System and method for controlling gas composition in reactor in olefin gas phase copolymerization process

Abstract

The invention belongs to the technical field of olefin gas phase polymerization, and discloses a system and a method for controlling gas composition in a reactor in the olefin gas phase copolymerization process, the system comprises a gas supply tank, the gas outlet of the gas supply tank is connected with the gas inlet of the reactor through a gas phase discharge pipeline, and the gas phase discharge pipeline is connected with the reactor. A gas outlet of the reactor is connected with the gas supply tank through a circulating gas return pipeline; the gas supply tank is connected with a liquid-phase olefin feeding unit and a gas-phase olefin feeding unit, the gas supply tank is provided with a temperature control unit and a first pressure control unit, and the gas-phase discharging pipeline or the circulating gas returning pipeline is provided with a circulating compressor. The system and the method provided by the invention can realize the stability of the gas composition in the reactor in the copolymerization process so as to produce a copolymer product with stable composition, and have the characteristics of less equipment, simple process and easiness in operation.

Description

Technical Field

[0001] The present invention belongs to the technical field of olefin gas phase polymerization, and in particular, relates to a system and method for controlling the gas composition in a reactor during olefin gas phase copolymerization. Background Art

[0002] Olefin gas phase copolymerization is an important part of polyolefin technology and is widely used in the production process of linear low-density polyethylene, random copolymer polypropylene and impact copolymer polypropylene. In the process of olefin gas phase copolymerization, the properties of the copolymers obtained by polymerization are also very different due to the different ratios of various olefins in the gas phase of the reactor. In the production of olefin copolymers, only by controlling the stable ratio of various olefins in the polymerization reactor can a copolymer product with stable performance be obtained.

[0003] Among the existing continuous polyolefin processes, the Spheripol process of Lyondell-Basell, the Hypol process of Prime Polymer, the Borstar process of Borealis and the loop process of ExxonMobil in the bulk-gas phase process, the Unipol process of Greace, the Novolen process of CB&I, the Innovene process of INEOS, the Horizone process of JPP, and the Spherizone process and Catalloy process of Lyondell-Basell in the gas phase process are all designed with a stage of olefin gas phase copolymerization, wherein the composition control of various gases in the copolymerization reactor is achieved by automatic control between the feed flow rate of each gas raw material and the chromatographic analysis result of the online sampling of the gas in the reactor. This method must be equipped with flow meters, regulating valves, online chromatographs and other equipment and instruments, which is expensive and complex to control. Summary of the invention

[0004] In response to the above problems, the present invention provides a system and method for controlling the gas composition in a reactor during the gas phase copolymerization of olefins. The system and method of the present invention can achieve the control of the gas composition of copolymerized olefins in the reactor, and the system contains few equipments, low cost, simple process and easy operation.

[0005] A first aspect of the present invention provides a system for controlling the gas composition in a reactor during gas phase copolymerization of olefins, the system comprising a gas supply tank, the gas outlet of the gas supply tank being connected to the gas inlet of the reactor via a gas phase discharge pipeline, and the gas outlet of the reactor being connected to the gas supply tank via a circulating gas return pipeline;

[0006] The gas supply tank is connected with a liquid-phase olefin feed unit and a gas-phase olefin feed unit, the gas supply tank is provided with a temperature control unit and a first pressure control unit, and a circulating compressor is provided on the gas-phase discharge pipeline or the circulating gas return pipeline.

