Low energy paraxylene production system and method

Abstract

The application discloses a system and method for producing p-xylene with low energy consumption, and belongs to the field of petroleum chemical industry. The system for producing p-xylene comprises a source of disulfonated aromatic hydrocarbon, a xylene rectification tower, a deep-cooling crystallization separation unit and an isomerization reactor. The source of disulfonated aromatic hydrocarbon is connected with the xylene rectification tower. The xylene rectification tower is provided with a gas phase outlet I, a solid phase outlet I and a liquid phase outlet I. The liquid phase outlet is connected with the deep-cooling crystallization separation unit. The deep-cooling crystallization separation unit is provided with a gas phase outlet II and a liquid phase outlet II. The liquid phase outlet II is connected with the isomerization reactor. The discharge port of the isomerization reactor is connected with the xylene rectification tower. The application adopts a rectification-crystallization-liquid phase isomerization combined process, fully gives play to the advantages and characteristics of the crystallization and liquid phase isomerization processes, has low equipment investment, few side reactions, greatly reduces the separation energy consumption and has important development significance and wide application prospect.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of petrochemical industry, and relates to a system and a method for producing p-xylene with low energy consumption. BACKGROUND

[0002] P-xylene is one of the main basic organic raw materials in the petrochemical industry, and is widely used in many chemical production fields such as chemical fibers, synthetic resins, pesticides, medicines and plastics. With the development of the petrochemical industry, aromatic hydrocarbons are mainly obtained from petroleum products. Mixed xylene is obtained from petroleum fractions by reforming, which is called reforming xylene. There is also xylene in the byproduct pyrolysis gasoline in the process of cracking petroleum fractions to produce ethylene, which is called pyrolysis xylene. In addition, mixed xylene is obtained by disproportionation and transalkylation of other aromatic hydrocarbons such as toluene and C9 aromatic hydrocarbons, which is called disproportionation xylene. Mixed xylene with low p-xylene content is converted into mixed xylene with high p-xylene content by isomerization process, which is called isomerization xylene.

[0003] A typical p-xylene production method is to separate p-xylene (PX) from a mixture of the remaining three isomers with similar boiling points by multi-stage deep cooling crystallization or molecular sieve simulated moving bed adsorption separation (simply referred to as adsorption separation) technology, to produce high-purity PX as a raw material for downstream pure terephthalic acid (PTA) devices. The adsorption separation process uses a simulated moving bed device and a rotary valve to switch the flow to separate mixed xylene. The switching flow is multiple, making the operation process complex, and the equipment requires a large one-time investment. At the same time, it needs to recover the desorbent, and the operation cost is high. For the treatment of ortho- and meta- xylene, the existing technology often adopts carbon eight aromatic hydrocarbon gas phase isomerization technology to isomerize it into p-xylene. The gas phase isomerization method needs to vaporize the raw material first, and then enter the reactor for reaction, which has high energy consumption. SUMMARY

[0004] The problems to be solved by the present application are as follows: 1) high energy consumption of PX production process; 2) high energy consumption of isomerization unit; 3) complex PX production process, large equipment investment and other problems. In view of these problems, a method for producing PX with low energy consumption is provided.

[0005] To solve the above problems, the present application provides a system for producing p-xylene, comprising: a source of disubstituted aromatic hydrocarbon, a xylene rectification tower, a deep cooling crystallization separation unit, and an isomerization reactor; the source of disubstituted aromatic hydrocarbon is connected to the xylene rectification tower; the xylene rectification tower has a tower top outlet, a tower bottom outlet, and a liquid phase outlet I; the liquid phase outlet I is connected to the deep cooling crystallization separation unit; the deep cooling crystallization separation unit has a solid phase outlet II and a liquid phase outlet II; the liquid phase outlet II is connected to the isomerization reactor; and the outlet of the isomerization reactor is connected to the xylene rectification tower. The system further comprises a heat exchanger I; the heat exchanger I has heat exchange pipelines P11 and P12; the liquid phase outlet II, the heat exchange pipeline P11, and the inlet of the isomerization reactor are sequentially connected; and the outlet of the isomerization reactor, the heat exchange pipeline P12, and the xylene rectification tower are sequentially connected.

