator 2. The remaining first paraxylene product e exited the crystallization system as the product.

[0087] Approximately 60% by weight of the first filtered mother liquor c enters the second crystallizer 4 for cooling and crystallization at a crystallization temperature of 7°C. The resulting second slurry g enters the second solid-liquid separator 5 for solid-liquid separation, yielding a second filtered mother liquor h and second crystals i. Approximately 70% by weight of the second filtered mother liquor h returns to the second crystallizer 4, with the remainder discharged from the crystallization system. The second crystals i are washed sequentially in the second solid-liquid separator 5 with feedstock a (at a flow rate of 288 kg / h) and a para-xylene product (at a flow rate of 288 kg / h). The second eluate k is then filtered and returned to the first crystallizer 1 for recrystallization. The washed second crystals i are melted in the second melter 6. Approximately 15% of the second para-xylene product j is returned to the second solid-liquid separator 5 as a wash liquid to wash the second crystals i. The remainder exits the crystallization system as product.

[0088] Comparative Example

[0089] A C8 mixture containing 95% paraxylene as feedstock a (at a flow rate of 4000 kg / h) was fed into the first crystallizer 1 for cooling crystallization at a crystallization temperature of 9°C. The resulting first slurry b entered the first solid-liquid separator 2 for solid-liquid separation, yielding first crystals d and a first filtered mother liquor c. 20% by weight of the first filtered mother liquor c was returned to the first crystallizer 1 for further cooling crystallization, while the remainder entered the second crystallizer 4 for further cooling crystallization. The first crystals d were washed sequentially in the first solid-liquid separator 2 with feedstock a (at a flow rate of 743 kg / h) and the paraxylene product (at a flow rate of 502 kg / h). The resulting eluate f was then filtered to yield a first eluate f, which was returned to the first crystallizer 1 for further recrystallization. After washing with feedstock 1 and the paraxylene product, the first crystals d were melted in the first melter 3. 40% by weight of the first paraxylene product e was returned to the first solid-liquid separator 2 as a wash liquid to wash the first crystals d within the first solid-liquid separator 2. The remaining first paraxylene product e exited the crystallization system as the product.

[0090] Approximately 80% by weight of the first filtered mother liquor c enters the second crystallizer 4 for cooling and crystallization at a crystallization temperature of 7°C. The resulting second slurry g enters the second solid-liquid separator 5 for solid-liquid separation, yielding a second filtered mother liquor h and second crystals i. Approximately 75% by weight of the second filtered mother liquor h returns to the second crystallizer 4, with the remainder discharged from the crystallization system. The second crystals i are washed sequentially in the second solid-liquid separator 5 with feedstock a (at a flow rate of 1338 kg / h) and a para-xylene product (at a flow rate of 1485 kg / h). The second eluate k is then filtered and returned to the first crystallizer 1 for recrystallization. The washed second crystals i are melted in the second melter 6. Approximately 40% of the second para-xylene product j is returned to the second solid-liquid separator 5 as a wash liquid to wash the second crystals i. The remainder exits the crystallization system as product.

[0091] As shown in the comparative examples above, for feeds below the concentration range, the wash ratio required to obtain a qualified product is significantly increased. This result is not predictable or achievable through conventional optimization or simple condition optimization. The type of wash solution, number of stages, and process structure are fundamental and critical to the efficient processing of feeds of varying concentrations.

[0092] The above description includes examples of one or more embodiments. Of course, it is not possible to describe all possible combinations of components or methods for the purpose of describing the above embodiments, but it will 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, to the extent that the term "comprising" is used in the specification or claims, the term is intended to be encompassed in a manner similar to the term "including", as explained by the use of "including" as a transitional word in the claims. In addition, any use of the term "or" in the specification of the claims is intended to mean a "non-exclusive or".

