A method for recovering solvent and dehydrating it in a solution polymerization process

Abstract

The present invention provides a method for recovering solvent and dehydrating water in a solution polymerization process, comprising: removing a first polymer solution obtained from the solution polymerization reaction, mixing the first polymer solution with a first polymerization reaction deactivator, and then conveying the mixture to a first separator; removing a first steam flow from the first separator and circulating the first steam flow to a polymerization reactor; mixing the second polymer solution and the second polymerization reaction deactivator, and conveying the mixture to a separation system composed of several stages of gas-liquid separators in series, wherein the steam flows obtained from each stage of the gas-liquid separators are conveyed to a first fractionator; obtaining a first top flow and a first bottom flow from the first fractionator, and recycling a portion of the first bottom flow as a second recycle flow to the polymerization reactor; and the mass content of water in the first recycle flow and the second recycle flow is less than 1 ppm. The present invention controls the amount of deactivator used to timely deactivate the polymerization reaction, thereby controlling the stable performance of the polymer product and reducing process costs.

Description

Technical Field

[0001] The present invention relates to a method for recovering solvent and dehydrating it in a solution polymerization process, in particular to a method for using water as a polymerization reaction deactivator and removing moisture from volatile hydrocarbons separated and recovered from a polymer solution. Background Art

[0002] In the olefin polymerization process, catalyst deactivation must be performed after the polymerization reaction to prevent continued reaction and potentially uncontrollable product performance. This process is typically accomplished by adding a deactivator. Typical deactivators include alcohols such as methanol and isopropanol, as well as water in vapor or liquid form. The polymer solution has a certain residence time in the separator, and the deactivator is typically added before entering the downstream separation system of the reactor, that is, before the monomers and solvent are removed from the polymer solution. This order of addition may also result in the recovered solvent containing the deactivator, requiring appropriate purification to remove the deactivator before reuse.

[0003] Patent US 20110172375A1 discloses a method using methanol as a deactivator during solution polymerization. The methanol in the recovered solvent must be removed through a drying bed. This can be done using 4A molecular sieves, or 3A molecular sieves can be used to remove both water and methanol. Patent CN 1176257A proposes a method for terminating gas-phase polymerization by adding a deactivator consisting of carbon monoxide, carbon dioxide, oxygen, water, an alcohol containing 1-6 carbon atoms, or an aldehyde containing 1-6 carbon atoms at a position above the gas distribution plate. The deactivator is added in gaseous form. Patent CN 101998967B discloses a method using a liquid deactivator comprising water / alcohol or a solid deactivator comprising sodium stearate / calcium stearate. The patent also mentions that if the deactivator is volatile, the amount added should be determined based on the monomer, solvent recovery method, and product characteristics, but the patent does not specify the method.

[0004] In addition to the method of adding a deactivating agent, patent WO 2021136629A1 proposes a method of deactivating the catalyst by heating a polymer solution. The product of the temperature rise (°C) and the heating time (minutes) of the polymer solution needs to be greater than 0.05, preferably greater than 0.1, and more preferably greater than 0.5. The method of deactivating the catalyst by heating the polymer solution can avoid the problem of introducing a deactivating agent into the recovered solvent. However, the properties of the polymer are greatly affected by temperature. Heating the polymer solution may result in the inability to obtain the product required for the process, and even thermal cracking of the polymer product. At the same time, heating the polymer solution will also increase the energy consumption of the process, resulting in poor economic efficiency.

[0005] While polymerization deactivation by adding a deactivating agent remains a viable method, there is still room for improvement in the method for adding the deactivating agent, the rate at which it is added, and how it is separated. This invention proposes a method for using water as the deactivating agent, controlling the amount and location of the deactivating agent, and removing the water via an azeotropic distillation column, resulting in a circulating solvent with a water content of less than 1 ppm. Summary of the Invention

[0006] In order to solve the problems in the prior art, the present invention proposes a method for recovering solvent and dehydrating it in a solution polymerization process.