[0007] A second aspect of the present invention provides a method for controlling the gas composition in a reactor during gas phase copolymerization of olefins using the above system, the method comprising:

[0008] 1) The liquid olefin feeding unit introduces liquid olefin into the gas supply tank, the gas supply tank is heated, and the liquid olefin is partially gasified; the gas olefin feeding unit introduces gaseous olefin into the gas supply tank, controls the temperature and pressure of the gas supply tank, and makes the gasified liquid olefin and the gaseous olefin form a copolymerized mixed olefin gas;

[0009] 2) Olefin gas phase copolymerization stage: the mixed olefin gas in the gas supply tank is introduced into the reactor, and the gas is circulated between the reactor and the gas supply tank through a circulation compressor, and the reactor performs a copolymerization reaction. As the copolymerization reaction proceeds, the gas phase olefin is continuously introduced into the gas supply tank, and the liquid phase olefin is continuously gasified. The temperature and pressure of the gas supply tank are maintained the same as those in step 1) through the temperature control unit and the first pressure control unit of the gas supply tank;

[0010] 3) After the copolymerization reaction is completed, the gas circulation is stopped and the unreacted monomers in the reactor are recovered.

[0011] The present invention provides a mixed gas of copolymerized olefins with a stable composition to the reactor through a gas-liquid two-phase gas supply tank, and by adding a circulating compressor between the reactor and the gas supply tank, the gas between the reactor and the gas supply tank is circulated, thereby achieving the effect of maintaining the gas composition in the reactor basically stable, thereby producing a copolymer product with a stable composition. Compared with the prior art, the system and method of the present invention have the characteristics of less equipment, simple process, and easy operation, and realize the stability of the gas composition in the reactor during the copolymerization process.

[0012] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 The present invention is a schematic diagram of the structure of a system for controlling the gas composition in a reactor during the gas phase copolymerization of olefins.

[0014] Figure 2 The schematic diagram of the structure of another system for controlling the gas composition in the reactor during the gas phase copolymerization of olefins.

[0015] Explanation of the reference numerals: 1 - reactor, 2 - gas supply tank, 3 - circulation compressor, 4 - liquid phase olefin feed unit, 5 - gas phase olefin feed unit, 6 - heat exchanger, 7 - pressure reducing valve, 8 - back pressure valve. DETAILED DESCRIPTION

[0016] The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0017] According to a first aspect of the present invention, the present invention provides a system for controlling the gas composition in a reactor during a gas phase copolymerization of olefins, the system comprising a gas supply tank, the gas outlet of the gas supply tank being connected to the gas inlet of the reactor via a gas phase discharge pipeline, and the gas outlet of the reactor being connected to the gas supply tank via a circulating gas return pipeline;

[0018] The gas supply tank is connected with a liquid-phase olefin feed unit and a gas-phase olefin feed unit, the gas supply tank is provided with a temperature control unit and a first pressure control unit, and a circulating compressor is provided on the gas-phase discharge pipeline or the circulating gas return pipeline.

[0019] In the present invention, the gas supply tank provides the reactor with gas raw materials of specific composition required for copolymerization, including copolymerized olefins and a small amount of hydrogen as a molecular weight regulator. The gas supply tank is provided with temperature, pressure and liquid level monitoring instruments. There is no strict requirement for the size of the gas supply tank, which is generally 0.1 to 10 times the volume of the reactor.

[0020] According to the present invention, the liquid olefin feed unit provides liquid olefin raw materials to the gas supply tank intermittently or continuously. If the pressure of the source of the liquid olefin raw materials is lower than the use pressure of the gas supply tank, the feed unit can use an explosion-proof diaphragm pump as a conveying power device; if the pressure of the source of the liquid olefin raw materials is higher than the use pressure of the gas supply tank, the feed unit can use the pressure difference to realize the raw material conveying without the need for additional power equipment. The liquid olefin feed unit can select an instrument including a flow control, which is automatically controlled with the liquid level of the gas supply tank to realize the continuous feeding of liquid olefins.

[0021] In the present invention, the gas phase olefin feed unit continuously supplies the gas phase olefin raw material to the gas supply tank. The pressure of the source of the gas phase olefin raw material is higher than the use pressure of the gas supply tank.

[0022] According to the present invention, the first pressure control unit is a pressure reducing valve or a first regulating valve arranged on the pipeline connecting the gas-phase olefin feed unit and the gas supply tank. The first regulating valve is electrically connected to the pressure instrument signal of the gas supply tank to stabilize the pressure of the gas supply tank at a set value.

