Method for recovering ethylene, ethylene oligomerization method and ethylene oligomerization system

Abstract

The invention discloses a method for recovering ethylene, an ethylene oligomerization method and an ethylene oligomerization system, the method for recovering ethylene comprises the following steps: carrying out oligomerization reaction on an ethylene-containing raw material to obtain an oligomerization reaction system of the ethylene-containing material; the ethylene-containing material is subjected to membrane separation and purification to obtain an ethylene-containing recycled material, and the membrane separation comprises two membrane separation processes in an optional sequence: a membrane separation process M and a membrane separation process M '; the membrane separation process M is used for separating C3 + hydrocarbons in the material; the membrane separation process M'is used for separating H2 in the material.In the separation and purification process of the ethylene oligomerization product, the combined membrane separation system is used for purifying circulating ethylene, energy is saved, consumption is reduced, and meanwhile the purpose of guaranteeing reaction selectivity during long-period operation is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for recycling ethylene, a method for ethylene oligomerization and an ethylene oligomerization system, in particular to a method for purifying recycled ethylene using a membrane separation system in the separation and purification process of ethylene oligomerization products, belonging to the technical field of petrochemical industry. BACKGROUND

[0002] Ethylene oligomerization reaction is a method for converting reactant ethylene into one or more products (usually C 2-20 Oligomerization products containing C 2-20 hydrocarbons), the reaction liquid discharged from the reactor usually contains a large amount of unreacted monomer ethylene in the gas phase after gas-liquid separation. When ethylene is in contact with other components (such as other reactants and / or catalysts), it can react to form by-products. When the boiling point of the by-products is low, they will enter the unreacted ethylene during gas-liquid separation to form a mixed gas. In addition, when there are non-ethylene gas phase reactants in the ethylene oligomerization reaction, they will also form a mixed gas with unreacted ethylene during gas-liquid separation. If this part of the gas is directly discharged to the flare system for burning, it not only causes serious waste of resources, but also increases the unit consumption of raw materials.

[0003] In order to save costs, the device will generally recycle and reuse ethylene through condensation. However, there will still be a small amount of non-ethylene gas in the recycled ethylene using conventional condensation process. With the accumulation of recycling, it will affect the subsequent reaction and generate more by-products, reducing product selectivity. If deep cooling separation technology is used, although the purity of recycled ethylene is further improved, the investment of deep cooling separation is large and the energy consumption is high. Therefore, it is of great significance to do a good job in energy saving and consumption reduction of ethylene oligomerization device on the basis of ensuring product quality. SUMMARY

[0004] In view of the shortcomings of the prior art, the present application provides a method for recycling ethylene, a method for ethylene oligomerization and an ethylene oligomerization system, which purifies recycled ethylene using a membrane separation system in the separation and purification process of ethylene oligomerization products, saves energy and reduces consumption, and achieves the purpose of ensuring reaction selectivity during long-term operation.

[0005] To achieve the above-mentioned purpose, on the one hand, the present application provides a method for recycling ethylene, which comprises the following steps: subjecting a material containing ethylene to an oligomerization reaction to obtain an oligomerization reaction system containing C 2-20 hydrocarbons; purifying the material containing ethylene by membrane separation to obtain recycled material containing ethylene; the membrane separation comprises two membrane separation processes in any order: membrane separation process M and membrane separation process M'; the membrane separation process M is used to separate C 3+ hydrocarbons in the material; and the membrane separation process M' is used to separate H2 in the material.

[0006] As a preferred technical solution, the membrane separation process is used for separating ethylene and C 3+ Membrane separation process of hydrocarbons, retaining C 3+ Membrane separation process of hydrocarbons, retaining C

[0007] The oligomerization system contains solvents, unreacted raw materials, catalysts, and reaction products (reaction products include α-olefins, low-boiling byproducts, heavy components, etc.).

[0008] As a preferred technical solution, the content of ethylene in the recovered material is ≥70%, and the water content is <10 ppm.

[0009] As a preferred technical solution, the flow rate of the ethylene-containing material subjected to membrane separation purification is 100-700 m 3 / h, preferably 300-600 m 3 / h.

