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

Abstract

Provided are: a gas separation membrane unit capable of performing gas separation that exhibits excellent recovery rate and purity of a product gas; and a gas separation system using the same. The gas separation membrane unit is supplied with a raw material mixture gas, and concentrates and enriches at least one of the gases included in the raw material mixture gas. The gas separation membrane unit is formed by combining, in parallel, a plurality of gas separation membrane modules. Each of the plurality of gas separation membrane modules has a gas inlet, an impermeable gas outlet, and a permeable gas outlet. The gas inlet, the impermeable gas outlet, and the permeable gas outlet of each of the modules are shared, thereby constituting the gas inlet, the impermeable gas outlet, and the permeable gas outlet of the gas separation membrane unit. The variation coefficient of gas separation selectivity or gas permeability of the plurality of gas separation membrane modules is 0.01-0.49.

Description

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

[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.

[0002] As a method for separating a mixed gas containing two or more different gases into individual gases, a membrane separation method utilizing the difference in the permeation rate of the gases through a membrane is known. 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 the non-permeable gas. The permeation rate, which is the permeation volume per unit membrane area, unit time, and unit partial pressure difference of each gas contained in the mixed gas through the membrane, 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 permeable gas / permeation rate of low permeable gas).

[0003] It is disclosed 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).

[0004] Patent Document 1: JP 2013-128868 A, US Patent Application Publication No. 2020 / 0316516

[0005] Prior art, including the case of evaluating gas separation performance by simulation as described in Patent Document 2, has not evaluated the variations in gas permeability and gas separation selectivity among multiple gas separation membrane modules in a gas separation membrane unit. However, in reality, in a gas separation membrane unit combining multiple gas separation membrane modules, there are variations in gas permeability and gas separation selectivity among the multiple gas separation membrane modules. The inventors have found that conventional gas separation membrane units do not achieve the expected product gas purity and / or product gas recovery rate due to variations in gas permeability and gas separation selectivity among the multiple gas separation membrane 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 overcome the drawbacks of the prior art and can achieve the expected product gas purity and product gas recovery rate.

[0006] The present invention provides the following configurations.

[0007] [1] A gas separation membrane unit that receives a raw material mixed gas and concentrates and enriches at least one gas contained in the raw material mixed gas, wherein the gas separation membrane unit is composed of a plurality of gas separation membrane modules combined 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, thereby constituting the gas inlet, non-permeate gas outlet, and permeate gas outlet of the gas separation membrane unit, and 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 gas separation membrane unit according to claim 1 or 2, which is the following (1) or (2): (1) The raw material mixed gas is a mixture of methane (CH4) and carbon dioxide (CO ２ ) wherein the gas separation selectivity is CO relative to CH ２Permeation rate ratio P'CO ２ / P'CH ４ and the gas permeability is ２ Permeation rate P'CO ２ (2) The raw material mixed gas is nitrogen (N ２ ) and oxygen (O ２ ) and the gas separation selectivity is N ２ O against ２ Permeation rate ratio P'O ２ / P'N ２ and the gas permeability is O ２ Permeation rate P'O ２ The gas separation membrane unit according to claim 1 or 2, wherein the permeation rate is expressed in units of 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, the gas separation system comprising: at least a first gas separation membrane unit; a raw material mixed gas supply line connected to a gas inlet of the first gas separation membrane unit; and a compression means disposed in the raw material mixed gas supply line, wherein the first gas separation membrane unit is the gas separation membrane unit according to any one of [1] to [3].

[0010] [5] A gas separation system for supplying a raw material mixed gas to a gas separation membrane unit and concentrating and enriching at least one gas contained in the raw material mixed gas, the gas separation system comprising 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, and a second line connecting a permeate gas outlet of the second gas separation membrane unit to the raw material mixed gas supply line, 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 units according to any one of [1] to [3].

[0011] [6] A gas separation system for supplying a raw material mixed gas to a gas separation membrane unit to concentrate and enrich at least one gas contained in the raw material mixed gas, comprising 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 the 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 to 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 to the gas inlet of the third gas separation membrane unit; and a fourth line connecting the 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 any one of [1] to [3].

[0012] [7] A method for producing an enriched gas, using the gas separation system according to [5] or [6], and extracting an enriched gas from the non-permeate gas outlet of the second gas separation membrane unit.

