Gas Separation Systems

Abstract

The present invention provides a technology that can achieve a high level of reduction in pressure loss and membrane area in a two-stage gas separation system using a hollow fiber gas separation membrane module, which concentrates and enriches methane contained in a raw material mixed gas containing carbon dioxide and methane. [Solution] Gas separation membrane units 11, 12 include one or more gas separation membrane modules 1, and are equipped with at least gas inlets 11a, 12a, permeate gas outlets 11b, 12b, and non-permeate gas outlets 11c, 12c. Gas separation membrane module 1 has a hollow fiber membrane bundle 22 consisting of a plurality of hollow fiber membranes 21 as gas separation membranes. The hollow fiber membranes 21 of the first gas separation membrane unit 11 satisfy the following formula (1): B 1.1 / A 1.4 ≦2.8 (1) In the formula (1), "A" is the inner diameter (μm) of the hollow fiber membrane, and "B" is the length (mm) of the gas separation membrane module.

Description

[Technical Field]

[0001] The present invention relates to a gas separation system for separating methane from a gas mixture containing carbon dioxide and methane. [Background technology]

[0002] Membrane separation methods using selectively permeable gas separation membranes are known as methods for separating specific gases from mixed gases, which are mixtures of multiple gases. The gas separation membranes used in membrane separation methods have a higher gas permeability for specific gas components than for other gas components, and utilize the difference in permeation rate between gas species to separate the mixed gas. Gas separation membrane modules used in membrane separation methods include plate and frame types, tubular types, and hollow fiber types. Among these, hollow fiber gas separation membrane modules have the advantage of a relatively large membrane area per unit volume and are also excellent in terms of pressure resistance and self-supporting properties, making them industrially advantageous and widely used. A hollow fiber gas separation membrane module has a hollow fiber membrane bundle consisting of a large number of selectively permeable hollow fiber membranes as gas separation membranes. A hollow fiber gas separation membrane module typically has a hollow fiber membrane element in which at least one end of the hollow fiber membrane bundle is fixed to a flat resin plate called a tube plate, and the hollow fiber membrane element is housed in a container equipped with a gas inlet and a gas outlet. A known gas separation system capable of recovering specific components from a mixed gas with high purity and high recovery rate is one that has multiple gas separation membrane units including gas separation membrane modules, and in which the multiple gas separation membrane units are connected in series to perform multiple gas separation steps.

[0003] Patent Documents 1 and 2 describe a two-stage gas separation system in which non-permeable gas from a first-stage gas separation membrane unit is introduced into a second-stage gas inlet, and a system for recovering a methane-enriched gas from a raw material mixed gas containing carbon dioxide and methane. Patent Document 1 describes the length of the gas separation membrane module used in Example 1, and Patent Document 2 describes preferred dimensions of the hollow fiber membrane. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-107005 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-128868 Summary of the Invention [Problem to be solved by the invention]

[0005] In gas separation membrane systems such as those described in Patent Documents 1 and 2, in which the non-permeate gas from the first stage is introduced into the second stage to concentrate methane from a mixed gas containing carbon dioxide and methane, such as biogas, environmental regulations regarding methane emissions require a lower methane concentration in the permeate gas discharged from the first-stage gas separation membrane unit, i.e., a higher methane recovery rate. However, in conventional two-stage gas separation membrane systems, attempts to increase the methane recovery rate have had problems such as increased pressure loss from the feed gas to the non-permeate gas from the first-stage separation membrane unit and the second-stage separation membrane unit, as well as the need to increase the membrane area of ​​the gas separation membranes in the system.

[0006] As a result of various investigations, the inventors have found that by using hollow fiber membranes constituting the gas separation membranes of the first-stage gas separation membrane unit that satisfy a specific mathematical formula including the inner diameter and length of the hollow fiber membrane as variables, it is possible to reduce the pressure loss It has also been found that it is possible to achieve a high level of both a reduction in membrane area and a high methane recovery rate.

[0007] An object of the present invention is to provide a two-stage gas separation system that effectively reduces the pressure loss and membrane area of ​​the gas separation membrane system and is capable of achieving a high recovery rate of methane. [Means for solving the problem]

[0008] The present invention has been made based on the above findings and provides the following configurations. [1] A gas separation system in which a raw material mixed gas containing at least carbon dioxide and methane is supplied to a gas separation membrane unit to concentrate and enrich methane contained in the raw material mixed gas, A first gas separation membrane unit and a second gas separation membrane unit are provided, Each gas separation membrane unit includes one or more gas separation membrane modules and is equipped with at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, and the gas separation membrane module has, as the gas separation membrane, a hollow fiber membrane bundle consisting of a plurality of hollow fiber membranes; a raw material mixed gas supply line is connected to the gas inlet of the first gas separation membrane unit, and a compression means is interposed in the raw material mixed gas supply line; the non-permeate gas outlet of the first gas separation membrane unit and the gas inlet of the second gas separation membrane unit are connected by a non-permeate gas outlet line, the permeate gas outlet of the second gas separation membrane unit and a position on the suction side of the compression means in the raw mixed gas supply line are connected by a permeate gas recycle line; Concentrated and enriched methane can be extracted from the non-permeate gas outlet of the second gas separation membrane unit, A gas separation system, wherein the hollow fiber membrane of the first gas separation membrane unit satisfies the following formula (1): B 1.1 / A 1.4 ≦2.8 (1) In the formula (1), "A" is the inner diameter (μm) of the hollow fiber membrane, and "B" is the length (mm) of the hollow fiber membrane.

