A multi-stage membrane separation process for comprehensive utilization of helium-containing natural gas liquefaction tail gas

Abstract

This invention provides a multi-stage membrane separation process for the comprehensive utilization of helium-containing natural gas liquefaction tail gas. It nests helium-preferred permeation membrane and methane-preferred permeation membrane separation units in a tiered manner. First, the liquefied tail gas is passed through two stages of helium-preferred permeation membrane separation units to produce crude helium. Then, the residual gas is passed through a methane-preferred permeation membrane separation unit, separating out methane-rich gas which is sent to the natural gas liquefaction plant to increase liquefied natural gas production. Finally, the residual gas from the methane-preferred permeation membrane is passed through a second stage of helium-preferred permeation membrane, recovering helium while reducing the separation scale and improving separation efficiency. This achieves high-efficiency and high-yield comprehensive utilization of helium-containing natural gas liquefaction tail gas. Using this multi-stage membrane separation process, for natural gas liquefaction tail gas with a helium concentration of 1.06 vol%, the helium yield reaches 99.4%, with a purity exceeding 80.0 vol%, and the methane yield exceeds 88.7%. The unit power consumption for separating each standard cubic meter of liquefied tail gas does not exceed 0.41 kWh.

Description

Technical Field

[0001] This invention relates to a multi-stage membrane separation process for the comprehensive utilization of helium-containing natural gas liquefaction tail gas, belonging to the field of petrochemicals. Background Technology

[0002] Natural gas is a mixture of hydrocarbon and non-hydrocarbon gases found in underground strata, and is a high-quality fuel and chemical feedstock. Natural gas liquefaction (LNG) processes can liquefy methane and condensable hydrocarbon components through cooling and pressurization, thereby separating them from non-condensable gases such as nitrogen and helium, ultimately effectively increasing the calorific value of commercial natural gas. Furthermore, liquefaction reduces the volume of natural gas by more than 600 times, which is highly advantageous for transportation and storage.

[0003] After processing by the natural gas purification and liquefaction units, ultra-low boiling point gases such as hydrogen, helium, and nitrogen are enriched in the non-condensable gas, forming natural gas liquefaction tail gas. The helium content can be concentrated from 0.10–0.20 vol% to 1.00–4.00 vol%, far exceeding the standard for helium-rich natural gas. Simultaneously, the methane content in the liquefied tail gas can reach 60.00–70.00 vol%. Taking a helium-containing natural gas liquefaction plant in China as an example, according to actual operating data, the helium content of the feed gas is approximately 0.17 vol%. After passing through the cryogenic liquefaction unit, the helium content in the liquefied tail gas can be increased to approximately 1.06 vol%, and the methane content reaches as high as 61.40 vol%. Based on the actual flow rate of the liquefied tail gas from this natural gas liquefaction plant, the total helium volume in the tail gas exceeds 66,000 standard cubic meters per year, and the total methane volume exceeds 380,000 standard cubic meters per year. Table 1 shows the specific composition of the helium-containing natural gas liquefaction tail gas produced as a byproduct of the helium-containing natural gas liquefaction plant. By using helium-containing natural gas liquefaction tail gas as raw material to recover helium and methane, crude helium can be produced while increasing the production of liquefied natural gas, which can create huge economic value.

[0004] Table 1. Composition of Helium-containing Natural Gas Liquefaction Tail Gas (By-product) from a Natural Gas Liquefaction Plant in a Certain Location in China

[0005]

[0006] High-efficiency and high-yield recovery of helium and methane is key to the comprehensive utilization of helium-containing natural gas liquefaction tail gas. Commonly used natural gas separation technologies include cryogenic distillation, pressure swing adsorption (PSA), and membrane separation. Since the boiling points of the components in helium-containing natural gas liquefaction tail gas are extremely low, cryogenic distillation requires cooling below -152°C, resulting in enormous energy consumption. While PSA has advantages in producing high-purity helium, its recovery rate is low, and most adsorbents have low nitrogen / methane selectivity, making it difficult to meet the recovery requirements of natural gas liquefaction tail gas. Membrane separation, a separation technology based on permeation rate differences, has the advantage of no phase change, avoiding the huge energy consumption caused by providing ultra-low temperatures for phase separation recovery of methane and helium, and holds promise for the synergistic recovery of helium and methane.

