Secondary production process and system of p-xylene

Abstract

The invention provides a secondary production process and system for p-xylene, and the secondary production process 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; sequentially washing the first crystal by using the raw material and the p-xylene product as washing liquid; melting the first crystal to obtain a first p-xylene product; carrying out solid-liquid separation on the second crystal mush to obtain a second crystal and a second filtered mother solution; returning part of the second filtered mother liquor to a second temperature condition for continuous cooling crystallization; sequentially washing the second crystal by using the raw material and the p-xylene product as washing liquid; and melting the washed second crystal to obtain a second p-xylene product. According to the secondary production process of p-xylene, the problem of large product washing liquid consumption caused by low washing efficiency during production of p-xylene in an existing production process can be solved.

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, and its products are widely used in various industries. In the paraxylene production process, separation of mixed xylene (commonly known as C8 fraction) is currently recognized by the industry as a key technical point and technical difficulty. Mixed xylene usually includes: paraxylene (PX), metaxylene (MX), orthoxylene (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 direct distillation method to separate the system requires high energy consumption and high cost; but 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 separation process, the main component paraxylene is precipitated from the solution, while other xylenes are present in the solution. High-purity paraxylene can be obtained by completely separating the paraxylene crystals from the xylene solution. However, since it is difficult to completely separate the solid-liquid phases, and the purity of paraxylene is generally required to be >99.7%, many post-processing processes 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 secondary production process and system of p-xylene, and adopts the following technical solutions:

[0006] In a first aspect, the present application discloses a secondary production process of 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 first crystals washed with the raw material and the para-xylene product to obtain a first para-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 to continue cooling and crystallizing, and discharging the remaining second filtered mother liquor out of the system;

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

[0015] Melting the second crystals washed with the raw material and the para-xylene product to obtain a second para-xylene product;

[0016] The operating temperature of the first temperature condition is greater than the 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 para-xylene product is returned to the first crystals of the previous step as washing liquid to wash the first crystals;

[0020] 10% to 20% of the second para-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 para-xylene 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 raw material and the paraxylene product are used as washing liquids to wash the second crystals in sequence to obtain a second eluate, and the second eluate is returned to the first temperature condition to continue 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 comprises a first crystallizer, a first solid-liquid separator and a first melter, and the secondary crystallization device comprises 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 communicated with the first solid-liquid separator inlet, and the first solid-liquid separator is used to separate the first slurry;

[0032] The first solid-liquid separator outlet is communicated with the first melter inlet, 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 para-xylene product as a washing liquid to wash the first crystal 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 crystal 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 for washing the second crystal slurry with a portion of the second paraxylene product as a washing liquid.

[0040] Optionally, the first melter outlet is also connected to the first solid-liquid separator inlet, so as to return part of the first para-xylene product as washing liquid to the first crystals of 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 connected to 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 of p-xylene, which is suitable for raw materials with a p-xylene concentration greater than 98%. It adopts step-by-step cooling and crystallization. After the slurry of the two crystallization stages is washed with raw materials and products, the product can be directly produced. 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 from 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 a solid-liquid separator, which can greatly reduce energy consumption while ensuring the 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 and crystallization, so as to stabilize the product purity and output and reduce the loss of p-xylene. Before product washing, using raw materials to wash 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 product washing efficiency and reduce the amount of product washing liquid, thereby greatly improving production efficiency. Due to the high feed concentration, a high recovery rate can be obtained without a low second-stage crystallization temperature, which also allows the second-stage crystallizer to directly wash out qualified products, thereby reducing the circulation volume of logistics within the process and achieving higher production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1It is a schematic flow 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 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;

[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] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] The inventors found that in the related art, mixed xylene is treated by a crystallization stage and an aging stage. The crystallizer is mainly responsible for recovering p-xylene, and its output p-xylene crystals go to the aging kettle. After solid-liquid separation and product washing, the crystal slurry in the aging kettle obtains a p-xylene product with purity that meets the requirements. The process of this method is simple, but due to the large temperature difference between the saturation temperature of the raw material and the operating temperature of the crystallizer, it is easy to cause the crystal particles to have a small particle size, and small-sized crystals are not conducive to solid-liquid separation, and 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 secondary 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 secondary production system and process for para-xylene provided in the embodiments of the present disclosure.

