A UNIPOL process polyethylene tail gas recovery and cryogenic separation system

Abstract

The present invention discloses a UNIPOL process polyethylene tail gas recovery and cryogenic separation system, which belongs to the technical field of polyethylene tail gas cryogenic separation. The system includes a tail gas compressor, a compressor inlet buffer tank, a cooler, a first liquid hydrocarbon separator, a normal temperature liquid hydrocarbon recovery pump and a cold box, wherein the cold box is provided with a main heat exchanger, a booster turbine expansion unit, a second liquid hydrocarbon separator, a third liquid hydrocarbon separator, an ethylene separator, a gas-liquid equalization separator and a low temperature liquid hydrocarbon recovery pump. The main heat exchanger is provided with a tail gas condensation channel, a liquid hydrocarbon recovery channel, an ethylene recovery channel, a nitrogen recovery channel, a liquid hydrocarbon cryogenic channel and a nitrogen reheating channel. The present invention adopts a cryogenic cold box and utilizes the physical properties of hydrocarbon gases at low temperatures to separate hydrocarbon gases in polyethylene tail gas from the gas phase. It recovers important components in polyethylene tail gas more effectively than existing devices, realizes high-value recovery of hydrocarbon gases in tail gas, reduces system energy consumption and reduces carbon emissions.

Description

Technical Field

[0001] The invention belongs to the technical field of polyethylene tail gas cryogenic separation, and in particular relates to a UNIPOL process polyethylene tail gas recovery cryogenic separation system. Background Art

[0002] Polyethylene is one of the five major synthetic resins and the most produced variety in the plastics industry. Based on the polymerization method and molecular weight, polyethylene is categorized as low-density polyethylene (LDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), and ultra-high molecular weight polyethylene (UHMWPE). Currently, the mainstream polyethylene production processes are slurry polyethylene (SPE) and gas-phase polyethylene (GPE). Domestic GPE plants utilize a high percentage of Univation's UNIPOL gas-phase fluidized bed production process. The UNIPOL process generates a large amount of hydrocarbon gas through polymerization reactions, which passes through a degassing chamber and an exhaust gas recovery system.

[0003] Chinese utility model patent application number CN202220735569.2 discloses a Unipol polyethylene process off-gas recovery system. The system comprises a first compressor, an inlet buffer tank, a multi-stream heat exchanger, a first expander, a second expander, a first brake end, a second brake end, a second compressor, and a degassing chamber. This system improves hydrocarbon gas recovery efficiency compared to previous systems, recovering 20 kg more ethylene per ton of mixer capacity while reducing fresh nitrogen usage, thereby lowering material consumption and production costs.

[0004] However, existing tail gas recovery systems are still not perfect, with low hydrocarbon gas recovery rates. Some hydrocarbon gas is still carried back into the system and discharged into the flare, resulting in carbon emissions. Therefore, the development of an advanced new UNIPOL process polyethylene tail gas recovery cryogenic separation unit can not only improve the recovery rate of hydrocarbon gas in the tail gas, bringing more substantial economic benefits to such polyethylene plants, but also reduce the unit's carbon emissions, making such polyethylene plants have excellent environmental properties. Summary of the Invention

[0005] The purpose of the present invention is to solve the problems in the prior art and provide a UNIPOL process polyethylene tail gas recovery cryogenic separation system, which uses a cryogenic separation device to improve the recovery rate of hydrocarbon gases in the tail gas and reduce the carbon emissions of the device.

[0006] The specific technical solutions adopted in the present invention are as follows:

[0007] The present invention provides a UNIPOL process polyethylene tail gas recovery and cryogenic separation system, comprising a tail gas compressor, a compressor inlet buffer tank, a cooler, a first liquid hydrocarbon separator, a room-temperature liquid hydrocarbon recovery pump, and a cold box. The cold box is equipped with a main heat exchanger, a booster turbine expander unit, a second liquid hydrocarbon separator, a third liquid hydrocarbon separator, an ethylene separator, a gas-liquid separator, and a low-temperature liquid hydrocarbon recovery pump.

