Gas separation membrane unit, gas separation system and method for producing enriched gas

Abstract

To provide a gas separation membrane unit which enables gas separation with an excellent recovery rate of product gas and purity, and a gas separation system using the same.SOLUTION: A gas separation membrane unit concentrates and enriches at least one kind of gas contained in raw material mixed gas by supplying the raw material mixed gas thereto, wherein the gas separation membrane unit is constituted by combining a plurality of gas separation membrane modules in parallel, each of the plurality of gas separation membrane modules has a gas inlet, a non-permeation gas discharge port, and a permeation gas discharge port, the gas inlet, the non-permeation gas discharge port and the permeation gas discharge port of the gas separation membrane unit are constituted by using the gas inlet, the non-permeation gas discharge port and the permeation gas discharge port of each of the modules in common, and a coefficient of fluctuation of gas separation selectivity or gas permeation of the plurality of gas separation membrane modules is 0.01 or more and 0.49 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gas separation membrane unit consisting of a plurality of modules, a gas separation system using the same to separate a mixed gas, and a method for producing an enriched gas using the gas separation system. [Background technology]

[0002] Membrane separation, which utilizes the difference in the permeation rate of gases through a membrane, is known as a method for separating a mixed gas containing two or more different gases into individual gases. In this method, the target gas, a high-purity high-permeability gas and / or a high-purity low-permeability gas, can be obtained by recovering the permeable gas and / or non-permeable gas. The permeation rate, which is the permeation volume per unit membrane area, unit time, and unit partial pressure difference for each gas contained in the mixed gas, is expressed as P' (units: × 10 -5 cm 3 (STP) / cm 2 The gas separation selectivity of a membrane can be expressed as (permeation rate of high permeability gas / permeation rate of low permeability gas).

[0003] It has been described that a raw mixed gas is separated using a gas separation membrane unit having predetermined gas permeability and gas separation selectivity, in which gas separation membrane modules are combined in parallel (for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

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

[0005] In conventional techniques, including the case where gas separation performance is evaluated by simulation as described in Patent Document 2, variations in gas permeability and gas separation selectivity in modules of gas separation membrane units are not evaluated. However, in a gas separation membrane unit that combines multiple gas separation membrane modules, there are variations in gas permeability and gas separation selectivity between modules, and the inventors have found that conventional gas separation membrane units do not achieve the originally expected product gas purity and product gas recovery rate due to the variations in gas permeability and gas separation selectivity between modules. Therefore, an object of the present invention is to provide a gas separation membrane unit, a gas separation system, and a method for producing an enriched gas that can eliminate the drawbacks of the prior art. [Means for solving the problem]

[0006] The present invention provides the following configurations.

[0007] [1] A gas separation membrane unit to which a raw material mixed gas is supplied and which concentrates and enriches at least one gas contained in the raw material mixed gas, the gas separation membrane unit is formed by combining a plurality of gas separation membrane modules in parallel, each of the plurality of gas separation membrane modules having a gas inlet, a non-permeate gas outlet, and a permeate gas outlet, and the gas inlet, non-permeate gas outlet, and permeate gas outlet of each module are shared, respectively, to form the gas inlet, non-permeate gas outlet, and permeate gas outlet of the gas separation membrane unit; A gas separation membrane unit, wherein the coefficient of variation of gas separation selectivity or gas permeability of the plurality of gas separation membrane modules is 0.01 or more and 0.49 or less.

[0008] [2] The gas separation membrane unit according to [1], wherein the gas separation membrane constituting the gas separation membrane module is a hollow fiber membrane having an asymmetric structure made of a polymer. [3] The raw material mixed gas contains CH4 and CO2, the gas separation selectivity is the ratio of the permeation rates of CO to CH, P'CO / P'CH; The gas separation membrane unit according to [1] or [2], wherein the gas permeability is a CO2 permeation rate P'CO2. However, the unit of permeation rate is cm 3 (STP) / cm 2 ·sec·cmHg.

[0009] [4] A gas separation system that supplies a raw material mixed gas to a gas separation membrane unit and concentrates and enriches at least one gas contained in the raw material mixed gas, At least a first gas separation membrane unit and a second gas separation membrane unit are provided, a raw material mixed gas supply line connected to a gas inlet of the first gas separation membrane unit; a compression means interposed in the raw material mixed gas supply line; a first line connecting the non-permeate gas outlet of the first gas separation membrane unit and the gas inlet of the second gas separation membrane unit; a second line connecting a permeate gas outlet of the second gas separation membrane unit to a raw material mixed gas supply line; A gas separation system, wherein one or more units selected from a first gas separation membrane unit and a second gas separation membrane unit are the gas separation membrane unit according to any one of [1] to [3].

[0010] [5] A gas separation system that supplies a raw material mixed gas to a gas separation membrane unit and concentrates and enriches at least one gas contained in the raw material mixed gas, The membrane reactor includes a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit, a raw material mixed gas supply line connected to a gas inlet of the first gas separation membrane unit; a compression means interposed in the raw material mixed gas supply line; a first line connecting the non-permeate gas outlet of the first gas separation membrane unit and the gas inlet of the second gas separation membrane unit; a second line connecting the permeate gas outlet of the second gas separation membrane unit to the raw material mixed gas supply line; a third line connecting the permeate gas outlet of the first gas separation membrane unit and the gas inlet of the third gas separation membrane unit; a fourth line connecting a non-permeate gas outlet of the third gas separation membrane unit to the raw material mixed gas supply line; A gas separation system in which one or more units selected from a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit are the gas separation membrane units according to any one of [1] to [3].

[0011] [6] A method for producing an enriched gas, using the gas separation system according to [4] or [5], and extracting the enriched gas from the non-permeate gas outlet of the second gas separation membrane unit. [Effects of the Invention]

[0012] According to the present invention, there are provided a gas separation membrane unit capable of gas separation with excellent recovery rate and purity of product gas, and a gas separation system using the same. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a gas separation system according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the structure of an example of a gas separation membrane module used in the gas separation system of the present invention. [Figure 3] FIG. 3 is a schematic diagram showing the configuration of a gas separation system according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will now be described based on preferred embodiments thereof with reference to the drawings. The upper and lower limits of the numerical values ​​in this specification can be combined without any restrictions. 1 and 3 includes a first gas separation membrane unit 11 and a second gas separation membrane unit 12. The gas separation system shown in FIG.

