Gas separation system and method for producing methane enriched gas

Abstract

Provided is a gas separation system 10 that comprises a first gas separation membrane unit 11 and a second gas separation membrane unit 12, and that is for producing CH4 enriched gas from a feedstock gas which contains CO2 and CH4, said gas separation system 10 comprising: a feedstock gas supply line 26 which is connected to a gas inlet 11a of the first gas separation membrane unit 11; a compression means 21 which is disposed in the feedstock gas supply line 26; a first line 14 which connects a non-permeating gas outlet 11b of the first gas separation membrane unit 11 and a gas inlet 12a of the second gas separation membrane unit 12; and a second line 17 which connects a permeating gas outlet 12c of the second gas separation membrane unit 12 and the feedstock gas supply line 26, wherein a cooling means 22 that reduces the operating temperature of the second gas separation membrane unit as compared to the operating temperature of the first gas separation membrane unit is provided.

Description

Gas separation system and method for producing methane-enriched gas

[0001] The present invention relates to a gas separation system that separates a mixed gas containing CO2 and methane (CH4) using a plurality of gas separation membrane units, and a method for producing a CH4-enriched gas using the same.

[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 the ratio of (permeation rate of high permeable gas / permeation rate of low permeable gas).

[0003] Generally, gas separation membranes with high gas separation selectivity have low gas permeation rates, and conversely, membranes with high gas permeation rates have low gas separation selectivity. Therefore, when recovering low-permeability gases from mixed gases using a single-stage gas separation membrane, if the purity of the recovered gas is constant, using a membrane with high gas separation selectivity will result in a high recovery rate. However, because the permeation rate is low, it is necessary to increase the membrane area or the operating pressure. On the other hand, membranes with high permeation rates do not require a large membrane area or a high operating pressure, but they have low gas separation selectivity and therefore a low recovery rate.

[0004] In general, gas separation membranes with selective gas permeability are used as gas separation membrane modules, which are constructed by housing the gas separation membrane in a container equipped with at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet. The gas separation membrane is mounted in the container so that the gas supply side and the gas permeate side are separated from each other. In gas separation systems, a gas separation membrane unit is typically constructed by combining multiple gas separation membrane modules in parallel to achieve the required membrane area. The multiple gas separation membrane modules that make up a gas separation membrane unit share the gas inlet, non-permeate gas outlet, and permeate gas outlet, so that the gas separation membrane unit essentially functions as a large gas separation membrane module.

[0005] A method using a system equipped with such gas separation membrane units in multiple stages is known for recovering the target low-permeability gas with high purity and high recovery rate. Examples of multistage gas separation systems include those that further separate the non-permeable gas from the first stage, which is enriched with the low-permeability gas, in order to improve purity, and those that recover the low-permeability gas contained in the permeable gas from the first stage in order to improve recovery rate.

[0006] A method for producing a CH4-enriched gas from a feed gas containing CO2 and CH4 by membrane separation using multiple gas separation membrane units is known (see, for example, Patent Document 1). Patent Document 1 describes the separation of CH4 and CO2 from biogas cooled to 0 to -60°C.

[0007] US2017 / 0304769A1

[0008] However, the gas separation system described in Patent Document 1 is not sufficient in terms of obtaining high-purity CH4 gas at low cost with a small membrane area.

[0009] The present invention aims to solve the above problems and provides the following configurations.

[0010] [1] A gas separation system comprising a first gas separation membrane unit and a second gas separation membrane unit, and used for producing a CH4-enriched gas from a raw material gas containing CO2 and CH4, wherein each gas separation membrane unit has at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, and the gas separation system comprises: a raw material gas supply line connected to the gas inlet of the first gas separation membrane unit; a compression means interposed in the raw material 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 gas supply line, and further comprising cooling means for lowering the operating temperature T2 of the second gas separation membrane unit compared to the operating temperature T1 of the first gas separation membrane unit.

[0011] [2] Gas permeation rate P1'CO2 of the first gas separation membrane unit at 40°C (40) Compared with the gas permeation rate P2'CO2 of the second gas separation membrane unit, (40) [3] The gas separation system according to [1], wherein the carbon dioxide gas permeation rate P1'CO2 at the operating temperature T1 of the first gas separation membrane unit is high. (T1) In comparison, the carbon dioxide gas permeation rate P1'CO2 at the operating temperature T2 of the second gas separation membrane unit (T2) The gas separation system according to [1] or [2], wherein

[0012] [4] A gas separation system according to any one of [1] to [3], wherein the temperature t2 of the gas supplied to the second gas separation membrane unit is 30°C or lower.

[0013] [5] A gas separation system according to any one of [1] to [4], wherein the gas temperature t2 supplied to the second gas separation membrane unit is lower than the gas temperature t1 supplied to the first gas separation membrane unit.

[0014] [6] The gas separation system according to any one of [1] to [5], wherein the difference T1-T2 between the operating temperatures T1 and T2 is 10°C or more.

[0015] [7] A gas separation system according to any one of [1] to [6], wherein the raw gas supply line is provided with a heating means for heating the raw gas to be supplied to the first gas separation membrane unit.

[0016] [8] During operation of the system, the separation selectivity P1'CO2 / P1'CH4 of the gas separation membrane constituting the second gas separation membrane unit is 15 or more and 3000 or less, and the CO2 permeation rate P1'CO2 is 3 x 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 80×10 -5 cm 3 (STP) / cm 2 The gas separation system according to any one of [1] to [7], wherein the saturation pressure is 1000 kJ / s or less.

[0017] [9] A gas separation system according to any one of [1] to [8], wherein the cooling means is interposed in the first line.

[0018]

[10] A gas separation system according to any one of [1] to [9], wherein the proportion of the gas flow rate returned to the first gas separation membrane unit out of the gas supply flow rate to the first gas separation membrane unit is 10 to 60%.

[0019]

[11] The gas separation system according to any one of [1] to

[10] , further comprising a third gas separation membrane unit, the third gas separation membrane unit having at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, 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 gas supply line.

[0020]

[12] The gas separation system according to

[11] , wherein the operating temperature of the second gas separation membrane unit is lower than the operating temperature of the third gas separation membrane unit.

[0021]

[13] The gas separation system according to any one of [1] to

[12] , further comprising a liquefaction device that further cools and liquefies the non-permeable gas of the second gas separation membrane unit.

[0022]

[14] A gas separation system according to any one of [1] to

[13] , wherein the operating temperature of the second gas separation membrane unit is lowered using cooled non-permeable gas from the second gas separation membrane unit.

[15] A method for producing an enriched gas enriched in CH by supplying a raw material gas containing at least CO2 and CH4 to a gas separation system, wherein the gas separation system comprises at least a first gas separation membrane unit and a second gas separation membrane unit, each gas separation membrane unit having at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a raw material gas supply line connected to the gas inlet of the first gas separation membrane unit, a compression means interposed in the raw material 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 gas supply line, and wherein the system is provided with cooling means for lowering the operating temperature of the second gas separation membrane unit to a temperature lower than the operating temperature of the first gas separation membrane unit.

