Gas separation system and method for separating mixed gas

Abstract

To provide a gas separation system suitable for reducing energy required for separating a mixed gas.SOLUTION: The gas separation system 100 of the present invention includes the first separation membrane unit 10 that separates the mixed gas 70 containing carbon dioxide and nitrogen into the first permeable gas 80 and the first non-permeable gas 81, the second separation membrane unit 20 that separates the first permeable gas 80 into the second permeable gas 90 and the second non-permeable gas 91, the first pressure reducing device 50 that reduces the pressure of the permeate-side space of the first separation membrane unit 10, and the second pressure reducing device 52 that reduces the pressure of the permeate-side space of the second separation membrane unit 20.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gas separation system and a method for separating a mixed gas. [Background technology]

[0002] In factories, power plants, and the like, energy obtained by burning fuel is utilized. Usually, burning fuel produces exhaust gases containing carbon dioxide and nitrogen. From the viewpoint of environmental regulations, it is desirable to remove carbon dioxide from the exhaust gases.

[0003] Membrane separation has been developed as a method for separating carbon dioxide from mixed gases containing carbon dioxide. Compared to absorption methods, which separate carbon dioxide contained in mixed gases by absorbing it into an absorbent, membrane separation methods can efficiently separate carbon dioxide while reducing operating costs.

[0004] In membrane separation, a gas separation system combining multiple separation membrane units is sometimes used to separate mixed gases with high accuracy. For example, Patent Document 1 discloses an apparatus using membrane separation, which includes a feed stream separation stage for separating a feed stream and a permeate separation stage for further separating the first permeate stream obtained in the feed stream separation stage. Patent Document 2 discloses a gas separation system including a first gas separation membrane unit for separating mixed gases and a second gas separation membrane unit for separating the retentate gas discharged from the first gas separation membrane unit. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2018-511472 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-128868 Summary of the Invention [Problem to be solved by the invention]

[0006] Conventional gas separation systems tend to consume a large amount of energy when separating mixed gases, particularly exhaust gases containing carbon dioxide and nitrogen.

[0007] Therefore, an object of the present invention is to provide a gas separation system suitable for reducing the energy required to separate a mixed gas. [Means for solving the problem]

[0008] As a result of extensive research, the present inventors have newly discovered that when the pressure of a mixed gas supplied to a gas separation system is significantly increased, the energy required to separate the mixed gas increases significantly. Based on this finding, the present inventors have furthered their research and have completed the present invention.

[0009] The present invention provides a first separation membrane unit that separates a mixed gas containing carbon dioxide and nitrogen into a first permeate gas and a first non-permeate gas; a second separation membrane unit that separates the first permeable gas into a second permeable gas and a second non-permeable gas; a first pressure reducing device that reduces the pressure in the permeate side space of the first separation membrane unit; a second pressure reducing device that reduces the pressure in the permeate side space of the second separation membrane unit; A gas separation system comprising:

[0010] Furthermore, the present invention provides a first separation step of supplying a mixed gas containing carbon dioxide and nitrogen to a first separation membrane unit and separating the mixed gas into a first permeate gas and a first non-permeate gas by reducing the pressure in the permeate side space of the first separation membrane unit; a second separation step of supplying the first permeate gas to a second separation membrane unit and reducing the pressure in the permeate side space of the second separation membrane unit, thereby separating the first permeate gas into a second permeate gas and a second non-permeate gas; The present invention provides a method for separating a mixed gas, comprising: [Effects of the Invention]

[0011] According to the present invention, a gas separation system suitable for reducing the energy required to separate a mixed gas can be provided. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a configuration diagram of a gas separation system according to one embodiment of the present invention. [Figure 2] FIG. 3 is a schematic cross-sectional view showing an example of a first separation membrane unit. [Figure 3] FIG. 2 is a cross-sectional view showing an example of a first separation membrane. [Figure 4] FIG. 3 is a schematic cross-sectional view showing an example of a second separation membrane unit. [Figure 5] 1 is a graph for explaining relational expression (A). [Figure 6] FIG. 4 is a schematic cross-sectional view showing another example of a first separation membrane unit. [Figure 7] FIG. 2 is a configuration diagram showing another example of a gas separation system. [Figure 8] FIG. 10 is a configuration diagram showing yet another example of a gas separation system. [Figure 9] FIG. 3 is a schematic cross-sectional view showing an example of a third separation membrane unit. [Figure 10] FIG. 1 is a diagram illustrating the configuration of a conventional gas separation system. [Figure 11] FIG. 1 is a diagram illustrating the configuration of a conventional gas separation system. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in detail below, but the following description is not intended to limit the present invention to a specific embodiment.

[0014] <Gas separation system> As shown in FIG. 1, the gas separation system 100 of this embodiment includes a first separation membrane unit 10, a second separation membrane unit 20, a first pressure reducing device 50, and a second pressure reducing device 52. The first separation membrane unit 10 is a membrane separation device that performs membrane separation on a mixed gas containing carbon dioxide and nitrogen using a first separation membrane. The first separation membrane of the first separation membrane unit 10 can separate the mixed gas into a first permeate gas and a first non-permeate gas. The second separation membrane unit 20 is a membrane separation device that performs membrane separation on the first permeate gas discharged from the first separation membrane unit 10 using a second separation membrane. The second separation membrane of the second separation membrane unit 20 can separate the first permeate gas into a second permeate gas and a second non-permeate gas.

[0015] The mixed gas may include, for example, a main component gas and a sub-component gas. The main component gas refers to the gas that is contained in the mixed gas in the largest amount by weight. The sub-component gas refers to the gas contained in the mixed gas other than the main component gas. The mixed gas may include, for example, nitrogen as the main component gas and carbon dioxide as the sub-component gas. An example of the mixed gas is exhaust gas produced by burning fuels (fossil fuels) such as natural gas (city gas) and propane gas. However, the mixed gas is not limited to exhaust gas and may also be air.

[0016] The first separation membrane of the first separation membrane unit 10 allows, for example, a minor component gas contained in the mixed gas, more specifically, at least one gas contained in the minor component gas, to preferentially permeate. Therefore, the first permeable gas separated by the first separation membrane has a higher minor component gas content than the mixed gas and a lower main component gas content than the mixed gas. On the other hand, the first non-permeable gas has a lower minor component gas content than the mixed gas and a higher main component gas content than the mixed gas. Note that, for ease of explanation, the names "main component gas" and "minor component gas" in the first permeable gas are defined based on their contents in the mixed gas before separation. For example, even if the content of a minor component gas (e.g., carbon dioxide) in the first permeable gas is higher than the content of a major component gas (e.g., nitrogen) in the mixed gas, the minor component gas in the mixed gas will be referred to as the "minor component gas" of the first permeable gas, and the major component gas in the mixed gas will be referred to as the "main component gas" of the first permeable gas.

[0017] The second separation membrane of the second separation membrane unit 20 can preferentially permeate, for example, a minor component gas contained in the first permeable gas, more specifically, at least one gas contained in the minor component gas. Therefore, the second permeable gas separated by the second separation membrane has a higher minor component gas content than the first permeable gas and a lower main component gas content than the first permeable gas. On the other hand, the second non-permeable gas has a lower minor component gas content than the first permeable gas and a higher main component gas content than the first permeable gas. Note that, in this specification, the terms "major component gas" and "minor component gas" for the second permeable gas and the second non-permeable gas, as with the first permeable gas, are defined based on their contents in the mixed gas before separation by the first separation membrane.

[0018] The first pressure reducing device 50 can reduce the pressure in the permeate side space of the first separation membrane unit 10. In other words, the first pressure reducing device 50 can generate or increase a pressure difference between the supply side space and the permeate side space of the first separation membrane unit 10. A specific example of the first pressure reducing device 50 is a pump. The first pressure reducing device 50 is preferably a vacuum device such as a vacuum pump. Vacuum pumps are typically gas transport vacuum pumps, and examples include reciprocating vacuum pumps and rotary vacuum pumps. Examples of reciprocating vacuum pumps include diaphragm and oscillating piston vacuum pumps. Examples of rotary vacuum pumps include liquid ring pumps; oil rotary pumps (rotary pumps); mechanical booster pumps; and various dry pumps such as roots, claw, screw, turbo, and scroll types. The pump serving as the first pressure reducing device 50 may be equipped with a variable speed mechanism for changing the rotation speed, etc. An example of the variable speed mechanism is an inverter that drives the pump motor. By controlling the rotation speed of the pump with the variable speed mechanism, the pressure in the permeate side space of the first separation membrane unit 10 can be adjusted appropriately.

[0019] The first pressure reducing device 50 may be an assembly of multiple pumps. That is, the first pressure reducing device 50 may have a configuration in which the pressure in the permeate side space of the first separation membrane unit 10 can be reduced by each of the multiple pumps. With this configuration, the pressure in the permeate side space of the first separation membrane unit 10 can be appropriately adjusted by adjusting the number of pumps in operation.

[0020] The second pressure reducing device 52 can reduce the pressure in the permeate side space of the second separation membrane unit 20. In other words, the second pressure reducing device 52 can generate or increase a pressure difference between the supply side space and the permeate side space of the second separation membrane unit 20. A specific example of the second pressure reducing device 52 is a pump. The second pressure reducing device 52 is preferably a vacuum device such as a vacuum pump. The vacuum pumps described above for the first pressure reducing device 50 can be used. The pump serving as the second pressure reducing device 52 may be equipped with a variable speed mechanism for changing the rotation speed, etc. An example of the variable speed mechanism is an inverter that drives the pump motor. The pressure in the permeate side space of the second separation membrane unit 20 can be appropriately adjusted by controlling the rotation speed, etc. of the pump using the variable speed mechanism.

[0021] The second pressure reducing device 52 may be an assembly of multiple pumps. That is, the second pressure reducing device 52 may have a configuration in which the pressure in the permeate side space of the second separation membrane unit 20 can be reduced by each of the multiple pumps. With this configuration, the pressure in the permeate side space of the second separation membrane unit 20 can be appropriately adjusted by adjusting the number of pumps in operation.