Claims

1. A secondary production process for p-xylene, characterized in that: include: Cooling and crystallizing the raw material containing paraxylene 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 a portion 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 slurry; Using the raw material and the p-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 slurry to obtain second crystals and a second filtered mother liquor; returning a portion of the second filtered mother liquor to the second temperature condition to continue cooling and crystallizing, and discharging the remaining second filtered mother liquor from the system; washing the second crystals in sequence using the raw material and the p-xylene product as washing liquids; melting the washed second crystals to obtain a second p-xylene product; An operating temperature of the first temperature condition is greater than an operating temperature of the second temperature condition.

2. The secondary production process of p-xylene according to claim 1, characterized in that: After obtaining the first p-xylene product, returning part of the first p-xylene product as a washing liquid to the first crystals in the previous step to wash the first crystals; After obtaining the second para-xylene product, part of the second para-xylene product is returned to the second crystals in the previous step as washing liquid to wash the second crystals.

3. The secondary production process of p-xylene according to claim 2, characterized in that: returning 10% to 20% of the first p-xylene product as a washing liquid to the first crystals in the previous step to wash the first crystals; 10% to 20% of the second p-xylene product is returned to the second crystals of the previous step as washing liquid to wash the second crystal slurry.

4. The secondary production process of p-xylene according to claim 2, characterized in that: Washing the first crystals with the raw material and the paraxylene product in sequence as washing liquids to obtain a first eluate, and returning the first eluate to the first temperature condition for further cooling and crystallization; The second crystals are washed in sequence with the raw material and the paraxylene product as washing liquids to obtain a second eluate, and the second eluate is returned to the first temperature condition for further cooling and crystallization.

5. The secondary production process of p-xylene according to claim 1, characterized in that: The concentration of p-xylene in the raw material is greater than 98%.

6. The secondary production process of p-xylene according to claim 1, characterized in that: The operating temperature range of the first temperature condition is 6°C to 12°C, and the operating temperature range of the second temperature condition is 0°C to 8°C.

7. The secondary production process of p-xylene according to claim 1, characterized in that: returning 30% to 50% of the first filtered mother liquor to the first temperature condition for further cooling and crystallization; 60% to 80% of the second filtered mother liquor is returned to the second temperature condition to continue cooling and crystallizing.

8. A secondary production system for p-xylene, for implementing the secondary production process for p-xylene according to any one of claims 1 to 7, characterized in that: It includes a primary crystallization device and a secondary crystallization device; The primary crystallization device includes a first crystallizer, a first solid-liquid separator and a first melter, and the secondary crystallization device includes a second crystallizer, a second solid-liquid separator and a second melter; The operating temperature of the second crystallizer is lower than the operating temperature of the first crystallizer; 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 crystallizer outlet is connected to the first solid-liquid separator inlet, and the first solid-liquid separator is used to perform solid-liquid separation and washing on the first slurry; The outlet of the first solid-liquid separator is communicated with the inlet of the first melter, and the first melter is used to melt the first crystals to obtain a first paraxylene product; The first solid-liquid separator outlet is also connected to the first crystallizer inlet and the second crystallizer inlet; The outlet of the second crystallizer is connected to the inlet of the second solid-liquid separator, and 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 slurry; The second solid-liquid separator is used for solid-liquid separation and washing of the second slurry; The outlet of the second solid-liquid separator is communicated with the inlet of the second melter, and the second melter is used to melt the second crystals to obtain a second paraxylene product; The second solid-liquid separator outlet is also communicated with the second crystallizer inlet.

9. The secondary production system of paraxylene according to claim 8, characterized in that: The first melter outlet is also connected to the first solid-liquid separator inlet, and is used to return part of the first paraxylene product as a washing liquid to the first crystals in the previous step to wash the first crystals; The second melter outlet is also connected to the second solid-liquid separator inlet for returning part of the second paraxylene product as washing liquid to the second crystals of the previous step to wash the second crystals.

10. The secondary production system of paraxylene according to claim 8, characterized in that: The first solid-liquid separator outlet is connected to the first crystallizer inlet, and is used to return the first eluate to the first temperature condition for further cooling and crystallization; The outlet of the second solid-liquid separator is communicated with the inlet of the first crystallizer, and is used to return the second eluate to the first temperature condition for further cooling and crystallization.

Images