A method for comprehensive recycling and utilization of the exhaust gas molecules from the degassing bin of a polyethylene plant

Abstract

The present invention relates to the field of petrochemical separation technology, and proposes a method for comprehensively recovering and utilizing gas molecules discharged from a degassing bin of a polyethylene device. The method comprises a polyethylene degassing bin, a dust filtration system, a raw gas cooling system, a raw gas buffer separation system, a raw gas compression system, a cryogenic separation system, a turboexpansion system, and a membrane separation system. The cryogenic separation system comprises a primary main heat exchanger, a primary separator, a secondary heat exchanger, and a secondary separator. The method utilizes a turboexpansion mechanism to extract low-temperature cooling energy of 0°C to 165°C, recovers the cooling energy through reflux through a high-efficiency heat exchanger, and establishes separators in different temperature zones based on the boiling points of various substances in the raw gas to liquefy and separate components that are easily liquefied. Since the method combines low-pressure gasification of liquefied hydrocarbons with turboexpansion refrigeration, the refrigeration temperature is lowered. Therefore, the method can achieve a recovery rate of heavy hydrocarbons and ethylene of over 99%, thereby solving the problems of low degassing recovery rate and high energy consumption in polyethylene production in the prior art.

Description

Technical Field

[0001] The present invention relates to the field of petrochemical separation technology, and in particular to a method for comprehensively recovering and utilizing gas molecules discharged from a degassing bin of a polyethylene device. Background Art

[0002] The ethylene industry is the core of the petrochemical industry. Ethylene is one of the world's most produced chemical products, accounting for over 75% of petrochemical output. It holds a crucial position in the national economy and is known as the "mother of the petrochemical industry." Ethylene production is widely considered a key indicator of a country's petrochemical industry development. Ethylene production is a key indicator of chemical industry production, and the scale of ethylene production determines the production scale of propylene, butadiene, and related aromatics. Ethylene significantly influences over 80% of China's chemical production and the expansion of its supply chain.

[0003] With the rapid development of ethylene technology in my country, domestic ethylene production capacity is expected to continue expanding after 2022, albeit at a slower pace compared to 2020 and 2021. A large number of ethylene projects are expected to come online in China from 2023 to 2025, with a total ethylene production capacity of 15.45 million tons / year, which is highly likely to come online. Polyethylene products are the primary focus of the ethylene industry. In recent years, annual demand for polyethylene has grown by over 8%, and my country continues to expand its polyethylene production facilities. Polyethylene production primarily utilizes gas-phase and liquid-phase polymerization processes, both of which generate exhaust gases. Exhaust gas recovery is a key component of the entire production process, significantly reducing operating costs and ultimately reducing material consumption while also conserving energy and protecting the environment.

[0004] At present, most of the exhaust gas from domestic polyethylene production equipment is recovered by compression, cooling and propylene or Freon pre-cooling. After recovery, a large amount of heavy hydrocarbons, ethylene and nitrogen are still discharged. This process has a low recovery rate and high energy consumption, which will cause considerable economic losses to the company in the long run. Summary of the Invention

[0005] The present invention provides a method for comprehensive recovery and utilization of gas molecules discharged from a degassing bin of a polyethylene device, which solves the problems of low degassing recovery rate and high energy consumption in polyethylene production in the prior art.

[0006] The technical solution of the present invention is as follows: a method for comprehensive recovery and utilization of gas molecules discharged from a degassing bin of a polyethylene device, comprising a polyethylene degassing bin, a dust filtration system, a feed gas cooling system, a feed gas buffer separation system, a feed gas compression system, a cryogenic separation system, a turbine expansion system, and a membrane separation system. The cryogenic separation system comprises a primary main heat exchanger, a primary separator, a secondary heat exchanger, and a secondary separator. The specific method is as follows:

