A separation device, separation method and application of separating mixed xylene by direct cooling crystallization

Abstract

This invention provides a separation apparatus, method, and application for separating mixed xylenes using a direct cooling crystallization process. The separation apparatus includes a crystallizer and a refrigerant pipeline connected to the side wall of the crystallizer. The refrigerant pipeline includes a horizontal straight section and an inclined section. The angle β formed by the inclined section of the refrigerant pipeline and the vertical line at the inlet of the crystallizer wall is 20–80°, and the angle α formed by the inclined section and the horizontal tangent at the refrigerant inlet of the crystallizer is 20–90°. Multiple refrigerant inlets are used, and the refrigerant liquid is dispersed by a liquid disperser at the inlet before entering the crystallizer. The high-speed entering refrigerant liquid agitates the material within the crystallizer, and a stirrer is installed inside the crystallizer. The technical solution provided by this invention improves heat exchange efficiency and effectively solves the problems of low indirect heat exchange efficiency, high precision requirements for the scraper during crystallizer preparation, and high investment costs in existing crystallization processes.

Description

Technical Field

[0001] This invention belongs to the field of mixture separation technology, and relates to a separation device, separation method and application of direct cooling crystallization separation of mixed xylenes. Specifically, it relates to the use of crystallizer and process design to ensure long-term stable operation of the crystallization separation process. Background Technology

[0002] Because the boiling points of the mixed xylene components are close, conventional separation methods cannot effectively separate them. Currently, the commonly used methods for separating mixed xylenes are adsorption separation and crystallization separation.

[0003] Compared to adsorption separation, crystallization offers advantages such as high product purity and a simple process flow. However, its drawback is low recovery rate. To improve recovery, crystallization is typically conducted at low temperatures. Currently, crystallization cooling is indirect, resulting in lower heat exchange efficiency. Furthermore, the size of the crystallizer itself limits the increase in heat exchange area, hindering the large-scale development and application of crystallizers. Therefore, direct cooling will effectively improve the efficiency of crystallization processes.

[0004] Patent CN104030880A discloses a method for direct cooling crystallization and separation of para-xylene, specifically addressing the problems of complex processes, numerous pieces of equipment, and high investment costs in existing technologies. This process involves directly introducing an inert cryogenic liquid into a crystallizer containing mixed xylene feedstock for direct heat exchange, causing the inert cryogenic liquid to vaporize and cool the feedstock, including the mixed xylene. While this method effectively improves heat exchange efficiency, the refrigerant is not effectively distributed after entering the crystallizer, leading to localized overcooling. This results in larger crystallized material with more impurities, affecting product purity.

[0005] Patent CN108794295A discloses a method for crystallizing and separating paraxylene. In this method, a condenser tube is installed on the crystallizer, with tubes extending both inside and outside the crystallizer. The tube inside the crystallizer has several micro-perforations for liquid flow. This micro-perforation design allows the inert coolant to gradually and slowly seep into the crystallizer from different heights during micro-flow, minimizing disturbance to the raw materials and promoting crystal production, thus improving crystal purity and product yield. While this process design uses a distribution pipe to effectively distribute the inert coolant, the relatively small disturbance within the system means that crystalline material can easily precipitate and accumulate on the crystallizer tubes, hindering continuous crystallization production. Summary of the Invention

[0006] The present invention addresses the problems of low refrigeration efficiency, difficulty in crystallizer manufacturing, and limited heat exchange area in current crystallization production processes that employ indirect refrigeration. To resolve these issues, a separation device and method for separating mixed xylenes during crystallization are provided. The mixed xylenes are primarily composed of C8 aromatic hydrocarbons and may contain small amounts of benzene, ethylbenzene, etc.

[0007] One objective of this invention is to provide a separation apparatus for separating mixed xylenes using a direct cooling crystallization method. The separation apparatus mainly includes a crystallizer and supporting facilities.

[0008] According to the present invention, the separation device includes: a crystallizer and a refrigerant pipeline connected to the side wall of the crystallizer, the refrigerant pipeline including a horizontal straight section and an inclined section, the inclined section of the refrigerant pipeline being inclined downwards and connected to the refrigerant inlet of the crystallizer.

[0009] According to the present invention, in the separation device for separating mixed xylenes by direct cooling crystallization:

[0010] The crystallizer can be a crystallization separation device commonly used in the prior art; for example, the tank of the crystallizer is cylindrical.