[0022] The gas separation system 100 further includes a mixed gas supply path 30. The mixed gas supply path 30 is connected to the mixed gas inlet (inlet 13a) of the first separation membrane unit 10 and is a path for supplying the mixed gas to the first separation membrane unit 10 from a tank (not shown) or the like that stores the mixed gas. The mixed gas supply path 30 may be directly connected to a mixed gas source, or may be configured to continuously supply the mixed gas from the source to the first separation membrane unit 10. The mixed gas supply path 30 may or may not include a pressurizing device 54 that pressurizes the supply space of the first separation membrane unit 10. Examples of the pressurizing device 54 include a compressor, a blower, and a back pressure valve. The pressurizing device 54 can pressurize the supply space of the first separation membrane unit 10, for example, by increasing the pressure of the mixed gas supplied to the first separation membrane unit 10.

[0023] A gas sensor for measuring the composition of the mixed gas may be disposed in the mixed gas supply path 30. This gas sensor can detect, for example, the carbon dioxide content of the mixed gas. The gas sensor is preferably disposed near the inlet 13a of the first separation membrane unit 10. By using the gas sensor to measure the composition of the mixed gas supplied to the first separation membrane unit 10, the operating conditions of the first separation membrane unit 10, particularly the degree of vacuum in the permeate side space of the first separation membrane unit 10, can be appropriately set.

[0024] The gas separation system 100 further includes a permeate gas supply path 32. The permeate gas supply path 32 is connected to the permeate gas outlet (outlet 14a) of the first separation membrane unit 10 and the permeate gas inlet (inlet 23a) of the second separation membrane unit 20, and is a path for supplying the first permeate gas from the first separation membrane unit 10 to the second separation membrane unit 20. A first pressure reducing device 50 is disposed in the permeate gas supply path 32.

[0025] The permeate gas supply path 32 has a first portion 32a extending from the first separation membrane unit 10 to the first pressure reducing device 50 and a second portion 32b extending from the first pressure reducing device 50 to the second separation membrane unit 20. The first pressure reducing device 50 can reduce the pressure in the permeate side space of the first separation membrane unit 10 through the first portion 32a. The first pressure reducing device 50, for example, sucks in the first permeate gas that has passed through the first portion 32a and discharges the first permeate gas to the second portion 32b. The first pressure reducing device 50 is configured, for example, not to discharge the gas that has passed through the first portion 32a to the outside of the gas separation system 100. The first pressure reducing device 50 is connected, for example, only to the first portion 32a and the second portion 32b. When the first pressure reducing device 50 is an assembly of multiple pumps, the first portion 32a of the permeate gas supply path 32 may branch and be connected to the inlets of the multiple pumps, respectively. Similarly, the second portion 32b of the permeate gas supply path 32 may branch and be connected to the outlets of multiple pumps, respectively.

[0026] The second section 32b may or may not be provided with a pressurizing device (not shown) that pressurizes the first permeate gas discharged from the first pressure reducing device 50. This pressurizing device can pressurize the supply side space of the second separation membrane unit 20. Examples of pressurizing devices include a compressor, a blower, and a back pressure valve. The second section 32b may be provided with a gas sensor for measuring the composition of the first permeate gas. This gas sensor can detect, for example, the carbon dioxide content of the first permeate gas. The gas sensor is preferably provided near the inlet 23a of the second separation membrane unit 20. By using the gas sensor to measure the composition of the first permeate gas supplied to the second separation membrane unit 20, the operating conditions of the second separation membrane unit 20, particularly the degree of pressure reduction in the permeate side space of the second separation membrane unit 20, can be appropriately set.

[0027] It is preferable that the permeable gas supply path 32 does not include a tank such as a collector for collecting the first permeable gas or an on-off valve for opening and closing the path. In particular, when a collector is not provided in the permeable gas supply path 32, the first permeable gas discharged from the first separation membrane unit 10 can be continuously supplied to the second separation membrane unit 20. With this configuration, the amount of mixed gas processed per unit time can be easily increased. As an example, the permeable gas supply path 32 may be composed of only the first pressure reducing device 50 and piping.

[0028] The gas separation system 100 further includes a first discharge path 34. The first discharge path 34 is connected to the non-permeate gas outlet (outlet 13b) of the first separation membrane unit 10, and is a path for discharging the first non-permeate gas from the first separation membrane unit 10. The first discharge path 34 has an opening (discharge port 42) formed therein for discharging the first non-permeate gas from the first discharge path 34. The gas separation system 100 may further include a tank (not shown) for storing the first non-permeate gas, and the first discharge path 34 may be connected to the tank.

[0029] The gas separation system 100 further includes a second discharge path 36 and a tank 60. The second discharge path 36 is connected to the permeate gas outlet (outlet 24a) of the second separation membrane unit 20 and the inlet of the tank 60, and is a path for sending the second permeate gas from the second separation membrane unit 20 to the tank 60. The tank 60 can store the second permeate gas sent from the second separation membrane unit 20. A second pressure reducing device 52 is arranged in the second discharge path 36.

[0030] The second discharge path 36 has a first portion 36a extending from the second separation membrane unit 20 to the second pressure reducing device 52 and a second portion 36b extending from the second pressure reducing device 52 to the tank 60. The second pressure reducing device 52 can reduce the pressure in the permeate side space of the second separation membrane unit 20 through the first portion 36a. The second pressure reducing device 52, for example, sucks the second permeate gas that has passed through the first portion 36a and discharges the second permeate gas to the second portion 36b. The second pressure reducing device 52 is configured, for example, not to discharge the gas that has passed through the first portion 36a to the outside of the gas separation system 100. The second pressure reducing device 52 is connected, for example, only to the first portion 36a and the second portion 36b. When the second pressure reducing device 52 is an assembly of multiple pumps, the first portion 36a of the second discharge path 36 may branch and be connected to the inlets of the multiple pumps, respectively. Similarly, the second portion 36b of the second discharge path 36 may branch and be connected to each of the outlets of a plurality of pumps.

[0031] The gas separation system 100 further includes a third discharge path 38. The third discharge path 38 is connected to the non-permeate gas outlet (outlet 23b) of the second separation membrane unit 20 and is a path for discharging the second non-permeate gas from the second separation membrane unit 20. The third discharge path 38 may merge with the mixed gas supply path 30 at a merging position 40. In FIG. 1 , the third discharge path 38 is connected to a pressurizing device 54 and merges with the mixed gas supply path 30 at the pressurizing device 54. By merging the third discharge path 38 with the mixed gas supply path 30, it is possible, for example, to reuse the second non-permeate gas containing minor component gases that were not completely separated by the second separation membrane unit 20.

[0032] Unless otherwise specified, each of the paths in the gas separation system 100 is made up of, for example, metal or resin piping.

[0033] The gas separation system 100 may further include a controller (not shown) that controls each component of the gas separation system 100. The controller is, for example, a DSP (Digital Signal Processor) including an A / D conversion circuit, an input / output circuit, an arithmetic circuit, a storage device, etc. The controller stores a program for appropriately operating the gas separation system 100. In detail, the controller may control, for example, the operation of the first pressure reducing device 50 and the second pressure reducing device 52. The controller may receive information from the gas sensor and determine the operating conditions of the pressure reducing devices 50 and 52 based on the information.

[0034] As an example, when the mixed gas is supplied to the first separation membrane unit 10 from the mixed gas supply path 30, the controller controls the first pressure reducing device 50 to reduce the pressure in the permeate side space of the first separation membrane unit 10. Specifically, the controller controls the first pressure reducing device 50 to continue reducing the pressure in the permeate side space of the first separation membrane unit 10 while the mixed gas is being supplied to the first separation membrane unit 10. Similarly, when the first permeate gas is supplied to the second separation membrane unit 20 from the permeate gas supply path 32, the controller controls the second pressure reducing device 52 to continue reducing the pressure in the permeate side space of the second separation membrane unit 20. Specifically, the controller controls the second pressure reducing device 52 to continue reducing the pressure in the permeate side space of the second separation membrane unit 20 while the first permeate gas is being supplied to the second separation membrane unit 20. Note that the gas separation system 100 does not necessarily have to include a controller. In gas separation system 100, an operator may control the operation of pressure reducing devices 50 and 52 by switching pressure reducing devices 50 and 52 on and off.

[0035] The gas separation system 100 of this embodiment is, for example, a continuous system. In this specification, a continuous system refers to a system that can continuously process a mixed gas without closing the paths that make up the gas separation system 100 with an on-off valve or the like. In other words, the gas separation system 100 can process a mixed gas in the first separation membrane unit 10, and can immediately process the first permeate gas obtained in the first separation membrane unit 10 in the second separation membrane unit 20 without collecting it in a tank or the like. In this way, the gas separation system 100 of this embodiment is capable of continuous operation. The gas separation system 100 that functions as a continuous system is suitable for applications in which a mixed gas is continuously supplied, particularly for treating exhaust gases.

[0036] In the gas separation system 100, for example, at least one of the following is satisfied: (I) the mixed gas is separated into a first permeate gas and a first non-permeate gas while the permeate side space of the first separation membrane unit 10 is depressurized by the first depressurization device 50, and (II) the first permeate gas is separated into a second permeate gas and a second non-permeate gas while the permeate side space of the second separation membrane unit 20 is depressurized by the second depressurization device 52. In the gas separation system 100, it is preferable that both of the above requirements (I) and (II) are satisfied.

[0037] [First separation membrane unit] As shown in FIG. 2, the first separation membrane unit 10 includes a first separation membrane 11 and a tank 12. The tank 12 has a first chamber 13 and a second chamber 14. The space within the first chamber 13 corresponds to the supply-side space, and the space within the second chamber 14 corresponds to the permeation-side space. The first separation membrane 11 is disposed inside the tank 12. Inside the tank 12, the first separation membrane 11 separates the first chamber 13 and the second chamber 14. The first separation membrane 11 extends from one to the other of a pair of wall surfaces of the tank 12.

[0038] The first chamber 13 has an inlet 13a and an outlet 13b. The second chamber 14 has an outlet 14a. The inlet 13a of the first chamber 13 is an opening for supplying the mixed gas 70 to the first separation membrane unit 10. The outlet 14a of the second chamber 14 is an opening for discharging a first permeate gas 80, which is obtained when the mixed gas 70 permeates the first separation membrane 11, from the first separation membrane unit 10. The outlet 13b of the first chamber 13 is an opening for discharging the mixed gas 70 (first non-permeate gas 81) that did not permeate the first separation membrane 11 from the first separation membrane unit 10. The inlet 13a, the outlet 13b, and the outlet 14a are each formed, for example, on a wall surface of the tank 12.