[0007] In existing commercial helium-preferred permeate (SPP) membranes, the permeation rate of helium is two orders of magnitude higher than that of nitrogen and methane, making helium enrichment a promising prospect. Commercial methane-preferred permeate (MPP) membranes achieve a methane / nitrogen selectivity of up to 3.5 at freezing temperatures, making them suitable for methane recovery. To simultaneously recover helium and methane, a multi-stage membrane separation process must first be designed. The helium concentration in liquefied petroleum gas (LPG) tail gas is low, and commercial MPP membranes cannot efficiently separate helium from nitrogen and methane. To ensure helium recovery, it is necessary to first pass the LPG tail gas through a two-stage MPP membrane to produce crude helium, and then pass the permeate from the MPP membrane through a methane-preferred permeate membrane to recover methane, thereby increasing LNG production. However, the large amount of methane in the LPG tail gas significantly increases the scale of helium separation, leading to a substantial decrease in separation efficiency. Utilizing a methane-preferred permeation membrane to recover methane from liquefied gas tail gas can significantly reduce the methane content in the residual gas, thereby reducing the separation scale of the helium recovery process. Simultaneously, the helium from the permeation process is recycled back to the natural gas liquefaction unit along with the methane-rich gas, preventing additional helium loss. Therefore, further feeding the residual gas from the methane-preferred permeation membrane into a second-stage helium-preferred permeation membrane, achieving a cascaded nesting of helium-preferred and methane-preferred permeation membranes, can significantly improve the separation efficiency of the helium recovery process. This enables high-yield and high-efficiency comprehensive utilization of helium-containing natural gas liquefaction tail gas, producing crude helium and liquefied natural gas with low energy consumption.

[0008] To address the practical need for comprehensive utilization of helium-containing natural gas liquefaction tail gas, this invention proposes a multi-stage membrane separation process that integrates helium-preferred permeation membrane and methane-preferred permeation membrane separation units. This process recovers helium through the helium-preferred permeation membrane while simultaneously recovering methane through the methane-preferred permeation membrane, reducing the separation scale of the helium recovery process and improving the separation efficiency of the helium-preferred permeation membrane. The multi-stage membrane separation process described in this invention can achieve high-efficiency and high-yield comprehensive utilization of helium-containing natural gas liquefaction tail gas, producing crude helium and liquefied natural gas with low energy consumption. Summary of the Invention

[0009] The purpose of this invention is to provide a multi-stage membrane separation process for co-producing crude helium and liquefied natural gas (LNG) using helium-containing natural gas liquefaction tail gas as feedstock. This process utilizes a nested series of helium-preferred and methane-preferred permeation membranes to efficiently concentrate low-concentration (>1.0 vol%) helium to produce crude helium (>80.0 vol%), which is then high-pressure bottled and sent to a helium purification unit. Methane in the liquefaction tail gas is separated and recycled to the natural gas liquefaction unit to produce LNG, achieving high-efficiency and high-yield comprehensive utilization of helium-containing natural gas liquefaction tail gas.

[0010] The technical solution of the present invention:

[0011] A multi-stage membrane separation process for comprehensively utilizing helium-containing natural gas liquefaction tail gas: Helium-containing natural gas liquefaction tail gas S1, a byproduct of the natural gas liquefaction unit, first passes through a first compressor 1, becoming pressurized helium-containing natural gas liquefaction tail gas S2. It then enters a first-stage helium-preferred permeation membrane separation unit 2, where methane and nitrogen are retained, yielding a first permeate gas S7. Hydrogen and helium preferentially permeate, yielding a first permeate gas S3. The first permeate gas S3 passes through a second compressor 3, becoming pressurized first permeate gas S4, and enters a second-stage helium-preferred permeation membrane separation unit 4. Hydrogen and helium preferentially permeate, yielding crude helium S5 with a concentration exceeding 80.0 vol%, which is then high-pressure bottled and sent to a helium purification unit. In the second-stage helium-preferred permeation membrane separation unit 4, methane and nitrogen are retained, yielding a second permeate gas S6, which is recycled back to the first-stage helium-preferred permeation membrane separation unit 2 for helium recovery. The first permeate gas S7 enters a first-stage methane-preferred permeation membrane separation unit 5, where nitrogen and helium are retained, yielding a third permeate gas S9. Methane... The first stage of methane preferential permeation produces methane-rich gas S8, which is recycled to the natural gas liquefaction unit to increase liquefied natural gas production. The third permeate gas S9 enters the second-stage methane preferential permeation membrane separation unit 6, where nitrogen and helium are retained, resulting in the fourth permeate gas S14. Methane preferentially permeates, producing the fourth permeate gas S10. The fourth permeate gas S10 passes through the third compressor 8 and is called the pressurized fourth permeate gas S11, which is recycled to the first-stage methane preferential permeation membrane separation unit 5 to recover methane again. The fourth permeate gas S14 enters the second-stage helium preferential permeation membrane separation unit 9, where hydrogen and helium preferentially permeate, resulting in the fifth permeate gas S15. Nitrogen and methane are retained, producing nitrogen-rich tail gas S12. The fifth permeate gas S15 passes through the fourth compressor 10 and is called the pressurized fifth permeate gas S16. After passing through the first compressor 1, it enters the first-stage helium preferential permeation membrane separation unit 2 to recover helium again. The nitrogen-rich tail gas S12 passes through the first expander 7 and is called the depressurized nitrogen-rich tail gas S13, which is sent to the natural gas liquefaction unit to recover cold energy.