[0006] Optionally, the system further comprises a heat exchanger II; the heat exchanger II has heat exchange pipelines P21 and P22; the liquid phase outlet II, the heat exchange pipeline P21, the heat exchange pipeline P11, and the inlet of the isomerization reactor are sequentially connected; and the liquid phase outlet I, the heat exchange pipeline P22, and the deep cooling crystallization separation unit are sequentially connected.

[0007] The source of disubstituted aromatic hydrocarbon in the present application is a feed pipeline or a storage tank that can provide disubstituted aromatic hydrocarbon.

[0008] Optionally, the liquid phase outlet I is located in the upper part of the xylene rectification tower, preferably at 1 / 3-2 / 3 of the height of the xylene rectification tower from top to bottom.

[0009] The present application also provides a method for producing p-xylene, comprising: S1 introducing disubstituted aromatic hydrocarbon into a xylene rectification tower for rectification; S2 separating a tower top component, a liquid phase component, and a tower bottom component in the xylene rectification tower; the tower top component is a C7-aromatic hydrocarbon component; the tower bottom component is a C9+ aromatic hydrocarbon component; and the liquid phase component is a C8 aromatic hydrocarbon component; S3 after the liquid phase component is subjected to deep cooling crystallization in a deep cooling crystallization separation unit, centrifugal separation is performed to obtain a crystallized solid and a crystallized mother liquor; S4 the crystallized mother liquor is introduced into an isomerization reactor to undergo isomerization to obtain an isomerization product; and S5 the isomerization product is returned to the xylene rectification tower.

[0010] Optionally, the method further comprises a heat exchange I process: the crystallized mother liquor is subjected to heat exchange with the isomerization product; and preferably, the method further comprises a heat exchange II process: the crystallized mother liquor is first subjected to heat exchange with the liquid phase component, and then subjected to heat exchange with the isomerization product.

[0011] Optionally, the composition of the dismutation aromatic hydrocarbon source is: toluene 0 wt%~2 wt%; ethylbenzene 0.5 wt%~3 wt%; dimethylbenzene 60 wt%~80 wt%; C9+ aromatic hydrocarbon 15 wt%~30 wt%.

[0012] Optionally, the operating conditions of the dimethylbenzene rectification tower include: tower top pressure 0~1 MPag, tower top temperature 100~200℃, reflux ratio 3~10; tower bottom temperature is 180~290℃; temperature of liquid phase outlet I is 160~260℃.

[0013] Optionally, the operating conditions of the crystallization separation unit include: crystallization temperature -100℃~20℃; preferably two-stage crystallization, one-stage crystallization temperature is -100℃~-20℃, and two-stage crystallization temperature is -30~20℃.

[0014] Optionally, the operating conditions of the isomerization reactor include: pressure 1.5~5 MPag, temperature 250~350℃, space velocity 0.5~10 h -1 ; the type of isomerization catalyst is quaternary ammonium salt Na type molecular sieve catalyst.

[0015] The present application utilizes the principle that the four C8 aromatic hydrocarbon isomers have a large difference in crystallization points (the crystallization point of PX is 13.26°C, and the crystallization points of the other three C8 aromatic hydrocarbon isomers are lower than -47.8°C), and high-purity PX is produced by the method of deep cooling. The deep cooling crystallization process needs two-stage deep cooling (one-stage crystallization temperature is about -65°C, and two-stage crystallization temperature is about -25°C).

[0016] The present application adopts the liquid-phase isomerization process to maximize the recovery of special xylene isomers from the C8 aromatic hydrocarbon isomerization mixture. The so-called “mixed xylene” is used to describe the C8 aromatic hydrocarbon mixture containing PX, o-xylene, m-xylene and some ethylbenzene equilibrium mixture. In the case of PX recovery, the mixed xylene feed is added to the PX device, and then the raffinate from the PX adsorption separation device or the deep cooling crystallization mother liquor (PX is almost completely depleted) is sent to the isomerization device. The isomerization device re-determines the thermodynamic equilibrium distribution of xylene isomerization. In fact, additional PX is produced from the remaining o-xylene and m-xylene, so that the o-position and m-position isomers are recycled until they are eliminated. The raw material of the liquid-phase isomerization method does not need to be vaporized, and the energy consumption is greatly reduced.