[0023] According to the present invention, the temperature control unit maintains the temperature of the material in the gas supply tank near the set value of the gas supply tank. The temperature control unit may include a jacket water system disposed on the outer surface of the gas supply tank and / or a heat exchanger disposed on the circulating gas return pipeline.

[0024] The jacket water system is a conventional temperature control system, which includes an instrument for controlling the jacket water temperature and flow rate so that the temperature of the gas supply tank can be maintained within a certain range of the set temperature value. The heat exchange medium of the heat exchanger is circulating water. If the temperature control unit only includes a heat exchanger, the heat exchanger must include an instrument for controlling the flow rate and temperature of the heat exchange medium, and the circulating gas pipeline must include a cross-line design connecting the reactor air inlet and outlet so that the circulating gas can circulate only between the gas supply tank, the compressor and the heat exchanger. These can be achieved with reference to the prior art.

[0025] In the present invention, the circulation compressor is a power source for realizing the circulation of the reaction gas between the reactor and the gas supply tank, and a centrifugal compressor can generally be selected.

[0026] According to a specific embodiment of the present invention, the pressure of the reactor is higher than the pressure of the gas supply tank, the circulating compressor is arranged on the gas phase discharge pipeline, and a second pressure control unit is arranged on the circulating gas return pipeline. The second pressure control unit is a second regulating valve or a back pressure valve (such as a self-operated back pressure valve), and the second regulating valve is electrically connected to the pressure instrument signal of the reactor.

[0027] According to another specific embodiment of the present invention, the pressure of the reactor is lower than the pressure of the gas supply tank, the circulating compressor is arranged on the circulating gas return pipeline, and the gas phase discharge pipeline is provided with a third pressure control unit, the third pressure control unit is a third regulating valve or a pressure reducing valve (such as a self-operated pressure reducing valve), and the third regulating valve is electrically connected to the pressure instrument signal of the reactor.

[0028] According to a second aspect of the present invention, the present invention provides a method for controlling the gas composition in a reactor during gas phase copolymerization of olefins using the above system, the method comprising:

[0029] 1) The liquid olefin feeding unit introduces liquid olefin into the gas supply tank, the gas supply tank is heated, and the liquid olefin is partially gasified; the gas olefin feeding unit introduces gaseous olefin into the gas supply tank, controls the temperature and pressure of the gas supply tank, and makes the gasified liquid olefin and the gaseous olefin form a copolymerized mixed olefin gas;

[0030] 2) Olefin gas phase copolymerization stage: the mixed olefin gas in the gas supply tank is introduced into the reactor, and the gas is circulated between the reactor and the gas supply tank through a circulation compressor, and the reactor performs a copolymerization reaction. As the copolymerization reaction proceeds, the gas phase olefin is continuously introduced into the gas supply tank, and the liquid phase olefin is continuously gasified. The temperature and pressure of the gas supply tank are maintained the same as those in step 1) through the temperature control unit and the first pressure control unit of the gas supply tank;

[0031] 3) After the copolymerization reaction is completed, the gas circulation is stopped and the unreacted monomers in the reactor are recovered.

[0032] In the present invention, the liquid phase olefin and the gas phase olefin can be selected from at least one of ethylene, propylene and α-olefins containing 4 to 10 carbon atoms, and the molecular weight of the liquid phase olefin is greater than that of the gas phase olefin.

[0033] According to the present invention, the feed of liquid olefin to the gas supply tank can be introduced only once before copolymerization and not introduced again during the copolymerization; it can also be introduced intermittently multiple times during the copolymerization; or it can be continuously introduced during the whole process through the automatic control of the flow control instrument and the liquid level of the gas supply tank.

[0034] In the present invention, the temperature, pressure and liquid olefin volume of the gas supply tank can be determined as needed, as long as the copolymerized mixed olefin gas can be continuously provided. For example, the liquid volume in the gas supply tank accounts for 10% to 90% of the volume of the gas supply tank, the temperature of the gas supply tank is 0 to 100°C, and the pressure of the gas supply tank is 0.1 to 10.0 MPa.

