Process and system for producing needle coke from ethylene tar

Abstract

This invention provides a method and system for producing needle coke from ethylene tar. The method includes the following steps: (1) ethylene tar feedstock is mixed with hydrogen-containing gas and then enters a hydrogenation reaction zone, where it is reacted sequentially with at least two hydrogenation catalyst beds; (2) the reaction effluent obtained in step (1) is separated into gaseous and liquid phase products; (3) the liquid phase product obtained in step (2) enters a coking unit, and the oil and gas generated in the reaction are separated to obtain gas, light component oil, middle distillate oil, and heavy distillate oil. After the reaction cycle is completed, the final needle coke product is obtained. In the hydrogenation reaction zone mentioned in step (1), compared with the hydrogenation catalyst in the adjacent upstream hydrogenation catalyst bed, the proportion of mesopores >30nm in the downstream hydrogenation catalyst bed is reduced, and the proportion of mesopores <5nm in the upstream hydrogenation catalyst bed is increased. The method can suppress the formation of coking precursors and produce high-quality needle coke products.

Description

Technical Field

[0001] This invention belongs to the field of petrochemical technology and relates to a method and system for producing needle coke from ethylene tar. Background Technology

[0002] Ethylene tar is a product of the high-temperature condensation of feedstock and products during the steam cracking process of ethylene pyrolysis. With the continuous expansion of domestic ethylene production capacity, the amount of by-product ethylene tar is also increasing. Therefore, how to efficiently and rationally utilize ethylene tar has gradually become a research hotspot. The technology of producing needle coke from ethylene tar not only provides a feasible way for the rational utilization of ethylene tar, but also alleviates the contradiction of insufficient raw materials for needle coke production.

[0003] Patent CN113755211A discloses a method for producing needle coke using optimized ethylene tar feedstock. The method includes feeding initial coking feedstock into a hydrogenation tower, condenser, gas-liquid separation, and distillation to obtain modified and optimized ethylene tar feedstock. After modification and optimization, the feedstock is then fed into a coking tower for coking reaction to obtain high-quality needle coke. This production process employs hydrogenation treatment and co-carbonization with the hydrogenated feedstock, stabilizing the free radical reaction in a hydrogen-rich environment. This increases the blending ratio of ethylene tar while simultaneously producing high-quality needle coke.

[0004] Patent CN114540059A discloses a combined process, system, and the needle coke produced from heavy oil. The combined process includes: a hydrogenation process, using heavy oil as raw material, hydrogenating it to remove easily coking components to obtain hydrogenated oil, wherein the heavy oil is one or more of catalytic slurry oil, ethylene tar, coal tar, and coking wax oil; a component separation process, separating the hydrogenated oil into heavy components and light components; an electrostatic desoldering process, passing the light components into an electrostatic desoldering device to remove ash to obtain deashed oil; a solvent extraction process, separating the deashed oil into extractable oil components and raffinate oil components; and a delayed coking process, subjecting the extractable oil to delayed coking treatment to obtain needle coke.

[0005] The needle coke products obtained by the above-mentioned patented method have a high content of embedded structure and a high coefficient of thermal expansion, and the quality of needle coke products needs to be further improved. Summary of the Invention

[0006] The inventors' in-depth research revealed that ethylene tar feedstock contains a large amount of mono- and polyolefins. These olefin molecules are highly reactive, especially polyolefins, which readily undergo cross-linking reactions to form cluster structures. These cluster structures increase the electron cloud density of polyolefins and even some monoolefin molecules, while simultaneously restricting the diffusion of ethylene tar molecules, making them more prone to condensation reactions to form coking precursors. These coking precursors contain a large number of long-chain olefin structures. As the material enters the coking unit, the presence of these long-chain olefin structures results in an excessive number of active sites, leading to a more intense coking reaction. This deteriorates the ordered packing of planar macromolecules in the mesophase pitch aromatics, reduces the proportion of streamlined structures, and increases the proportion of mosaic structures, resulting in an increased coefficient of thermal expansion and higher ash content in the needle coke product.

[0007] Based on the above research results, the present invention provides a method and system for producing needle coke from ethylene tar. The method can suppress the formation of coking precursors and produce high-quality needle coke products.

[0008] According to a first aspect of the present invention, the present invention provides a method for producing needle coke from ethylene tar, the method comprising the following steps:

[0009] (1) After the ethylene tar feedstock is mixed with hydrogen-containing gas, it enters the hydrogenation reaction zone and reacts with at least two hydrogenation catalyst beds in sequence.