[0007] The technical solutions of the present invention are as follows:

[0008] The present invention first provides a method for recovering solvent and dehydrating water in a solution polymerization process, the method comprising the following steps:

[0009] 1) performing a solution polymerization reaction in a polymerization reactor to obtain a first polymer solution; taking out the first polymer solution, mixing it with a first polymerization reaction deactivator, and then transferring it to a first separator;

[0010] 2) removing a first steam stream and a second polymer solution from the first separator, conveying the first steam stream to a first heat recovery device to obtain a first recycle stream, and conveying the first recycle stream to a polymerization reactor;

[0011] 3) mixing the second polymer solution and the second polymerization deactivator, and then conveying the mixed mixture to a separation system composed of several stages of gas-liquid separators in series, wherein each stage of the gas-liquid separator obtains a steam flow as a gas phase product, and the liquid phase separated by the previous stage of the gas-liquid separator is fed as a feed to the next stage of the gas-liquid separator for gas-liquid separation;

[0012] 4) The steam obtained from each gas-liquid separator is transported to the first fractionator after heat recovery; the liquid phase obtained by the last gas-liquid separator is output as a concentrated polymer solution;

[0013] 5) obtaining a first top stream and a first bottom stream from the first fractionator, recovering at least a portion of the first bottom stream as a second recycle stream, and conveying the second recycle stream to the polymerization reactor;

[0014] The first polymerization reaction deactivator and the second polymerization reaction deactivator are both water, the mass content of water in the first recycling stream and the second recycling stream is less than 1 ppm, and the first fractionator is an azeotropic distillation tower.

[0015] As a preferred embodiment of the present invention, the amount of the first polymerization reaction deactivator added per unit time is less than the total amount of deactivator required for the catalyst and co-catalyst added per unit time, preferably 50 to 90% of the total amount (by mass) of the deactivator required for the catalyst and co-catalyst added per unit time;

[0016] The mass flow rate of the second polymerization deactivating agent added is less than 5% of the mass flow rate of the second polymer solution, preferably less than 2%, and more preferably less than 1%.

[0017] As a preferred embodiment of the present invention, the solvent used in the solution polymerization reaction and water exhibit a binary azeotropic phenomenon; the solvent is selected from C4-C12 chain alkanes or cycloalkanes, C6-C9 aromatic hydrocarbons or mixed solvents thereof, preferably C5-C8 chain alkanes or cycloalkanes.

[0018] As a preferred embodiment of the present invention, the first polymerization reaction deactivator is added before the polymer solution enters the first separator, or before the polymer solution enters the heat exchanger upstream of the first separator; the second polymerization reaction deactivator is added before the polymer solution enters the separation system, or before the polymer solution enters the heat exchanger upstream of the separation system.

[0019] As a preferred embodiment of the present invention, a cooler and a collecting tank are provided on the top of the first fractionator. The collecting tank is used to collect condensate and transport at least a portion of the material in the collecting tank back to the first fractionator.

[0020] As a preferred embodiment of the present invention, the first separator is a flash unit. The separation system is a two-stage system comprising a second separator as a front stage and a third separator as a back stage, wherein the second separator is a flash unit and the third separator is a deep devolatilizer, preferably a falling film devolatilizer or a scraped film devolatilizer.

[0021] As a preferred embodiment of the present invention, the first separator operates at a pressure of 5 to 16 bar, preferably 8 to 12 bar; the second separator operates at a pressure of 1 to 5 bar, preferably 1 to 3 bar; and the third separator operates at a pressure of 0.1 to 1 bar, preferably 0.1 to 0.5 bar.

[0022] As a preferred embodiment of the present invention, in step 3), a stripping agent is further added to the second polymer solution, and the second separator and the third separator are separated in the presence of the stripping agent, which is steam, water, CO2, or nitrogen; the amount of the stripping agent added is 0 to 5% of the mass flow rate of the polymer solution.

[0023] As a preferred embodiment of the present invention, the polymerization reactor is in the form of multiple combined reactors, including two reactors connected in series and two reactors connected in parallel.

[0024] (1) The present invention precisely controls the amount of deactivating agent used to deactivate the polymerization reaction in a timely manner, thereby overcoming the problem of unstable polymer product performance caused by excessive deactivating agent or untimely addition of deactivating agent in the prior art. (2) The present invention controls the location and amount of deactivating agent added, thereby overcoming the problem of deactivating agent being present in multiple locations in the process stream in the prior art, thereby reducing the processing volume required for deactivating agent removal. (3) The present invention removes the deactivating agent by azeotropic distillation, thereby overcoming the problem of high deactivating agent removal costs in the prior art and reducing economic costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention is a flowchart illustrating a solvent recovery and dehydration process in a solution polymerization process according to an embodiment of the present invention. DETAILED DESCRIPTION

[0026] The present invention will be further described and illustrated below in conjunction with specific embodiments. The embodiments are merely illustrative of the present disclosure and do not limit its scope. The technical features of the various embodiments of the present invention may be combined accordingly, provided that there is no conflict between them.

