Membrane coupling separation process integrating pipeline natural gas pressure regulation and light hydrocarbon recovery process

Abstract

The invention belongs to the field of petrochemical engineering, and provides a membrane coupling separation process integrating pipeline natural gas pressure regulation and light hydrocarbon recovery processes. The method comprises the following steps: firstly, enriching light dydrocarbon through a light dydrocarbon preferential permeable membrane while directly utilizing pressure energy, then removing methane through a low-temperature absorption unit, and finally separating ethane from light dydrocarbon condensate through a desorption tower. Meanwhile, the supercharging device driven by the expansion machine is adopted as a work recovery device, cold energy converted in the expansion process provides cold energy for a subsequent absorption unit, and energy consumption and operation cost of the light hydrocarbon separation process are remarkably reduced. According to the light hydrocarbon separation technology integrating multi-stage turbine expansion, low-temperature absorption and membrane separation coupling, low-cost light hydrocarbon separation is achieved under the condition that extra energy does not need to be consumed, meanwhile, the production cost of ethane per ton is reduced, and remarkable economic value is achieved.

Description

Technical Field

[0001] The present invention belongs to the field of petrochemical engineering and relates to a membrane-coupled separation process integrating pipeline natural gas pressure regulation and light hydrocarbon recovery processes. Background Art

[0002] High-pressure pipeline transportation is the main method for long-distance and large-scale transportation of natural gas. Taking the natural gas in the China-Russia Eastern Route pipeline as an example, the design pressure of the natural gas transportation pipeline can reach 12 MPa. However, when the natural gas transported by the high-pressure pipeline reaches the demand area, considering the safety and usage requirements of users, the gas pressure needs to be reduced to below 1.6 MPa before it can be supplied to the urban gas pipeline network and various industrial users.

[0003] Table 1 Composition of high-pressure pipeline natural gas at a pressure reduction station in China

[0004]

[0005] The high-pressure pipeline natural gas contains a certain amount of light hydrocarbon components such as ethane and propane. Table 1 shows the specific composition of the high-pressure pipeline natural gas at a pressure reduction station in China. These light hydrocarbon components are good raw materials for industrial ethylene production. However, due to the low content of light hydrocarbons in pipeline natural gas, it is difficult to directly utilize and they are often directly burned for energy supply. Taking the high-pressure pipeline natural gas at a pressure reduction station in Northeast China as an example, according to the actual flow rate of the pipeline natural gas, the total amount of light hydrocarbons in the natural gas exceeds 120 tons per day, which can provide about 50,000 tons of raw materials for the ethylene plant every year, and the economic value generated will exceed 230 million yuan. Therefore, light hydrocarbon recovery is imperative.

[0006] Currently, the commonly used natural gas light hydrocarbon recovery technologies include cryogenic distillation, shallow cold absorption, and membrane separation, etc. Since the boiling point of methane in pipeline natural gas is relatively low, using the cryogenic distillation method requires providing a cooling capacity below -100 °C, which results in complex cryogenic devices, high equipment requirements, and low operating stability; while the shallow cold absorption method can reduce the operating temperature of light hydrocarbon separation, but the light hydrocarbon concentration in pipeline natural gas is low, and the amount of light hydrocarbons that can be separated by a unit absorbent is small, resulting in a large absorbent circulation volume required to achieve a high light hydrocarbon recovery rate; membrane separation is a separation technology based on the difference in permeation rates, which can directly use the pressure energy to concentrate light hydrocarbons and has the advantage of no phase change, but there are also limitations such as low light hydrocarbon / methane permeation selectivity. Therefore, for the efficient separation of light hydrocarbons in high-pressure pipeline natural gas, the following problems need to be solved simultaneously: 1) Avoid extreme operating conditions such as ultra-low temperature and improve the energy efficiency of the process; 2) While achieving high-concentration and high-recovery separation of light hydrocarbons, reduce the process operating costs and other expenses; 3) Reasonably and efficiently utilize the pressure energy contained in high-pressure pipeline natural gas.

