Gas separation system and method for producing enriched gas.

Description

/ 45 Gas separation system and method for producing enriched gas. Technical field

[001] The present invention relates to a gas separation system for separating a mixed gas, the system including a plurality of gas separation membrane units, and a method for producing an enriched gas using the gas separation system. Fundamentals of the technique

[002] PTL 1 describes a three-stage gas separation system that recovers a product gas as the retentate gas from the second stage. PTL 1 describes that the product gas can be obtained using the system with a reduced membrane area at a high recovery rate and high purity.

[003] PTL 2 describes a two-stage gas separation system in which a portion of the permeate gas from the first stage is recycled to a mixed feedstock gas supply line. Citation List Patent Literature

[004] [PTL. 1] US 2015 / 0273388A

[005] [PTL. 2] WO 2024 / 014493 Summary of the invention

[006] In the gas separation system described in PTL 1, when the flow rate of the mixed feedstock gas decreases, the recovery rate of the enriched gas (product gas) decreases if operation continues without changing the configuration of each gas separation membrane unit. To maintain the recovery rate of the enriched gas (product gas) within a certain range, it is necessary to reduce the number of modules operated in the gas separation membrane unit or reduce the operating pressure. To reduce the number of modules operated, a valve is required. Petition 870250121014, dated 12 / 29 / 2025, page 69 / 113 / 45 closure for each module, and also, when the number of modules operated is reduced, there is a difference (variation) in the degree of contamination (gas permeation performance) over time between the module that continues to operate and the closed module, which can lead to a decrease in system performance. Although reducing the operating pressure can be considered a countermeasure, there is a limit since the resulting operating pressure may fall outside the appropriate operating pressure range of the compressor. Reducing the operating pressure can also cause the following problems: the product gas pressure may fall below the standard value and, therefore, recompression of the product gas may be necessary.

[007] On the other hand, the present inventors observed that when a portion of the permeate gas from the first stage is recycled to the mixed feedstock gas supply line as in PTL 2 in a three-stage gas separation system in which the retentate gas from the second stage is the product gas, the effect of maintaining the recovery rate is not sufficient.

[008] Thus, an objective of the present invention is to provide a gas separation system and a method for producing an enriched gas that are capable of overcoming the aforementioned drawbacks of the related technique.

[009] As a result of intensive studies to achieve the above-described objective, the present inventors surprisingly observed that, in a three-stage gas separation system in which the retentate gas from the second stage is recovered as a product gas, the product gas recovery rate in the gas separation system can be effectively maintained by recycling a portion of the permeate gas from the third stage, not the permeate gas from the first stage, to a mixed feedstock gas supply line. Petition 870250121014, dated 12 / 29 / 2025, page 70 / 113 / 45

[0010] The present invention was made based on the above observations and provides the following [1] to [9]. rn

[0011] A gas separation system for enriching at least one of the gases contained in the mixed feedstock gas by suppressing the mixed feedstock gas to a gas separation membrane unit included in the gas separation system, wherein the gas separation system comprises a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit, each gas separation membrane unit comprising at least one gas inlet, one permeate gas outlet, and one retentate gas outlet, the retentate gas outlet of the first gas separation membrane unit and the gas inlet of the second gas separation membrane unit are connected by means of a first retentate gas line,The permeate gas outlet of the first gas separation membrane unit and the gas inlet of the third gas separation membrane unit are connected by means of a first permeate gas line, and a mixed raw material gas supply line is connected to the gas inlet of the first gas separation membrane unit, and a compression medium is interposed in the mixed raw material gas supply line, a third permeate gas line is connected to the permeate gas outlet of the third gas separation membrane unit, the permeate gas outlet of the second membrane unit of, Petition 870250121014, dated 12 / 29 / 2025, page 71 / 113 / 45 gas separation and a position on one side of the suction of the compression medium in the mixed raw material gas supply line are connected to each other by means of a second permeate gas recycling line, and the retentate gas outlet of the third gas separation membrane unit and a position on one side of the suction of the compression medium in the mixed raw material gas supply line are connected to each other by means of a third retentate gas recycling line, the third permeate gas line comprises a flow path that allows a portion of the permeate gas discharged from the third gas separation membrane unit to be recycled to a position on one side of the suction of the compression medium in the mixed raw material gas supply line, and an enriched gas is recovered from the retentate gas outlet of the second gas separation membrane unit. [2]

[0012] The gas separation system according to [1], wherein the third permeate gas line comprises a bypass portion of the flow path, and a first flow path, second flow path, and third flow path downstream, and further comprises a flow adjustment unit that adjusts the flow rate of the permeate gas flowing into the second flow path; First flow path: a flow path connecting the bypass portion of the flow path and the permeate gas outlet of the third gas separation membrane unit; second flow path: a flow path connecting the bypass portion of the flow path and a position on the suction side of the compression medium in the mixed feedstock gas supply line. Petition 870250121014, dated 12 / 29 / 2025, page 72 / 113 / 45 third flow path: a flow path connecting the derivation portion of the flow path and the outside of the system. [3]

[0013] The gas separation system according to [2], wherein the gas separation system comprises: a flow detection unit configured to detect a flow rate of mixed feedstock gas flowing into the mixed feedstock gas supply line; and a control unit configured to instruct the flow adjustment unit to increase the flow rate of permeate gas flowing into the second flow path when the flow rate of mixed feedstock gas detected by the flow detection unit decreases. [4]

[0014] The gas separation system according to any one of [1] to [3], wherein the mixed feedstock gas includes a gas A and a gas B, each of the first gas separation membrane unit, the second gas separation membrane unit, and the third gas separation membrane unit has greater permeability to gas A than to gas B, the gas separation membrane of the first gas separation membrane unit has a gas separation selectivity at 40°C (P1'A(40) / P1'B(40)) of 30 or more and 150 or less, and the gas separation membrane of the third gas separation membrane unit has a gas separation selectivity at 40°C (P3'A(40) / P3'B(40)) of 30 or more and 150 or less. [5]

[0015] The gas separation system according to any one of [1] to [4], wherein the mixed feedstock gas is a biogas, and an enriched methane gas is recovered from the retentate gas outlet of the second gas separation membrane unit. Petition 870250121014, dated 12 / 29 / 2025, p. 73 / 113 / 45 [6]

[0016] The gas separation system according to any one of [1] to [5], wherein the permeate gas discharged from the third gas separation membrane unit has a permeate gas flow rate P3; a gas recycled to the position on the suction side of the compression medium in the mixed feedstock gas supply line has a flow rate PR3, the gas being a portion of the permeate gas discharged from the third gas separation membrane unit; and a ratio of PR3 to P3 is 1% or more and 80% or less. [7]

[0017] The gas separation system according to any one of [1] to [6], in which the amount of mixed feedstock gas flowing into the gas separation system fluctuates by 5% or more during operation. [8]

[0018] The gas separation system according to any one of [1] to [7], wherein the first permeate gas line comprises a flow path that allows a portion of permeate gas discharged from the first gas separation membrane unit to be recycled to a position on the suction side of the compression medium in the mixed feedstock gas supply line. [9]

[0019] A method for producing enriched gas, the method comprising supplying mixed feedstock gas to a gas separation system to enrich at least one of the gases contained in the mixed feedstock gas, wherein the gas separation system comprises a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit, Petition 870250121014, dated 12 / 29 / 2025, page 74 / 113 / 45 each gas separation membrane unit comprises at least one gas inlet, one permeate gas outlet, and one retentate gas outlet; the retentate gas outlet of the first gas separation membrane unit and the gas inlet of the second gas separation membrane unit are connected by means of a first retentate gas line; the permeate gas outlet of the first gas separation membrane unit and the gas inlet of the third gas separation membrane unit are connected by means of a first permeate gas line; and a mixed raw material gas supply line is connected to the gas inlet of the first gas separation membrane unit, and a compression means is interposed in the mixed raw material gas supply line; a third permeate gas line is connected to the permeate gas outlet of the third gas separation membrane unit.The permeate gas outlet of the second gas separation membrane unit and a position on one side of the suction of the compression medium in the mixed feedstock gas supply line are connected to each other by means of a second permeate gas recycling line, and the retentate gas outlet of the third gas separation membrane unit and a position on one side of the suction of the compression medium in the mixed feedstock gas supply line are connected to each other by means of a third retentate gas recycling line, the third permeate gas line comprising a flow path that allows at least a portion of the discharged permeate gas, Petition 870250121014, dated 12 / 29 / 2025, page 75 / 113 / 45, from the third gas separation membrane unit, is recycled to a position on one side of the suction of the compression medium in the mixed raw material gas supply line, and an enriched gas is recovered from the retentate gas outlet of the second gas separation membrane unit. Advantageous effects of the invention

[0020] According to the present invention, in the three-stage gas separation system that recovers the product gas as the second-stage retentate gas, a high product gas recovery rate can be maintained even when the flow rate of the mixed feedstock gas decreases. Brief description of the drawings

[0021] FIG. 1 is a schematic diagram showing a configuration of a gas separation system according to a first embodiment of the present invention.

[0022] FIG. 2 is a schematic diagram showing a configuration of a gas separation system according to a second embodiment of the present invention.

[0023] FIG. 3 is a schematic diagram showing a configuration of a gas separation system from Comparative Example 1.

[0024] FIG. 4 is a schematic diagram showing a configuration of a gas separation system from Comparative Example 2.

[0025] FIG. 5 is a schematic view showing an exemplary structure of a gas separation membrane module used in the gas separation system of the present invention. Description of modalities

[0026] The present invention will now be described based on preferred embodiments thereof with reference to the drawings.

[0027] Here, the expression connecting A and B by means of a line C or a line C connecting A and B includes both the case where A and B and a Petition 870250121014, dated 12 / 29 / 2025, p. 76 / 113 / 45: Line C are separate members and the case where A and / or C is a single continuous member. The same applies to the expression "connect a line E to D," which includes both the case where D and line E are separate members and the case where D and line E are a single member.