[0007] Step 1: After the polyethylene conveying medium (mainly isopentane or isobutane) and the incompletely polymerized gas (mainly ethylene containing a small amount of hydrogen) exit the polymerization reactor, they first enter the polyethylene degassing bin. The gas pressure in the polyethylene degassing bin is reduced to 30kPa (the raw gas pressure of the degassing bin system is about 35-50kPa, and its raw gas composition is mostly isopentane or isobutane, butene, ethylene, ethane, nitrogen, hydrogen and other effective gases). The polyethylene conveying medium and the incompletely polymerized gas are fully gasified and fully separated from the powder entrained in the conveying medium. The powder is discharged from the bottom of the polyethylene degassing bin from top to bottom. In order to prevent flammable gas from entering the powder, nitrogen is blown into the bottom of the polyethylene degassing bin. The nitrogen blown into the polyethylene degassing bin draws the flammable gas (polyethylene conveying medium such as isobutane or isopentane) entrained in the powder from the top of the polyethylene degassing bin together with the gasified conveying medium.

[0008] Step 2: To prevent polyethylene dust from being carried out in the gas, the raw gas drawn from the top of the polyethylene degassing bin first enters the dust filtration system;

[0009] Step 3: The dust-filtered gas is cooled to room temperature by the raw gas cooling system and enters the raw gas buffer separation system. The raw gas buffer separation system is mainly used to buffer the raw gas to reduce the flow rate of the raw gas and further reduce the dust in the raw gas through the tank top filter at a low flow rate;

[0010] Step 4: After being stabilized by the raw gas buffer separation system, the raw gas enters the raw gas compression system. The compressed gas enters the gas-liquid separator after cooling, and the separated liquid product is pumped to the reactor system through the heavy hydrocarbon pump;

[0011] In step 5, the separated gaseous product enters the cryogenic separation system. The raw gas entering the cryogenic separation system is cooled to a certain temperature by the first-stage main heat exchanger. The cooled gas is partially liquefied and enters the first-stage separator. The liquid heavy hydrocarbon product separated by the first-stage separator is discharged from the bottom of the first-stage separator, decompressed by the throttle valve, enters the first-stage main heat exchanger for reheating, and then is sent out of the cold box and returned to the compressor inlet. The separated gaseous product continues to enter the secondary heat exchanger for further secondary cooling to a certain temperature. The cooled gas is partially liquefied into liquid and enters the secondary separator. The liquid product separated by the secondary separator is decompressed by the pressure reducing valve, returns to the secondary heat exchanger for further reheating, and then is sent out of the secondary heat exchanger. The gaseous product at the top of the secondary separator is reheated to a certain temperature through the secondary heat exchanger and enters the turbine expansion system.

[0012] Step 6: After cooling and depressurizing through the turboexpansion system, the raw gas returns to the secondary heat exchanger to recover the low-temperature cold energy after the turboexpansion. The reheated gas is pressurized and heated at the braking end of the turboexpander before being sent to the membrane separation system.

[0013] In step seven, the gas from the braking end of the turboexpander first enters the nitrogen heater of the membrane separation system to heat the raw gas to a certain temperature. The heated gas enters the membrane separation container, and the small molecule hydrogen permeates the membrane separation device, which is called permeate gas and is sent to the flare system through the vent pipeline. The gas that is not permeated is called recovered nitrogen, which is cooled by the ethylene heat exchanger and then sent back to the polyethylene degassing chamber.

[0014] Preferably, the filtering equipment of the dust filtering system can be one or more of a security filter, a basket filter, a self-cleaning filter, and a condensing filter, and the filtering accuracy is <30 μm.

[0015] Preferably, the cooling equipment of the raw gas cooling system can be a shell-and-tube heat exchanger or a coil-and-tube heat exchanger, and the cooling medium can be circulating water or chilled water.

[0016] Preferably, the raw gas compression system can be a centrifugal compressor, a reciprocating compressor and a screw compressor, and the compressor exhaust pressure is between 1.0 MPa and 3.5 MPa.