[0023] 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 the configuration of a gas separation system in a second embodiment of the present invention. Fig. 4 is a schematic diagram showing the configuration of a gas separation system in yet another embodiment of the present invention. Fig. 5 is a schematic diagram showing the configuration of a gas separation system in yet another embodiment of the present invention. Fig. 6 is a schematic diagram showing the configuration of a gas separation system in yet another embodiment of the present invention.

[0024] The present invention will be described below based on preferred embodiments with reference to the drawings. The upper and lower limits of the numerical values ​​in this specification can be combined without any restrictions. Each gas separation system shown in Figures 1 and 3 to 5 includes a first gas separation membrane unit 11 and a second gas separation membrane unit 12. Furthermore, the gas separation systems shown in Figures 3 and 5 further include a third gas separation membrane unit 13.

[0025] As shown in FIG. 2, each of the gas separation membrane units 11, 12, and 13 can be, for example, a module 40 formed 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 may, for example, use one gas separation membrane module 40 shown in FIG. 2, or may comprise a plurality of such modules 40 arranged 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.

[0026] 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 extension direction of the hollow fiber membrane. 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. 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 as an example, the present invention can naturally be applied to separation membrane modules of other configurations, such as shell feed type modules.

[0027] 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 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.

[0028] 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" below refer to the direction of flow of the raw material gas. In addition, in each gas separation membrane unit in the system 10, CO2 is a gas that has a high permeation rate through the gas separation membrane, i.e., a highly permeable gas, and CH4 is a gas that has a low permeation rate through the gas separation membrane, i.e., a low permeable gas.

[0029] A raw gas supply line 26 is connected to the first gas inlet 11a of the first gas separation membrane unit 11 to supply raw gas from a raw gas source (not shown) to the first gas separation membrane unit 11. A compression means 21 is interposed in the raw gas supply line 26. A permeable gas discharge port 12c of the second gas separation membrane unit 12 is connected to the raw 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 on the raw gas supply line 26.

[0030] The compression means 21 is installed for the purpose of compressing the raw material gas supplied from the raw material gas source. It is also installed for the purpose of compressing the second permeate gas discharged from the second gas separation membrane unit 12 when the second permeate gas is returned to the first gas separation membrane unit 11 via the second line 17. The compression means 21 may be the same as means conventionally used in the relevant technical field. For example, a compressor may be used. The raw material gas typically contains approximately 0 to 10% by mass of moisture when supplied to the system.

[0031] A recovery line 15 for extracting non-permeate gas enriched in CH4 is connected to the non-permeate gas outlet 12b of the second gas separation membrane unit 12. The recovery line 15 may be provided with a liquefaction means 50 downstream of the non-permeate gas outlet 12b for liquefying the CH4 gas (CH4-enriched gas) discharged from the second non-permeate gas outlet 12b. The CH4 gas used for liquefied methane production is required by standards to be high-purity methane with impurity concentrations suppressed to around several tens of ppmv, and the present invention makes it possible to achieve this purity at low cost.

[0032] Downstream of the compression means 21 in the raw gas flow direction in the raw gas supply line 26, there is provided a cooling means 23 capable of cooling the raw gas compressed by the compression means 21. Also, downstream of the cooling means 23, there are provided a condensate flow line 24 for separating and removing a condensate flow of moisture and the like generated by cooling by the cooling means from the raw gas supply line 26, and a heating means 25 for heating the raw gas from which the condensate flow has been separated to a desired temperature. As the cooling means 23, a conventionally known means can be used, such as a heat exchanger. As the heating means 25, a conventionally known heater can be used.

[0033] One of the features of the present system 10 is the provision of cooling means 22 that lowers the operating temperature of the second gas separation membrane unit 12 below the operating temperature of the first gas separation membrane unit 11. The operating temperature here refers to the temperature of the gas separation membrane that constitutes the corresponding gas separation membrane unit. The cooling means 22 may directly cool the gas separation membrane that constitutes the second gas separation membrane unit 12, or may cool the gas separation membrane by cooling the non-permeate gas (second supply gas) of the first gas separation membrane unit 11 that is supplied to the second gas separation membrane unit 12. In this embodiment, the cooling means 22 is interposed in the first line 14 and cools the second supply gas.

[0034] Gas separation membranes made of, for example, polymeric materials are used as the gas separation membranes of the gas separation membrane modules that make up the first gas separation membrane unit 11 and the second gas separation membrane unit 12. Gas separation membranes can be used in the form of, for example, flat film membranes or hollow fiber membranes. In particular, it is preferable to use hollow fiber membranes as the gas separation membranes and bundle a plurality of these membranes into a hollow fiber membrane element in order to increase the throughput.

[0035] The polymeric material constituting the gas separation membrane is preferably one that has the ability to selectively permeate CO2 relative to CH4. Examples of such polymeric materials include glassy polymer materials and rubbery polymer materials. Examples of glassy polymer materials include polyimide, polyamide, polyamideimide, cellulose-based materials such as cellulose acetate, polysulfone, polyethersulfone, polyphenylsulfone, and polycarbonate. Examples of rubbery polymer materials include silicone resin and polybutadiene resin. In particular, from the viewpoint of further enhancing the effects of the present invention, it is preferable to use polyimide as the gas separation membrane constituting the second gas separation membrane unit 12. It is also preferable to use polyimide as the material for the gas separation membranes constituting the first gas separation membrane unit 11 and the third gas separation membrane unit 13 described below.

[0036] When polyimide is used as the gas separation membrane, it can be obtained by synthesizing a polyamic acid using a tetracarboxylic acid component and a diamine component, and then heat-treating the polyamic acid to dehydrate and imidize it.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] In the present invention, when polyimide is used as the material for the gas separation membrane constituting the second gas separation membrane unit 12, aromatic polyimide may be particularly used. The aromatic polyimide can be obtained using an aromatic tetracarboxylic acid component and an aromatic diamine component.

[0043] In the present invention, when polyimide is used as the material for the gas separation membranes constituting the first gas separation membrane unit 11 and the third gas separation membrane unit 13 described below, aromatic polyimide may also be used.

[0044] When gas separation membranes made of polyimide are used in the first gas separation membrane unit 11, the second gas separation membrane unit 12, and the third gas separation membrane unit 13, asymmetric membranes may be used as the gas separation membranes. Asymmetric membranes generally have a two-layer structure consisting of a skin layer with a dense structure that separates the target gas and a porous layer that supports it.