[0009] [2] The gas separation system according to [1], wherein the hollow fiber membrane satisfies the following formula (2): 1.0≦B 1.1 / A 1.4 ≦2.8 (2) [3] The gas separation system according to [1] or [2], wherein A is 50 μm or more and 300 μm or less. [4] The gas separation system according to any one of [1] to [3] above, wherein B is 300 mm or more and 2500 mm or less. [5] The gas separation system according to any one of [1] to [4] above, wherein the gas separation membrane module of the first gas separation membrane unit is of a hollow feed type. [6] The carbon dioxide permeation rate P' of the gas separation membrane of the first gas separation membrane unit at 40°C CO2 and the methane permeation rate P' of the gas separation membrane at 40°C. CH4 The ratio of CO2 / P' CH4 The gas separation system according to any one of [1] to [5] above, wherein the β-amino acid value is 30 or more and 150 or less. [7] The gas separation system according to any one of [1] to [6], wherein the ratio A1:A2 between the total carbon dioxide permeability A1 of the gas separation membrane modules of the first gas separation membrane unit and the total carbon dioxide permeability A2 of the gas separation membrane modules of the second gas separation membrane unit is 1:2 or more and 1:12 or less. [8] The gas separation system according to any one of [1] to [7] above, wherein the hollow fiber membrane of the second gas separation membrane unit satisfies formula (1). [Effects of the Invention]

[0010] According to the present invention, there is provided a two-stage gas separation system that effectively reduces the pressure loss and membrane area of ​​the gas separation membrane system and is capable of high methane recovery. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of one embodiment of a gas separation system of the present invention. [Figure 2] FIG. 2 is a diagram of an example of a gas separation membrane module in the gas separation system shown in FIG. 1, and is a cross-sectional view schematically showing a cross section along the longitudinal direction of a hollow fiber membrane bundle in the gas separation membrane module. [Figure 3]FIG. 3 is an end view schematically showing an end face of one end in the longitudinal direction of the hollow fiber membrane bundle (hollow fiber membrane element) shown in FIG. [Figure 4] FIG. 4 is a schematic diagram showing an example of the configuration of each gas separation membrane unit in the gas separation system shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described below based on preferred embodiments with reference to the drawings. The upper and lower limits of the numerical values ​​in this specification can be combined without any restrictions.

[0013] In this specification, the expressions "A and B are connected by line C" or "line C connecting A and B" include both cases where A and B and line C are separate members, and cases where A and / or C are one continuous member. Similarly, the expression "line E is connected to D" includes both cases where D and line E are separate members and cases where D and line E are one continuous member.

[0014] The gas separation system of the present invention supplies a raw material mixed gas containing at least carbon dioxide (CO2) and methane (CH4) to a gas separation membrane unit having a gas separation membrane, and concentrates and enriches the methane contained in the raw material mixed gas.

[0015] In the present invention, the type of raw material mixed gas is not particularly limited, provided that it contains carbon dioxide and methane, and examples thereof include biogas and natural gas. The "biogas" is a gas generated by decomposing a biomass material, for example, by contacting the biomass material with microorganisms under anaerobic conditions to subject the biomass material to fermentation such as methane fermentation. Examples of the biomass material include organic matter such as food waste, agricultural residues, sewage sludge, and livestock waste. Landfill gas generated by microbial decomposition of organic matter in waste landfills is also included in biogas. The "natural gas" is gas derived from fossil fuels buried deep underground.

[0016] In the present invention, the composition of the raw material mixed gas is not particularly limited, but from the viewpoint of efficiently recovering the target methane with high purity and high recovery rate by making the most of the features of the gas separation system of the present invention, the raw material mixed gas preferably contains 30 mol% or more of methane, more preferably 40 to 99 mol%. From the same viewpoint, the raw material mixed gas preferably contains 3 mol% or more of carbon dioxide, more preferably 5 mol% or more. The upper limit of the carbon dioxide content is, for example, preferably 70 mol% or less, more preferably 60 mol% or less. In particular, when the raw material mixed gas is biogas, it preferably contains 30 mol% or more, more preferably 40 to 95 mol% of methane. When the raw material mixed gas is natural gas, it preferably contains 80 mol% or more, more preferably 85 to 99 mol% of methane.

[0017] 1 shows a schematic configuration of a gas separation system 10, which is one embodiment of the gas separation system of the present invention. The gas separation system 10 includes a first gas separation membrane unit 11 and a second gas separation membrane unit 12 as means for separating a raw mixed gas. The two units 11 and 12 are connected in series. In this embodiment, the first gas separation membrane unit 11 is disposed upstream in the supply direction Y1 of the raw mixed gas, and the second gas separation membrane unit 12 is disposed downstream in the supply direction.

[0018] Both units 11 and 12 have at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet. Specifically, the first gas separation membrane unit 11 has a gas inlet 11a, a permeate gas outlet 11b, and a non-permeate gas outlet 11c, and the second gas separation membrane unit 12 has a gas inlet 12a, a permeate gas outlet 12b, and a non-permeate gas outlet 12c.

[0019] As shown in Figure 1, a raw material mixed gas supply line 13 is connected to a gas inlet 11a of the first gas separation membrane unit 11, for supplying a raw material mixed gas from a raw material mixed gas source (not shown) to the first gas separation membrane unit 11. A compression means 14 is interposed in the raw material mixed gas supply line 13. A non-permeate gas discharge port 11c of the first gas separation membrane unit 11 is connected to a gas inlet 12a of the second gas separation membrane unit 12 by a non-permeate gas discharge line 16. A permeate gas discharge port 12b of the second gas separation membrane unit 12 is connected to a position on the suction side of the compression means 14 on the raw material mixed gas supply line 13 (upstream of the compression means 14 in the supply direction of the raw material mixed gas, which is the gas to be separated).

[0020] Compression means 14 is provided for the purpose of compressing the raw material mixed gas supplied from the raw material mixed gas source, and also for the purpose of compressing the permeable gas discharged from second gas separation membrane unit 12 when returning the permeable gas to first gas separation membrane unit 11 through permeable gas recycle line 17. Compression means 14 is not particularly limited as long as it is a known means, and for example, a compressor can be used.

[0021] In this embodiment, the gas separation system 10 further includes a permeate gas discharge line 15 and a non-permeate gas recovery line 18 in addition to the above-described components such as the raw material mixed gas supply line 13. The permeate gas discharge line 15 is used to discharge the permeate gas separated in the first gas separation membrane unit 11 to the outside of the unit 11, and is connected to a permeate gas outlet 11b of the unit 11. The non-permeate gas recovery line 18 is used to extract the non-permeate gas (product gas) separated in the second gas separation membrane unit 12, and is connected to a non-permeate gas outlet 12c of the unit 12. If necessary, flow control valves may be provided along each of the above-described lines 13, 15, 16, 17, and 18 to adjust the gas flow rate.

[0022] The operation of the gas separation system 10 of this embodiment having the above configuration will be described. The raw mixed gas to be separated contains at least carbon dioxide and methane. The gas to be recovered (product gas) is enriched in methane, a low-permeability component, compared to the raw mixed gas.

[0023] First, the raw material mixed gas is supplied from the raw material mixed gas source to the first gas separation membrane unit 11 through the raw material mixed gas supply line 13. Before being supplied to the first gas separation membrane unit 11, the raw material mixed gas is compressed by the compression means 14, and the pressure thereof increases.