[0028] First, a gas separation system 10 according to a first embodiment of the system of the present invention and a method for producing enriched gas using the same according to a first embodiment of the method of the present invention will be described with reference to FIG. 1. A gas separation system 10 shown in FIG. 1 includes three gas separation membrane units: a first gas separation membrane unit 11, a second gas separation membrane unit 12, and a third gas separation membrane unit 13. As each of the gas separation membrane units 11, 12, and 13, it is possible to use, for example, a module 40 including a gas separation membrane 20 composed of hollow fiber membranes or similar and having selective gas permeability and a jacket 31 housing the gas separation membrane 20, as shown in FIG. 5.Each of the gas separation membrane units 11, 12, and 13 of the present embodiment includes a single gas separation membrane module 40 shown in FIG. 5 or includes a plurality of modules 40 arranged in parallel. Two opposite surfaces of the sheath 31 of module 40 are open to form an opening 32. It should be noted that the opening 32 is for inserting the gas separation membrane 20 into the sheath 31 and is not an opening of the gas separation membrane 20. The gas separation membrane 20 is inserted into the sheath 31 through the opening 32. In the case where the gas separation membrane 20 is a bundle of hollow fiber membranes, the gas separation membrane 20 is housed in the sheath 31 so that each end portion of the hollow fiber membrane is open in the vicinity of each opening 32 of the sheath 31 in the housed state. Petition 870250121014, dated 12 / 29 / 2025, page 77 / 113 / 45 gas separation membrane 20.

[0029] In a state where the gas separation membrane 20 is housed in the casing 31, the gas separation membrane 20 is fixed to the inner wall of the casing 31 by the tube plates 33 and 34 in positions of both end portions in the Y direction, which is the direction of extension of the hollow fiber membrane. Each opening 32 of the casing 31 is closed by a cover 35, 36. The cover 35 is provided with a gas inlet 37. Conversely, the cover 36 is provided with a retentate gas outlet 38. The mixed gas to be separated is introduced into the module through the gas inlet 37 of the cover 35. Of the introduced gas, the gas that permeated the gas separation membrane 20 is discharged to the outside of the module through a permeate gas outlet 39 provided in the casing 31. Conversely, the retentate gas that did not permeate the gas separation membrane 20 is discharged through the retentate gas outlet 38 of the cover 36 to the outside of the module.A purge gas supply inlet (not shown) may optionally be provided in the casing 31. Although the separation membrane module shown in FIG. 5 has been described as an example, the present invention can be applied to any separation membrane module with other configurations, and can be applied, for example, to a clamshell-type feed module.

[0030] Returning to FIG. 1, the first gas separation membrane unit 11 and the second gas separation membrane unit 12 are connected in series, as shown in FIG. 1. Specifically, the first gas separation membrane unit 11 and the second gas separation membrane unit 12 are connected by connecting the retentate gas outlet 11b of the first gas separation membrane unit 11 and the gas inlet 12a of the second gas separation membrane unit 12 via the first retentate gas line 14.

[0031] As shown in FIG. 1, the first membrane unit Petition 870250121014, dated 12 / 29 / 2025, page 78 / 113 / 45 of gas separation 11 and the third gas separation membrane unit 13 are connected in series. Specifically, the first gas separation membrane unit 11 and the third gas separation membrane unit 13 are connected by connecting the permeate gas outlet 11c of the first gas separation membrane unit 11 and the gas inlet 13a of the third gas separation membrane unit 13 via the first permeate gas line 15.

[0032] A mixed gas feedstock supply line 16 for supplying a mixed gas feedstock, as a feedstock, from a mixed gas source (not shown) to the first gas separation membrane unit 11 is connected to the gas inlet 11a of the first gas separation membrane unit 11. A compression medium 21 is interposed in the mixed gas feedstock supply line 16. The compression medium 21 is provided for the purpose of pressurizing the mixed gas supplied from the mixed gas source. Furthermore, the compression means 21 is provided for the following purposes: when the permeate gas discharged from the second gas separation membrane unit 12 is returned to the first gas separation membrane unit 11, the permeate gas is pressurized, and when the retentate gas discharged from the third gas separation membrane unit 13 is returned to the first gas separation membrane unit 11, the retentate gas is pressurized.

[0033] In the second gas separation membrane unit 12, the permeate gas outlet 12c is connected at the suction side of the compression medium 21 to the mixed raw material gas supply line 16 via the second permeate gas recycling line 17. Conversely, in the third gas separation membrane unit 13, the retentate gas outlet 13b is connected at the suction side of the compression medium 21 to the mixed raw material gas supply line 16 via the third retentate gas recycling line 18. Petition 870250121014, dated 12 / 29 / 2025, page 79 / 113 / 45

[0034] A third permeate gas line 70 is connected to the permeate gas outlet 13c of the third gas separation membrane unit 13.

[0035] A second retentate gas line 80 is connected to the retentate gas outlet 12b of the second gas separation membrane unit 12.

[0036] As shown in FIG. 1, the third permeate gas line 70 includes a flow path 72 that allows a portion of the permeate gas discharged from the third gas separation membrane unit 13 to be recycled to the suction side position of the compression medium 21 in the mixed feedstock gas supply line 16.

[0037] Specifically, the third permeate gas line 70 includes a bypass portion of the flow path 74, and includes the following first flow path 71, second flow path 72 and third flow path 73.

[0038] First flow path 71: a flow path connecting the bypass portion of flow path 74 and the permeate gas outlet 13c of the third gas separation membrane unit 13.

[0039] Second flow path 72: a flow path connecting the bypass portion of flow path 74 and the position on the suction side of the compression medium 21 in the mixed feedstock gas supply line 16.

[0040] Third flow path 73: a flow path connecting the branch portion of flow path 74 and the outside of the system.

[0041] The expression “connect to the outside of the system” and its derivatives mean that the connection to recycle the permeate gas in the third flow path to any of the first to third gas separation membrane units 11 to 13 is not carried out. The expression “connect to the external system” and its derivatives encompass introducing the permeate gas in the third flow path into a concentrator (not shown), introducing it into a storage vessel, a compressor or a blower, or an oxidation device or a combustion device, releasing it into Petition 870250121014, dated 12 / 29 / 2025, page 80 / 113 / 45 atmosphere, and introduce into a fourth gas separation membrane unit, for example.

[0042] The third permeate gas line 70 preferably includes a flow adjustment unit to adjust the flow rate of the permeate gas flowing to the second flow path 72. The flow adjustment unit may be provided in the branch portion of the flow path 74, or it may be provided in a position close to the mixed feedstock gas supply line 16 away from the branch portion 74 in the flow path 72, or it may be provided in the flow path 73. As the flow adjustment unit, a three-way valve 77 (see FIG. 2) capable of adjusting the flow rate may be used, or two two-way valves may be provided in the branch portion, for example. In the example shown in FIG.1. Two-way valves 75 and / or 76 capable of adjusting the flow rate are provided, and these valves are controlled by the control unit 5 to thereby control the permeate gas of the third gas separation membrane unit 13 (unit: Nm3 / h), and the ratio of the flow rate of the recycled gas through the second flow path 72 (PR3) to the flow rate of the permeate gas of the third gas separation membrane unit 13 (P3) (i.e., the ratio PR3 / P3). Examples of two-way valves capable of adjusting the flow rate include a needle valve, a ball valve, and a butterfly valve.

[0043] It should be noted that P3 is the flow rate (unit: Nm3 / h) of the gas discharged from the permeate gas outlet of the third gas separation membrane unit 13 (hereinafter, the gas is also referred to as the third permeate gas; the other gases may also be designated in the same way hereafter). PR3 is the flow rate (Nm3 / h) of the gas recycled to the mixed feedstock gas supply line 16, the gas being a portion of the third permeate gas.

[0044] The operation of the gas separation system 10 of the present embodiment with the configuration described above will be described. The mixed gas of Petition 870250121014, dated 12 / 29 / 2025, page 81 / 113 / 45 The raw material to be separated is supplied from a mixed gas source (not shown) in the first gas separation membrane unit 11 through a mixed gas raw material supply line 16. Before supply, the mixed gas raw material is pressurized by the compression medium 21 so that its pressure is increased. As with the compression medium 21, the same means used so far in the art can be used. For example, a compressor can be used.

[0045] The mixed gas feedstock includes at least one gas A and one gas B, which are two different types of gases to be separated. When the mixed gas pressurized by the compression medium 21 is supplied to the first gas separation membrane unit 11, the mixed gas is separated into a permeate gas, which is a gas that has permeated the gas separation membrane, and a retentate gas, which is a gas that has not permeated the gas separation membrane, due to a difference in the permeation rate through the gas separation membrane. Gas A is a gas with a higher permeation rate than gas B through each of the gas separation membranes that constitute units 11 to 13, i.e., each of the gas separation membranes has a higher permeability to gas A than to gas B.Gas B has a lower permeation rate than gas A through each of the gas separation membranes that make up units 11 to 13; in other words, each of the gas separation membranes has lower permeability to gas B than to gas A.

[0046] The retentate gas discharged from the first gas separation membrane unit 11 is a gas that has a higher concentration of gas B than the mixed gas as a feedstock. The retentate gas is discharged from the retentate gas outlet 11b of the first gas separation membrane unit 11 and is supplied to the second gas separation membrane unit 12 via the first retentate gas line 14.

[0047] On the other hand, the permeate gas of the first unit of Petition 870250121014, dated 12 / 29 / 2025, p. 82 / 113 / 45 gas separation membrane 11 is a gas that has a higher concentration of gas A than the mixed gas as a feedstock. The permeate gas is discharged from the permeate gas outlet 11c of the first gas separation membrane unit 11 and is supplied to the third gas separation membrane unit 13 via the first permeate gas line 15.

[0048] The retentate gas from the first gas separation membrane unit introduced into the second gas separation membrane unit 12 is separated into a permeate gas and a retentate gas by unit 12. The retentate gas is richer in gas B than the gas introduced into the second gas separation membrane unit 12, and is recovered as a product gas (gas B - enriched gas) from the retentate gas outlet 12b of unit 12. Conversely, the permeate gas is discharged from the permeate gas outlet 12c of the second gas separation membrane unit 12, and is recycled to the suction side of the compression medium 21 in the mixed feedstock gas supply line 16 via the second permeate gas recycling line 17 connected to outlet 12c. The recycled permeate gas is mixed with the mixed gas as a feedstock and then pressurized by the compression medium 21.