[0013] Fig. 1 is a schematic diagram showing the configuration of a gas separation system in a first embodiment of the present invention. 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. Fig. 3 is a schematic diagram showing an example of a method for connecting gas separation membrane modules used in the gas separation system of the present invention. Fig. 4 is a schematic diagram showing the configuration of a gas separation system in a second embodiment of the present invention. Fig. 5 is a schematic diagram showing the configuration of a gas separation system in a third embodiment of the present invention.

[0014] The present invention will be described below based on preferred embodiments with reference to the drawings. In this specification, the term "expected product gas purity and product gas recovery rate" refers to, for example, the purity and recovery rate calculated by simulation assuming that the coefficient of variation of the gas permeation rate or gas separation selectivity of each gas separation membrane module constituting a gas separation membrane unit is 0, or a purity and recovery rate close to that. The upper and lower limits of the numerical values ​​in this specification can be combined without any restrictions. In this specification, the expressions "connecting A and B by line C" or "line C connecting A and B" include both cases where A and B and line C are separate components, and cases where line C is a single component continuous with A and / or C. Similarly, the expression "connecting line E to D" includes both cases where D and line E are separate components, and cases where D and line E are a single component. Each gas separation system shown in FIGS. 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. 4 further includes a third gas separation membrane unit 13. The gas separation system shown in FIG. 5 includes a first gas separation membrane unit 11 .

[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 4 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, and 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 (more preferably about 100 to 200,000) hollow fiber membranes of an appropriate length, fixing both ends of the hollow fiber membrane bundle with a tube sheet 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 sheet 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 communicating with the inside of the hollow fiber membranes is separated from the space communicating with 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 plates 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. The lid 35 is provided with a gas inlet 37. Meanwhile, the lid 36 is provided with a non-permeate gas outlet 38. 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 outside 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 outside the module through the non-permeate gas outlet 38 of the lid 36. As described above, the gas separation membrane unit 11 (or 12 or 13) in this specification is formed by combining multiple gas separation membrane modules 40 in parallel. Each of the gas separation membrane modules 40 has a gas inlet 37, a non-permeate gas outlet 38, and a permeate gas outlet 39. The gas inlet 37, the non-permeate gas outlet 38, and the permeate gas outlet 39 of each module are shared, forming the gas inlet 11a (or 12a or 13a), the non-permeate gas outlet 11b (or 12b or 13b), and the permeate gas outlet 11c (or 12c or 13c) of the gas separation membrane unit. In some cases, a purge gas supply port (not shown) may be provided in the casing 31. While the separation membrane module of FIG. 2 has been used as an example, the present invention can of course be applied to separation membrane modules of other configurations, such as shell-feed modules.

[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 a unit are not controlled by valves or the like. When the total length of the longest module constituting a unit is L1 and the total length of the shortest module 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 S1:S2 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 constituting a unit 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.

[0019] FIG. 3 is a schematic diagram illustrating a gas separation membrane unit in which gas separation membrane modules are connected in parallel. For example, the gas inlet 37 of each module is connected to a main pipe via a distribution pipe dp, which connects to the gas outlet of another unit or to the outside of the system. Furthermore, the non-permeate gas outlet 38 of each module constituting one unit is connected to the main pipe via a confluence pipe that connects to the gas inlet of another unit or to the outside of the system. For example, in the system of FIG. 1 described below, as shown in FIG. 3, non-permeate gases discharged from the non-permeate gas outlets 38 of each module of the first gas separation membrane unit 11 are joined in line 14 via a confluence pipe cp1 and supplied to the second gas separation membrane unit 12. Similarly, non-permeate gases discharged from the non-permeate gas outlets 38 of each module of the second gas separation membrane unit 12 are joined in line 15 via a confluence 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 constituting one unit is connected to the 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 the embodiment shown in FIG. 1 (described below), as shown in FIG. 3 , 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 junction pipe cp2 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), supplied to the suction side of compressor 21, compressed, and supplied to the first gas separation membrane unit 11.

[0020] The gas separation membrane module used in the present invention preferably comprises hollow fiber membranes. In the manufacturing process of such a gas separation membrane module, a hollow fiber membrane bundle consisting of a large number of hollow fiber membranes is bundled and fixed with a tube sheet made of a thermosetting resin. During this process, the bundled hollow fibers come into contact with each other and / or the hollow fibers themselves may bend, deform, or break. Due to the complex effects of these 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 module often varies to a certain extent. Therefore, it is preferable that the gas separation membrane module used in the present invention comprises hollow fiber membranes, because this has the technical significance of keeping the coefficient of variation of the gas permeability and / or gas separation selectivity of the gas separation membrane module below a certain level.