[0024] When the raw mixed gas pressurized by the compression means 14 is supplied to the first gas separation membrane unit 11, it is separated into a first permeate gas, which is a gas that permeates the gas separation membrane (hollow fiber membrane bundle 22) of the first gas separation membrane module 1, and a first non-permeate gas, which is a gas that does not permeate the gas separation membrane, due to the difference in permeation rate through the gas separation membrane (hollow fiber membrane bundle 22) of the first gas separation membrane module 1. The first permeate gas is enriched in carbon dioxide compared to the raw mixed gas. The first non-permeate gas is enriched in methane compared to the raw mixed gas. The first permeate gas is discharged to the outside of the first gas separation membrane unit 11 through the non-permeate gas outlet 11c and discharged to the outside of the gas separation system 10 through the permeate gas discharge line 15. Meanwhile, the first non-permeate gas is discharged to the outside of the first gas separation membrane unit 11 through the non-permeate gas outlet 11c and supplied to the second gas separation membrane unit 12 through the non-permeate gas discharge line 16.

[0025] In this embodiment, the entire first permeable gas is discharged to the outside of the gas separation system 10, but like the second permeable gas described later, at least a portion of the first permeable gas may be returned to the suction side of the compression means 14 in the raw material mixed gas supply line 13 via a permeable gas recycle line (not shown).

[0026] Each of the units 11 and 12 is composed of one gas separation membrane module or a plurality of gas separation membrane modules arranged in parallel. The gas separation membrane module, like the gas separation membrane module 1 shown in Figure 2, has a hollow fiber membrane bundle 22 consisting of a plurality of hollow fiber membranes 21 as gas separation membranes with selective permeability. Each of the plurality of hollow fiber membranes 21 has a hollow shape that is elongated in one direction. The hollow fiber membrane bundle 22 is a bundle of the plurality of hollow fiber membranes 21.

[0027] In this embodiment, the hollow fiber membrane bundle 22 has a longitudinal direction Y along the extension direction of the hollow fiber membranes 21, and a width direction X that is a direction perpendicular to the longitudinal direction Y. In this embodiment, the longitudinal direction of the hollow fiber membranes 21 is the same as the longitudinal direction of the hollow fiber membrane bundle 22.

[0028] In this embodiment, the gas separation membrane module 1 includes a hollow fiber membrane element 2 including a hollow fiber membrane bundle 22, and a container 3 that houses the hollow fiber membrane element 2. The hollow fiber membrane element 2 is detachably disposed in the container 3.

[0029] The hollow fiber membrane element 2 is formed by fixing at least one end in the longitudinal direction Y of a hollow fiber membrane bundle 22 consisting of a bundle of multiple hollow fiber membranes 21 with selective permeability to a tube sheet 23 while maintaining the openings of the hollow fiber membranes 21. In this embodiment, each of the multiple hollow fiber membranes 21 is fixed to the tube sheet 23 at an end on the side of a gas inlet 34 (described later) in the longitudinal direction Y of the module 1 (FIG. 2), and at an end on the side of a non-permeate gas outlet 36 (described later), and together with the pair of tube sheets 23, 23, constitutes the hollow fiber membrane element 2. In the hollow fiber membrane element 2 of this embodiment, each of the multiple hollow fiber membranes 21 is fixed with both longitudinal ends penetrating the tube sheet 23 and open.

[0030] In the present invention, there are no particular limitations on the shape of the tube sheet 23. In this embodiment, the tube sheet 23 has a disk-like shape in plan view, as shown in Fig. 3. The material of the tube sheet 23 is not particularly limited as long as it can be bonded to the hollow fiber membranes 21, and examples of the material include resins such as epoxy resins and metals.

[0031] In the present invention, the material of the hollow fiber membrane 21 is not particularly limited, provided that it is capable of concentrating and enriching methane contained in a raw material mixed gas containing carbon dioxide and methane, and examples thereof include rubbery polymer materials such as silicone resin and polybutadiene resin; and glassy polymer materials such as polyimide, polyetherimide, polyamide, polyamideimide, polysulfone, polycarbonate, cellulose, etc. Among these, polyimide is particularly suitable as the material of the hollow fiber membrane 21 in terms of heat resistance and mechanical properties.

[0032] In the present invention, the structure of the hollow fiber membrane 21 is not particularly limited, and examples thereof include homogeneous membranes such as symmetric membranes and non-porous membranes; and non-homogeneous asymmetric membranes. From the viewpoint of making the most of the characteristics of the present invention described below and achieving the desired effects of the present invention, namely, achieving high levels of reduction in pressure loss and membrane area of ​​the gas separation membrane, the hollow fiber membrane 21 is preferably an asymmetric membrane. An asymmetric membrane refers to a membrane having a very thin, dense surface layer (skin layer) of the selective layer responsible for gas separation and a thick, porous support layer responsible for mechanical strength. Generally, the skin layer is much denser than the porous layer and is usually very thin, preferably 0.001 to 5 μm thick. In asymmetric hollow fiber membranes, a dense layer generally constitutes at least one of the inner and outer surfaces.

[0033] In the present invention, the number of hollow fiber membranes 21 constituting the hollow fiber membrane bundle 22 is not particularly limited and is typically about 100 to 1,000,000. From the viewpoint of the balance between gas separation performance and production costs, the number of hollow fiber membranes 21 constituting the hollow fiber membrane bundle 22 is preferably 1,000 or more, more preferably 10,000 or more, and even more preferably 30,000 or more. Also, the number is preferably 800,000 or less, more preferably 700,000 or less, and even more preferably 600,000 or less.

[0034] In the present invention, the shape of the hollow fiber membrane bundle 22, i.e., the shape of the bundle of hollow fiber membranes 21, is not particularly limited and may be, for example, a columnar shape such as a cylindrical or rectangular columnar shape; or a flat plate shape. From the viewpoints of ease of production and improvement of the pressure resistance of the hollow fiber membranes, a cylindrical shape is preferred for the hollow fiber membrane bundle 22.

[0035] In the present invention, the arrangement of the hollow fiber membranes 21 in the hollow fiber membrane bundle 22 is not particularly limited, and the plurality of hollow fiber membranes 21 may be arranged parallel to one another in one direction, or may be arranged so as to cross one another. In this embodiment, the former is adopted, and specifically, all of the plurality of hollow fiber membranes 21 constituting the hollow fiber membrane bundle 22 are arranged parallel to the axial direction of the container 3 (or the axial direction of the core tube if the hollow fiber membrane bundle 22 has a core tube described below).