[0010] (2) The reaction effluent obtained in step (1) is separated into gas phase product and liquid phase product by gas-liquid separation;

[0011] (3) The liquid product obtained in step (2) enters the coking unit. The oil and gas generated by the reaction are separated to obtain gas, light component oil, middle distillate oil and heavy distillate oil. After the reaction cycle is completed, the final needle coke product is obtained.

[0012] In the hydrogenation reaction zone described in step (1), an upstream hydrogenation catalyst bed and a downstream hydrogenation catalyst bed are sequentially arranged along the liquid phase flow direction. Compared with the hydrogenation catalyst in the adjacent upstream hydrogenation catalyst bed, the hydrogenation catalyst in the downstream hydrogenation catalyst has a lower proportion of mesopores >30nm in the total pore volume, preferably 30-60 percentage points lower; and a higher proportion of mesopores <5nm in the total pore volume, preferably 30-50 percentage points higher.

[0013] Furthermore, in the above method, in the hydrogenation reaction zone described in step (1), an upstream hydrogenation catalyst bed and a downstream hydrogenation catalyst bed are sequentially arranged along the liquid phase flow direction. Compared with the hydrogenation catalyst in the adjacent upstream hydrogenation catalyst bed, the hydrogenation catalyst in the downstream hydrogenation catalyst has an increased total infrared acidity, preferably 0.1 to 0.2 mmol / g higher; a decreased B / L acid ratio, preferably 0.1 to 0.3 lower; and an increased mass fraction of active metal components (calculated as oxides) in the hydrogenation catalyst, preferably 3 to 20 wt%.

[0014] Furthermore, in the above method, in the hydrogenation reaction zone described in step (1), taking the setting of two hydrogenation catalyst beds as an example, the material passes through catalyst bed A and catalyst bed B in sequence according to the contact order with the material.

[0015] Furthermore, in the above method, in step (1), the proportion of mesopores with a diameter >30 nm in the hydrogenation catalyst loaded in catalyst bed A is 40-90% of the total pore volume, and the proportion of mesopores with a diameter <5 nm in the total pore volume is 10-30%; the total infrared acid content is 0.20-0.30 mmol / g, the B / L acid ratio is 0.6-0.8, and the mass content of active metal components as oxides is 1.0-5.0 wt%.

[0016] Furthermore, in the above method, in step (1), the proportion of mesopores >30nm in the hydrogenation catalyst packed in catalyst bed B is 10-30% of the total pore volume, and the proportion of mesopores <5nm in the total pore volume is 40-80%; the total infrared acid content is 0.30-0.50 mmol / g, the B / L acid ratio is 0.3-0.5, and the mass content of active metal components as oxides is 4.0-25.0 wt%.

[0017] Furthermore, in the above method, the volume ratio of catalyst bed A to catalyst bed B in step (1) is (10-40):(60-90).

[0018] Furthermore, in the above method, in step (1), catalyst bed A and catalyst bed B can be filled with only one type of hydrogenation catalyst, or multiple hydrogenation catalysts can be filled in a graded manner.

[0019] Furthermore, in the above method, in the hydrogenation reaction zone described in step (1), hydrogen injection points are set between each catalyst bed to adjust the reaction temperature.

[0020] Furthermore, in the above method, the olefin content in the ethylene tar feedstock in step (1) is 4-25 mol%, preferably 4-20 mol%; the saturated content is no more than 10 wt%, preferably 0.5-8.0 wt%; and the total content of aromatics and gums is more than 75 wt%, preferably 80-98 wt%.

[0021] Furthermore, in the above method, step (1) may also be blended with aromatic oil products, wherein the aromatic oil products are one or more of catalytic diesel oil, catalytic wax oil, catalytic slurry oil and catalytic cracking clarified oil; the blending ratio of the aromatic oil products is 1 to 50 wt%, preferably 15 to 45 wt%.

[0022] Furthermore, in the above method, the hydrogen-containing gas mentioned in step (1) is hydrogen or a mixture of hydrogen and other gases; the other gases may be high-part gas and / or low-part gas from the hydrogenation unit; furthermore, the high-part gas may come from hydrogenation units with a hydrogen partial pressure in the range of 4.0 to 20.0 MPa, including hydrogenation units for naphtha, diesel, wax oil and residue oil.

[0023] Furthermore, in the above method, the hydrogenation reaction zone in step (1) is equipped with at least one hydrogenation reactor, preferably 1 to 2 hydrogenation reactors; even further, the hydrogenation reactor can be a trickle bed reactor, a fluidized bed reactor, or a tubular reactor, which can be a single reactor type or a combination of one or more of them.