[0049] The gas introduced into the third gas separation membrane unit 13 (this gas is rich in gas A) is separated into a permeate gas and a retentate gas by unit 13. The retentate gas is discharged from the retentate gas outlet 13b of the third gas separation membrane unit 13, and is recycled to the suction side of the compression medium 21 in the mixed feedstock gas supply line 16 via the third retentate gas recycling line 18 connected to outlet 13b.

[0050] On the other hand, in system 10 of the present embodiment, the permeate gas from the third gas separation membrane unit 13 is richer in gas A than the gas introduced into the third gas separation membrane unit 13, and a portion of this gas is recycled to the suction side. Petition 870250121014, dated 12 / 29 / 2025, page 83 / 113 / 45 of the compression medium 21 in the mixed raw material gas supply line 16 through the first flow path 71, the branch portion of the flow path 74 and the second flow path 72. On the other hand, the remaining part is discharged to the outside of the system through the first flow path 71, the branch portion of the flow path 74 and the third flow path 73.

[0051] Conventionally, when the amount of mixed feedstock gas flowing into the system (hereinafter also simply referred to as mixed feedstock gas flow rate or mixed feedstock gas flow rate) decreases, the membrane area of ​​the first gas separation membrane unit 11 and the third gas separation membrane unit 13 is excessive relative to the amount of mixed feedstock gas, and gas B thus tends to remain contained in the permeate gas discharged from units 11 and 13. As a result, disadvantageously, the recovery rate of gas B decreases, and also the amount of gas B in the gas discharged on the outside of the system increases. The only solution to these problems was a reduction in membrane area or operating pressure.However, this solution can unfortunately result in decreased system performance, or a product gas pressure lower than a standard value, leading to the need for product gas recompression.

[0052] On the other hand, in the present invention, the flow path 72 is provided, and a portion of the third permeate gas can be recycled to the suction side of the compression medium 21 in the mixed raw material gas supply line 16 (hereinafter, it is also simply referred to as recycled).

[0053] This allows the first permeate gas and the third permeate gas to flow in the respective quantities of gas appropriate to the membrane area of ​​the first gas separation membrane unit 11 and the third gas separation membrane unit 13 without reducing the areas of Petition 870250121014, dated 12 / 29 / 2025, p. 84 / 113 / 45 membrane of the first gas separation membrane unit 11 and the third gas separation membrane unit 13, even when the flow rate of the mixed feedstock gas is reduced. The third permeate gas normally contains gas A at an extremely high concentration. Recycling the gas containing gas A at a high concentration to the mixed feedstock gas supply line 16 makes it less likely that gas B will be contained in the permeate gases discharged from units 11 and 13, and the recovery rate of gas B can thus be effectively improved.

[0054] With regard to the points described above, since the third permeate gas contains gas A at a very high concentration as described herein, recycling this permeate gas to the first gas separation membrane unit 11 reduces the operating efficiency of the system, and was therefore considered unthinkable in the art.

[0055] As is clear from the comparison between Comparative Example 2 and Example 1 described below, there is an advantage in the present invention in that the effect of improving the recovery rate of the product gas (gas B) is easily achieved by increasing the recycling rate of the third permeate gas, PR3 / P3. On the other hand, in the case where the first permeate gas is recycled, the recovery rate of gas B will likely stabilize regardless of how much the recycling rate PR1 / P1 is increased. Here, P1 is the flow rate of the first permeate gas (the flow rate of the permeate gas discharged from the permeate gas outlet of the first gas separation membrane unit 11, Nm3 / h), and PR1 is a flow rate (Nm3 / h) of the recycled gas on the suction side of the compression medium 21 in the mixed feedstock gas supply line 16, the gas being a portion of the first permeate gas.

[0056] As shown in Example 5 described below, a portion of the first permeate gas and a portion of the third permeate gas can both be recycled in the present invention. In view of the intensification of Petition 870250121014, dated 12 / 29 / 2025, p. 85 / 113 / 45 above-described advantage in improving the recovery rate of gas B, the ratio of the recycling gas flow rate (Nm3 / h) of the third permeate gas (PR3) to the total recycling rate (Nm3 / h) of the first permeate gas (PR1) and the recycling rate (Nm3 / h) of the third permeate gas (PR3) (i.e., the ratio PR3 / (PR1+PR3) (%)) is preferably 1% or more, more preferably 5% or more, and even more preferably 10% or more.

[0057] In the present invention, since the third permeate gas with a high concentration of gas A is used, the recovery rate of the product gas can be increased without significantly increasing the flow rate of the third permeate gas (PR3) for recycling, even if the flow rate of the mixed feedstock gas is considerably reduced (see each of the Examples). In other words, the present invention has the advantage that the power required for the operation of the system and the recovery rate are easily balanced.

[0058] Additionally, in the present invention, since the third permeate gas with a high concentration of gas A is recycled, the product gas recovery rate can be increased without requiring such a large increase in the recycling gas flow rate C, even if the concentration of gas B in the mixed feedstock gas becomes relatively high (see comparison between the run at 140 Nm3 / h in Example 1 and Example 2; also at 100 Nm3 / h and 60 Nm3 / h). Here, the recycling gas flow rate C (Nm3 / h) is the total of the second permeate gas flow rate P2, the third retentate gas flow rate R3, the recycling gas flow rate of the first permeate gas, PR1, and the recycling gas flow rate of the third permeate gas, PR3.The second permeate gas flow rate P2 is the flow rate of gas discharged from the permeate gas outlet of the second gas separation membrane unit 12, and the third retentate gas flow rate R3 is the flow rate of gas discharged from the retentate gas outlet of the third gas separation membrane unit 13. Petition 870250121014, dated 12 / 29 / 2025, p. 86 / 113 / 45

[0059] In the present invention, the mixed feedstock gas preferably contains 30 mol% or more of gas B, and particularly preferably contains 40 to 95 mol% of gas B. In the present invention, the mixed feedstock gas preferably contains 3 to 70 mol% of gas A in view of the high technical significance of the gas separation system of the present invention, and particularly preferably contains 5 to 60 mol% of gas A. If the gas flow rate fluctuates during operation and / or between operations, the concentrations may fall within the above range at least once.

[0060] In the gas separation system of the present invention, the flow rate of the permeate gas recycle from the third gas separation membrane unit (PR3) is preferably defined so that the ratio P3L / FL is small, where P3L is the decrease in the flow rate of the permeate gas from the third gas separation membrane unit 13, and FL is the decrease in the flow rate of the mixed feedstock gas; and PR3 is more preferably defined so that the ratio P3L / FL is a negative value. This allows control of the flow rate of the permeate gas recycle from the third gas separation membrane unit (PR3) in the gas separation system of the present invention so that the flow rate of the permeate gas from the third gas separation membrane unit 13 does not change or increase relative to FL, even when the flow rate of the mixed feedstock gas decreases.More specifically, when neither the first permeate gas nor the third permeate gas is recirculated, the defined value of the mixed feedstock gas flow rate (hereinafter also referred to as the “standard mixed feedstock gas flow rate”; the state in which the operation is carried out at this flow rate without recirculation of either the first or third permeate gas is referred to as the “standard state”) is designated F0a. When this flow rate is decreased from F0a to F0b without changing the above-described constant conditions (FL = F0a - F0b), it is preferable to define the third permeate gas flow rate P3 such that the amount of decrease in the third permeate gas flow rate P3. Petition 870250121014, dated 12 / 29 / 2025, p. 87 / 113 / 45 of the flow rate P30 in the standard state (P3L = P30 - P3) is small, or that the third permeate gas flow rate P3 increases from the standard state; in particular, it is more preferable that the third permeate gas flow rate P3 be defined in such a way as to increase from the standard state. In Examples 1, 3 and 5 described below, the standard state is a state in which F0 is 200 Nm3 / h (F0a is 200 Nm3 / h) in the state of Comparative Example 1, and P30 is 78 Nm3 / h.

[0061] Here, defined mixed feedstock gas flow rate refers to a flow rate defined as a standard for the gas separation system. For example, defined mixed feedstock gas flow rate is a flow rate defined as a standard for the gas separation system in a state where neither the first permeate gas nor the third permeate gas is recycled.

[0062] It should be noted that the standard state here is defined under a condition where the operation is carried out at a constant operating pressure, a constant membrane area, a constant operating temperature, a constant concentration of gas A and a constant concentration of gas B without changing the module or compressor used, except for the recycling flow rates of the first permeate gas and the third permeate gas (hereinafter, it is also referred to as under steady conditions). The defined mixed feedstock gas flow rate is a flow rate at which a recovery rate and purity appropriate for the performance of the gas separation system under steady conditions can be obtained.

[0063] Similarly, it is preferable to define the first permeate gas flow rate P1 so that, when the flow rate of the introduced mixed feedstock gas decreases from Fa to Fb, the decrease in flow rate P1 to that of the standard state (P10; 95 Nm3 / h in Examples 1, 3 and 5 described below) is small (the decrease is designated as PL1; P1L = P10 - P1) or so that the flow rate P1 increases relative to that of the standard state. It is particularly preferable to define the first permeate gas flow rate P1 so that the flow rate P1 increases relative to that of the standard state. Petition 870250121014, dated 12 / 29 / 2025, page 88 / 113 / 45

[0064] Here, the expression “the decrease in the third permeate gas flow rate is small” and its derivatives means, for example, that the value of P3L / FL is preferably 12% or less, more preferably 10% or less, even more preferably 7% or less, even more preferably 5% or less, preferably 1% or less, particularly preferably 0% or less. Above all, preferably means that the third permeate gas flow rate increases from this value in the standard state, and specifically the value of P3L / FL is -1% or less.

