Systems and operating methods for crystallization separation of xylene

Abstract

This invention relates to the field of xylene separation, and discloses a system and operating method for crystallizing and separating xylene. The system includes a crystallizer for contacting a circulating refrigerant with a mixed xylene feedstock to obtain a crystallized slurry and a gaseous refrigerant. The crystallizer is equipped with a refrigerant distributor, an ultrasonic device, and a guide tube. The refrigerant distributor is located at the refrigerant inlet of the crystallizer and is used to spray the circulating refrigerant into the crystallizer. The ultrasonic device is connected to the refrigerant distributor to prevent localized overcooling and icing of the refrigerant distributor. The guide tube is positioned above the refrigerant distributor, and its outer wall is jacketed. This system enables direct contact cooling between the refrigerant and the material, solving the problem of heat exchange area limitations in indirect heat exchange methods. Furthermore, the system does not use a scraper agitator, which is beneficial for continuous production.

Description

Technical Field

[0001] This invention relates to the field of xylene separation, and more specifically to a system and operating method for crystallizing and separating xylene. Background Technology

[0002] Para-xylene (PX) is an important raw material in the polyester industry. The production of PX generates a mixture of para-xylene, m-xylene, o-xylene, and ethylbenzene. Currently, the main industrial processes for separating PX are adsorption separation and crystallization separation. Compared to adsorption separation, crystallization offers higher product purity and a simpler process, but its drawback is a lower recovery rate. To improve the recovery rate, crystallization is typically performed at low temperatures.

[0003] In industrial production, C8 aromatics are cooled and crystallized in a crystallizer. The refrigerant passes through a jacket, exchanging heat and cooling the crystallizer wall. The crystallizer uses a scraper-type agitator to scrape off PX crystals from the wall. This indirect cooling method has low heat exchange efficiency, and the crystallizer's size limits the increase in heat exchange area, hindering its large-scale development and application. The scraper agitator is difficult to design and has a complex form; the scraper often freezes and breaks under stress, causing frequent shutdowns for maintenance and affecting continuous production. Therefore, a direct cooling method without agitation would effectively promote the efficient development of the crystallization process.

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

[0005] CN108794295A discloses a method for crystallizing and separating para-xylene. This invention employs a crystallization tank with a condenser tube extending from both inside and outside the tank. The condenser tube inside the tank has several micro-perforations for liquid flow. This micro-perforation design allows the inert coolant to gradually and slowly seep into the crystallization tank 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 crystallization tube, which is detrimental to continuous crystallization production.

[0006] Therefore, there is an urgent need to develop a new process or device. Summary of the Invention

[0007] The purpose of this invention is to overcome the problems of low refrigeration efficiency, limited heat exchange area, and difficulty in manufacturing and easy damage to the crystallizer scraper agitator, which affect continuous production, caused by the use of indirect refrigeration in existing xylene crystallization separation processes. This invention provides a system and operating method for xylene crystallization separation. This system enables direct contact cooling between the refrigerant and the material, solving the problem of heat exchange area limitations in indirect heat exchange methods. Furthermore, the absence of a scraper agitator in this system facilitates continuous production.

[0008] To achieve the above objectives, a first aspect of the present invention provides a system for crystallizing and separating xylene, the system comprising a crystallizer for contacting a circulating refrigerant with a mixed xylene feedstock to obtain a crystallized slurry and a gaseous refrigerant; the crystallizer is provided with a refrigerant distributor, an ultrasonic device, and a flow guide tube; The refrigerant distributor is located at the refrigerant inlet of the crystallizer and is used to spray circulating refrigerant into the crystallizer. The ultrasonic device is connected to the refrigerant distributor to prevent localized overcooling and icing of the refrigerant distributor; The guide tube is positioned above the refrigerant distributor, and the outer wall of the guide tube is provided with a jacket.

[0009] A second aspect of the present invention provides a method for operating the system described in the first aspect, the method comprising: contacting a circulating refrigerant with a mixed xylene feedstock in a crystallizer to obtain a crystallization slurry and a gaseous refrigerant; the crystallization product falling between a guide tube and the inner wall of the crystallizer.