[0012] The beneficial effects of this invention are as follows: Through a multi-stage membrane separation process with nested helium-preferred permeation membrane separation units and methane-preferred permeation membrane separation units, helium and methane are synergistically recovered while reducing the separation scale of helium recovery and improving the separation efficiency of the helium-preferred permeation membrane. Ultimately, this achieves high-efficiency and high-yield comprehensive utilization of helium-containing natural gas liquefaction tail gas, producing crude helium and liquefied natural gas with low energy consumption. Using the process of this invention, for helium-containing natural gas liquefaction tail gas with a helium concentration of 1.06 vol% and a methane concentration of 61.40 vol%, simulation results show that the helium recovery rate can reach up to 99.4%, and the purity reaches over 80.0 vol%, meeting the concentration requirements of centralized refining strategies. Simultaneously, the methane recovery rate of the liquefied tail gas can reach over 88.7%. According to the simulation results, the power consumption per separation unit does not exceed 0.41 kWh / Nm³. 3 Helium-containing natural gas liquefaction tail gas. Attached Figure Description

[0013] Figure 1 This is a flowchart illustrating the principle of a two-stage membrane separation process for producing crude helium using helium-containing natural gas liquefaction tail gas as raw material.

[0014] Figure 2 This is a flowchart illustrating the principle of a two-stage membrane separation process for producing liquefied natural gas using helium-containing natural gas liquefaction tail gas as raw material.

[0015] Figure 3 This is a flowchart illustrating the principle of a multi-stage membrane separation process that comprehensively utilizes the tail gas from helium-containing natural gas liquefaction.

[0016] In the diagram: 1 First compressor; 2 First-stage helium preferential permeate membrane separation unit; 3 Second compressor; 4 Second-stage helium preferential permeate membrane separation unit; 5 First-stage methane preferential permeate membrane separation unit; 6 Second-stage methane preferential permeate membrane separation unit; 7 First expander; 8 Third compressor; 9 Second-stage helium preferential permeate membrane separation unit; 10 Fourth compressor; S1 Helium-containing natural gas liquefaction tail gas; S2 Pressurized helium-containing natural gas liquefaction tail gas; S3 First permeate gas; S4 Pressurized first permeate gas; S5 Crude helium; S6 Second residual gas; S7 First residual gas; S8 Methane-rich gas; S9 Third residual gas; S10 Fourth permeate gas; S11 Pressurized fourth permeate gas; S12 Nitrogen-rich tail gas; S13 Depressurized nitrogen-rich tail gas; S14 Fourth residual gas; S15 Fifth permeate gas; S16 Pressurized fifth permeate gas. Detailed Implementation

[0017] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0018] Example 1

[0019] Example 1 describes a two-stage membrane separation process for producing crude helium from helium-containing natural gas after it has been processed by a natural gas purification unit in a certain area. The process flow structure is as follows: Figure 1 As shown, helium in the tail gas of helium-containing natural gas liquefaction is concentrated to produce crude helium, which is then bottled under high pressure and sent to the helium purification unit.

