Gas separation system and method for producing methane-enriched gas

Abstract

The gas separation membrane unit 11 and the gas separation membrane unit 12 are provided. 2 and C.H. 4 From the feed gas containing CH 4 1. A gas separation system 10 for use in producing an enriched gas, comprising: a raw material gas supply line 26 connected to the gas inlet 11a of the first gas separation membrane unit 11; A compression means 21 disposed in a raw material gas supply line 26; a first line 14 connecting a non-permeate gas outlet 11b of the first gas separation membrane unit 11 and a gas inlet 12a of the second gas separation membrane unit 12; a second line 17 connecting the permeation gas outlet 12c of the second gas separation membrane unit 12 to the raw material gas supply line 26; A cooling means 22 is provided for lowering the operating temperature of the second gas separation membrane unit compared to the operating temperature of the first gas separation membrane unit.

Description

[Technical field]

[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 an enriched gas enriched in CH4 using the same. [Background technology]

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

[0003] In general, 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 a low permeable gas is recovered from a mixed gas using a single-stage gas separation membrane, if the purity of the recovered gas is constant, the recovery rate will be high if a membrane with high gas separation selectivity is used. However, since the permeation rate is low, it is necessary to increase the membrane area or the operating pressure. On the other hand, a membrane with a high permeation rate does not need to have a large membrane area or a high operating pressure, but has low gas separation selectivity, resulting in a low recovery rate.

[0004] In general, a gas separation membrane having selective gas permeability is used as a gas separation membrane module in which the gas separation membrane is housed in a container having 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 space of the gas supply side and the gas permeate side are isolated. In a gas separation system, a gas separation membrane unit is usually used in which a plurality of gas separation membrane modules are combined in parallel to obtain a required membrane area. Since the plurality of gas separation membrane modules constituting the gas separation membrane unit share the gas inlet, the non-permeate gas outlet, and the permeate gas outlet, the gas separation membrane unit essentially functions as a large gas separation membrane module.

[0005] In order to recover the target low permeable gas with high purity and high recovery rate, a method using a system equipped with this gas separation membrane unit in multiple stages is known. Examples of multi-stage gas separation systems include those that further separate the non-permeable gas in the first stage, which is enriched with the low permeable gas, in order to improve purity, and those that recover the low permeable gas contained in the permeable gas in the first stage in order to improve recovery rate.

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

[0007] [Patent Document 1] US2017 / 0304769A1 Summary of the Invention

[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 is intended to solve the above problems and provides the following configuration.

[0010] [1] A gas separation system comprising a first gas separation membrane unit and a second gas separation membrane unit, and used to produce a CH4-enriched gas from a raw gas containing CO2 and CH4, Each gas separation membrane unit has at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a raw material gas supply line connected to a gas inlet of the first gas separation membrane unit; A compression means disposed in the raw material gas supply line; a first line connecting a non-permeate gas outlet of the first gas separation membrane unit and a gas inlet of the second gas separation membrane unit; a second line connecting a permeation gas outlet of the second gas separation membrane unit to a raw material gas supply line; A gas separation system comprising a cooling means for lowering an operating temperature T2 of the second gas separation membrane unit compared to an operating temperature T1 of the first gas separation membrane unit.

[0011] [2] At 40°C, the gas permeation rate P1'CO2 of the first gas separation membrane unit (40) Compared to the gas permeation rate P2'CO2 of the second gas separation membrane unit, (40) The gas separation system according to [1], wherein [3] Carbon dioxide gas permeation rate P1'CO2 at the operating temperature T1 of the first gas separation membrane unit (T1) In comparison, the gas permeation rate P1'CO2 of carbon dioxide 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] The 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] The gas separation system according to any one of [1] to [4], wherein a temperature t2 of the gas supplied to the second gas separation membrane unit is lower than a temperature t1 of the gas 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] The gas separation system according to any one of [1] to [6], wherein the raw material gas supply line is provided with a heating means for heating the raw material 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 3×10 -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 pressure drop is 0.01 sq. cmHg or less.