[0027] The following describes preferred embodiments of the present invention in more detail. Those skilled in the art should understand that the present invention is not limited by the embodiments described herein, and the scope of the present invention is not limited to the following examples. These embodiments are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0028] The following will refer to Figure 1 The method of the present invention is described as follows: Figure 1 An exemplary process for using the method of the present invention is shown. The system can be equipped with additional pipelines and / or additional equipment as needed, such as adding series and / or parallel reactors, and / or other devices for operating the system, such as valves and pumps.

[0029] like Figure 1 As shown, the present invention provides a method for recovering solvent and dehydrating water in a solution polymerization process, and the implementation steps of the method include:

[0030] 1) performing a solution polymerization reaction in a polymerization reactor to obtain a first polymer solution; taking out the first polymer solution, mixing it with a first polymerization reaction deactivator, and then transferring it to a first separator;

[0031] 2) removing a first steam stream and a second polymer solution from the first separator, conveying the first steam stream to a first heat recovery device to obtain a first recycle stream, and conveying the first recycle stream to a polymerization reactor;

[0032] 3) mixing the second polymer solution and the second polymerization deactivator, and then conveying the mixed mixture to a separation system composed of several stages of gas-liquid separators in series, wherein each stage of the gas-liquid separator obtains a steam flow as a gas phase product, and the liquid phase separated by the previous stage of the gas-liquid separator is fed as a feed to the next stage of the gas-liquid separator for gas-liquid separation;

[0033] 4) The steam obtained from each gas-liquid separator is transported to the first fractionator after heat recovery; the liquid phase obtained by the last gas-liquid separator is output as a concentrated polymer solution;

[0034] 5) obtaining a first top stream and a first bottom stream from the first fractionator, recovering at least a portion of the first bottom stream as a second recycle stream, and conveying the second recycle stream to the polymerization reactor;

[0035] The first polymerization reaction deactivator and the second polymerization reaction deactivator are both water, the mass content of water in the first recycling stream and the second recycling stream is less than 1 ppm, and the first fractionator is an azeotropic distillation tower.

[0036] In one embodiment of the present invention, the separation system is divided into two stages, including a second separator 6 and a third separator 8. Under this design, the system devices used to implement the method mainly include a feeding unit 1, a polymerization reaction unit 2, a first heat exchanger 3, a first separator 4, a second heat exchanger 5, a second separator 6, a third heat exchanger 7, a third separator 8, a first heat recovery device 9, a second heat recovery device 10, a third heat recovery device 11, and a first fractionator 12.

[0037] exist Figure 1In the process, ethylene monomer, α-olefin comonomer, solvent, catalyst, cocatalyst and hydrogen are introduced into the feed unit 1, wherein the monomer, comonomer and solvent are from the first recycling stream and also from the fresh feed 13; then, they are transported to the polymerization reaction unit 2 through pipeline 15; a first polymer solution is taken out from the polymerization reaction unit 2 and transported to the first separator 4 through pipeline 16, the first heat exchanger 3 and the pipeline 17, and before the first polymer solution enters the first heat exchanger 3, a deactivating agent is injected through pipeline 32 and mixed with the polymer solution; a first steam flow and a second polymer solution (first concentrated polymer solution) are obtained from the first separator 4; the first steam flow is transported to the first heat recovery device 9 through pipeline 18 to obtain a first recycling flow; the first recycling flow is transported to the feed unit 1 through pipeline 31; the second polymer solution from the first separator 4 is transported to the second separator 6 through pipeline 19, the second heat exchanger 5 and the pipeline 20; before the second polymer solution enters the second heat exchanger 5, a deactivating agent is injected through pipeline 33 and mixed with the first The two polymer solutions are mixed; a second steam flow and a third polymer solution (a second concentrated polymer solution) are obtained from the second separator 6; the second steam flow is conveyed to the second heat recovery device 10 through pipeline 21 to obtain a first condensate; the third polymer solution from the second separator 6 is conveyed to the third separator 8 through pipeline 22, the third heat exchanger 7 and pipeline 23; a third steam flow and a fourth polymer solution (a third concentrated polymer solution) are obtained from the third separator 8; the third steam flow is conveyed to the third heat recovery device 11 through pipeline 24 to obtain a second condensate; the first condensate and the second condensate are conveyed to the first fractionator 12 through pipelines 26 and 27 respectively; a first top flow and a first bottom flow are obtained from the top of the first fractionator, the first top flow is conveyed to the downstream separation unit through pipeline 28, and at least a portion of the first bottom flow is recovered as a second recycling flow. In this embodiment, at least 20% of the mass flow of the first bottom flow is circulated to the feed unit 1 through pipeline 29, and the remaining first bottom flow is conveyed to the downstream separation unit through pipeline 30.