[0024] Furthermore, in the above method, the operating conditions of the hydrogenation reaction zone in step (1) are: reaction pressure 3.0–15.0 MPa, preferably 4.0–12.0 MPa; volume hourly space velocity 0.1–10.0 h⁻¹. -1 Preferably 0.2–3.0 h -1 The hydrogen-to-oil volume ratio is 200–1000, preferably 250–800; the reaction temperature is 100–400℃, preferably 110–360℃. Those skilled in the art can adjust the operating conditions of the hydrogenation reaction zone according to the properties of the feedstock oil or product requirements.

[0025] Furthermore, in the above method, the gaseous product obtained in step (2) is a hydrogen-rich gas, which can be returned to the hydrogenation reaction zone as recycled hydrogen.

[0026] Furthermore, in the above method, the coking device in step (3) is equipped with at least one heating furnace, two coke towers and one fractionation tower, and at least one coke tower is always kept in the reaction stage and at least one coke tower is in the decoking stage.

[0027] Furthermore, in the above method, the reaction conditions of the coking unit in step (3) are as follows: the outlet temperature of the heating furnace adopts variable temperature operation, with a temperature range of 380℃~550℃, preferably 400℃~520℃; the top pressure of the coke tower is 0.01MPa~3.0MPa, preferably 0.5MPa~1.0MPa, and can be constant pressure or variable pressure operation. If it is variable pressure operation, the pressure change rate is 0.01MPa / h~60MPa / h, preferably 0.2MPa / h~10MPa / h; the reaction cycle is 10h~48h, preferably 18h~36h.

[0028] Furthermore, in the above method, the liquid phase product in step (3) can be blended with distillate oil and then enter the coking unit. The distillate oil is selected from one or more of atmospheric residue, vacuum residue, catalytic gasoline, catalytic diesel, catalytic wax oil, catalytic slurry, lubricating oil furfural refined extract oil and catalytic cracking clarified oil, or it can be the middle distillate oil or heavy distillate oil obtained by fractionation in the delayed coking fractionation tower in this method; the weight ratio of distillate oil to ethylene tar is 0.5 to 4.0.

[0029] Furthermore, in the above method, one or more of the light component oil, middle distillate oil and heavy distillate oil obtained in step (3) can be mixed with ethylene tar feedstock and introduced into the hydrogenation reaction zone.

[0030] According to a second aspect of the present invention, a system for producing needle coke from ethylene tar is provided, the system comprising:

[0031] The hydrogenation reaction zone is used to receive ethylene tar feedstock and hydrogen-containing gas, and to perform hydrogenation treatment.

[0032] The gas-liquid separation unit is used to receive the reaction effluent from the hydrogenation reaction zone and separate it to obtain gaseous and liquid products.

[0033] The coking reaction zone includes at least one heating furnace, two coke towers and one fractionation tower, with at least one coke tower always in the reaction stage and at least one coke tower in the decoking stage.

[0034] Furthermore, in the above system, the hydrogenation reaction zone is equipped with at least one hydrogenation reactor; even further, the hydrogenation reactor can be a trickle bed reactor, a fluidized bed reactor, or a tubular reactor, and can be a single reactor type or a combination of one or more of these types.

[0035] Furthermore, in the above system, the gas-liquid separation unit and the separation tower in the coking reaction zone can be configured using existing technologies in the field. Specifically, in this invention, the gas-liquid separation unit typically includes a hot high-pressure separator, a hot low-pressure separator, a cold high-pressure separator, and a cold low-pressure separator.

[0036] Compared with the prior art, the beneficial effects of the method of the present invention are as follows:

[0037] 1. Based on the inventor's research, and considering the characteristics of the cluster structures formed in ethylene tar, hydrogenation catalysts are graded along the liquid phase flow direction. In the upper reaction zone of the reactor, where the cluster structure molecules are large, hydrogenation catalysts with a high proportion of mesoporous pore volume (>30nm) are preferentially loaded. This facilitates the dissociation of cluster structure molecules, reduces the diffusion restriction caused by the cluster structure in ethylene tar, significantly reduces the probability of coking precursors with long-chain olefin structures, inhibits the formation of embedded structures in mesophase microspheres, and helps reduce the thermal expansion coefficient and increase the true density of needle coke. In the lower reaction zone of the reactor, where the cluster structure has been dissociated and the molecular size is small, hydrogenation catalysts with a high proportion of mesoporous pore volume (<5nm) are loaded. This increases the contact probability between reactant molecules and active sites, improves the olefin saturation efficiency of the downstream bed, thereby improving the feed quality of the coking unit and the performance of needle coke products.