[0036] In the present invention, the hollow fiber membrane bundle 22 may have a core tube (not shown) inside, which is a tubular core having a large number of through holes on its circumferential surface. For example, the hollow fiber membrane bundle 22 may have an internal space surrounded by a plurality of hollow fiber membranes 21, and the core tube extending in the longitudinal direction Y of the hollow fiber membrane bundle 22 may be disposed in this internal space.

[0037] The casing 31 in the module 1 is open on two opposing sides, forming openings 32A and 32B. In the module 1, the gas separation membranes are housed in the casing 31 so that, in the housed state, each end of the hollow fiber membrane is open near each of the openings 32A and 32B at both ends of the longitudinal direction Y of the casing 31. When the hollow fiber membrane element 2 is housed in the casing 31, the hollow fiber membrane element 2 is fixed to the inner wall of the casing 31 by tube sheets 23 at both ends in the Y direction, which is the direction in which the hollow fiber membranes extend. Covers 33A and 33B are attached to the openings 32A and 32B of the casing 31, respectively. A gas inlet 34 is provided in the cover 33A. A non-permeate gas outlet 36 is provided in the cover 33B.

[0038] The gas to be separated is introduced into the gas separation membrane module 1 through a gas inlet 34. Of the introduced gas, the gas that has permeated the gas separation membrane (hollow fiber membrane bundle 22) is discharged to the outside of the gas separation membrane module 1 through a permeate gas outlet 35. On the other hand, the non-permeate gas that has not permeated the gas separation membrane is discharged to the outside of the gas separation membrane module 1 through a non-permeate gas outlet 36.

[0039] The gas inlet 34 of the vessel 3 functions as the gas inlets 11a and 12a of both units 11 and 12. The permeate gas outlet 35 of the vessel 3 functions as the permeate gas outlets 11b and 12b of both units 11 and 12. The non-permeate gas outlet 36 of the vessel 3 functions as the non-permeate gas outlets 11c and 12c of both units 11 and 12.

[0040] The materials of the components of the container 3, such as the casing 31 and the lids 33A and 33B, are preferably metal, plastic, glass fiber composite material, ceramic, or the like.

[0041] The gas separation membrane module 1, in which the hollow fiber membrane elements 2 are housed in the container 3, has an internal space that is separated by a tube plate 23, sealed except for the gas flow path, and maintained airtight. The sealing method is not particularly limited, but it is preferable to place airtight means such as an O-ring or packing made of elastic resin between the openings 32A, 32B and the lids 33A, 33B, for example. In the internal space, a space that communicates with the internal space of the hollow fiber membranes 21 and a space that communicates with the external space of the hollow fiber membranes 21 are isolated from each other, maintaining airtightness.

[0042] In the present invention, the gas separation membrane modules 1 constituting the first gas separation membrane unit 11 and the second gas separation membrane unit 12 may be of a hollow feed type in which the internal space of the hollow fiber membrane 21 is the primary side (the raw material mixed gas supply side), or of a shell feed type in which the external space of the hollow fiber membrane 21 is the primary side, but are preferably of the hollow feed type.

[0043] FIG. 4 shows an example of a connection method for a gas separation membrane unit in which multiple gas separation membrane modules 1 are connected in parallel. In the configuration shown in FIG. 4, a single gas supply line 40 that supplies the gas to be separated to the gas separation membrane unit branches into multiple lines, which are connected one-to-one to the gas inlets 34 of the multiple gas separation membrane modules 1, allowing the gas to be introduced into each gas separation membrane module 1 through the gas supply line 40. The multiple gas separation membrane modules 1 share a single permeate gas discharge line 41, and the permeate gas that permeates through the hollow fiber membrane bundles 22 of each gas separation membrane unit and is discharged from the permeate gas discharge port 35 is discharged to the outside of the unit through the permeate gas discharge line 41. The multiple gas separation membrane modules 1 share a single non-permeate gas discharge line 42, and the non-permeate gas that does not permeate through the hollow fiber membrane bundles 22 of each gas separation membrane unit and is discharged from the non-permeate gas discharge port 36 is discharged to the outside of the unit through the non-permeate gas discharge line 42.

[0044] One of the main features of the gas separation system 10 is that the hollow fiber membrane 21 of the first gas separation membrane unit 11 satisfies the following formula (1): In the following formula (1), "A" is the inner diameter (μm) of the hollow fiber membrane 21, and "B" is the length (mm) of the hollow fiber membrane 21. B 1.1 / A 1.4 ≦2.8 (1)

[0045] The present inventors have conducted extensive research into a configuration for a two-stage gas separation system 10 that achieves both a high methane recovery rate and a reduced membrane area. In the process, they discovered that by ensuring a predetermined relationship between the inner diameter and length of the hollow fiber membranes in the first-stage gas separation membrane unit, the two-stage gas separation system 10 can achieve a high methane recovery rate while effectively reducing the pressure drop from the first feed gas to the second non-permeate gas. Specifically, in the present invention, the hollow fiber membranes 21 constituting the hollow fiber membrane bundle 22 (gas separation membrane) satisfy the above-mentioned formula (1), so that even if the methane recovery rate is increased, the pressure drop in the gas separation membrane does not become excessive, and increases in energy loss and membrane area can be suppressed. Furthermore, the present invention has the advantage of being able to ensure the pressure required for supplying the product gas from the second non-permeate gas outlet. In contrast, in conventional two-stage gas separation systems using hollow-fiber gas separation membrane modules, increasing the methane recovery rate requires a certain level of reflux rate for the permeate gas in the second stage, which tends to increase the gas flow rate supplied to the first stage, resulting in a problem of a large pressure loss from the first feed gas to the second non-permeate gas. In this type of two-stage gas separation system, the main driving force for the system is the differential pressure between the supply side (primary side) and the permeate side (secondary side) of the gas separation membrane module, which is generated by the compression means. To ensure stable system operation, this differential pressure must be maintained above a certain level. However, if the pressure loss in the first-stage gas separation membrane unit is large, the pressure on the supply side of the second-stage gas separation membrane unit drops, making it difficult to maintain the differential pressure between the supply side and the permeate side of the second-stage gas separation membrane unit above a certain level, leading to a decrease in the overall system throughput. Therefore, to maintain throughput, it was necessary to increase the membrane area in the system, especially the membrane area in the second stage, where pressure is likely to drop.