[0039] (1st separation membrane) The first separation membrane 11 is capable of preferentially transmitting, for example, a minor component gas contained in the mixed gas 70. Hereinafter, as an example, a first separation membrane 11 that is capable of preferentially transmitting an acidic gas, particularly carbon dioxide, contained in the mixed gas 70 will be described.

[0040] 3, the first separation membrane 11 includes, for example, a separation functional layer 1. The first separation membrane 11 may further include a porous support 3 that supports the separation functional layer 1, and an intermediate layer 2 disposed between the separation functional layer 1 and the porous support 3. The intermediate layer 2 is in direct contact with, for example, both the separation functional layer 1 and the porous support 3.

[0041] The membrane area of ​​the first separation membrane 11 can be set appropriately depending on the conditions for separating the mixed gas 70.

[0042] (separation functional layer) The separation functional layer 1 is a layer that allows preferential permeation of acidic gases contained in the mixed gas 70. In a preferred embodiment, the separation functional layer 1 contains a resin. Examples of resins contained in the separation functional layer 1 include polyether block amide resins, polyamide resins, polyether resins, polyimide resins, cellulose acetate resins, silicone resins, and fluororesins. The separation functional layer 1 preferably contains a polyether block amide resin. In this embodiment, the separation functional layer 1 preferably consists essentially of a resin. In this specification, "consisting essentially of" means excluding other components that alter the essential characteristics of the referenced material, and means that the material is composed of, for example, 95% or more, or even 99% or more.

[0043] In another preferred embodiment, the separation functional layer 1 contains an ionic liquid. The separation functional layer 1 has, for example, a double-network gel containing an ionic liquid. The double-network gel is a gel having two types of network structures that are independent of each other. The double-network gel includes, for example, a first network structure mainly composed of an organic material, a second network structure mainly composed of an inorganic material, and an ionic liquid. In this specification, "mainly composed" means that 50 wt% or more, or even 70 wt% or more, of the material in question is composed.

[0044] The organic material for forming the first network structure includes, for example, a polymer such as polyacrylamide (particularly, polydialkylacrylamide such as polydimethylacrylamide). The polymer contained in the organic material has a structural unit derived from an acrylamide derivative and may further include a crosslinked structure. The polymer including a crosslinked structure can be produced by a known method. For example, first, a prepolymer having a structural unit having an N-hydroxysuccinimide ester group is prepared. The structural unit having an N-hydroxysuccinimide ester group is derived from, for example, N-acryloxysuccinimide. Next, the prepolymer is reacted with an amine-based crosslinking agent to obtain a polymer including a crosslinked structure. The amine-based crosslinking agent is a compound having two or more primary amino groups, such as ethylene glycol bis(3-aminopropyl) ether.

[0045] The second network structure may include a network of a plurality of particles. The network of a plurality of particles may be formed, for example, by a plurality of particles being bonded to each other by hydrogen bonds. The particles included in the second network structure may be particles exemplified as nanoparticles, which will be described later. As an example, the particles included in the second network structure are silica particles.

[0046] In this embodiment, specific examples of the ionic liquid include an ionic liquid having imidazolium, pyridinium, ammonium, or phosphonium and a substituent having one or more carbon atoms.

[0047] In the ionic liquid having an imidazolium and a substituent having one or more carbon atoms, examples of the substituent having one or more carbon atoms include an alkyl group having from 1 to 20 carbon atoms, a cycloalkyl group having from 3 to 14 carbon atoms, and an aryl group having from 6 to 20 carbon atoms, which may be further substituted with a hydroxy group, a cyano group, an amino group, a monovalent ether group, or the like (for example, a hydroxyalkyl group having from 1 to 20 carbon atoms).

[0048] Examples of alkyl groups having 1 to 20 carbon atoms include methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, n-octadecyl, n-nonadecyl, and n-eicosadecyl. Examples of such groups include an alkyl group, an i-propyl group, a sec-butyl group, an i-butyl group, a 1-methylbutyl group, a 1-ethylpropyl group, a 2-methylbutyl group, an i-pentyl group, a neopentyl group, a 1,2-dimethylpropyl group, a 1,1-dimethylpropyl group, a t-pentyl group, a 2-ethylhexyl group, and a 1,5-dimethylhexyl group, which may be further substituted with a hydroxy group, a cyano group, an amino group, a monovalent ether group, or the like.

[0049] The alkyl group may be substituted with a cycloalkyl group. The number of carbon atoms in the alkyl group substituted with a cycloalkyl group is, for example, 1 or more and 20 or less. Examples of the alkyl group substituted with a cycloalkyl group include a cyclopropylmethyl group, a cyclobutylmethyl group, a cyclohexylmethyl group, and a cyclohexylpropyl group, which may be further substituted with a hydroxy group, a cyano group, an amino group, a monovalent ether group, or the like.

[0050] Examples of cycloalkyl groups having 3 to 14 carbon atoms include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclododecyl, norbornyl, bornyl, and adamantyl groups, which may be further substituted with a hydroxy group, a cyano group, an amino group, a monovalent ether group, or the like.

[0051] Examples of the aryl group having 6 to 20 carbon atoms include a phenyl group, a toluyl group, a xylyl group, a mesityl group, an anisyl group, a naphthyl group, and a benzyl group, which may be further substituted with a hydroxy group, a cyano group, an amino group, a monovalent ether group, or the like.

[0052] The imidazolium and the compound having a substituent with one or more carbon atoms may further have a substituent such as an alkyl group, and may form a salt with a counter anion. Examples of the counter anion include alkyl sulfate, tosylate, methanesulfonate, acetate, bis(fluorosulfonyl)imide, bis(trifluoromethanesulfonyl)imide, thiocyanate, dicyanamide, tricyanomethanide, tetracyanoborate, hexafluorophosphate, tetrafluoroborate, and halide. From the viewpoint of gas separation performance, bis(fluorosulfonyl)imide, bis(trifluoromethanesulfonyl)imide, dicyanamide, tricyanomethanide, and tetracyanoborate are preferred.

[0053] Specific examples of the ionic liquid having an imidazolium and a substituent having one or more carbon atoms include 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, 1-ethyl-3-methylimidazolium dicyanamide, 1-butyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-butyl-3-methylimidazolium tetrachloroferrate, 1-butyl-3-methylimidazolium iodide, 1-butyl-2,3-dimethylimidazolium chloride, 1-butyl-2,3-dimethylimidazolium hexafluorophosphate, 1-butyl-2,3-dimethylimidazolium tetra ...fluoromethanesulfonate, 1-butyl-3-methylimidazolium tetrafluoromethanesulfonate, 1-butyl-3-methylimidazolium tetrafluoromethanesulfonate, 1-butyl-3-methylimidazolium tetrafluoromethanesulfonate, 1-butyl-3-methylimidazolium tetrafluoromethanesulfonate, 1-butyl-3-methylimidazolium tetrafluoromethanesulfonate, 1-butyl-3-methylimidazolium tetrafluoromethanesulfonate, 1-butyl-3-methylimidazolium tetrafluoro tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylimidazolium trifluoro(trifluoromethyl)borate, 1-butyl-3-methylimidazolium tribromide, 1,3-dimesitylimidazolium chloride, 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride, 1,3-diisopropylimidazolium tetrafluoroborate, 1,3-di-tert-butylimidazolium tetrafluoroborate, 1,3-dicyclohexylimidazolium tetrafluoroborate, 1,3-dicyclohexylimidazolium chloride, 1,2-Dimethyl-3-propylimidazolium iodide, 1-hexyl-3-methylimidazolium chloride, 1-hexyl-3-methylimidazolium hexafluorophosphate, 1-hexyl-3-methylimidazolium tetrafluoroborate, 1-hexyl-3-methylimidazolium bromide, 1-methyl-3-propylimidazolium iodide, 1-methyl-3-n-octylimidazolium bromide, 1-methyl-3-n-octylimidazolium Examples of suitable imidazolium compounds include 1-methyl-3-n-octylimidazolium hexafluorophosphate, 1-methyl-3-[6-(methylsulfinyl)hexyl]imidazolium p-toluenesulfonate, 1-ethyl-3-methylimidazolium tricyanomethanide, 1-ethyl-3-methylimidazolium tetracyanoborate, and 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

[0054] Among these, from the viewpoint of gas separation performance, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([EMI][FSI]), 1-ethyl-3-methylimidazolium dicyanamide ([EMI][DCA]), 1-ethyl-3-methylimidazolium tricyanomethanide ([EMI][TCM]), 1-ethyl-3-methylimidazolium tetracyanoborate ([EMI][TCB]), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][TF2N]), and 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C2OHim][TF2N]) are particularly preferred.

[0055] The method for producing the double network gel is not particularly limited, and for example, the method disclosed in E. Kamio et al., Adv. Mater, 29, 1704118 (2017) can be used.

[0056] The content of the ionic liquid in the double-network gel is, for example, 50 wt% or more, preferably 60 wt% or more, more preferably 70 wt% or more, and even more preferably 80 wt% or more. The higher the content of the ionic liquid, the more the separation functional layer 1 can preferentially transmit acidic gases contained in the mixed gas. The upper limit of the content of the ionic liquid is not particularly limited and is, for example, 95 wt%.

[0057] The content of the first network structure, which is primarily made of organic material, in the double-network gel is, for example, 1 wt% or more, preferably 5 wt% or more, and more preferably 10 wt% or more. The upper limit of the content of the first network structure is, for example, 15 wt%. The content of the second network structure, which is primarily made of inorganic material, in the double-network gel is, for example, 1 wt% or more from the viewpoint of improving the strength of the double-network gel. The upper limit of the content of the second network structure is, for example, 5 wt%. The ratio of the total weight of the first network structure and the second network structure to the weight of the double-network gel is, for example, 2 wt% or more, preferably 5 wt% or more, and more preferably 10 wt% or more. This ratio is preferably 20 wt% or less. In this embodiment, the separation functional layer 1 preferably consists essentially of the double-network gel.

[0058] The thickness of the separation functional layer 1 is, for example, 50 μm or less, preferably 25 μm or less, and more preferably 15 μm or less. In some cases, the thickness of the separation functional layer 1 may be 10 μm or less, 5.0 μm or less, or 2.0 μm or less. The thickness of the separation functional layer 1 may be 0.05 μm or more, or 0.1 μm or more.