[0038] 2. In this invention, the B / L ratio of the catalyst is graded along the direction of liquid phase flow. The hydrogenation catalyst packed in the upstream bed has a low total infrared acidity and a high B / L ratio, which helps the isomerization reaction to occur, reduces the free radical activity of ethylene tar molecules, thereby reducing the probability of the formation of coking precursors with long-chain olefin structures, and thus improving the quality of needle coke products.

[0039] 3. In this invention, the active metal content of the catalyst is graded along the direction of liquid flow, and the active metal content in the hydrogenation catalyst gradually increases, which can achieve a stable transition of the reaction temperature of the device and reduce the harshness of the device operation. Detailed Implementation

[0040] The technical solutions and effects of the present invention will be further illustrated below with reference to the embodiments, but the invention is not limited to the following embodiments.

[0041] The specific embodiments of the present invention will be described in detail below. However, it should be noted that the scope of protection of the present invention is not limited to these specific embodiments, but is determined by the claims in the appendix.

[0042] All publications, patent applications, patents, and other references mentioned in this specification are incorporated herein by reference. Unless otherwise defined, all technical and scientific terms used in this specification have the meanings commonly understood by those skilled in the art. In case of conflict, the definitions in this specification shall prevail.

[0043] When this specification uses the prefixes “known to those skilled in the art,” “prior art,” or similar terms to derive materials, substances, methods, steps, apparatus, or components, the objects derived from such prefixes cover those commonly used in the art at the time of this application, but also include those that are not currently commonly used but will become generally recognized in the art as suitable for similar purposes.

[0044] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprises" shall be understood to include the stated elements or components without excluding other elements or other components.

[0045] In this document, all numeric values ​​of parameters (e.g., quantity or condition) should be understood to be modified by the term “about” in all cases, regardless of whether “about” actually appears before the numeric value.

[0046] Unless otherwise specified, all percentages, parts, ratios, etc. mentioned in this instruction manual are based on weight, and the pressure is gauge pressure.

[0047] In the context of this specification, any two or more embodiments of the present invention can be arbitrarily combined, and the resulting technical solutions are part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0048] In the method of this invention, the coefficient of thermal expansion of the needle coke product is determined according to the international standard GB / T3074.4 "Method for Determination of Coefficient of Thermal Expansion (CTE) of Graphite Electrode", and the true density is determined according to the international standard GB / T6155 "Method for Determination of True Density of Carbon Materials".

[0049] In the method of this invention, the process of analyzing the embedded structure content of needle coke products is as follows: After the raw coke used to prepare needle coke is calcined in a tube furnace at 900℃, a sample is taken and ground into coke particles of 0.2-0.8mm. Then, after being made into a light slide, the microstructure of needle coke is observed under an oil immersion lens with cross-polarized light as the light source, 10x eyepiece and 50x objective lens.

[0050] The properties of the ethylene tar feedstock used in the following examples and comparative examples are shown in Table 1; the hydrogenation catalyst used is supported on alumina and the active metal components are molybdenum and nickel, and its properties are shown in Table 2.

[0051] Table 1 Properties of Ethylene Tar Feedstock

[0052] crude oil <![CDATA[Density, g.cm -3 > 1.0625 S, wt% 0.12 Alkenes, mol% 10.83 <![CDATA[Viscosity (100 °C), mm 2 / s]]> 590.3 Residual carbon, wt% 20.81 Ash content, wt% 0.006 Four components, wt% Saturated fraction 5.8 Aromatic components 82.17 gelatinous 11.05 Asphalt 0.98

[0053] Table 2 Properties of hydrogenation catalysts

[0054]

[0055] Examples 1-4

[0056] (1) After the ethylene tar feedstock is mixed with hydrogen, it enters the hydrogenation reactor. Along the direction of liquid phase flow, the hydrogenation reactor is loaded with hydrogenation catalyst in sequence. The catalyst loading scheme and operating conditions are shown in Table 3.

[0057] (2) The reaction effluent obtained in step (1) is separated into gas phase product and liquid phase product by gas-liquid separation;

[0058] (3) The liquid product obtained in step (2) enters the coking unit, is heated in the heater, and then enters the coke tower. The outlet temperature of the coking heater is 500℃, the heating rate is 2.5℃ / h, the coke tower is operated under constant pressure, the top pressure is 0.5MPa, and the reaction cycle is 36h. The oil and gas generated in the reaction are separated to obtain gas, light component oil, middle distillate oil and heavy distillate oil. After the reaction cycle is completed, the final needle coke product is obtained. The product properties are shown in Table 4.