[0008] The main heat exchanger is equipped with an exhaust gas condensation channel, a liquid hydrocarbon recovery channel, an ethylene recovery channel, a nitrogen recovery channel, a liquid hydrocarbon deep cooling channel and a nitrogen reheating channel.

[0009] The compressor inlet buffer tank receives polyethylene tail gas. The inlet of the tail gas compressor is connected to the outlet of the compressor inlet buffer tank, and the outlet is divided into two paths, one of which returns to the compressor inlet buffer tank through a pipeline. The other path is connected to the inlet of the cooler, and the cooler receives the polyethylene tail gas pressurized by the tail gas compressor for cooling. The outlet of the cooler is connected to the tail gas inlet of the first liquid hydrocarbon separator. The liquid phase outlet of the first liquid hydrocarbon separator is connected to the external liquid recovery device through a normal temperature liquid hydrocarbon recovery pump. The gas phase outlet of the first liquid hydrocarbon separator is divided into two paths, one of which enters the tail gas condensation channel of the main heat exchanger through a pipeline for cooling. The other path is connected to the compressor inlet buffer tank, and the pipeline is equipped with a normal temperature reflux regulating valve. The outlet of the tail gas condensation channel is connected to the inlet of the second liquid hydrocarbon separator, and the nitrogen-rich gas outlet of the second liquid hydrocarbon separator is connected to the nitrogen reheating channel of the main heat exchanger through a pipeline for nitrogen reheating. The liquid hydrocarbon outlet of the second liquid hydrocarbon separator is connected to the liquid hydrocarbon inlet of the gas-liquid equalization separator through a pipeline.

[0010] The outlet of the nitrogen reheating channel is connected via a pipeline to the expansion inlet of the booster turbine expander unit for expansion refrigeration. The expansion outlet of the booster turbine expander unit is connected via a pipeline to the inlet of the third liquid hydrocarbon separator. The nitrogen-rich gas outlet of the third liquid hydrocarbon separator is connected via a pipeline to the inlet of the nitrogen recovery channel of the main heat exchanger for nitrogen reheating. The liquid hydrocarbon outlet of the third liquid hydrocarbon separator is connected via a pipeline to the liquid hydrocarbon inlet of the gas-liquid equalization separator.

[0011] The outlet of the nitrogen recovery channel is connected to the compression end inlet of the booster turbine expander unit through a pipeline for compression and boosting. The compression end outlet of the booster turbine expander unit is connected to the cold box through a pipeline.

[0012] The liquid outlet of the gas-liquid equalization separator is connected via a pipeline to the liquid hydrocarbon cryogenic channel of the main heat exchanger for cold recovery. The outlet of the liquid hydrocarbon cryogenic channel is connected via a pipeline to the inlet of the ethylene separator. The ethylene outlet of the ethylene separator is connected via a pipeline to the inlet of the ethylene recovery channel of the main heat exchanger for ethylene rewarming. The outlet of the ethylene recovery channel is connected via a pipeline to an ethylene recovery unit outside the cold box. The liquid hydrocarbon outlet of the ethylene separator is connected via a pipeline equipped with a cryogenic liquid hydrocarbon recovery pump to the inlet of the liquid hydrocarbon recovery channel of the main heat exchanger for rewarming. The outlet of the liquid hydrocarbon recovery channel returns to the first liquid hydrocarbon separator via a pipeline.

[0013] Preferably, a first liquid level regulating valve is provided on the pipeline between the liquid hydrocarbon outlet of the second liquid hydrocarbon separator and the liquid hydrocarbon inlet of the gas-liquid equalization separator.

[0014] Preferably, a second liquid level regulating valve is provided on the pipeline between the liquid hydrocarbon outlet of the third liquid hydrocarbon separator and the liquid hydrocarbon inlet of the gas-liquid equalization separator.