[0021] 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 thinner than the porous layer that 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, preferably having 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 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 the matrix of the porous layer and skin layer to be controlled on the nm and μm scale, for example, by the dry-wet phase inversion method described below. Furthermore, because the skin layer, which contributes to gas separation performance, is extremely thin, differences in, for example, the presence or absence of fine defects in the skin layer, the thickness and shape of the skin layer, etc., will occur at least between production lots, even if the production conditions are carefully adjusted using the same production method. In addition, the purity and / or impurity and water content of the polymer raw materials themselves are not necessarily constant, so various properties, such as the viscosity of the polymer itself obtained by polymerization using these raw materials or its concentration relative to the solvent, are almost never identical between lots. Therefore, hollow fiber membranes having an asymmetric structure manufactured using polymers with different properties between lots 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 / or gas separation selectivity of the gas separation membrane module below a certain level.

[0022] One method for keeping the coefficient of variation of a gas separation membrane module at or 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 / or impurity content of the polymer raw materials used to manufacture the hollow fiber membranes to a certain level and / or by carefully controlling the polymer polymerization conditions and the manufacturing conditions for 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 at or 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 at or below a certain level.

[0023] The hollow fiber membrane preferably has an inner diameter of about 10 to 3,000 μm, and more preferably 30 to 500 μm. The hollow fiber membrane preferably has an outer diameter of about 30 to 7,000 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] The aromatic tetracarboxylic acid dianhydride is preferably one having two to three aromatic rings, and examples thereof include 3,3',4,4'-biphenyltetracarboxylic acid 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 acid dianhydride, 2,2',3,3'-biphenyltetracarboxylic acid dianhydride, pyromellitic dianhydride, benzophenonetetracarboxylic acid dianhydride, 4,4'-oxydiphthalic dianhydride, diphenylsulfonetetracarboxylic acid dianhydride, p-terphenyltetracarboxylic acid dianhydride, and m-terphenyltetracarboxylic acid dianhydride.

[0029] 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.

[0030] Examples of 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 as the main component, with isomers having 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.

[0031] 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.

[0032] <Suitable Method for Producing Gas Separation Membrane> For example, a suitable method for producing a gas separation membrane is a method in which a gas separation membrane is produced by a dry-wet phase inversion method using a polymer solution containing a polymer and a solvent.

[0033] 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.

[0034] Suitable examples of organic polar solvents include phenol-based solvents such as phenols such as phenol, 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.

[0035] 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.

[0036] 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 to form a porous layer. This method was proposed by Loeb et al. (e.g., U.S. Patent No. 3,133,132).

[0037] The polymer contained in the polymer solution may be any of the various polymer materials listed above.

[0038] 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 of the organic polar solvent used in preparing the polyimide solution described above can be mentioned.

[0039] The aliphatic alcohol contained as a solvent in the polymer solution preferably has a boiling point of 60° C. or higher. The aliphatic alcohol may be a monohydric aliphatic alcohol or a dihydric or higher aliphatic polyhydric alcohol.

[0040] 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.

[0041] 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.

[0042] 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 type and viscosity of the polymer and solvent 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 to the inside of the hollow fiber extruded from the nozzle.

[0043] In the coagulation process that follows the spinning process, the hollow fiber-shaped body discharged from the nozzle is once exposed to the atmosphere or N ２ The polymer is then extruded into an inert gas atmosphere such as ethanol, and subsequently introduced into a coagulation bath where it is immersed in the 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 ethanol and propyl alcohol; ketones having lower alkyl groups such as acetone, diethyl ketone, and methyl ethyl ketone; and mixtures thereof. Furthermore, when the polymer solution is a polyimide solution and the solvent of the polyimide solution is an amide-based solvent, an aqueous solution of the amide-based solvent is also preferred.

[0044] 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, for example, 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.

[0045] 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, as described above, it is obtained through a multi-stage process of spinning from a polymer solution, and therefore there is significant variation in the thickness and / or permeability of the skin layer during production, and in the shape, such as the thickness and / or length, of the hollow fiber membrane. Therefore, there is great technical significance in keeping the coefficient of variation of the gas permeability and / or gas separation selectivity of the gas separation membrane module at a certain level or less. The present invention can be realized by selecting a module to be used so that the coefficient of variation is at a certain level or less.

[0046] 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 to 0.49. Having the coefficient of variation of the gas separation membrane module 40 be 0.49 or less can improve the recovery rate and purity of the resulting product gas. The reason why the present invention achieves the above effects is thought to be that variations in the feed gas flow rate flowing into each module within the same unit can be reduced, and the gas separation load in each module can be leveled out, thereby preventing a state in which some modules receive too much or too little gas relative to the gas permeability and / or gas separation selectivity of that module, allowing the original permeation performance to be exhibited. To further enhance the above 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.