[0059] Comparative Example 1

[0060] Compared with Example 1, the difference is that the catalyst loading order is reversed, that is, catalyst 2 and catalyst 1 are loaded sequentially along the direction of liquid phase flow, and the other conditions are the same as in Example 1. The product properties are shown in Table 4.

[0061] Comparative Example 2

[0062] The difference from Example 1 is that only catalyst 1 was loaded, while all other conditions were the same as in Example 1. The properties of the product of Comparative Example 2 are shown in Table 4.

[0063] Comparative Example 3

[0064] Compared with Example 1, the difference is that only catalyst 2 is loaded, and the other conditions are the same as in Example 1. The product properties of Comparative Example 3 are shown in Table 4.

[0065] Table 3 Catalyst loading scheme and process conditions of the examples

[0066] project Example 1 Example 2 Example 3 Example 4 Catalyst loading scheme Catalyst 1 15 30 20 40 Catalyst 2 85 70 -- -- Catalyst 3 -- -- 80 60 Operating conditions Reaction pressure, MPa 10 12 8 6 Reaction temperature, °C 240 340 240 260 Hydrogen-to-oil volume ratio 500 500 500 500 <![CDATA[Space velocity, h -1 > 1.0 1.0 1.0 1.0

[0067] Table 4. Properties of Products from Examples and Comparative Samples

[0068] needle-like char properties Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 <![CDATA[Coefficient of thermal expansion, 10 -6 / °C]]> 1.23 0.82 0.91 1.04 1.66 1.78 1.95 True density, g / mL 2.12 2.13 2.12 2.12 2.11 2.10 2.10 Ash content, wt% 0.018 0.014 0.015 0.016 0.025 0.023 0.028 Inlay structure content, wt% 3.2 1.1 1.4 2.8 6.9 9.2 10.7

Claims

1. A method for producing needle coke from ethylene tar, characterized in that: The method includes the following steps: (1) After the ethylene tar feedstock is mixed with hydrogen-containing gas, it enters the hydrogenation reaction zone and reacts in sequence with at least two hydrogenation catalyst beds; (2) The reaction effluent obtained in step (1) is separated into gas phase product and liquid phase product by gas-liquid separation; (3) The liquid product obtained in step (2) enters the coking unit. The oil and gas generated by the reaction are separated to obtain gas, light component oil, middle distillate oil and heavy distillate oil. After the reaction cycle is completed, the final needle coke product is obtained. In step (1), the hydrogenation reaction zone consists of an upstream hydrogenation catalyst bed and a downstream hydrogenation catalyst bed arranged sequentially along the liquid phase flow direction. Compared with the hydrogenation catalyst in the adjacent upstream hydrogenation catalyst bed, the downstream hydrogenation catalyst exhibits the following characteristics: the proportion of mesopores >30 nm in the total pore volume decreases by 30-60 percentage points; the proportion of mesopores <5 nm in the total pore volume increases by 30-50 percentage points; the infrared total acidity of the hydrogenation catalyst increases by 0.1-0.2 mmol / g; the B / L acid ratio decreases by 0.1-0.3; and the mass fraction of active metal components in the hydrogenation catalyst, calculated as oxides, increases by 3-20 wt%.

2. The method according to claim 1, characterized in that: In the hydrogenation reaction zone described in step (1), two hydrogenation catalyst beds are set up. The material passes through catalyst bed A and catalyst bed B in sequence according to the contact order with the material.

3. The method according to claim 2, characterized in that: In step (1), the hydrogenation catalyst packed in catalyst bed A has a mesopore volume >30nm accounting for 40~90% of the total pore volume, and a mesopore volume <5nm accounting for 10~30% of the total pore volume; the total infrared acid content is 0.20~0.30 mmol / g, the B / L acid ratio is 0.6~0.8, and the mass content of active metal components as oxides is 1.0~5.0 wt%.

4. The method according to claim 2, characterized in that: In step (1), the hydrogenation catalyst packed in catalyst bed B has a mesopore volume >30 nm accounting for 10-30% of the total pore volume, and a mesopore volume <5 nm accounting for 40-80% of the total pore volume; the total infrared acid content is 0.30-0.50 mmol / g, the B / L acid ratio is 0.3-0.5, and the mass content of active metal components as oxides is 4.0-25.0 wt%.