[0007] A large number of studies have shown that the concentration of light hydrocarbons in the feed gas has a significant impact on the selection of the advantageous region of the separation technology. The existing commercial light hydrocarbon preferential permeation membranes can achieve a light hydrocarbon / methane selectivity of 4.5 under low-temperature conditions. When the light hydrocarbon concentration is low, membrane separation technology is suitable for separating light hydrocarbons to enrich them in the high-concentration region; while when the light hydrocarbons reach the high-concentration region, the shallow cold absorption method is in an advantageous position. At the same time, the light hydrocarbon separation process is under low-temperature conditions, and the refrigeration process requires a large amount of energy, increasing the process operation cost. However, the pipeline natural gas is in a high-pressure state, and turboexpansion can be used for efficient refrigeration to provide cold energy for the light hydrocarbon separation process. Moreover, the mechanical work recovered by turboexpansion can synchronously drive the compressor to further provide cold energy and mechanical energy for the light hydrocarbon separation process.

[0008] For the actual demand of comprehensive utilization of the pressure energy of high-pressure pipeline natural gas, the present invention proposes a light hydrocarbon separation process that synergistically combines a light hydrocarbon preferential permeation membrane separation and a low-temperature absorption unit to comprehensively utilize the pressure energy of high-pressure pipeline natural gas. The light hydrocarbons are enriched while directly utilizing the pressure energy through the light hydrocarbon preferential permeation membrane, then the removal of methane is achieved through the low-temperature absorption unit, and finally the separation of ethane and light hydrocarbon condensate is achieved through the desorption tower. The light hydrocarbon separation process described in the present invention can reasonably and efficiently achieve the comprehensive utilization of the pressure energy of high-pressure pipeline natural gas, and achieve the efficient separation of light hydrocarbons without consuming external mechanical energy and cold energy. Summary of the Invention

[0009] The object of the present invention is to provide a method for separating light hydrocarbons by reasonably designing a multi-technology coupling process without consuming additional energy by utilizing the mechanical energy and cold energy generated after the expansion and pressure reduction of high-pressure pipeline natural gas. This process enriches light hydrocarbons by directly utilizing the pressure energy, reduces the energy consumption of subsequent separation units, and improves the product quality of light hydrocarbons. At the same time, a booster device driven by an expander is used as a work recovery device, and the cold energy converted during the expansion process provides cold energy for the membrane separation and low-temperature absorption units to meet the electricity and refrigeration requirements in the process flow, realizing the efficient and reasonable utilization of the pressure energy of high-pressure pipeline natural gas and the low-cost separation of light hydrocarbons.

[0010] The technical solution of the present invention:

[0011] A membrane-coupled separation process integrating the pressure regulation of pipeline natural gas and the light hydrocarbon recovery process. The high-pressure pipeline natural gas S1 first enters the first expander 1, and after pressure reduction, it enters the liquid separation tank 2. The first condensate S4 is obtained at the bottom of the liquid separation tank 2, and the first non-condensable gas S3 is obtained at the top. The first non-condensable gas S3 enters the cold box 3. After recovering the cold energy generated during the pressure reduction of the high-pressure pipeline natural gas S1, it enters the first heat exchanger 4. After combining with the bottom condensate S16 of the distillation column and utilizing the cold energy, it enters the second expander 5 to further reduce the pressure to the pressure specified by the urban gas pipeline network, forming the pressure-reduced low-temperature frozen natural gas S5. The pressure-reduced low-temperature frozen natural gas S5 enters the membrane separation unit 6 to utilize the pressure energy of the natural gas itself and the mechanical energy formed during the pressure reduction process. The retentate gas S6 of the membrane separation unit and the tail gas S9 at the top of the absorber are combined and then pass through the second heat exchanger 13 and the third heat exchanger 16 in sequence to utilize the cold energy and become the low-pressure pipeline natural gas S11. At the same time, the light hydrocarbons are enriched on the permeate side of the membrane separation unit 6 to form the permeate gas S7 of the membrane separation unit. The permeate gas S7 of the membrane separation unit then enters the cold box 3 to further recover the cold energy. The light hydrocarbon-rich gas S8 coming out of the cold box 3 and the tail gas S12 at the top of the stripper enter from the bottom of the absorber 7 and contact countercurrently with the lean absorbent S17 entering from the top of the absorber 7. After absorbing most of the light hydrocarbons, it is withdrawn from the bottom of the absorber 7 and is called the light hydrocarbon-rich liquid S10 at the bottom of the absorber. The light hydrocarbon-rich liquid S10 at the bottom of the absorber is further pressurized by the first pump 8, combined with the first condensate S4, and enters from the top of the stripper 9, and contacts countercurrently with the rising steam generated by the first reboiler 10. The light hydrocarbon-rich liquid S13 at the bottom of the stripper is obtained at the bottom of the stripper 9. The light hydrocarbon-rich liquid S13 at the bottom of the stripper enters the distillation column 12 through the throttle valve 11. The gas withdrawn from the top of the distillation column 12 enters the second heat exchanger 13. After utilizing the cold energy with the retentate gas S6 of the membrane separation unit, it becomes the ethane product gas S14. The liquid-phase product at the bottom of the distillation column 12 is partially vaporized by the second reboiler 14, and the gas-phase component returns to the distillation column 12 to maintain the gas-liquid balance in the column. The unvaporized liquid phase exchanges heat with the first reboiler 10 at the bottom of the stripper 9. Finally, one stream is produced as the light hydrocarbon condensate S15, and the other stream, the bottom condensate S16 of the distillation column, passes through the second pump 15, the third heat exchanger 16, the first heat exchanger 4, and the first cooler 17 in sequence and then enters the cold box 3. After utilizing the cold energy, it becomes the lean absorbent S17 of the absorber and is recycled as the absorbent.