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

[0018]

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

[0019]

[11] Further comprising a third gas separation membrane unit; The third gas separation membrane unit includes at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a third line connecting a permeate gas outlet of the first gas separation membrane unit and a gas inlet of the third gas separation membrane unit; The gas separation system according to any one of [1] to

[10] , further comprising a fourth line connecting a 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-permeated gas from the second gas separation membrane unit.

[0022]

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

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

[15] A method for producing an enriched gas enriched in CH4 by supplying a raw gas containing at least CO2 and CH4 to a gas separation system, comprising: The gas separation system comprises: At least a first gas separation membrane unit and a second gas separation membrane unit are provided, Each gas separation membrane unit has at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a raw material gas supply line connected to a gas inlet of the first gas separation membrane unit; A compression means disposed in the raw material gas supply line; a first line connecting a non-permeate gas outlet of the first gas separation membrane unit and a gas inlet of the second gas separation membrane unit; a second line connecting a permeation gas outlet of the second gas separation membrane unit to a raw material gas supply line; A method for producing an enriched gas, the system being provided with a cooling means for lowering the operating temperature of the second gas separation membrane unit below the operating temperature of the first gas separation membrane unit. [Brief description of the drawings]

[0023] [Figure 1]FIG. 1 is a schematic diagram showing the configuration of a gas separation system in a first embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing the structure of an example of a gas separation membrane module used in the gas separation system of the present invention. [Diagram 3] FIG. 3 is a schematic diagram showing the configuration of a gas separation system in a second embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram showing the configuration of a gas separation system in still another embodiment of the present invention. [Diagram 5] FIG. 5 is a schematic diagram showing the configuration of a gas separation system in still another embodiment of the present invention. [Figure 6] FIG. 6 is a schematic diagram showing the configuration of a gas separation system in still another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The present invention will now be described based on preferred embodiments with reference to the drawings. The upper and lower numerical limits in this specification can be combined without any restrictions. Each of the gas separation systems 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 each of the gas separation membrane units 11, 12, and 13, for example, as shown in FIG. 2, a module 40 formed by housing a gas separation membrane 30 having selective gas permeability, such as a hollow fiber membrane, in a casing 31 can be used. The gas separation membrane units 11, 12, and 13 shown in FIG. 1 and FIG. 3 are, for example, a gas separation membrane module 40 shown in FIG. 2, or a plurality of modules 40 arranged in parallel. The casing 31 in the module 40 has two opposing sides open to form an opening 32. It should be noted that the opening 32 is for inserting the gas separation membrane 30 into the casing 31, and is not an opening of the gas separation membrane 30. The gas separation membrane 30 is housed in the casing 31 through the opening 32. When the gas separation membrane 30 is made of a hollow fiber membrane bundle, the gas separation membrane 30 is housed in the casing 31 so that each end of the hollow fiber membrane opens near each opening 32 of the casing 31 in the housed state.

[0026] In a state where 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 membrane extends. Each opening 32 of the casing 31 is closed by a cover 35 and 36. The cover 35 is provided with a gas inlet 37. Meanwhile, the cover 36 is provided with a non-permeate gas outlet 38. The mixed gas to be separated is introduced into the module from the gas inlet 37 of the cover 35. Among the introduced gases, the gas that has permeated the gas separation membrane 30 is discharged to the outside of the module from a permeate gas outlet 39 provided in the casing 31. Meanwhile, the non-permeate gas that has not permeated the gas separation membrane 30 is discharged to the outside of the module from the non-permeate gas outlet 38 of the cover 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, for example, to a shell feed type module.

[0027] 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 connecting a non-permeate gas outlet 11b of the first gas separation membrane unit 11 to a gas inlet 12a of the second gas separation membrane unit 12 by a first line 14.

[0028] In the following description, the gas inlets, exhaust ports, supplied gas, permeate gas, and non-permeate 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 gas. In the system 10, in each gas separation membrane unit, CO2 is a gas that has a high permeation rate through the gas separation membrane, i.e., a high permeability gas, and CH4 is a gas that has a low permeation rate through the gas separation membrane, i.e., a low permeability gas.