[0059] (middle class) The intermediate layer 2 may contain, for example, a resin and may further contain nanoparticles dispersed in the resin (matrix). The nanoparticles may be spaced apart within the matrix or may be partially aggregated. The material of the matrix is ​​not particularly limited, and examples thereof include silicone resins such as polydimethylsiloxane; fluororesins such as polytetrafluoroethylene; epoxy resins such as polyethylene oxide; polyimide resins; polysulfone resins; polyacetylene resins such as polytrimethylsilylpropyne and polydiphenylacetylene; and polyolefin resins such as polymethylpentene. The matrix preferably contains a silicone resin.

[0060] The nanoparticles may contain an inorganic material or an organic material. Examples of inorganic materials contained in the nanoparticles include silica, titania, and alumina. The nanoparticles preferably contain silica.

[0061] The thickness of the intermediate layer 2 is not particularly limited and is, for example, less than 50 μm, preferably 40 μm or less, and more preferably 30 μm or less. The lower limit of the thickness of the intermediate layer 2 is not particularly limited and is, for example, 1 μm. The intermediate layer 2 is, for example, a layer having a thickness of less than 50 μm.

[0062] (porous support) The porous support 3 supports the separation function layer 1 via the intermediate layer 2. Examples of the porous support 3 include nonwoven fabrics, porous polytetrafluoroethylene, aromatic polyamide fibers, porous metals, sintered metals, porous ceramics, porous polyesters, porous nylons, activated carbon fibers, latex, silicones, silicone rubbers, permeable (porous) polymers containing at least one selected from the group consisting of polyvinyl fluoride, polyvinylidene fluoride, polyurethane, polypropylene, polyethylene, polystyrene, polycarbonate, polysulfone, polyether ether ketone, polyacrylonitrile, polyimide, and polyphenylene oxide, metal foams with open or closed cells, polymer foams with open or closed cells, silica, porous glass, and mesh screens. The porous support 3 may be a combination of two or more of these materials.

[0063] The porous support 3 has an average pore size of, for example, 0.01 to 0.4 μm. The thickness of the porous support 3 is not particularly limited and is, for example, 10 μm or more, preferably 20 μm or more, and more preferably 50 μm or more. The thickness of the porous support 3 is, for example, 300 μm or less, preferably 200 μm or less, and more preferably 150 μm or less.

[0064] (Method for manufacturing the first separation membrane) The first separation membrane 11 can be produced, for example, by the following method. First, a coating solution containing the material for the intermediate layer 2 is prepared. Next, the coating solution containing the material for the intermediate layer 2 is applied onto the porous support 3 to form a coating film. The method for applying the coating solution is not particularly limited, and for example, a wire bar can be used. The thickness of the formed intermediate layer 2 can be adjusted by adjusting the wire diameter of the wire bar and the concentration of the material for the intermediate layer 2 in the coating solution. The coating film may also be formed by immersing the porous support 3 in the coating solution. Next, the coating film is dried to form the intermediate layer 2. The coating film can be dried, for example, under heating conditions. The heating temperature for the coating film is, for example, 50°C or higher. The heating time for the coating film is, for example, 1 minute or more, and may be 5 minutes or more.

[0065] If necessary, an adhesion-improving treatment may be applied to the surface of the intermediate layer 2. The adhesion-improving treatment may include surface treatment such as application of a primer, corona discharge treatment, or plasma treatment.

[0066] Next, a coating liquid containing the material for the separation functional layer 1 is prepared. The coating liquid containing the material for the separation functional layer 1 is applied onto the intermediate layer 2 to obtain a coating film. This coating film is dried to form the separation functional layer 1. The coating method and drying conditions for the coating liquid can be the same as those described above for the intermediate layer 2. The coating liquid containing the material for the separation functional layer 1 may also be applied by spin coating. This results in a first separation film 11.

[0067] The method for producing the first separation membrane 11 is not limited to the above method. For example, the first separation membrane 11 can also be produced by the following method. For example, a coating liquid containing the material of the separation functional layer 1 is applied onto a transfer film to obtain a coating film. The coating film is dried to form the separation functional layer 1. Next, a coating liquid containing the material of the intermediate layer 2 is applied onto the separation functional layer 1 and dried to form the intermediate layer 2. The laminate of the intermediate layer 2 and the separation functional layer 1 is transferred to the porous support 3. In this way, the first separation membrane 11 is obtained.

[0068] (Characteristics of the first separation membrane) The permeation rate of the acidic gas permeating through the first separation membrane 11 is not particularly limited. For example, the permeation rate T1 of carbon dioxide permeating through the first separation membrane 11 is, for example, 50 GPU or more, preferably 100 GPU or more, more preferably 150 GPU or more, even more preferably 200 GPU or more, and particularly preferably 300 GPU or more. The upper limit of the permeation rate T1 is not particularly limited, and is, for example, 500 GPU. However, the GPU is not limited to 10 -6 ·cm 3 (STP) / (sec cm 2 cm 3 (STP) refers to the volume of carbon dioxide at 1 atmosphere and 0°C.

[0069] The permeation rate T1 can be calculated by the following method. First, a mixed gas consisting of carbon dioxide and nitrogen is supplied to a space adjacent to one surface of the first separation membrane 11 (for example, the main surface 11a of the first separation membrane 11 on the separation function layer side). As a result, a permeated gas that has permeated the first separation membrane 11 is obtained in a space adjacent to the other surface of the first separation membrane 11 (for example, the main surface 11b of the first separation membrane 11 on the porous support side). The composition and weight of the permeated gas are measured. The permeation rate T1 can be calculated from the measurement results. In the above operation, the carbon dioxide concentration in the mixed gas is 50 vol% under standard conditions (0°C, 101 kPa). The mixed gas supplied to the space adjacent to one surface of the first separation membrane 11 has a temperature of 30°C and a pressure of 0.1 MPa.

[0070] According to the above method, it is also possible to calculate the permeation rate T2 of nitrogen permeating through the first separation membrane 11. The permeation rate T2 is, for example, 25 GPU or less, preferably 15 GPU or less, and more preferably 5 GPU or less. The lower limit of the permeation rate T2 is not particularly limited, and is, for example, 1 GPU.

[0071] Under the measurement conditions for the permeation rate T1, the separation factor α of the first separation membrane 11 for carbon dioxide relative to nitrogen is, for example, 15 or more, preferably 20 or more, more preferably 30 or more, even more preferably 40 or more, and particularly preferably 50 or more. The upper limit of the separation factor α is not particularly limited, and is, for example, 100. The separation factor α can be calculated from the following formula. In the following formula, X A and X B are the volume fractions of carbon dioxide and nitrogen in the mixed gas, respectively. Y A and Y B are the volume ratio of carbon dioxide and the volume ratio of nitrogen in the permeated gas that has permeated the first separation membrane 11, respectively. Separation factor α=(Y A / Y B ) / (X A / X B )

[0072] [Second separation membrane unit] As shown in FIG. 4, the second separation membrane unit 20 includes a second separation membrane 21 and a tank 22. The tank 22 has a third chamber 23 and a fourth chamber 24. The space within the third chamber 23 corresponds to the supply-side space, and the space within the fourth chamber 24 corresponds to the permeation-side space. The second separation membrane 21 is disposed inside the tank 22. Inside the tank 22, the second separation membrane 21 separates the third chamber 23 and the fourth chamber 24. The second separation membrane 21 extends from one to the other of a pair of wall surfaces of the tank 22.

[0073] The third chamber 23 has an inlet 23a and an outlet 23b. The fourth chamber 24 has an outlet 24a. The inlet 23a of the third chamber 23 is an opening for supplying the first permeate gas 80 to the second separation membrane unit 20. The outlet 24a of the fourth chamber 24 is an opening for discharging the second permeate gas 90, which is obtained when the first permeate gas 80 permeates the second separation membrane 21, from the second separation membrane unit 20. The outlet 23b of the third chamber 23 is an opening for discharging the first permeate gas 80 (second non-permeate gas 91) that did not permeate the second separation membrane 21 from the second separation membrane unit 20. The inlet 23a, the outlet 23b, and the outlet 24a are each formed, for example, on a wall surface of the tank 22.

[0074] The second separation membrane 21 can, for example, preferentially permeate the minor component gas contained in the first permeation gas 80. The separation membranes exemplified as the first separation membrane 11 can be used as the second separation membrane 21. The membrane area of ​​the second separation membrane 21 can be set appropriately depending on the conditions for separating the first permeation gas 80.

[0075] Except for the membrane area, the second separation membrane 21 may be the same as or different from the first separation membrane 11. As an example, at least one selected from the group consisting of the first separation membrane 11 and the second separation membrane 21 may contain a polyether block amide resin or an ionic liquid as a separation functional layer material, and it is preferable that both the first separation membrane 11 and the second separation membrane 21 contain a polyether block amide resin or an ionic liquid as a separation functional layer material.

[0076] The first separation membrane unit 10 and the second separation membrane unit 20 included in the gas separation system 100 are suitable for a flow-through (continuous) membrane separation method. However, these separation membrane units may also be used for a batch-type membrane separation method.

[0077] [Method for separating mixed gases] In this embodiment, the method for separating the mixed gas 70 includes a first separation step using the first separation membrane unit 10 and a second separation step using the second separation membrane unit 20.

[0078] The first separation step is performed, for example, as follows. First, the mixed gas 70 is supplied to the first chamber 13 (supply-side space) of the first separation membrane unit 10 through the mixed gas supply path 30. The mixed gas 70 is supplied to the first chamber 13 at atmospheric pressure (e.g., 101 kPa) in the measurement environment without being pressurized. However, the mixed gas 70 may be pressurized by the pressurizing device 54 as long as the pressure does not exceed 700 kPa. Pressurizing the mixed gas 70 to this extent tends to reduce the membrane area of ​​the first separation membrane 11 required for separating the mixed gas 70 while reducing the energy required to separate the mixed gas 70. The pressure of the mixed gas 70, i.e., the pressure in the supply-side space of the first separation membrane unit 10, may be, for example, 101 kPa or more, 150 kPa or more, 200 kPa or more, 300 kPa or more, or 400 kPa or more. The upper limit of the pressure of the mixed gas 70 may be 600 kPa or 500 kPa. In this specification, unless otherwise specified, "pressure" refers to absolute pressure.