[0020] The helium-containing natural gas liquefaction tail gas (S1, 0.60 MPaG), a byproduct of the natural gas liquefaction unit, is first pressurized by the first compressor 1 (from 0.60 MPaG to 2.40 MPaG) and referred to as pressurized helium-containing natural gas liquefaction tail gas S2. It then enters the first-stage helium preferential permeation membrane separation unit 2, where methane and nitrogen are retained to obtain the first permeate gas S7, and hydrogen and helium preferentially permeate to obtain the first permeate gas S3. The first permeate gas S3 is then pressurized by the second compressor 3 (from 0.01 MPaG to 2.40 MPaG) and referred to as pressurized first permeate gas S4. It then enters the second-stage helium preferential permeation membrane separation unit 4, where helium and hydrogen preferentially permeate to obtain crude helium (S5, helium concentration 81.64 mol%). This crude helium is then bottled and sent to the helium purification unit, where methane and nitrogen are retained to obtain the second permeate gas S6, which is recycled back to the first-stage helium preferential permeation membrane separation unit 2 to recover helium again. The actual key material composition and operating parameters in Example 1 are shown in Table 2.

[0021] Table 2. Summary of the composition and operating parameters of key materials in Example 1

[0022]

[0023] In this implementation case, the total power consumption of the first and second compressors is approximately 92kW. Combined with the crude helium production, the power consumption per unit of purification is approximately 9.24kWh / Nm³. 3 Crude helium. According to the simulation results given in Table 2, the helium recovery rate can reach 97.9%. Based on the helium price in 2022 (considering the cost of further purification), the annual output value generated by the production of crude helium is expected to reach 20.7 million yuan, the annual operating cost is about 1.69 million yuan, the annual equipment depreciation is about 860,000 yuan, and an additional annual profit of 18.15 million yuan can be generated on the basis of the natural gas liquefaction plant.

[0024] Example 2

[0025] Example 2 addresses helium-containing natural gas processed by a natural gas purification unit in a certain area. It employs the two-stage membrane separation process described in this invention, using helium-containing natural gas liquefaction tail gas as raw material to produce liquefied natural gas. The principle flow structure is as follows: Figure 2 As shown, methane in the tail gas of helium-containing natural gas liquefaction is recovered and sent to the natural gas liquefaction unit to produce liquefied natural gas.

[0026] The helium-containing natural gas liquefaction tail gas (S1, 0.60 MPaG) produced by the natural gas liquefaction unit first passes through the first compressor 1 (pressurized from 0.60 MPaG to 1.90 MPaG), and is then called pressurized helium-containing natural gas liquefaction tail gas S2. It then enters the first-stage methane preferential permeation membrane separation unit 5, where nitrogen and helium are retained, yielding the third permeate gas S9. Methane preferentially permeates this to obtain methane-rich gas (S8, methane concentration 75.92 mol%), which is recycled back to the natural gas liquefaction unit to increase liquefied natural gas production. The third permeate gas S9 then enters the second-stage methane preferential permeation membrane separation unit 6, where nitrogen and helium are retained, yielding nitrogen-rich tail gas. (S12, nitrogen concentration 78.25 mol%), methane preferentially permeates to obtain the fourth permeate gas S10; the nitrogen-rich tail gas S12 is depressurized to 0.05 MPaG by the first expander 7 (reduced from 1.90 MPaG to 0.05 MPaG) and is called depressurized nitrogen-rich tail gas S13, which is sent to the natural gas liquefaction unit to recover cold energy; the fourth permeate gas S10 is pressurized to 0.60 MPaG by the third compressor 8 (repressurized from 0.01 MPaG to 0.60 MPaG) and then pressurized to 1.90 MPaG by the first compressor 1 (repressurized from 0.60 MPaG to 1.90 MPaG) to obtain pressurized fourth permeate gas S11, which enters the first stage methane preferential permeate membrane separation unit 5 to recover methane again. After the methane-rich gas is recycled to the natural gas liquefaction unit, the actual key material composition and operating parameters in Example 2 are shown in Table 3.

[0027] Table 3. Summary of the composition and operating parameters of key materials in Example 2

[0028]

[0029] In this embodiment, the total power consumption of the first compressor is approximately 141 kW. The power consumption for pressurizing the methane-rich gas inside the natural gas liquefaction unit and the power consumption for cooling the methane-rich gas at low temperatures, converted to electricity, totals approximately 125 kW. Considering the average flow rate of methane in the methane-rich gas, the purification unit power consumption (electricity) is approximately 0.55 kWh / Nm³. 3 Methane. According to the simulation results given in Table 3, the methane recovery rate in the tail gas can reach 85.3%. Based on the 2022 liquefied natural gas price, the increased liquefied natural gas production is expected to generate an annual output value of RMB 10.73 million, with annual operating costs of approximately RMB 2.53 million and annual equipment depreciation of approximately RMB 380,000. This will result in an additional annual profit of RMB 7.82 million based on the natural gas liquefaction plant.