[0047] Furthermore, since a further decrease in the coefficient of variation does not result in improved recovery and purity, the coefficient of variation is preferably 0.01 or greater, more preferably 0.015 or greater. Identifying this critical point allows for reduced manufacturing costs when manufacturing gas separation membrane modules identically down to the smallest detail. For example, this allows for the mitigation of conditions such as the temperature conditions during spinning and / or the conditions of the solvent used and the type of manufacturing equipment used, thereby allowing for slight differences between lots and improving yield. In this specification, the term "gas permeability or gas separation selectivity of a gas separation membrane module" refers to the gas permeability or gas separation selectivity of the gas separation membrane that constitutes the gas separation membrane module. Because gas permeability and gas separation selectivity are correlated, the effects of the present invention can be achieved by specifying either one of the coefficients of variation.

[0048] For example, in the case of gas permeability, the coefficient of variation refers to a 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 a 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.

[0049] Gas permeability refers to the gas permeation rate of highly permeable gas A. As described 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 the membrane can be expressed as (permeation rate of high permeable gas A / permeation rate of low permeable gas B). In the unit of the present invention, for each module, gas A has high permeability and gas B has low permeability. In the system of the present invention, for each unit and each module constituting it, gas A usually has high permeability and gas B has low permeability. In the gas separation membrane unit of the present invention, it is preferable that the gas permeability and gas separation selectivity of the gas separation membrane module satisfy the above-mentioned ranges of coefficient of variation, in order to further increase the recovery rate and purity of the obtained gas.

[0050] In the present invention, the highly permeable gas A is CO ２ (carbon dioxide), and low-permeability gas B is CH ４ (methane), or the highly permeable gas A is O ２ (oxygen), and the low-permeability gas B is N ２ It is preferable that the highly permeable gas A is CO (nitrogen) in view of the effect of adopting a predetermined coefficient of variation. ２ and the low permeable gas B is CH ４ That is CH ４ This is also preferable in terms of reducing the environmental load by increasing the recovery rate.

[0051] Gas separation selectivity (P') of each gas separation membrane module at 40°C Ａ / P' Ｂ ) is preferably 2 or more and 1000 or less, more preferably 2.5 or more and 800 or less, and particularly preferably 3 or more and 500 or less. ２ and the low permeable gas B is CH ４ When the ４ ) of each gas separation membrane module at 40°C. ＣＯ２ / P' ＣＨ４ ) is preferably 5 or more and 150 or less, more preferably 10 or more and 130 or less, further preferably 15 or more and 120 or less, and particularly preferably 20 or more and 110 or less. ２ and the low permeable gas B is N ２ In order to obtain a high-purity product gas with a high recovery rate more efficiently, the gas separation selectivity (P' Ｏ２ / P' Ｎ２ ) is preferably 2 or more and 10 or less, more preferably 3 or more and 9 or less, particularly preferably 4 or more and 8 or less, and particularly preferably 4.5 or more and 7.5 or less.

[0052] In the present invention, in order to obtain a high-purity product gas with a high recovery rate more efficiently, the permeation rate P' of gas A through each gas separation membrane module at 40° C. Ａis 0.1 x 10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 500×10 －５ cm ３ (STP) / cm ２ sec cmHg or less, preferably 0.5 × 10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 400×10 －５ cm ３ (STP) / cm ２ sec cmHg or less is more preferable, and 0.8 × 10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 350×10 －５ cm ３ (STP) / cm ２ sec cmHg or less is more preferable, and 1×10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 300×10 －５ cm ３ (STP) / cm ２ In the present invention, in order to obtain a high-purity product gas with a high recovery rate more efficiently, the permeation rate P' of gas B through each gas separation membrane module at 40° C. is set to 1 / 2 sec·cmHg or less. Ｂ is 0.01 x 10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 15×10 －５ cm ３ (STP) / cm ２ sec cmHg or less, preferably 0.01 x 10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 10×10 －５ cm ３ (STP) / cm ２ sec cmHg or less, and more preferably 0.02 × 10 －５ cm ３ (STP) / cm ２・sec・cmHg or more 7×10 －５ cm ３ (STP) / cm ２ sec cmHg or less is more preferable, and 0.02 × 10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 5×10 －５ cm ３ (STP) / cm ２ It is particularly preferable that the pressure is .times. ...