[0079] As described above, the mixed gas 70 contains carbon dioxide and nitrogen. Typically, the mixed gas 70 contains nitrogen as a main component gas and carbon dioxide as a secondary component gas. However, the gas separation system 100 of this embodiment can also be used to separate a mixed gas 70 with a composition other than the above. For example, the main component gas may be one type of gas selected from non-polar gases such as hydrogen and methane, and inert gases such as helium. In some cases, the main component gas may be one type of gas selected from acidic gases such as carbon dioxide. The mixed gas 70 may contain, as a secondary component gas, an acidic gas other than carbon dioxide or oxygen instead of or in addition to carbon dioxide. Examples of acidic gases other than carbon dioxide include hydrogen sulfide, carbonyl sulfide, and sulfur oxides (SO ). x ), hydrogen cyanide, nitrogen oxides (NO x ) In some cases, the mixed gas 70 may contain a non-polar gas such as nitrogen, hydrogen, methane, or oxygen, or an inert gas such as helium, as a secondary component gas. As an example, the mixed gas may contain hydrogen as the primary component gas and carbon dioxide as a secondary component gas. The mixed gas may contain methane as the primary component gas and carbon dioxide as a secondary component gas.

[0080] The content of the main component gas (e.g., nitrogen) in the mixed gas 70 is, for example, 50 wt% or more, preferably 60 wt% or more, more preferably 70 wt% or more, even more preferably 75 wt% or more, particularly preferably 80 wt% or more, especially preferably 85 wt% or more, and may be 90 wt% or more, and in some cases may be 95 wt% or more, or may be 99 wt% or more. The upper limit of the content of the main component gas in the mixed gas 70 is not particularly limited, and may be, for example, 99.9 wt%, 99 wt%, or 95 wt%. The content of the minor component gas (e.g., carbon dioxide) in the mixed gas 70 is, for example, less than 50 wt%, preferably 40 wt% or less, more preferably 30 wt% or less, even more preferably 25 wt% or less, particularly preferably 20 wt% or less, especially preferably 15 wt% or less, and may be 10 wt% or less, and in some cases may be 5 wt% or less, or may be 1 wt% or less. The lower limit of the minor component gas content in the mixed gas 70 is not particularly limited, and may be, for example, 0.1 wt%, 1 wt%, or 5 wt%. The content of the minor component gas, particularly carbon dioxide, in the mixed gas 70 is preferably 5 wt% or more. In this specification, unless otherwise specified, the term "content" refers to a value under standard conditions (0°C, 101 kPa).

[0081] Carbon dioxide content in mixed gas 70 C CO2 (wt%), and the nitrogen content C in the mixed gas 70 N2 (wt%) may satisfy the following relational expression (1): When relational expression (1) is satisfied, the energy required in the gas separation system 100 tends to be significantly reduced. C CO2 <C N2 / 4 (1)

[0082] Next, while the mixed gas 70 is being supplied to the first chamber 13 of the first separation membrane unit 10, the pressure inside the second chamber 14 (permeation side space) is reduced. Specifically, the pressure inside the second chamber 14 is reduced through the outlet 14a using the first pressure reducing device 50. The pressure inside the permeation side space is, for example, 70 kPa or less, preferably 50 kPa or less, more preferably 30 kPa or less, even more preferably 10 kPa or less, and particularly preferably 5 kPa or less. The pressure inside the permeation side space may be 1 kPa or more, 2 kPa or more, or 5 kPa or more, taking into account the energy (power) required to operate the first pressure reducing device 50.

[0083] By reducing the pressure inside the second chamber 14, a pressure difference is generated or increases between the supply side space and the permeate side space. As a result, the mixed gas 70 is separated by the first separation membrane 11, and a first permeate gas 80 is supplied to the second chamber 14. While the mixed gas 70 is being separated, the pressure inside the second chamber 14 may continue to be reduced by the first pressure reducing device 50. From the viewpoint of reducing the energy consumed by the gas separation system 100, the pressure difference between the supply side space and the permeate side space of the first separation membrane unit 10 is adjusted to, for example, 500 kPa or less, preferably 400 kPa or less, more preferably 300 kPa or less, even more preferably 200 kPa or less, and particularly preferably 100 kPa or less. The lower limit of the pressure difference between the supply side space and the permeate side space of the first separation membrane unit 10 is not particularly limited and is, for example, 10 kPa.

[0084] The first permeable gas 80 supplied to the second chamber 14 passes through the first portion 32a of the permeable gas supply path 32 and is sucked into, for example, the first pressure reducing device 50. The first pressure reducing device 50, for example, discharges the sucked first permeable gas 80 to the second portion 32b of the permeable gas supply path 32. Because the first permeable gas 80 is supplied to the second portion 32b, the pressure within the second portion 32b is usually maintained at approximately atmospheric pressure in the measurement environment. The first permeable gas 80 is supplied to the second separation membrane unit 20 through the second portion 32b.

[0085] As described above, the first separation membrane 11 of the first separation membrane unit 10 preferentially allows, for example, a minor component gas contained in the mixed gas 70 to permeate. Therefore, the first permeable gas 80 obtained by the first separation step has a higher minor component gas content than the mixed gas 70. The minor component gas (e.g., carbon dioxide) content in the first permeable gas 80 obtained by the first separation step is not particularly limited, and is, for example, 30 wt% to 80 wt%. The ratio of the minor component gas content (wt%) in the first permeable gas 80 to the minor component gas content (wt%) in the mixed gas 70 is not particularly limited, and is, for example, 2 to 10.

[0086] Meanwhile, the concentration of the main component gas in the mixed gas 70 gradually increases from the inlet 13a to the outlet 13b of the first chamber 13. The content of the main component gas (e.g., nitrogen) in the mixed gas 70 (first non-permeate gas 81) treated in the first chamber 13 is, for example, 95 wt% or more, preferably 97 wt% or more, more preferably 98 wt% or more, and even more preferably 99 wt% or more. The first non-permeate gas 81 is discharged to the outside of the first separation membrane unit 10 through the outlet 13b. The first non-permeate gas 81 is discharged through the first discharge path 34 and from the discharge port 42.

[0087] The second separation step is performed, for example, as follows. First, the first permeable gas 80 is supplied to the third chamber 23 (supply-side space) of the second separation membrane unit 20 through the second portion 32b of the permeable gas supply path 32. The first permeable gas 80 is supplied to the third chamber 23 at atmospheric pressure (e.g., 101 kPa) in the measurement environment without being pressurized. However, the first permeable gas 80 may be pressurized by a pressurizing device disposed in the second portion 32b as long as the pressure does not exceed 700 kPa. The pressure of the first permeable gas 80, i.e., the pressure in the supply-side space of the second separation membrane unit 20, may be, for example, 101 kPa or more, 150 kPa or more, 200 kPa or more, 300 kPa or more, or 400 kPa or more. The upper limit of the pressure of the first permeable gas 80 may be 600 kPa or 500 kPa.

[0088] Next, while the first permeate gas 80 is being supplied to the third chamber 23 of the second separation membrane unit 20, the fourth chamber 24 (permeation side space) is depressurized. Specifically, the second depressurization device 52 is used to depressurize the fourth chamber 24 through the outlet 24a. The pressure in the permeation side space is, for example, 70 kPa or less, preferably 50 kPa or less, more preferably 30 kPa or less, even more preferably 10 kPa or less, and particularly preferably 5 kPa or less. Taking into account the energy (power) required to operate the second depressurization device 52, the pressure in the permeation side space may be 1 kPa or more, 2 kPa or more, 5 kPa or more, 10 kPa or more, 15 kPa or more, 20 kPa or more, or 30 kPa or more.

[0089] By reducing the pressure inside the fourth chamber 24, a pressure difference is generated or increases between the supply side space and the permeate side space. As a result, the first permeate gas 80 is separated by the second separation membrane 21, and the second permeate gas 90 is supplied to the fourth chamber 24. While the first permeate gas 80 is being separated, the pressure inside the fourth chamber 24 may continue to be reduced by the second pressure reducing device 52. From the viewpoint of reducing the energy consumed by the gas separation system 100, the pressure difference between the supply side space and the permeate side space of the second separation membrane unit 20 is adjusted to, for example, 500 kPa or less, preferably 400 kPa or less, more preferably 300 kPa or less, even more preferably 200 kPa or less, and particularly preferably 100 kPa or less. The lower limit of the pressure difference between the supply side space and the permeate side space of the second separation membrane unit 20 is not particularly limited and is, for example, 10 kPa.

[0090] The second permeable gas 90 supplied to the fourth chamber 24 passes through the first portion 36a of the second discharge path 36 and is sucked into, for example, the second pressure reducing device 52. The second pressure reducing device 52, for example, discharges the sucked second permeable gas 90 into the second portion 36b of the second discharge path 36. The second permeable gas 90 is sent to the tank 60 through the second portion 36b.

[0091] As described above, the second separation membrane 21 of the second separation membrane unit 20 preferentially allows, for example, minor component gases contained in the first permeable gas 80 to permeate. Therefore, the second permeable gas 90 obtained by the second separation step has a higher minor component gas content than the first permeable gas 80. The minor component gas (e.g., carbon dioxide) content of the second permeable gas 90 obtained by the second separation step is not particularly limited and is, for example, 80 wt% or more, preferably 85 wt% or more, more preferably 90 wt% or more, and even more preferably 95 wt% or more. The ratio of the minor component gas content (wt%) in the second permeable gas 90 to the minor component gas content (wt%) in the first permeable gas 80 is not particularly limited and is, for example, 1.2 to 3.

[0092] Meanwhile, the concentration of the main component gas in the first permeable gas 80 gradually increases from the inlet 23a to the outlet 23b of the third chamber 23. The content of the main component gas (e.g., nitrogen) in the first permeable gas 80 (second non-permeable gas 91) treated in the third chamber 23 is not particularly limited, and is, for example, 60 wt% to 90 wt%. The second non-permeable gas 91 is discharged to the outside of the second separation membrane unit 20 through the outlet 23b.

[0093] In the separation method of this embodiment, the first separation step and the second separation step are preferably performed consecutively. That is, the first permeable gas 80 separated in the first separation step is preferably immediately subjected to the second separation step without being collected in a tank or the like. By performing the first separation step and the second separation step consecutively, the amount of mixed gas 70 processed per unit time can be easily increased.