[0017] The present application aims at the characteristics of high PX content in dismutation C8 aromatic hydrocarbon, and develops a PX production process with low production cost and competitive products by combining the rectification, deep cooling crystallization and liquid-phase isomerization process, which has important development significance and broad application prospect:

[0018] (1) The PX is produced by the combined process of rectification-crystallization-liquid phase isomerization. The present application is aimed at the characteristics of the disproportionated C8 raw material. The rectification method is used to refine the C8 aromatic hydrocarbons, and then the PX is separated by the method of deep cooling crystallization. The crystallization mother liquor is matched with the liquid phase isomerization unit to re-generate the C8 aromatic hydrocarbons rich in PX, and is returned to the xylene rectification column to complete the C8 aromatic hydrocarbon cycle. (Different from adsorption + gas phase isomerization)

[0019] (2) The C7-, C8 aromatic hydrocarbons and C9+ components are separated by a single column, and the deheptane column is omitted, so the equipment investment is low.

[0020] (3) The crystallization mother liquor is first heat-exchanged with the product stream of the liquid phase isomerization unit before entering the liquid phase isomerization unit, that is, the hot stream of the rectification unit and the cold stream of the crystallization unit are heat-exchanged, so as to further reduce the energy consumption.

[0021] (4) The isomerization unit adopts the liquid phase isomerization mode, so the energy consumption required for the gasification of the reaction material can be omitted, and the side reactions are few and the PX yield is high.

[0022] (5) The whole process adopts the hot combined production mode, and the heat integration pinch point technology is matched, so the hot stream can be heat-exchanged with the cold stream of other units, which can greatly reduce the separation energy consumption, and has the advantages of simple process, small investment and low energy consumption. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The process flow diagram for producing PX according to the present application;

[0024] Figure 1 In the figure, 1 is a xylene rectification column, 2 is a deep cooling crystallization separation unit (a conventional crystallizer), 3 is an isomerization reactor; 4 is a heat exchanger I; 5 is a heat exchanger II.

[0025] Reaction stream: 101 is a disproportionated aromatic hydrocarbon mixture, 102 is a heavy component, 103 is a light component, 104 is a refined C8 aromatic hydrocarbon mixture, 105 is a crystallized p-xylene product, 106 is a crystallization mother liquor, and 107 is a liquid phase isomerization product. DETAILED DESCRIPTION

[0026] As shown in the figure, the process method for producing PX according to the present application is as follows: Figure 1

[0027] ​The dismutation aromatic hydrocarbon mixture 101 from the dismutation or isomerization unit enters a xylene rectification column 1 for separation, light components 103 (C7) are withdrawn from the overhead of the xylene rectification column 1, and heavy components 102 (C9+) are withdrawn from the column bottom. The C8 mixture 104 after purification is withdrawn from the liquid phase outlet I (1 / 3-2 / 3 height of the rectification column 1) of the xylene rectification column 1 and enters the deep cooling crystallization separation unit 2 for frozen crystallization. The crystallized solid and liquid are separated by centrifugation, the solid is the crystallized para-xylene product 105, and the liquid is the crystallization mother liquor 106 (liquid C8 aromatic hydrocarbon mixture containing a small amount of PX), which enters the isomerization reactor 3, and the reaction produces the isomerization product 107 (C8 aromatic hydrocarbon mixture containing a balanced concentration of PX), which returns to the xylene rectification column 1 to complete the C8 aromatic hydrocarbon cycle.