5. The method according to claim 2, characterized in that: In step (1), the volume ratio of catalyst bed A to catalyst bed B is (10~40):(60~90).

6. The method according to claim 1, characterized in that: The olefin content in the ethylene tar feedstock described in step (1) is 4-25 mol%; the saturated content does not exceed 10 wt%; and the total content of aromatics and gums exceeds 75 wt%.

7. The method according to claim 6, characterized in that: The olefin content in the ethylene tar feedstock described in step (1) is 4~20 mol%; the saturated content is 0.5~8.0 wt%; and the total content of aromatics and gums is 80~98 wt%.

8. The method according to claim 1, characterized in that: In step (1), aromatic oil products are also blended, which are one or more of catalytic diesel, catalytic wax oil, catalytic slurry oil and catalytic cracking clarified oil; the blending ratio of aromatic oil products is 1~50 wt%.

9. The method according to claim 8, characterized in that: In step (1), the blending ratio of aromatic oil is 15~45 wt%.

10. The method according to claim 1, characterized in that: The hydrogen-containing gas mentioned in step (1) is hydrogen or a mixture of hydrogen and other gases; the other gases are high-splitting gas and / or low-splitting gas from the hydrogenation unit.

11. The method according to claim 10, characterized in that: In step (1), the high-density gas comes from hydrogenation units with hydrogen partial pressure in the range of 4.0~20.0 MPa, including naphtha, diesel, wax oil and residue oil hydrogenation units.

12. The method according to claim 1, characterized in that: The hydrogenation reaction zone described in step (1) shall be equipped with at least one hydrogenation reactor.

13. The method according to claim 12, characterized in that: The hydrogenation reaction zone described in step (1) is equipped with 1 to 2 hydrogenation reactors.

14. The method according to claim 12, characterized in that: In step (1), the hydrogenation reactor is one or more of the following: a trickle bed reactor, a fluidized bed reactor, and a tubular reactor.

15. The method according to claim 1, characterized in that: The operating conditions of the hydrogenation reaction zone described in step (1) are as follows: reaction pressure 3.0~15.0 MPa; volume hourly space velocity 0.1~10.0 h⁻¹. -1 The hydrogen-to-oil volume ratio is 200-1000; the reaction temperature is 100-400℃.

16. The method according to claim 15, characterized in that: The operating conditions of the hydrogenation reaction zone described in step (1) are as follows: reaction pressure 4.0~12.0 MPa; volume hourly space velocity 0.2~3.0 h⁻¹. -1 The hydrogen-to-oil volume ratio is 250-800; the reaction temperature is 110-360℃.

17. The method according to claim 1, characterized in that: The gaseous product obtained in step (2) is a hydrogen-rich gas, which is returned to the hydrogenation reaction zone as recycled hydrogen.

18. The method according to claim 1, characterized in that: The coking unit in step (3) is equipped with at least one heating furnace, two coke towers and one fractionation tower, and at least one coke tower is always kept in the reaction stage and at least one coke tower is in the decoking stage.

19. The method according to claim 1, characterized in that: The reaction conditions of the coking unit in step (3) are as follows: the outlet temperature of the heating furnace is operated by temperature variation, with a temperature range of 380℃~550℃; the top pressure of the coke tower is 0.01MPa~3.0MPa, and the operation is either constant pressure or variable pressure; the reaction cycle is 10h~48h.

20. The method according to claim 19, characterized in that: The reaction conditions of the coking unit in step (3) are as follows: the outlet temperature of the heating furnace adopts variable temperature operation, with a temperature range of 400℃~520℃; the top pressure of the coke tower is 0.5MPa~1.0MPa, constant pressure or variable pressure operation; the reaction cycle is 18h~36h.

21. The method according to claim 19, characterized in that: When using variable pressure operation in step (3), the pressure change rate is 0.01MPa / h to 60MPa / h.

22. The method according to claim 19, characterized in that: When using variable pressure operation in step (3), the pressure change rate is 0.2MPa / h to 10MPa / h.

23. The method according to claim 1, characterized in that: In step (3), the liquid phase product is blended with distillate oil and then enters the coking unit. The distillate oil is selected from one or more of atmospheric residue, vacuum residue, catalytic gasoline, catalytic diesel, catalytic wax oil, catalytic slurry, lubricating oil furfural refined extract oil and catalytic cracking clarified oil. The weight ratio of distillate oil to ethylene tar is 0.5 to 4.0.

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