[0094] In a configuration in which the separation membrane of the separation membrane unit (first separation membrane unit 10 or second separation membrane unit 20) preferentially allows minor component gases to permeate, the higher the content of the minor component gas in the gas supplied to the separation membrane unit, the easier it is to increase the partial pressure difference between the minor component gas in the supply-side space and the permeate-side space of the separation membrane unit by reducing the pressure in the permeate-side space of the separation membrane unit. Therefore, when the content of the minor component gas in the gas supplied to the separation membrane unit is high, the energy (power) required to operate the pressure-reducing device (first pressure-reducing device 50 or second pressure-reducing device 52) can be reduced without significantly reducing the separation performance of the separation membrane unit by setting the pressure in the permeate-side space of the separation membrane unit relatively high. In this way, it is preferable to adjust the pressure in the permeate-side space of the separation membrane unit depending on the composition of the gas supplied to the separation membrane unit, particularly the content of the minor component gas.

[0095] As an example, in the separation method of this embodiment, when the minor component gas is carbon dioxide, at least one of the following may be true: (i) in the first separation step, when the pressure in the permeate side space of the first separation membrane unit 10 is expressed as P (kPa) and the carbon dioxide content in the mixed gas 70 is expressed as x (wt%), the following relational formula (A) is satisfied; and (ii) in the second separation step, when the pressure in the permeate side space of the second separation membrane unit 20 is expressed as P (kPa) and the carbon dioxide content in the first permeate gas 80 is expressed as x (wt%), the following relational formula (A) is satisfied. 0.2e 0.0536x ≦P≦0.55e 0.0536x (A)

[0096] FIG. 5 is a graph illustrating the relational expression (A). In this graph, the horizontal axis represents the carbon dioxide content x in the gas supplied to the separation membrane unit. The vertical axis represents the pressure P in the permeate side space of the separation membrane unit. According to the studies of the present inventors, by adjusting the pressure P within a range that satisfies the relational expression (A), it is possible to reduce the power required for the pressure reduction device without significantly reducing the separation performance of the separation membrane unit. In particular, by setting the pressure P within a range that satisfies the relational expression (A) based on the carbon dioxide content x, it is possible to reduce the power required for the pressure reduction device without significantly increasing the membrane area required for gas separation for the separation membranes provided in the separation membrane unit. The pressure P (kPa) in the permeate side space of the separation membrane unit and the carbon dioxide content x (wt%) in the gas supplied to the separation membrane unit may satisfy the following relational expression (B). P=0.4116e 0.0536x (B)

[0097] Relational formula (B) shows the relationship between the carbon dioxide content x and the pressure P in the permeate side space, which is suitable for reducing the power required for the pressure reducing device without significantly reducing the separation performance of the separation membrane unit. More specifically, relational formula (B) is an approximation curve created based on the results of a simulation.

[0098] As can be seen from the relational expressions (A) and (B), the higher the carbon dioxide content x, the higher the pressure P in the permeate side space can be set. Typically, the carbon dioxide content in the first permeate gas 80 is higher than the carbon dioxide content in the mixed gas 70. Therefore, the pressure P2 in the permeate side space of the second separation membrane unit 20 in the second separation step may be the same as or higher than the pressure P1 in the permeate side space of the first separation membrane unit 10 in the first separation step. It is preferable that the pressure P2 is higher than the pressure P1.

[0099] In the first and second separation steps, the pressure P in the permeate space of the separation membrane unit may be changed over time in response to changes in the composition of the gas supplied to the separation membrane unit, particularly the carbon dioxide content x. By constantly adjusting the pressure P in the permeate space to an appropriate value in response to the composition of the gas supplied to the separation membrane unit, the power required for the pressure reduction device can be effectively reduced. Changes in the composition of the gas supplied to the separation membrane unit can be detected, for example, by a gas sensor located near the inlet of the separation membrane unit.

[0100] The separation method of the present embodiment further includes, for example, a mixing step of mixing the second non-permeable gas 91 with the mixed gas 70. The mixing step can be carried out by sending the second non-permeable gas 91 to the joining position 40 of the mixed gas supply path 30 through the third discharge path 38. The mixing step makes it possible to reuse the second non-permeable gas 91, which tends to improve the recovery rates of the main component gas and the sub-component gas.

[0101] According to the method for separating a mixed gas 70 of this embodiment, for example, a second permeable gas 90 in which a minor component gas is concentrated and a first non-permeable gas 81 in which a major component gas is concentrated can be recovered. The recovery rate of the minor component gas (e.g., carbon dioxide) by this separation method is not particularly limited and is, for example, 80% or more, preferably 90% or more. The recovery rate of the major component gas is not particularly limited and is, for example, 90% or more, preferably 95% or more. The second permeable gas 90 in which carbon dioxide is concentrated can be used, for example, to produce dry ice. The first non-permeable gas 81 in which nitrogen is concentrated can be used, for example, for industrial purposes.

[0102] In conventional gas separation systems, the mixed gas supplied is typically pressurized to at least 700 kPa or more. This is evident from the fact that the working pressure is set to at least 7 bar in the examples of Patent Document 1. However, when pressurizing the mixed gas, the non-permeating gas that does not permeate the separation membrane in the separation membrane unit is also pressurized. In this case, the energy consumed to pressurize the non-permeating gas is wasted. In the gas separation system 100 of this embodiment, the pressure reduction devices 50 and 52 can reduce the pressure in the permeation side spaces of the separation membrane units 10 and 20, respectively, to perform the separation process, eliminating the need to significantly pressurize the feed side space. Because the energy waste caused by pressurizing the feed side space can be eliminated, the gas separation system 100 of this embodiment is suitable for reducing the energy required to separate the mixed gas 70. In particular, when the first separation membrane 11 of the first separation membrane unit 10 preferentially permeates the minor component gas in the mixed gas 70, the gas separation system 100 tends to significantly reduce the energy required to separate the mixed gas 70.

[0103] [Modification of the first separation membrane unit] In the gas separation system 100, the first separation membrane unit 10 may be a spiral-type membrane element as shown in Fig. 6. The first separation membrane unit 15 in Fig. 6 includes a central tube 16 and a stack 17. The stack 17 includes a first separation membrane 11.

[0104] The central tube 16 has a cylindrical shape. A plurality of holes or slits are formed on the surface of the central tube 16 to allow the first permeable gas 80 to flow into the interior of the central tube 16. Examples of materials for the central tube 16 include resins such as acrylonitrile-butadiene-styrene copolymer resin (ABS resin), polyphenylene ether resin (PPE resin), and polysulfone resin (PSF resin); and metals such as stainless steel and titanium. The inner diameter of the central tube 16 is, for example, in the range of 20 to 100 mm.

[0105] In addition to the first separation membrane 11, the laminate 17 further includes a feed-side channel material 18 and a permeate-side channel material 19. The laminate 17 is wound around a central tube 16. The first separation membrane unit 15 may further include an exterior material (not shown).

[0106] The feed-side flow path material 18 and the permeate-side flow path material 19 may be, for example, a resin net, woven fabric, or knitted fabric made of polyethylene, polypropylene, polyethylene terephthalate (PET), polyphenylene sulfide (PPS), or ethylene-chlorotrifluoroethylene copolymer (ECTFE).

[0107] Membrane separation using the first separation membrane unit 15 is performed, for example, by the following method. First, a mixed gas 70 is supplied to one end of the wound stack 17. The space inside the central tube 16 is depressurized. As a result, a first permeable gas 80 that has permeated the first separation membrane 11 of the stack 17 moves into the inside of the central tube 16. The first permeable gas 80 is discharged to the outside through the central tube 16. The mixed gas 70 (first non-permeable gas 81) that has been treated in the first separation membrane unit 15 is discharged to the outside from the other end of the wound stack 17.

[0108] In the gas separation system 100, the second separation membrane unit 20 may be a spiral membrane element having a configuration similar to that of the first separation membrane unit 15.

[0109] <Modification of Gas Separation System> The gas separation system 100 may include multiple first separation membrane units 10. The gas separation system 110 of this embodiment shown in FIG. 7 includes two first separation membrane units 10a and 10b. In the gas separation system 110, the mixed gas supply path 30 branches into a first portion 30a and a second portion 30b at a branch position 44. The permeate gas supply path 32 further includes a third portion 32c. The gas separation system 110 further includes a fourth discharge path 35. Except for the above, the structure of the gas separation system 110 of this embodiment is the same as that of the gas separation system 100. Therefore, elements common to the gas separation system 100 and the gas separation system 110 of this embodiment are denoted by the same reference numerals, and their description may be omitted. In other words, the descriptions of the respective embodiments may be mutually applicable unless technically inconsistent. Furthermore, the respective embodiments may be mutually combined unless technically inconsistent.

[0110] The branching position 44 in the mixed gas supply path 30 is located, for example, downstream of the confluence position 40 (and the pressurizing device 54), i.e., between the confluence position 40 and the first separation membrane unit 10a. The first portion 30a extends from the branching position 44 to the mixed gas inlet (inlet 13a) of the first separation membrane unit 10a. The mixed gas 70 can be supplied to the first separation membrane unit 10a through the first portion 30a. The second portion 30b extends from the branching position 44 to the mixed gas inlet (inlet 13c) of the first separation membrane unit 10b. The mixed gas 70 can be supplied to the first separation membrane unit 10b through the second portion 30b.

[0111] The third portion 32c of the permeate gas supply path 32 extends from the permeate gas outlet (outlet 14b) of the first separation membrane unit 10b to the first pressure reducing device 50. The first pressure reducing device 50 can reduce the pressure in the permeate side space of the first separation membrane unit 10a through the first portion 32a and the permeate side space of the first separation membrane unit 10b through the third portion 32c. The first pressure reducing device 50, for example, sucks the first permeate gas that has passed through the third portion 32c and discharges the first permeate gas to the second portion 32b. In this manner, in the gas separation system 110, the first pressure reducing device 50 may be shared by multiple first separation membrane units 10. A configuration in which the first pressure reducing device 50 is shared by multiple first separation membrane units 10 reduces the installation area of ​​the pressure reducing device and is suitable for securing space. Furthermore, this configuration allows components such as valves to be omitted, which is advantageous in terms of cost. When the first pressure reducing device 50 is shared by multiple first separation membrane units 10, the gas separation system 110 is also suitable for further reducing the energy required to separate the mixed gas 70. However, the gas separation system 110 may be configured to include multiple first pressure reducing devices 50, and each of the multiple first pressure reducing devices 50 may be configured to reduce the pressure in the permeate side spaces of the multiple first separation membrane units 10.