[0028] The specific steps include:

[0029] a) introducing a dismutation aromatic hydrocarbon feed into a xylene rectification column, the dismutation aromatic hydrocarbon feed comprising benzene, toluene, mixed xylene, ethylbenzene, C7-, C9+ aromatic hydrocarbons, and combinations thereof; the xylene rectification column operating conditions are a column top pressure of 0-1 MPag, a column top temperature of 100-200°C, and a reflux ratio of 3-10;

[0030] b) separating C7- and C9+ aromatic components in the xylene rectification column, C7- light components are withdrawn from the column top, and C9+ heavy components are withdrawn from the column bottom. The purified C8 aromatic hydrocarbon mixture is withdrawn from the side line, including mixed xylene and a small amount of ethylbenzene;

[0031] c) introducing the purified C8 aromatic hydrocarbon mixture into a deep cooling crystallization separation unit for deep cooling crystallization, and further separating the crystallized solid and mother liquor by a centrifuge. The centrifuged mother liquor contains a small amount of PX C8 aromatic hydrocarbons, and the centrifuged filter cake is a PX solid; the crystallization separation unit operating conditions are two-stage crystallization, the first-stage crystallization temperature is -100°C to -20°C, and the second-stage crystallization temperature is -30 to 20°C;

[0032] d) introducing the centrifuged mother liquor into an isomerization reactor, converting the meta-xylene (MX) and ortho-xylene (OX) in the lean para-xylene stream in the isomerization reactor (to PX) to produce an isomerization effluent, wherein the isomerization effluent contains PX-rich C8 aromatic hydrocarbons; the isomerization unit operating conditions are a pressure of 1.5-5 MPag, a temperature of 250-350°C, an operating phase state of liquid phase, and a space velocity of 0.5-10 h-1; the isomerization reactor contains an isomerization catalyst.

[0033] e) reintroducing the isomerization effluent into the xylene rectification column, which is configured to separate C7- to C9+ aromatic hydrocarbons, to complete the C8 aromatic hydrocarbon cycle;

[0034] f) Separating C7- to C9+ aromatics from the isomerization effluent in a xylene distillation column to produce a xylene cycle and C7-, C9+, wherein the xylene cycle includes mixed xylenes, wherein C7- includes C7- aromatics and C7-alkanes, and C9+ includes C9+ aromatics;

[0035] g) Xylene is recycled to the cryogenic crystallization separation unit; and C7- and C9+ are recycled and extracted.

[0036] To further reduce energy consumption, the streams from steps d) and e) undergo heat exchange before entering their respective units. The hot stream from the xylene distillation column exchanges heat with the purified stream of PX product in the crystallization unit.

[0037] The present invention will be further illustrated by the following embodiments, but is not limited to these embodiments.

[0038] Single-pass yield: (PX content in product - PX content in feedstock) / PX content in feedstock.

[0039] The method for calculating the purity of p-xylene is: p-xylene purity = PX content in the product / total content of the product.

[0040] Example 1: System for producing p-xylene

[0041] The system for producing para-xylene includes: a disproportionated aromatic source (which can provide a feed line or storage tank for disproportionated aromatics), xylene distillation column 1, cryogenic crystallization separation unit 2, isomerization reactor 3, and heat exchanger I 4.

[0042] The disproportionated aromatic source is connected to the inlet of xylene distillation column 1; xylene distillation column 1 has a top outlet, a bottom outlet, and a liquid phase outlet I; wherein, liquid phase outlet I is located at the top of the xylene distillation column and is connected to cryogenic crystallization separation unit 2; cryogenic crystallization separation unit 2 has a solid phase outlet II and a liquid phase outlet II; wherein, liquid phase outlet II is connected to the feed inlet (located at the bottom) of isomerization reactor 3; the discharge outlet (located at the top) of isomerization reactor 3 is connected to the xylene distillation column. Heat exchanger I4 has heat exchange pipes P11 and P12 for heat exchange; liquid phase outlet II, heat exchange pipe P11, and the feed inlet of isomerization reactor are sequentially connected; the discharge outlet of isomerization reactor, heat exchange pipe P12, and xylene distillation column are sequentially connected.