[0045] A particularly suitable gas separation membrane for use in the first gas separation membrane unit 11, the second gas separation membrane unit 12, and the third gas separation membrane unit 13 is a polyimide hollow fiber gas separation membrane having an asymmetric structure with a skin layer thickness of 10 nm or more and 200 nm or less, a porous layer thickness of 20 μm or more and 200 μm or less, and an inner diameter of approximately 30 μm or more and 500 μm or less.

[0046] The operation of the gas separation system 10 of this embodiment having the above configuration will be described. A feed gas containing CH and CO to be separated is supplied from a feed gas source (not shown) through a feed gas supply line 26 to the first gas separation membrane unit 11. The feed gas is compressed by the compression means 21, and its pressure increases.

[0047] The raw material gas is compressed by compression means 21 and then cooled by cooling means 23, and a condensate 53 containing moisture is separated and removed from the raw material gas through condensate flow line 24. The raw material gas from which the condensate 53 has been separated and removed is heated to a constant temperature by heating means 25 and supplied to first gas separation membrane unit 11.

[0048] When the raw 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.

[0049] The first non-permeate gas discharged from the first gas separation membrane unit 11 is enriched in CH compared to the feed 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 enriched in CO compared to the feed gas. The first permeate gas is taken out of the system through a third line 18.

[0050] The first non-permeate gas discharged from the first gas separation membrane unit 11 is cooled by the cooling means 22 and supplied to the second gas separation membrane unit 12 at a temperature lower than the operating temperature T1 of the first gas separation membrane unit 11, where it comes into contact with the gas separation membrane of the second gas separation membrane unit 12. As a result, the operating temperature T2 of the second gas separation membrane unit 12 becomes lower than the operating temperature of the first gas separation membrane unit 11. Under these temperature conditions, the first non-permeate gas (second feed gas) 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 is further enriched in CH4. The second non-permeate gas may be supplied to the liquefaction device 50 from the second non-permeate gas outlet 12b of the second gas separation membrane unit 12 through the recovery line 15, where it is further cooled and liquefied. On the other hand, 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 gas supply line 26 via the second line 17 connected to the outlet 12c.

[0051] 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 gas and then compressed by the compressor 21 .

[0052] As described above, the gas separation system of the present invention uses the cooling means 22 to lower the operating temperature T2 of the second gas separation membrane unit below the operating temperature T1 of the first gas separation membrane unit 11. In the second gas separation membrane unit 12, the separation selectivity for CO2 and CH4 improves as the operating temperature decreases. This allows the second non-permeable gas to achieve high CH4 purity while reducing the amount of second permeable gas merging into the feed gas supply line 26 via the second line 17 (hereinafter, the total amount of gas merging into the feed gas from units other than the first gas separation membrane unit 11 is also referred to as the "reflux flow rate"). Because the reduction in the reflux flow rate reduces compression power, the present invention enables the production of high-purity CH4 at low cost. In this embodiment, conventionally used polyimide hollow fiber membranes are used as the gas separation membranes in the second gas separation membrane unit 12, and CH4 purity can be effectively improved by controlling the operating temperature.

[0053] In terms of lowering the operating temperature of second gas separation membrane unit 12 with a simple system, it is preferable to cool the second supply gas by cooling means 22 so that the temperature of the first non-permeable gas (second supply gas) supplied to the second gas separation membrane unit is lower than the temperature of the raw material gas (first supply gas) supplied to first gas separation membrane unit 11. In this specification, the temperature of the supply gas to each gas separation membrane unit refers to the temperature at the time it is supplied to the gas inlet of each gas separation membrane unit.

[0054] In the present invention, the operating temperature T1 of the first gas separation membrane unit 11 is set higher than the operating temperature T2 of the second gas separation membrane unit 12. This allows high treatment capacity to be obtained even with a small membrane area due to the high permeation rates of CO and CH through the first gas separation membrane unit 11, thereby reducing equipment costs.

[0055] Furthermore, in this embodiment, the second non-permeable gas, whose temperature has already been lowered to a certain degree by the cooling means 22, can also be cooled in the liquefaction device 50. In this case, the energy required for liquefaction in the liquefaction device can be reduced, and liquefied methane can be produced at low cost.

[0056] In order to further increase the CH purity in the second non-permeable gas, the temperature t2 of the second feed gas is preferably higher than the dew point of water in the gas. The use of the cooling means 23, condensate flow line 24, and heating means 25 described above allows the water dew point of the second feed gas to be sufficiently lowered. The temperature t2 of the second feed gas refers to the gas temperature of the first non-permeable gas at the time of supply to the second gas separation membrane unit 12. Meanwhile, the first feed gas temperature t1 referred to in this specification refers to the gas temperature of the raw material gas at the time of supply to the first gas separation membrane unit 11. When the temperature of the separation membrane is controlled by the temperature of the gas supplied to the unit, the operating temperatures of the first gas separation membrane unit 11 and the second gas separation membrane unit 12 can be considered to be the same as the temperature t1 of the first feed gas and the temperature t2 of the second feed gas, respectively.

[0057] The CO2 permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit at the operating temperature T2 of the second gas separation membrane unit (T2) is the CO2 permeation rate P1'CO2 constituting the first gas separation membrane unit at the operating temperature T1. (T1) In particular, the CO2 permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit at the operating temperature T2 of the second gas separation membrane unit is (T2) is the CO2 permeation rate P1'CO2 constituting the first gas separation membrane unit at the operating temperature T1. (T1) It is preferable that the P2'CO2 (T2) is P1'CO2 (T1) On the other hand, the CO2 permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit at the operating temperature T2 of the second gas separation membrane unit is (T2) is the CO2 permeation rate P1'CO2 constituting the first gas separation membrane unit at the operating temperature T1. (T1)is preferably 7 or less, more preferably 6 or less, even more preferably 5 or less, and particularly preferably 4 or less. Taking the above into consideration, the CO2 permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit at the operating temperature T2 of the second gas separation membrane unit is (T2) is the CO2 permeation rate P1'CO2 constituting the first gas separation membrane unit (T1) With respect to the CO2 permeation rate, it is preferably 0.5 or more and 7 or less, more preferably 0.65 or more and 7 or less, even more preferably 0.75 or more and 7 or less, particularly preferably more than 1 and 7 or less, even more preferably 1.4 or more and 6 or less, particularly preferably 1.5 or more and 5 or less, and most preferably 1.7 or more and 4 or less. By setting the CO2 permeation rate in such a range, it is possible to further enhance the effect of obtaining high-purity CH4 gas at low cost.