[0015] As shown in FIG. 2, each of the gas separation membrane units 11, 12, and 13 can be, for example, a module 40 configured by housing a gas separation membrane 30, which is made of a hollow fiber membrane or the like and has selective gas permeability, in a casing 31. The gas separation membrane units 11, 12, and 13 shown in FIGS. 1 and 3 are, for example, configured by arranging a plurality of gas separation membrane modules 40, as shown in FIG. 2, in parallel. The casing 31 in the module 40 has openings 32 on two opposing sides. It should be noted that these openings 32 are for inserting the gas separation membrane 30 into the casing 31, but are not openings in the gas separation membrane 30 itself. The gas separation membrane 30 is housed in the casing 31 through these openings 32. When the gas separation membrane 30 is made of a bundle of hollow fiber membranes, the gas separation membrane 30 is housed in the casing 31 so that, in the housed state, each end of the hollow fiber membrane is open near each opening 32 in the casing 31.

[0016] A hollow fiber gas separation membrane module such as module 40 can be obtained, for example, by bundling preferably about 100 to 1,000,000 hollow fiber membranes (more preferably about 100 to 200,000 hollow fiber membranes) of an appropriate length, fixing both ends of the hollow fiber membrane bundle to a tube plate made of a thermosetting resin or the like so that at least one end of the hollow fibers remains open, and storing and attaching the resulting hollow fiber membrane element consisting of the hollow fiber membrane bundle and the tube plate in a container equipped with at least a mixed gas inlet, a permeate gas outlet, and a non-permeate gas outlet so that the space leading to the inside of the hollow fiber membranes is separated from the space leading to the outside of the hollow fiber membranes.

[0017] When the gas separation membrane 30 is housed in the casing 31, the gas separation membrane 30 is fixed to the inner wall of the casing 31 by tube sheets 33 and 34 at both ends in the Y direction, which is the direction in which the hollow fiber membranes extend. Each opening 32 of the casing 31 is closed by a lid 35 or 36. A gas inlet 37 is provided in the lid 35. Meanwhile, a non-permeate gas outlet 38 is provided in the lid 36. The mixed gas to be separated is introduced into the module through the gas inlet 37 of the lid 35. Of the introduced gases, the gas that permeates the gas separation membrane 30 is discharged to the outside of the module through a permeate gas outlet 39 provided in the casing 31. Meanwhile, the non-permeate gas that does not permeate the gas separation membrane 30 is discharged to the outside of the module through the non-permeate gas outlet 38 of the lid 36. As described above, the gas separation membrane unit 11 (or 12, 13) in this specification is formed by combining in parallel a plurality of gas separation membrane modules 40. Each of the plurality of gas separation membrane modules 40 has a gas inlet 37, a non-permeate gas outlet 38, and a permeate gas outlet 39, and the gas inlet 37, non-permeate gas outlet 38, and permeate gas outlet 39 of each module are shared, forming the gas inlet 11a (or 12a, 13a), non-permeate gas outlet 11b (or 12b, 13b), and permeate gas outlet 11c (or 12c, 13c) of the gas separation membrane unit. In some cases, a purge gas supply port (not shown) may be provided in the casing 31. Although the separation membrane module of Fig. 2 has been described above as an example, the present invention can naturally be applied to separation membrane modules of other configurations, such as a shell feed type module.

[0018] In the present invention, the flow rates of the modules may be the same or different. In the gas separation membrane system of the present invention, the flow rates of the modules constituting one unit are not controlled by valves or the like. When the total length of the longest module among the modules is L1 and the total length of the shortest module among the modules is L2, {(L1-L2) / L1} x 100 [unit: %] is preferably 10% or less, more preferably 5% or less, even more preferably 3% or less, and most preferably 1% or less. Furthermore, the ratio of the membrane area S1 of the module with the largest membrane area to the membrane area S2 of the module with the smallest membrane area among the modules is preferably S1:S2 = 1:1 to 5, more preferably 1:1 to 3, even more preferably 1:1 to 1.5, and most preferably 1:1 to 1.1. In this system, the non-permeate gas outlet 38 of each module is connected to a main pipe via a junction pipe (not shown) that joins the main pipe that connects to the gas inlet of another unit or to the outside of the system. For example, in FIG. 1 (described below), the non-permeate gas discharged from the non-permeate gas outlet 38 of each module of the first gas separation membrane unit 11 is joined in line 14 via a junction pipe (not shown) and supplied to the second gas separation membrane unit 12. Similarly, the non-permeate gas discharged from the non-permeate gas outlet 38 of each module of the second gas separation membrane unit 12 is joined in line 15 via a junction pipe (not shown) and supplied to the second gas separation membrane unit 12. Similarly, the permeate gas discharged from the permeate gas outlet 39 of each module is connected to a main pipe via a junction pipe (not shown) that joins with the main pipe that connects to the gas inlet of another unit or to the outside of the system. For example, in FIG. 1 (described later), the permeate gases discharged from the permeate gas outlet 39 of each module of the first gas separation membrane unit 11 are joined in line 18 via a junction pipe (not shown) and supplied to the outside of the system. Furthermore, the permeate gases discharged from the permeate gas outlet 39 of each module of the second gas separation membrane unit 12 are joined in line 17 via a junction pipe (not shown) and supplied to the suction side of compressor 21, and then supplied to the first gas separation membrane unit 11.

[0019] The gas separation membrane module used in the present invention is preferably made of hollow fiber membranes. In the manufacturing processes of such gas separation membrane modules, such as bundling a hollow fiber membrane bundle consisting of a large number of hollow fiber membranes and securing them with a tube sheet made of a thermosetting resin, contact between the bundled hollow fibers and bending, deformation, and fiber breakage of the hollow fibers themselves occur. Due to the complex effects of such differences in the state of the hollow fibers, even when the same number of hollow fibers are manufactured using the same process, the performance of the manufactured hollow fiber gas separation membrane modules often varies to a certain extent. Therefore, it is preferable that the gas separation membrane module used in the present invention be made of hollow fiber membranes, as this has excellent technical significance in keeping the coefficient of variation of the gas permeability and gas separation selectivity of the gas separation membrane module below a certain level.

[0020] Furthermore, in the gas separation membrane module used in the present invention, hollow fiber membranes made of polymers and having an asymmetric structure can be suitably used as the gas separation membrane. Hollow fiber membranes having an asymmetric structure have a skin layer and a porous layer. The skin layer refers to a layer that is thinner than the porous layer and is primarily responsible for gas separation performance. The skin layer is a much denser layer than the porous layer and is usually extremely thin, preferably having a thickness of 1 nm to 5 μm, more preferably 10 nm to 200 nm. The porous layer refers to a relatively thick porous layer that supports the skin layer and preferably has a thickness of 10 μm to 2000 μm, more preferably 20 μm to 200 μm. The diameter of the pores in the porous layer is not particularly limited, but is generally about 0.01 to 100 μm, more preferably 0.01 to 50 μm. Such hollow fiber membranes have a large effective surface area, high pressure resistance, and are excellent as gas separation membranes. Furthermore, as mentioned above, hollow fiber membranes made of polymers and having an asymmetric structure require that the matrices of the porous layer and skin layer be controlled on the nm and μm scale, for example, by the dry-wet phase inversion method described below. Furthermore, since the skin layer that contributes to gas separation performance is extremely thin, even if the manufacturing conditions are carefully adjusted using the same manufacturing method, differences will arise at least between manufacturing lots, such as the presence or absence of fine defects in the skin layer, and differences in the thickness and shape of the skin layer. In addition, since the purity of the polymer raw materials themselves, the amount of impurities contained, and the amount of water are not necessarily constant, various properties such as the viscosity of the polymer itself obtained by polymerization using these raw materials and its concentration relative to the solvent are almost never the same for each lot. Therefore, hollow fiber membranes having an asymmetric structure manufactured using polymers with different properties for each lot are prone to variations in their gas separation properties due to the complex influence of the above factors. Therefore, it is preferable that the gas separation membrane unit used in the present invention be composed of a gas separation membrane module manufactured from hollow fiber membranes having an asymmetric structure, because this has excellent technical significance in keeping the coefficient of variation of the gas permeability and gas separation selectivity of the gas separation membrane module below a certain level.