[0011] The angle β formed by the inclined section of the refrigerant pipeline and the vertical line at the inlet of the crystallizer wall is 20 to 80°. For example, it can be 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80° or any value between the above two values. Preferably, the angle β formed by the inclined section of the refrigerant pipeline and the vertical line at the inlet of the crystallizer wall is 30 to 75°.

[0012] The angle α formed by the inclined section of the refrigerant pipeline and the horizontal tangent at the refrigerant inlet of the crystallizer is 20-90°. For example, it can be 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80°, 85°, 90° or any value between the above two. Preferably, the angle α formed by the inclined section of the refrigerant pipeline and the horizontal tangent at the refrigerant inlet of the crystallizer is 30-90°.

[0013] In this invention, the downward feeding method using an inclined tube section can effectively reduce the problem of crystal precipitation and deposition at the refrigerant inlet, and prevent pipe blockage.

[0014] In a preferred embodiment, the inclined pipe section of the refrigerant pipeline adopts a reduced diameter design. Preferably, the ratio of the pipe diameter at the end of the inclined pipe section to the pipe diameter at the beginning of the inclined pipe section is 1:(1.05~2). For example, it can be 1:1.05, 1:1.1, 1:1.15, 1:1.2, 1:1.25, 1:1.3, 1:1.35, 1:1.4, 1:1.45, 1:1.5, 1:1.55, 1:1.6, 1:1.65, 1:1.7, 1:1.75, 1:1.8, 1:1.85, 1:1.9, 1:1.95, 1:2, or any value between the above two values. Preferably, the ratio of the pipe diameter at the end of the inclined pipe section to the pipe diameter at the beginning of the inclined pipe section is 1:(1.2~1.8).

[0015] In a preferred embodiment, the length of the refrigerant line inclined section is not particularly limited, and the end of the refrigerant line inclined section is optionally connected to a nozzle.

[0016] The end of the inclined section of the refrigerant pipeline is fitted with a porous filling material. This porous filling material can be any commonly used porous material in the prior art; for example, it can be, but is not limited to, commonly used wire mesh filler. According to the present invention, the refrigerant inlet uses an inclined pipe, the inclined pipe section is reduced in diameter, and it is fitted with a porous filling material, which disperses the refrigerant into fine particles before entering the crystallizer and causes the liquid inside the crystallizer to be in a turbulent state.

[0017] According to the present invention, in the separation device for separating mixed xylene by direct cooling crystallization, the side wall of the crystallizer is provided with multiple refrigerant inlets and refrigerant pipelines connected thereto. Preferably, the refrigerant inlets are provided at different heights on the side wall of the crystallizer, and at least two refrigerant inlets are provided on a ring line at the same height on the side wall of the crystallizer. According to an embodiment of the present invention, the refrigerant inlets at the same height on the side wall of the crystallizer are symmetrically or at equal angles. The provision of multiple refrigerant inlets can prevent problems such as refrigerant flow deviation, which could cause axial temperature gradients within the crystallizer and uneven crystallization of materials.

[0018] According to the present invention, in the separation device for separating mixed xylene by direct cooling crystallization, the refrigerant pipelines are provided with parallel branches, and refrigerant heaters are provided on the branches; when the material crystallizes and precipitates and blocks the refrigerant inlet, the heaters on the branches heat and vaporize the refrigerant, and the temperature of the heated gas is 30-100°C, forming a heated gas purge port at the refrigerant inlet, which can effectively melt the blocked crystalline material;

[0019] Of the refrigerant inlets at different heights on the sidewall of the crystallizer, the highest refrigerant inlet is located below the liquid level in the crystallizer. Preferably, the distance between the highest refrigerant inlet and the liquid level is 1 / 8 to 1 / 2 of the length of the straight pipe section of the crystallizer, more preferably 1 / 5 to 1 / 2. The distance between the highest refrigerant inlet and the upper end cap of the straight pipe section of the crystallizer is 1 / 15 to 3 / 5 of the length of the straight pipe section of the crystallizer, preferably 1 / 10 to 1 / 2. The distance between the lowest refrigerant inlet and the lower end cap of the straight pipe section of the crystallizer is 1 / 20 to 2 / 5 of the length of the straight pipe section of the crystallizer, preferably 1 / 15 to 1 / 3.