[0035] According to the present invention, during the copolymerization reaction, the temperature of the material in the gas supply tank fluctuates less than 10° C., preferably less than 5° C., and more preferably less than 2° C. relative to the set temperature of the gas supply tank. The temperature of the material in the gas supply tank is the actual temperature displayed on the temperature monitoring instrument.

[0036] Components, components and parameters not limited in the present invention may be selected according to the prior art and belong to conventional technical means in the field.

[0037] The present invention will be further described below with reference to the following examples, but is not limited to these examples.

[0038] Example 1

[0039] Process flow such as Figure 1 As shown. Reactor 1 is a 5.0-liter stirred stainless steel kettle with jacketed hot water for temperature control; gas supply tank 2 is a 2.5-liter stainless steel tank with a constant temperature water bath for temperature control, and the temperature control range is 5 to 90°C; circulating compressor 3 is an air-driven piston gas booster with a flow rate of 63NL / min; a circulating water jacket heat exchanger 6 is installed on the pipeline of circulating gas returning to the gas supply tank, and the circulating water temperature is controllable at 5 to 40°C, and the circulating water flow rate is 4.0L / min.

[0040] Preparation of gas phase copolymerization raw materials: At room temperature, the liquid olefin feeding unit 4 presses 1.5 liters of room temperature liquid propylene into the gas supply tank, turns on the constant temperature water bath of the gas supply tank, sets the temperature to 30°C and starts to heat up; when the temperature in the gas supply tank stabilizes at 30°C, the pressure of the gas supply tank is about 1.25MPa; add 0.05g of hydrogen to the gas supply tank; finally, set the pressure of the ethylene feed pressure reducing valve to 1.7MPa, and the gas phase olefin feeding unit 5 adds ethylene gas to the gas supply tank, and finally the gauge pressure of the gas supply tank is 1.7MPa.

[0041] Propylene liquid phase homopolymerization: at room temperature, add 12.9 mg of DQ catalyst, 1.5 mmol of triethylaluminum, 0.2 mmol of methylcyclohexyldimethoxysilane, and then add 0.1 g of hydrogen into the reactor; start stirring, add 2.0 liters of liquid propylene into the reactor through the metering tank, and then start the reactor temperature control system to quickly raise the reactor temperature to 70°C, and carry out liquid phase bulk homopolymerization of propylene for 0.5 hour.

[0042] Gas phase copolymerization of propylene and ethylene: After the homopolymerization of propylene is completed, the unreacted monomers are vented, the valve on the pipeline from the gas supply tank to the reactor is opened, and the mixed gas is introduced into the reactor. The reactor temperature is set to 70°C; then the valve from the reactor outlet to the circulating compressor is opened, the circulating compressor is started, and finally the valve from the outlet of the circulating compressor to the gas supply tank is opened, and the circulating water valves at the inlet and outlet of the heat exchanger on the circulating gas return pipeline are opened. The circulating water temperature is 6°C. As the copolymerization reaction proceeds, gaseous olefins are continuously introduced into the gas supply tank, and liquid olefins are continuously vaporized. The temperature of the gas supply tank is maintained at 30°C and the gauge pressure is 1.7MPa. During the polymerization process, the actual temperature of the gas supply tank fluctuates by less than 2°C relative to the set temperature. Adjust the pressure reducing valve 7 on the gas phase discharge line from the gas supply tank to the reactor to stabilize the reactor pressure at 1.0 MPa. After the reactor pressure is stable, take a gas sample R-1 from the reactor, and take a gas sample R-2 in the reactor after 60 minutes. Then stop the circulating compressor, close the valves on the reactor inlet and outlet pipelines, vent the unreacted monomers in the reactor and then discharge the material to obtain 420 grams of impact copolymer polypropylene. The polymer was analyzed and tested, and the ethylene mass content was 7.35% and the rubber phase mass content was 17.25%. The relevant experimental results are shown in Table 1.