[0053] In the present invention, the highly permeable gas A is CO ２ and the low permeable gas B is CH ４ When the ４ ) of each gas separation membrane module at 40°C. ２ Permeation rate P' 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, and 2 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 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.

[0054] In the present invention, the highly permeable gas A is CO ２ and the low permeable gas B is CH ４ In order to obtain high purity CH4 with a high recovery rate more efficiently, the CH4 of each gas separation membrane module at 40°C was ４ Permeation rate P' 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, preferably 0.05 × 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more. 2.5×10 -5 cm 3 (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 pressure is .times. ...

[0055] In the present invention, the highly permeable gas A is O ２ and the low permeable gas B is N ２ When the concentration is 100%, the product gas (N ２ ) of each gas separation membrane module at 40°C. ２ Permeation rate P' Ｏ２ is 0.1 x 10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 30×10－５ cm ３ (STP) / cm ２ sec cmHg or less, preferably 0.5 × 10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 25×10 －５ cm ３ (STP) / cm ２ sec cmHg or less is more preferable, and 0.8 × 10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 20×10 －５ cm ３ (STP) / cm ２ sec cmHg or less is more preferable, and 1×10 －５ cm ３ (STP) / cm ２ ・sec・cmHg or more 18×10 －５ cm ３ (STP) / cm ２ sec·cmHg or less is particularly preferred.

[0056] In the present invention, the highly permeable gas A is O ２ and the low permeable gas B is N ２ When the concentration is 100%, the product gas (N ２ ) of each gas separation membrane module at 40 °C. ２ Permeation rate P' Ｎ２ is 0.03 x 10 －５ cm ３ (STP) / cm 2 ・sec・cmHg or more 5×10 -5 cm 3 (STP) / cm 2 sec cmHg or less, preferably 0.05 × 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more. 4×10 -5 cm 3 (STP) / cm 2 sec cmHg or less is more preferable, and 0.05 × 10 -5 cm3 (STP) / cm 2 ・sec・cmHg or more 3.5×10 -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 3×10 -5 cm 3 (STP) / cm 2 It is particularly preferable that the pressure is .times. ...

[0057] In the present invention, the gas permeability of a gas separation membrane module can be measured, for example, by the following method. ４ and CO ２ Although an example is shown for CH, other gases can be measured in a similar manner. ４ and CO ２ Measurement method for gas permeability of CH ４ The gas permeability of the membrane is ４ When a pure gas of 1000 kJ / cm2 is pressurized at a predetermined pressure, the flow rate of the gas that permeates a membrane having a predetermined membrane area at a temperature of 40°C is measured using a thin-film flow meter. ４ Gas permeability P' to the membrane ＣＨ４ (Unit: x 10 －５ cm ３ (STP) / cm ２ Calculate the CO ２ The gas permeability of the membrane is ２ When each of the pure gases is pressurized at a predetermined pressure, the flow rate of the gas that permeates a membrane having a predetermined membrane area at a temperature of 40°C is measured using a thin-film flow meter. ２ Gas permeability P' to the membrane ＣＯ２ (Unit: x 10 －５ cm ３ (STP) / cm ２ sec cmHg).

[0058] In addition, CH ４ and CO ２During the separation operation of a gas containing ４ and CO ２ The concentration of CO in each module is measured using a known flow model. ２ 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 using the average values. The inner diameter can be, for example, 50 to 800 μm, and the outer diameter can be, for example, 100 to 1000 μm.

[0059] As an exception, CH ４ and CO ２ The coefficient of variation when separating gas containing N is ２ and O ２ For example, CO ２ / CH ４ Before the separation operation, remove the module and supply compressed air to ２ Enriched gas and N ２ The gas flow rates at the gas inlet, permeate gas outlet, and non-permeate gas outlet at this time and the N in the gas are ２ and O ２ The concentration of O may be measured, and the gas permeability and gas separation selectivity may be calculated from the results using a general formula such as a counterflow model. ２ The coefficient of variation of the gas permeation rate is ２ and the gas separation selectivity (P') at 40°C. Ｏ２ / P' Ｎ２ 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.

[0060] 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.

[0061] 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.

[0062] Next, the gas separation system of the present invention will be further described. Referring back to Figure 1, as shown in the figure, 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 the non-permeate gas outlet 11b of the first gas separation membrane unit 11 to the gas inlet 12a of the second gas separation membrane unit 12.

[0063] 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.