[0021] One method for keeping the coefficient of variation of a gas separation membrane module below a certain level is to minimize the variation in the gas separation selectivity and gas permeability of the manufactured hollow fiber membranes and gas separation membrane modules. For example, as described above, this can be achieved by controlling the purity and impurity content of the polymer raw materials used to manufacture the hollow fiber membranes to a certain level, or by carefully controlling the polymer polymerization conditions and the manufacturing conditions of the hollow fiber membranes and gas separation membrane modules. Another method involves measuring the gas separation selectivity and gas permeability of the manufactured module and selecting a combination that results in a coefficient of variation below a certain level from the measurement data. By simultaneously performing these methods, the coefficient of variation of the gas separation membrane unit can be more efficiently kept below a certain level.

[0022] The hollow fiber membrane preferably has an inner diameter of about 10 to 3000 μm, and more preferably 30 to 500 μm. The hollow fiber membrane preferably has an outer diameter of about 30 to 7000 μm, and more preferably 35 to 700 μm. The thicknesses of the skin layer and porous layer, the inner and outer diameters of the hollow fiber membrane, and the diameter of the pores can be measured using an optical microscope or an electron microscope.

[0023] The material of the gas separation membrane constituting the gas separation membrane module of the present invention is not particularly limited, but examples thereof include polyimide, polyamide, polysulfone, polyethersulfone, polyamideimide, polyetherimide, polycarbonate, etc. Among these, it is preferable that the gas separation membrane is made of polyimide in terms of durability, heat resistance, separation performance, etc.

[0024] When a polyimide is used as the gas separation membrane, it can be obtained by dehydrating and imidizing a tetracarboxylic acid component and a diamine component.

[0025] The tetracarboxylic acid component may be an aliphatic tetracarboxylic dianhydride or an aromatic tetracarboxylic dianhydride, while the diamine component may be an aliphatic diamine or an aromatic diamine.

[0026] Examples of the aliphatic tetracarboxylic dianhydride include cyclobutane-1,2,3,4-tetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, dicyclohexyl-3,3',4,4'-tetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic-1,2:4,5-dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, and bicyclo[2.2.2]oct-7-ene-2,3;5,6-tetracarboxylic dianhydride.

[0027] The aromatic tetracarboxylic dianhydride is preferably one having two to three aromatic rings, and examples thereof include 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride) (this compound is also called 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride), 2,3,3',4'-biphenyltetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, diphenylsulfonetetracarboxylic dianhydride, p-terphenyltetracarboxylic dianhydride, and m-terphenyltetracarboxylic dianhydride.

[0028] Examples of the aliphatic diamine include trans-1,4-diaminocyclohexane, cis-1,4-diaminocyclohexane, 1,6-hexamethylenediamine, 1,10-decamethylenediamine, 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, and polyoxypropylenediamines having a weight-average molecular weight of 500 or less.

[0029] Aromatic diamines include paraphenylenediamine, metaphenylenediamine, 4,4'-oxydianiline, 3,4'-oxydianiline, 4,4'-diaminodiphenylmethane, 2,4-toluenediamine, 3,3'-dihydroxy-4,4'-diaminobiphenyl, bis(4-amino-3-carboxyphenyl)methane, 2,4-diaminotoluene, 3,5-diaminobenzoic acid, and 3,7-diamino-2,8-dimethyldibenzothiophene=5,5-dioxide, which are the main components, and are also available as isomers with different methyl group positions: 3,7-diamino-2,6-dimethyldibenzothiophene=5,5-dioxide and 3,7-diamino-4,6-dimethyldibenzothiophene=5, 5-dioxide, 2,2',5,5'-tetrachlorobenzidine, 3,3',5,5'-tetrachlorobenzidine, 3,3'-dichlorobenzidine, 2,2'-dichlorobenzidine, 2,2',3,3',5,5'-hexachlorobenzidine, 2,2',5,5'-tetrabromobenzidine, 3,3',5,5'-tetrabromobenzidine, 3,3'-dibromobenzidine, 2,2'-dibromobenzidine, 2,2',3,3',5,5'-hexachlorobenzidine, 3,3'-diaminodiphenyl sulfone, 3,3'-diamino-4,4'-dimethyl-diphenyl sulfone, 3,3'-diamino-4,4'-diethyl-diphenyl sulfone, and the like.

[0030] In the present invention, when a polyimide is used as a material for the gas separation membrane constituting the gas separation membrane module, an aromatic polyimide may be used in particular. The aromatic polyimide can be obtained using an aromatic tetracarboxylic acid component and an aromatic diamine component.

[0031] <Suitable manufacturing method for gas separation membrane> For example, a suitable method for producing a gas separation membrane is to produce the gas separation membrane by a dry-wet phase inversion method using a polymer solution containing a polymer and a solvent.

[0032] As one embodiment of the method for preparing the polymer solution used in the present invention, a method for preparing a polyimide solution will be described. The polyimide solution is preferably prepared by a two-stage method in which a tetracarboxylic acid component and a diamine component are added to an organic polar solvent in a predetermined composition ratio, polymerized at a low temperature around room temperature to produce a polyamic acid, and then heated for thermal imidization or by adding pyridine or the like for chemical imidization; or a one-stage method in which a tetracarboxylic acid component and a diamine component are added to an organic polar solvent in a predetermined composition ratio, and polymerized and imidized at a high temperature around 100 to 250°C, preferably around 130 to 200°C. When the imidization reaction is carried out by heating, it is preferable to carry out the reaction while removing the water or alcohol that is eliminated.

[0033] Suitable examples of organic polar solvents include phenol-based solvents such as phenols, cresol, and xylenol; catechols such as catechol and resorcinol having two hydroxyl groups directly on the benzene ring; and halogenated phenols such as 3-chlorophenol, 4-chlorophenol (the same as parachlorophenol described below), 3-bromophenol, 4-bromophenol, and 2-chloro-5-hydroxytoluene; and amide-based solvents such as amides, such as N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, N,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetamide, and N,N-diethylacetamide; and mixed solvents thereof.