[0057] In the first aspect, the present application embodiment provides a secondary production process of p-xylene, referring to Figure 1 ,include:

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

[0059] S102. performing solid-liquid separation on the first slurry b to obtain first crystals d and a first filtered mother liquor c;

[0060] 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;

[0061] S104. Using the raw material a and the para-xylene product as washing liquids, sequentially washing the first crystal d;

[0062] S105. Melting the washed first crystals d to obtain a first paraxylene product e;

[0063] S106. performing solid-liquid separation on the second slurry g to obtain second crystals i and a second filtered 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 the raw material a and the para-xylene product as washing liquids, sequentially washing the second crystal i;

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

[0067] Among them, 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 the first filtered mother liquor c and the first crystal d are separated, in step S104, the first crystal d is first washed with raw material a, and then washed with a para-xylene product to obtain a first eluate f. Correspondingly, in step S108, the second crystal i is first washed with raw material a, and then washed with a para-xylene product to obtain a second eluate k, and both the first eluate f and the second eluate k are returned to the first temperature condition to continue cooling and crystallization. Using raw material a for washing can increase the crystal temperature and reduce the impurity inclusion caused by explosive nucleation in the product washing stage. At the same time, the washing of raw material a can first discharge some impurities, reduce the amount of product washing liquid, and thus greatly improve production efficiency. In addition, the solid-liquid separation of the slurry and the washing operation of the crystal can be integrated in one device to complete, greatly reducing energy consumption.

[0070] Further, in an optional embodiment, 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, and the obtained first eluate f is returned to the first temperature condition to continue cooling and crystallization. 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.

[0071] In an optional embodiment, in step S103, for the first filtered mother liquor c, 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 the subsequent separation efficiency. The remaining first filtered mother liquor c enters the second temperature condition for cooling and crystallization.

[0072] Correspondingly, after the second crystal slurry g undergoes solid-liquid separation to obtain the second crystal i and the second filtered mother liquor h, 60% to 80% of the second filtered mother liquor h is returned to the second temperature condition to continue cooling and crystallizing, which is conducive to promoting the continued growth of the crystals at the second temperature condition and improving the 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, and finally the second eluate k is returned to the first temperature condition to continue cooling and crystallization.

[0074] 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.

[0075] In a second aspect, the present application discloses a secondary production system for p-xylene, which is used to implement the secondary production process for 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 are successively reduced. 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, 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.

[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 further wash and filter 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, and the outlet of the first solid-liquid separator 2 is further connected to the inlet of the first melter 3. The first crystallizer 1 is used to cool and crystallize the raw material a containing 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.

[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 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.

[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, so as to return part of the first paraxylene product e to the first solid-liquid separator 2, so as to wash the first crystals d in the first solid-liquid separator 2 and obtain the first eluate f. The outlet of the first solid-liquid separator 2 is also connected to the inlet of the first crystallizer 1, so as to return the first eluate f to the first crystallizer 1 for further 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, and the outlet of the second solid-liquid separator 5 is connected to the inlet of the second melter 6. The second crystallizer 4 is used to cool and crystallize part of the first filtered mother liquor c under the second temperature condition to obtain the 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.