[0058] Furthermore, the separation selectivity P2'CO2 / P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit at the operating temperature T2 of the second gas separation membrane unit (T2) is the separation selectivity P1'CO2 / P1'CH4 of the gas separation membrane constituting the first gas separation membrane unit at the operating temperature T1 of the first gas separation membrane unit. (T1) With respect to the CO2 permeation rate, a value of 0.4 to 13 is preferred in that it effectively reduces the membrane area and the compression power required to obtain high-purity CH4 gas at low cost, a value of 0.5 to 12 is more preferred, a value of 0.6 to 7 is even more preferred, and a value of 0.7 to 5 is particularly preferred in terms of reducing the compression power required. By achieving such separation selectivity and CO2 permeation rate, the effect of obtaining high-purity CH4 gas at low cost can be further enhanced.

[0059] At 40°C, the permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit (40) is the permeation rate P1'CO2 of the gas separation membrane constituting the first gas separation membrane unit. (40) It is preferable that the permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit 12 at 40°C is higher than that of the gas separation membrane constituting the second gas separation membrane unit 12. (40)is the permeation rate P1'CO2 of the gas separation membrane constituting the first gas separation membrane unit 11 at 40°C. (40) is set to 1, it is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.4 or more. (40) is the permeation rate P1'CO2 of the gas separation membrane constituting the first gas separation membrane unit 11 at 40°C. (40) In terms of availability of gas separation membranes, it is preferable that the ratio is 12 or less, where P2'CO2 / P2'CH4 is 1. In terms of CH4 gas purity, it is preferable that the ratio is 5 or less. (40) The separation selectivity P1'CO2 / P1'CH4 of the gas separation membrane constituting the first gas separation membrane unit (40) In order to more effectively reduce the membrane area of ​​the second gas separation membrane unit 12, the separation selectivity P2'CO2 / P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit 12 at 40°C is preferably lower than (40) The separation selectivity P1'CO2 / P1'CH4 of the gas separation membrane constituting the first gas separation membrane unit 11 at 40°C (40) When P2'CO2 / P2'CH4 is taken as 1, it is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.70 or less, and particularly preferably 0.60 or less. (40) is the separation selectivity P1'CO2 / P1'CH4 of the gas separation membrane constituting the first gas separation membrane unit 11 at 40°C. (40) When the selectivity is set to 1, it is preferably 0.2 or more in terms of availability of gas separation membranes, and is preferably 0.3 or more in terms of CH gas purity. Since 40°C is a general operating temperature for membrane separation of CO2 and CH4, in this embodiment, the selectivity at this temperature is also specified as an index.

[0060] The temperature of the first feed gas t1 is preferably 0 to 60°C, more preferably 10 to 50°C, and even more preferably 20 to 40°C. Setting the temperature t1 of the first feed gas within the above range is preferable because it is easy to adjust the CO permeation rate and increase the CH purity while suppressing the membrane area. Furthermore, since the gas separation membrane of the first gas separation membrane unit 11 of this embodiment usually has a higher selectivity for HO gas compared to CH, setting the first feed gas within this temperature range prevents water condensation in the first gas separation membrane unit 11 and enables water permeation within the unit, thereby reducing the water content in the second feed gas and lowering the dew point. Preferred temperature ranges for the operating temperature T1 of the first gas separation membrane unit 11 include the same range as the preferred range for the temperature of the first feed gas t1, with 0 to 60°C being preferred, 10 to 50°C being more preferred, and 20 to 40°C being even more preferred.

[0061] The temperature t2 of the second feed gas is preferably 30°C or less, because this provides a better CH4 purity in the second non-permeable gas and a better effect of reducing the required compression power. From this viewpoint, the temperature of the second feed gas is preferably 30°C or less, more preferably 25°C or less, even more preferably 20°C or less, and particularly preferably 10°C or less. Furthermore, the temperature t2 of the second feed gas is preferably -50°C or higher, because this avoids a deterioration in the gas separation function of the second gas separation membrane unit 12 and makes it easier to achieve a temperature above the dew point of water in the first non-permeable gas supplied to the second gas separation membrane unit 12; -40°C or higher is more preferred, -30°C or higher is even more preferred, and -20°C or higher is particularly preferred. Preferred temperature ranges for the operating temperature T2 of the second gas separation membrane unit 12 include the same ranges as the preferred range for the temperature t2 of the second feed gas, with -50 to 30°C being preferred, -40 to 25°C being more preferred, -30 to 20°C being even more preferred, and -20 to 10°C being particularly preferred.

[0062] The temperature difference t1-t2 between the temperature t1 of the first feed gas and the temperature t2 of the second feed gas is preferably 10°C or more, from the viewpoint of reducing the required compression power and enhancing the effect of obtaining high-purity CH4 gas at low cost, and is particularly preferably 15°C or more. The temperature difference t1-t2 is preferably 110°C or less, more preferably 90°C or less, and particularly preferably 55°C or less, from the viewpoint of preventing a deterioration in the function of the gas separation membrane and reducing the membrane area by not making the temperature difference t1-t2 too large. The temperature difference T1-T2 between the operating temperature T1 of the first gas separation membrane unit 11 and the operating temperature T2 of the second gas separation membrane unit 12 is preferably 10°C or more and 110°C or less, more preferably 15°C or more and 90°C or less, and even more preferably 15°C or more and 55°C or less, similar to t1-t2.

[0063] Furthermore, the temperature difference T0-T0' between the temperature T0 of the raw material gas supplied to the system and the temperature T0' of the raw material gas at the time of intake into the compression means 21 is preferably 2° C. or more, from the viewpoint of reducing the required compression power and enhancing the effect of obtaining high-purity CH4 gas at low cost, more preferably 3° C. or more, even more preferably 7° C. or more, and particularly preferably 10° C. or more. From the viewpoint of preventing freezing and clogging in the raw material line by not making the temperature difference T0-T0' too large, it is preferably 30° C. or less.

[0064] Although not limited thereto, for example, the water dew point of the gas supplied to the second gas separation membrane unit 12 (hereinafter also referred to as the "second supply gas") is preferably -70°C or higher and 0°C or lower, and more preferably -50°C or higher and -8°C or lower. This dew point is the temperature under pressure at which the gas is supplied to the second gas separation membrane unit 12. Here, the water dew point of the second supply gas tends to decrease when the reflux rate is reduced. This is because reducing the reflux rate tends to reduce the water content of the first supply gas, which in turn reduces the water dew point of the second supply gas. For these reasons, the present invention makes it easier to obtain high-purity CH4 gas.

[0065] In this embodiment, the proportion of the gas supply flow rate (first supply gas flow rate F1) to the first gas separation membrane unit that is refluxed from gas separation membrane units other than the first gas separation membrane unit (F4) (hereinafter also referred to as the "reflux rate") is preferably 60% or less in order to further reduce operating costs, and more preferably 50% or less. Furthermore, a reflux rate of, for example, a lower limit of 10% or more is preferred in terms of suppressing the membrane area, and more preferably 20% or more.