[0017] Preferably, the primary main heat exchanger and the secondary heat exchanger in the cryogenic separation system can be a heat exchanger or a combination of a heat exchanger or more. The heat exchanger can be a plate-fin type or a coiled tube type heat exchanger, or a combination of the two types. The primary main heat exchanger can be pre-cooled with propylene or with Freon or without pre-cooling. The raw gas from the raw gas compression system enters the primary heat exchanger of the cryogenic separation system to be cooled to a certain temperature and enters the primary separator. The liquid phase heavy hydrocarbon product at the bottom of the primary separator enters the primary main heat exchanger for reheating. The reheated heavy hydrocarbon can be returned to the compressor inlet or to the last separator of the compressor. The separated gas phase enters the secondary heat exchanger to be cooled to a certain temperature and enters the secondary separator. The liquid phase ethylene product at the bottom of the secondary separator is cooled to a certain temperature and enters the secondary separator. After the throttle valve reduces the pressure, it is reheated in the secondary and primary heat exchangers and then sent out of the boundary area. The gaseous nitrogen product at the top of the secondary separator enters the secondary heat exchanger to be reheated to a certain temperature and then enters the turbo expansion system. The turbo expansion system is closely integrated with the primary and secondary heat exchangers of the cryogenic separation system. The nitrogen products from the primary and secondary heat exchangers first pass through the primary turbo expander to expand to a certain pressure and temperature, and then reheat to a certain temperature through the primary and secondary heat exchangers and enter the secondary turbo expander to continue cooling and reducing pressure. The cooled and decompressed gas returns to the primary heat exchanger and the secondary heat exchanger for reheating. The reheated nitrogen product enters the braking end of the primary and secondary turbo expanders to increase temperature and pressure. The heated and pressurized gas is sent to the membrane separation system.

[0018] Preferably, the turboexpander in the turboexpander system can be a gas bearing centrifugal expander or an oil-lubricated bearing centrifugal expander, the first-stage exhaust pressure of the expander is between 0.5MPa and 2MPa, and the second-stage exhaust pressure of the expander is between 0.01MPa and 1MPa. The turboexpander braking end can be a centrifugal impeller booster or a straight-tooth speed-reducing impeller, and the operating pressure of the working impeller is between 0.01 and 1MPa. The first and second-stage turboexpanders can be used in series or in parallel.

[0019] Preferably, the membrane separation system includes a pre-membrane heater and a membrane separation system. The pre-membrane heater can be selected from one or a combination of two or more of a coiled tube heat exchanger, a shell-and-tube heat exchanger, a plate heat exchanger, a plate-fin heat exchanger, and an electric heater. The membrane separation system is composed of a membrane container and a membrane assembly, and is used to separate the hydrogen in the nitrogen product and the hydrocarbons that are not fully recovered by deep cooling. The membrane separation system can be a single component of the membrane container and the membrane assembly or a combination of the two components.

[0020] Preferably, the first-stage cooling temperature in step five is between -45°C and -70°C.

[0021] Preferably, the secondary cooling temperature in step five is between -120°C and -160°C.

[0022] The beneficial effects of the present invention are:

[0023] 1. The present invention utilizes a turboexpansion mechanism to extract low-temperature cooling energy at 0°C to -165°C, recovers the cooling energy through reflux through a high-efficiency heat exchanger, and, based on the boiling points of various substances in the feed gas, sets up separators in different temperature zones to liquefy and separate components that are easily liquefied. Since the present invention effectively combines low-pressure gasification of liquefied hydrocarbons with turboexpansion refrigeration, the refrigeration temperature is lowered. Therefore, the present invention can achieve a recovery rate of heavy hydrocarbons and ethylene exceeding 99%, solving the problems of low degassing recovery rate and high energy consumption in polyethylene production in the prior art.