[0029] A raw gas supply line 26 for supplying raw gas from a raw gas source (not shown) to the first gas separation membrane unit 11 is connected to the first gas inlet 11a of the first gas separation membrane unit 11. Compression means 21 is interposed in the raw gas supply line 26. The permeable gas outlet 12c of the second gas separation membrane unit 12 is connected to the raw material gas supply line 26 by a second line 17. Specifically, the second line 17 connects the second permeable gas outlet 12c to a position on the suction side of the compression means 21 of the raw material gas supply line 26.

[0030] The compression means 21 is provided for the purpose of compressing the raw material gas supplied from the raw material gas source. It is also provided for the purpose of compressing the second permeable gas discharged from the second gas separation membrane unit 12 when the second permeable gas is returned to the first gas separation membrane unit 11 through the second line 17. As the compression means 21, the same means as those used in the technical field can be used. For example, a compressor can be used. The raw material gas usually contains about 0 to 10 mass % moisture when it is supplied to the system.

[0031] A recovery line 15 for extracting non-permeate gas enriched with 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. High-purity methane with impurity concentrations suppressed to about several tens of ppmv is required for CH4 gas for liquefied methane production according to the standard, and the present invention can 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 one 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, a cooling means 22 is interposed in the first line 14 to cool the second feed gas.

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

[0035] As the polymeric material constituting the gas separation membrane, one having a selective permeability of CO2 with respect to CH4 is preferably used. Such polymeric materials include glassy polymeric materials and rubbery polymeric materials. For example, glassy polymeric materials include polyimide, polyamide, polyamideimide, cellulose-based materials such as cellulose acetate, polysulfone, polyethersulfone, polyphenylsulfone, polycarbonate, etc. Examples of rubbery polymeric materials include silicone resin, polybutadiene resin, etc. In particular, from the viewpoint of making the effects of the present invention more pronounced, it is preferable to use polyimide as the gas separation membrane constituting the second gas separation membrane unit 12. Polyimide is also preferably used as the material for the gas separation membranes constituting first gas separation membrane unit 11 and 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 dianhydride is preferably one having 2 to 3 aromatic rings, and examples thereof include 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride) (this compound is also called 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride), 2,3,3',4'-biphenyltetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, diphenylsulfonetetracarboxylic dianhydride, p-terphenyltetracarboxylic dianhydride, and m-terphenyltetracarboxylic dianhydride.

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

[0042] In the present invention, aromatic polyimides may be used in particular when polyimides are used as the material of the gas separation membrane constituting the second gas separation membrane unit 12. The aromatic polyimides 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 of the gas separation membrane constituting first gas separation membrane unit 11 and third gas separation membrane unit 13 described below, aromatic polyimide may be used.

[0044] When gas separation membranes made of polyimide are used for the first gas separation membrane unit 11, the second gas separation membrane unit 12, and the third gas separation membrane unit 13, an asymmetric membrane may be used as the gas separation membrane. An asymmetric membrane generally has 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 raw gas containing CH4 and CO2 to be separated is supplied from a raw gas source (not shown) through a raw gas supply line 26 to the first gas separation membrane unit 11. The raw gas is compressed by the compression means 21, and the pressure of the raw gas increases.

[0047] The raw material gas is compressed by compression means 21 and then cooled by cooling means 23, and condensate 53 containing moisture is separated and removed from the raw material gas through condensate flow line 24. The raw material gas from which 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 CH4 compared to the raw material gas. The first non-permeate gas is discharged from the non-permeate gas outlet 11b of the first gas separation membrane unit 11 and supplied to the second gas separation membrane unit 12 through the first line 14. On the other hand, the first permeable gas discharged from the first gas separation membrane unit 11 has a higher concentration of CO2 than the raw material gas. The first permeable gas is taken out of the system via a third line 18.