[0064] A raw material mixed gas supply line 26 is connected to the first gas inlet 11a of the first gas separation membrane unit 11 to supply 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 21 is interposed in the raw material mixed gas supply line 26. A permeable gas discharge port 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 permeable gas discharge port 12c to a position on the suction side of the compression means 21 of the raw material mixed gas supply line 26.

[0065] Compression means 21 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 second permeable gas discharged from second gas separation membrane unit 12 when the second permeable gas is returned to first gas separation membrane unit 11 through second line 17. A compressor can be used as compression means 21.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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 a first line 14. On the other hand, the first permeate 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 permeate gas is taken out of the system through a third line 18.

[0070] 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 as an enriched gas enriched in gas B from the second non-permeate gas outlet 12b of the second gas separation membrane unit 12 through the recovery line 15. Meanwhile, the second permeate gas is discharged from the second permeate 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 mixed gas supply line 26 via the second line 17 connected to the outlet 12c.

[0071] 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 .

[0072] In this embodiment, the gas separation membrane unit of the present invention is used in 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 with the number of units. To particularly enhance this effect, it is preferable that both of the first gas separation membrane unit 11 and the second gas separation membrane unit 12 be gas separation membrane units of the present invention. Furthermore, for example, the coefficients of variation in 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.

[0073] Next, a gas separation system 10' according to a second embodiment of the present invention will be described with reference to Figure 4. 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.

[0074] 4 has a first gas separation membrane unit 11 and a third gas separation membrane unit 13 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.

[0075] In the third gas separation membrane unit 13, the non-permeate gas outlet 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. 4, a third permeate gas discharge line 19 is connected to the permeate gas outlet 13c of the third gas separation membrane unit 13.

[0076] The compression means 21 is installed for the purpose of compressing the raw material mixed gas supplied from the gas source and the second permeable gas returned from the second gas separation membrane unit 12, as well as the third non-permeable gas returned from the third gas separation membrane unit 13.

[0077] The gas paths 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.

[0078] 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 further enriched in Gas A compared to the first permeate gas introduced into the third gas separation membrane unit 13, and is extracted from the permeate gas outlet 13c of the third gas separation membrane unit 13 through a third permeate gas outlet line 19 to the outside of the system. 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 .

[0079] 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.

[0080] In the configurations of Figures 1 and 4, the pressure of the compression means 21 is preferably 0.2 MPaG or more and 3.0 MPaG or less, and 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.

[0081] 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.

[0082] 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.

[0083] 4, 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 to use the units of the present invention in two or more of the first gas separation membrane unit 11 to the third gas separation membrane unit 13, and it is preferable to use the units of the present invention in all three.

[0084] 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 microbial fermentation, such as 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.

[0085] 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.

[0086] In the present invention, the raw material mixed gas contains N ２ and O ２ It is also preferable that the compound contains N ２ If it contains ２ In the present invention, the raw material mixed gas preferably contains 1 mol % or more, and more preferably 5 to 95 mol %. ２ If it contains ２ It is preferable that the gas separation system of the present invention contains 0.1 to 50 mol % of the above, and it is particularly preferable that the gas separation system of the present invention contains 0.5 to 30 mol % of the above.

[0087] 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 reducing means may be provided on the permeation side of one or two of the gas separation membrane units to provide motive force for passing through the separation membrane to the mixed gas supplied to each gas separation membrane unit. A known vacuum pump or the like can be used as such a pressure reducing means. Furthermore, the gas separation system of the present invention is not limited to the configurations shown in FIGS. 1 and 4 , and may also be configured as shown in FIG. 5 , which has only one gas separation membrane unit. In the configuration shown in FIG. 5 , the permeate gas is discharged from the system via the first permeate gas discharge line, but the permeate gas from the first permeate gas discharge line may be returned to the raw material supply line 26.

[0088] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to these examples. This test is a model experiment to demonstrate the variations in gas separation selectivity and gas permeability, and was determined 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.

[0089] 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.

[0090] <Gas Separation Membrane Module> Table 1 shows the gas separation properties at 40°C of gas separation membrane modules A and B, which were used as references in the examples and comparative examples. 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.

[0091]

[0092] [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. Each gas separation membrane module had a gas permeability (CO ２ The permeation rate (P'CO2) of the reference gas separation membrane module A at the same temperature (CO ２The gas separation selectivity (P'CO2 / P'CH4) shown in Table 2 is the average gas separation selectivity (P'CO2 / P'CH4) 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 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 shown in Table 2. In the following, the flow rate of the raw material mixed gas 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 CO in the product gas ２ Concentration and CH ４ The recovery rate is shown in Table 2. (Conditions) Flow rate of raw material mixed gas flowing into the system (F0): 320 Nm ３ / h Composition of raw material mixed gas: CO ２ 40 mol%, CH ４ is 60 mol %, the pressure (operating pressure) of the gas flowing into the first gas separation membrane unit 11 is 1.0 MPaG, the operating temperature of the first gas separation membrane unit 11 is 40° C., and the operating temperature of the second gas separation membrane unit 12 is 40° C.