[0034] The polyimide solution obtained by polymerization-imidization can be directly used for spinning as described below. Alternatively, the polyimide solution obtained can be poured into a solvent insoluble in the polyimide to precipitate and isolate the polyimide, and then the polyimide can be dissolved in an organic polar solvent to a predetermined concentration to prepare an aromatic polyimide solution, which can then be used for spinning.

[0035] The hollow fiber membrane having the asymmetric structure can be suitably obtained by a dry-wet phase inversion method using a polymer solution. The dry-wet phase inversion method is a known method for forming a membrane by bringing a polymer solution into contact with a coagulation liquid to cause phase inversion. The dry-wet phase inversion method is a phase inversion method in which the solvent on the surface of a membrane-shaped polymer solution is evaporated to form a thin, dense layer, and then the membrane is immersed in a coagulation liquid (a solvent that is miscible with the solvent of the polymer solution but insoluble in the polymer), and the resulting phase separation phenomenon is utilized to form micropores and form a porous layer. This method was proposed by Loeb et al. (e.g., U.S. Patent No. 3,133,132).

[0036] The polymer contained in the polymer solution may be any of the various polymer materials listed above.

[0037] In the present invention, the solvent of the polymer solution used in the dry-wet phase inversion method preferably contains a solvent capable of dissolving the polymer and an aliphatic alcohol. The solvent capable of dissolving the polymer is preferably an organic polar solvent, and may be the organic polar solvent used when synthesizing the polymer by polymerizing the monomer. In one embodiment of the present invention, when the polymer is a polyimide, the organic polar solvent may be any organic polar solvent capable of dissolving the polyimide, and examples thereof include the organic polar solvents used in preparing the polyimide solution described above.

[0038] The aliphatic alcohol contained as a solvent in the polymer solution preferably has a boiling point of at least 60° C. The aliphatic alcohol may be a monohydric aliphatic alcohol or a dihydric or higher aliphatic polyhydric alcohol.

[0039] The monohydric aliphatic alcohol used as a solvent for the polymer solution is preferably acyclic, may be linear or branched, and has, for example, 1 to 7 carbon atoms. The aliphatic polyhydric alcohol used as a solvent for the polymer solution is preferably acyclic, may be linear or branched, and has, for example, preferably 2 to 10 carbon atoms, more preferably 3 to 8 carbon atoms.

[0040] As one embodiment of the present invention, a method for producing a hollow fiber membrane by a dry-wet phase inversion method using a polymer solution will be described below.

[0041] First, in the spinning step (spinning dope discharging step), the spinning nozzle used to discharge the spinning dope may be any nozzle capable of extruding the spinning dope into a hollow fiber, and a tube-in-orifice nozzle is suitable. The temperature range of the polymer solution (preferably a polyimide solution) during extrusion usually depends on the polymer and solvent type and viscosity contained in the dope, but is preferably about 20°C to 150°C, more preferably 30°C to 120°C. Spinning is performed while supplying a gas or liquid into the hollow fiber extruded from the nozzle.

[0042] In the coagulation process that follows the spinning process, the hollow fiber-shaped body discharged from the nozzle is first extruded into the air or an inert gas atmosphere such as nitrogen, and then introduced into a coagulation bath and immersed in a coagulation liquid. The coagulation liquid is preferably one that does not substantially dissolve the polymer component and is compatible with the solvent of the polymer component. While not particularly limited, preferred coagulation liquids include water, lower alcohols such as methanol, ethanol, and propyl alcohol, and ketones having lower alkyl groups such as acetone, diethyl ketone, and methyl ethyl ketone, as well as mixtures thereof. Furthermore, when the polymer solution is a polyimide solution and the solvent for the polyimide solution is an amide-based solvent, an aqueous solution of the amide-based solvent is also preferred.

[0043] The fibers obtained in the coagulation step are washed with a washing solvent such as ethanol if necessary, and then the coagulation liquid and / or washing solvent on the outside and inside of the hollow fibers are replaced with a replacement solvent, such as an aliphatic hydrocarbon such as isopentane, n-hexane, isooctane, n-heptane, etc. The hollow fibers are then dried at 100 to 200°C to obtain the hollow fibers.

[0044] As described above, in the present invention, when the gas separation membrane constituting the gas separation membrane module is a hollow fiber membrane made of a polymer and having an asymmetric structure, it is obtained through a multi-stage process of spinning from a polymer solution, as described above, and therefore there is significant variation in the thickness and permeability of the skin layer during production, and in the shape of the hollow fiber membrane, such as the thickness and length. For this reason, there is great technical significance in keeping the coefficient of variation of the gas permeability and gas separation selectivity of the gas separation membrane module at a certain level or less.

[0045] The gas separation membrane unit of the present invention is a combination of multiple gas separation membrane modules 40 described above arranged in parallel, and in this unit, the coefficient of variation of the gas permeability or gas separation selectivity of each gas separation membrane module 40 is 0.01 or more and 0.49 or less. When the coefficient of variation of the gas separation membrane module 40 is 0.49 or less, the recovery rate and purity of the resulting product gas can be increased. The reason why the present invention achieves the above effects is thought to be that it reduces variations in the supply gas flow rate flowing into each module in the same unit and equalizes the gas separation load in each module, thereby preventing a state in which too much or too little gas flows into some modules relative to the performance of the module, such as gas permeability or gas separation selectivity, and allowing the original permeation performance to be exhibited. In order to further enhance the above-mentioned effects, the coefficient of variation is preferably 0.45 or less, more preferably 0.40 or less, particularly preferably 0.35 or less, and even more preferably 0.30 or less.

[0046] Furthermore, the coefficient of variation is preferably 0.01 or more, more preferably 0.015 or more, since a further reduction in the coefficient of variation would not result in any further improvement in recovery rate or purity. The identification of such critical points has the advantage of reducing the manufacturing costs that would be incurred if gas separation membrane modules were manufactured with exact same specifications, such as by relaxing the need to change the temperature conditions during spinning, the solvent conditions used, and the type of manufacturing equipment used, thereby allowing for slight differences between lots and improving yields.

[0047] For example, in the case of gas permeability, the coefficient of variation refers to the value (S / AV) obtained by dividing the standard deviation value S by the average value AV of the measured gas permeability of each gas separation membrane module. Similarly, in the case of gas separation selectivity, the coefficient of variation refers to the value (S / AV) obtained by dividing the standard deviation value S by the average value AV of the measured gas separation selectivity of each gas separation membrane module.