[0010] As a preferred technical solution, the membrane separation includes two membrane separation processes in optional sequence: membrane separation process M and membrane separation process M';

[0011] The conditions of membrane separation process M include: feed pressure 0.01-2 MPa, feed temperature 5-40℃; the feed pressure is selected from any value or a range value between any two of 0.01 MPa, 0.06 MPa, 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa, 1 MPa, 2 MPa.

[0012] The conditions of membrane separation process M' include: feed pressure 0.006-1 MPa, feed temperature 20-40℃; the feed pressure is selected from any value or a range value between any two of 0.006 MPa, 0.03 MPa, 0.06 MPa, 0.17 MPa, 0.4 MPa, 0.6 MPa, 1 MPa.

[0013] As a preferred technical solution, the ethylene-containing material is first heated and then subjected to membrane separation; preferably, the heating causes the temperature of the ethylene-containing material to rise by 5-10℃. The temperature rise of the gas (ethylene-containing material) is selected from any value or a range value between any two of 5℃, 6℃, 7℃, 8℃, 9℃, 10℃. In a preferred embodiment of the present application, the temperature of the gas rises by 7℃.

[0014] As a preferred technical solution, the C 2-20 The olefin is preferably C 4-12 The olefin is more preferably 1-hexene and / or 1-octene.

[0015] As a preferred technical solution, the oligomerization reaction conditions include: a reaction pressure of 0.1-20 MPa and a reaction temperature of 0-200℃.

[0016] In another aspect, the application provides an ethylene oligomerization method, comprising the following steps in sequence:

[0017] S1: the ethylene-containing raw material is subjected to oligomerization to obtain a C 2-20 An oligomerization reaction system of olefins, which is contacted with a reaction termination agent to obtain a terminated reaction system; the contact of the oligomerization reaction system with the reaction termination agent deactivates the catalyst to obtain the terminated reaction system; preferably, the reaction termination agent is an oxygen-containing compound, preferably deionized water;

[0018] S2: the terminated reaction system is subjected to gas-liquid separation to obtain a gas phase and a liquid phase, and the gas phase is condensed to obtain an ethylene-containing material,

[0019] S3: the ethylene-containing material is recovered to obtain a recovered material containing ethylene, which is returned to the oligomerization reaction; the recovery is performed by using any of the above-mentioned methods for recovering ethylene.

[0020] The reaction process is controlled by adjusting the total amount of ethylene fed into the reactor; when the amount of recycled ethylene is insufficient, fresh ethylene is added to make up for it; the amount of recycled ethylene feed varies according to the reaction conditions and is not particularly limited.

[0021] As a preferred technical solution, the gas-liquid separation in S2 includes flash separation and fractionation; the terminated reaction system is subjected to flash separation to obtain a flash gas phase and a flash liquid phase; the flash gas phase includes an ethylene-containing material I; the oligomerization reaction system of the terminated reaction is input into a flash system to separate the unreacted raw material from the reaction liquid of the terminated reaction. The flash liquid phase is subjected to fractionation to obtain an overhead distillate and a bottom distillate, and the overhead distillate includes an ethylene-containing material II; the flash gas phase and the overhead distillate are mixed, and then subjected to the membrane separation purification.

[0022] As a preferred technical solution, the flash separation conditions include a flash temperature of 100-150℃ and an operating pressure of 0.1-2 MPa.

[0023] As a preferred technical solution, the fractionation conditions include an overhead temperature of 40-180℃, a bottom temperature of 50-200℃, a pressure of 0.2-2 MPa, and a reflux ratio of 5-80.

[0024] As a preferred technical solution, the total selectivity of 1-hexene and 1-octene prepared by the ethylene oligomerization method is ≥88%, preferably ≥94.9%.

[0025] The total selectivity of the 1-hexene and 1-octene is selected from any value of 88%, 89.6%, 92.8%, 94.9% or a range value between any two of them.

[0026] As a preferred technical solution, the S2 further comprises a condensing I process; the flash gas phase after the condensing I process obtains a gas-liquid mixture; the gas-liquid mixture contains condensed gas phase and condensed liquid phase; the condensed gas phase contains material I containing ethylene; the condensed liquid phase is mixed with the flash liquid phase, and then is fractionated; the condensed gas phase is mixed with the overhead distillate, and then is subjected to the membrane separation and purification; the condensing I process is carried out at a temperature of 0-55°C, preferably 0.5-40°C.