[0112] The fourth discharge path 35 is connected to the non-permeate gas outlet (outlet 13d) of the first separation membrane unit 10b, and is a path for discharging the first non-permeate gas from the first separation membrane unit 10b. An opening (discharge port 46) for discharging the first non-permeate gas from the fourth discharge path 35 is formed in the fourth discharge path 35. The gas separation system 110 may further include a tank (not shown) for storing the first non-permeate gas, and the first discharge path 34 and the fourth discharge path 35 may each be connected to the tank.

[0113] In the gas separation system 110, the multiple first separation membrane units 10 may be the same as or different from each other. By having the gas separation system 110 have multiple first separation membrane units 10, the membrane area of ​​the separation membrane (first separation membrane 11) that treats the mixed gas 70 can be easily increased.

[0114] <Another modified example of the gas separation system> 8 shows a configuration diagram of a modified gas separation system 120. As shown in FIG. 8, the gas separation system 120 further includes a third separation membrane unit 25, a third pressure reducing device 56, a fifth discharge path 37, and a sixth discharge path 39. The second discharge path 36 is connected to the permeate gas inlet (inlet 28a) of the third separation membrane unit 25. Except for the above, the structure of the gas separation system 120 is the same as the structure of the gas separation system 100 in FIG. 1.

[0115] The third separation membrane unit 25 is a membrane separation device that uses a third separation membrane to perform membrane separation on the second permeate gas discharged from the second separation membrane unit 20. The third separation membrane included in the third separation membrane unit 25 can further separate the second permeate gas into a third permeate gas and a third non-permeate gas.

[0116] The third separation membrane of the third separation membrane unit 25 allows, for example, a minor component gas contained in the second permeable gas, more specifically, at least one gas contained in the minor component gas, to preferentially permeate. Therefore, the third permeable gas separated by the third separation membrane has a higher minor component gas content than the second permeable gas and a lower main component gas content than the second permeable gas. On the other hand, the third non-permeable gas has a lower minor component gas content than the second permeable gas and a higher main component gas content than the second permeable gas. Note that, in this specification, as with the first permeable gas, the terms "main component gas" and "minor component gas" for the third permeable gas and the third non-permeable gas are defined based on their contents in the mixed gas before separation by the first separation membrane.

[0117] The third pressure reducing device 56 can reduce the pressure in the permeate side space of the third separation membrane unit 25. In other words, the third pressure reducing device 56 can generate or increase a pressure difference between the supply side space and the permeate side space of the third separation membrane unit 25. A specific example of the third pressure reducing device 56 is a pump. The third pressure reducing device 56 is preferably a vacuum device such as a vacuum pump. As the vacuum pump, the pumps described above for the first pressure reducing device 50 can be used. The pump serving as the third pressure reducing device 56 may be equipped with a variable speed mechanism for changing the rotation speed, etc. An example of the variable speed mechanism is an inverter that drives the pump motor. By controlling the rotation speed, etc. of the pump using the variable speed mechanism, the pressure in the permeate side space of the third separation membrane unit 25 can be appropriately adjusted.

[0118] The third pressure reducing device 56 may be an assembly of multiple pumps. That is, the third pressure reducing device 56 may have a configuration in which the pressure in the permeate side space of the third separation membrane unit 25 can be reduced by each of multiple pumps. With this configuration, the pressure in the permeate side space of the third separation membrane unit 25 can be appropriately adjusted by adjusting the number of pumps in operation.

[0119] As described above, the second discharge path 36 is connected to the inlet 28a of the third separation membrane unit 25. The second discharge path 36 functions as a path for supplying the second permeate gas from the second separation membrane unit 20 to the third separation membrane unit 25. A pressurizing device (not shown) for pressurizing the second permeate gas discharged from the second pressure reducing device 52 may or may not be disposed in the second portion 36b of the second discharge path 36. This pressurizing device can pressurize the supply side space of the third separation membrane unit 25. Examples of pressurizing devices include a compressor, a blower, and a back pressure valve. A gas sensor for measuring the composition of the second permeate gas may be disposed in the second portion 36b. This gas sensor can detect, for example, the carbon dioxide content of the second permeate gas. The gas sensor is preferably disposed near the inlet 28a of the third separation membrane unit 25. By using a gas sensor to measure the composition of the second permeate gas supplied to the third separation membrane unit 25, the operating conditions of the third separation membrane unit 25, particularly the degree of pressure reduction in the permeation side space of the third separation membrane unit 25, can be appropriately set.

[0120] It is preferable that the second discharge path 36 does not include a tank such as a collector for collecting the second permeate gas or an on-off valve for opening and closing the path. In particular, when a collector is not provided in the second discharge path 36, the second permeate gas discharged from the second separation membrane unit 20 can be continuously supplied to the third separation membrane unit 25. With this configuration, the amount of mixed gas 70 processed per unit time can be easily increased. As an example, the second discharge path 36 may be composed of only the second pressure reducing device 52 and piping.

[0121] The fifth discharge path 37 is connected to the permeate gas outlet (outlet 29a) of the third separation membrane unit 25 and the inlet of the tank 60, and is a path for sending the third permeate gas from the third separation membrane unit 25 to the tank 60. The tank 60 can store the third permeate gas sent from the third separation membrane unit 25. A third pressure reducing device 56 is arranged on the fifth discharge path 37.

[0122] The fifth discharge path 37 has a first portion 37a extending from the third separation membrane unit 25 to the third pressure reducing device 56 and a second portion 37b extending from the third pressure reducing device 56 to the tank 60. The third pressure reducing device 56 can reduce the pressure in the permeate side space of the third separation membrane unit 25 through the first portion 37a. The third pressure reducing device 56, for example, sucks the third permeate gas that has passed through the first portion 37a and discharges the third permeate gas to the second portion 37b. The third pressure reducing device 56 is configured, for example, not to discharge the gas that has passed through the first portion 37a to the outside of the gas separation system 120. The third pressure reducing device 56 is connected, for example, only to the first portion 37a and the second portion 37b. When the third pressure reducing device 56 is an assembly of multiple pumps, the first portion 37a of the fifth discharge path 37 may branch and be connected to the inlets of the multiple pumps, respectively. Similarly, the second portion 37b of the fifth discharge path 37 may branch and be connected to each of the outlets of a plurality of pumps.

[0123] The sixth discharge path 39 is connected to the non-permeate gas outlet (outlet 28b) of the third separation membrane unit 25, and is a path for discharging the third non-permeate gas from the third separation membrane unit 25. The sixth discharge path 39 may merge with the permeate gas supply path 32, specifically the second portion 32b of the permeate gas supply path 32, at a merging position 41. By merging the sixth discharge path 39 with the permeate gas supply path 32, it is possible, for example, to reuse the third non-permeate gas containing minor component gases that were not completely separated by the third separation membrane unit 25.

[0124] If the gas separation system 120 is equipped with a controller, the controller can also control the operation of, for example, the third pressure reducing device 56. As an example, when the second permeate gas is supplied to the third separation membrane unit 25 from the second discharge path 36, the controller controls the third pressure reducing device 56 so that the permeate side space of the third separation membrane unit 25 is depressurized. In particular, the controller controls the third pressure reducing device 56 so that the permeate side space of the third separation membrane unit 25 continues to be depressurized while the second permeate gas is being supplied to the third separation membrane unit 25. In the gas separation system 120, for example, (III) the second permeate gas is separated into a third permeate gas and a third non-permeate gas while the permeate side space of the third separation membrane unit 25 is depressurized by the third pressure reducing device 56.

[0125] The gas separation system 120 of this embodiment has a configuration in which a third separation membrane unit 25 and a third pressure reduction device 56 are further arranged downstream of the second separation membrane unit 20. In the gas separation system 120, a combination of a separation membrane unit and a pressure reduction device may be further arranged downstream of the third separation membrane unit 25. The number of combinations of separation membrane units and pressure reduction devices arranged downstream of the third separation membrane unit 25 can be appropriately set depending on the composition of the mixed gas 70 to be separated. When the gas separation system 120 is equipped with a controller, the controller controls the operation of each pressure reduction device arranged downstream of the third separation membrane unit 25, for example.

[0126] [Third separation membrane unit] As shown in FIG. 9 , the third separation membrane unit 25 includes a third separation membrane 26 and a tank 27. The tank 27 has a fifth chamber 28 and a sixth chamber 29. The space within the fifth chamber 28 corresponds to the supply-side space, and the space within the sixth chamber 29 corresponds to the permeation-side space. The third separation membrane 26 is disposed inside the tank 27. Inside the tank 27, the third separation membrane 26 separates the fifth chamber 28 and the sixth chamber 29. The third separation membrane 26 extends from one to the other of a pair of wall surfaces of the tank 27.

[0127] The fifth chamber 28 has an inlet 28a and an outlet 28b. The sixth chamber 29 has an outlet 29a. The inlet 28a of the fifth chamber 28 is an opening for supplying the second permeate gas 90 to the third separation membrane unit 25. The outlet 29a of the sixth chamber 29 is an opening for discharging a third permeate gas 95, which is obtained when the second permeate gas 90 permeates the third separation membrane 26, from the third separation membrane unit 25. The outlet 28b of the fifth chamber 28 is an opening for discharging the second permeate gas 90 (third non-permeate gas 96) that did not permeate the third separation membrane 26 from the third separation membrane unit 25. The inlet 28a, the outlet 28b, and the outlet 29a are each formed, for example, on a wall surface of the tank 27.

[0128] The third separation membrane 26 can, for example, preferentially permeate the minor component gas contained in the second permeable gas 90. The separation membranes exemplified as the first separation membrane 11 can be used as the third separation membrane 26. The membrane area of ​​the third separation membrane 26 can be set appropriately depending on the conditions for separating the second permeable gas 90.

[0129] Except for the membrane area, the third separation membrane 26 may be the same as or different from the first separation membrane 11 or the second separation membrane 21. As an example, at least one selected from the group consisting of the first separation membrane 11, the second separation membrane 21, and the third separation membrane 26 may contain a polyether block amide resin or an ionic liquid as a material for the separation functional layer, and it is preferable that each of the first separation membrane 11, the second separation membrane 21, and the third separation membrane 26 contains a polyether block amide resin or an ionic liquid as a material for the separation functional layer.