[0012] Advantages of the present invention: Through the coupled integration of operation units such as multi-stage turbine expansion, membrane separation, and cryogenic absorption, the present invention can separate high-value-added products such as ethane from natural gas at low cost while reasonably and efficiently utilizing the pressure energy of high-pressure pipeline natural gas. The membrane separation unit directly utilizes the pressure energy to enrich light hydrocarbons, reducing the energy consumption of subsequent separation units. At the same time, through the turbine expansion refrigeration process driven by high-pressure pipeline natural gas and the compression process driven by the shaft work output of the turbine, sufficient cold energy and mechanical energy are provided for the cryogenic membrane separation process, significantly reducing the external energy demand during the light hydrocarbon enrichment process. The cryogenic absorption unit efficiently utilizes the cold energy during the expansion process to separate components such as methane and ethane without additional cold energy. Taking 45,000 Nm 3 / h of high-pressure pipeline natural gas as an example, based on the separation process designed according to the present invention, it consumes 4. Five tons per hour of 0.35 MPaG steam and 32.8 tons per hour of circulating water, and is expected to produce 6,127.2 tons of ethane products and 2,027.2 tons of light hydrocarbon condensate per year, with the expected economic benefit exceeding 17 million yuan. In summary, the membrane-coupled separation process integrating the pressure regulation of pipeline natural gas and the light hydrocarbon recovery process provided by the present invention can reasonably and efficiently utilize the pressure energy during the natural gas pressure reduction process, creating significant economic benefits for the separation of light hydrocarbons. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is the principle flow chart of the membrane-coupled separation process integrating the pressure regulation of pipeline natural gas and the light hydrocarbon recovery process.

[0014] Figure 2 is the principle flow chart of the primary membrane separation unit of the membrane-coupled separation process integrating the pressure regulation of pipeline natural gas and the light hydrocarbon recovery process.

[0015] Figure 3 is the principle flow chart of the primary two-stage membrane separation unit of the membrane-coupled separation process integrating the pressure regulation of pipeline natural gas and the light hydrocarbon recovery process.

[0016] In the figure: 1 First expander; 2 Liquid separation tank; 3 Cold box; 4 First heat exchanger; 5 Second expander; 6 Membrane separation unit; 7 Absorption tower; 8 First pump; 9 Stripping tower; 10 First reboiler; 11 Throttle valve; 12 Rectifying tower; 13 Second heat exchanger; 14 Second reboiler; 15 Second pump; 16 Third heat exchanger; 17 First cooler; 101 First-stage light hydrocarbon preferentially permeating membrane separation unit; 102 Fourth heat exchanger; 103 First compressor; 104 Second cooler; 201 First-stage light hydrocarbon preferentially permeating membrane separation unit; 202 Fifth heat exchanger; 203 Second compressor; 204 Third cooler; 205 Second-stage light hydrocarbon preferentially permeating membrane separation unit; 206 Sixth heat exchanger; 207 Third compressor; 208 Fourth cooler; S1 High-pressure pipeline natural gas; S2 Depressurized natural gas; S3 First non-condensable gas; S4 First condensate; S5 Depressurized low-temperature refrigerated natural gas; S6 Permeate gas of membrane separation unit; S7 Permeate gas of membrane separation unit; S8 Light hydrocarbon-rich gas; S9 Tail gas at the top of absorption tower; S10 Light hydrocarbon-rich liquid at the bottom of absorption tower; S11 Low-pressure pipeline natural gas; S12 Tail gas at the top of stripping tower; S13 Light hydrocarbon-rich liquid at the bottom of stripping tower; S14 Ethane product gas; S15 Light hydrocarbon condensate; S16 Condensate at the bottom of rectifying tower; S17 Lean absorbent of absorption tower; S101 First permeate gas; S102 Pressurized first permeate gas; S201 Second permeate gas; S202 Pressurized second permeate gas; S203 First retentate gas; S204 Third permeate gas; S205 Pressurized third permeate gas. Specific embodiments

[0017] The following further describes the specific embodiments of the present invention in conjunction with the accompanying drawings and technical solutions.