[0065] The expression “the decrease in the flow rate of the permeate gas of the first gas separation membrane unit 11 is small” and its derivatives means, for example, that the value of P1L / FL is preferably 12% or less, more preferably 10% or less, even more preferably 7% or less, even more preferably 5% or less, preferably 1% or less, particularly preferably 0% or less. Above all, preferably means that the flow rate of the permeate gas of the first gas separation membrane unit 11 increases from the flow rate in the standard state, and specifically the value of P1L / FL is -2% or less.

[0066] The value of P3L / FL is preferably -70% or more, more preferably -60% or more, even more preferably -50% or more, and even more preferably -40% or more. The value of P1L / FL is preferably -100% or more, and most preferably -90% or more.

[0067] The value of P3L / P30 is preferably 12% or less, more preferably 7% or less, even more preferably 5% or less, even more preferably 1% or less, and particularly preferably 0% or less. The value of P3L / P30 is preferably -70% or more, more preferably -60% or more, even more preferably -50% or more, and even more preferably -40% or more.

[0068] The value of P1L / P10 is preferably 12% or less, more preferably 7% or less, even more preferably 5% or less, up to Petition 870250121014, dated 12 / 29 / 2025, page 89 / 113 / 45 even more preferably 1% or less, and particularly preferably 0% or less. The value of P1L / P10 is preferably -100% or more, and more preferably -90% or more.

[0069] The values ​​of the first permeate gas flow rate Pi and the third permeate gas flow rate P3 at the time of the predetermined decrease in the mixed gas flow rate of the raw material can be measured, for example, by a flow sensor (not shown).

[0070] The term “constant operating pressure” (unit: MPaG) means that the fluctuation in operating pressure is, for example, within ±1%. The term “constant membrane area” means that the number of modules has not changed, or that the fluctuation in membrane area (unit: m2) in each unit is within ±1%. The term “constant composition” means that the fluctuation in the concentration (unit: % mol) of the flowing gas A is within ±1% or less and that the fluctuation in the concentration (unit: % mol) of the flowing gas B is within ±1% or less. The term “constant operating temperature” means that the fluctuation in operating temperature is within ±1% or less.

[0071] Fluctuations in the concentrations of gas A and gas B are expressed by the following formula when the concentration fluctuates from concentration c1 (mol %) to concentration c2 (mol%): Fluctuation in concentration = (c2-c1) / c1*100 (%)

[0072] The gas separation system can be configured so that the recovery rate of gas B is much higher than in the standard state of the system. For example, the recycling rate of the third permeate gas (PR3 / P3) can be set so that the concentration of gas A in the first permeate gas and / or the concentration of gas A in the third permeate gas is higher than the respective concentrations in the standard state of the system, whereby the recovery rate of gas B as the product gas can be improved.

[0073] The ratio of PR3 to P3 can fluctuate depending on the gas flow rate. Petition 870250121014, dated 12 / 29 / 2025, p. 90 / 113 / 45 mixed raw material, wherein P3 is the flow rate (Nm3 / h) of the permeate gas from the third gas separation membrane unit 13, and PR3 is the flow rate (Nm3 / h) of the recycled gas to the mixed raw material gas supply line 16, the gas being a portion of the third permeate gas. The system of the present invention allows a recycling rate of the third permeate gas (PR3 / P3) greater than 0% and less than 100%. The upper limit of the PR3 / P3 recycling rate during system operation is not particularly limited and is appropriately defined according to the intended gas separation purpose. However, for example, the system is preferably operated in such a way that the PR3 / P3 recycling rate is 80% or less, and more preferably 60% or less, in view of improving the system's operating efficiency.The PR3 / P3 recycling rate is preferably 1% or more, and more preferably 5% or more, in view of demonstrating the effects of the present invention.

[0074] In the present invention, the ratio of the flow rate (Nm3 / h) of the third permeation gas (PR3) to the total flow rate of C recycling gas (Nm3 / h) (i.e., the PR / C ratio (%)) can be 0.1% or more, 0.5% or more, or 1% or more. In view of the system's operating efficiency, PR3 / C is preferably 80% or less, more preferably 70% or less, and particularly preferably 60% or less.

[0075] In the system of the present invention, the decrease in the flow rate of the mixed raw material gas flowing to the gas separation system 10 (the flow rate is designated as F0; Nm3 / h) (the decrease is (F0a-F0b) / F0a (%)) can be, for example, 5% or more, and can be 10% or more. The decrease in flow rate referred to by this decrease rate can occur during a single operation, for example. The operating time of the single operation can be 10 to 30,000 hours, for example.

[0076] In the system of the present invention, the recycling rate of the third permeate gas (PR3 / P3) is preferably a rate such that the Petition 870250121014, dated 12 / 29 / 2025, p. 91 / 113 / 45 recycling rate C / F1 is 90% or less, and more preferably 80% or less, in view of the system's operating efficiency, wherein the recycling rate C / F1 is the ratio of the above-described recycling gas flow rate C to the flow rate of the first supply gas, F1, and the recycling gas flow rate C (unit: Nm3 / h) is the total of the second permeate gas flow rate P2, the third retentate gas flow rate R3, the recycling gas flow rate of the first permeate gas, PR1, and the recycling gas flow rate of the third permeate gas, PR3.

[0077] In order to further enhance the effect of maintaining the recovery rate of gas B, it is preferable to use a separation membrane module with high separation performance as the first gas separation membrane unit 11. A high separation performance of the first gas separation membrane unit 11 can reliably prevent gas B from being included on the permeation side along with a large amount of gas A, thereby improving the recovery rate of gas B. From this point of view, the gas separation selectivity of the first gas separation membrane unit 11 (Pi'A / Pi'B) is preferably 30 or more, more preferably 35 or more, even more preferably 40 or more, and particularly preferably 50 or more. The gas separation selectivity Pi'A / Pi'B is preferably equal to or less than a certain value in view of reducing the required compression power.Specifically, from this point of view, the gas separation selectivity P1'A / P1'B is preferably 150 or less, more preferably 130 or less, even more preferably 120 or less, and particularly preferably 110 or less. Here, the gas separation selectivity of a unit refers to the gas separation selectivity of the gas separation membrane that constitutes the unit at 40°C.

[0078] The permeation rate of gas A through the first gas separation membrane unit 11 (P1'A) is preferably 1.5*10' Petition 870250121014, dated 12 / 29 / 2025, p. 92 / 113 / 455cm3(STP) / cm2· s · cmHg or more, more preferably 2*10'5cm3(STP) / cm2· s · cmHg or more, even more preferably 3 10'5cm3(STP) / cm2· s · cmHg or more, and particularly preferably 7*10'5cm3(STP) / cm2· s · cmHg or more, in view of reducing the required compression power. In view of improving the recovery rate of gas B, the permeation rate of gas A (Pi'A) is preferably 100*10'5cm3(STP) / cm2· s · cmHg or less, more preferably 45*10'5cm3(STP) / cm2· s · cmHg or less, even more preferably 25*10'5cm3(STP) / cm2· s · cmHg or less, and particularly preferably 15*10'5cm3(STP) / cm2· s · cmHg or less. Here, the gas permeability of a unit refers to the gas permeability of the gas separation membrane constituting the unit at 40°C.

[0079] It is also preferable to use a high-performance separation membrane module as the third gas separation membrane unit 13. A high separation performance of the third gas separation membrane unit 13, in addition to the high separation performance of the first gas separation membrane unit 11, can further increase the recovery rate of gas B. From this point of view, the gas separation selectivity of the third gas separation membrane unit (P3'A / P3'B) is preferably 30 or more, more preferably 35 or more, even more preferably 40 or more, and particularly preferably 50 or more. The gas separation selectivity of the third gas separation membrane unit (Ps'A / Ps'B) is preferably 150 or less, more preferably 130 or less, even more preferably 120 or less, and particularly preferably 110 or less, in view of reducing the required compression power.

[0080] The permeation rate of gas A through the third gas separation membrane unit 13 (Pi'A) is preferably 1.5*10'5cm3(STP) / cm2· s · cmHg or more, more preferably 2*10'5cm3 Petition 870250121014, dated 12 / 29 / 2025, p. 93 / 113 / 45 (STP) / cm2· s · cmHg or more, even more preferably 3*10'5cm3(STP) / cm2· s · cmHg or more, and particularly preferably 7*10'5cm3(STP) / cm2· s · cmHg or more, in view of the compression power required. In view of improving the recovery rate of gas B, the permeation rate of gas A (P3'A) is preferably 100*10'5cm3(STP) / cm2· s · cmHg or less, more preferably 45*10'5cm3(STP) / cm2· s · cmHg or less, even more preferably 25*10'5cm3(STP) / cm2s cmHg or less, and particularly preferably 15*10'5cm3(STP) / cm2s cmHg or less.

[0081] In the present invention, the gas separation selectivity of the gas separation membrane of the second gas separation membrane unit (P2'A / P2'B) is preferably 5 or more, more preferably 10 or more, even more preferably 15 or more, and particularly preferably 20 or more, in order to easily obtain a gas separation system capable of maintaining the recovery rate of gas B without increasing the required compression power even when the flow rate of the mixed feedstock gas is reduced. In view of reducing the required compression power, the gas separation selectivity of the gas separation membrane of the second gas separation membrane unit (P2'A / P2'B) is preferably 150 or less, more preferably 90 or less, even more preferably 65 or less, and particularly preferably 55 or less.

[0082] Here, the permeation rate through the second gas separation membrane unit (P2'B) is preferably 3*10⁵ cm³(STP) / cm²·s·cmHg or less, more preferably 2.5*10⁵ cm³(STP) / cm²·s·cmHg or less, and even more preferably 2.0*10⁵ cm³(STP) / cm²·s·cmHg or less in order to easily obtain a gas separation system capable of maintaining the recovery rate of gas B without increasing the required compression power even when the gas flow rate Petition 870250121014, dated 12 / 29 / 2025, page 94 / 113 / 45. The mixture of raw materials is reduced. The permeation rate through the gas separation membrane of the second gas separation membrane unit (P2'B) is preferably 0.03 x 10⁻⁵ cm³(STP) / cm²·s·cmHg or more, more preferably 0.08 x 10⁻⁵ cm³(STP) / cm²·s·cmHg or more, even more preferably 0.3 x 10⁻⁵ cm³(STP) / cm²·s·cmHg or more, and particularly preferably 0.8 x 10⁻⁵ cm³(STP) / cm²·s·cmHg or more, in view of reducing the required membrane area.