[0050] The first non-permeable 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, and contacts 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 this temperature condition, the first non-permeable gas (second supply gas) is separated into a second permeable gas and a second non-permeable gas by the gas separation membrane of the second gas separation membrane unit 12. The second non-permeable gas discharged from the second gas separation membrane unit 12 is further enriched in CH4. The second non-permeable gas may be supplied to the liquefaction device 50 from the second non-permeable gas outlet 12b of the 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 compression means 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 of CO2 and CH4 improves with the decrease in operating temperature. As a result, the amount of the second permeable gas that merges into the raw gas supply line 26 through the second line 17 (hereinafter, the total amount of gas that merges into the raw gas from units other than the first gas separation membrane unit 11 is also referred to as the "reflux amount") can be reduced, while achieving high CH4 purity of the second non-permeable gas. Since the compression power can be reduced by reducing the reflux amount, the present invention allows the production of high purity CH4 at low cost. In this embodiment, the conventionally used polyimide hollow fiber membrane is used as the gas separation membrane of the second gas separation membrane unit 12, and the 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 the 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 processing capacity to be obtained even with a small membrane area due to the high permeation rates of CO2 and CH4 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 reduced 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 CH4 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 above-mentioned cooling means 23, condensate flow line 24, and heating means 25 can be used to sufficiently lower the dew point of water in the second feed gas. 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 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 regarded as the same temperature 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 permeation rate P1'CO2 of the 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 preferably 0.5 or more, more preferably 0.65 or more, and even more preferably 0.75 or more. (T2) is the permeation rate P1'CO2 of the CO2 constituting the first gas separation membrane unit at the operating temperature T1 (T1) It is preferable that P2'CO2 is larger than P2'CO2 in order to reduce the membrane area. (T2) 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 preferably more than 1, more preferably 1.4 or more, even more preferably 1.5 or more, and particularly preferably 1.7 or more. (T2) is the permeation rate P1'CO2 of the 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 especially 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 permeation rate P1'CO2 of the first gas separation membrane unit (T1) On the other hand, 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 such a CO2 permeation rate, 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) 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) On the other hand, a ratio of 0.4 to 13 is preferable in that it effectively reduces the membrane area and the compression power, thereby enabling high-purity CH4 gas to be obtained at low cost, a ratio of 0.5 to 12 is more preferable, a ratio of 0.6 to 7 is even more preferable, and a ratio of 0.7 to 5 is particularly preferable in that it reduces the compression power. By achieving such separation selectivity and CO2 permeation rate, it is possible to further enhance the effect of obtaining high-purity CH4 gas at low cost.

[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)In order to reduce the membrane area of ​​the second gas separation membrane unit 12, 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 first gas separation membrane unit 12. (40) The permeation rate P1'CO2 of the gas separation membrane constituting the first gas separation membrane unit 11 at 40°C (40) When P2'CO2 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) When the purity of the CH4 gas is taken as 1, it is preferably 12 or less in terms of availability of the gas separation membrane, and it is preferably 5 or less in terms of CH4 gas purity. In addition, at 40°C, the separation selectivity P2'CO2 / P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit (40) The separation selectivity P1'CO2 / P1'CH4 of the gas separation membrane constituting the first gas separation membrane unit (40) In view of reducing the membrane area of ​​the second gas separation membrane unit 12, it is preferable that the above-mentioned value is lower than that of the first gas separation membrane unit 12. 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 (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 set to 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 purity of the CH4 gas is taken as 1, the ratio is preferably 0.2 or more in terms of availability of the gas separation membrane, and is preferably 0.3 or more in terms of CH4 gas purity. Since 40° C. is a typical 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 supply 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 supply gas within the above range is preferable in that it is easy to increase the CH4 purity while adjusting the CO2 permeation rate and suppressing the membrane area. In addition, since the gas separation membrane of the first gas separation membrane unit 11 of this embodiment usually has a higher selectivity for H2O gas than CH4, setting the first supply gas in such a temperature range can prevent water condensation in the first gas separation membrane unit 11 and realize water permeation within the unit, thereby reducing the water content in the second supply gas and lowering the dew point. The preferred temperature range for the operating temperature T1 of the first gas separation membrane unit 11 is the same as the preferred temperature range for the first supply gas t1, and is preferably 0 to 60°C, more preferably 10 to 50°C, and even more preferably 20 to 40°C.