[0027] Preferably, the S2 further comprises a condensing II process; the overhead distillate is subjected to the condensing II process to obtain material II containing ethylene; the condensed gas phase is mixed with the material II containing ethylene, and then is subjected to the membrane separation and purification; the condensing II process is carried out at a temperature of 0-55°C, preferably 0.5-40°C.

[0028] In another aspect, the present application provides an ethylene oligomerization system, comprising an oligomerization reactor, a separation device and a membrane separation assembly; the oligomerization reaction system is output from the oligomerization reactor; the oligomerization reactor is connected to the separation device, and the gas phase outlet of the separation device is connected to the membrane separation assembly; the membrane separation assembly comprises two membrane assemblies in optional sequence: membrane assembly M and membrane assembly M'; the membrane assembly M is used for retaining C 3+ hydrocarbons; and the membrane assembly M' is used for permeating H2.

[0029] As a preferred technical solution, the membrane material of the membrane assembly M is selected from polyimide and modified polymers thereof, polysulfone and modified polymers thereof, cellulose acetate and modified polymers thereof or polycarbonate and modified polymers thereof, preferably polyether block polyamide membrane and cross-linked sulfonated polyimide membrane; the membrane parameter of the membrane assembly M is a separation coefficient of ethylene and C 3+ hydrocarbons ≥ 30; preferably ≥ 100; more preferably ≥ 300;

[0030] As a preferred technical solution, the membrane material of the membrane assembly M' is carbon molecular sieve membrane; the membrane parameter of the carbon molecular sieve membrane is a separation coefficient of ethylene and H2 ≥ 10; preferably ≥ 50; preferably ≥ 100; as known in the art, the carbon molecular sieve membrane is prepared by pyrolysis of a polymer precursor, and the polymer precursor comprises a rigid, glassy polymer; the polymer precursor of the carbon molecular sieve membrane of the present application is preferably polyimide and its derivatives, polyvinylidene, and metal-doped mixed polymers.

[0031] As a preferred technical solution, a heat tracing device is arranged on the pipeline through which the recycled ethylene enters the membrane separation system, so that the temperature of the gas is raised.

[0032] In the present application, the flow rate of the recycled ethylene when entering the membrane separation system is 100-700 m3 / h. In a preferred embodiment of the present application, the circulating ethylene has a flow rate of 300-600 m 3 / h when entering the membrane separation system.

[0033] In a preferred embodiment of the present application, the circulating ethylene has an ethylene content of not less than 70% and a water content of not higher than 10 ppm when entering the membrane separation system.

[0034] As a preferred technical solution, the separation device comprises a flash device and a light-removing unit column; the flash device comprises at least one flash tank; the outlet of the oligomerization reactor is communicated with the flash device; the gas phase outlet of the flash device is communicated with the membrane separation assembly; the liquid phase outlet of the flash device is communicated with the light-removing unit column; and the top of the light-removing unit column is communicated with the membrane assembly.

[0035] As a preferred technical solution, the system further comprises a cooler, a gas-liquid separator and a condenser; the gas phase outlet of the flash device, the cooler and the gas-liquid separator are sequentially communicated; the gas phase outlet of the gas-liquid separator is communicated with the membrane separation assembly; the liquid phase outlet of the gas-liquid separator is communicated with the light-removing unit column; and the top of the light-removing unit column, the condenser and the membrane assembly are sequentially communicated.

[0036] The gas phase flowing out of the flash tank is sent to the gas-liquid separation tank after being cooled by the cooler (carried out at normal temperature and pressure), the gas phase is sent to the membrane separation buffer tank, and the liquid phase is jointly sent to the light-removing unit column with the flash liquid phase. The top product of the light-removing column is sent to the reflux tank after being condensed by the condenser. In order to ensure that the top material can be fully cooled, the cooling medium of the top condenser is chilled water. The gas phase component in the reflux tank of the light-removing column is separated into gas and liquid phases, and the gas phase is also sent to the membrane separation buffer tank. The gas phase in the membrane separation buffer tank is sent to the membrane separation system after being heated to increase the temperature by 5-10°C.