[0015] Preferably, the pipeline at the compression end outlet of the above-mentioned booster turbine expander unit is divided into two routes, one route is connected to the nitrogen recovery device outside the cold box, and the other route is discharged through a pipeline equipped with a flow regulating valve.

[0016] Preferably, a heat reflux regulating valve is provided on the pipe connecting the outlet of the exhaust gas compressor to the compressor inlet buffer tank. A normal temperature reflux regulating valve is provided on the pipe connecting the gas phase outlet of the first liquid hydrocarbon separator to the compressor inlet buffer tank. A compression reflux regulating valve is provided on the compression end outlet of the booster turbine expander unit.

[0017] Preferably, a temperature regulating valve is provided on the bypass of the inlet and outlet pipes of the liquid hydrocarbon cryogenic channel of the main heat exchanger. A bypass regulating valve is provided on the bypass of the inlet and outlet pipes of the expansion end of the booster turbine expander unit.

[0018] Preferably, a first pressure regulating valve is provided on the pipeline connecting the nitrogen-rich gas outlet of the third liquid hydrocarbon separator and the inlet of the nitrogen recovery channel of the main heat exchanger. A second pressure regulating valve is provided on the pipeline connecting the outlet of the nitrogen recovery channel and the inlet of the expansion end of the booster turbine expander unit.

[0019] Preferably, the outlet pressure of the exhaust gas compressor is set in the range of 1.5-1.7 MPaG. The temperature of the recovered exhaust gas at the inlet of the exhaust gas condensation channel is set in the range of 10-40°C, and the temperature of the exhaust gas at the outlet of the exhaust gas condensation channel after condensation is set in the range of -110--130°C.

[0020] Preferably, the pressure setting range of the nitrogen recovery channel is 0.7-0.8 MPaG, and the temperature setting range is 15-45°C. The pressure setting range of the ethylene recovery channel is 0.04-0.08 MPaG, and the temperature setting range is -5-25°C. The pressure setting range of the liquid hydrocarbon recovery channel is 1.5-2 MPaG, and the temperature setting range is 0-25°C.

[0021] Preferably, the differential pressure setting range of the expansion end of the above-mentioned booster turbine expander unit is 0.75 to 1.0 MPa, and the expansion end outlet temperature setting range is -135 to -142°C.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention provides a novel UNIPOL process polyethylene tail gas recovery and cryogenic separation system. This system utilizes advanced cryogenic separation technology and exploits the physical properties of hydrocarbon gases at low temperatures to separate hydrocarbon gases from the gas phase through a series of separation and condensation steps. Through rational process design, the present invention overcomes the low tail gas hydrocarbon recovery rate of existing systems and efficiently recovers highly economical hydrocarbon gases from polyethylene plant tail gas. While existing technologies have ethylene recovery rates of only 70-80%, the present system achieves ethylene recovery rates exceeding 90%, while also reducing carbon emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the polyethylene tail gas recovery and cryogenic separation system of the UNIPOL process provided by the present invention;

[0025] Figure 2 A schematic diagram of the internal structure of the cold box provided by the present invention;

[0026] In the figure: 1. exhaust gas compressor; 2. compressor inlet buffer tank; 3. cooler; 4. first liquid hydrocarbon separator; 5. normal temperature liquid hydrocarbon recovery pump; 6. cold box; 7. main heat exchanger; 8. booster turbine expander unit; 9. second liquid hydrocarbon separator; 10. third liquid hydrocarbon separator; 11. ethylene separator; 12. gas-liquid equalization separator; 13. low temperature liquid hydrocarbon recovery pump; 14. first pressure regulating valve; 15. second pressure regulating valve; 16. first liquid level regulating valve; 17. second liquid level regulating valve; 18. temperature regulating valve; 19. bypass regulating valve; 20. compression reflux regulating valve; 21. normal temperature reflux regulating valve; 22. hot reflux regulating valve; 23. flow regulating valve. DETAILED DESCRIPTION

[0027] The present invention will be further described and illustrated below with reference to the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention may be combined accordingly, provided that there is no conflict between them.