[0061] The temperature t2 of the second supply gas is preferably 30° C. or less, since 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 supply 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. The temperature t2 of the second supply gas is preferably −50° C. or more, since this avoids a decrease in the gas separation function of the second gas separation membrane unit 12 and makes it easier to make the temperature of the first non-permeable gas supplied to the second gas separation membrane unit 12 equal to or higher than the dew point of water, more preferably −40° C. or more, even more preferably −30° C. or more, and particularly preferably −20° C. or more. 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 supply gas, preferably -50 to 30°C, more preferably -40 to 25°C, even more preferably -30 to 20°C, and particularly preferably -20°C to 10°C.

[0062] The temperature difference t1-t2 between the temperature t1 of the first supply gas and the temperature t2 of the second supply gas is more preferably 10° C. or more in terms of increasing the effect of obtaining high-purity CH4 gas at low cost with less required compression power, 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 in terms of preventing deterioration of the function of the gas separation membrane and reducing the membrane area by not making it 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, similar to t1-t2, 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.

[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 being sucked into the compression means 21 is preferably 2° C. or more in terms 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 dew point of water in 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 the pressure supplied to the second gas separation membrane unit 12. Here, the dew point of water in the second supply gas tends to decrease when the reflux amount is reduced, because the moisture content of the first supply gas is likely to decrease when the reflux amount is reduced, and therefore the dew point of water in the second supply gas decreases. For these reasons, the present invention makes it easier to obtain high-purity CH4 gas.

[0065] In this embodiment, the ratio of the flow rate (F4) of the gas separation membrane unit other than the first gas separation membrane unit to the gas supply flow rate (flow rate F1 of the first supply gas) to the first gas separation membrane unit (hereinafter also referred to as the "reflux rate") is preferably 60% or less in terms of further reducing operating costs, and more preferably 50% or less. In addition, for example, a lower limit of the reflux rate of 10% or more is preferable 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, further 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 2×10 -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 2sec cmHg or less, and more preferably 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.

[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, and 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 less than 1.5 sec cmHg.

[0069] Preferred ranges of 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 of 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 (permeation rate P2' of carbon dioxide) 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 1. 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 7×10 -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 15×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 80×10 -5 cm 3 (STP) / cm 2 It is particularly preferable that the concentration is less than 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 blood pressure is less than 1.5 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 1. 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, and 0.08×10 -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 0.3×10 -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 super 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 supply 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 supply gas to a temperature of 20 to 1400, further preferably 30 to 800, and particularly preferably 30 to 700. Even at the temperature t2 of the second supply 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 3×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more 80×10 -5 cm 3 (STP) / cm 2 Cooling the second feed 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 obtaining high-purity CH4 gas 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 a temperature of 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 6×10 -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 equal to or less than 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 CH4 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 feed gas to a pressure of 0.005×10 sec cmHg or less is preferable because it effectively reduces the membrane area and the required compression power, thereby enabling 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 temperature 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 equal to or less than t2. (T2) It is preferable that the numerical range is similar to that of the above.

[0076] In the present invention, the CO2 concentration in the second non-permeable gas obtained is preferably 100 ppm or less by volume, more preferably 50 ppm or less, because such a high-purity CH4 gas with few impurities makes it possible to liquefy it.

[0077] In addition, the H2O concentration in the second non-permeable gas obtained is preferably 50 ppm or less by volume, more preferably 20 ppm or less, because such a 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 a microorganism under anaerobic conditions and subjected to a fermentation process such as CH4 fermentation by the microorganism. 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 landfill sites. Biogas and landfill gas are usually composed mainly of CO2 and CH4.

[0079] In the present invention, the raw material gas preferably contains 30 mol% or more of CH4, and more preferably 40 to 95 mol%. In addition, in the present invention, the raw material gas preferably contains 3 to 70 mol% of CO2, and more 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-permeating 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, as the pressure of the gas normally supplied to the first gas separation membrane unit 11, 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.