[0024] 2. The present invention adopts membrane separation technology, which can use the partial pressure difference of gas on both sides of the membrane as the driving force for mass transfer, and utilize the difference in permeation rate of different gases through the membrane material to achieve component separation. Combining this feature, the present invention can effectively separate hydrogen and organic hydrocarbons in the feed gas;

[0025] 3. The process of the present invention is simple and reliable, has great operational flexibility, high recovery rate, low energy consumption, low operating pressure and good separation effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] Figure 1 This is a schematic diagram of the process flow of the propane dehydrogenation product cryogenic separation system implemented in the present invention;

[0028] Figure 2 This is a schematic diagram of the process connection of the cryogenic separation system implemented in the present invention;

[0029] In the figure: 1. Polyethylene degassing chamber; 2. Dust filtration system; 3. Raw gas cooling system; 4. Raw gas buffer separation system; 5. Raw gas compression system; 6. Cryogenic separation system; 601. Primary main heat exchanger; 602. Primary separator; 603. Secondary heat exchanger; 604. Secondary separator; 7. Turbine expansion system; 8. Membrane separation system. DETAILED DESCRIPTION

[0030] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0031] See also Figure 1 and Figure 2 The present invention provides a technical solution: a method for comprehensive recovery and utilization of gas molecules discharged from a degassing bin of a polyethylene device, comprising a polyethylene degassing bin 1, a dust filtration system 2, a raw gas cooling system 3, a raw gas buffer separation system 4, a raw gas compression system 5, a cryogenic separation system 6, a turbine expansion system 7, and a membrane separation system 8. The cryogenic separation system 6 comprises a primary main heat exchanger 601, a primary separator 602, a secondary heat exchanger 603, and a secondary separator 604. The specific method is as follows:

[0032] Step 1: After the polyethylene conveying medium (mainly isopentane or isobutane) and the incompletely polymerized gas (mainly ethylene containing a small amount of hydrogen) exit the polymerization reactor, they first enter the polyethylene degassing bin 1. The gas pressure in the polyethylene degassing bin 1 drops to 30kPa (the raw gas pressure of the polyethylene degassing bin 1 is about 35-50kPa, and its raw gas composition is mostly isopentane or isobutane, butene, ethylene, ethane, nitrogen, hydrogen and other effective gases). The polyethylene conveying medium and the incompletely polymerized gas are fully gasified and fully separated from the powder entrained in the conveying medium. The powder is discharged from the bottom of the polyethylene degassing bin 1 from top to bottom, and nitrogen is blown into the bottom of the polyethylene degassing bin 1. The nitrogen blown into the polyethylene degassing bin 1 draws out the combustible gas (polyethylene conveying medium such as isobutane or isopentane) entrained in the powder from the top of the polyethylene degassing bin 1 together with the gasified conveying medium.

[0033] Step 2: The raw gas drawn from the top of the polyethylene degassing bin 1 first enters the dust filtration system 2;

[0034] Step 3: The dust-filtered gas is cooled to room temperature by the raw gas cooling system 3 and enters the raw gas buffer separation system 4. The raw gas buffer separation system 4 is mainly used to buffer the raw gas to reduce the flow rate of the raw gas and further reduce the dust in the raw gas through the tank top filter at a low flow rate;

[0035] Step 4: After being stabilized by the raw gas buffer separation system 4, the raw gas enters the raw gas compression system 5. The compressed gas enters the gas-liquid separator after cooling, and the separated liquid product is pumped to the reactor system through the heavy hydrocarbon pump;

[0036] In step five, the separated gaseous product enters the cryogenic separation system 6. The raw gas entering the cryogenic separation system 6 is cooled to a certain temperature by the first-stage main heat exchanger 601. The cooled gas is partially liquefied and enters the first-stage separator 602. The liquid heavy hydrocarbon product separated by the first-stage separator 602 is discharged from the bottom of the first-stage separator 602, decompressed by the throttle valve, enters the first-stage main heat exchanger 601 for reheating, and is then sent out of the cold box and returned to the compressor inlet. The separated gaseous product continues to enter the secondary heat exchanger 603 for further secondary cooling to a certain temperature. The cooled gas is partially liquefied into liquid and enters the secondary separator 604. The liquid product separated by the secondary separator 604 is decompressed by the pressure reducing valve, returns to the secondary heat exchanger 603, continues to be reheated, and is then sent out of the secondary heat exchanger 603. The gaseous product at the top of the secondary separator 604 is reheated to a certain temperature by the secondary heat exchanger 603 and enters the turbo expansion system 7.