[0038] As described above, the feedstock of the present invention primarily comprises ethylene monomer, α-olefin comonomer, solvent, catalyst, cocatalyst, and hydrogen. As is known in the art, copolymerization of ethylene and α-olefins is a method for producing polymers with varying molecular weights, densities, and other properties, such as linear low-density polyethylene (LLDPE) produced by copolymerizing ethylene and 1-octene, and high-density polyethylene (HDPE) produced by copolymerizing ethylene and 1-butene. The α-olefin can be an α-olefin containing 3-12 carbon atoms, and the comonomer is typically a single α-olefin, but can also be two or more α-olefins. Copolymerization of ethylene and multiple α-olefins can produce multi-component copolymers. As is known in the art, the solvent must be inert to the catalyst system and reactants and remain stable during the reaction. The solvent is selected from C4-C12 chain alkanes or cycloalkanes, C6-C9 aromatic hydrocarbons, or mixed solvents thereof, preferably C5-C8 chain alkanes or cycloalkanes. N-hexane, cyclohexane, and Isopar E are all known and widely used solvents in the art. As is known in the art, solution polymerization requires a large amount of solvent to maintain the ethylene monomer, α-olefin comonomer, catalyst, molecular weight regulator and polymer product in a single phase in the reactor.

[0039] As is known in the art, the polymerization reaction of ethylene monomer and α-olefin comonomer must be carried out in the presence of a catalyst. The catalyst can be any catalyst known in the art suitable for solution copolymerization of ethylene and α-olefin comonomers, including metallocene catalysts and ZN catalysts, such as a constrained geometry catalyst (CGC) as the primary catalyst and methylaluminoxane (MAO) as the cocatalyst. As is known in the art, the addition of a small amount of a substance with a large chain transfer constant can reduce the molecular weight of the polymer. Various suitable chain transfer agents exist, with hydrogen being a common example.

[0040] Before entering the separation system, the polymer solution must be heated in a heat exchanger to achieve better separation and remove volatile components from the polymer solution. Patent CN 109563187B describes a method using a spiral heat exchanger as a polymer solution preheater. In this method, the heat exchanger can be a shell-and-tube heat exchanger with flow-enhancing internals.

[0041] As is known in the art, polymer solution separation requires multiple separation and recovery steps, typically including multiple flash evaporation, distillation, circulation, and extrusion processes. The main purpose is to remove volatile components from the polymer solution to obtain a polymer that meets product requirements. Patent CN106414509 A discloses a solution polymerization method involving two-stage flash devolatilization. The heated polymer solution from the heat exchanger outlet is fed to a first-stage flash tank for separation. The top vapor stream is conveyed to a heat recovery unit, and the bottom liquid stream is fed to a second-stage flash tank for further separation. The temperature is adjusted by a heat exchanger before entering the second-stage flash unit. In the present invention, the first separator operates at a pressure of 5 to 16 bar, preferably 8 to 12 bar; the second separator operates at a pressure of 1 to 5 bar, preferably 1 to 3 bar; and the third separator operates at a pressure of 0.1 to 1 bar, preferably 0.1 to 0.5 bar. In the present invention, the polymer solution is heated by a heat exchanger before entering the first, second, and third separators to provide the energy required for separation. In addition, a deactivating agent needs to be injected into the polymer solution before it enters the heat exchanger upstream of the first separator and the second separator.

[0042] The method of the present invention is further described below with reference to examples and comparative examples. It should be noted that the examples herein are only used to illustrate the present invention and are not intended to limit the present invention.