[0081] Next, a gas separation system 10' according to a second embodiment of the present invention will be described with reference to Fig. 3. In the description of the second embodiment, the same components as those in the first embodiment are given the same reference numerals and the description will be omitted, and the 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 a permeation gas outlet 11c of the first gas separation membrane unit 11 to a gas inlet 13a of the third gas separation membrane unit 13 by a third line 18. As the gas separation membrane module constituting the third gas separation membrane unit 13, a gas separation membrane module similar to that of the first gas separation membrane unit 11 and the second gas separation membrane unit 12 can be used.

[0083] In the third gas separation membrane unit 13, the non-permeate gas discharge port 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 a position on the suction side of the compression means 21 in the raw gas supply line 26. In the example shown in Fig. 3, a third permeate gas discharge line 19 is connected to the permeate gas discharge port 13c of the third gas separation membrane unit 13.

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

[0086] As in the first embodiment, 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 and a first non-permeable gas. The first permeable gas discharged from the first gas separation membrane unit 11 is supplied to the third gas separation membrane unit 13 through the third line 18. The first permeable gas introduced into the third gas separation membrane unit 13 is separated into a third permeable gas and a third non-permeable gas by the unit 13. The third permeable gas is further enriched in CO2 compared to the first permeable gas introduced into the third gas separation membrane unit 13, and is taken out of the system from the permeable gas outlet 13c of the unit 13 through the third permeable gas outlet line 19. On the other hand, the third non-permeable gas is discharged from the non-permeable 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 gas supply line 26 via the 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 rates of CO2 and CH4 through the third gas separation membrane unit 13 and reduces the membrane area of ​​the third gas separation membrane unit 13, 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 supply gas using cooling means 22 so that the temperature t2 of the second supply gas supplied to the second gas separation membrane unit is lower than the temperature t3 of the first permeable gas (third supply gas) supplied to the third gas separation membrane unit 13, in order to simplify 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 permeation rate P3'CO2 of 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 permeation rate P3'CO2 of the third gas separation membrane unit at the operating temperature T3 (T3) It is preferable that P2'CO2 is larger than P2'CO2 in order to reduce the membrane area. (T2) 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 preferably more than 1, more preferably 1.4 or more, even more preferably 1.5 or more, and particularly preferably 1.7 or more. (T2) is the permeation rate P3'CO2 of the third gas separation membrane unit at the operating temperature T3 (T3) is preferably 7 or less, more preferably 6 or less, even more preferably 5 or less, and especially preferably 4 or less. By setting such a CO2 permeation rate, it is possible to further enhance the effect of obtaining high-purity CH4 gas at low cost.

[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 (T3) On the other hand, a ratio of 0.4 to 13 is preferable in terms of effectively reducing the membrane area to obtain high-purity CH4 gas at low cost, and a ratio of 0.5 to 12 is more preferable. From the viewpoint of reducing the compression power, a ratio of 0.6 to 7 is even more preferable, and a ratio of 0.7 to 5 is particularly preferable.

[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) In order to reduce the membrane area of ​​the second gas separation membrane unit 12, 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 first 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) When P2'CO2 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 purity of the CH4 gas is taken as 1, it is preferably 12 or less in terms of availability of the gas separation membrane, and it is preferably 5 or less in terms of CH4 gas purity. At 40°C, the separation selectivity P2'CO2 / P2'CH4 of the gas separation membrane constituting the second gas separation membrane unit (40) However, 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 membrane area of ​​the second gas separation membrane unit 12 is further reduced by using such a configuration. Since 40°C is a typical operating temperature for membrane separation of CO2 and CH4, in this embodiment, the selectivity at this temperature is defined as an index. 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 (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) is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.70 or less, and particularly preferably 0.60 or less, where P2'CO2 / P2'CH4 is 1. 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 purity of the CH4 gas is taken as 1, the ratio is preferably 0.2 or more in terms of availability of the gas separation membrane, and 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 further preferably 20 to 40° C. By setting the temperature t3 of the third supply gas within the above range, it is preferable in that it is easy to increase the CH4 purity while controlling the membrane area by adjusting the CO2 permeation rate. A preferred range of the operating temperature of the third gas separation membrane unit 13 is the same as the preferred range of 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 in terms of reducing the compression power and enhancing the effect of obtaining high purity CH4 gas at low cost, and more preferably 15° C. or more. 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 terms of preventing deterioration of the function of the gas separation membrane and reducing the membrane area by not making it too large. In addition, 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 flow rate (F4) refluxed from gas separation membrane units other than the first gas separation membrane unit among the gas supply flow rate (flow rate F1 of the first supply gas) to the first gas separation membrane unit (hereinafter also referred to as the "reflux rate") is preferably 70% or less in terms of further reducing operating costs, and more preferably 60% or less. In addition, the reflux rate is preferably, for example, preferably 10% or more in terms of reducing the membrane area, and is preferably 20% or more. In this embodiment, F4 refers to the total amount of the second permeable gas of the second gas separation membrane unit 12 and the third non-permeable gas of 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, further 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 2×10 -5 cm 3 (STP) / cm2 ·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, and more preferably 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, and 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) / cm2 ·sec·cmHg or more 0.2×10 -5 cm 3 (STP) / cm 2 It is particularly preferable that the pressure is less than 1.5 sec cmHg.