[0037] Through the above technical solution, the present disclosure provides an ethylene oligomerization reaction method and an ethylene oligomerization system. In the separation and purification process of the ethylene oligomerization product, the circulating ethylene is purified by using a combined membrane assembly, which can remove low-boiling-point by-products and non-ethylene gas-phase reactants under mild conditions, saves energy and reduces consumption, improves the purity of the circulating ethylene, and to some extent, guarantees the product selectivity during long-period operation. DETAILED DESCRIPTION

[0038] The technical solutions of the present application are further described below according to specific embodiments. The protection scope of the present application is not limited to the following embodiments, and these examples are listed only for exemplary purposes and do not limit the present application in any way.

[0039] In the membrane separation system of the present application, at least two groups of membrane assemblies are independently connected in series, wherein the recycled ethylene is heated by a pipeline heater and then enters the membrane assembly M, the material of the membrane is inorganic membrane, polymer membrane and composite membrane, preferably polymer membrane and composite membrane, more preferably polymer membrane, the polymer membrane includes polyimide and modified polymer thereof, polysulfone and modified polymer thereof, cellulose acetate and modified polymer thereof, polycarbonate and modified polymer thereof, in a preferred embodiment of the present application, the material of the membrane assembly M is polyether block polyamide, cross-linked sulfonated polyimide, etc. After the recycled ethylene passes through the membrane assembly M, the permeate phase passes through the membrane assembly M', and in a preferred embodiment of the present application, the material of the membrane assembly M' is carbon molecular sieve membrane.

[0040] In the following examples and comparative examples,

[0041] Ethylene, polymerization grade, commercially available;

[0042] Chromium acetylacetonate, purchased from Beijing Bailingwei Chemical Reagent Co., Ltd.;

[0043] Methylcyclohexane, purchased from Beijing Bailingwei Chemical Reagent Co., Ltd.;

[0044] Modified methylaluminoxane (MMAO), purchased from AkzoNobel;

[0045] Chromium-based catalyst (acetylacetone chromium mixed with bridged diphosphine ligand at a molar ratio of 1:2), wherein the bridged diphosphine ligand is prepared according to the method of Preparation Example 3 in Chinese patent document CN111434669A.

[0046] The total selectivity of the reaction activity, 1-hexene and 1-octene is calculated according to the following formula:

[0047]

[0048] Wherein, m 总产物 refers to the mass of all products obtained by reaction;

[0049] n Cr refers to the molar amount of the catalyst calculated as chromium metal;

[0050] h refers to the hours of the oligomerization reaction of the ethylene-containing raw material;

[0051] m 1-己烯 refers to the mass of 1-hexene obtained by reaction;

[0052] m 1-辛烯 refers to the mass of 1-octene obtained by reaction.

[0053] Example 1 Ethylene oligomerization system

[0054] The ethylene oligomerization system includes an oligomerization reactor (high-pressure reactor), a separation device, and a membrane separation assembly; the oligomerization reactor is connected to the separation device, and the gas phase outlet of the separation device is connected to the membrane separation assembly; the membrane separation assembly includes membrane module M and membrane module M'; the permeate side of membrane module M is connected to the feed side of membrane module M'; the membrane material of membrane module M is a polyether block polyamide membrane; the membrane material of membrane module M' is a carbon molecular sieve membrane.

[0055] Example 2 Ethylene Oligomerization System

[0056] The ethylene oligomerization system includes an oligomerization reactor, a separation unit, and a membrane separation component; the oligomerization reactor is connected to the separation unit, and the gas phase outlet of the separation unit is connected to the membrane separation component.

[0057] The separation device includes a flash evaporator, a light-weight component removal tower, a cooler, a gas-liquid separator, and a condenser; the outlet of the oligomerization reactor is connected to the flash evaporator; the gas phase outlet of the flash evaporator, the cooler, and the gas-liquid separator are connected in sequence; the gas phase outlet of the gas-liquid separator is connected to a membrane separation module; the liquid phase outlet of the gas-liquid separator is connected to the light-weight component removal tower; the top of the light-weight component removal tower, the condenser, and the membrane module are connected in sequence, and the liquid phase outlet of the flash evaporator is connected to the light-weight component removal tower.