[0043] Example 2: System for producing p-xylene

[0044] like Figure 1 As shown, the system for producing para-xylene includes: a disproportionated aromatic source (which can provide a feed line or storage tank for disproportionated aromatics), a xylene distillation column 1, a cryogenic crystallization separation unit 2, an isomerization reactor 3, heat exchanger I 4, and heat exchanger II 5.

[0045] The disproportionated aromatic source is connected to the inlet of xylene distillation column 1; xylene distillation column 1 has a top outlet, a bottom outlet and a liquid phase outlet I; wherein, liquid phase outlet I is located at the top of xylene distillation column and is connected to cryogenic crystallization separation unit 2; cryogenic crystallization separation unit 2 has a solid phase outlet II and a liquid phase outlet II; wherein, liquid phase outlet II is connected to the feed inlet (located at the bottom) of isomerization reactor 3; the discharge outlet (located at the top) of isomerization reactor 3 is connected to xylene distillation column.

[0046] Heat exchanger I 4 has heat exchange pipes P11 and P12 for heat exchange; liquid phase outlet II, heat exchange pipe P11, and the feed inlet of the isomerization reactor are sequentially connected; the discharge outlet of the isomerization reactor, heat exchange pipe P12, and the xylene distillation column are sequentially connected. Heat exchanger II 5 has heat exchange pipes P21 and P22 for heat exchange; liquid phase outlet II, heat exchange pipe P21, heat exchange pipe P11, and the feed inlet of the isomerization reactor are sequentially connected; liquid phase outlet I, heat exchange pipe P22, and the cryogenic crystallization separation unit are sequentially connected.

[0047] Example 3

[0048] like Figure 1 As shown, the process flow for producing para-xylene using Example 2 is as follows: A mixture of disproportionated aromatics 101 (composition: toluene 0.5 wt%, ethylbenzene 1.45 wt%, xylene 70.81 wt%, C9+ aromatics 27.24 wt%) enters the xylene distillation column from the middle section. The column top temperature is 135°C, the bottom temperature is 185°C, and the reflux ratio is 3. The purified C8 aromatics mixture 104 is collected from the liquid outlet at 160°C. After heat exchange with the mother liquor, it enters the cryogenic crystallization separation unit 2. This unit consists of two crystallizers connected in series: crystallizer 1 at -60°C and crystallizer 2 at -15°C. After separation, the crystallized para-xylene product 105 is obtained, with a single-pass yield of 65 wt% and a purity of 99.87%. The crystallization mother liquor 106 is first heated in heat exchanger II with the refined C8 aromatic mixture 104, then heat-exchanged in heat exchanger I with the liquid-phase isomerization product 107, and then enters isomerization reactor 3. The reactor temperature is 270℃ and the space velocity is 2 h⁻¹. -1 The pressure was 2 MPa, and the catalyst was a quaternary ammonium salt Na-type molecular sieve catalyst prepared according to Example 1 in patent document CN114425412 A. After the reaction, the PX-rich stream was returned to the xylene distillation column to complete the cycle.

[0049] The heat exchange process of this invention is reflected in two aspects. First, the refined C8 aromatic mixture 104 and the mother liquor 106 exchange heat. Because the temperature difference between the hot and cold streams is large, multi-stage heat exchange (multi-stage refers to several consecutive heat exchanges) can be used to fully utilize the heat. Second, the mother liquor 106, after heat exchange, exchanges heat again with the liquid-phase isomerization product 107 at the isomerization outlet before entering the isomerization reactor. Because the isomerization requires a high temperature, this allows for full utilization of the heat. Preferably, after heat exchange in heat exchanger I 4, the mother liquor is heated to the isomerization temperature (270°C) by a heater before entering the isomerization reactor 3.