[0020] According to the present invention, in the separation device for separating mixed xylenes by direct cooling crystallization:

[0021] The crystallizer is provided with a gas phase outlet at the top; the refrigerant absorbs heat and vaporizes in the crystallizer, existing in the crystallizer in the form of bubbles. The gas phase outlet at the top of the crystallizer is used to discharge the vaporized refrigerant. The vaporized refrigerant is distributed in the crystallizer in the form of bubbles, which is beneficial to the uniform mixing of materials. The gaseous refrigerant discharged from the crystallizer can be further reused through a compression system to improve the overall economy.

[0022] The top of the crystallizer is also provided with a material inlet connected to a material pipeline. Preferably, a precooler and a material heat exchanger are provided on the material pipeline. Optionally, a liquid distributor is provided at the end of the material pipeline. The liquid distributor can be in the form of a disc type or a branch type, so that the mixed xylene raw material is evenly distributed.

[0023] A stirrer is provided inside the crystallizer; preferably, the stirrer is a radial stirrer and / or an axial stirrer; the stirrer has a blade type, including but not limited to paddle type and anchor type, and the coolant enters the crystallizer in an oblique shape, which can make the material distribution in the crystallizer uniform and facilitate the uniform precipitation of the crystallized material.

[0024] The crystallizer is provided with a crystallization slurry outlet at the bottom and is connected to a centrifuge through a pipeline; the centrifuge is provided with a centrifugal mother liquor outlet and a centrifugal filter cake outlet. Preferably, the centrifugal mother liquor outlet is connected to a material heat exchanger on the material pipeline through a pipeline and is used as the heat exchange medium of the material heat exchanger.

[0025] A second objective of this invention is to provide a method for separating mixed xylenes using a direct cooling crystallization process. This method employs the separation apparatus described in the first objective of this invention to separate para-xylene from the mixed xylenes. A direct contact cooling process is used, where the refrigerant is directly introduced into the crystallizer to contact and cool the mixed xylenes, effectively improving the heat transfer efficiency between materials.

[0026] According to the present invention, the separation method for separating mixed xylene by direct cooling crystallization includes: pre-cooling the mixed xylene and then contacting it with a refrigerant for heat exchange to obtain a slurry containing para-xylene crystals, and then separating the para-xylene crystals.

[0027] According to the present invention, in the direct cooling crystallization method for separating mixed xylenes:

[0028] Pre-cooling treatment is performed before feeding mixed xylene. The pre-cooling temperature is not lower than the crystallization temperature of mixed xylene. The pre-cooling medium can be air cooling, water cooling, chilled water, etc., which can effectively reduce the amount of high-grade refrigerant used.

[0029] The refrigerant is selected from inert cryogenic liquids, preferably at least one of liquid nitrogen and liquid carbon dioxide;

[0030] In the mixed xylenes, the concentration of p-xylene is not less than 20 wt%, preferably 20 to 90 wt%;

[0031] The mass ratio of the refrigerant to the mixed xylene is 1:(3-10), preferably 1:(4-8);

[0032] The inlet pressure of the refrigerant is 0.01 to 0.5 MPaG, preferably 0.1 to 0.3 MPaG;

[0033] The temperature at which the refrigerant enters the crystallizer is -80 to -10°C, preferably -78 to 0°C;

[0034] The temperature at which the mixed xylene enters the crystallizer is -38 to 20°C, preferably -38 to 10°C.

[0035] The third objective of this invention is to provide a separation apparatus for separating mixed xylenes by direct cooling crystallization as described in the first objective of this invention, or a separation method for separating mixed xylenes by direct cooling crystallization as described in the second objective of this invention, for application in the preparation of para-xylene from mixed xylenes.

[0036] Compared with the prior art, the present invention has the following advantages:

[0037] (1) The present invention adopts a downward feeding method with inclined tube section, which can effectively reduce the problem of crystal precipitation and deposition at the refrigerant inlet and prevent pipe blockage;

[0038] (2) The refrigerant inlet adopts an inclined tube, the inclined tube section is treated with a reduced diameter, and a porous filling material is built in it to disperse the refrigerant into fine particles and enter the crystallizer and make the liquid in the crystallizer in a disturbed state.