[0043] Example 2

[0044] Process flow such as Figure 2 As shown. Reactor 1 is a 5.0-liter stirred stainless steel kettle with jacketed hot water for temperature control; gas supply tank 2 is a 2.5-liter stainless steel tank with a constant temperature water bath for temperature control, and the temperature control range is 4 to 90°C; circulating compressor 3 is an air-driven piston gas booster with a maximum flow rate of 63NL / min; a circulating water jacket heat exchanger 6 is installed on the pipeline of circulating gas returning to the gas supply tank, and the circulating water temperature is controllable at 5 to 40°C, and the circulating water flow rate is 4.0L / min.

[0045] Preparation of gas phase copolymerization raw materials: At room temperature, the liquid olefin feeding unit 4 presses 1.5 liters of room temperature liquid butene into the gas supply tank, turns on the constant temperature water bath of the gas supply tank, sets the temperature to 40°C and starts to heat up; when the temperature in the gas supply tank stabilizes at 40°C, the pressure of the gas supply tank is about 0.35MPa, the pressure of the ethylene feed pressure reducing valve is set to 0.55MPa, and the gas phase olefin feeding unit 5 adds ethylene gas into the gas supply tank, and finally the gas supply tank gauge pressure is 0.55MPa.

[0046] Propylene liquid phase homopolymerization: at room temperature, add 19.8 mg of DQ catalyst, 1.5 mmol of triethylaluminum, 0.2 mmol of methylcyclohexyldimethoxysilane, and then add 0.1 g of hydrogen into the reactor; start stirring, add 2.0 liters of liquid propylene into the reactor through the metering tank, and then start the reactor temperature control system to quickly raise the reactor temperature to 70°C, and carry out liquid phase bulk homopolymerization of propylene for 0.5 hour.

[0047] Gas phase copolymerization of ethylene and butene: After the homopolymerization of propylene is completed, the unreacted monomers are vented, the valve on the gas supply tank-circulation compressor-reactor pipeline is opened, and the circulation compressor is started to introduce mixed gas into the reactor, and the reactor temperature is set to 70°C; then the valve from the reactor outlet to the gas supply tank is opened, and the circulating water valve of the heat exchanger inlet and outlet on the circulating gas return pipeline is opened, and the circulating water temperature is 15°C. As the copolymerization reaction proceeds, the gas phase olefin is continuously introduced into the gas supply tank, and the liquid phase olefin is continuously gasified, and the temperature of the gas supply tank is maintained at 40°C and the gauge pressure is 0.55MPa. During the polymerization process, the actual temperature of the gas supply tank fluctuates less than 2°C relative to the set temperature. The back pressure valve 8 on the circulating gas return pipeline is adjusted to stabilize the reactor pressure at 0.8MPa. After the reactor pressure is stabilized, a gas sample R-1 is taken from the reactor, and a gas sample R-2 in the reactor is taken 60 minutes later, and then the circulation compressor is stopped, the valves on the reactor inlet and outlet pipelines are closed, and the unreacted monomers in the reactor are vented and discharged to obtain 679 grams of impact copolymerized polypropylene. The polymer was analyzed and tested, and the ethylene molar content was 11.39%, and the butene molar content was 1.06%. The relevant experimental results are shown in Table 1.

[0048] Example 3

[0049] The difference from Example 1 is that the pressure of the gas supply tank is controlled to be 2.25MPa, the copolymerization pressure is 1.4MPa, the copolymerization time is 30min, and the actual temperature of the gas supply tank fluctuates by a maximum of 3.2°C relative to the set temperature during the copolymerization. A total of 479 grams of polymer was obtained in the experiment, and the polymer was analyzed and tested, with an ethylene mass content of 8.68% and a rubber phase mass content of 16.04%. The rest is the same as Example 1. The relevant experimental results are shown in Table 1.