[0050] This invention employs a combined distillation-crystallization-liquid phase isomerization process, fully leveraging the advantages and characteristics of both crystallization and liquid phase isomerization processes, maximizing strengths and minimizing weaknesses, resulting in low initial equipment investment and simple operation. The mixed aromatics separation unit uses a single tower to separate C7- (aromatics with ≤7 carbons), C8 aromatics, and C9+ (aromatics with >9 carbons) components, resulting in low equipment investment. The isomerization unit uses a liquid phase isomerization method, which eliminates the energy consumption required for reactant gasification and minimizes side reactions. The entire process adopts a combined thermal production method, utilizing pinch technology to match hot and cold streams, which can significantly reduce separation energy consumption, offering advantages such as simple process, low investment, and low energy consumption.

[0051] Comparative Example 1: System and process for producing p-xylene

[0052] according to Figure 1 The described PX production process differs in that heat exchangers I 4 and II 5 are not installed, and no heat exchange is performed. The disproportionated aromatic mixture 101 (toluene 0.5 wt%, ethylbenzene 1.45 wt%, xylene 70.81 wt%, C9+ aromatics 27.24 wt%) enters the xylene distillation column in the middle section of the toluene distillation column. The top temperature of the xylene distillation column is 135℃, the bottom temperature is 185℃, and the reflux ratio is 3. The purified C8 aromatic mixture 104 is collected at liquid outlet I at 160℃ and directly enters the cryogenic crystallization separation unit 2 without heat exchange. This cryogenic crystallization separation unit 2 consists of two crystallizers connected in series: crystallizer 1 at -60℃ and crystallizer 2 at -15℃. After separation, the crystallized product is obtained as crystallized para-xylene product 105, with a single-pass yield of 65 wt% and a purity of 99.87%. The crystallization mother liquor 106 enters the isomerization reactor 3 at a temperature of 270°C and a space velocity of 2 h⁻¹. -1 The pressure was 2 MPa, and the catalyst was CN 114425412 A (Example 1). After the reaction, the PX-rich stream was returned to the xylene distillation column to complete the cycle. The energy consumption increased by 62% compared to Example 3 (energy consumption is calculated based on heat load, part of which is the cooling required for crystallization after the xylene column side stream is discharged, and the other part is the heat required for heating the crystallization mother liquor in the cryogenic crystallization unit before it enters the isomerization reactor).

[0053] Any numerical values recited herein include all values from the lower value and up to the upper value. Values that are recited herein also include values that are "framed" by the property limits. For example, if a concentration range is stated as 1% to 50%, it is intended that values such as 25%, 30%, and 35%, as well as 1%, 2%, 3%, and 5%, are expressly enumerated. For values which are less than one, one unit in the high 10s place is considered the lowest value. For values which are fractions of one, one unit in the low 10s place is considered the lowest value. For numerical ranges recited herein, any numerical value implicitly recited as a limit of a stated range can be explicitly recited. For example, a range of 1.0% to 50% should be explicitly understood to include 1.0%, 1.1%, 1.2%,..., 5.9%, and 6.0%, etc. In addition, any numerical value recited herein can be explicitly recited as a limit of a stated range or implicitly understood as such. For example, a range of "about 1.0% to 10%" can be explicitly recited as a range of "about 1.0% to about 10%", or, alternatively, can be understood to include the end points 1.0% and 10%, unless otherwise indicated. Other than in the operating and comparative examples, or where otherwise explicitly indicated, all numerical quantities in this description are meant to be interpreted in an "open term" manner. For example, the expression "about 1% to 10%" is intended to mean "about 1% to about 10%". Numerical quantities given herein are by weight of the total composition, unless otherwise specified.

[0054] It should be noted that the foregoing examples have been provided merely for the purposes of explanation and are in no way to be construed as limiting. The present application is described with reference to exemplary embodiments, but it is understood that the words which have been used herein are words of description, and that they are being used to describe the preferred embodiments of the present application. The application is, however, capable of modifications in various obvious respects, all without departing from the scope and spirit of the present application. While the application has been described with reference to particular methods, materials and examples, the application is not to be construed as being limited thereto. Other variations and modifications of the application will occur to those skilled in the art upon consideration of the foregoing description, and it is the intent to use all methods and materials consistent with the scope of the application to the fullest extent allowed by the appended claims.