[0028] In the description of the present invention, it should be understood that the terms "first" and "second" are used solely for descriptive purposes and are not to be construed as indicating or implying relative importance or implicitly specifying the number of technical features being described. Therefore, features defined as "first" or "second" may explicitly or implicitly include at least one of such features.

[0029] In the description of the present invention, it should be understood that when an element is considered to be "connected" to another element, it can be directly connected to the other element or indirectly connected, that is, there are intermediate elements. On the contrary, when an element is said to be "directly" connected to another element, there are no intermediate elements.

[0030] like Figure 1 As shown, as a preferred embodiment of the present invention, a novel UNIPOL process polyethylene tail gas recovery and cryogenic separation system is provided. Specifically, it includes a tail gas compressor 1, a compressor inlet buffer tank 2, a cooler 3, a first liquid hydrocarbon separator 4 (i.e., the compressor outlet liquid hydrocarbon separator), a normal temperature liquid hydrocarbon recovery pump 5, and a cold box 6. The equipment in the cold box 6 includes a main heat exchanger 7, a booster turbine expander unit 8, a second liquid hydrocarbon separator 9, a third liquid hydrocarbon separator 10 (i.e., the expander outlet liquid hydrocarbon separator), an ethylene separator 11, a gas-liquid equalization separator 12, and a low-temperature liquid hydrocarbon recovery pump 13. The cold box 6 also includes two pressure regulating valves, two liquid level regulating valves, three reflux regulating valves, a temperature regulating valve 18, and a bypass regulating valve 19.

[0031] The main heat exchanger 7 is provided with an exhaust gas condensation channel A, a liquid hydrocarbon recovery channel B, an ethylene recovery channel C, a nitrogen recovery channel D, a liquid hydrocarbon deep cooling channel E and a nitrogen reheating channel F.

[0032] Compressor inlet buffer tank 2 receives polyethylene exhaust gas. Its outlet is connected to the inlet of exhaust compressor 1. After being pressurized by exhaust compressor 1, the polyethylene exhaust gas returns to compressor inlet buffer tank 2 via a pipeline equipped with a heat return regulating valve 22. Another pipeline is connected to the inlet of cooler 3, which receives and cools the pressurized polyethylene exhaust gas from exhaust compressor 1.

[0033] The outlet of cooler 3 is connected to the tail gas inlet of the first liquid hydrocarbon separator 4. The polyethylene tail gas enters the first liquid hydrocarbon separator 4 for separation. The liquid phase outlet of the first liquid hydrocarbon separator 4 is connected to an external liquid recovery device via a room-temperature liquid hydrocarbon recovery pump 5, and is transported outside the device for recovery. The gas phase outlet of the first liquid hydrocarbon separator 4 is divided into two channels: one is connected to the compressor inlet buffer tank 2, and the other is piped into a cold box for further recovery and cooling.

[0034] The outlet of the tail gas condensation channel A is connected to the inlet of the second liquid hydrocarbon separator 9, performing gas-liquid separation to separate nitrogen-rich gas and liquid hydrocarbons. The nitrogen-rich gas outlet of the second liquid hydrocarbon separator 9 is connected via a pipeline to the nitrogen reheating channel F of the main heat exchanger 7 for nitrogen reheating. The liquid hydrocarbon outlet of the second liquid hydrocarbon separator 9 is connected via a pipeline to the liquid hydrocarbon inlet of the gas-liquid equalization separator 12. This pipeline is equipped with a first liquid level regulating valve 16 for flow control.