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

[0099] Preferred ranges of 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 of the separation selectivity and CO2 and CH4 permeation rates at 40°C described above.

[0100] The pressure of the compression means 21, as the pressure of the gas normally 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.

[0101] Although the present invention has been described above based on its preferred embodiments, the present invention is not limited to the above 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 a gas separation membrane unit of another form 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. A known vacuum pump or the like can be used as such a pressure reducing means.

[0103] For example, when biogas is used as the raw gas, the raw gas may be pre-treated to remove impurities such as siloxane, volatile organic compounds, and organic compounds with a C2 or higher content, and the raw gas after removal may then 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-permeable gas is introduced into a heat exchanger 54 through a line 52 branched from the recovery line 15, and a liquid medium 55 separately introduced into the heat exchanger 54 is cooled. The cooled liquid medium 55 is used to cool the cooling means 22 through a line 56. The second non-permeable gas used to cool the liquid medium 55 is returned to the recovery line 15 through a line 53. Alternatively, as shown in FIG. 5, the second non-permeable gas liquefied in a liquefaction device 50 may be introduced into a heat exchanger 54 serving as a methane vaporizer through a line 59 as the cooled second non-permeable gas, and the liquid medium 55 may be cooled by the heat of vaporization of the liquefied second non-permeable gas.

[0105] Alternatively, as shown in FIG. 6, the second non-permeable gas liquefied in the liquefaction device 50 may be supplied to the cooling means 22 through a line 57 and used as a cooling refrigerant for the cooling means 22, where it is cooled by heat exchange with the first non-permeable gas. Other than the above, the embodiments in Figs. 4 to 6 are similar to the embodiments in Figs. 1 and 3, respectively. EXAMPLES

[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 by these examples. In each example and comparative example, the H2O concentration in the obtained gas product (second non-permeable gas) was 2 ppm or less on a volume basis. 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] [Table 1]

[0109] [Comparative Example 1] Using the gas separation system 10 shown in FIG. 1, a mixed gas containing carbon dioxide and CH4 was separated to obtain CH4 as a product gas. The raw gas used was 39.8 mol% CO2, 59.7 mol% CH4, and 0.5 mol% H2O. The flow rate of the gas entering the system was 200 Nm 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 constructed of the gas separation membrane module A. As the compression means 21 in the system 10, a compressor was used.

[0110] [Examples 1-1 to 1-4] The cooling means 22 was operated to change the temperature of the second supply gas as shown in the temperature drop in Table 2 (first non-permeate gas ⇒ second supply 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 1. Note that the temperature drop in Table 2 is the value obtained by subtracting the temperature of the second supply 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 with 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, whereby 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. Except for this, the product gas was obtained in the same manner as in Comparative Example 2.

[0113] [Comparative Example 3] The same procedure was followed as in Comparative Example 1, except that a gas separation system 10' shown in FIG. 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 % H2O. 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 second supply gas temperature as shown in Table 3, whereby the operating temperature of the second gas separation membrane unit 12 was made lower than 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 with the gas separation membrane module B.