[0046] In the present invention, the "inner diameter" of one hollow fiber membrane refers to the average value (=(Ax+An) / 2) of the maximum diameter Ax and the minimum diameter An of the hollow portion 21a of the hollow fiber membrane 21 in the direction perpendicular to the longitudinal direction (radial direction) of the hollow fiber membrane 21. When the planar shape of the hollow portion 21a (cross-sectional shape along the direction perpendicular to the longitudinal direction of the hollow fiber membrane 21) is elliptical, the inner diameter A is the average value of the major axis length and the minor axis length of the elliptical hollow portion 21a. When the planar shape is a perfect circle, Ax and An have the same value, and the inner diameter A is the diameter as shown in Figure 3. The inner diameter A of the hollow fiber membrane can be measured using an optical microscope. A measurement magnification of, for example, 20 to 150 times is suitable.

[0047] In the present invention, the inner diameters of five hollow fiber membranes arbitrarily selected from the hollow fiber membrane bundle constituting the first gas separation membrane unit are measured, and the average of these measurements can be used as the inner diameter A of the hollow fiber membranes. For example, when measuring the inner diameter A of the hollow fiber membranes in the hollow fiber membrane bundle (gas separation membrane), the inner diameters of the open ends of the hollow fiber membranes that open into the tube sheet 23 at one longitudinal end of the hollow fiber membrane element 2 described above can be measured, and the average of these measurements can be used as the inner diameter A of the hollow fiber membranes.

[0048] In the present invention, the "length B of the hollow fiber membrane" refers to the length between one end and the other end in the longitudinal direction (direction perpendicular to the radial direction) of the hollow fiber membrane bundle 22 in the gas separation membrane module. In this embodiment, as shown in Fig. 2, one longitudinal end of the hollow fiber membrane bundle 22 is located near the gas inlet 34, and the other longitudinal end of the hollow fiber membrane bundle 22 is located near the non-permeate gas outlet 36, so that the length B of the hollow fiber membrane bundle 22 corresponds to the distance between the one end and the other end. In the present invention, the hollow fiber membranes are preferably arranged without being folded back from one end to the other end (preferably arranged in a straight line), as shown in Fig. 2. The length B of the hollow fiber membrane can be measured using a measuring means such as a ruler, a laser distance meter, etc. As another example of a method for measuring the length B of the hollow fiber membrane, the length between the open ends of the hollow fiber membranes (the end faces of the tube sheet) can be measured with a tape measure or the like, or the length B' (not shown) between the open ends of the lid (the length between the gas inlet 34 and the gas outlet 36) can be measured with a tape measure or the like, and then, with the lid still attached to the casing, the distance from the open ends of the lid (the gas inlet 34, the gas outlet 36) to the open ends of the hollow fibers (the end faces of the tube sheet) can be measured with a laser distance meter or the like, and this can be subtracted from B' to obtain B.

[0049] From the viewpoint of more reliably achieving the effects of satisfying the above formula (1) (reducing the pressure loss of the gas separation membrane and reducing the membrane area), B 1.1 / A 1.4 The value of is preferably 2.6 or less, more preferably 2.5 or less, and even more preferably 2.4 or less.

[0050] The hollow fiber membranes 21 of the first gas separation membrane unit 11 preferably satisfy the following formula (2). 1.0≦B 1.1 / A 1.4 ≦2.8 (2) B 1.1 / A 1.4 The smaller the value of B, the greater the effect of reducing pressure loss of the gas separation membrane. However, if the value is too small, the manufacturing cost increases and there is a concern that the gas separation effect of the gas separation membrane may decrease. By having the hollow fiber membranes 21 of the first gas separation membrane unit 11 satisfy the above formula (2), such concerns are more reliably eliminated, and the desired effects of the present invention are more reliably achieved. 1.1 / A 1.4 The value of is preferably 1.1 or more, and more preferably 1.2 or more.

[0051] From the viewpoint of ensuring that the desired effects of the present invention are achieved, the inner diameter A of the hollow fiber membrane is preferably 50 μm or more and 300 μm or less, more preferably 70 μm or more and 300 μm or less, even more preferably 70 μm or more and 260 μm or less, and even more preferably 80 μm or more and 230 μm or less.

[0052] In the present invention, the outer diameter A1 (see FIG. 3) of the hollow fiber membrane is not particularly limited, but from the viewpoint of mechanical strength and handleability, it is preferably 100 μm or more and 450 μm or less, more preferably 130 μm or more and 400 μm or less. The outer diameter A1 refers to the average value (=(ax+an) / 2) of the length ax (not shown) of the maximum transverse line segment in the direction (radial direction) perpendicular to the longitudinal direction of the hollow fiber membrane 21 of the outer surface portion 21b of the hollow fiber membrane 21 and the length an (not shown) of the minimum transverse line segment. When the cross-sectional shape of the hollow fiber membrane along the direction perpendicular to the longitudinal direction is a perfect circle, the outer diameter A1 is the diameter of the hollow fiber membrane. The outer diameter of the hollow fiber membranes can be measured using an optical microscope. The measurement magnification is preferably, for example, 20 to 150 times. As with the inner diameter, the outer diameters of five hollow fiber membranes arbitrarily selected from the hollow fiber membrane bundle constituting the first gas separation membrane unit can be measured, and the average of these measurements can be used as the outer diameter A1 of the hollow fiber membranes.

[0053] From the viewpoint of ensuring that the desired effects of the present invention are achieved, the length B of the hollow fiber membrane 21 is preferably 300 mm or more and 2500 mm or less, more preferably 300 mm or more and 2300 mm or less, even more preferably 300 mm or more and 2000 mm or less, and even more preferably 500 mm or more and 2000 mm or less.

[0054] In a particular system of the present invention, in order to achieve the specified objective of suppressing pressure loss and reducing membrane area to a high level, the hollow fiber membrane 21 of the first gas separation membrane unit 11, which is directly subjected to gas pressure from the compression means 14, must satisfy the above formula (1). Although the hollow fiber membranes 21 of the second gas separation membrane unit 12 do not necessarily need to satisfy the formula (1), the above-mentioned problem can be solved more reliably if the hollow fiber membranes 21 of the second gas separation membrane unit 12 also satisfy the formula (1). It is more preferable that the hollow fiber membranes 21 of the second gas separation membrane unit 12 also satisfy the formula (2). In the gas separation system 10 of this embodiment, all of the gas separation membrane modules used in both units 11, 12 are gas separation membrane modules 1 that use hollow fiber membranes 21 that satisfy the formula (2). Unless otherwise specified, the explanation for the first gas separation membrane unit 11 applies, as appropriate, to the second gas separation membrane unit 12.