[0066] In the present invention, in order to efficiently obtain high-purity CH4 with a high recovery rate, the gas separation selectivity of the gas separation membrane of the first gas separation membrane unit at 40°C (CO2 permeation rate P1'CO2 / CH4 permeation rate P1'CH4 (40) ) is preferably 30 or more and 150 or less, more preferably 35 or more and 130 or less, even more preferably 40 or more and 120 or less, and particularly preferably 50 or more and 110 or less.

[0067] In the present invention, in order to obtain high-purity CH4 at a high recovery rate while suppressing the compression power and membrane area, the carbon dioxide permeation rate P1' of the gas separation membrane of the first gas separation membrane unit at 40°C is set to CO2 (40) is 1.5 x 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 100×10 -5 cm 3 (STP) / cm 2 sec cmHg or less, and 2 x 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 45×10 -5 cm 3 (STP) / cm 2 sec cmHg or less is more preferable, and 3×10 -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 7 × 10 -5cm 3 (STP) / cm 2 ・sec・cmHg or more 15×10 -5 cm 3 (STP) / cm 2 sec·cmHg or less is particularly preferred.

[0068] In the present invention, in order to obtain high-purity CH4 while suppressing the compression power and membrane area, the CH4 permeation rate P1' of the gas separation membrane of the first gas separation membrane unit at 40°C is set to CH4 (40) 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 0.8×10 -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 0.5×10 -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 0.2×10 -5 cm 3 (STP) / cm 2 It is particularly preferable that the pressure is .times. ...

[0069] Preferred ranges for the separation selectivity and CO2 and CH4 permeation rates of the first gas separation membrane unit 11 at the operating temperature T1 of the first gas separation membrane unit include the same ranges as the preferred ranges for the separation selectivity and CO2 and CH4 permeation rates at 40°C described above.

[0070] In the present invention, in order to efficiently obtain high-purity CH4 with a high recovery rate, the gas separation selectivity (carbon dioxide permeation rate P2') of the gas separation membrane of the second gas separation membrane unit at 40°C is CO2 / CH4 permeation rate P2' CH4 (40) ) is preferably 5 or more and 150 or less, more preferably 10 or more and 90 or less, even more preferably 15 or more and 65 or less, and particularly preferably 20 or more and less than 55.

[0071] In the present invention, in order to obtain high-purity CH4 at a high recovery rate while suppressing the compression power and membrane area, the carbon dioxide permeation rate P2' of the gas separation membrane of the second gas separation membrane unit at 40°C is set to CO2 (40) is 1.5 x 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 100×10 -5 cm 3 (STP) / cm 2 sec cmHg or less, and 7 x 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 90×10 -5 cm 3 (STP) / cm 2 sec cmHg or less, and more preferably 15 x 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 preferable, and 20 × 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg super 75×10 -5 cm 3 (STP) / cm 2 It is most preferable that the pressure is less than 1 / sec / cmHg.

[0072] In the present invention, in order to obtain high-purity CH4 while suppressing the compression power and membrane area, the CH4 permeation rate P2' of the gas separation membrane of the second gas separation membrane unit at 40°C is set to CH4 (40) 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 0.08 x 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 2.5×10 -5 cm 3 (STP) / cm 2 sec cmHg or less, and more preferably 0.3 × 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 1.5×10 -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 over 1.5×10 -5 cm 3 (STP) / cm 2 sec·cmHg or less is particularly preferred.

[0073] Furthermore, at the operating temperature T2 of the second gas separation membrane unit 12, the separation selectivity P2'CO2 / P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit (T2)However, it is preferable to cool the second feed gas to a temperature of 15 to 3000, in order to effectively reduce the membrane area and the compression power, thereby obtaining high-purity CH4 gas at low cost, and it is more preferable to cool the second feed gas to a temperature of 20 to 1400, even more preferably 30 to 800, and particularly preferably 30 to 700. Even at the temperature t2 of the second feed gas, the separation selectivity P2'CO2 / P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit (T2) It is preferable that the numerical range is similar to that of the above.

[0074] At the operating temperature T2 of the second gas separation membrane unit 12, the CO2 permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit (T2) is 3 x 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 80×10 -5 cm 3 (STP) / cm 2 Cooling the second supply gas to a pressure of 4×10 sec cmHg or less is preferable because it effectively reduces the membrane area and the compression power, thereby enabling high-purity CH4 gas to be obtained at low cost. -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 60×10 -5 cm 3 (STP) / cm 2 It is more preferable to cool the second supply gas to 5×10 sec cmHg or less. -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 55×10 -5 cm 3 (STP) / cm 2 sec cmHg or less, and more preferably 6 x 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 50×10 -5 cm 3 (STP) / cm 2It is particularly preferable that the CO2 permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit is 1 / sec.cmHg or less even at the second supply gas temperature t2. (T2) It is preferable that the numerical range is similar to that of the above.

[0075] At the operating temperature T2 of the second gas separation membrane unit 12, the CH permeation rate P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit is 0.002 × 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 1×10 -5 cm 3 (STP) / cm 2 Cooling the second supply gas to a viscosity of 0.005 × 10 sec cmHg or less is preferable because it effectively reduces the membrane area and the required compression power, thereby allowing high-purity CH4 gas to be obtained at low cost. -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 0.8×10 -5 cm 3 (STP) / cm 2 It is more preferable to cool the second supply gas to a value of 0.01×10 sec cmHg or less. -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 0.6×10 -5 cm 3 (STP) / cm 2 sec cmHg or less, and more preferably 0.01 × 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 0.5×10 -5 cm 3 (STP) / cm 2 It is particularly preferable that the CO2 permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit is 1 / sec.cmHg or less even at the second supply gas temperature t2. (T2) It is preferable that the numerical range is similar to that of the above.

[0076] In the present invention, the CO concentration in the obtained second non-permeable gas is preferably 100 ppm or less by volume, and more preferably 50 ppm or less, because such a high-purity CH gas with few impurities makes it possible to liquefy it.

[0077] The H2O concentration in the resulting second non-permeable gas is preferably 50 ppm or less, more preferably 20 ppm or less, by volume, because such high-purity CH4 gas with few impurities can be liquefied.

[0078] The raw material gas used in the present invention is a gas containing at least CO2 and CH4, and examples thereof 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 a fermentation process such as CH4 fermentation by the microorganisms. 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.

[0079] In the present invention, the feed gas preferably contains 30 mol% or more of CH4, and particularly preferably 40 to 95 mol%. Furthermore, in the present invention, the feed gas preferably contains 3 to 70 mol% of CO2, and particularly preferably 5 to 60 mol%, in view of the high technical significance of the gas separation system of the present invention. The CO2 concentration of the first permeable gas is preferably 85 to 99 mol%, and more preferably 88 to 98 mol%. The CO2 concentration of the first non-permeable gas is preferably 5 to 45 mol%, and more preferably 8 to 40 mol%. The CO2 concentration of the second permeable gas is preferably 20 to 85 mol%, and more preferably 25 to 80 mol%.