[0043] In Example 1, the process of the present invention was carried out. The reactor feed composition included 13,313.6 kg / h of ethylene, 8.3 kg / h of methane, 592.2 kg / h of ethane, 7,455.6 kg / h of 1-octene, 3,655.2 kg / h of 2-octene, 500 kg / h of octane, 58,448.5 kg / h of n-hexane as solvent, 0.625 kg / h of CGC as the primary catalyst, 4.89 kg / h of MAO as the co-catalyst, and 2.3 kg / h of hydrogen as a molecular weight regulator. The feed stream was split into two streams: one stream was cooled to -28°C by a cooler, and the other stream was cooled to 50°C by a heater. After mixing, the feed streams reached the specified temperature of -25°C. The reactor was an adiabatic autoclave reactor equipped with a stirrer. The reaction pressure was 40 bar, the residence time was 10 minutes, and the reaction temperature was 145.7°C. The polymer solution in the reactor contains 12500 kg / h of polymer, with a mass fraction of 14.9%.

[0044] The separation system utilizes a three-stage devolatilization process. The first stage is a medium-pressure flash evaporation at 16 bar. Before entering the first-stage flash tank, the polymer solution is mixed with 1.9 kg / h of deactivating agent and heated to 200°C via a heat exchanger. The first steam stream generated in the first-stage flash tank is conveyed to a first heat recovery unit to obtain a first recycle stream, which is then conveyed to the polymerization reactor. The second stage is a low-pressure flash evaporation at 3 bar. Before entering the second-stage flash tank, the polymer solution is mixed with 20 kg / h of deactivating agent and 980 kg / h of stripping agent and heated to 220°C via a heat exchanger. The third stage utilizes a falling-film devolatilizer at 1.3 bar, maintained at 190°C, and fed with 500 kg / h of stripping agent. After the three-stage devolatilization, the polymer solution enters an extruder at a pressure of 0.1 bar and a temperature of 200°C. The polymer is extruded and granulated to obtain the polymer. In the embodiment, the feed stream of the reactor is obtained by mixing the recycled stream and the fresh stream, and the mass flow rate of the recycled stream is 70976.5 kg / h, of which the mass flow rate of the first recycled stream is 35117.8 kg / h, the mass flow rate of the second recycled stream is 8946.4 kg / h, and the other recycled streams come from other equipment in the separation unit. The water content of all recycled streams is less than 1 ppm.

[0045] In Example 1, an azeotropic distillation tower is selected as the first fractionator; the mass flow rate entering the azeotropic distillation tower is 36399.0 kg / h, including 77.3 kg / h of water. In Example 1, the number of plates of the azeotropic distillation tower is 15, the feed plate is the first, and the operating pressure is 4.5 bar. The mass ratio of the overhead distillate to the feed is 0.04. A condenser and a separator are set at the top of the tower. The condenser temperature is 40°C. The separator is an oil-water separation tank. The oil phase with a lower water content is completely refluxed to the azeotropic distillation tower, and the water phase with a higher water content is transported to the wastewater treatment unit. The mass of the stream obtained at the bottom of the azeotropic distillation tower is 35786.0 kg / h, and the mass fraction of the water content is 2.66x10 -8 .

[0046] Comparative Example 1 uses the same method as Example 1, except that the mass flow ratio of the overhead distillate to the feed stream of the azeotropic distillation column is 0.02. The water content of the bottom liquid obtained by this method is 0.0015 by mass.

[0047] Comparative Example 2 adopts the same method as Example 1, except that no azeotropic distillation tower is provided, and water in the recovered solvent is removed by providing a drying bed.

[0048] Table 1 shows the water content of the kettle liquid, the utility consumption of the reboiler and the condenser of the azeotropic distillation tower of Example 1 and Comparative Example 1. The utility used in the azeotropic distillation tower is 160°C saturated steam, whose calorific value is 2111.6kJ / kg. In Example 1, the reboiler energy consumption required for each kg of water is 118875kJ, and 56.30kg of steam is required to remove 1kg of water. One ton of steam costs 230 yuan, and it costs 12.95 yuan to remove 1kg of water. Comparing Example 1 with Comparative Example 1, the marginal thermal utility consumption for each additional kg of water removed is

[0049]

[0050] Only 5.7 kg steam / kg H2O is required, which is much less than the average consumption in Comparative Example 1. That is, increasing the distillate and feed ratio in Example 1 can achieve better dehydration effect, and the average energy consumption for dehydration is lower.