[0048] Gas permeability refers to the gas permeation rate of highly permeable gas A. As mentioned above, the permeation rate, which is the permeation volume per unit membrane area, unit time, and unit partial pressure difference, is expressed as P' (unit: × 10 -5 cm 3 (STP) / cm 2 The gas separation selectivity of a membrane can be expressed as (permeation rate of high permeability gas A / permeation rate of low permeability gas B). In the unit of the present invention, each module has high permeability to gas A and low permeability to gas B. In the system of the present invention, each unit and each module constituting it usually has high permeability to gas A and low permeability to gas B. In the gas separation membrane unit of the present invention, it is preferable that the gas separation membrane module satisfy the above-mentioned ranges of coefficient of variation for both gas permeability and gas separation selectivity, in order to further increase the recovery rate and purity of the resulting gas.

[0049] In the present invention, it is preferable that the high permeability gas A is CO2 and the low permeability gas B is CH4, since the effect of adopting a predetermined coefficient of variation is high. In this case, in order to obtain a high-purity product gas (CH4) with a high recovery rate more efficiently, the gas separation selectivity (P') of each gas separation membrane module at 40°C is CO2 / P' CH4 ) is preferably 5 or more and 150 or less, more preferably 10 or more and 130 or less, even more preferably 15 or more and 120 or less, and particularly preferably 20 or more and 110 or less.

[0050] In the present invention, in order to obtain a high-purity product gas (CH4) with a high recovery rate more efficiently, the carbon dioxide permeation rate P' of each gas separation membrane module at 40°C is CO2 is 1.5 x 10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 275×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is preferable, and 2×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 100×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is more preferable, and 3×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 90×10 -5 cm 3 (STP) / cm 2 sec cmHg or less is more preferable, and 7×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 80×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is particularly preferred.

[0051] In the present invention, in order to obtain high-purity CH4 with a high recovery rate more efficiently, the CH4 permeation rate P' of each gas separation membrane module at 40°C is CH4 is 0.03 x 10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 3×10 -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.2.5×10 -5 cm3 (STP) / cm 2 ·sec·cmHg or less is more preferable, and 0.05×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 2.0×10 -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 1.5×10 -5 cm 3 (STP) / cm 2 It is particularly preferable that the RH is 1.5 sec.cmHg or less.

[0052] In the present invention, the gas permeability of a gas separation membrane module can be measured, for example, by the following method. Although examples are shown below for CH4 and CO2, other gases can also be measured by the same method. (Method for measuring gas permeability of CH4 and CO2) The gas permeability of a CH4 membrane is measured by measuring the flow rate of gas passing through a membrane with a specified membrane area at a temperature of 40°C when pure CH4 gas is pressurized at a specified pressure using a thin-film flowmeter. The gas permeability P' of CH4 through the membrane is calculated from the pressure, membrane area, and permeated gas flow rate. CH4 (Unit: × 10 -5 cm 3 (STP) / cm 2 sec cmHg). The gas permeability of a CO2 membrane is measured by using a thin-film flowmeter to measure the flow rate of gas passing through a membrane with a specified membrane area at a temperature of 40°C when pure CO2 gas is pressurized at a specified pressure. The gas permeability P' of CO2 through the membrane is calculated from the pressure, membrane area, and permeated gas flow rate. CO2 (Unit: × 10 -5 cm 3 (STP) / cm 2 sec cmHg).

[0053] During the separation operation of a gas containing CH4 and CO2, the gas flow rate and the concentrations of CH4 and CO2 in the gas were measured at the gas inlet, permeate gas outlet, and non-permeate gas outlet of each module in the same unit, and the CO2 permeability and gas separation selectivity (P' CO2 / P' CH4 ) may be calculated. For example, it can be calculated using formulas (1) to (20) described in THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING., VOLUME 90, 2011, pp. 1253-1268. The inner and outer diameters of the hollow fibers used in the calculation can be calculated by, for example, measuring the inner and outer diameters of any five hollow fiber membranes from a module using a measuring microscope and averaging the measured values. The inner diameter can be, for example, 50 to 800 μm, and the outer diameter can be, for example, 100 to 1000 μm.

[0054] As an exception, the coefficient of variation when separating gases containing CH4 and CO2 may be measured using a combination of N2 and O2 at the same temperature. For example, before CO2 / CH4 separation operation, the module is removed, compressed air is supplied, and the gas is separated into oxygen-rich gas and nitrogen-rich gas. The gas flow rates at the gas inlet, permeate gas outlet, and non-permeate gas outlet, as well as the concentrations of N2 and O2 in the gas, are measured, and the gas permeability and gas separation selectivity can be back-calculated from the results using a general formula such as a counterflow model. In this case, the coefficient of variation of the O2 gas permeation rate can be regarded as the coefficient of variation of the CO2 gas permeation rate, and the gas separation selectivity (P' O2 / P' N2 ) is calculated as the coefficient of variation of the gas separation selectivity (P' CO2 / P' CH4 ) can be considered as the coefficient of variation at the same temperature.

[0055] If the coefficient of variation of a gas separation membrane unit falls within the range of 0.01 to 0.49 by any of the above methods, the coefficient of variation of the module of the unit is deemed to fall within the specified range.

[0056] The number of gas separation membrane modules in the gas separation membrane unit of the present invention is preferably 2 to 100, more preferably 4 to 70. In addition, the gas separation membrane module has a membrane area of ​​0.1 to 700 m 2 It is preferable that the length is 1 to 500 m. 2 Furthermore, the length of the gas separation membrane module is preferably 0.1 to 4 m, more preferably 0.2 to 3 m. When the gas separation membrane module is made of hollow fiber membranes, the preferred length of the hollow fiber membranes is the same as the preferred length of the module described above.

[0057] Next, the gas separation system of the present invention will be further described. 1, as shown, a first gas separation membrane unit 11 and a second gas separation membrane unit 12 are connected in series. Specifically, the first gas separation membrane unit 11 and the second gas separation membrane unit 12 are connected by a first line 14 connecting a non-permeate gas outlet 11b of the first gas separation membrane unit 11 to a gas inlet 12a of the second gas separation membrane unit 12.

[0058] In the following description, the gas inlets, outlets, supplied gas, permeated gas, and non-permeated gas of the first gas separation membrane unit 11 and the second gas separation membrane unit 12 may be referred to as "first" and "second," respectively. The same applies to the configuration of the third gas separation membrane unit 13, which will be described later. In addition, "upstream" and "downstream" in the following description are based on the flow direction of the raw material mixed gas.

[0059] A raw material mixed gas supply line 26 is connected to the first gas inlet 11a of the first gas separation membrane unit 11 for supplying the raw material mixed gas from a raw material mixed gas source (not shown) to the first gas separation membrane unit 11. Compression means 21 is interposed and disposed midway along the raw material mixed gas supply line 26. The permeate gas outlet 12c of the second gas separation membrane unit 12 is connected to the raw material mixed gas supply line 26 by a second line 17. Specifically, the second line 17 connects the second permeate gas outlet 12c to a position on the suction side of the compression means 21 of the raw material mixed gas supply line 26.