[0083] The gas separation selectivity of the second gas separation membrane unit 12 is preferably equal to or less than that of the first gas separation membrane unit 11 and / or the third gas separation membrane unit 13 in view of reducing the membrane area while still maintaining the gas B recovery rate.

[0084] In view of the foregoing, it is preferable that the separation selectivity of the gas separation membrane that constitutes the second gas separation membrane unit (P2'A / P2'B) be less than the separation selectivity of the gas separation membrane that constitutes the first gas separation membrane unit (P1'A / P1'B), in view of the reduction of the membrane area of ​​the second gas separation membrane unit 12.

[0085] In order to further effectively reduce the membrane area of ​​the second gas separation membrane unit 12, the separation selectivity of the gas separation membrane constituting the second gas separation membrane unit 12 (P2'A / P2'B) is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.70 or less, and particularly preferably 0.60 or less, where the separation selectivity of the gas separation membrane constituting the first gas separation membrane unit 11 (P1'A / P1'B) is defined as 1. The gas separation selectivity P2'A / P2'B is preferably 0.2 or more in order to suppress an increase in compression power, and more preferably 0.3 or more, where the selectivity of. Petition 870250121014, dated 12 / 29 / 2025, p. 95 / 113 / 45 gas separation of the first gas separation membrane unit 11 (Pi'A / Pi'B) is defined as 1.

[0086] The permeation rate through the gas separation membrane constituting the second gas separation membrane unit (P2'A) is preferably greater than the permeation rate through the gas separation membrane constituting the first gas separation membrane unit (P1'A) in view of the reduction of the membrane area of ​​the second gas separation membrane unit 12. From this point of view, the permeation rate through the gas separation membrane constituting the second gas separation membrane unit 12 (P2'A) is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.4 or more where the permeation rate through the gas separation membrane constituting the first gas separation membrane unit 11 (Pi'A) at 40°C is defined as 1.Additionally, in order to suppress an increase in compression power, P2'A at 40°C is preferably 12 or less, and more preferably 7 or less, where the permeation rate through the gas separation membrane that constitutes the first gas separation membrane unit 11 (P1'a) is defined as 1.

[0087] The separation selectivity of the second gas separation membrane unit (P2'A / P2'B) is preferably lower than the separation selectivity of the gas separation membrane that constitutes the third gas separation membrane unit (Ps'A / Pi'B). Such a configuration may further reduce the membrane area of ​​the second gas separation membrane unit 12.

[0088] In order to further effectively reduce the membrane area of ​​the second gas separation membrane unit 12, the gas separation selectivity of the second gas separation membrane unit 12 (Pi'A / Pi'B) is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.70 or less, and Petition 870250121014, dated 12 / 29 / 2025, p. 96 / 113 / 45 particularly preferably 0.60 or less, where the gas separation selectivity of the third gas separation membrane unit 13 (Ps'A / Ps'B) is defined as 1. Where the separation selectivity of the gas separation membrane that constitutes the third gas separation membrane unit 13 (P3'A / P3'B) is defined as 1, the separation selectivity of the gas separation membrane (Pi'A / Pi'B) is preferably 0.2 or more in view of the ease of availability of the gas separation membrane, and is preferably 0.3 or more in view of the purity of the product gas.

[0089] The permeation rate through the gas separation membrane that constitutes the second gas separation membrane unit (P2'A) is preferably greater than the permeation rate through the gas separation membrane that constitutes the third gas separation membrane unit (P3'a) in view of the reduction of the membrane area of ​​the second gas separation membrane unit 12. From this point of view, the permeation rate through the gas separation membrane that constitutes the second gas separation membrane unit 12 (P2'A) is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.4 or more, where the permeation rate through the gas separation membrane that constitutes the third gas separation membrane unit 13 (P3'A) is defined as 1.Where the permeation rate through the gas separation membrane constituting the third gas separation membrane unit 13 (P3'A) is defined as 1, P2'A is preferably 12 or less in view of ease of availability of the gas separation membrane, and is preferably 5 or less in view of the purity of the product gas.

[0090] The ratio between the membrane area of ​​the first gas separation membrane unit 11 (S1) and the membrane area of ​​the third gas separation membrane unit (S3) is not particularly limited. In order to increase the gas recovery rate B, S1:S3 is Petition 870250121014, dated 12 / 29 / 2025, p. 97 / 113 / 45 preferably 1:0.3 or more, and more preferably 1:0.5 or more. S1:S3 is preferably 1:2.0 or less, and more preferably 1:1.5 or less.

[0091] The ratio between the membrane area of ​​the second gas separation membrane unit 12 (S2) and the membrane area of ​​the third gas separation membrane unit (S3) is not particularly limited. In order to increase the gas recovery rate B, S3:S2 is preferably 1:0.2 or more, and more preferably 1:0.4 or more. S3:S2 is preferably 1:4.0 or less, and more preferably 1:3.0 or less.

[0092] The fluctuation in the flow rate of the mixed raw material gas can be the value during operation as well as between operations, and the present invention is adaptable to the fluctuation in the flow rate of the third permeate gas recycle in either case.

[0093] Fluctuations in the flow rate of the mixed feedstock gas can be detected by a sensor (not shown).

[0094] As described herein, the system 10 may include a control unit 5 configured to detect a fluctuation in the flow rate of the mixed feedstock gas or a fluctuation in the gas concentration in the first permeate gas or the third permeate gas caused by the fluctuation in the flow rate of the mixed feedstock gas, and to control the flow adjustment valves 75 and 76.

[0095] The control unit 5 in the present embodiment includes a flow / composition data acquisition unit 50 and an adjustment instruction unit 55. The control unit 5 is electrically connected to the flow adjustment valves 75 and 76, a flow sensor (not shown) provided in the mixed feedstock gas supply line, or a gas component concentration sensor provided at the permeate gas outlet of the first gas separation membrane unit 11 or at the permeate gas outlet of the third permeate gas, and the control unit 5 controls the operation. of these.The control of each unit in control unit 5 is performed by a CPU, a ROM, a RAM, and others, and is carried out by... Petition 870250121014, dated 12 / 29 / 2025, page 98 / 113 / 45 example, by the CPU, which loads a program stored in ROM, on disk, or similar in RAM and executes the program.

[0096] The flow / composition data acquisition unit 50 acquires a flow value from the mixed raw material gas supply line 16 measured by a flow sensor (not shown), and stores the flow value data in a time series. The flow / composition data acquisition unit 50 acquires the flow value in real time during the operation of the system 10. Hereinafter, the data containing the flow values ​​stored in a time series are also simply referred to as flow data. The flow / composition data acquisition unit 50 stores the flow data in a storage unit (not shown). In addition, the flow / composition data acquisition unit 50 performs control related to the flow data, such as control of recording the flow data to the storage unit and control of reading the flow data from the storage unit.

[0097] When the flow rate value of the mixed feedstock gas supply line 16 detected by the sensor (not shown) becomes a predetermined value, the adjustment instruction unit 55 controls the flow adjustment valves 75 and / or 76 to adjust the recycling rate of the third permeate gas (PR3 / P3). In the control unit 5, the flow / composition data acquisition unit 50 can additionally analyze the composition (the concentration of gas A or the concentration of gas B) of the mixed feedstock gas in the mixed feedstock gas supply line using a sensor (a flow detection unit) (not illustrated), and the adjustment instruction unit 55 can determine the recycling rate of the third permeate gas (PR3 / P3) based on the composition of the mixed feedstock gas obtained in addition to the flow rate value.

[0098] Alternatively, instead of measuring the flow rate of the mixed feedstock gas, the concentration of gas B in the first permeate gas and in Petition 870250121014, dated 12 / 29 / 2025, p. 99 / 113 / 45 third permeate gas can be detected by the gas component concentration sensor provided at the permeate gas outlet of the first gas separation membrane unit 11 and at the permeate gas outlet for the third permeate gas, respectively, to control the recycling rate of the third permeate gas (PR3 / P3) based on these gas B concentrations.

[0099] As can be seen from the examples described below, the product gas recovery rate can be maintained and the product gas purity can be improved in the system by adjusting the recycling rate of the third permeate gas (PR3 / P3) in response to fluctuations in the flow rate of the mixed feedstock gas, without increasing or decreasing the number of gas separation membrane modules or reducing the operating pressure. As a result, the operating efficiency of the system can be maintained and the number of gas separation membrane modules used in the gas separation system can be stabilized. In this way, it is possible to prevent variation in the degree of contamination between modules as described above and also reduce the overall cost, including the manufacturing and maintenance costs of the gas separation system.

[00100] Additionally, in the present invention, it is possible to easily address a fluctuation in the composition of the mixed feedstock gas. For example, if there is an increase in the proportion of gas B and a decrease in the proportion of gas A in the mixed feedstock gas during the supply of the mixed feedstock gas and / or a period during which the supply is suspended, the recovery rate of gas B can be maintained by increasing the recycling rate of the third permeate gas (PR3 / P3) in the present invention, without increasing the number of modules of the third gas separation membrane unit 13.

[00101] As a flow sensor for mixed raw material gas, a thermal mass flowmeter, an ultrasonic flowmeter, or a vortex flowmeter can be used, for example. Petition 870250121014, dated 12 / 29 / 2025, pp. 100 / 113 / 45

[00102] A fluctuation in the composition of the mixed gas of the feedstock (in particular, a fluctuation in the concentration of gas A or gas B) can be detected by a sensor (not shown). The sensor is not particularly limited, provided it can measure the concentration of gas A or gas B, and a known sensor can be used depending on the environment of use or the type of gas. Examples of these include a CH4 densitometer, a CH4 detector, a CO2 densitometer, and a CO2 detector. Analytical equipment to which a gas chromatography analysis technique or an infrared spectroscopy technique is applied is also preferably used.