[0050] Example 4

[0051] The difference from Example 1 is that the pressure of the gas supply tank is controlled to be 1.90MPa, the copolymerization pressure is 1.4MPa, the copolymerization time is 30min, and the actual temperature of the gas supply tank fluctuates by a maximum of 4.5°C relative to the set temperature during the copolymerization. A total of 536 grams of polymer was obtained in the experiment, and the polymer was analyzed and tested, with an ethylene mass content of 6.52% and a rubber phase mass content of 14.30%. The rest is the same as Example 1. The relevant experimental results are shown in Table 1.

[0052] Example 5

[0053] The difference from Example 1 is that the pressure of the gas supply tank is controlled to be 1.75MPa, the copolymerization pressure is 1.4MPa, the copolymerization time is 30min, the inlet and outlet circulating water valves of the heat exchanger are not opened during the copolymerization process, and the actual temperature of the gas supply tank fluctuates by a maximum of 6.3°C relative to the set temperature. A total of 476 grams of polymer was obtained in the experiment, and the polymer was analyzed and tested, with an ethylene mass content of 5.53% and a rubber phase mass content of 12.89%. The rest is the same as Example 1. The relevant experimental results are shown in Table 1.

[0054] Comparative Example 1

[0055] The difference from Example 1 is that the circulating compressor and the valve of the reactor outlet returning to the gas supply tank were not opened during the copolymerization process. A total of 404 grams of polymer was obtained in the experiment. The polymer was analyzed and tested, and the ethylene mass content was 5.74% and the rubber phase mass content was 14.76%. The rest was the same as Example 1. The relevant experimental results are shown in Table 1.

[0056] Comparative Example 2

[0057] The difference from Example 2 is that the pressure of the gas supply tank is controlled to be 0.80 MPa, the circulating compressor and the valve of the reactor gas outlet returning to the gas supply tank are not opened during the copolymerization process, and the copolymerization pressure is 0.7 MPa. A total of 359 grams of polymer was obtained in the experiment. The polymer was analyzed and tested, and the ethylene molar content was 12.07% and the butene molar content was 0.62%. The rest is the same as Example 1. The relevant experimental results are shown in Table 1.

[0058] Table 1

[0059]

[0060]

[0061] Note: “E / P” is the volume ratio of ethylene to propylene obtained by chromatographic analysis of gas samples; “E / B” is the volume ratio of ethylene to butene obtained by chromatographic analysis of gas samples.

[0062] As shown in Table 1, the system and method for controlling the gas composition in the reactor during the gas phase copolymerization of olefins of the present invention can make the E / P or E / B fluctuation range less than 15%, and can effectively control the gas composition in the reactor during the copolymerization process, thereby producing a copolymer product with stable composition. Moreover, when the temperature fluctuation of the gas supply tank is less than 5°C and a circulating gas compressor is used during the copolymerization process, the fluctuation range of the gas phase composition in the reactor at the start and end of the copolymerization process is less than 10%; in a more optimal case (such as the ethylene-propylene copolymerization of Example 1), when the temperature fluctuation of the gas supply tank is less than 2°C, the fluctuation range of the gas phase composition in the reactor at the start and end of the copolymerization process is less than 5%. On the contrary, it can be seen from the comparative example that when there is no gas circulation between the gas supply tank and the reactor during the copolymerization process, whether it is ethylene-propylene gas phase copolymerization or ethylene-butyl gas phase copolymerization, the fluctuation range of the gas phase composition in the reactor at the start and end of the copolymerization exceeds 40%. In this case, the structural composition of the copolymer obtained by polymerization is very inhomogeneous, and the product quality and stability will also be greatly affected.

[0063] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A system for controlling the gas composition in a reactor during olefin gas phase copolymerization, characterized in that: The system comprises a gas supply tank, the gas outlet of the gas supply tank is connected to the gas inlet of the reactor through a gas phase discharge pipeline, and the gas outlet of the reactor is connected to the gas supply tank through a circulating gas return pipeline; The gas supply tank is connected with a liquid-phase olefin feed unit and a gas-phase olefin feed unit, the gas supply tank is provided with a temperature control unit and a first pressure control unit, and a circulating compressor is provided on the gas-phase discharge pipeline or the circulating gas return pipeline.