[0116] [Examples 4-1 to 4-4] The cooling means 22 was operated to change the second supply gas temperature as shown in Table 3, whereby the operating temperature of the second gas separation membrane unit 12 was made lower than 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] [Table 2]

[0118] [Table 3]

[0119] From the results in Tables 2 and 3, it is seen that the gas separation system of the present invention is capable of obtaining CH4 of a purity high enough to be liquefied from a feed gas containing carbon dioxide and CH4 while reducing the required compression power. In addition, 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, and it is seen that it is possible to achieve both low compression power and a small area while maintaining high purity. [Industrial Applicability]

[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 first gas separation membrane unit and a second gas separation membrane unit are provided, 2 and C.H. 4 from a source gas containing CH 4 1. A gas separation system for use in producing an enriched gas, comprising: Each gas separation membrane unit has at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a raw material gas supply line connected to a gas inlet of the first gas separation membrane unit; A compression means disposed in the raw material gas supply line; a first line connecting a non-permeate gas outlet of the first gas separation membrane unit and a gas inlet of the second gas separation membrane unit; a second line connecting a permeation gas outlet of the second gas separation membrane unit to a raw material gas supply line; A gas separation system comprising a cooling means for lowering an operating temperature T2 of the second gas separation membrane unit compared to an operating temperature T1 of the first gas separation membrane unit.

2. At 40° C., the gas permeation rate P of the first gas separation membrane unit 1 'CO 2 (40) Compared to the gas permeation rate P 2 'CO 2 (40) The gas separation system of claim 1 ,

3. The gas permeation rate P of carbon dioxide at the operating temperature T1 of the first gas separation membrane unit 1 'CO 2 (T1) , the gas permeation rate P of carbon dioxide at the operating temperature T2 of the second gas separation membrane unit 1 'CO 2 (T2) The gas separation system of claim 1 , wherein

4. 3. 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. 3. The gas separation system according to claim 1, wherein a temperature t2 of the gas supplied to the second gas separation membrane unit is lower than a temperature t1 of the gas 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 more.

7. 3. The gas separation system according to claim 1, 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.

8. During operation of the gas separation system, the separation selectivity P of the gas separation membrane constituting the second gas separation membrane unit 1 'CO 2 / P 1 'CH 4 is 15 or more and 3000 or less, and CO 2 Permeation rate P 1 'CO 2 is 3 x 10 -5 cm 3 (STP) / cm 2 ・sec・cmHg or more 80×10 -5 cm 3 (STP) / cm 2 3. The gas separation system according to claim 1 or 2, wherein the pressure drop is less than or equal to 1.5 sec cmHg.

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

10. 3. The gas separation system according to claim 1, wherein the proportion of the flow rate of the gas returned to the first gas separation membrane unit in the flow rate of the gas supplied to the first gas separation membrane unit is 10 to 60%.

11. Further comprising a third gas separation membrane unit; The third gas separation membrane unit includes at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a third line connecting a permeation gas outlet of the first gas separation membrane unit and a gas inlet of the third gas separation membrane unit; 3. The gas separation system according to claim 1, further comprising a fourth line connecting a non-permeate gas outlet of the third gas separation membrane unit to 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. 3. The gas separation system according to claim 1, further comprising a liquefaction device for further cooling and liquefying the non-permeate gas from the second gas separation membrane unit.

14. 3. 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 a cooled non-permeate gas of the second gas separation membrane unit.

15. At least CO 2 and C.H. 4 A raw gas containing CH 4 A method for producing an enriched gas enriched with The gas separation system comprises: At least a first gas separation membrane unit and a second gas separation membrane unit are provided, Each gas separation membrane unit has at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a raw material gas supply line connected to a gas inlet of the first gas separation membrane unit; A compression means disposed in the raw material gas supply line; a first line connecting a non-permeate gas outlet of the first gas separation membrane unit and a gas inlet of the second gas separation membrane unit; a second line connecting a permeation gas outlet of the second gas separation membrane unit to a raw material gas supply line; A method for producing an enriched gas, the system being provided with a cooling means for lowering the operating temperature of the second gas separation membrane unit below the operating temperature of the first gas separation membrane unit.