[0058] The membrane separation unit includes membrane module M and membrane module M'; the permeate side of membrane module M is connected to the feed side of membrane module M'; the membrane material of membrane module M is a polyether block polyamide membrane; the membrane material of membrane module M' is a carbon molecular sieve membrane.

[0059] Example 3: Ethylene oligomerization method

[0060] The method for ethylene oligomerization using the ethylene oligomerization system of Example 2.

[0061] Materials used in the reaction: ethylene (flow rate, 200 kg / h); methylcyclohexane (solvent, flow rate, 380 kg / h); chromium-based catalyst (concentration, 5 μmol / L based on chromium); modified methylaluminoxane (MMAO) (concentration, 2 mmol / L based on aluminum).

[0062] Reaction conditions: All reagents were fed under nitrogen protection, the reaction temperature was 40℃, and the reaction pressure was 3MPa.

[0063] After the reaction residence time reaches 30 minutes, the reaction liquid discharged from the polymerization reactor outlet comes into contact with the terminator (deionized water) and is then fed into a flash tank for flash evaporation. The temperature of the flash tank is 120℃ and the pressure of the flash tank is 0.9MPa. The flash gas phase (mainly including unreacted ethylene, low-boiling-point byproducts and a small amount of C6-C8 olefin products, etc.) enters the cooler from the top for condensation and gas-liquid separation. The temperature of the circulating water in the cooler is 10℃. The gas phase obtained from the gas-liquid separation (mainly ethylene and low-boiling-point byproducts) is sent to the membrane separation buffer tank.

[0064] The liquid phase material from the gas-liquid separation (mainly including unreacted ethylene, low-boiling-point byproducts generated during the reaction, C6-C8 olefins and components above C8, solvent, and deactivated catalyst components) enters the light phase stripping tower together with the flash liquid phase. This further separates the remaining unreacted ethylene and low-boiling-point byproducts in the reaction liquid. The top temperature of the light phase stripping tower is 108.6℃, and the bottom temperature is 186.7℃. The top products of the light phase stripping tower, ethylene and low-boiling-point byproducts, are sent to the reflux tank after passing through a condenser. To ensure sufficient cooling of the top material, the cooling medium in the top condenser is 5℃ circulating chilled water. The gas phase components (mainly ethylene and low-boiling-point byproducts) in the light phase stripping tower reflux tank also enter the membrane separation buffer tank after gas-liquid separation.

[0065] After confirming that there are sufficient gaseous components in the flash evaporation and light-duty removal towers to enter the membrane separation buffer tank (ethylene content 96.7%, water content 5 ppm in the buffer tank), the membrane separation system is started, and the gaseous phase in the membrane separation buffer tank is diverted at a rate of 400 m³ / h. 3 The flow rate of [amount] / h is heated by 7°C via pipeline before being fed into membrane module A (Pebax 1657 roll-up membrane, ethylene / C3+ hydrocarbon separation coefficient 300), with a feed pressure of 1MPa and a feed temperature of 33°C. The permeate phase of membrane module M is fed into membrane module M' (CMSM membrane obtained by pyrolysis of a mixture of polyvinylpyrrolidone and polyimide, H2 / ethylene separation coefficient 104, prepared according to the literature JAPPLPOLYM SCI, 2001, 79:836-841), with a feed pressure of 0.4MPa and a feed temperature of 33°C. The retentate phase (recycled ethylene) of membrane module M' is sent to the polymerization reactor for the next round of reaction. When ethylene absorption is stable (i.e., when the gas flow rate at the outlet of the flash phase fluctuates by no more than 5%), the reaction activity is 4.2 × 10⁻⁶. 8 g·mol(Cr) -1 ·h -1 The overall selectivity for 1-hexene and 1-octene was 94.9%, and the reactivity was 4.2 × 10⁻⁶ after 180 h of reaction. 8 g·mol(Cr) -1 ·h -1 The overall selectivity for 1-hexene and 1-octene was 94.9%.