[0030] Example 3

[0031] Example 3 addresses helium-containing natural gas processed by a natural gas purification unit in a certain area. It employs the multi-stage membrane separation process described in this invention, which comprehensively utilizes the tail gas from the liquefaction of helium-containing natural gas. The principle flow structure is as follows: Figure 3As shown, helium in the tail gas of helium-containing natural gas liquefaction is concentrated to produce crude helium, which is then bottled under high pressure and sent to a helium purification unit. Methane is separated and sent to a natural gas liquefaction unit to produce liquefied natural gas.

[0032] The helium-containing natural gas liquefaction tail gas (S1, 0.60 MPaG), a byproduct of the natural gas liquefaction unit, is first pressurized by the first compressor 1 (from 0.60 MPaG to 2.40 MPaG) and referred to as pressurized helium-containing natural gas liquefaction tail gas S2. This gas then enters the first-stage helium preferential permeation membrane separation unit 2, where methane and nitrogen are retained to obtain the first permeate gas S7, while hydrogen and helium preferentially permeate to obtain the first permeate gas S3. The first permeate gas S3 is then pressurized by the second compressor 3 (from 0.01 MPaG to 2.40 MPaG) and referred to as pressurized first permeate gas S4, which enters the second-stage helium preferential permeation membrane separation unit. In membrane separation unit 4, hydrogen and helium preferentially permeate to obtain crude helium (S5, helium concentration 81.69 mol%), which is then high-pressure bottled and sent to the helium purification unit. Methane and nitrogen are retained, resulting in second permeate gas S6, which is recycled back to the first-stage helium preferential permeate membrane separation unit 2 for helium recovery. The first permeate gas S7 enters the first-stage methane preferential permeate membrane separation unit 5, where nitrogen and helium are retained, resulting in third permeate gas S9. Methane preferentially permeates to obtain methane-rich gas (S8, methane concentration 76.21 mol%), which is recycled to the natural gas liquefaction unit to increase liquefied natural gas production. The third permeate gas S9 then enters... In the second stage methane preferential permeation membrane separation unit 6, nitrogen and helium are retained, yielding the fourth permeate gas S14. Methane preferentially permeates, yielding the fourth permeate gas S10. The fourth permeate gas S10 is then pressurized by the third compressor 8 (from 0.01 MPaG to 2.40 MPaG) and becomes the pressurized fourth permeate gas S11, which is recycled back into the first stage methane preferential permeation membrane separation unit 5 to recover methane again. The fourth permeate gas S14 enters the second stage helium preferential permeation membrane separation unit 9, where hydrogen and helium preferentially permeate, yielding the fifth permeate gas (S15, helium concentration 3.51 mol%). Nitrogen and methane are retained. Nitrogen-rich tail gas (S12, nitrogen concentration 84.01 mol%) is obtained. The fifth permeate gas S15 enters the fourth compressor 10 (pressurized from 0.01 MPaG to 0.60 MPaG) and is called pressurized fifth permeate gas S16. It is then circulated through the first compressor 1 (pressurized from 0.60 MPaG to 2.40 MPaG) and enters the first-stage helium preferential permeate membrane separation unit 2 to recover helium again. The nitrogen-rich tail gas S12 is formed and is then depressurized through the first expander 7 (depressurized from 2.40 MPaG to 0.05 MPaG) and is called depressurized nitrogen-rich tail gas S13, which is sent to the natural gas liquefaction unit to recover cold energy. After the methane-rich gas is circulated to the natural gas liquefaction unit, the actual key material composition and operating parameters in Example 3 are shown in Table 4.