[0060] Compression means 21 is installed 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 second permeable gas discharged from second gas separation membrane unit 12 when returning the second permeable gas to first gas separation membrane unit 11 through second line 17. A compressor can be used as compression means 21.

[0061] A recovery line 15 is connected to the non-permeate gas outlet 12b of the second gas separation membrane unit 12 for taking out the non-permeate gas in which gas B has been concentrated and enriched.

[0062] The operation of the gas separation system 10 of this embodiment having the above configuration will be described. A raw material mixed gas containing gas A and gas B to be separated is supplied from a raw material mixed gas source (not shown) through a raw material mixed gas supply line 26 to the first gas separation membrane unit 11. The raw material mixed gas is pressurized by the compression means 21, and its pressure increases.

[0063] When the raw material mixed gas pressurized by the compression means 21 is supplied to the first gas separation membrane unit 11, it is separated into a first permeable gas, which is the gas that has permeated the gas separation membrane, and a first non-permeable gas, which is the gas that has not permeated the gas separation membrane, due to the difference in permeation rate through the gas separation membrane.

[0064] The first non-permeate gas discharged from the first gas separation membrane unit 11 is a gas in which gas B is concentrated compared to the raw material mixed gas. The first non-permeate gas is discharged from the non-permeate gas outlet 11b of the first gas separation membrane unit 11 and supplied to the second gas separation membrane unit 12 through the first line 14. On the other hand, the first permeable gas discharged from the first gas separation membrane unit 11 is a gas in which gas A is concentrated compared to the raw material mixed gas. The first permeable gas is taken out of the system via a third line 18.

[0065] The first non-permeate gas discharged from the first gas separation membrane unit 11 is supplied to the second gas separation membrane unit 12, where it comes into contact with the gas separation membrane of the second gas separation membrane unit 12 and is separated into a second permeate gas and a second non-permeate gas by the gas separation membrane of the second gas separation membrane unit 12. The second non-permeate gas discharged from the second gas separation membrane unit 12 has been further concentrated and enriched in gas B. The second non-permeate gas is taken out of the system from the second non-permeate gas outlet 12b of the second gas separation membrane unit 12 through a recovery line 15 as an enriched gas enriched in gas B. Meanwhile, the second permeable gas is discharged from the second permeable gas outlet 12c of the second gas separation membrane unit 12 and returned to the suction side of the compression means 21 in the raw material mixed gas supply line 26 via the second line 17 connected to the outlet 12c.

[0066] The second permeable gas discharged from the second gas separation membrane unit 12 and returned via the second line 17 is mixed with the raw material mixed gas, and then compressed by the compression means 21 .

[0067] In this embodiment, the gas separation membrane unit of the present invention is used for either or both of the first gas separation membrane unit 11 and the second gas separation membrane unit. This provides excellent improvements in purity and recovery rate. In the present invention, an advantage of using a unit with a specific coefficient of variation in a system in which a product gas is extracted as a second non-permeate gas in two or more stages is that it prevents the variation in the supply gas flow rate flowing into each module from increasing as multiple units are used. To particularly enhance this effect, it is preferable that at least the first gas separation membrane unit 11 of the first gas separation membrane unit 11 and the second gas separation membrane unit 12 be a gas separation membrane unit of the present invention. Furthermore, for example, the coefficient of variation of gas separation selectivity or gas permeability may be the same or different between the first gas separation membrane unit 11 and the second gas separation membrane unit 12, but if they are different, it is preferable that the coefficient of variation of the first gas separation membrane unit 11 is lower than the coefficient of variation of the second gas separation membrane unit 12.

[0068] Next, a gas separation system 10' according to a second embodiment of the present invention will be described with reference to Fig. 3. In the description of the second embodiment, the same components as those in the first embodiment will be assigned the same reference numerals and description thereof will be omitted, and differences from the first embodiment will be mainly described.

[0069] 3, a first gas separation membrane unit 11 and a 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 to the gas inlet 13a of the third gas separation membrane unit 13 via a third line 18. The gas separation membrane module constituting the third gas separation membrane unit 13 can be the same as that of the first gas separation membrane unit 11 and the second gas separation membrane unit 12.

[0070] In the third gas separation membrane unit 13, the non-permeate gas discharge port 13b is connected to the raw mixed gas supply line 26 by a fourth line 41. In the embodiment shown in FIG. 3, the fourth line 41 is connected to the raw mixed gas supply line 26 at a position on the suction side of the compression means 21. In the example shown in FIG. 3, a third permeate gas discharge line 19 is connected to the permeate gas discharge port 13c of the third gas separation membrane unit 13.

[0071] The compression means 21 is installed for the purpose of pressurizing the raw material mixed gas supplied from the gas source, the second permeable gas returned from the second gas separation membrane unit 12, and the third non-permeable gas returned from the third gas separation membrane unit 13.

[0072] The gas path during operation for gas separation in the gas separation system 10' of this embodiment having the above configuration will be described with reference to Figure 3. The raw mixed gas to be separated is supplied from a mixed gas source (not shown) through a raw mixed gas supply line 26 to the first gas separation membrane unit 11. Prior to supply, the raw mixed gas is pressurized by compression means 21, and its pressure increases.

[0073] As in the first embodiment, when the raw mixed gas pressurized by the compression means 21 is supplied to the first gas separation membrane unit 11, it is separated into a first permeate gas and a first non-permeate gas. The first permeate gas discharged from the first gas separation membrane unit 11 is supplied to the third gas separation membrane unit 13 via a third line 18. The first permeate gas introduced into the third gas separation membrane unit 13 is separated into a third permeate gas and a third non-permeate gas by the third gas separation membrane unit 13. The third permeate gas is more concentrated and enriched in Gas A than the first permeate gas introduced into the third gas separation membrane unit 13, and is taken out of the system from the permeate gas outlet 13c of the third gas separation membrane unit 13 through a third permeate gas outlet line 19. Meanwhile, the third non-permeate gas is discharged from the non-permeate gas outlet 13b of the third gas separation membrane unit 13 and returned to the suction side of the compression means 21 in the raw mixed gas supply line 26 via a fourth line 41 connected to the outlet 13b. The third non-permeate gas returned through line 41 is mixed with the raw material mixed gas and then compressed by compression means 21 .

[0074] The third line 18 may or may not include a second compression means for compressing the first permeate gas and sending it to the third gas separation membrane unit.

[0075] In the configurations of FIGS. 1 and 3, the pressure of the compression means 21 is preferably 0.2 MPaG or more and 3.0 MPaG or less, more preferably 0.3 MPaG or more and 2.4 MPaG or less, as the pressure of the gas supplied to the first gas separation membrane unit 11.