[0035] The outlet of the nitrogen reheating channel F is connected to the expansion inlet of the booster turbine expander unit 8 via a pipeline equipped with a second pressure regulating valve 15, allowing expansion cooling and cooling of the nitrogen-rich gas. The expansion outlet of the booster turbine expander unit 8 is connected to the inlet of the third liquid hydrocarbon separator 10 via a pipeline. The nitrogen-rich gas outlet of the third liquid hydrocarbon separator 10 is connected to the inlet of the nitrogen recovery channel D of the main heat exchanger 7 via a pipeline for nitrogen reheating. The nitrogen enters the compression end of the booster turbine expander unit 8 for compression and pressurization, and then leaves the device for nitrogen recovery and utilization. A first pressure regulating valve 14 is installed in the pipeline connecting the nitrogen-rich gas outlet of the third liquid hydrocarbon separator 10 and the inlet of the nitrogen recovery channel D of the main heat exchanger 7.

[0036] The outlet of nitrogen recovery channel D is connected to the compression inlet of the booster turbine expander unit 8 via a pipeline for compression and boosting. The compression outlet of the booster turbine expander unit 8 is connected to the cold box 6 via a pipeline. The pipeline leading to the compression outlet of the booster turbine expander unit 8 is bifurcated into two paths: one path connects to the nitrogen recovery device outside the cold box 6, and the other path discharges through a pipeline equipped with a flow control valve 23. The compression outlet of the booster turbine expander unit 8 is equipped with a compression reflux control valve 20, and a bypass control valve 19 is installed in the bypass of the expansion inlet and outlet pipelines.

[0037] The liquid hydrocarbon outlet of the third liquid hydrocarbon separator 10 is connected to the liquid hydrocarbon inlet of the gas-liquid separator 12 via a pipeline. A second liquid level regulating valve 17 for regulating flow is provided on the pipeline.

[0038] The liquid outlet of the gas-liquid separator 12 is connected to the liquid hydrocarbon cryogenic channel E of the main heat exchanger 7 via a pipeline for cold recovery. A temperature regulating valve 18 is provided on the bypass of the inlet and outlet pipelines of the liquid hydrocarbon cryogenic channel E. The outlet of the liquid hydrocarbon cryogenic channel E is connected to the inlet of the ethylene separator 11 via a pipeline. The ethylene outlet of the ethylene separator 11 is connected to the inlet of the ethylene recovery channel C of the main heat exchanger 7 via a pipeline for ethylene rewarming. The outlet of the ethylene recovery channel C is connected to the ethylene recovery device outside the cold box 6 via a pipeline. The liquid hydrocarbon outlet of the ethylene separator 11 is connected to the inlet of the liquid hydrocarbon recovery channel B of the main heat exchanger 7 via a pipeline equipped with a low-temperature liquid hydrocarbon recovery pump 13 for rewarming. The outlet of the liquid hydrocarbon recovery channel B returns to the first liquid hydrocarbon separator 4 via a pipeline.

[0039] In order to improve the efficiency of the polyethylene tail gas recovery cryogenic separation system and ensure the recovery rate of liquid hydrocarbons, the following parameters need to be properly controlled during the separation process.

[0040] The outlet pressure of the exhaust compressor 1 is set in the range of 1.5 to 1.7 MPaG. The temperature of the recovered exhaust gas at the inlet of the exhaust condensation channel A is set in the range of 10 to 40°C, and the temperature of the exhaust gas at the outlet of the exhaust condensation channel A after condensation is set in the range of -110 to -130°C.

[0041] The pressure setting range of nitrogen recovery channel D is 0.7-0.8 MPaG, and the temperature setting range is 15-45°C. The pressure setting range of ethylene recovery channel C is 0.04-0.08 MPaG, and the temperature setting range is -5-25°C. The pressure setting range of liquid hydrocarbon recovery channel B is 1.5-2 MPaG, and the temperature setting range is 0-25°C. The differential pressure setting range of the expansion end of booster turbine expander unit 8 is 0.75-1.0 MPa, and the expansion end outlet temperature setting range is -135--142°C.

[0042] The embodiment described above is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Persons skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent substitution or equivalent transformation falls within the scope of protection of the present invention.