[0055] As described above, the gas separation membrane modules 1 constituting both units 11 may be either hollow feed type or shell feed type, but from the viewpoint of further enhancing the pressure drop reduction effect, the gas separation membrane module 1 of the first gas separation membrane unit 11 is preferably of the hollow feed type. From the same viewpoint, the gas separation membrane module 1 of the second gas separation membrane unit 12 is preferably of the hollow feed type.

[0056] In general, the gas separation selectivity of a gas mixture containing a highly permeable component and a less permeable component by a gas separation membrane can be expressed by the ratio of the permeation rate of the highly permeable component to the permeation rate of the less permeable component (the former / the latter). The "permeation rate" is the permeation volume of the component per unit membrane area, unit time, and unit partial pressure difference [unit: × 10 -5 cm3(STP) / cm2 ·sec·cmHg], and is referred to herein as "P' ○○ " (○○ is the name of the ingredient). The raw mixed gas to be separated by the gas separation system 10 contains carbon dioxide and methane, with carbon dioxide being a highly permeable component and methane being a low permeable component. From the viewpoint of efficiently recovering methane, a low permeable component, with a high recovery rate and high purity by making the most of the effect of satisfying the above formula (1), the permeation rate P' of carbon dioxide (CO2) through the gas separation membrane of the first gas separation membrane unit 11 at 40°C is CO2 and the permeation rate P' of methane (CH4) through the gas separation membrane at 40°C. CH4 The ratio (separation selectivity) is P' CO2 / P' CH4 is preferably 30 or more and 150 or less, more preferably 35 or more and 130 or less, even more preferably 40 or more and 120 or less, and even more preferably 50 or more and 110 or less. The preferred separation selectivity for the gas separation membrane of the second gas separation membrane unit 12 is the same as the ranges listed above for the first gas separation membrane unit 11.

[0057] In the gas separation membrane of the first gas separation membrane unit 11, the carbon dioxide permeation rate P' at 40°C CO2 is 1.5 x 10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 100×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is preferable, and 2.0×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 45×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is more preferable, and 3.0×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 25×10 -5 cm 3 (STP) / cm2 sec cmHg or less, and more preferably 7.0 × 10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 15×10 -5 cm 3 (STP) / cm 2 The preferred carbon dioxide permeation rate P' of the gas separation membrane of the second gas separation membrane unit 12 is 1.5 sec.cmHg or less. CO2 are the same as the ranges given above for the first gas separation membrane unit 11.

[0058] In the gas separation membrane (hollow fiber membrane bundle 22) of the first gas separation membrane unit 11, the methane permeation rate P' at 40°C CH4 is 0.030 x 10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 3.0×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is preferable, and 0.05×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 0.80×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is more preferable, and 0.0510 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 0.50×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is more preferable, and 0.08×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 0.20×10 -5 cm 3 (STP) / cm 2 The preferred methane permeation rate P' for the gas separation membrane of the second gas separation membrane unit 12 is 1.5 sec.cmHg or less.CH4 are the same as the ranges given above for the first gas separation membrane unit 11.

[0059] Carbon dioxide permeation rate P' at 40°C CO2 can be calculated by the following procedure: Pure carbon dioxide gas is pressurized at a predetermined pressure, the temperature of the pure gas is set to 40°C, and the flow rate of the pure gas that has permeated a gas separation membrane (hollow fiber membrane bundle) having a predetermined membrane area is measured using a thin-film flowmeter, and the permeation rate P' is calculated from the pressure of the pure gas, the membrane area of ​​the gas separation membrane, and the flow rate of the permeated pure gas. CO2 Calculate the methane permeation rate P' at 40°C. CH4 can be calculated by using pure methane gas instead of pure carbon dioxide gas in the above procedure.

[0060] The ratio A1:A2 of the total carbon dioxide permeability A1 of the gas separation membrane modules of the first gas separation membrane unit 11 to the total carbon dioxide permeability A2 of the gas separation membrane modules of the second gas separation membrane unit 12 is preferably 1:2 or more and 12 or less, more preferably 1:3 or more and 12 or less, even more preferably 1:5 or more and 12 or less, still more preferably 1:7 or more and 12 or less, still more preferably 1:8 or more and 12 or less, and most preferably 1:9 or more and 12 or less. The total carbon dioxide permeabilities A1 and A2 are respectively calculated by multiplying the total membrane area S1 of the gas separation membrane modules of the first gas separation membrane unit 11 and the carbon dioxide permeation rate P' of the gas separation membrane of the first gas separation membrane unit 11 at 40°C. CO2 the product of the total membrane area S2 of the gas separation membrane modules of the second gas separation membrane unit 12 and the carbon dioxide permeation rate P' of the gas separation membrane of the second gas separation membrane unit 12 at 40°C. CO2 It is expressed as the product of A1 and A2. When the ratio A1:A2 is within the above range, pressure loss can be suppressed even at a high methane recovery rate, making it easier to achieve a high level of membrane area reduction. The ratio A1:A2 can be adjusted, for example, by adjusting the type and size (inner diameter, length, etc.) of the hollow fiber membranes that make up the gas separation membrane (hollow fiber membrane bundle) and the number of gas separation membrane modules.

[0061] The "total membrane area S1" and the "total membrane area S2" are each calculated by multiplying the membrane area of ​​the hollow fiber membranes 21 by the number of membranes. For example, the "total membrane area S1" can be calculated as follows: As described above, in an asymmetric hollow fiber membrane, a dense layer generally constitutes at least one of the inner and outer surfaces. The membrane area of ​​the hollow fiber membrane 21 can be calculated as the product of the inner or outer circumferential length of the hollow fiber membrane 21, whichever is the circumferential length of the side having the dense layer, and the length B of the hollow fiber membrane 21. For example, when the dense layer is present on the inside of the hollow fiber membrane, the membrane area can be calculated from the inner circumferential length of the hollow fiber membrane (the outer circumferential length of the hollow portion 21a). For example, the inner circumferential length can be calculated by the following formula (3) using the inner diameter A of the hollow fiber membrane measured using an optical microscope. When the dense layer is present on the outside of the hollow fiber membrane, the outer circumferential length can be calculated by the following formula (4) using the outer diameter A1 of the hollow fiber membrane. Inner circumference length = Inner diameter A x 3.14 (3) Outer circumference length = outer diameter A1 x 3.14 (4) When the dense layer is present on both sides of the hollow fiber membrane, the surface that serves as the supply side (feed side) during operation is determined in the same manner as above, depending on whether it is the inner or outer surface of the membrane. Similarly to the "total membrane area S1," the "total membrane area S2" can be calculated by determining the inner diameter A or outer diameter A1 and the length B of the hollow fiber membrane 21.