[0018] Example 1

[0019] Example 1 is directed to the high-pressure pipeline natural gas at a pressure reduction station in Northeast China. The integrated first-stage membrane coupling separation process for pipeline natural gas pressure regulation and light hydrocarbon recovery described in the present invention is adopted. The principle process structure is as Figure 1 、 2 shown. Under the condition of reasonably and efficiently utilizing the pressure energy of high-pressure pipeline natural gas, ethane gas, low-pressure natural gas and light hydrocarbon condensate are produced.

[0020] High-pressure pipeline natural gas S1 first enters the first expander 1, where the pressure is reduced to 3.90 MPaG and the temperature is reduced to -57.70 °C. Then it enters the liquid separation tank 2, where the first condensate S4 is obtained at the bottom and the first non-condensable gas S3 is obtained at the top. The first non-condensable gas S3 is heated to 20.74 °C through the cold box 3 and the first heat exchanger 4, and then enters the second expander 5, where the pressure is reduced to 1.50 MPaG and the temperature is reduced to -40.00 °C, becoming pressure-reduced low-temperature refrigerated natural gas S5. Then it enters the first-stage light hydrocarbon preferential permeation membrane separation unit 101, where methane is retained to obtain the retentate gas S6 of the membrane separation unit, and light hydrocarbons preferentially permeate to obtain the first permeate gas S101. The first permeate gas S101 enters the fourth heat exchanger 102 after passing through the fourth heat exchanger 102, the first compressor 103, and the second cooler 104. After the cold energy is utilized, the temperature is reduced to -35.64 °C, and then it enters the cold box 3 to further reduce the temperature to -54.70 °C. Together with the stripping tower top tail gas S12, it enters the absorption tower 7 from the bottom. The absorption lean liquid S17 of the absorption tower enters from the top. The absorption tower top tail gas S9 and the retentate gas S6 of the membrane separation unit are combined and then pass through the second heat exchanger 13 and the third heat exchanger 16 in sequence, and the temperature is increased to 15.00 °C to become low-pressure pipeline natural gas S11. The light hydrocarbon rich liquid S10 at the bottom of the absorption tower is pressurized to 1.50 MPaG by the first pump 8, combined with the first condensate S4, and enters the top of the stripping tower 9. After obtaining the stripping tower bottom light hydrocarbon rich liquid S13 at the bottom of the tower, it enters the rectifying tower 12 with a top pressure of 1.15 MPaG through the throttle valve 11. The gas extracted from the top of the tower enters the second heat exchanger 13, and after the cold energy is utilized with the retentate gas S6 of the membrane separation unit, the temperature is reduced to -12.87 °C to become ethane product gas S14. The liquid extracted from the bottom of the tower is partially heated and vaporized by the second reboiler 14, and the gas phase component returns to the rectifying tower 12. The unvaporized liquid phase exchanges heat with the first reboiler 10 at the bottom of the stripping tower, and one part is produced as light hydrocarbon condensate S15, and the other part of the rectifying tower bottom condensate S16 passes through the second pump 15, the third heat exchanger 16, the first heat exchanger 4, and the first cooler 17 in sequence and then enters the cold box 3. After the cold energy is utilized, the temperature is reduced to -54.70 °C to become the absorption lean liquid S17 of the absorption tower.

[0021] The actual key material compositions and operating parameters in Example 1 are shown in Table 2.

[0022] Table 2 List of Compositions and Operating Parameters of Key Materials in Example 1

[0023]

[0024]

[0025] In this embodiment, the total output work of the first expander and the second expander is approximately 1694 kW, while the total consumed work of the first compressor, the first pump, and the second pump is approximately 1261 kW. According to the simulation results given in Table 2, the cold energy of -57.7 °C generated after the high-pressure natural gas is depressurized can be reasonably utilized, and the light hydrocarbon recovery rate can reach 57.9%. According to the prices of ethane and light hydrocarbon condensate in 2023, the annual output value created by producing ethane is expected to reach 25.83 million yuan, the value created by producing light hydrocarbon condensate per year is expected to reach 7.62 million yuan, the operating cost is approximately 8.25 million yuan per year, the equipment depreciation is approximately 2.2 million yuan per year, and the annual profit is approximately 14.83 million yuan.