[0066] Example 4

[0067] The experimental method was the same as in Example 3, except that the gas phase in the membrane separation buffer tank was not heated through pipelines (i.e., the temperature was not increased) and was directly fed into membrane module M. All other conditions remained unchanged. After 180 hours of reaction, the reaction activity was 4.0 × 10⁻⁶. 8 g·mol(Cr) -1 ·h -1The overall selectivity for 1-hexene and 1-octene was 89.6%.

[0068] Example 5

[0069] The experimental method was the same as in Example 3, except that the gas phase in the membrane separation buffer tank was heated by 3°C via a pipeline before being sent to membrane module M. All other conditions remained unchanged. After 180 hours of reaction, the reaction activity was 4.1 × 10⁻⁶. 8 g·mol(Cr) -1 ·h -1 The overall selectivity for 1-hexene and 1-octene was 92.8%.

[0070] Example 6

[0071] The experimental method was the same as in Example 3, except that the gas phase in the membrane separation buffer tank was heated by 15°C via a pipeline before being sent to the membrane module M'. All other conditions remained unchanged. After 180 hours of reaction, the reaction activity was 4.1 × 10⁻⁶. 8 g·mol(Cr) -1 ·h -1 The overall selectivity for 1-hexene and 1-octene was 88.0%.

[0072] Comparative Example 1

[0073] The experimental method was the same as in Example 3, except that the recycled ethylene was not separated through membrane module M, and all other conditions remained unchanged. After 180 hours of reaction, the reaction activity was 3.0 × 10⁻⁶. 8 g·mol(Cr) -1 ·h -1 The overall selectivity for 1-hexene and 1-octene was 77.0%.

[0074] Comparative Example 2

[0075] The experimental method was the same as in Example 3, except that the recycled ethylene was not separated through membrane module M', and all other conditions remained unchanged. After 180 hours of reaction, the reaction activity was 6.5 × 10⁻⁶. 7 g·mol(Cr) -1 ·h -1 The overall selectivity for 1-hexene and 1-octene was 82.1%.

[0076] Comparative Example 3

[0077] The experimental method was the same as in Example 3, except that the recycled ethylene did not pass through any membrane module, and all other conditions remained unchanged. After 180 hours of reaction, the reactivity was 5.2 × 10⁻⁶. 7 g·mol(Cr) -1 ·h -1 The overall selectivity for 1-hexene and 1-octene was 73.5%.

[0078] 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.

[0079] 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 method for recovering ethylene, characterized in that, The raw material containing ethylene is subjected to an oligomerization reaction to obtain a C 2-20 Oligomerization reaction system of olefins; the oligomerization reaction system comprises a material containing ethylene; the material containing ethylene is purified by membrane separation to obtain a recovered material containing ethylene, the membrane separation comprises two membrane separation procedures in optional sequence: membrane separation procedure M and membrane separation procedure M'; The membrane separation process M is used to separate C 3+ hydrocarbons; the membrane separation procedure M' is used for separating H2 in the material.

2. The method according to claim 1, characterized in that, the content of ethylene in the recovered material is ≥70%, and the water content is <10 ppm; And / or, the flow rate of the ethylene-containing material subjected to membrane separation purification is 100-700 m 3 / h, preferably 300-600 m 3 / h.

3. The method according to claim 1 or 2, characterized in that, the conditions of the membrane separation procedure M comprise: feed pressure 0.01-2 MPa, and feed temperature 5-40℃; the conditions of the membrane separation procedure M' comprise: feed pressure 0.006-1 MPa, and feed temperature 20-40℃.

4. The method according to any one of claims 1-3, characterized in that, the material containing ethylene is heated before the membrane separation; preferably, the heating makes the temperature of the material containing ethylene rise by 5-10℃.

5. The method according to any one of claims 1-4, characterized in that, The C 2-20 The olefin is preferably a C 4-12 The olefin is more preferably 1 -hexene and / or 1 -octene. and / or, the oligomerization reaction conditions comprise: reaction pressure 0.1-20 MPa, and reaction temperature 0-200℃.