[0080] 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] Next, a gas separation system 10' according to a second embodiment of the present invention will be described with reference to Figure 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.

[0082] 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.

[0083] In the third gas separation membrane unit 13, the non-permeate gas outlet 13b is connected to the raw 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 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 outlet 13c of the third gas separation membrane unit 13.

[0084] The compression means 21 is installed for the purpose of compressing the raw 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.

[0085] 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 material gas to be separated is supplied from a mixed gas source (not shown) through a raw material gas supply line 26 to the first gas separation membrane unit 11. Prior to supply, the raw material gas is pressurized by a compression means 21, and its pressure increases.

[0086] As in the first embodiment, when the feed 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 CO2 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 feed 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 feed gas and then compressed by compression means 21 .

[0087] The third line 18 may or may not have a second compression means interposed therein.

[0088] In the gas separation system 10′ of this embodiment, it is preferable to use the cooling means 22 to lower the operating temperature T2 of the second gas separation membrane unit below the operating temperature T3 of the third gas separation membrane unit 13. This increases the permeation rate of CO and CH through the third gas separation membrane unit 13, thereby reducing the membrane area of ​​the third gas separation membrane unit 13 and thereby reducing equipment costs. In order to lower the operating temperature T2 of the second gas separation membrane unit below the operating temperature T3 of the third gas separation membrane unit 13, it is preferable to cool the second feed gas using the cooling means 22 so that the temperature t2 of the second feed gas supplied to the second gas separation membrane unit is lower than the temperature t3 of the first permeate gas (third feed gas) supplied to the third gas separation membrane unit 13, in terms of simplifying the system.

[0089] The CO2 permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit at the operating temperature T2 of the second gas separation membrane unit (T2) is the CO2 permeation rate P3'CO2 constituting the third gas separation membrane unit at the operating temperature T3. (T3) The CO2 permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit at the operating temperature T2 of the second gas separation membrane unit is preferably 0.5 or more, more preferably 0.65 or more, and even more preferably 0.75 or more. (T2) is the CO2 permeation rate P3'CO2 constituting the third gas separation membrane unit at the operating temperature T3. (T3) It is preferable that the P2'CO2 (T2) is P3'CO2 (T3) On the other hand, the CO2 permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit at the operating temperature T2 of the second gas separation membrane unit is (T2) is the CO2 permeation rate P3'CO2 constituting the third gas separation membrane unit at the operating temperature T3. (T3)The ratio to is preferably 7 or less, more preferably 6 or less, even more preferably 5 or less, and particularly preferably 4 or less. By setting the CO2 permeation rate in this range, the effect of obtaining high-purity CH4 gas at low cost can be further enhanced.

[0090] Furthermore, at the temperature of the second feed gas, the separation selectivity P2'CO2 / P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit at the operating temperature T2 of the second gas separation membrane unit is (T2) The separation selectivity P3'CO2 / P3'CH4 of the gas separation membrane constituting the third gas separation membrane unit at the operating temperature T3 of the third gas separation membrane unit is (T3) On the other hand, a value of 0.4 or more and 13 or less is preferred in that the membrane area can be effectively reduced to obtain high-purity CH4 gas at low cost, and a value of 0.5 or more and 12 or less is more preferred. In terms of reducing the compression power, a value of 0.6 or more and 7 or less is even more preferred, and a value of 0.7 or more and 5 or less is particularly preferred.

[0091] At 40°C, the permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit (40) The permeation rate P3'CO2 of the gas separation membrane constituting the third gas separation membrane unit (40) It is preferable that the permeation rate P2'CO2 of the gas separation membrane constituting the second gas separation membrane unit 12 at 40°C is higher than that of the gas separation membrane constituting the second gas separation membrane unit 12. (40) The permeation rate P3'CO2 of the gas separation membrane constituting the third gas separation membrane unit 13 at 40°C (40) is set to 1, it is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.4 or more. (40) is the permeation rate P3'CO2 of the gas separation membrane constituting the third gas separation membrane unit 13 at 40°C. (40) When the ratio of P2'CO2 to P2'CH4 is 1, it is preferable that the ratio is 12 or less from the viewpoint of availability of the gas separation membrane, and it is preferable that the ratio is 5 or less from the viewpoint of CH4 gas purity. (40)The separation selectivity of the gas separation membrane constituting the third gas separation membrane unit is P3'CO2 / P3'CH4 (40) It is preferable that the selectivity P2'CO2 / P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit 12 at 40°C is lower than that of the gas separation membrane constituting the second gas separation membrane unit 12. By adopting such a configuration, the membrane area of ​​the second gas separation membrane unit 12 can be further reduced. Since 40°C is a general operating temperature for membrane separation of CO2 and CH4, in this embodiment, the selectivity at this temperature is specified as an index. In order to further effectively reduce the membrane area of ​​the second gas separation membrane unit 12, the separation selectivity P2'CO2 / P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit 12 at 40°C is (40) The separation selectivity P3'CO2 / P3'CH4 of the gas separation membrane constituting the third gas separation membrane unit 13 at 40°C (40) When the ratio is taken as 1, it is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.70 or less, and particularly preferably 0.60 or less. In addition, the separation selectivity P2'CO2 / P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit 12 at 40°C is (40) is the separation selectivity P3'CO2 / P3'CH4 of the gas separation membrane constituting the third gas separation membrane unit 13 at 40°C. (40) When the ratio is taken as 1, it is preferably 0.2 or more in terms of availability of gas separation membranes, and it is preferably 0.3 or more in terms of CH4 gas purity.

[0092] The temperature t3 of the third supply gas is preferably 0 to 60°C, more preferably 10 to 50°C, and even more preferably 20 to 40°C. Setting the temperature t3 of the third supply gas within the above range is preferable because it makes it easier to increase the CH purity while adjusting the CO permeation rate and reducing the membrane area. Preferred temperature ranges for the operating temperature of the third gas separation membrane unit 13 include the same range as the preferred range for the temperature t3 of the third supply gas.

[0093] The temperature difference t3-t2 between the temperature t3 of the third supply gas and the temperature t2 of the second supply gas is preferably 10°C or more, and more preferably 15°C or more, in order to reduce the compression power and thereby enhance the effect of obtaining high-purity CH4 gas at low cost. The temperature difference t3-t2 is preferably 110°C or less, more preferably 90°C or less, and particularly preferably 55°C or less, in order to prevent a deterioration in the function of the gas separation membrane and reduce the membrane area by not making the temperature difference t3-t2 too large. Furthermore, the temperature difference T3-T2 between the operating temperature T3 of the third gas separation membrane unit 13 and the operating temperature T2 of the second gas separation membrane unit 12 is preferably 10°C or more and 110°C or less, more preferably 15°C or more and 90°C or less, and even more preferably 15°C or more and 55°C or less, similar to t3-t2.