Claims

1. A UNIPOL process polyethylene tail gas recovery and cryogenic separation system, characterized in that: The invention comprises an exhaust gas compressor (1), a compressor inlet buffer tank (2), a cooler (3), a first liquid hydrocarbon separator (4), a normal temperature liquid hydrocarbon recovery pump (5) and a cold box (6); the cold box (6) is provided with a main heat exchanger (7), a booster turbine expansion unit (8), a second liquid hydrocarbon separator (9), a third liquid hydrocarbon separator (10), an ethylene separator (11), a gas-liquid equalization separator (12) and a low temperature liquid hydrocarbon recovery pump (13); The main heat exchanger (7) is provided with an exhaust gas condensation channel (A), a liquid hydrocarbon recovery channel (B), an ethylene recovery channel (C), a nitrogen recovery channel (D), a liquid hydrocarbon deep cooling channel (E) and a nitrogen reheating channel (F); The compressor inlet buffer tank (2) receives polyethylene tail gas; the inlet of the tail gas compressor (1) is connected to the outlet of the compressor inlet buffer tank (2), and the outlet is divided into two paths, one path returns to the compressor inlet buffer tank (2) through a pipeline; the other path is connected to the inlet of the cooler (3), and the cooler (3) receives the polyethylene tail gas pressurized by the tail gas compressor (1) for cooling; the outlet of the cooler (3) is connected to the tail gas inlet of the first liquid hydrocarbon separator (4); the liquid phase outlet of the first liquid hydrocarbon separator (4) is connected to an external liquid recovery device through a normal temperature liquid hydrocarbon recovery pump (5); the first The gas phase outlet of the liquid hydrocarbon separator (4) is divided into two paths, one of which enters the tail gas condensation channel (A) of the main heat exchanger (7) through a pipeline for cooling; the other path is connected to the compressor inlet buffer tank (2), and a normal temperature reflux regulating valve (21) is provided on the pipeline; the outlet of the tail gas condensation channel (A) is connected to the inlet of the second liquid hydrocarbon separator (9), the nitrogen-rich gas outlet of the second liquid hydrocarbon separator (9) is connected to the nitrogen reheating channel (F) of the main heat exchanger (7) through a pipeline for nitrogen reheating, and the liquid hydrocarbon outlet of the second liquid hydrocarbon separator (9) is connected to the liquid hydrocarbon inlet of the gas-liquid equalization separator (12) through a pipeline; The outlet of the nitrogen reheating channel (F) is connected to the expansion end inlet of the booster turbine expansion unit (8) through a pipeline to perform expansion refrigeration; the expansion end outlet of the booster turbine expansion unit (8) is connected to the inlet of the third liquid hydrocarbon separator (10) through a pipeline; the nitrogen-rich gas outlet of the third liquid hydrocarbon separator (10) is connected to the inlet of the nitrogen recovery channel (D) of the main heat exchanger (7) through a pipeline to perform nitrogen reheating; the liquid hydrocarbon outlet of the third liquid hydrocarbon separator (10) is connected to the liquid hydrocarbon inlet of the gas-liquid equalization separator (12) through a pipeline; The outlet of the nitrogen recovery channel (D) is connected to the compression end inlet of the booster turbine expander unit (8) through a pipeline for compression and pressurization; the compression end outlet of the booster turbine expander unit (8) is connected to the cold box (6) through a pipeline; The liquid outlet of the gas-liquid uniform separator (12) is connected to the liquid hydrocarbon cryogenic channel (E) of the main heat exchanger (7) through a pipeline for cold recovery; the outlet of the liquid hydrocarbon cryogenic channel (E) is connected to the inlet of the ethylene separator (11) through a pipeline; the ethylene outlet of the ethylene separator (11) is connected to the inlet of the ethylene recovery channel (C) of the main heat exchanger (7) through a pipeline for ethylene rewarming; the outlet of the ethylene recovery channel (C) is connected to the ethylene recovery device outside the cold box (6) through a pipeline; the liquid hydrocarbon outlet of the ethylene separator (11) is connected to the inlet of the liquid hydrocarbon recovery channel (B) of the main heat exchanger (7) through a pipeline provided with a low-temperature liquid hydrocarbon recovery pump (13) for rewarming; the outlet of the liquid hydrocarbon recovery channel (B) returns to the first liquid hydrocarbon separator (4) through a pipeline.