[0033] Table 4. Summary of the composition and operating parameters of key materials in Example 3

[0034]

[0035] In this embodiment, the total power consumption of the first, second, third, and fourth compressors is approximately 190 kW. The power consumption for pressurizing the methane-rich gas inside the natural gas liquefaction unit and the power consumption for cooling the methane-rich gas (converted to low-temperature cooling energy) totals approximately 128 kW. Considering the average flow rate of the helium-containing natural gas liquefaction tail gas before the methane-rich gas circulation mixing, the purification unit power consumption is approximately 0.41 kWh / Nm³. 3 Intake. According to the simulation results given in Table 4, the helium recovery rate in the exhaust gas can reach 99.4%, an increase of 1.5% compared to Example 1, and the methane recovery rate can reach 88.7%, an increase of 3.4% compared to Example 2. Based on the 2022 prices of liquefied natural gas and helium (considering the cost of further purification), the annual output value generated by the increased liquefied natural gas production is expected to reach RMB 11.27 million, and the annual output value generated by the production of crude helium is expected to reach RMB 21.02 million. The annual operating cost is approximately RMB 3.02 million, which is RMB 1.2 million less than the total annual operating cost of Examples 1 and 2. The annual equipment depreciation is approximately RMB 680,000, which is RMB 560,000 less than the total annual equipment depreciation of Examples 1 and 2. Based on the natural gas liquefaction plant, an additional annual profit of RMB 28.6 million can be generated, which is RMB 2.63 million more than the total annual profit of Examples 1 and 2, demonstrating better economic benefits.

[0036] In summary, the multi-stage membrane separation process for the comprehensive utilization of helium-containing natural gas liquefaction tail gas described in this invention, through the nested separation units of helium-preferred permeation membrane and methane-preferred permeation membrane, synergistically recovers helium and methane from the liquefaction tail gas while significantly improving the separation efficiency of the helium-preferred permeation membrane. This achieves comprehensive utilization of helium-containing natural gas liquefaction tail gas with high efficiency and high yield, produces crude helium with low energy consumption, and increases liquefied natural gas production, resulting in significant economic benefits.

Claims

1. A multi-stage membrane separation process for comprehensively utilizing helium-containing natural gas liquefaction tail gas, characterized in that: The helium-containing natural gas liquefaction tail gas (S1), a byproduct of the natural gas liquefaction unit, first passes through the first compressor (1) and is then referred to as pressurized helium-containing natural gas liquefaction tail gas (S2). It then enters the first-stage helium preferential permeation membrane separation unit (2), where methane and nitrogen are retained to obtain the first permeate gas (S7), while hydrogen and helium preferentially permeate to obtain the first permeate gas (S3). The first permeate gas (S3) passes through the second compressor (3) and is then referred to as pressurized first permeate gas (S4). It then enters the second-stage helium preferential permeation membrane separation unit (4), where hydrogen... Helium is preferentially permeated to obtain crude helium with a concentration exceeding 80.0 vol% (S5), which is then high-pressure bottled and sent to the helium purification unit. In the second-stage helium preferential permeation membrane separation unit (4), methane and nitrogen are retained, resulting in a second permeate gas (S6), which is recycled back into the first-stage helium preferential permeation membrane separation unit (2) to recover helium again. The first permeate gas (S7) enters the first-stage methane preferential permeation membrane separation unit (5), where nitrogen and helium are retained, resulting in a third permeate gas (S9). Methane preferentially permeates to obtain methane-rich gas (S8). The gas is recycled to the natural gas liquefaction unit to increase liquefied natural gas production; the third permeate gas (S9) enters the second stage methane preferential permeation membrane separation unit (6), where nitrogen and helium are retained to obtain the fourth permeate gas (S14), which undergoes preferential permeation by methane to obtain the fourth permeate gas (S10); the fourth permeate gas (S10) is then passed through the third compressor (8) and is referred to as the pressurized fourth permeate gas (S11), which is recycled back to the first stage methane preferential permeation membrane separation unit (5) to recover methane again; the fourth permeate gas (S14) enters the second stage helium preferential permeation membrane separation unit. In the separation unit (9), hydrogen and helium are preferentially permeated to obtain the fifth permeate gas (S15), while nitrogen and methane are retained to obtain nitrogen-rich tail gas (S12). The fifth permeate gas (S15) is called the pressurized fifth permeate gas (S16) after passing through the fourth compressor (10). After circulating through the first compressor (1), it enters the first-stage helium preferential permeate membrane separation unit (2) to recover helium again. The nitrogen-rich tail gas (S12) is called the depressurized nitrogen-rich tail gas (S13) after passing through the first expander (7) and is sent to the natural gas liquefaction unit to recover cold energy.

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