6. A method for ethylene oligomerization, characterized in that, comprising continuous steps: S1 contains an ethylene-containing raw material, which is subjected to an oligomerization reaction to obtain a C 2-20 an oligomerization reaction system of an olefin, which is contacted with a reaction terminator to obtain a terminated reaction system; S2: the terminated reaction system is subjected to gas-liquid separation to obtain a gas phase and a liquid phase, and the gas phase is condensed to obtain a material containing ethylene, S3: the material containing ethylene is recovered to obtain a recovered material containing ethylene, which is returned to the oligomerization reaction; the recovery adopts the method for recovering ethylene according to any one of claims 1-5.

7. The method according to claim 6, characterized in that, the gas-liquid separation in S2 comprises flash separation and fractionation, the terminated reaction system is subjected to flash separation to obtain a flash gas phase and a flash liquid phase; the flash gas phase comprises a material containing ethylene I; the flash liquid phase is subjected to fractionation to obtain an overhead distillate and a bottom distillate, and the overhead distillate comprises a material containing ethylene II; the flash gas phase and the overhead distillate are mixed before the membrane separation purification, preferably, the conditions of the flash separation comprise: temperature 100-150℃, and pressure 0.1-2 MPa; preferably, the conditions of the fractionation comprise: overhead temperature 40-180℃, bottom temperature 50-200℃, pressure 0.2-2 MPa, and reflux ratio 5-80.

8. The method according to claim 6 or 7, characterized in that, in the method, the total selectivity of 1-hexene and 1-octene is ≥88%, preferably ≥94.9%.

9. The method according to claim 6 or 7 or 8, characterized in that, in S2, a step of condensing I is further included; the flash gas phase is subjected to condensing I to obtain a gas-liquid mixture; the gas-liquid mixture comprises a condensed gas phase and a condensed liquid phase; the condensed gas phase comprises the material containing ethylene I; the condensed liquid phase is mixed with the flash liquid phase before the fractionation; the condensed gas phase is mixed with the overhead distillate before the membrane separation purification; the temperature of condensing I is 0-55℃, preferably 0.5-40℃. Preferably, the S2 further comprises a condensation II process, the overhead of the condensation II process produces a material II containing ethylene, and the condensed gas phase is mixed with the material II containing ethylene, and then the membrane separation and purification process is performed again. The condensation II process is performed at a temperature of 0-55°C, preferably 0.5-40°C.

10. An ethylene oligomerization system, characterized in that, it comprises an oligomerization reactor, a separation device, and a membrane separation assembly; the oligomerization reactor is connected to the separation device, and the gas phase outlet of the separation device is connected to the membrane separation assembly; the membrane separation assembly comprises two membrane assemblies, a membrane assembly M and a membrane assembly M', which are connected in series in an optional order; The membrane module M is used to retain C 3+ hydrocarbons; the membrane assembly M' is used for permeating H2.

11. The ethylene oligomerization system according to claim 10, characterized in that, The membrane material of the membrane module M is selected from the group consisting of polyimide and modified polymers thereof, polysulfone and modified polymers thereof, cellulose acetate and modified polymers thereof or polycarbonate and modified polymers thereof, preferably polyether block polyamide membrane, cross-linked sulfonated polyimide membrane; the membrane parameters are ethylene and C 3+ The separation factor of hydrocarbons is > 30; preferably > 100, more preferably > 300; the membrane material of the membrane assembly M' is a carbon molecular sieve membrane; the separation coefficient of the carbon molecular sieve membrane for ethylene and H2 is ≥10, preferably ≥50, and more preferably ≥100.

12. The ethylene oligomerization system according to claim 10 or 11, characterized in that, the separation device comprises a flash device and a light-removing unit column; the outlet of the oligomerization reactor is connected to the flash device; the gas phase outlet of the flash device is connected to the membrane separation assembly; the liquid phase outlet of the flash device is connected to the light-removing unit column; the top of the light-removing unit column is connected to the membrane assembly.

13. The ethylene oligomerization system according to claim 12, characterized in that, the system further comprises a cooler, a gas-liquid separator, and a condenser; the gas phase outlet of the flash device, the cooler, and the gas-liquid separator are connected in series; the gas phase outlet of the gas-liquid separator is connected to the membrane separation assembly; and the liquid phase outlet of the gas-liquid separator is connected to the light-removing unit column; the top of the light-removing unit column, the condenser, and the membrane assembly are connected in series.

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