[0039] (3) The setting of multiple refrigerant inlets can prevent problems such as refrigerant flow deviation, which can cause axial temperature gradients in the crystallizer and uneven crystallization of materials. Attached Figure Description

[0040] Figure 1 This is a diagram of the separation apparatus for separating mixed xylenes using the direct cooling method in this invention. Figure 1 In the diagram, I is the crystallizer, II is the refrigerant heater, III is the stirrer, IV is the centrifuge, V is the precooler, VI is the material heat exchanger, and VII is the centrifugal pump. 1 is the refrigerant, 2 is the mixed xylene feedstock, 3 is the gaseous refrigerant, 4 is the mixed crystal slurry, 5 is the centrifugal filter cake, 6 is the centrifugal mother liquor, 7 is the mother liquor after heat exchange, and 8 is a branch of the refrigerant pipeline.

[0041] Figure 2 This is a top view of the crystallizer used in this invention. Figure 2 In the diagram, α is the angle between the inclined section of the refrigerant pipeline and the horizontal tangent of the refrigerant inlet on the side wall of the crystallizer, 9 is the side wall of the crystallizer, 10 is the refrigerant inlet on the side wall of the crystallizer, 11 is the horizontal straight section of the refrigerant pipeline, and 12 is the inclined section of the refrigerant pipeline.

[0042] Figure 3 This is a cross-sectional view of the crystallizer sidewall used in this invention. Figure 3 In the diagram, β is the angle between the inclined section of the refrigerant pipeline and the vertical line of the crystallizer sidewall, 9 is the crystallizer sidewall, 10 is the refrigerant inlet of the crystallizer sidewall, 11 is the horizontal straight section of the refrigerant pipeline, and 12 is the inclined section of the refrigerant pipeline. Detailed Implementation

[0043] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0044] Furthermore, various embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention. The resulting technical solutions are part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0045] Unless otherwise specified, the raw materials used in the examples and comparative examples are all disclosed in the prior art, such as those that can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0046] To better understand the present invention, the present invention will be further described below with reference to the embodiments. The following embodiments are only for illustration and not for limiting the present invention.

[0047] According to a preferred embodiment of the present invention, the production apparatus mainly includes a crystallizer and supporting facilities.

[0048] According to a preferred embodiment of the present invention, the crystallizer adopts a direct contact cooling process, in which the refrigerant is directly introduced into the crystallizer for cooling, effectively improving the heat transfer efficiency between materials.

[0049] According to a preferred embodiment of the present invention, the refrigerant pipeline includes a horizontal straight pipe section and an inclined pipe section. The inclined pipe section of the refrigerant pipeline is inclined downward and connected to the refrigerant inlet of the crystallizer. A liquid disperser is provided at the refrigerant inlet of the crystallizer. The inlet pipe is reduced in diameter. The disperser includes, but is not limited to, a pipe-embedded filler, a nozzle, etc., so that the refrigerant is dispersed into fine particles and enters the crystallizer, and the liquid in the crystallizer is in a disturbed state.

[0050] According to a preferred embodiment of the present invention, the refrigerant enters the crystallizer in segments, with no fewer than two segments. Refrigerant inlets are provided at different heights on the sidewall of the crystallizer, and at least two refrigerant inlets are provided on the ring line of the sidewall at the same height. The refrigerant inlets at the same height are symmetrically or at equal angles to prevent problems such as refrigerant flow deviation, which could cause axial temperature gradients in the crystallizer and uneven crystallization of materials.

[0051] According to a preferred embodiment of the present invention, among the refrigerant inlets at different heights on the sidewall of the crystallizer, the highest refrigerant inlet is located below the liquid level of the crystallizer. Preferably, the distance between the highest refrigerant inlet and the liquid level is 1 / 8 to 1 / 2 of the length of the straight pipe section of the crystallizer, more preferably 1 / 5 to 1 / 2. The distance between the highest refrigerant inlet and the upper end cap of the straight pipe section of the crystallizer is 1 / 15 to 3 / 5 of the length of the straight pipe section of the crystallizer, preferably 1 / 10 to 1 / 2. The distance between the lowest refrigerant inlet and the lower end cap of the straight pipe section of the crystallizer is 1 / 20 to 2 / 5 of the length of the straight pipe section of the crystallizer, preferably 1 / 15 to 1 / 3.

[0052] According to a preferred embodiment of the present invention, the angle β formed by the inclined section of the refrigerant pipeline and the vertical line at the inlet of the crystallizer wall is 20 to 80°, preferably 30 to 75°;

[0053] According to a preferred embodiment of the present invention, the angle α formed by the inclined section of the refrigerant pipeline and the horizontal tangent at the refrigerant inlet of the crystallizer is 20 to 90°, preferably 30 to 90°. The downward-sloping feed method of the pipeline opening can effectively reduce the problem of crystal precipitation and deposition at the refrigerant inlet and prevent pipeline blockage.