[00103] In the present invention, it is possible to operate the gas separation system without changing the number of operating modules of the third gas separation membrane unit 13 during operation and / or between operations of the gas separation system. The term “during operation” means a period from the beginning to the end of the supply of mixed feedstock gas to the gas separation system, during which the mixed feedstock gas is continuously supplied to allow gas to flow from the mixed feedstock gas supply line 16 to each of the units 11, 12 and 13. The term “between operations” means a period from a point in time when the gas flowing from the mixed feedstock gas supply line 16 to each of the units 11, 12 and 13 is interrupted until a point in time when the gas flow is restarted. The operating module is a module that contributes to gas separation.

[00104] In some conventional systems, the number of operating modules is adjusted by an on / off valve or similar device provided in each module in response to a fluctuation in the composition of the mixed feedstock gas or a fluctuation in the gas flow rate. However, in such systems, on / off valves are required for individual modules, and the degree of deterioration of membrane permeability or similar devices varies between Petition 870250121014, dated 12 / 29 / 2025, pp. 101 / 113 / 45 a module that continues operation and a closed module, which may lead to a decrease in system performance.

[00105] In the present invention, on the other hand, it is not necessary to change the number of operating modules during operation and / or between operations, and the decrease in system performance is thus avoided.

[00106] The gas separation membrane in each of the gas separation membrane units 11, 12 and 13 can be appropriately selected according to the type of mixed gas to be supplied or the gas product targeted. Like the gas separation membrane, the membranes used in the technique can be used without particular limitation. Examples of these include: rubber-type polymer materials such as silicone resins and polybutadiene resins; glass-type polymer materials such as polyimide, polyetherimide, polyamide, polyamideimide, polysulfone, polycarbonate, and cellulose; and ceramic materials such as zeolite. The gas separation membrane can be any of a homogeneous membrane, an asymmetric membrane composed of a homogeneous layer and a porous layer, and a microporous membrane. The gas separation membrane can be housed in the jacket in any form of a sheet-and-frame type, a spiral type, and a hollow fiber type.A particularly suitable gas separation membrane is an aromatic polyimide hollow fiber gas separation membrane with an asymmetric structure including a homogeneous layer with a thickness of 10 nm or more and 200 nm or less, a porous layer with a thickness of 20 μm or more and 200 μm or less and also with an inner diameter of about 30 μm or more and 500 μm or less.

[00107] A gas separation membrane module or a plurality of gas separation membrane modules may be included in a gas separation membrane unit. When two or more gas separation membrane modules are included in a unit Petition 870250121014, dated 12 / 29 / 2025, pp. 102 / 113 / 45 gas separation membrane, these are preferably connected in parallel in the unit. When each gas separation membrane unit includes a plurality of gas separation membrane modules, the membrane area in the unit can be easily adjusted by changing the number of gas separation membrane modules.

[00108] The types of mixed feedstock gas and separation membrane module are not limited. Examples of their combination are as follows: a separation membrane module with a higher permeation rate of carbon dioxide (CO2) than methane (CH4) is used as the separation membrane module 40 to which the mixed feedstock gas is supplied, and a gas containing CH4 and CO2 is used as the mixed feedstock gas; and a separation membrane module with a higher permeation rate of oxygen gas (O2) than nitrogen gas (N2) is used as the separation membrane module 40 to which the mixed feedstock gas is supplied, and a gas containing N2 and O2 is used as the mixed feedstock gas. Examples of mixed feedstock gas containing CH4 and CO2 include biogas, landfill gas, and natural gas. Examples of mixed feedstock gas containing N2 and O2 include air.Biogas is a gas generated when biomass is brought into contact with a microorganism under anaerobic conditions to carry out a fermentation process such as methane fermentation by the microorganism. Examples of biomass include organic matter such as food waste, agricultural residue, sewage sludge, and animal waste. Landfill gas refers to a gas generated by microbial or similar decomposition of organic matter in a landfill. In general, biogas and landfill gas are each composed primarily of methane and carbon dioxide. The present invention is preferably applied to biogas or landfill gas. The flow rate of these gases may not be constant; however, even in such a case, the gas separation system in the present invention can be operated at a constant rate. Petition 870250121014, dated 12 / 29 / 2025, pp. 103 / 113 / 45 extremely stable methane recovery rate as a product gas without changing the number of modules in each gas separation membrane unit.

[00109] In the case where gas B is methane, the upper limit of methane concentration in a discharged gas is regulated since methane is a greenhouse gas. Thus, when the methane concentration in a discharged gas exceeds the regulatory limit, direct discharge is impossible, and combustion is therefore necessary in some cases. Therefore, in the case where methane is gas B, the methane concentration in the system's discharged gas is particularly important, as it serves as a measure of environmental regulations in the system. In the present invention, the concentration of gas B in the discharged gas can be controlled in response to fluctuations in the flow rate of the mixed feedstock gas, as described herein, and it is thus possible to prevent an increase in the amount of methane in the third permeate gas discharged on the outside of the system. This easily allows for an environmentally sound system design.

[00110] The gas separation system of the present embodiment can be suitably used, for example, in a method of separating and recovering methane gas, which permeates less readily, from biogas basically containing methane gas and carbon dioxide gas. Methane gas corresponds to gas B, which permeates less readily, and carbon dioxide gas corresponds to gas A, which permeates readily. In this case, when the separated and recovered methane gas is supplied, for example, to the municipal gas line, a gas compressor as a means of compression can be provided at the retentate gas outlet 12b of the second gas separation membrane unit 12 to increase the pressure of the methane gas.

[00111] The pressure of the gas flowing into the first gas separation membrane unit 11 through the compression medium is generally preferably 0.2 MPaG or more and 3.0 MPaG or less, and more Petition 870250121014, dated 12 / 29 / 2025, pp. 104 / 113 / 45 preferably 0.3 MPaG or more and 2.4 MPaG or less.

[00112] The pressure of the gas flowing to the third gas separation membrane unit 13 is generally preferably 0.01 MPaG or more and 0.7 MPaG or less, and more preferably 0.02 MPaG or more and 0.5 MPaG or less.

[00113] The operating temperature of each gas separation membrane unit is normally preferably 0°C or higher and 80°C or lower, and more preferably 5°C or higher and 60°C or lower. Here, the operating temperature refers to the temperature of the gas at the inlet of the gas separation membrane unit (gas separation membrane module) while the mixed gas is supplied to the gas separation membrane unit.

[00114] In order to obtain a lower separation selectivity of the second gas separation membrane unit 12 than of the first gas separation membrane unit 11 during operation, the operating temperature of the second gas separation membrane unit 12 may be higher than that of the first gas separation membrane unit 11.

[00115] Next, a second embodiment of the present invention will be described with reference to FIG. 2. The second embodiment of the present invention is such that the first permeate gas line 15 connecting the permeate gas outlet 11c of the first gas separation membrane unit 11 and the gas inlet 13a of the third gas separation membrane unit 13 in the first embodiment shown in FIG. 1 is branched to provide the flow path 90. The flow path 90 connects the first permeate gas line 15 and the suction side of the compression medium 21 of the mixed feedstock gas supply line 16. A flow adjustment valve 91 capable of adjusting the flow rate of the gas flowing to the flow path 90 of the first permeate gas line 15 is provided in the branch portion of the flow path 92 that branches from the first permeate gas line 15 to the flow path 90. In FIG. 2, the flow control valve 91 is a Petition 870250121014, dated 12 / 29 / 2025, pp. 105 / 113 / 45 three-way valve. The flow path 90 allows a portion of the first permeate gas to be recycled to the suction side of the compression medium 21 in the mixed feedstock gas supply line 16. In this embodiment, the control unit 5 can control the flow adjustment valve (three-way valve) 91 in addition to the flow adjustment valve (three-way valve) 77 to thereby control both the recycling rate of the first permeate gas (PR1 / P1) and the recycling rate of the third permeate gas (PR3 / P3). In this way, fine control according to various purposes can be made under various conditions such as the recovery rate and purity of gas B, the recycling rate (C / F1), and the operating pressure.Furthermore, recycling not only the third permeate gas but also the first permeate gas offers an advantage in that the amount of flow to be compressed is kept within the appropriate range for compressor operation.

[00116] Additionally, in the embodiment of FIG. 2, the compression medium 22 is interposed in the first permeate gas line 15. This can also be provided in the embodiment shown in FIG. 1 (an embodiment in which the first permeate gas is not recycled). In an embodiment in which the first permeate gas in the first permeate gas line 15 is partially recycled to the mixed feedstock gas supply line 16 as shown in FIG. 2, it is advantageous to provide the compression medium 22 closer to the third gas separation membrane unit 13 than to the bypass portion 91 of the first permeate gas line 15, in view of reducing the compression power.

[00117] Although the present invention has been described based on preferred embodiments thereof, the present invention is not limited to those embodiments. For example, in the embodiments described above, a unit consisting of a gas separation membrane module with hollow fiber membranes is used as an example of each. Petition 870250121014, dated 12 / 29 / 2025, page 106 / 113 / 45 gas separation membrane unit, and instead, however, a gas separation membrane unit in another form may be used.

[00118] In addition to the compression means described above, a compression means may be provided on the permeation side of any one or more of the units in order to thereby apply power to the flow of mixed gas to pass through the separation membrane. As such a compression means, a known vacuum pump may be used, for example. Examples

[00119] The present invention will now be described in more detail by means of Examples. However, the scope of the present invention is not limited by these examples. Comparative Examples 1 and 3

[00120] A mixed gas containing carbon dioxide and methane (model biogas gas) was separated using a conventional gas separation system 10 shown in FIG. 3. A compressor was used as the compression medium 21 in system 10. The flow rate of the mixed feedstock gas (flow rate of gas flowing into the system) and the composition of the mixed feedstock gas were as shown in Table 2 below. The mixed feedstock gas was heated lengthwise by the compression medium, cooled by a cooling device (not shown), and supplied to each gas separation membrane unit. The conditions were as follows.