[0094] In this embodiment, the proportion of the gas supply flow rate to the first gas separation membrane unit (first supply gas flow rate F1) that is refluxed from gas separation membrane units other than the first gas separation membrane unit (F4) (hereinafter also referred to as the "reflux rate") is preferably 70% or less in order to further reduce operating costs, and more preferably 60% or less. Furthermore, a reflux rate of, for example, a lower limit of 10% or more is preferred in terms of reducing membrane area, and a reflux rate of 20% or more is preferred. In this embodiment, F4 refers to the total amount of the second permeable gas in the second gas separation membrane unit 12 and the third non-permeable gas in the third gas separation membrane unit 13.

[0095] In the present invention, in order to efficiently obtain high-purity CH4 with a high recovery rate, the gas separation selectivity (P3') of the gas separation membrane of the third gas separation membrane unit at 40°C is CO2 / CH4 permeation rate P3' CH4 (40) ) is preferably 30 or more and 150 or less, more preferably 35 or more and 130 or less, even more preferably 40 or more and 120 or less, and particularly preferably 50 or more and 110 or less.

[0096] In the present invention, in order to obtain high-purity CH4 at a high recovery rate while suppressing the compression power and membrane area, the carbon dioxide permeation rate P3' of the gas separation membrane of the third gas separation membrane unit at 40°C is set to CO2 (40) is 1.5 x 10 -5 cm 3(STP) / cm 2 ・sec・cmHg or more 100×10 -5 cm 3 (STP) / cm 2 sec cmHg or less, and 2 x 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 45×10 -5 cm 3 (STP) / cm 2 sec cmHg or less is more preferable, and 3×10 -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 7 × 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 15×10 -5 cm 3 (STP) / cm 2 sec·cmHg or less is particularly preferred.

[0097] In the present invention, in order to obtain high-purity CH4 while suppressing the compression power and membrane area, the CH4 permeation rate P3' of the gas separation membrane of the third gas separation membrane unit at 40°C is set to CH4 (40) 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 0.8×10 -5 cm 3 (STP) / cm 2 sec cmHg or less is more preferable, and 0.05 × 10 -5 cm 3 (STP) / cm2 ・sec・cmHg or more 0.5×10 -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 0.2×10 -5 cm 3 (STP) / cm 2 It is particularly preferable that the pressure is .times. ...

[0098] The CO2 concentration of the first permeable gas is preferably 80 to 99 mol%, more preferably 83 to 95 mol%. The CO2 concentration of the first non-permeable gas is preferably 5 to 30 mol%, more preferably 10 to 25 mol%. The CO2 concentration of the second permeable gas is preferably 25 to 65 mol%, more preferably 30 to 60 mol%. The CO2 concentration of the third permeable gas is preferably 95 to 99.8 mol%, more preferably 98 to 99.5 mol%. The CO2 concentration of the third non-permeable gas is preferably 55 to 90 mol%, more preferably 60 to 85 mol%.

[0099] Preferred ranges for the separation selectivity and CO2 and CH4 permeation rates of the third gas separation membrane unit 13 at the operating temperature T3 of the third gas separation membrane unit include the same ranges as the preferred ranges for the separation selectivity and CO2 and CH4 permeation rates at 40°C described above.

[0100] The pressure of the compression means 21 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, as the pressure of the gas supplied to the third gas separation membrane unit 13 .

[0101] Although the present invention has been described above based on preferred embodiments thereof, the present invention is not limited to these embodiments. For example, in the above embodiments, a unit constituted by a gas separation membrane module having hollow fiber membranes is used as an example of each gas separation membrane unit, but other types of gas separation membrane units may be used instead.

[0102] In addition to the compression means in the above embodiment, a pressure reducing 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. Such pressure reducing means may include a known vacuum pump or the like.

[0103] For example, when biogas is used as the raw material gas, the raw material gas may be pre-treated to remove impurities such as siloxane, volatile organic compounds, and organic compounds of C2 or higher, and then the removed raw material gas may be supplied to the system 10.

[0104] The cooled second non-permeate gas may also be used to lower the operating temperature of the second gas separation membrane unit 12. For example, as shown in FIG. 4 , the cooled second non-permeate gas is introduced into a heat exchanger 54 via a line 52 branching from the recovery line 15, and a liquid medium 55 separately introduced into the heat exchanger 54 is cooled. One example of a method is to use the cooled liquid medium 55 for cooling the cooling means 22 via a line 56. The second non-permeate gas used to cool the liquid medium 55 is returned to the recovery line 15 via a line 53. Alternatively, as shown in FIG. 5 , the second non-permeate gas liquefied in a liquefaction unit 50 may be introduced into a heat exchanger 54, which serves as a methane vaporizer, via a line 59, and the liquid medium 55 may be cooled by the heat of vaporization of the liquefied second non-permeate gas.

[0105] Alternatively, as shown in Figure 6, the second non-permeable gas liquefied in liquefaction device 50 may be supplied to cooling means 22 via line 57 and used as a cooling refrigerant for cooling means 22, where it is cooled by heat exchange with the first non-permeable gas. The embodiments shown in Figures 4 to 6 are similar to the embodiments shown in Figures 1 and 3, respectively, in all other respects.

[0106] 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. In each example and comparative example, the HO concentration in the resulting gas product (second non-permeate gas) was 2 ppm or less by volume. In each example and comparative example, the first feed gas temperature was 40°C.

[0107] <Gas Separation Membrane Module> The gas separation properties at 40°C of the gas separation membrane modules used in the examples and comparative examples are shown in Table 1. This gas separation membrane module has a gas separation membrane made of an aromatic polyimide hollow fiber membrane housed in a case. Gas separation membrane module A is composed of a gas separation membrane with higher gas separation selectivity and lower gas permeation rate than gas separation membrane module B. P' in Table 1 CO2 and P' CH4 The unit is x 10 -5 cm 3 (STP) / cm 2 sec cmHg.

[0108]

[0109] Comparative Example 1 Using the gas separation system 10 shown in Figure 1, a mixed gas containing carbon dioxide and CH4 was separated to obtain CH4 as a product gas. The feed gas used had a composition of 39.8 mol% CO2, 59.7 mol% CH4, and 0.5 mol% HO. The flow rate of the gas entering the system was 200 Nm3. 3 / h. The cooling means 22 was not operated. The first gas separation membrane unit 11 and the second gas separation membrane unit 12 were both configured with the gas separation membrane module A. A compressor was used as the compression means 21 in the system 10.