[0026] Example 2

[0027] In Example 2, for the high-pressure pipeline natural gas at a pressure reduction station in Northeast China, the integrated one-stage and two-stage membrane coupling separation process for pipeline natural gas pressure regulation and light hydrocarbon recovery described in the present invention is adopted. The principle process structure is as Figure 1 、 3 shown. Under the condition of reasonably and efficiently utilizing the pressure energy of the high-pressure pipeline natural gas, ethane gas, low-pressure natural gas, and light hydrocarbon condensate are produced.

[0028] High-pressure pipeline natural gas S1 first enters the first expander 1, where the pressure is reduced to 3.90 MPaG and the temperature is reduced to -57.70 °C. Then it enters the liquid separation tank 2, where the first condensate S4 is obtained at the bottom and the first non-condensable gas S3 is obtained at the top. The first non-condensable gas S3 is heated to 20.74 °C through the cold box 3 and the first heat exchanger 4, and then enters the second expander 5, where the pressure is reduced to 1.50 MPaG and the temperature is reduced to -40.00 °C, becoming depressurized low-temperature refrigerated natural gas S5. Then it enters the first-stage light hydrocarbon preferential permeation membrane separation unit 201, where methane is intercepted to obtain the first retentate gas S203, and light hydrocarbons preferentially permeate to obtain the second permeate gas S201. The first retentate gas S203 enters the second-stage light hydrocarbon preferential permeation membrane separation unit 205, where methane is intercepted to obtain the membrane separation unit retentate gas S6, and light hydrocarbons preferentially permeate to obtain the third permeate gas S204. The third permeate gas S204 circulates through the sixth heat exchanger 206, the third compressor 207, and the fourth cooler 208 and then enters the first-stage light hydrocarbon preferential permeation membrane separation unit 201 again for light hydrocarbon separation. The second permeate gas S201 enters the fifth heat exchanger 202, the second compressor 203, and the third cooler 204 and then enters the fifth heat exchanger 202. After the cold energy is utilized, the temperature is reduced to -33.43 °C, and then it enters the cold box 3 to further reduce the temperature to -54.70 °C. It enters the absorption tower 7 from the bottom together with the stripping tower top tail gas S12. The absorption tower absorbs the lean liquid S17 entering from the top. The absorption tower top tail gas S9 and the membrane separation unit retentate gas S6 are combined and then pass through the second heat exchanger 13 and the third heat exchanger 16 in sequence, and the temperature is increased to 15.00 °C, becoming low-pressure pipeline natural gas S11. The light hydrocarbon rich liquid S10 at the bottom of the absorption tower is pressurized to 1.50 MPaG by the first pump 8, combined with the first condensate S4, and enters the top of the stripping tower 9. After the stripping tower bottom light hydrocarbon rich liquid S13 is obtained at the bottom, it enters the rectification tower 12 with a top pressure of 1.15 MPaG through the throttle valve 11. The gas extracted from the top of the tower enters the second heat exchanger 13, and after the cold energy is utilized with the membrane separation unit retentate gas S6, the temperature is reduced to -12.87 °C, becoming ethane product gas S14. The liquid extracted from the bottom of the tower is partially heated and vaporized by the second reboiler 14, and the gas phase component returns to the rectification tower 12. The unvaporized liquid phase exchanges heat with the stripping tower bottom first reboiler 10, and one part is produced as light hydrocarbon condensate S15, and the other part of the rectification tower bottom condensate S16 passes through the second pump 15, the third heat exchanger 16, the first heat exchanger 4, and the first cooler 17 in sequence and then enters the cold box 3. After the cold energy is utilized, the temperature is reduced to -54.70 °C, becoming the absorption tower absorption lean liquid S17.

[0029] In Example 2, the actual key material compositions and operating parameters are shown in Table 3.