[00121] The operating temperature is the temperature of the gas separation membrane in each gas separation membrane unit.

[00122] - Temperature of the mixed gas of raw material flowing into the system: 40°C

[00123] - Pressure (operating pressure) of gas flowing to the first gas separation membrane unit 11: 1.0 MPi-rG

[00124] - Pressure (operating pressure) of gas flowing into the Petition 870250121014, dated 12 / 29 / 2025, pp. 107 / 113 / 45 third gas separation membrane unit 13: 0.19 MPa-G

[00125] - Operating temperature of the first gas separation membrane unit 11: 40°C

[00126] - Operating temperature of the second gas separation membrane unit 12: 40°C

[00127] - Operating temperature of the third gas separation membrane unit 13: 40°C

[00128] - The number of modules of the first gas separation membrane unit 11: 11

[00129] - The number of modules of the third gas separation membrane unit 13: 11

[00130] - Gas separation selectivity of the first gas separation membrane unit 11: use of Module A.

[00131] - Gas separation selectivity of the third gas separation membrane unit 13: use of Module A.

[00132] As for each of the modules constituting the first and third gas separation membrane units 11 and 13, a Module A was used, Module A including a gas separation membrane composed of polyimide hollow fiber membrane and a case housing the separation membrane. As for a module constituting the second gas separation membrane unit 12, either a Module A or B was used, modules A and B each including a gas separation membrane composed of polyimide hollow fiber membrane and a case housing the separation membrane. As shown in Table 1, gas separation membrane module A has a P'co2 of 9.9, a P'ch4 of 0.18, and a P'co2 / P'ch4 of 55. These numerical values ​​are at an operating temperature of 40°C. The membrane area per module was 24 m2.

[00133] Table 2 shows the number of modules of the second gas separation membrane unit 12, the total number of modules used in Petition 870250121014, dated 12 / 29 / 2025, pages 108 / 113 / 45 units 11 to 13, the gas flow rate for each of the gas separation membrane units, and the CO2 concentration.

[00134] In addition, the purity of the product gas CH4 (the retentate gas from the second gas separation membrane unit 12) and the product gas recovery rate (the flow rate, in percentage, of the CH4 component contained in the retentate gas from the second gas separation membrane unit 12 where the flow rate of the CH4 component contained in the mixed feedstock gas is defined as 100%) are shown in Table 2. Comparative Example 2

[00135] A mixed gas was separated in the same manner as in Comparative Example 1, except that the gas separation system 10' shown in FIG. 4 was used, and that the recycling gas flow rate of the first permeate gas (PR1) was set to the value shown in Table 2. Comparative Example 4

[00136] A mixed gas was separated in the same manner as in Comparative Example 1, except that the module used in the second gas separation membrane unit 12 was changed to module B in Table 1. Examples 1 to 3

[00137] A mixed gas was separated in the same manner as in Comparative Example 1, except that system 10 in FIG. 1 was used, and that the recycling gas flow rate of the third permeate gas (PR3) was set to the value shown in Table 2. Example 4

[00138] A mixed gas was separated in the same manner as in Example 1, except that the module used in the second gas separation membrane unit 12 was changed to module B in Table 1. Example 5

[00139] A mixed gas was separated in the same manner as in Comparative Example 1, except that a gas separation system 10' shown in Petition 870250121014, dated 12 / 29 / 2025, pp. 109 / 113 42 / 45 FIG. 2 was used, although compression medium 22 was not provided, and the recycling gas flow rate of the third permeation gas (PR3) and the recycling gas flow rate of the first permeation gas (PR1) were each set to the value shown in Table 2. Table 1 Membrane module for P'CO2 P'CB4 P'co: / P'{?H4 separation. Membrane area per module m2. A 9.9 0.18 55 24 B 21 0 82 26 20 PT(ünít:Tlo'cnPfSTPf / cm* > see * cm Hq) Petition 870250121014, dated 12 / 29 / 2025, pages 110 / 113 / 45 Table 2 Gás produto * =3 £ O o HT? H s 2 â o.^3 u ..C o'- 99.5 98.7 97.6 95.0 98.6 98.6 98.6 99.2 98.5 97.6 95.4 99.5 98.7 conc . de CH4 . 0 c .2 0 S ° E VI VI Vi Vi Vi Vi Vi Vi Vi Vi Vi Vi Number of modules Total Q- Cd Cd cd cd cd cd cd cd cd cd cd cd CP CP Second gas separation membrane unit Q- θ θ θ θ θ θ θ θ θ θ θ Ch Ch CO2 concentration P Ο v 0 0 .3 '«> 0 *S 0 μ υ oí) ü % em mol £ Cd CP CP cd r- cd OO cd CP cd £ £ cd C*7 PO vu £ o 55 u -cs 0 2 .2 H 42 &D & g 73 % em mol g> 03 S CP ΟΦ ΟΦ ΟΦ ΟΦ ΟΦ & £ o 03 O fi Ο -Λ O 2 .2 CZ) □ &D Q- g ^3 % em mol F7 £ CP cd ΟΦ s $ F7 ΟΦ cd s &í) Ό vt) 0 CZ) 5 OD^3 g .2 c % em mol 03 Cd cd r- cd cd cd CP cd cd so cd CP cd S Ο V u 0 .SU -cs ü OQO d) G. g % em mol Cd ΟΦ cd ΟΦ CP 00 CP s ΟΦ CP ΟΦ ΟΦ !£ cd ΟΦ .§ 2 0 g S 2 £ '55 od^3 = .S α % em mol 0 CP C*7 £> 3 0 cn CP C*“l cd 03 CP Vazão PR3 / P3 (%) O 0 0 0 0 O 0 0 0 0 OO 0 *•=1 ,2 '5 &ügo 2 2 *·=§ δ 2-2 S Φ^3 2·— c 2 m & g 73 2; '' NmJ / h O 0 0 0 0 O 0 0 0 0 OO 0 PO vu £ o 0 u -cs 0 2 .2 H ao & g 73 NmJ / h 03 £ cd £ £ £ £ cd cd C*“l θ 03 £ 2 2 32 0 a 2 PH '0 M-73 gl c NmJ / h £ oo £ fQ £ £ cd cd £ QJ 'g O -CS 0 2-rtoj CZ) □ oi Q. G Ph NmJ / h F7 OF CP cd 0 0 O cd CP &í) Ό Vi ... hs t) 0 OCO .2 & P rC CZ) □ &D ^3 jS .g C NmJ / h CP cd CP ss σζ 0 0 CP cd ΟΦ 0 $ cd cs Ό Q &P g H CS 2 — C CP θ cd C*7 CP C*“l £ cd θ CP C*“l cR cn PR3 / C (%) 0 o 0 0 0 0 0 0 o 0 O 0 0 0 -cs '5 & S <»?cSO ojjO Q cS g 4 03 03 0 ΟΦ CP ΟΦ s ΟΦ 0 ΟΦ ΟΦ CP Cd 03 P3L / P30 (%) • Cd !£ cd cd cd • cd 2 • cd P3L / FL (%) • cR cn cn cR cR cR • cd cd cd • fQ P1L / P10 (%) • cd s cd cd 2 • CP cd O ΟΦ • cd P1L / FL (%) • CP σζ CP cd cd • cd cd ΟΦ cd • V- .2 D _ S 0 v> u «3 -cS_2 Q ©OfiO.S £ -cs δ Ί> Q r»> Ò 2 43 c ^3 η,'55 0Ό 0, g ^3 NmJ / h 0 0 0 0 o 0 0 0 OOO 0 0 SO OQ í— Λ _ • g -cs 0 2 -2 £ 0 &ΰ Λ g Ό NmJ / h £ £ fQ P 0 cd cd £ I £.a O a õ 2 £ '55 m-73 Sl « h NmJ / h 03 03 Cd 03 Φ cd ΟΦ cd ΟΦ cd θ ¢0 03 CP cd cd cd Gas separation selectivity 1) Third separation membrane unit Gas separation unit • 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 Second gas separation membrane unit • 001 001 001 001 001 001 001 001 001 001 001 0.46 0.46 First gas separation membrane unit • 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 Raw material gas Composition CH4 % in mol 0 o 0 0 0 0 o 0 o 0 0 0 0 o o % in mol 0 0 0 0 0 0 0 θ θ θ θ 0 0 Flow rate (F0) NmJ / h θ 0 0 0 0 0 0 θ ooo θ 0 Ex. With p. 1 Ex. With p. 2 Ex. With p. 3 Ex. With Petition 870250121014, dated 12 / 29 / 2025, pp. 111 / 113 / 45 97.6 95.0 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 97.7 97.7 97.7 97.7 99.5 99.5 99.5 99.5 99.5 99.5 VI VI VI VI VI VI VI VI VI VI VI VI Vi Vi Vi Vi Vi V! V! V! CT) CT) Cd cd cd cd cd cd cd cd cd cd cd cd cd nn h h cd ex ex θ θ θ θ θ θ θ θ θ θ ex ex ex ex θ θ n Cd r- OO cd η t? £ $ o ο ο Ο CT) g> g> g> g> g> g> g> g> g> oooo CT) Cd cd s cd cR η 3 £ 2 £ 0Φ C<) 3 σζ SC*“) OO cd cd cd cd cd r- cd cd cd cd 0Φ θ cd cd cd cd cd cd θ θ Cd ΟΦ C*“) cd cd cd Ο Ο oo CT) nn cd cd cd cd cd σί ΟΦ C*“) C*“) cd so CT) £ o 2 0Φ £ £ £ S ο o O cd $ 00 ο £ ex rQ n cd cd $ η C<) oon cd £> η ΟΦ cd o CT) 0Φ θ sn cd £> cd £ Cd $ $ of: OO OO ΟΦ ΟΦ ΟΦ ex £ rQ & & 00 00 cd 00 η ΟΦ oo OO Γ- r- r- & & 0Φ ΟΦ οο ex cd £> σζ Ρ ΟΦ rQ r- £ rQ 0Φ s 3 £ ç- CT) OO 0> CT) $ cd Β r- θ η O cd s cd cd ΟΦ CT) o 3 η C*“) o S cR £> cR £> cd oo cd 00 £ cd ΟΦ cd o cd t- cd cn 0Φ cd ΟΦ cd 00 OO oo O Ο g> o cd § C<) cd 0Φ cd ex θ ο S !£ -γ cd cd cd cd CT) cd cd ex ex ex ex -γ Γ- • cd -γ -γ • cd CT) o • cd tq • -γ -γ -γ ΟΦ η s cd cd n η cd cd CT) cd cd Cd η cd n cd cd cd η η ο ΟΦ CT) • nn cd • cd cd • cR cd cd .<o • n η cd οφ o o ο ο 00 £ rq & 0φ oo oo r- γ- 168 cr θ s 1.00 0.46 001 100 ã * -h ex. 2 3 4 5 Note: The standard state (P1o and P3o) in each of Comparative Examples 1 and 2 and Examples 1, 3 and 5 corresponds to the state when the mixed feedstock gas flows at 200 Nm3 / h in Comparative Example 1. Note: The standard state in each of Comparative Examples 3 and 2 corresponds to Comparative Example 3. Note: The standard state in each of Comparative Example 4 and Example 4 corresponds to the state when the mixed raw material gas flows at 200 Nm3 / h in Comparative Example 4. Petition 870250121014, dated 12 / 29 / 2025, pages 112 / 113 / 45