[0110] Examples 1-1 to 1-4 The operating temperature of the second gas separation membrane unit 12 was lowered below the operating temperature of the first gas separation membrane unit 11 by operating the cooling means 22 and changing the temperature of the second feed gas as shown in Table 2 (first non-permeate gas ⇒ second feed gas). Except for this, the product gas was obtained in the same manner as in Comparative Example 1. The temperature drop in Table 2 is the value obtained by subtracting the temperature of the second feed gas supplied to the second gas separation membrane unit 12 from the temperature of the non-permeate gas discharged from the first gas separation membrane unit 11.

[0111] Comparative Example 2 A product gas was obtained in the same manner as in Comparative Example 1, except that the second gas separation membrane unit 12 was constructed using the gas separation membrane module B.

[0112] Examples 2-1 to 2-4 The cooling means 22 was operated to change the temperature of the second feed gas as shown in Table 2 (first non-permeable gas ⇒ second feed gas), thereby lowering the operating temperature of the second gas separation membrane unit 12 below the operating temperature of the first gas separation membrane unit 11. Except for this, the product gas was obtained in the same manner as in Comparative Example 2.

[0113] Comparative Example 3 was the same as Comparative Example 1, except that a gas separation system 10' shown in Figure 3 was used. The third gas separation membrane unit 13 was composed of a gas separation membrane module A. A compressor was used as the compression means 21 in the system 10. The raw material gas used had a composition of 39.8 mol% CO2, 59.7 mol% CH4, and 0.5 mol% HO. The flow rate of the raw material gas flowing into the system was 200 Nm 3 / h.

[0114] Examples 3-1 to 3-4 The cooling means 22 was operated to change the temperature of the second feed gas as shown in Table 3 (first non-permeable gas ⇒ second feed gas), thereby lowering the operating temperature of the second gas separation membrane unit 12 below the operating temperatures of the first gas separation membrane unit 11 and the third gas separation membrane unit 13. Except for this, the product gas was obtained in the same manner as in Comparative Example 3.

[0115] Comparative Example 4 A product gas was obtained in the same manner as in Comparative Example 3, except that the second gas separation membrane unit 12 was constructed using the gas separation membrane module B.

[0116] [Examples 4-1 to 4-4] By operating the cooling means 22 and changing the temperature of the second feed gas as shown in Table 3 (first non-permeable gas ⇒ second feed gas), the operating temperature of the second gas separation membrane unit 12 was lowered below the operating temperatures of the first gas separation membrane unit 11 and the third gas separation membrane unit 13. Except for this, the product gas was obtained in the same manner as in Comparative Example 4.

[0117]

[0118]

[0119] The results in Tables 2 and 3 show that the gas separation system of the present invention can obtain CH4 of such high purity that it can be liquefied from a feed gas containing carbon dioxide and CH4 while reducing the required compression power. Furthermore, by lowering the separation selectivity of the second gas separation membrane unit at 40°C compared to the separation selectivity of the first gas separation membrane unit at the same temperature, it is possible to reduce the number of gas separation membrane modules used in the second gas separation membrane unit while maintaining the purity, recovery rate, and compression power of the recovered CH4, thereby achieving both low compression power and a small area while maintaining high purity.

[0120] According to the present invention, high-purity CH4 can be obtained from a raw material gas containing CO2 and CH4 at low cost with a small membrane area.

Claims

1. A gas separation system comprising a first gas separation membrane unit and a second gas separation membrane unit and used for producing a CH4-enriched gas from a raw material gas containing CO2 and CH4, each gas separation membrane unit comprising at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a raw material gas supply line connected to the gas inlet of the first gas separation membrane unit, compression means interposed in the raw material 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, and a second line connecting the permeate gas outlet of the second gas separation membrane unit and the raw material gas supply line, and provided with cooling means for reducing the operating temperature T2 of the second gas separation membrane unit as compared with the operating temperature T1 of the first gas separation membrane unit.

2. At 40°C, the gas permeation rate P2'CO2 of the second gas separation membrane unit is higher than the gas permeation rate P1'CO2 of the first gas separation membrane unit. (40) (40) The gas separation system according to claim 1.

3. The carbon dioxide gas permeation rate P1'CO2 at the operating temperature T1 of the first gas separation membrane unit (T1) is greater than the carbon dioxide gas permeation rate P1'CO2 at the operating temperature T2 of the second gas separation membrane unit (T2) The gas separation system according to claim 1.

4. The gas separation system according to claim 1 or 2, wherein the gas temperature t2 supplied to the second gas separation membrane unit is 30°C or lower.

5. The gas separation system according to claim 1 or 2, wherein the gas temperature t2 supplied to the second gas separation membrane unit is lower than the gas temperature t1 supplied to the first gas separation membrane unit.

6. The gas separation system according to claim 1 or 2, wherein the difference T1 - T2 between the operating temperatures T1 and T2 is 10°C or higher.

7. The gas separation system according to claim 1 or 2, wherein heating means for heating the raw material gas supplied to the first gas separation membrane unit is provided in the raw material gas supply line.

8. During the operation of the gas separation system, the separation selectivity P1'CO2 / P1'CH4 of the gas separation membrane constituting the second gas separation membrane unit is 15 or more and 3000 or less, and the permeation rate P1'CO2 of CO2 is 3×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more and 80×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less. The gas separation system according to claim 1 or 2.

9. The gas separation system according to claim 1 or 2, wherein the cooling means is interposed in the first line.

10. The gas separation system according to claim 1 or 2, wherein the ratio of the flow rate refluxed to the first gas separation membrane unit in the gas supply flow rate to the first gas separation membrane unit is 10 - 60%.

11. The gas separation system according to claim 1 or 2, further comprising a third gas separation membrane unit, the third gas separation membrane unit comprising at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, and comprising 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, and a fourth line connecting the non-permeate gas outlet of the third gas separation membrane unit and the raw material gas supply line.

12. The gas separation system according to claim 11, wherein the operating temperature of the second gas separation membrane unit is lower than the operating temperature of the third gas separation membrane unit.

13. The gas separation system according to claim 1 or 2, further comprising a liquefaction device for further cooling and liquefying the non-permeating gas of the second gas separation membrane unit.

14. The gas separation system according to claim 1 or 2, wherein the operating temperature of the second gas separation membrane unit is reduced by using the non-permeating gas of the cooled second gas separation membrane unit.

15. A method for producing a gas enriched in CH4 by supplying a raw material gas containing at least CO2 and CH4 to a gas separation system, wherein the gas separation system comprises at least a first gas separation membrane unit and a second gas separation membrane unit, each gas separation membrane unit includes at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a raw material gas supply line connected to the gas inlet of the first gas separation membrane unit, a compression means disposed in the raw material 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, and a second line connecting the permeate gas outlet of the second gas separation membrane unit and the raw material gas supply line, and the system is provided with a cooling means for reducing the operating temperature of the second gas separation membrane unit to be lower than the operating temperature of the first gas separation membrane unit.

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