[0093]

[0094] As can be seen from the comparison of each Example with Comparative 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 CO ２ The concentration of CO2 is less than 4.0 mol %, which is an effective improvement in purity. 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 is reduced to 0.01, any further reduction in the coefficient of variation is not sufficient to improve the purity. ２ The concentration did not change, and ４ Therefore, by setting the coefficient of variation to 0.01 or more, the uniformity of the production conditions can be relaxed compared to Comparative Examples 2 and 5, and the production cost can be reduced.

[0095] [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 (CO ２ The gas permeability (CO permeation rate) of the four gas separation membrane modules at 40°C was ２ The average value of the gas permeability (CO ２ 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 shown in Table 3. A simulation was performed. ２ The concentrations and CH4 recovery rates are shown in Table 3.

[0096]

[0097] [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 four gas separation membrane modules were configured to have a gas permeability (CO ２ The gas separation selectivity (P'CO permeation rate) shown in Table 4 was the relative value shown in Table 4 with respect to the value of the reference gas separation membrane module A at 40°C. ２ / P'CH ４ ) is the gas separation selectivity (P′CO ) of gas separation membrane module A at 40°C, calculated by taking the average value of the values ​​of the four gas separation membrane modules at 40°C. ２ / P'CH ４ 4 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 a simulation was performed in which the system was operated under the conditions shown in Table 4. ２ Concentration and CH ４ The recovery rate is shown in Table 4. (Conditions) Flow rate of raw material mixed gas flowing into the system (F0): 320 Nm ３ / h Composition of raw material mixed gas: CO ２40 mol%, CH ４ is 60 mol%, - pressure (operating pressure) of gas flowing into the first gas separation membrane unit 11: 1.26 MPaG - pressure (operating pressure) of gas flowing into the third gas separation membrane unit 13: 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

[0098]

[0099] [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 each had four gas separation membrane modules, each with a gas separation selectivity (P'CO2 / P'CH4) at 40°C that was the ratio shown in Table 5 relative to the value at 40°C of the reference gas separation membrane module A. The gas permeability (CO2 permeation rate) 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. 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 4, and a simulation was performed operating under the following conditions and the conditions shown in Table 5. CO in the product gas ２ The concentration and CH4 recovery rate are shown in Table 5. (Conditions) Flow rate of raw material mixed gas flowing into the system (F0): 320 Nm ３ / h Composition of raw material mixed gas: CO ２ 40 mol%, CH ４ is 60 mol%, - pressure (operating pressure) of gas flowing into the first gas separation membrane unit 11: 1.26 MPaG - pressure (operating pressure) of gas flowing into the third gas separation membrane unit 13: 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

[0100]

[0101] [Comparative Examples 12 and 13, Example 18] 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. Each gas separation membrane module had a gas permeability (CO ２ Permeation rate (P'CO ２ )) is the gas permeability (CO ２ The gas separation selectivity (P'CO ２ / P'CH ４ ) is the gas separation selectivity (P'CO ２ / P'CH ４ ) of the gas separation membrane module B at 40°C (P'CO ２ / P'CH ４ ) is the value obtained by dividing the CO in the product gas by the CO in the first gas separation membrane unit 11 and the second gas separation membrane unit 12. 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. In the following, the flow rate of the raw material mixed gas 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. ２ Concentration and CH ４ The recovery rate is shown in Table 6. (Conditions) Flow rate of raw material mixed gas flowing into the system (F0): 320 Nm ３ / h Composition of raw material mixed gas: CO ２ 40 mol%, CH ４ is 60 mol %, the pressure (operating pressure) of the gas flowing into the first gas separation membrane unit 11 is 1.0 MPaG, the operating temperature of the first gas separation membrane unit 11 is 40° C., and the operating temperature of the second gas separation membrane unit 12 is 40° C.

[0102]

[0103] As can be seen from the comparison with Example 18 and Comparative Example 12 in Table 6, 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 CO ２It can be seen that the concentration is significantly reduced and the purity can be effectively improved. Furthermore, as shown by the comparison between Example 18 and Comparative Example 13, once the coefficient of variation is reduced to 0.01, even if it is reduced further, the CO ２ The concentration did not change, and ４ Therefore, by setting the coefficient of variation to 0.01 or more, the uniformity of the production conditions can be relaxed compared to Comparative Example 13, and the production cost can be reduced.