[0054] According to a preferred embodiment of the present invention, a stirrer is provided inside the crystallizer. The stirrer is radial and / or axial in form, and the stirring paddle is not limited to blade type and anchor type. Combined with the coolant entering the crystallizer in an oblique manner, it can make the material distribution in the crystallizer uniform and facilitate the uniform precipitation of the crystallized material.

[0055] According to a preferred embodiment of the present invention, the refrigerant is an inert cryogenic liquid, including but not limited to at least one of liquid carbon dioxide and liquid nitrogen.

[0056] According to a preferred embodiment of the present invention, the refrigerant absorbs heat and vaporizes in the crystallizer, existing in the crystallizer in the form of bubbles. A gas phase outlet is provided at the top of the crystallizer for discharging the vaporized refrigerant. The vaporized refrigerant is distributed in the crystallizer in the form of bubbles, which is beneficial to the uniform mixing of materials.

[0057] According to a preferred embodiment of the present invention, the gaseous refrigerant discharged from the crystallizer can be further reused through a compression system, thereby improving the overall economy.

[0058] According to a preferred embodiment of the present invention, the concentration of p-xylene in the mixed xylene is not less than 20 wt%.

[0059] According to a preferred embodiment of the present invention, a liquid distributor is provided at the mixed xylene feed inlet. The distributor may be of the form including but not limited to disc type or branch type, so as to make the mixed xylene raw material evenly distributed.

[0060] According to a preferred embodiment of the present invention, the mixed xylene is pre-cooled before feeding, and the pre-cooling temperature is not lower than the crystallization temperature of the mixed xylene. The pre-cooling medium can be air cooling, water cooling, chilled water, or other cooling methods, which can effectively reduce the amount of high-grade refrigerant used.

[0061] According to a preferred embodiment of the present invention, the main component of the mixed xylene is C8 aromatic hydrocarbon, and it may contain small amounts of benzene, ethylbenzene and other components.

[0062] Examples and comparative examples:

[0063] This embodiment illustrates the separation apparatus and method for separating mixed xylenes by direct cooling crystallization provided by the present invention.

[0064]

Example 1

[0065] like Figure 1As shown, a separation device for separating mixed xylene by direct cooling crystallization includes: a crystallizer I and a refrigerant pipeline 1 connected to the side wall of the crystallizer I. The refrigerant pipeline 1 includes a horizontal straight section and an inclined section. The inclined section of the refrigerant pipeline 1 is inclined downwards and connected to the refrigerant inlet of the crystallizer. Multiple refrigerant inlets and connected refrigerant pipelines are provided on the side wall of the crystallizer I. Refrigerant inlets are provided at different heights on the side wall of the crystallizer I, and at least two refrigerant inlets are provided on a loop at the same height on the side wall of the crystallizer I. The refrigerant inlets at the same height are distributed at equal angles. Parallel branches 8 are provided between the refrigerant pipelines 1, and refrigerant heaters II are provided on the branches 8. A gas phase outlet 3 is provided at the top of the crystallizer I; a material inlet connected to a material pipeline 2 is also provided at the top of the crystallizer I. A precooler V and a material heat exchanger VI are provided on the material pipeline 2; a stirrer III is provided inside the crystallizer I. The bottom of crystallizer I is provided with a crystallization slurry outlet, which is connected to centrifuge IV through a pipeline; centrifuge IV is provided with a centrifugal mother liquor outlet 6 and a centrifugal filter cake outlet 5. The centrifugal mother liquor outlet 6 is connected to a material heat exchanger VI on the material pipeline through a pipeline, and is used as the heat exchange medium of the material heat exchanger.

[0066] like Figures 2-3 As shown, the refrigerant pipeline includes a horizontal straight section and an inclined section. The inclined section of the refrigerant pipeline is inclined downwards and connected to the refrigerant inlet of the crystallizer. The angle β formed by the inclined section of the refrigerant pipeline and the vertical line at the inlet of the crystallizer wall is 20-80°, preferably 30-75°. The angle α formed by the inclined section of the refrigerant pipeline and the horizontal tangent at the refrigerant inlet of the crystallizer is 20-90°, preferably 30-90°.