[0010] The beneficial effects of the present invention through the above technical solution include: The system provided by this invention enables direct cooling by directly introducing refrigerant into the crystallizer to mix with the material, thereby improving the heat exchange efficiency of the crystallizer. It also eliminates the need for a scraper agitator, reducing power consumption and minimizing abnormal shutdowns caused by frequent scraper drops, which is beneficial for long-term operation of the equipment. Furthermore, it specifically addresses the problems of high precision requirements and high investment costs associated with existing crystallizers for scraper agitator equipment. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the system provided by the present invention.

[0012] Explanation of reference numerals in the attached figures Detailed Implementation

[0013] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0014] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "left," "right," "top," and "bottom," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. In addition, "inner" and "outer" refer to the inner and outer contours relative to the outline of each component itself.

[0015] In this invention, the terms "first" and "second" do not limit the substances and operations, but are only used to distinguish the substances introduced in different steps and the operations performed in different stages.

[0016] The first aspect of the present invention provides a system for crystallizing and separating xylene, the system comprising a crystallizer for contacting a circulating refrigerant with a mixed xylene feedstock to obtain a crystallization slurry and a gaseous refrigerant; the crystallizer is provided with a refrigerant distributor, an ultrasonic device and a flow guide tube; The refrigerant distributor is located at the refrigerant inlet of the crystallizer and is used to spray circulating refrigerant into the crystallizer. The ultrasonic device is connected to the refrigerant distributor to prevent localized overcooling and icing of the refrigerant distributor; The guide tube is positioned above the refrigerant distributor, and the outer wall of the guide tube is provided with a jacket.

[0017] The system provided by this invention involves a circulating refrigerant that is dispersed by a refrigerant distributor within the crystallizer and enters the crystallizer at high speed in a jet-like manner. The circulating refrigerant vaporizes upon contact with the mixed xylene raw material inside the guide tube, causing the xylene to crystallize and precipitate, forming a three-phase mixture of gas, liquid, and solid. This mixture rises to the top of the crystallizer for separation. The gaseous refrigerant is discharged from the top of the crystallizer, while the liquid-solid phase falls between the outer wall of the guide tube and the inner wall of the crystallizer, ultimately being collected at the bottom of the crystallizer. A heating medium is circulated through the jacket of the guide tube to prevent icing on the wall surface. The refrigerant distributor is connected to an ultrasonic device to achieve intermittent vibration, preventing localized overcooling at the refrigerant distributor outlet and subsequent icing and blockage of the refrigerant distributor.

[0018] According to a preferred embodiment of the present invention, the ratio of the diameter of the guide tube to the diameter of the crystallizer is (1-9):10, preferably (5-8):10.

[0019] According to a preferred embodiment of the present invention, the ratio of the height of the guide tube to the height of the crystallizer is (1-9):10, preferably (3-7):10.

[0020] Controlling the size of the guide tube within the above-mentioned preferred range can improve the gas-liquid-solid distribution, enhance mass transfer performance, and improve mixing performance. By optimizing the structure of the guide tube, the gas holdup can be increased, the bubble size can be reduced, the gas-liquid-solid distribution effect can be improved, and the mixing of gas, liquid, and solid in the axial and radial directions of the crystallizer can be promoted, thereby increasing the mass transfer rate between gas-liquid, liquid-solid, and even gas-liquid-solid phases.

[0021] Moreover, by adjusting the position and structure of the guide tube, more room for adjustment of operating conditions can be provided to adapt to different process requirements.

[0022] According to a preferred embodiment of the present invention, the refrigerant distributor is arranged directly below the guide tube.

[0023] According to a preferred embodiment of the present invention, the lateral coverage area of ​​the refrigerant distributor is 50-90% of the cross-sectional area of ​​the guide tube.

[0024] According to a preferred embodiment of the present invention, the shortest vertical distance between the refrigerant distributor inlet and the guide tube is 200-1000 mm.

[0025] With the above preferred embodiment, the refrigerant distributor and the guide tube cooperate with each other, which is conducive to the complete entry of circulating refrigerant from the refrigerant distributor into the guide tube. This is more conducive to improving the gas phase fraction inside the guide tube and promoting the circulation inside and outside the guide tube.

[0026] In this invention, when the refrigerant distributor is arranged above the guide tube, the distance between them refers to the vertical distance between the refrigerant distributor inlet and the top of the guide tube. When the refrigerant distributor is arranged below the guide tube, the distance between them refers to the vertical distance between the refrigerant distributor inlet and the bottom of the guide tube.