2. The UNIPOL process polyethylene tail gas recovery and cryogenic separation system according to claim 1, characterized in that: A first liquid level regulating valve (16) is provided on the pipeline between the liquid hydrocarbon outlet of the second liquid hydrocarbon separator (9) and the liquid hydrocarbon inlet of the gas-liquid separator (12).

3. The UNIPOL process polyethylene tail gas recovery and cryogenic separation system according to claim 1, characterized in that: A second liquid level regulating valve (17) is provided on the pipeline between the liquid hydrocarbon outlet of the third liquid hydrocarbon separator (10) and the liquid hydrocarbon inlet of the gas-liquid separator (12).

4. The UNIPOL process polyethylene tail gas recovery and cryogenic separation system according to claim 1, characterized in that: The pipeline at the compression end outlet of the booster turbine expander unit (8) is divided into two routes, one route is connected to the nitrogen recovery device outside the cold box (6), and the other route is discharged through a pipeline provided with a flow regulating valve (23).

5. The UNIPOL process polyethylene tail gas recovery and cryogenic separation system according to claim 1, characterized in that: A heat reflux regulating valve (22) is provided on the pipeline from the outlet of the tail gas compressor (1) back to the compressor inlet buffer tank (2); a normal temperature reflux regulating valve (21) is provided on the pipeline connecting the gas phase outlet of the first liquid hydrocarbon separator (4) and the compressor inlet buffer tank (2); and a compression reflux regulating valve (20) is provided on the compression end outlet of the booster turbine expander unit (8).

6. The UNIPOL process polyethylene tail gas recovery and cryogenic separation system according to claim 1, characterized in that: A temperature regulating valve (18) is provided on the bypass of the inlet and outlet pipes of the liquid hydrocarbon cryogenic channel (E) of the main heat exchanger (7); a bypass regulating valve (19) is provided on the bypass of the inlet and outlet pipes of the expansion end of the booster turbine expansion unit (8).

7. The UNIPOL process polyethylene tail gas recovery and cryogenic separation system according to claim 1, characterized in that: A first pressure regulating valve (14) is provided on the pipeline connecting the nitrogen-rich gas outlet of the third liquid hydrocarbon separator (10) and the inlet of the nitrogen recovery channel (D) of the main heat exchanger (7); and a second pressure regulating valve (15) is provided on the pipeline connecting the outlet of the nitrogen reheating channel (F) and the expansion end inlet of the booster turbine expander unit (8).

8. The UNIPOL process polyethylene tail gas recovery and cryogenic separation system according to claim 1, characterized in that: The outlet pressure of the exhaust gas compressor (1) is set in the range of 1.5 to 1.7 MPaG; the temperature of the recovered exhaust gas at the inlet of the exhaust gas condensation channel (A) is set in the range of 10 to 40°C, and the temperature of the exhaust gas condensation channel (A) outlet after condensation is set in the range of -110 to -130°C.

9. The UNIPOL process polyethylene tail gas recovery and cryogenic separation system according to claim 1, characterized in that: The pressure setting range of the nitrogen recovery channel (D) is 0.7-0.8 MPaG, and the temperature setting range is 15-45°C; the pressure setting range of the ethylene recovery channel (C) is 0.04-0.08 MPaG, and the temperature setting range is -5-25°C; the pressure setting range of the liquid hydrocarbon recovery channel (B) is 1.5-2 MPaG, and the temperature setting range is 0-25°C.

10. The UNIPOL process polyethylene tail gas recovery and cryogenic separation system according to claim 1, characterized in that: The differential pressure setting range of the expansion end of the booster turbine expander unit (8) is 0.75 to 1.0 MPa, and the expansion end outlet temperature setting range is -135 to -142°C.

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