[0037] Step 6: After cooling and depressurizing through the turboexpansion system 7, the raw gas returns to the secondary heat exchanger 603 to recover the low-temperature cold energy after the turboexpansion. The reheated gas is pressurized and heated at the braking end of the turboexpander before being sent to the membrane separation system 8.

[0038] In step seven, the gas from the braking end of the turboexpander first enters the nitrogen heater of the membrane separation system 8 to heat the raw gas to a certain temperature. The heated gas enters the membrane separation container, and the small molecule hydrogen passes through the membrane separation device and is sent to the flare system through the vent pipeline. The non-permeated gas is cooled by the ethylene heat exchanger and then sent back to the polyethylene degassing chamber 1.

[0039] The filtering equipment of the dust filtering system 2 can be one or more of a safety filter, a basket filter, a self-cleaning filter, and a condensing filter, and the filtering accuracy is less than 30 μm.

[0040] In this embodiment, the cooling equipment of the raw gas cooling system 3 can be a shell-and-tube heat exchanger or a coil-and-tube heat exchanger, and the cooling medium can be circulating water or chilled water.

[0041] In this embodiment, the raw gas compression system 5 can be a centrifugal compressor, a reciprocating compressor, or a screw compressor, and the exhaust pressure of the compressor is between 1.0 MPa and 3.5 MPa.

[0042] In this embodiment, the primary main heat exchanger 601 and the secondary heat exchanger 603 in the cryogenic separation system 6 can be one heat exchanger or a combination of two or more heat exchangers. The heat exchanger can be a plate-fin type or a coil-wound type heat exchanger, or a combination of the two types. The primary main heat exchanger 601 can be pre-cooled with propylene or with Freon or without pre-cooling. The raw gas from the raw gas compression system 5 enters the primary main heat exchanger 601 of the cryogenic separation system 6 to be cooled to a certain temperature and enters the primary separator 602. The liquid phase heavy hydrocarbon product at the bottom of the primary separator 602 enters the primary main heat exchanger 601 for reheating. The reheated heavy hydrocarbon can be returned to the compressor inlet or returned to the last stage separator of the compressor. The separated gas phase enters the secondary heat exchanger 603 to be cooled to a certain temperature and enters the secondary separator 604. The secondary separator 6 04 The liquid ethylene product at the bottom is depressurized by the throttle valve and then reheated by the secondary and primary heat exchangers before being sent out of the boundary area. The gaseous nitrogen product at the top of the secondary separator 604 enters the secondary heat exchanger 603 to be reheated to a certain temperature and then enters the turbo expansion system. The turbo expansion system is closely integrated with the primary and secondary heat exchangers of the cryogenic separation system. The nitrogen products from the primary and secondary heat exchangers first pass through the primary turbo expander to expand to a certain pressure and temperature, and then reheat to a certain temperature through the primary and secondary heat exchangers and enter the secondary turbo expander to continue cooling and depressurizing. The gas after cooling and depressurizing returns to the primary main heat exchanger 601 and the secondary heat exchanger 603 for reheating. The reheated nitrogen product enters the braking end of the primary and secondary turbo expanders to increase temperature and pressure. The gas after temperature and pressure increase is sent to the membrane separation system.

[0043] In this embodiment, the turboexpander in the turboexpander system 7 can be a gas-bearing centrifugal expander or an oil-lubricated bearing centrifugal expander. The exhaust pressure of the first stage of the expander is between 0.5 MPa and 2 MPa, and the exhaust pressure of the second stage of the expander is between 0.01 MPa and 1 MPa. The turboexpander braking end can be a centrifugal impeller booster or a straight-tooth speed-reducing impeller. The operating pressure of the working impeller is between 0.01 and 1 MPa. The first and second stage turboexpanders can be used in series or in parallel.