2. The system for controlling the gas composition in a reactor during olefin gas phase copolymerization according to claim 1, wherein: The gas supply tank is respectively provided with temperature, pressure and liquid level monitoring instruments.

3. The system for controlling the gas composition in a reactor during olefin gas phase copolymerization according to claim 1, wherein: The temperature control unit includes a jacket water system arranged on the outer surface of the gas supply tank and / or a heat exchanger arranged on the circulating gas return pipeline.

4. The system for controlling the gas composition in a reactor during olefin gas phase copolymerization according to claim 1, wherein: The first pressure control unit comprises a pressure reducing valve or a first regulating valve arranged on a pipeline connecting the gas-phase olefin feeding unit and the gas supply tank, and the first regulating valve is electrically connected to a pressure instrument signal of the gas supply tank.

5. The system for controlling the gas composition in a reactor during olefin gas phase copolymerization according to claim 1, wherein: The circulating compressor is arranged on the gas phase discharge pipeline, and the circulating gas return pipeline is provided with a second pressure control unit, which is a second regulating valve or a back pressure valve, and the second regulating valve is electrically connected to the pressure instrument signal of the reactor.

6. The system for controlling the gas composition in a reactor during olefin gas phase copolymerization according to claim 1, wherein: The circulating compressor is arranged on the circulating gas return pipeline, and the gas phase discharge pipeline is provided with a third pressure control unit, which is a third regulating valve or a pressure reducing valve, and the third regulating valve is electrically connected to the pressure instrument signal of the reactor.

7. A method for controlling the gas composition in a reactor during gas phase copolymerization of olefins using the system described in any one of claims 1 to 6, characterized in that: The method includes: 1) The liquid olefin feeding unit introduces liquid olefin into the gas supply tank, the gas supply tank is heated, and the liquid olefin is partially gasified; the gas olefin feeding unit introduces gaseous olefin into the gas supply tank, controls the temperature and pressure of the gas supply tank, and makes the gasified liquid olefin and the gaseous olefin form a copolymerized mixed olefin gas; 2) Olefin gas phase copolymerization stage: the mixed olefin gas in the gas supply tank is introduced into the reactor, and the gas is circulated between the reactor and the gas supply tank through a circulation compressor, and the reactor performs a copolymerization reaction. As the copolymerization reaction proceeds, the gas phase olefin is continuously introduced into the gas supply tank, and the liquid phase olefin is continuously gasified. The temperature and pressure of the gas supply tank are maintained the same as those in step 1) through the temperature control unit and the first pressure control unit of the gas supply tank; 3) After the copolymerization reaction is completed, the gas circulation is stopped and the unreacted monomers in the reactor are recovered.

8. The method for controlling the gas composition in the reactor during olefin gas phase copolymerization according to claim 7, wherein: The liquid phase olefin and the gas phase olefin are respectively selected from at least one of ethylene, propylene and α-olefins containing 4 to 10 carbon atoms, and the molecular weight of the liquid phase olefin is greater than that of the gas phase olefin.

9. The method for controlling the gas composition in the reactor during olefin gas phase copolymerization according to claim 7, wherein: The volume of liquid in the gas supply tank accounts for 10% to 90% of the volume of the gas supply tank, the temperature of the gas supply tank is 0 to 100° C., and the pressure of the gas supply tank is 0.1 to 10.0 MPa.

10. The method for controlling the gas composition in a reactor during olefin gas phase copolymerization according to claim 7, wherein: During the copolymerization reaction, the temperature of the material in the gas supply tank fluctuates by less than 10°C relative to the set temperature of the gas supply tank.

11. The method for controlling the gas composition in a reactor during olefin gas phase copolymerization according to claim 10, wherein: During the copolymerization reaction, the temperature of the material in the gas supply tank fluctuates less than 5°C, preferably less than 2°C, relative to the set temperature of the gas supply tank.

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