[0027] The present invention does not have any particular limitation on the type of refrigerant distributor. Various refrigerant distributors commonly used in the art can be used, including but not limited to pipe packing materials, nozzles, etc., so as to make the circulating refrigerant sprayed into the crystallizer and make the liquid in the crystallizer in a turbulent state.

[0028] The present invention does not impose any particular limitation on the ultrasonic device, and various ultrasonic devices commonly used in the art can be used.

[0029] According to a preferred embodiment of the present invention, the outer wall of the crystallizer is provided with a jacket, and a heating medium or a cooling medium is circulated inside the jacket to prevent the wall surface from freezing or to assist in refrigeration.

[0030] Generally, when the crystallizing slurry falls from the top of the crystallizer, a heating medium is circulated inside the jacket to prevent ice formation on the wall surface; When the cooling effect of the refrigerant is insufficient to meet the crystallization requirements of the mixed xylene raw materials, a cooling medium is circulated inside the jacket for auxiliary cooling.

[0031] According to a preferred embodiment of the present invention, the system further includes a refrigerant heat exchanger, the outlet of which is connected to the refrigerant inlet of the crystallizer.

[0032] According to a preferred embodiment of the present invention, the system further includes a refrigerant compressor, the outlet of which is connected to the inlet of a refrigerant heat exchanger.

[0033] According to a preferred embodiment of the present invention, the system further includes a refrigerant buffer tank, the first outlet of which is connected to the inlet of the refrigerant compressor.

[0034] According to a preferred embodiment of the present invention, the first inlet of the refrigerant buffer tank is connected to the outside world for providing fresh refrigerant from the outside.

[0035] According to a preferred embodiment of the present invention, the second inlet of the refrigerant buffer tank is connected to the gas phase outlet of the crystallizer for recovering the gas phase refrigerant.

[0036] In this invention, when the purity of the refrigerant in the refrigerant buffer tank is less than 99.9 wt%, a portion of it (i.e., the substandard refrigerant) is discharged from the second outlet of the refrigerant buffer tank, and the remaining portion is recycled; and a corresponding amount of fresh refrigerant is added to meet the crystallization requirements.

[0037] According to a preferred embodiment of the present invention, the system further includes a centrifuge for separating the crystallized product obtained from the crystallizer to obtain the crystallization mother liquor and the para-xylene product.

[0038] According to a specific embodiment of the present invention, reference may be made to Figure 1The circulating refrigerant stored in the refrigerant buffer tank VII (provided by fresh refrigerant 2 and part of the gaseous refrigerant 3 obtained from the crystallizer described below) is compressed by the refrigerant compressor VIII and heat-exchanged by the refrigerant heat exchanger IX. It is then injected into the crystallizer I through the refrigerant distributor IV at the refrigerant inlet of the crystallizer I. After the circulating refrigerant comes into contact with the mixed xylene raw material, it vaporizes and precipitates p-xylene crystals. It becomes a gas-liquid-solid three-phase mixture that rises inside the guide tube III and rises to the top of the crystallizer I for separation. The gaseous refrigerant 3 is discharged from the top of the crystallizer, and the crystallization slurry 5 falls between the outer wall of the guide tube III and the inner wall of the crystallizer I and is finally collected at the bottom of the crystallizer I. Heating medium is introduced into both the crystallizer jacket II and the guide tube jacket V to prevent ice formation on the walls; the refrigerant distributor IV is connected to an ultrasonic device to prevent local overcooling at the outlet of the refrigerant distributor, which could cause material to freeze and block the refrigerant distributor. The crystallization slurry 5 collected at the bottom of crystallizer I is sent to centrifuge VI for separation to obtain crystallization mother liquor 6 and paraxylene product 7.

[0039] A second aspect of the present invention provides a method for operating the system described in the first aspect, the method comprising: contacting a circulating refrigerant with a mixed xylene feedstock in a crystallizer to obtain a crystallization slurry and a gaseous refrigerant; the crystallization product falling between a guide tube and the inner wall of the crystallizer.

[0040] According to a preferred embodiment of the present invention, circulating refrigerant is injected into the crystallizer via a refrigerant distributor.