[0062] If the first gas separation membrane unit and the second gas separation membrane unit use the same module grade, the membrane area ratio can be substituted by the ratio of the number of modules. Examples of modules of the same grade include those with the same product name or model number, or those with the same module outer diameter and module length, which are highly likely to have the same membrane area.

[0063] In order to avoid an excessive increase in the number of modules, the membrane area (m 2 / (Nm 3 / h)) is 3.5(m 2 / (Nm 3 / h)) or less, and 3.3 (m 2 / (Nm 3 / h) or less. 2 / (Nm 3 / h)) is preferably 1.0 or more, and more preferably 2.0 or more. The raw material mixed gas flow rate here refers to the raw material mixed gas flow rate Y flowing into the system.

[0064] According to the gas separation system 10, the hollow fiber membrane 21 of the first gas separation membrane unit 11 satisfies the above formula (1), so that the membrane area of ​​the gas separation membrane of the system can be reduced while maintaining the purity and recovery rate of the recovered methane at a high level.

[0065] The pressure of the gas flowing into the first gas separation membrane unit is preferably 0.60 MPaG or more, more preferably 0.80 MPaG or more, from the viewpoints of easily ensuring the pressure at which the second non-permeable gas is extracted as a product gas and further reducing the membrane area. Furthermore, the pressure of the gas flowing into the first gas separation membrane unit is preferably 2.0 MPaG or less, more preferably 1.8 MPaG or less, from the viewpoints of more easily reducing pressure loss and reducing energy costs when the gas pressure is below a certain level.

[0066] The pressure of the gas discharged from the non-permeate gas outlet of the second gas separation membrane unit is preferably 0.50 MPaG or more, and more preferably 0.70 MPaG or more, because it is easier to ensure the pressure at which the second non-permeate gas is extracted as a product gas. The pressure of the gas discharged from the non-permeate gas outlet of the second gas separation membrane unit is preferably 1.90 MPaG or less, and more preferably 1.70 MPaG or less, because a pressure of the gas below a certain level makes it easier to reduce pressure loss and reduces energy costs.

[0067] The operating temperature of each separation membrane unit 11, 12 (the temperature of the gas separation membrane during operation) is preferably in the range of 0 to 80°C, more preferably 5 to 60°C, and even more preferably 10 to 50°C.

[0068] Furthermore, the amount of gas X (Nm 3 / h) and the amount of gas flowing into the system Y (Nm 3 / h) is the amount of gas flowing into the system X (Nm 3 The recycle rate ((X-Y) / Y x 100(%)), which is the value obtained by dividing the total methane recovery rate by the total methane recovery rate (X-Y) / Y x 100(%), is preferably 60% or higher, and more preferably 70% or higher, from the viewpoint of high methane recovery rate. Furthermore, from the viewpoint of reducing the energy cost of the system, it is preferably 85% or lower, and more preferably 80% or lower.

[0069] According to the gas separation system of the present invention, the recovery rate of methane from a raw mixed gas containing carbon dioxide and methane is preferably at least 98.0%, more preferably 98.5% or more under preferable conditions, and even more preferably 99.0% or more under preferable conditions. The recovery rate of methane can be calculated by the following formula. Methane recovery rate (%) = (second non-permeate gas flow rate × CH4 concentration in the non-permeate gas discharged from the second non-permeate gas outlet) / (raw material mixed gas flow rate Y flowing into the system × CH4 concentration in the raw material mixed gas flowing into the system)

[0070] In the gas separation system of the present invention, represented by gas separation system 10, the purity of methane obtained by this system is preferably 96% by volume or more, more preferably 97% by volume or more, and particularly preferably 99% by volume or more. In addition, the concentration of carbon dioxide contained in the product gas obtained by this system is preferably 5% by volume or less, more preferably 3% by volume or less, and even more preferably 1% by volume or less. [Example]

[0071] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0072] (Examples 1 to 6, Comparative Examples 1 to 3) A simulation was performed using a model to recover methane-enriched gas as a product gas (non-permeable gas separated by the second gas separation membrane unit) from a raw material mixed gas containing carbon dioxide and methane using a gas separation system having the same basic configuration as the above-described gas separation system 10. The model used was that described in THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING., VOLUME 90, 2011, pp. 1253-1268. The simulation conditions were as follows:

[0073] (Simulation conditions) <Conditions for gas separation membrane module performance> Raw material gas mixture Flow rate Y of raw material mixed gas (flow rate Y into system 10): 1000 Nm 3 / h Carbon dioxide permeation rate P' of gas separation membrane at 40℃ CO2 :9.9×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg ·Methane permeation rate P' of gas separation membrane at 40℃ CH4 :0.20×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg P' CO2 / P' CH4 :49.5 Membrane area per gas separation membrane module: 24m 2 The gas separation membrane module was assumed to be a hollow feed type gas separation membrane module, with the gas separation membrane being a hollow fiber membrane bundle made of asymmetric hollow fiber membranes using aromatic polyimide as the material. The dense layer was located on the inner surface of the hollow fiber membrane. The gas separation membrane module was common to the first gas separation membrane unit and the second gas separation membrane unit. Each unit used gas separation membrane modules that met the above conditions and the configuration shown in the table below, connected in parallel in the number shown in the table below.

[0074] In Tables 1 and 2 below, the outer diameter of a hollow fiber membrane with an inner diameter A of 100 μm was 200 μm, the outer diameter of a hollow fiber membrane with an inner diameter A of 300 μm was 150 μm, and the outer diameter of a hollow fiber membrane with an inner diameter A of 200 μm was 400 μm.