[0044] In this embodiment, the membrane separation system 8 includes a pre-membrane heater and a membrane separation system. The pre-membrane heater can be selected from one or a combination of two or more of a coiled tube heat exchanger, a shell-and-tube heat exchanger, a plate heat exchanger, a plate-fin heat exchanger, and an electric heater. The membrane separation system is composed of a membrane container and a membrane assembly, and is used to separate the hydrogen in the nitrogen product and the hydrocarbons that are not fully recovered by deep cooling. The membrane separation system can be a single component of the membrane container and the membrane assembly or a combination of the two components.

[0045] In this embodiment, the first-stage cooling temperature in step five is between -45°C and -70°C.

[0046] In this embodiment, the secondary cooling temperature in step five is between -120°C and -160°C.

[0047] It should be noted that the pressure units 'kPa (gauge pressure)' and 'MPa (gauge pressure)' used in this application refer to pressure values relative to atmospheric pressure, i.e., gauge pressure. The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention are intended to be included within the scope of protection of the present invention.

Claims

1. A method for comprehensive recovery and utilization of gas molecules discharged from a degassing bin of a polyethylene device, characterized in that: The invention comprises a polyethylene degassing chamber (1), a dust filtration system (2), a raw gas cooling system (3), a raw gas buffer separation system (4), a raw gas compression system (5), a cryogenic separation system (6), a turbine expansion system (7) and a membrane separation system (8), wherein the cryogenic separation system (6) comprises a primary main heat exchanger (601), a primary separator (602), a secondary heat exchanger (603) and a secondary separator (604). The specific method is as follows: Step 1: After the polyethylene conveying medium and the incompletely polymerized gas exit the polymerization reactor, they first enter the polyethylene degassing bin (1). The gas pressure in the polyethylene degassing bin (1) is reduced to 30 kPa. The polyethylene conveying medium and the incompletely polymerized gas are fully gasified and fully separated from the powder entrained in the conveying medium. The powder is discharged from the bottom of the polyethylene degassing bin (1) from top to bottom. Nitrogen is blown into the bottom of the polyethylene degassing bin (1). The nitrogen blown into the polyethylene degassing bin (1) draws the combustible gas entrained in the powder from bottom to top together with the gasified conveying medium and is then drawn out from the top of the polyethylene degassing bin (1). Step 2: The raw gas drawn from the top of the polyethylene degassing chamber (1) first enters the dust filtration system (2); Step 3: The dust-filtered gas is cooled to room temperature by the raw gas cooling system (3) and enters the raw gas buffer separation system (4); Step 4: After being stabilized by the raw gas buffer separation system (4), the raw gas enters the raw gas compression system (5). The compressed gas enters the gas-liquid separator after being cooled, and the separated liquid product is pumped to the reactor system through the heavy hydrocarbon pump; Step 5: The separated gaseous product enters the cryogenic separation system (6). The raw gas entering the cryogenic separation system (6) is cooled to a certain temperature by the first-stage main heat exchanger (601). The cooled gas is partially liquefied and enters the first-stage separator (602). The liquid heavy hydrocarbon product separated by the first-stage separator (602) is discharged from the bottom of the first-stage separator (602) and is reduced in pressure by the throttle valve and enters the first-stage main heat exchanger (601). After reheating, it is sent out of the cold box and returned to the compressor inlet. The separated gaseous product continues to enter the second-stage heat exchanger (603) and continues to be cooled to a certain temperature by the second stage. The cooled gas is partially liquefied into liquid and enters the second-stage separator (604). The liquid product separated by the second-stage separator (604) is reduced in pressure by the pressure reducing valve and returns to the second-stage heat exchanger (603). After further reheating, it is sent out of the second-stage heat exchanger (603). The gaseous product at the top of the second-stage separator (604) is reheated to a certain temperature by the second-stage heat exchanger (603) and enters the turbine expansion system (7). Step 6: After cooling and depressurizing the raw gas through the turboexpansion system (7), the raw gas returns to the secondary heat exchanger (603) to recover the low-temperature cold energy after the raw gas is expanded through the turbine. The reheated gas is pressurized and heated at the braking end of the turboexpander and then sent to the membrane separation system (8); In step seven, the gas from the braking end of the turboexpander first enters the nitrogen heater of the membrane separation system (8), and the raw gas is heated to a certain temperature. The heated gas enters the membrane separation container, and the small molecule hydrogen passes through the membrane separation device and is sent to the flare system through the venting pipeline. The non-permeated gas is cooled by the ethylene heat exchanger and then sent back to the polyethylene degassing chamber (1).