[0041] The present invention does not impose any particular limitation on the specific operating conditions of the refrigerant distributor, as long as the crystallization requirements are met. According to a preferred embodiment of the present invention, the operating conditions of the refrigerant distributor include: an inlet pressure of 0.06-2.54 MPaG and an inlet temperature of -80 to -10°C.

[0042] According to a preferred embodiment of the present invention, the ultrasonic device is activated in an intermittent vibration mode.

[0043] According to a preferred embodiment of the present invention, the operating conditions of the ultrasonic device include: an ultrasonic frequency of 33-120kHz, an ultrasonic on-off interval of 30-60min, and an ultrasonic duration of 5-20min.

[0044] According to a preferred embodiment of the present invention, the circulating refrigerant is in liquid form.

[0045] According to a preferred embodiment of the invention, the circulating refrigerant is provided by fresh refrigerant and optionally a portion of the gaseous refrigerant obtained from a crystallizer.

[0046] According to a preferred embodiment of the present invention, the fresh refrigerant is selected from at least one of liquid carbon dioxide, liquid nitrogen and liquid helium.

[0047] According to a preferred embodiment of the present invention, the content of para-xylene in the mixed xylene raw material is 15-88 wt%.

[0048] The main component of the mixed xylene raw material described in this invention is C8 aromatic hydrocarbons, and it may also contain small amounts of benzene, ethylbenzene, etc.

[0049] The present invention will be described in detail below through embodiments.

[0050] Example 1 System such as Figure 1 As shown.

[0051] Mixed xylene feedstock (PX purity 21wt%) at -36℃ enters the crystallizer from the top. The fresh refrigerant is liquid carbon dioxide, which passes sequentially through a refrigerant buffer tank, a refrigerant compressor, and a refrigerant heat exchanger before being injected into the crystallizer through a refrigerant distributor. The inlet pressure of the refrigerant distributor is 0.12 MPaG, and the inlet temperature is -75℃. The mass flow ratio of the mixed xylene feedstock to the liquid carbon dioxide is 2.2:1. The liquid carbon dioxide and mixed xylene are mixed and cooled, while the gaseous carbon dioxide is discharged from the top of the crystallizer for circulation.

[0052] The ratio of the diameter of the guide tube to the diameter of the crystallizer is 6 / 10, the ratio of the height of the guide tube to the height of the crystallizer is 7 / 10, and the refrigerant distributor is arranged directly below the guide tube; the lateral coverage area of ​​the refrigerant distributor is 70% of the cross-sectional area of ​​the guide tube; the distance between the inlet of the refrigerant distributor and the bottom of the guide tube is 400mm.

[0053] The operating conditions for the ultrasonic equipment include: ultrasonic frequency of 80kHz, ultrasonic start interval of 30 min, and duration of 5 min.

[0054] -36℃ gaseous CO2 is introduced into the guide tube jacket and crystallizer jacket to prevent icing on the wall surface.

[0055] The crystallization slurry is discharged from the bottom of the crystallizer and then separated in a centrifuge to obtain filter cake and crystallization mother liquor. The PX purity in the filter cake is 86.3 wt%, and the filter cake recovery rate is 65.8% (solid PX / PX in mixed xylene feedstock × 100%).

[0056] Example 2 System such as Figure 1 As shown.

[0057] A mixed xylene feedstock (PX purity 88wt%) at 9℃ enters the crystallizer from the top. The fresh refrigerant is liquid carbon dioxide, which passes sequentially through a refrigerant buffer tank, a refrigerant compressor, and a refrigerant heat exchanger before being injected into the crystallizer from a refrigerant distributor. The inlet pressure of the refrigerant distributor is 1.42 MPaG, and the inlet temperature is -28℃. The mass flow ratio of the mixed xylene feedstock to the liquid carbon dioxide is 4.2:1. The liquid carbon dioxide and mixed xylene are mixed and cooled, while the gaseous carbon dioxide is discharged from the top of the crystallizer for circulation.

[0058] The ratio of the diameter of the guide tube to the diameter of the crystallizer is 7 / 10, the ratio of the height of the guide tube to the height of the crystallizer is 6 / 10, and the refrigerant distributor is arranged directly below the guide tube; the lateral coverage area of ​​the refrigerant distributor is 80% of the cross-sectional area of ​​the guide tube; the distance between the inlet of the refrigerant distributor and the bottom of the guide tube is 400mm.