[0104] <Gas Separation Membrane Module> Table 7 shows the gas separation characteristics at 40°C of gas separation membrane module C, which was used as the reference in the examples and comparative examples. 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 7 Ｏ２ and P' Ｎ２ The unit is x 10 －５ cm ３ (STP) / cm ２ sec cmHg.

[0105]

[0106] [Comparative Examples 14 and 15, Example 19] The first gas separation membrane unit 11 was constructed by combining four gas separation membrane modules in parallel. Each gas separation membrane module had a gas permeability (O ２ Permeation rate (P'O ２ )) is the gas permeability (O ２ The gas separation selectivity (P'O permeation rate) shown in Table 8 was the relative value shown in Table 8. ２ / P'N ２ ) is the gas separation selectivity (P′O ) of the four gas separation membrane modules at 40°C. ２ / P'N ２ ) of the gas separation membrane module C at 40°C (P'O ２ / P'N ２) is the value obtained by dividing the O in the product gas by the O in the first gas separation membrane unit 11. A simulation was conducted in which the first gas separation membrane unit 11 was configured in the system of FIG. 5 and operated under the following conditions and the conditions in Table 8. In the following, the flow rate of the raw material mixed gas 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 the first gas separation membrane unit. ２ Concentration and N ２ The recovery rate is shown in Table 8. (Conditions) Flow rate of raw material mixed gas flowing into the system (F0): 100 Nm ３ / h Composition of raw material mixed gas: O ２ 21 mol%, N ２ is 79 mol%, the pressure of the gas flowing into the first gas separation membrane unit 11 (operating pressure) is 1.0 MPaG, and the operating temperature of the first gas separation membrane unit 11 is 40° C.

[0107]

[0108] As can be seen from the comparison with Example 19 and Comparative Example 14 in Table 8, when the coefficient of variation of gas permeability of the module of the first gas separation membrane unit 11 is 0.49 or less, the O ２ It can be seen that the concentration is significantly reduced, and the purity can be effectively improved. Furthermore, as shown by the comparison with Example 19 and Comparative Example 15, once the coefficient of variation is reduced to 0.01, even if it is further reduced, the O ２ The concentration does not change, and ２ Therefore, by setting the coefficient of variation to 0.01 or more, the uniformity of the production conditions can be relaxed compared to Comparative Example 15, and the production cost can be reduced.

[0109] According to the present invention, there are provided a gas separation membrane unit capable of obtaining the expected product gas purity and product gas recovery rate, a gas separation system using the same, and a method for producing an enriched gas.

Claims

1. A gas separation membrane unit which receives a raw material mixed gas and concentrates and enriches at least one gas contained in the raw material mixed gas, the gas separation membrane unit comprising a plurality of gas separation membrane modules connected 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, the gas inlet, non-permeate gas outlet, and permeate gas outlet of each module being shared, thereby constituting the gas inlet, non-permeate gas outlet, and permeate gas outlet of the gas separation membrane unit, and 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. 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 gas separation membrane unit according to claim 1 or 2, which is either (1) or (2) below: (1) The raw material mixed gas is CH4 and CO ２ wherein the gas separation selectivity is CO relative to CH ２ Permeation rate ratio P'CO ２ / P'CH ４ and the gas permeability is ２ Permeation rate P'CO ２ (2) The raw material mixed gas is N ２ and O ２ The gas separation selectivity is ２ O against ２ Permeation rate ratio P'O ２ / P'N ２ and the gas permeability is O ２ Permeation rate P'O ２ The gas separation membrane unit according to claim 1 or 2, wherein the permeation rate is expressed in units of 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, comprising at least a first gas separation membrane unit, a raw material mixed gas supply line connected to a gas inlet of the first gas separation membrane unit, and a compression means interposed in the raw material mixed gas supply line, wherein the first gas separation membrane unit is the gas separation membrane unit described in 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, comprising at least a first gas separation membrane unit and a second gas separation membrane unit, a raw material mixed gas supply line connected to the 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 to the gas inlet of the second gas separation membrane unit, and a second line connecting the permeate gas outlet of the second gas separation membrane unit to the raw material mixed gas supply line, 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 units described in claim 1.

6. 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, comprising 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 the 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 to 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 to the gas inlet of the third gas separation membrane unit, and a fourth line connecting the 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.

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

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