[0076] The pressure of the gas supplied to the third gas separation membrane unit 13 is preferably 0.05 MPaG or more and 1.2 MPaG or less, and more preferably 0.1 MPaG or more and 1.0 MPaG or less.

[0077] The operating temperature of each separation membrane unit 11, 12, 13 (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.

[0078] In the embodiment shown in Figure 3, the units of the present invention are used in one or more, two or more, or all three of the first gas separation membrane unit 11 to the third gas separation membrane unit 13. This provides an excellent effect of improving purity and recovery rate. To particularly enhance this effect, it is preferable that at least one of the first gas separation membrane unit 11 and the second gas separation membrane unit 12 be a gas separation membrane unit of the present invention. For example, the coefficients of variation of gas separation selectivity or gas permeability may be the same or different for the first gas separation membrane unit 11, the second gas separation membrane unit 12, and the third gas separation membrane unit, but if they are different, it is preferable that the coefficient of variation of the first gas separation membrane unit 11 and / or the second gas separation membrane unit 12 is lower than the coefficient of variation of the third gas separation membrane unit 13, and it is more preferable that the coefficients of variation of both the first gas separation membrane unit 11 and the second gas separation membrane unit 12 are lower than the coefficient of variation of the third gas separation membrane unit 13.

[0079] The raw material mixed gas used in the present invention is preferably a gas containing at least CO2 and CH4, as this is highly effective in keeping the coefficient of variation in the present invention below a specific value. Suitable examples include biogas, landfill gas, and natural gas. Biogas is a gas generated when a biomass raw material is brought into contact with microorganisms under anaerobic conditions and subjected to fermentation treatment such as microbial CH4 fermentation. Examples of biomass raw materials include organic matter such as food waste, agricultural residues, sewage sludge, and livestock waste. Landfill gas refers to gas generated by microbial decomposition of organic matter in waste landfills. Biogas and landfill gas are usually composed mainly of CO2 and CH4.

[0080] In the present invention, when the raw material mixed gas contains CH4, it preferably contains 30 mol% or more of CH4, and particularly preferably 40 to 95 mol%. Furthermore, in the present invention, when the raw material mixed gas contains CO2, it is preferable that it contains 3 to 70 mol% of CO2, and particularly preferably 5 to 60 mol% of CO2, in view of the high technical significance of the gas separation system of the present invention.

[0081] Although the present invention has been described above based on its preferred embodiments, the present invention is not limited to these embodiments. For example, in addition to the compression means in the above embodiments, a pressure reduction means may be provided on the permeation side of one or two of the gas separation membrane units to provide the mixed gas supplied to each gas separation membrane unit with power to pass through the separation membrane. A known vacuum pump or the like can be used as such a pressure reduction means. [Example]

[0082] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to these examples. This test was a model experiment to show the variations in gas separation selectivity and gas permeability, and was obtained by simulation according to the above-mentioned method using the model described in THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING., VOLUME 90, 2011, pp. 1253-1268.

[0083] The gas permeability and gas separation selectivity ratios of the gas separation membrane modules mentioned below are all values ​​measured at 40° C. The membrane area of ​​each gas separation membrane module used in the examples and comparative examples was 95 m 2 and the overall length of the modules was the same.

[0084] <Gas separation membrane module> The gas separation properties at 40°C of gas separation membrane module A, which was used as the reference in the examples and comparative examples, are shown in Table 1. This gas separation membrane module was assumed to be one in which a gas separation membrane made of an aromatic polyimide hollow fiber membrane was housed in a case. P' in Table 1 CO2 and P' CH4 The unit is x 10 -5 cm 3 (STP) / cm 2 ·sec·cmHg.

[0085] [Table 1]

[0086] [Comparative Examples 1 to 5, Examples 1 to 11] The first gas separation membrane unit 11 and the second gas separation membrane unit 12 were each constructed by combining four gas separation membrane modules in parallel. The gas permeability (CO2 permeation rate (P'CO2)) of each gas separation membrane module at 40°C was the relative value shown in Table 2 relative to the gas permeability (CO2 permeation rate) of the reference gas separation membrane module A at the same temperature. The gas separation selectivity (P'CO2 / P'CH4) shown in Table 2 is the average gas separation selectivity (P'CO2 / P'CH4) of the four gas separation membrane modules at 40°C divided by the gas separation selectivity (P'CO2 / P'CH4) of gas separation membrane module A at 40°C. A simulation was conducted in which the first gas separation membrane unit 11 and the second gas separation membrane unit 12 were configured in the system shown in Figure 1 and operated under the following conditions and the conditions in Table 2. Note that the flow rate of the raw material mixed gas below refers to the flow rate of the raw material mixed gas flowing into the system. The operating pressure refers to the pressure of the gas flowing into the first gas separation membrane unit 11. The operating temperature refers to the temperature of the gas separation membrane in each gas separation membrane unit. The CO2 concentration in the product gas and the CH4 recovery rate are shown in Table 2. (Terms and Conditions) Flow rate of raw material mixed gas entering the system (F0): 320Nm 3 / h Composition of raw material mixed gas: CO2 40 mol%, CH4 60 mol%, The pressure of the gas flowing into the first gas separation membrane unit 11 (operating pressure) is 1.0 MPaG. Operating temperature of the first gas separation membrane unit 11: 40°C Operating temperature of the second gas separation membrane unit 12: 40°C

[0087] [Table 2]

[0088] As can be seen from the comparison with each example and comparison examples 1, 3 and 4 in Table 2, when the coefficient of variation of gas permeability of the module of the first gas separation membrane unit 11 or the second gas separation membrane unit 12 is 0.49 or less, the CO2 concentration in the product is less than 4.0 mol%, and the purity can be effectively improved. Furthermore, as shown by the comparison between Example 3 and Comparative Example 2, and the comparison between Example 13 and Comparative Example 5, once the coefficient of variation decreases to 0.01, further reductions do not result in a change in the CO2 concentration or an increase in the CH4 recovery rate. Therefore, by setting the coefficient of variation to 0.01 or more, the uniformity of the manufacturing conditions can be relaxed compared to Comparative Examples 2 and 5, and the manufacturing cost can be reduced.

[0089] [Comparative Examples 6 and 7, Examples 14 and 15] The first gas separation membrane unit 11 and the second gas separation membrane unit 12 were each constructed by combining four gas separation membrane modules in parallel. The four gas separation membrane modules had gas separation selectivities (P'CO2 / P'CH4) relative to the reference gas separation membrane module A, as shown in Table 3. The gas permeabilities (CO2 permeation rates) shown in Table 3 are values ​​obtained by dividing the average gas permeabilities (CO2 permeation rates) of the four gas separation membrane modules at 40°C by the gas permeability (CO2 permeation rate) of gas separation membrane module A at 40°C. A simulation was performed in which the first gas separation membrane unit 11 and the second gas separation membrane unit 12 were configured in the system shown in Figure 1 and operated under the conditions in Table 3. Table 3 shows the CO2 concentration in the product gas and the CH4 recovery rate.