[0075] <System operating conditions> Composition of raw material mixed gas: 40 mol% carbon dioxide, 60 mol% methane Operating pressure: 1.4MPaG Methane concentration in product gas: 99% by volume Methane recovery rate: Values ​​shown in Table 1 Operating temperature of the first gas separation membrane unit: 25°C Operating temperature of the second gas separation membrane unit: 25°C The "operating pressure" refers to the pressure of the raw material mixed gas flowing into the first gas separation membrane unit. In this specification, "MPaG" refers to a gauge pressure with atmospheric pressure as the reference (zero). The "operating temperature" refers to the temperature of the gas separation membrane (hollow fiber membrane bundle) in the gas separation membrane unit.

[0076] The results of the simulation are shown in Table 1 below. In Table 1 below, "first unit" refers to the first gas separation membrane unit, and "second unit" refers to the second gas separation membrane unit. In Table 1, "supply flow rate to first unit" refers to the flow rate of the raw mixed gas flowing into the first gas separation membrane unit. "Product gas pressure" refers to the pressure of the second non-permeate gas at the second non-permeate gas outlet. The "recycling rate" in Table 1 is calculated using the following formula. Recycle rate (%) = (supply flow rate to first unit x (Nm 3 / h) - Inflow rate Y (Nm 3 / h) / inflow rate Y (Nm 3 / h)×100(%) The "pressure loss rate" in Table 1 is calculated using the following formula. Pressure loss rate (%) = {(operating pressure (MPaG) - product gas pressure (MPaG)) / operating pressure (MPaG)} x 100

[0077] [Table 1]

[0078] As shown in Table 1, in each of the Examples and Comparative Examples, the methane recovery rate was at a particularly high level of 99.0% or more. 1.1 / A 1.4 The gas separation systems of each Example in which "B" is 2.8 or less had a significantly smaller pressure loss rate than the gas separation systems of each Comparative Example which did not satisfy this requirement, and it was possible to significantly reduce the number of gas separation membrane modules constituting the second gas separation membrane unit. From this, it can be seen that in a two-stage gas separation system using hollow fiber gas separation membrane modules, in order to reduce the pressure loss of the gas separation membrane and achieve a high level of membrane area reduction even under conditions of high methane recovery rate, "B" of the hollow fiber membrane of the first gas separation membrane unit 1.1 / A 1.4 It can be seen that it is effective to keep " below 2.8.

[0079] (Examples 7 to 12, Comparative Examples 4 to 6) The recycle rate was changed to the value shown in Table 2, and the methane recovery rate was changed to the value shown in Table 2. Except for this, simulations were carried out in the same manner as in Examples 1 to 6 and Comparative Examples 1 to 3. The results are shown in Table 2.

[0080] [Table 2]

[0081] As shown in Table 2, even when the recycling rate was improved to a very high level of 99.2%, the "B" of the hollow fiber membrane of the first gas separation membrane unit was still not recovered. 1.1 / A 1.4 By making the ratio 2.8 or less, the pressure loss rate can be reduced to 20% or less, and the membrane area can be reduced. [Explanation of symbols]

[0082] 1. Gas separation membrane module 2. Hollow fiber membrane element 21 Hollow fiber membrane 22 Hollow fiber membrane bundle 23 Tube plate 3 containers 31 Casing 32A,32B opening 33A,33B Lid body 34 Gas inlet 35 Permeation gas outlet 36 Non-permeable gas outlet 10 Gas Separation System 11 First gas separation membrane unit 11a Gas inlet 11b Permeation gas outlet 11c Non-permeable gas outlet 12 Second gas separation membrane unit 12a Gas inlet 12b Permeation gas outlet 12c Non-permeable gas outlet 13 Raw material mixed gas supply line 14 Compression Methods 15 Permeation gas exhaust line 16 Non-permeate gas discharge line 17 Permeate gas recycle line 18 Non-permeate gas recovery line

Claims

1. A gas separation system that supplies a raw material mixed gas containing at least carbon dioxide and methane to a gas separation membrane unit to concentrate and enrich methane contained in the raw material mixed gas, a first gas separation membrane unit and a second gas separation membrane unit; Each of the gas separation membrane units includes one or more gas separation membrane modules and is equipped with at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, and the gas separation membrane module has, as the gas separation membrane, a hollow fiber membrane bundle consisting of a plurality of hollow fiber membranes; a raw material mixed gas supply line is connected to the gas inlet of the first gas separation membrane unit, and a compression means is interposed in the raw material mixed gas supply line; the non-permeate gas outlet of the first gas separation membrane unit and the gas inlet of the second gas separation membrane unit are connected by a non-permeate gas outlet line; the permeate gas outlet of the second gas separation membrane unit and a position on the suction side of the compression means in the raw mixed gas supply line are connected by a permeate gas recycle line; Concentrated and enriched methane can be extracted from the non-permeate gas outlet of the second gas separation membrane unit, A gas separation system, wherein the hollow fiber membrane of the first gas separation membrane unit satisfies the following formula (1): B １．１ / A １．４ ≦2.8 ・・・(1) In the formula (1), "A" is the inner diameter (μm) of the hollow fiber membrane, and "B" is the length (mm) of the hollow fiber membrane.

2. The gas separation system according to claim 1, wherein the hollow fiber membrane satisfies the following formula (2): 1.0≦B １．１ / A １．４ ≦2.8 ・・・(2)

3. 2. The gas separation system according to claim 1, wherein A is 50 μm or more and 300 μm or less.

4. 2. The gas separation system according to claim 1, wherein B is 300 mm or more and 2500 mm or less.

5. 2. The gas separation system according to claim 1, wherein the gas separation membrane module of the first gas separation membrane unit is of a hollow feed type.

6. The carbon dioxide permeation rate P' of the gas separation membrane of the first gas separation membrane unit at 40°C ＣＯ２ and the methane permeation rate P' of the gas separation membrane at 40°C. ＣＨ４ The ratio of ＣＯ２ / P' ＣＨ４ The gas separation system according to claim 1, wherein the β-glucan content is 30 or more and 150 or less.

7. 2. The gas separation system according to claim 1, wherein the ratio A1:A2 of the total carbon dioxide permeability A1 of the gas separation membrane modules of the first gas separation membrane unit to the total carbon dioxide permeability A2 of the gas separation membrane modules of the second gas separation membrane unit is 1:2 or more and 1:12 or less.

8. The gas separation system according to claim 1 , wherein the hollow fiber membrane of the second gas separation membrane unit satisfies formula (1).

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