[0030] Table 3 List of Compositions and Operating Parameters of Key Materials in Example 2

[0031]

[0032] In this embodiment, the total output work of the first expander and the second expander is approximately 1694 kW, while the total consumed work of the first compressor, the second compressor, the first pump and the second pump is approximately 1655 kW. According to the simulation results given in Table 3, the cold energy of -57.7 °C generated after the high-pressure natural gas is depressurized can be reasonably and efficiently utilized. At the same time, the light hydrocarbon recovery rate can reach 61.9%, which is 6.9% higher than that in Example 1. According to the prices of ethane and light hydrocarbon condensate in 2023, the annual output value created by producing ethane is expected to reach 27.57 million yuan, the value created by producing light hydrocarbon condensate per year is expected to reach 8.11 million yuan, the operating cost is about 6.8 million yuan per year, which is 1.45 million yuan less than that in Example 1 per year, and the equipment depreciation is about 2.66 million yuan per year. Compared with Example 1, the annual profit can be increased by 2.03 million yuan, showing good economic benefits.

[0033] In summary, the membrane-coupled separation process integrating the pipeline natural gas pressure regulation and light hydrocarbon recovery process described in the present invention, through the mutual coupling of the light hydrocarbon preferential permeation membrane separation and the low-temperature absorption unit, while synergistically separating the light hydrocarbons in the pipeline natural gas, reasonably and efficiently realizes the comprehensive utilization of mechanical energy, cold energy and pressure energy in the process of pipeline natural gas depressurization. Without consuming external mechanical energy and cold energy, the light hydrocarbons are separated at low cost, showing significant economic benefits.

Claims

1. A membrane-coupled separation process integrating the pressure regulation of pipeline natural gas and the recovery of light hydrocarbons, characterized in that: High-pressure pipeline natural gas (S1) first enters the first expander (1), is depressurized and then enters the liquid separation tank (2). The first condensate (S4) is obtained at the bottom of the liquid separation tank (2), and the first non-condensable gas (S3) is obtained at the top. The first non-condensable gas (S3) enters the cold box (3), after recovering the cold energy generated during the pressure reduction of the high-pressure pipeline natural gas (S1), enters the first heat exchanger (4), combines with the bottom condensate of the rectification column (S16) to utilize the cold energy, and then enters the second expander (5) to further reduce the pressure to the pressure specified by the urban gas pipeline network, forming depressurized low-temperature frozen natural gas (S5). The depressurized low-temperature frozen natural gas (S5) enters the membrane separation unit (6) to utilize the pressure energy of the natural gas itself and the mechanical energy formed during the pressure reduction process. The retentate gas (S6) of the membrane separation unit and the tail gas at the top of the absorber (S9) are combined and then pass through the second heat exchanger (13) and the third heat exchanger (16) to utilize the cold energy, and then become low-pressure pipeline natural gas (S11). At the same time, light hydrocarbons are enriched on the permeate side of the membrane separation unit (6) to form the permeate gas of the membrane separation unit (S7). The permeate gas of the membrane separation unit (S7) then enters the cold box (3) to further recover the cold energy. The light hydrocarbon-rich gas (S8) coming out of the cold box (3) and the tail gas at the top of the stripper (S12) enter from the bottom of the absorber (7) and contact countercurrently with the lean absorbent of the absorber (S17) entering from the top of the absorber (7). After absorbing most of the light hydrocarbons, it is taken out from the bottom of the absorber (7), which is called the light hydrocarbon-rich liquid at the bottom of the absorber (S10). The light hydrocarbon-rich liquid at the bottom of the absorber (S10) is further pressurized by the first pump (8), combined with the first condensate (S4), and enters from the top of the stripper (9), and contacts countercurrently with the rising steam generated by the first reboiler (10). The light hydrocarbon-rich liquid at the bottom of the stripper (S13) is obtained at the bottom of the stripper (9). The light hydrocarbon-rich liquid at the bottom of the stripper (S13) enters the rectification column (12) through the throttle valve (11). The gas taken out from the top of the rectification column (12) enters the second heat exchanger (13), and after utilizing the cold energy with the retentate gas (S6) of the membrane separation unit, it becomes ethane product gas (S14). After partial vaporization of the liquid-phase product at the bottom of the rectification column (12) by the second reboiler (14), the gas-phase component returns to the rectification column (12) to maintain the gas-liquid balance in the column, and the unvaporized liquid-phase exchanges heat with the first reboiler (10) at the bottom of the stripper (9). Finally, one stream is produced as light hydrocarbon condensate (S15), and the other stream of rectification column bottom condensate (S16) passes through the second pump (15), the third heat exchanger (16), the first heat exchanger (4), and the first cooler (17) in sequence and then enters the cold box (3). After utilizing the cold energy, it becomes the lean absorbent of the absorber (S17) and is recycled as the absorbent.

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