[0090] [Table 3]

[0091] [Comparative Examples 8 and 9, Example 16] The first gas separation membrane unit 11, the second gas separation membrane unit 12, and the third gas separation membrane unit were each constructed by combining four gas separation membrane modules in parallel. The gas permeabilities (CO2 permeation rates) of the four gas separation membrane modules at 40°C were the relative values ​​shown in Table 4 relative to the values ​​at 40°C of the reference gas separation membrane module A. The gas separation selectivity (P'CO2 / P'CH4) shown in Table 4 is the average value of the values ​​at 40°C of the four gas separation membrane modules divided by the gas separation selectivity (P'CO2 / P'CH4) of gas separation membrane module A at 40°C. A simulation was performed in which the first gas separation membrane unit 11, the second gas separation membrane unit 12, and the third gas separation membrane unit were configured into the system shown in Figure 3 and operated under the conditions shown in Table 4. The CO2 concentration in the product gas and the CH4 recovery rate are shown in Table 4. (Terms and Conditions) Flow rate of raw material mixed gas entering the system (F0): 320Nm 3 / h Composition of raw material mixed gas: CO2 40 mol%, CH4 60 mol%, The pressure of the gas flowing into the first gas separation membrane unit 11 (operating pressure) is 1.26 MPaG. The pressure of the gas flowing into the third gas separation membrane unit 13 (operating pressure) is 0.20 MPaG. Operating temperature of the first gas separation membrane unit 11: 40°C Operating temperature of the second gas separation membrane unit 12: 40°C Operating temperature of the third gas separation membrane unit 13: 40°C

[0092] [Table 4]

[0093] [Comparative Examples 10 and 11, Example 17] The first gas separation membrane unit 11, the second gas separation membrane unit 12, and the third gas separation membrane unit were each constructed by combining four gas separation membrane modules in parallel. The gas permeabilities (CO2 permeation rates) of the four gas separation membrane modules at 40°C were the ratios shown in Table 5 relative to the value at 40°C of the reference gas separation membrane module A. The gas separation selectivity (P'CO2 / P'CH4) shown in Table 5 is the ratio of the average value of the four gas separation membrane modules at 40°C to the value at 40°C of the reference gas separation membrane module A. A simulation was performed in which the system shown in Figure 3 was configured with the first gas separation membrane unit 11, the second gas separation membrane unit 12, and the third gas separation membrane unit, and operated under the conditions below and those shown in Table 5. The CO2 concentration in the product gas and the CH4 recovery rate are shown in Table 5. (Terms and Conditions) Flow rate of raw material mixed gas entering the system (F0): 320Nm 3 / h Composition of raw material mixed gas: CO2 40 mol%, CH4 60 mol%, The pressure of the gas flowing into the first gas separation membrane unit 11 (operating pressure) is 1.26 MPaG. The pressure of the gas flowing into the third gas separation membrane unit 13 (operating pressure) is 0.20 MPaG. Operating temperature of the first gas separation membrane unit 11: 40°C Operating temperature of the second gas separation membrane unit 12: 40°C Operating temperature of the third gas separation membrane unit 13: 40°C

[0094] [Table 5] [Explanation of symbols]

[0095] 10, 10' Gas Separation System 11 First gas separation membrane unit 11a Gas inlet 11b Non-permeable gas outlet 11c Permeation gas outlet 12 Second gas separation membrane unit 12a Gas inlet 12b Non-permeable gas outlet 12c Permeation gas outlet 13 Third gas separation membrane unit 13a Gas inlet 13b Non-permeable gas outlet 13c Permeation gas outlet

Claims

1. A gas separation membrane unit to which a raw material mixed gas is supplied and which concentrates and enriches at least one gas contained in the raw material mixed gas, the gas separation membrane unit is formed by combining a plurality of gas separation membrane modules in parallel, each of the plurality of gas separation membrane modules having a gas inlet, a non-permeate gas outlet, and a permeate gas outlet, and the gas inlet, non-permeate gas outlet, and permeate gas outlet of each module are shared, respectively, to form the gas inlet, non-permeate gas outlet, and permeate gas outlet of the gas separation membrane unit; A gas separation membrane unit, wherein the coefficient of variation of gas separation selectivity or gas permeability of the plurality of gas separation membrane modules is 0.01 or more and 0.49 or less.

2. 2. The gas separation membrane unit according to claim 1, wherein the gas separation membrane constituting the gas separation membrane module is a hollow fiber membrane made of a polymer and having an asymmetric structure.

3. The raw material mixed gas is CH 4 and CO ２ Contains The gas separation selectivity is CH 4 CO ２ Permeation rate ratio P'CO ２ / P'CH ４ and The gas permeability is ２ Permeation rate P'CO ２ The gas separation membrane unit according to claim 1 or 2, However, the unit of permeation rate is cm ３ (STP) / cm ２ sec cmHg.

4. A gas separation system that supplies a raw material mixed gas to a gas separation membrane unit and concentrates and enriches at least one gas contained in the raw material mixed gas, at least a first gas separation membrane unit and a second gas separation membrane unit; a raw material mixed gas supply line connected to a gas inlet of the first gas separation membrane unit; a compression means interposed in the raw material mixed gas supply line; a first line connecting a non-permeate gas outlet of the first gas separation membrane unit to a gas inlet of the second gas separation membrane unit; a second line connecting a permeated gas outlet of the second gas separation membrane unit to the raw material mixed gas supply line, A gas separation system, wherein one or more units selected from the first gas separation membrane unit and the second gas separation membrane unit are the gas separation membrane unit according to claim 1.

5. A gas separation system that supplies a raw material mixed gas to a gas separation membrane unit and concentrates and enriches at least one gas contained in the raw material mixed gas, a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit; a raw material mixed gas supply line connected to a gas inlet of the first gas separation membrane unit; a compression means interposed in the raw material mixed gas supply line; a first line connecting a non-permeate gas outlet of the first gas separation membrane unit to a gas inlet of the second gas separation membrane unit; a second line connecting the permeate gas outlet of the second gas separation membrane unit to the raw material mixed gas supply line; a third line connecting the permeate gas outlet of the first gas separation membrane unit and the gas inlet of the third gas separation membrane unit; a fourth line connecting a non-permeate gas outlet of the third gas separation membrane unit to the raw material mixed gas supply line; A gas separation system, wherein one or more units selected from a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit are the gas separation membrane unit according to claim 1.

6. A method for producing an enriched gas, comprising using the gas separation system according to claim 4 or 5, and extracting the enriched gas from the non-permeate gas outlet of the second gas separation membrane unit.

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