[0059] The operating conditions for the ultrasonic equipment include: ultrasonic frequency of 80kHz, ultrasonic start interval of 30 min, and duration of 5 min.

[0060] 9°C gaseous CO2 is introduced into the guide tube jacket and crystallizer jacket to prevent icing on the wall surface.

[0061] The crystallization slurry is discharged from the bottom of the crystallizer and then separated in a centrifuge to obtain filter cake and crystallization mother liquor. The PX purity in the filter cake is 96.7 wt%, and the filter cake recovery rate is 91% (solid PX / PX in mixed xylene feedstock × 100%).

[0062] Comparative Example 1 The conditions are the same as in Example 1, except that there is no ultrasonic equipment and no gaseous CO2 is introduced into the guide tube jacket and crystallizer jacket.

[0063] The material separated from the centrifuge outlet showed that the PX purity in the filter cake was 69.4 wt%, and the filter cake recovery rate was 45% (solid PX / PX in mixed xylene feedstock × 100%).

[0064] However, after one day of operation, the refrigerant distributor became blocked by ice, and ice formed on the outer wall of the guide tube and the inner wall of the crystallizer. The environment inside the crystallizer deteriorated, and the machine had to be shut down.

[0065] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A system for crystallizing and separating xylene, characterized in that, The system includes a crystallizer for contacting a circulating refrigerant with a mixed xylene feedstock to obtain a crystallized slurry and a gaseous refrigerant; the crystallizer is equipped with a refrigerant distributor, an ultrasonic device, and a flow guide tube. The refrigerant distributor is located at the refrigerant inlet of the crystallizer and is used to spray circulating refrigerant into the crystallizer. The ultrasonic device is connected to the refrigerant distributor to prevent localized overcooling and icing of the refrigerant distributor; The guide tube is positioned above the refrigerant distributor, and the outer wall of the guide tube is provided with a jacket.

2. The system according to claim 1, wherein, The ratio of the diameter of the guide tube to the diameter of the crystallizer is (1-9):10, preferably (5-8):10; Preferably, the ratio of the height of the guide tube to the height of the crystallizer is (1-9):10, and more preferably (3-7):

10.

3. The system according to claim 1 or 2, wherein, The refrigerant distributor is located directly below the flow guide tube; Preferably, the lateral coverage area of ​​the refrigerant distributor is 50-90% of the cross-sectional area of ​​the guide tube; Preferably, the shortest vertical distance between the refrigerant distributor inlet and the guide tube is 200-1000mm.

4. The system according to any one of claims 1-3, wherein, The outer wall of the crystallizer is equipped with a jacket to prevent ice formation on the wall or to assist in refrigeration.

5. The system according to any one of claims 1-4, wherein, The system also includes a refrigerant heat exchanger, the outlet of which is connected to the refrigerant inlet of the crystallizer.

6. The system according to any one of claims 1-5, wherein, The system also includes a centrifuge for separating the crystallization slurry obtained from the crystallizer to obtain the crystallization mother liquor and paraxylene product.

7. A method of operating the system according to any one of claims 1-6, the method comprising: In the crystallizer, the circulating refrigerant is brought into contact with the mixed xylene feedstock to obtain crystallization slurry and gaseous refrigerant; The crystallization slurry falls between the guide tube and the inner wall of the crystallizer.

8. The method according to claim 7, wherein, The circulating refrigerant is injected into the crystallizer via a refrigerant distributor; Preferably, the operating conditions of the refrigerant distributor include: an inlet pressure of 0.06-2.54 MPaG and an inlet temperature of -80 to -10°C.

9. The method according to claim 7, wherein, The ultrasonic device is switched to intermittent vibration mode. Preferably, the operating conditions of the ultrasonic device include: an ultrasonic frequency of 33-120kHz, an ultrasonic start-up interval of 30-60min, and an ultrasonic duration of 5-20min.

10. The method according to any one of claims 7-9, wherein, The circulating refrigerant is in liquid form; Preferably, the circulating refrigerant is provided by fresh refrigerant and optionally a portion of the gaseous refrigerant obtained from the crystallizer; Preferably, the fresh refrigerant is selected from at least one of liquid carbon dioxide, liquid nitrogen, and liquid helium; Preferably, the content of para-xylene in the mixed xylene raw material is 15-88 wt%.

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