[0067] Using the above Figures 1-3 The separation methods for p-xylene using separation devices and for separating mixed xylenes by direct cooling crystallization include:

[0068] The raw material, mixed xylene (PX purity 21wt%), sequentially passes through the V precooler, VI material heat exchanger, and liquid distributor before entering the crystallizer. The temperature of the mixed xylene entering the crystallizer is -38℃. The refrigerant is liquid carbon dioxide, with an inlet pressure of 0.15 MPaG and a temperature of -75℃. The inlet pipe diameter is 0.8 times the diameter of the pipeline, and the pipe is filled with wire mesh. The crystallizer has three refrigerant sections, with four inlets in each section on the same horizontal loop. Each inlet is designed at a uniform 90° angle, with the inlet pipe tangent to the crystallizer wall at α = 45° and inclined downwards at β = 30°. The height of the straight section of the crystallizer is 15000mm. The distance between the bottom refrigerant inlet and the bottom end cap of the straight section is 1000mm, and the distance between the top refrigerant inlet and the end cap of the upper straight section is 6000mm. The distance from the liquid level in the crystallizer is 4000mm. The middle refrigerant inlet is located between the top and bottom refrigerant inlets. Liquid carbon dioxide and mixed xylene were cooled together (the mass ratio of carbon dioxide to xylene was 0.22:1). After separating the material at the crystallizer outlet, it was found that the purity of PX in the filter cake was 85 wt%, and the filter cake yield accounted for 15% of the total feed.

[0069]

Example 2

[0070] The operating conditions in this embodiment are the same as in embodiment 1. The difference is that two opposite refrigerant inlets are set on the same horizontal loop. After separating the material at the crystallizer outlet, it can be found that the PX purity in the filter cake is 78.5 wt%, and the total yield of the filter cake accounts for 14% of the total feed.

[0071]

Example 3

[0072] The operating conditions in this embodiment are the same as in Embodiment 1, except that two refrigerant inlets are set up. The distance between the lower refrigerant inlet and the lower end cap of the straight pipe section of the crystallizer is 5000mm, and the distance between the upper refrigerant inlet and the upper end cap of the straight pipe section of the crystallizer is 8000mm. The distance from the liquid level of the upper crystallizer is the same as in Embodiment 1. It can be seen from the separation of materials at the crystallizer outlet that the purity of PX in the filter cake is 75wt%, and the total yield of the filter cake accounts for 13% of the feed.

[0073]

Example 4

[0074] The operating conditions in this embodiment are the same as in embodiment 1. The difference is that the refrigerant enters the crystallizer tangentially to the crystallizer wall (α is 90°) and tilts downward at β is 30°. After separating the material at the crystallizer outlet, it can be found that the PX purity in the filter cake is 78wt%, and the total yield of the filter cake accounts for 13.5% of the total feed.

[0075] Comparative Example 1

[0076] The operating conditions in this embodiment are the same as in Embodiment 1, except that the refrigerant inlet pipe is not specially treated and only one refrigerant inlet is provided. The refrigerant inlet is located 300 mm from the lower end of the straight pipe section of the crystallizer. Liquid carbon dioxide and mixed xylene are used for cooling. Analysis of the separated materials at the crystallizer outlet shows that the PX purity in the filter cake is 70%, and the filter cake yield accounts for 10% of the total feed.

[0077] Any numerical value mentioned in this invention, if there is only a two-unit interval between any minimum and any maximum value, includes all values ​​that increase by one unit each time from the minimum to the maximum value. For example, if the amount of a component, or the value of a process variable such as temperature, pressure, or time, is stated as 50-90, in this specification it means specifically listing values ​​such as 51-89, 52-88… and 69-71 and 70-71, etc. For non-integer values, it may be appropriately considered that a unit is 0.1, 0.01, 0.001, or 0.0001. These are merely some specifically specified examples. In this application, in a similar manner, all possible combinations of numerical values ​​between the listed minimum and maximum values ​​are considered to have been disclosed.

[0078] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A separation apparatus for separating mixed xylenes by direct cooling crystallization, comprising: The crystallizer and the refrigerant piping connected to the side wall of the crystallizer, the refrigerant piping including a horizontal straight section and an inclined section, the inclined section of the refrigerant piping being inclined downwards and connected to the refrigerant inlet of the crystallizer.