[00140] The system of the present invention, in which a portion of the third permeate gas was recycled to the first gas separation membrane unit, was used in each of Examples 1 to 5, and, in these Examples, even if the flow rate of the mixed feedstock gas is decreased, the CH4 recovery rate as high as 99.5% or more is maintained while still maintaining the purity of the product gas without increasing or decreasing the number of modules, as shown in Table 2. Particularly, in Example 4, high purity and high recovery rate were maintained at an operating pressure comparable to that of Example 1 with a reduction in the required membrane area compared to Example 1.

[00141] In Comparative Examples 1, 3 and 4, the CH4 recovery rate was significantly reduced with a decrease in the flow rate of the mixed feedstock gas. In the three-stage system, the effect of improving the CH4 recovery rate was not sufficient in Comparative Example 2, where the permeate gas from the first gas separation membrane unit 11 was recycled to the suction side of the compression medium 21 in the mixed feedstock gas supply line 16 as described in PTL 1, instead of the third gas separation membrane unit 13. Industrial applicability

[00142] According to the present invention, a three-stage gas separation system is provided that is capable of operating without changing the number of modules to be operated in a gas separation membrane unit, while still suppressing the influence on the purity and recovery rate of an enriched gas (product gas) even when the flow rate of a mixed feedstock gas decreases. Petition 870250121014, dated 12 / 29 / 2025, p. 113 / 113< / o>

Claims

1 / 5 CLAIMS 1. Gas separation system (10, 10'), characterized in that it is for enriching at least one of the gases contained in mixed feedstock gas by suppressing the mixed feedstock gas to a gas separation membrane unit included in the gas separation system (10, 10'), wherein the gas separation system (10, 10') comprises a first gas separation membrane unit (11), a second gas separation membrane unit (12), and a third gas separation membrane unit (13), each gas separation membrane unit (11, 12, 13) comprising at least one gas inlet (11a, 12a, 13a), a permeate gas outlet (11c, 12c, 13c), and a retentate gas outlet (11b, 12b, 13b), the retentate gas outlet (11b) of the first membrane unit. gas separation unit (11) and the gas inlet (12a) of that second gas separation membrane unit (12) are connected by means of a first retentate gas line (14),The permeate gas outlet (11c) of the first gas separation membrane unit (11) and the gas inlet (13a) of that third gas separation membrane unit (13) are connected by means of a first permeate gas line (15), and a mixed raw material gas supply line (16) is connected to the gas inlet (11a) of that first gas separation membrane unit (11), and a compression medium (21) is interposed in the mixed raw material gas supply line (16), a third permeate gas line (70) is connected to the permeate gas outlet (13c) of the third gas separation membrane unit (13), the permeate gas outlet (12c) of the second gas separation membrane unit (12) and a position on a suction side of the compression medium (21) in the mixed raw material gas supply line (16) Petition 870260074370, of 27 / 07 / 2026, page 11 / 21 2 / 5 are connected to each other by means of a second permeate gas recycling line (17),and the retentate gas outlet (13b) of the third gas separation membrane unit (13) and a position on a suction side of the compression medium (21) in the mixed feedstock gas supply line (16) are connected to each other by means of a third retentate gas recycling line (18), the third permeate gas line (70) comprises a flow path that allows a portion of the permeate gas discharged from the third gas separation membrane unit (13) to be recycled to a position on a suction side of the compression medium (21) in the mixed feedstock gas supply line (16), and an enriched gas is recovered from the retentate gas outlet (12b) of the second gas separation membrane unit (12).

2. Gas separation system (10, 10') according to claim 1, characterized in that the third permeate gas line (70) comprises a flow path bypass portion (74), and a first flow path (71), second flow path (72), and third flow path (73) below, and further comprises a flow adjustment unit that adjusts the flow rate of the permeate gas flowing to the second flow path (72);first flow path (71): a flow path connecting the flow path bypass portion (74) and the permeate gas outlet (13c) of the third gas separation membrane unit (13), second flow path (72): a flow path connecting the flow path bypass portion (74) and a position on the suction side of the compression medium (21) in the mixed feedstock gas supply line (16), third flow path (73): a flow path connecting the flow path bypass portion (74) and the outside of the system. Petition 870260074370, dated 07 / 27 / 2026, page 12 / 21 3 / 5; 3. Gas separation system (10, 10') according to claim 2, characterized in that the gas separation system (10, 10') comprises: a flow detection unit configured to detect a flow rate of the mixed raw material gas flowing into the mixed raw material gas supply line (16); and a control unit (5) configured to instruct the flow adjustment unit to increase the flow rate of the permeate gas flowing into the second flow path (72) when the flow rate of the mixed raw material gas detected by the flow detection unit decreases.

4. Gas separation system (10, 10') according to claim 1 or 2, characterized in that the mixed feedstock gas includes a gas A and a gas B, each of the first gas separation membrane unit (11), the second gas separation membrane unit (12), and the third gas separation membrane unit (13) having greater permeability to gas A than to gas B, the gas separation membrane of that first gas separation membrane unit (11) having a gas separation selectivity (P1-A / P1-B) of 30 or more at 40°C, and the gas separation membrane of that third gas separation membrane unit (13) having a gas separation selectivity (P3'A / P3'B) of 30 or more at 40°C.

5. Gas separation system (10, 10') according to claim 1 or 2, characterized in that the mixed feedstock gas is a biogas, and a methane-enriched gas is recovered from the retentate gas outlet (12b) of the second gas separation membrane unit (12).

6. Gas separation system (10, 10') according to claim 1 or 2, characterized in that the permeate gas discharged from the third gas separation membrane unit (13) has a permeate gas flow rate P3; Petition 870260074370, dated 07 / 27 / 2026, page 13 / 21 4 / 5 a gas recycled to the position on the suction side of the compression medium (21) in the mixed raw material gas supply line (16) has a flow rate PR3, the gas being a portion of the permeate gas discharged from the third gas separation membrane unit (13); and a ratio of PR3 to P3 is 1% or more and 80% or less.

7. Gas separation system (10, 10') according to claim 1 or 2, characterized in that the amount of mixed feedstock gas flowing into the gas separation system (10, 10') fluctuates by 5% or more during operation.

8. Gas separation system (10') according to any one of claims 1 to 2, characterized in that the first permeate gas line (15) comprises a flow path that allows a portion of permeate gas discharged from the first gas separation membrane unit (11) to be recycled to a position on the suction side of the compression medium (21) in the mixed feedstock gas supply line (16).

9. Method for producing enriched gas, characterized in that the method comprises supplying mixed feedstock gas to a gas separation system (10, 10') as defined in claim 1 to enrich at least one of the gases contained in the mixed feedstock gas, wherein the gas separation system (10, 10') comprises a first gas separation membrane unit (11), a second gas separation membrane unit (12), and a third gas separation membrane unit (13), each gas separation membrane unit (11, 12, 13) comprising at least one gas inlet (11a, 12a, 13a), a permeate gas outlet (11c, 12c, 13c), and a retentate gas outlet (11b, 12b, 13b), the retentate gas outlet (11b) of the first gas separation membrane unit (11) and the gas inlet (12a) of that unit. second gas separation membrane unit (12) are connected by means of a first Petition 870260074370, dated 07 / 27 / 2026,page. 14 / 21 5 / 5 retentate gas line (14), the permeate gas outlet (11c) of the first gas separation membrane unit (11) and the gas inlet (13a) of that third gas separation membrane unit (13) are connected by means of a first permeate gas line (15), and a mixed raw material gas supply line (16) is connected to the gas inlet (11a) of that first gas separation membrane unit (11), and a compression medium (21) is interposed in the mixed raw material gas supply line (16), a third permeate gas line (70) is connected to the permeate gas outlet (13c) of the third gas separation membrane unit (13), the permeate gas outlet (12c) of the second gas separation membrane unit (12) and a position on a suction side of the compression medium (21) in the mixed raw material gas supply line (16) are connected to each other by means of a second permeate gas recycling line (17),and the retentate gas outlet (13b) of the third gas separation membrane unit (13) and a position on a suction side of the compression medium (21) in the mixed feedstock gas supply line (16) are connected to each other by means of a third retentate gas recycling line (18), the third permeate gas line (70) comprises a flow path that allows a portion of the permeate gas discharged from the third gas separation membrane unit (13) to be recycled to a position on a suction side of the compression medium (21) in the mixed feedstock gas supply line (16), and an enriched gas is recovered from the retentate gas outlet (12b) of the second gas separation membrane unit (12). Petition 870260074370, dated 27 / 07 / 2026, pp. 15 / 21,

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