2. The method for comprehensive recovery and utilization of gas molecules discharged from the degassing bin of a polyethylene device according to claim 1, characterized in that: The filtering equipment of the dust filtering system (2) is one or more of a safety filter, a basket filter, a self-cleaning filter, and a condensing filter, and the filtering accuracy is less than 30 μm.

3. The method for comprehensive recovery and utilization of gas molecules discharged from the degassing bin of a polyethylene device according to claim 1, characterized in that: The cooling equipment of the raw gas cooling system (3) is a shell-and-tube heat exchanger or a coiled-tube heat exchanger, and the cooling medium uses circulating water or chilled water.

4. The method for comprehensive recovery and utilization of gas molecules discharged from a degassing bin of a polyethylene device according to claim 1, characterized in that: The raw gas compression system (5) is one of a centrifugal compressor, a reciprocating compressor, and a screw compressor, and the compressor exhaust pressure is between 1.0 MPa and 3.5 MPa.

5. The method for comprehensive recovery and utilization of gas molecules discharged from a degassing bin of a polyethylene device according to claim 1, characterized in that: The primary main heat exchanger (601) and the secondary heat exchanger (603) in the cryogenic separation system (6) are one heat exchanger or a combination of two or more heat exchangers. The heat exchangers are plate-fin type or coil-wound type heat exchangers, or a combination of the two types. The primary main heat exchanger (601) is pre-cooled with propylene, pre-cooled with freon, or not pre-cooled.

6. The method for comprehensive recovery and utilization of gas molecules discharged from a degassing bin of a polyethylene device according to claim 1, characterized in that: The turboexpander in the turboexpander system (7) is a gas bearing centrifugal expander or an oil-lubricated bearing centrifugal expander, the first-stage exhaust pressure of the expander is between 0.5 MPa and 2 MPa, the second-stage exhaust pressure of the expander is between 0.01 MPa and 1 MPa, the turboexpander braking end is a centrifugal impeller booster or a straight tooth speed reduction impeller, and the operating pressure of the working impeller is between 0.01 and 1 MPa.

7. The method for comprehensive recovery and utilization of gas molecules discharged from a degassing bin of a polyethylene device according to claim 1, characterized in that: The membrane separation system (8) includes a pre-membrane heater and a membrane separation system. The pre-membrane heater is selected from one or a combination of two or more of a coiled tube heat exchanger, a shell-and-tube heat exchanger, a plate heat exchanger, a plate-fin heat exchanger, and an electric heater. The membrane separation system is composed of a membrane container and a membrane assembly, and is used to separate the hydrogen in the nitrogen product and the hydrocarbons that are not fully recovered by deep cooling. The membrane separation system is a single component of the membrane container and the membrane assembly or a combination of the two components.

8. The method for comprehensive recovery and utilization of gas molecules discharged from a degassing bin of a polyethylene device according to claim 1, characterized in that: The first-stage cooling temperature in step 5 is between -45°C and -70°C.

9. The method for comprehensive recovery and utilization of gas molecules discharged from a degassing bin of a polyethylene device according to claim 1, characterized in that: The secondary cooling temperature in step 5 is between -120°C and -160°C.

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