Claims

1. A system for producing para-xylene, characterized by Comprising: a disproportionated aromatic hydrocarbon source, a xylene rectification column, a deep cooling crystallization separation unit, an isomerization reactor; the disproportionated aromatic hydrocarbon source is communicated with the inlet of the xylene rectification column; the xylene rectification column has a top outlet, a bottom outlet and a liquid phase outlet I; the liquid phase outlet I is communicated with the deep cooling crystallization separation unit; the deep cooling crystallization separation unit has a solid phase outlet II and a liquid phase outlet II; the liquid phase outlet II is communicated with the isomerization reactor; the outlet of the isomerization reactor is communicated with the xylene rectification column; the system further comprises a heat exchanger I; the heat exchanger I has heat exchange pipelines P11 and P12; the outlet of the isomerization reactor, the heat exchange pipeline P12 and the xylene rectification column are sequentially communicated; the system further comprises a heat exchanger II; the heat exchanger II has heat exchange pipelines P21 and P22; the liquid phase outlet II, the heat exchange pipeline P21, the heat exchange pipeline P11 and the inlet of the isomerization reactor are sequentially communicated; the liquid phase outlet I, the heat exchange pipeline P22 and the deep cooling crystallization separation unit are sequentially communicated.

2. The system for producing p-xylene according to claim 1, wherein the liquid phase outlet I is located at the upper part of the xylene rectification column.

3. The system for producing p-xylene according to claim 1 or 2, wherein the liquid phase outlet I is located at 1 / 3-2 / 3 height from top to bottom of the xylene rectification column.

4. A method for producing p-xylene, comprising: S1: disproportionated aromatic hydrocarbons provided by the disproportionated aromatic hydrocarbon source are introduced into the xylene rectification column for rectification; S2: the xylene rectification column separates top components, liquid phase components and bottom components; the top components are C7-aromatic hydrocarbon components; the bottom components are C9+ aromatic hydrocarbon components; the liquid phase components are C8 aromatic hydrocarbon components; S3: after the liquid phase components are subjected to deep cooling crystallization in the deep cooling crystallization separation unit, centrifugal separation is performed to obtain crystallized solids and a crystallized mother liquor; S4: the crystallized mother liquor is introduced into the isomerization reactor to perform isomerization reaction to obtain isomerization products; S5: the isomerization products are returned to the xylene rectification column; the method comprises a heat exchange II process: the crystallized mother liquor is first subjected to heat exchange with the liquid phase components, and then subjected to heat exchange with the isomerization products.

5. The method for producing p-xylene according to claim 4, wherein the disproportionated aromatic hydrocarbon source comprises: 0 wt%-2 wt% of toluene, 0.5 wt%-3 wt% of ethylbenzene, 60 wt%-80 wt% of xylene and 15 wt%-30 wt% of C9+ aromatic hydrocarbons.

6. The method for producing p-xylene according to claim 4 or 5, wherein the operation conditions of the xylene rectification column comprise: a top pressure of 0-1 MPag, a top temperature of 100-200℃, a reflux ratio of 3-10, a bottom temperature of 180-290℃ and a temperature of the liquid phase outlet I of 160-260℃.

7. The method for producing p-xylene according to claim 4, wherein the operation conditions of the deep cooling crystallization separation unit comprise: a crystallization temperature of -100℃-20℃. ​ ​ ​ ​ ​ 8. The method for producing p-xylene according to claim 7, wherein the deep cooling crystallization is two-stage crystallization, and the first-stage crystallization temperature is -100°C to -20°C, and the second-stage crystallization temperature is -30°C to 20°C.

9. The method for producing p-xylene according to claim 4, wherein the deep cooling crystallization is two-stage crystallization, and the first-stage crystallization temperature is -100°C to -20°C, and the second-stage crystallization temperature is -30°C to 20°C. ​ The operation conditions of the isomerization reactor include: pressure 1.5-5 MPag, temperature 250-350℃, space velocity 0.5-10 h -1 The type of the isomerization catalyst is a quaternary ammonium salt type Na type molecular sieve catalyst.

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