2. The separation device according to claim 1, characterized in that, The crystallizer tank is cylindrical; and / or, The angle β formed by the inclined section of the refrigerant pipeline and the vertical line at the inlet of the crystallizer wall is 20–80°, preferably 30–75°; and / or, The angle α formed by the inclined section of the refrigerant pipeline and the horizontal tangent at the refrigerant inlet of the crystallizer is 20–90°, preferably 30–90°; and / or, The inclined section of the refrigerant pipeline adopts a reduced diameter design. Preferably, the ratio of the pipe diameter at the end of the inclined section to the pipe diameter at the beginning of the inclined section is 1:(1.05~2), more preferably 1:(1.2~1.8); and / or, The end of the inclined section of the refrigerant pipeline is filled with a porous filling material.

3. The separation device according to claim 1, characterized in that, The side wall of the crystallizer is provided with multiple refrigerant inlets and refrigerant pipelines connected thereto. Preferably, refrigerant inlets are provided at different heights on the side wall of the crystallizer, and at least two refrigerant inlets are provided on a loop at the same height on the side wall of the crystallizer.

4. The separation device according to claim 3, characterized in that, The refrigerant inlets at the same height on the sidewall of the crystallizer are symmetrically or at equal angles; and / or, The refrigerant pipelines are connected in parallel with branch lines, and refrigerant heaters are installed on the branch lines; and / or, Of the refrigerant inlets at different heights on the sidewall of the crystallizer, the highest refrigerant inlet is located below the liquid level in the crystallizer. Preferably, the distance between the highest refrigerant inlet and the liquid level is 1 / 8 to 1 / 2 of the length of the straight pipe section of the crystallizer; and / or, the distance between the highest refrigerant inlet and the upper end cap of the straight pipe section of the crystallizer is 1 / 15 to 3 / 5 of the length of the straight pipe section of the crystallizer, preferably 1 / 10 to 1 / 2; and / or, the distance between the lowest refrigerant inlet and the lower end cap of the straight pipe section of the crystallizer is 1 / 20 to 2 / 5 of the length of the straight pipe section of the crystallizer, preferably 1 / 15 to 1 / 3.

5. The separation device according to claim 1, characterized in that, The crystallizer is provided with a gas phase outlet at the top; and / or, The top of the crystallizer is also provided with a material inlet connected to a material pipeline. Preferably, a precooler and a material heat exchanger are provided on the material pipeline, and optionally, a liquid distributor is provided at the end of the material pipeline; and / or, A stirrer is provided inside the crystallizer; preferably, the stirrer is a radial stirrer and / or an axial stirrer; and / or, The crystallizer has a crystallization slurry outlet at the bottom and is connected to a centrifuge via a pipeline.

6. The separation device according to claim 5, characterized in that, The centrifuge is provided with a centrifugal mother liquor outlet and a centrifugal filter cake outlet. Preferably, the centrifugal mother liquor outlet is connected by a pipeline to a material heat exchanger on the material pipeline and is used as the heat exchange medium of the material heat exchanger.

7. A method for separating mixed xylenes by direct cooling crystallization, wherein the separation device for separating mixed xylenes by direct cooling crystallization as described in any one of claims 1 to 6 is used to separate p-xylene from the mixed xylenes.

8. The separation method according to claim 7, characterized in that, include: After pre-cooling the mixed xylenes, they were contacted with a refrigerant for heat exchange to obtain a slurry containing p-xylene crystals, which were then separated to obtain p-xylene crystals.

9. The separation method according to claim 8, characterized in that, The refrigerant is selected from inert cryogenic liquids, preferably at least one of liquid nitrogen and liquid carbon dioxide; and / or, In the mixed xylenes, the concentration of p-xylene is not less than 20 wt%, preferably 20–90 wt%; and / or, The mass ratio of the refrigerant to the mixed xylene is 1:(3-10), preferably 1:(4-8).

10. The separation method according to claim 8, characterized in that, The inlet pressure of the refrigerant is 0.01–0.5 MPaG, preferably 0.1–0.3 MPaG; and / or, The temperature at which the refrigerant enters the crystallizer is -80 to -10°C, preferably -78 to 0°C; and / or, The temperature at which the mixed xylene enters the crystallizer is -38 to 20°C, preferably -38 to 10°C.

11. The separation apparatus for separating mixed xylenes by direct cooling crystallization according to any one of claims 1 to 6, or the separation method for separating mixed xylenes by direct cooling crystallization according to any one of claims 7 to 10, for use in the preparation of para-xylene from mixed xylenes.

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