[0051] The desiccant used in Comparative Example 2 is a widely used molecular sieve desiccant. Each kilogram of desiccant absorbs 0.04 kilograms of water. The desiccant costs 14 yuan per kilogram, has an average lifespan of three years, and requires regeneration every three months, for a total of 12 cycles. Therefore, the maximum absorption capacity per kilogram of desiccant is 1 * 0.04 * 12 = 0.48 kilograms. Therefore, the desiccant cost required to remove 1 kilogram of water is 1 / 0.48 * 14 = 29 yuan, which is much higher than the cost of Example 1.

[0052]

[0053] The above-described embodiments merely illustrate several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. Persons skilled in the art will readily appreciate that variations and modifications may be made without departing from the scope of the present invention, all of which fall within the scope of protection of the present invention.

Claims

1. A method for recovering solvent and dehydrating in a solution polymerization process, characterized in that: The method comprises the following steps: 1) performing a solution polymerization reaction in a polymerization reactor to obtain a first polymer solution; taking out the first polymer solution, mixing it with a first polymerization reaction deactivator, and then transferring it to a first separator; The first polymer solution is obtained by reacting ethylene monomer, α-olefin comonomer, solvent, catalyst, cocatalyst and hydrogen; 2) removing a first steam stream and a second polymer solution from the first separator, conveying the first steam stream to a first heat recovery device to obtain a first recycle stream, and conveying the first recycle stream to a polymerization reactor; 3) mixing the second polymer solution and the second polymerization deactivator, and then conveying the mixed mixture to a separation system composed of several stages of gas-liquid separators in series. In the separation system, each stage of the gas-liquid separator obtains a vapor flow as a gas phase product, and the liquid phase separated by the previous stage of the gas-liquid separator is conveyed as a feed to the next stage of the gas-liquid separator for gas-liquid separation; 4) The steam obtained from each stage of the gas-liquid separator is transported to the first fractionator after heat recovery; the liquid phase separated by the last stage of the gas-liquid separator is output as a concentrated polymer solution; 5) obtaining a first top stream and a first bottom stream from the first fractionator, recovering at least a portion of the first bottom stream as a second recycle stream, and conveying the second recycle stream to the polymerization reactor; The first polymerization reaction deactivator and the second polymerization reaction deactivator are both water, the mass content of water in the first recycle stream and the second recycle stream is less than 1 ppm, and the first fractionator is an azeotropic distillation column; the mass ratio of the overhead distillate to the feed of the azeotropic distillation column is 0.04; The amount of the first polymerization reaction deactivator added per unit time is less than the total amount of deactivator required for the catalyst and co-catalyst added per unit time; The mass flow rate of the added second polymerization deactivating agent is less than 5% of the mass flow rate of the second polymer solution; The first separator is a flash unit; The separation system is two-stage, including a second separator as the front stage and a third separator as the back stage, the second separator is a flash unit, and the third separator is a deep devolatilizer; The first separator operates at a pressure of 5 to 16 bar; the second separator operates at a pressure of 1 to 5 bar; and the third separator operates at a pressure of 0.1 to 1 bar.

2. The method for recovering solvent and dehydrating the solution polymerization process according to claim 1, characterized in that: The solvent used in the solution polymerization reaction and water have a binary azeotropic phenomenon; the solvent is selected from C4~C12 chain alkanes or cycloalkanes, C6~C9 aromatic hydrocarbons or mixed solvents thereof.

3. The method according to claim 1, characterized in that The first polymerization reaction deactivator is added before the polymer solution enters the first separator, or before the polymer solution enters the heat exchanger upstream of the first separator; the second polymerization reaction deactivator is added before the polymer solution enters the separation system, or before the polymer solution enters the heat exchanger upstream of the separation system.

4. The method according to claim 1, wherein A cooler and a collecting tank are provided on the top of the first fractionator. The collecting tank is used to collect condensate and transport at least a portion of the material in the collecting tank back to the first fractionator.

5. The method according to claim 1, wherein In step 3), a stripping agent is added to the second polymer solution, and the second separator and the third separator are separated in the presence of the stripping agent, which is steam, water, CO2, or nitrogen. The amount of the stripping agent added is 0-5% of the mass flow rate of the polymer solution.

6. The method according to claim 1, characterized in that The polymerization reactor is in the form of a single reactor or multiple reactors, including two reactors connected in series or two reactors connected in parallel.

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