[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 part of the second filtered mother liquor h to the second crystallizer 4, and discharge the remaining part 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, 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 to obtain the second eluate k. The outlet of the second solid-liquid separator 5 is connected to the first crystallizer 1, so as to return the second eluate k to the first crystallizer 1 for further 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 the product yield. The relatively high concentration eluate is treated separately from the relatively low concentration filtered mother liquor, 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 and 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 with a paraxylene content of 98% is added as raw material a (flow rate of 4000 kg / h) to the first crystallizer 1 for cooling and crystallization. The crystallization temperature is 11°C. The obtained first crystal slurry b enters the first solid-liquid separator 2 for solid-liquid separation to obtain the first crystal d and the first filtered mother liquor c. 40% of the weight of the first filtered mother liquor c returns to the first crystallizer 1 for further cooling and crystallization, 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 523 kg / h) and paraxylene product (flow rate of 523 kg / h) in sequence, and then filtered to obtain the first eluate f, which is returned to the first crystallizer 1 for further recrystallization. The first crystal d washed with raw material 1 and paraxylene product is melted in the first melter 3, and 15% of the weight of the first paraxylene product e is returned to the first solid-liquid separator 2 as washing liquid to wash the first crystal d in the first solid-liquid separator 2, and the remaining first paraxylene product e leaves the crystallization system as a product.

[0087] About 60% 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. About 70% by weight of the second filtered mother liquor h returns to the second crystallizer 4, and the remaining part is discharged from the crystallization system; the second crystal i is washed in the second solid-liquid separator 5 by raw material a (flow rate of 288 kg / h) and para-xylene product (flow rate of 288 kg / h), filtered 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 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, and the remaining part leaves the crystallization system as a product.

[0088] Comparative Example

[0089] A C8 mixture with a paraxylene content of 95% is added as raw material a (flow rate of 4000 kg / 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 for solid-liquid separation to obtain the first crystal d and the first filtered mother liquor c. 20% of the weight of the first filtered mother liquor c returns to the first crystallizer 1 for further cooling and crystallization, 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 743 kg / h) and paraxylene product (flow rate of 502 kg / h) in sequence, and then filtered to obtain the first eluate f, which is returned to the first crystallizer 1 for further recrystallization. The first crystal d washed with raw material 1 and paraxylene product is melted in the first melter 3, and 40% of the weight of the first paraxylene product e is returned to the first solid-liquid separator 2 as washing liquid to wash the first crystal d in the first solid-liquid separator 2, and the remaining first paraxylene product e leaves the crystallization system as a product.

[0090] About 80% 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. About 75% by weight of the second filtered mother liquor h returns to the second crystallizer 4, and the remaining part is discharged from the crystallization system; the second crystal i is washed in the second solid-liquid separator 5 by raw material a (flow rate of 1338 kg / h) and para-xylene product (flow rate of 1485 kg / h), filtered 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 about 40% 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, and the remaining part leaves the crystallization system as a product.

[0091] From the above comparative examples, it can be seen that for feeds below the concentration range, the washing ratio required to obtain qualified products is greatly increased. This result is not predictable by conventional condition optimization or can be solved by simple condition optimization. The type of washing liquid, the number of stages and the process structure are more basic and critical for the efficient treatment of feeds of different concentrations.

[0092] 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 secondary production process for p-xylene, 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 to continue cooling and crystallizing, and discharging the remaining second filtered mother liquor out of the system; 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; The operating temperature of the first temperature condition is greater than the operating temperature of the second temperature condition.

2. The secondary 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; 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 paraxylene according to claim 2, characterized in that: returning 10% to 20% of the first para-xylene product as washing liquid to the first crystals in the previous step to wash the first crystals; 10% to 20% of the second para-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 paraxylene according to claim 2, characterized in that: Using the raw material and the paraxylene product as washing liquids to wash the first crystals in sequence to obtain a first eluate, and returning the first eluate to the first temperature condition to continue cooling and crystallizing; The raw material and the paraxylene product are used as washing liquids to wash the second crystals in sequence to obtain a second eluate, and the second eluate is returned to the first temperature condition to continue cooling and crystallization.

5. The secondary production process of paraxylene 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 of p-xylene, used for implementing the secondary production process of 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 comprises a first crystallizer, a first solid-liquid separator and a first melter, and the secondary crystallization device comprises 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 first solid-liquid separator outlet is communicated with the first melter inlet, 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 crystal 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 connected to the second crystallizer inlet.

9. The secondary production system of paraxylene according to claim 8, characterized in that: The first melter outlet is also 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 of the previous step, so as 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 connected to the inlet of the first crystallizer, and is used to return the second eluate to the first temperature condition for further cooling and crystallization.

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