[0130] The third separation membrane unit 25 is suitable for a flow-through (continuous) membrane separation method. However, the third separation membrane unit 25 may also be used for a batch-type membrane separation method. The third separation membrane unit 25 may be the spiral membrane element described above for the first separation membrane unit 15.

[0131] [Method for separating mixed gases] In this embodiment, the method for separating the mixed gas 70 further includes a third separation step using a third separation membrane unit 25, in addition to the first and second separation steps described above.

[0132] The third separation step is performed, for example, as follows. First, the second permeable gas 90 is supplied to the fifth chamber 28 (supply-side space) of the third separation membrane unit 25 through the second portion 36b of the second discharge path 36. The second permeable gas 90 is supplied to the fifth chamber 28 at atmospheric pressure in the measurement environment (e.g., 101 kPa) without being pressurized. However, the second permeable gas 90 may be pressurized by a pressurizing device disposed in the second portion 36b as long as the pressure does not exceed 700 kPa. The pressure of the second permeable gas 90, i.e., the pressure in the supply-side space of the third separation membrane unit 25, may be, for example, 101 kPa or more, 150 kPa or more, 200 kPa or more, 300 kPa or more, or 400 kPa or more. The upper limit of the pressure of the second permeable gas 90 may be 600 kPa or 500 kPa.

[0133] Next, with the second permeate gas 90 being supplied to the fifth chamber 28 of the third separation membrane unit 25, the sixth chamber 29 (permeation side space) is depressurized. Specifically, the sixth chamber 29 is depressurized through the outlet 29a using the third depressurization device 56. The pressure in the permeation side space is, for example, 70 kPa or less, preferably 50 kPa or less, more preferably 30 kPa or less, even more preferably 10 kPa or less, and particularly preferably 5 kPa or less. In consideration of the energy (power) required to operate the third depressurization device 56, the pressure in the permeation side space may be 1 kPa or more, 2 kPa or more, 5 kPa or more, 10 kPa or more, 15 kPa or more, 20 kPa or more, or 30 kPa or more.

[0134] By reducing the pressure inside the sixth chamber 29, a pressure difference is generated or increases between the supply-side space and the permeate-side space. As a result, the second permeate gas 90 is separated by the third separation membrane 26, and a third permeate gas 95 is supplied to the sixth chamber 29. While the second permeate gas 90 is being separated, the pressure inside the sixth chamber 29 may continue to be reduced by the third pressure reducing device 56. From the viewpoint of reducing the energy consumed by the gas separation system 120, the pressure difference between the supply-side space and the permeate-side space of the third separation membrane unit 25 is adjusted to, for example, 500 kPa or less, preferably 400 kPa or less, more preferably 300 kPa or less, even more preferably 200 kPa or less, and particularly preferably 100 kPa or less. The lower limit of the pressure difference between the supply-side space and the permeate-side space of the third separation membrane unit 25 is not particularly limited and is, for example, 10 kPa.

[0135] The third permeable gas 95 supplied to the sixth chamber 29 passes through the first portion 37a of the fifth discharge path 37 and is sucked into, for example, the third pressure reducing device 56. The third pressure reducing device 56, for example, discharges the sucked third permeable gas 95 into the second portion 37b of the fifth discharge path 37. The third permeable gas 95 is sent to the tank 60 through the second portion 37b.

[0136] As described above, the third separation membrane 26 of the third separation membrane unit 25 preferentially allows, for example, minor component gases contained in the second permeable gas 90 to permeate. Therefore, the third permeable gas 95 obtained by the third separation step has a higher minor component gas content than the second permeable gas 90. The minor component gas (e.g., carbon dioxide) content in the third permeable gas 95 obtained by the third separation step is not particularly limited and is, for example, 80 wt% or more, preferably 85 wt% or more, more preferably 90 wt% or more, and even more preferably 95 wt% or more. The ratio of the minor component gas content (wt%) in the third permeable gas 95 to the minor component gas content (wt%) in the second permeable gas 90 is not particularly limited and is, for example, 1.2 to 3.

[0137] Meanwhile, the concentration of the main component gas in the second permeable gas 90 gradually increases from the inlet 28a toward the outlet 28b of the fifth chamber 28. The content of the main component gas (e.g., nitrogen) in the second permeable gas 90 (third non-permeable gas 96) treated in the fifth chamber 28 is not particularly limited, and is, for example, 60 wt% to 90 wt%. The third non-permeable gas 96 is discharged to the outside of the third separation membrane unit 25 through the outlet 28b.

[0138] In the separation method of this embodiment, the first separation step to the third separation step are preferably performed consecutively. That is, the second permeable gas 90 separated in the second separation step is preferably not collected in a tank or the like but is immediately subjected to the third separation step. By performing the first separation step to the third separation step consecutively, the amount of mixed gas 70 processed per unit time can be easily increased.

[0139] The pressure in the permeate side space of the third separation membrane unit 25 may be adjusted depending on the composition, particularly the content of the minor component gas, of the second permeate gas 90 supplied to the third separation membrane unit 25. As an example, in the separation method of the present embodiment, when the minor component gas is carbon dioxide, in the third separation step, when the pressure in the permeate side space of the third separation membrane unit 25 is represented as P (kPa) and the content of carbon dioxide in the second permeate gas 90 is represented as x (wt%), either the above-mentioned relational formula (A) or the above-mentioned relational formula (B) may be satisfied.

[0140] The pressure P3 in the permeate side space of the third separation membrane unit 25 in the third separation step may be the same as or higher than the pressure P2 in the permeate side space of the second separation membrane unit 20 in the second separation step. The pressure P3 is preferably higher than the pressure P2.

[0141] In the third separation step, the pressure P3 in the permeate side space of the third separation membrane unit 25 may be changed over time in response to changes in the composition of the gas supplied to the third separation membrane unit 25, particularly the carbon dioxide content x. By constantly adjusting the pressure P3 in the permeate side space to an appropriate value in response to the composition of the gas supplied to the third separation membrane unit 25, it is possible to effectively reduce the power required for the third pressure reducing device 56. Changes in the composition of the gas supplied to the third separation membrane unit 25 can be detected, for example, by a gas sensor located near the inlet 28a of the third separation membrane unit 25.

[0142] The separation method of this embodiment further includes, for example, a second mixing step of mixing the third non-permeable gas 96 with the first permeable gas 80. The second mixing step can be carried out by sending the third non-permeable gas 96 to the joining position 41 of the permeable gas supply path 32 through the sixth discharge path 39. The second mixing step makes it possible to reuse the third non-permeable gas 96, which tends to improve the recovery rates of the main component gas and the minor component gas.

[0143] According to the method for separating a mixed gas 70 of this embodiment, for example, a third permeable gas 95 in which the minor component gas is concentrated and a first non-permeable gas 81 in which the major component gas is concentrated can be recovered. The recovery rate of the minor component gas (e.g., carbon dioxide) by this separation method is not particularly limited and is, for example, 80% or more, preferably 90% or more. The recovery rate of the major component gas is not particularly limited and is, for example, 90% or more, preferably 95% or more.

[0144] The method for separating the mixed gas 70 of this embodiment using the gas separation system 120 is suitable for separating the mixed gas 70 having a low content of minor component gases. Specifically, the method for separating the mixed gas 70 of this embodiment is suitable for separating the mixed gas 70 (e.g., air) having a content of minor component gases, particularly carbon dioxide, of 1 wt % or less. [Example]

[0145] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0146] (Separation membrane A) First, 10 g of silicone solution (YSR3022 manufactured by Momentive Performance Materials) was diluted with 140 g of normal decane (manufactured by Sankyo Chemical Co., Ltd.) to prepare a 2 mass % silicone solution. An ultrafiltration membrane (NTU-3175M manufactured by Nitto Denko) was immersed in the prepared silicone solution for 5 seconds, drained for 40 seconds, and dried in a dryer at 120°C for 2 minutes. This process was repeated twice to form a 2 μm thick silicone layer (intermediate layer) on the ultrafiltration membrane. The surface of the formed silicone layer was treated with a table-type corona treatment device (manufactured by Kasuga Electric Works) at 1 J / cm. 2 The hydrophilization treatment was carried out at a strength of .

[0147] Next, 2 g of polyether block amide (Pebax MH1657 manufactured by Arkema) was added to 98 g of a 70% by weight aqueous isopropanol solution and stirred at 80 ° C for 3 hours to prepare a 2% by weight Pebax solution. The prepared solution was spread on the silicone layer and spin-coated at 2000 rpm for 40 seconds. The resulting coating film was then dried in an oven at 60 ° C for 30 minutes to form a separation functional layer. This resulted in separation membrane A.

[0148] (Separation membrane B) Separation membrane B was prepared by the same method as separation membrane A, except that a coating solution containing a double-network gel was used instead of the coating solution containing polyether block amide. The double-network gel contained a first network structure, a second network structure, and an ionic liquid. The first network structure was composed of a polymer having structural units derived from dimethylacrylamide and further including a crosslinked structure. The second network structure was composed of silica particles (AEROSIL 200 manufactured by Aerosil). 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA) was used as the ionic liquid. The double-network gel contained 15 wt% of the first network structure, 5 wt% of the second network structure, and 80 wt% of the ionic liquid.

[0149] (Separation membrane C) Separation membrane C was produced in the same manner as separation membrane B, except that 1-ethyl-3-methylimidazolium tricyanomethanide (EMI-C(CN)3) was used as the ionic liquid instead of EMI-DCA.

[0150] [Evaluation of separation membrane characteristics] Next, the carbon dioxide permeation rate T1, nitrogen permeation rate T2, and carbon dioxide / nitrogen separation factor α (CO2 / N2) were measured for each of separation membranes A to C using the following method. First, the separation membrane was placed in a metal cell and sealed with an O-ring to prevent leakage. Next, a mixed gas was injected into the metal cell so that the mixed gas contacted the main surface of the separation membrane on the separation function layer side. The mixed gas consisted essentially of carbon dioxide and nitrogen. The carbon dioxide concentration in the mixed gas was 50 vol% under standard conditions. The temperature of the mixed gas injected into the metal cell was 30°C. The mixed gas pressure was 0.1 MPa. As a result, permeated gas was obtained from the main surface of the separation membrane on the porous support side. The carbon dioxide permeation rate T1, nitrogen permeation rate T2, and separation factor α were calculated based on the composition and weight of the obtained permeated gas. The results are shown in Table 1.

[0151] [Table 1]

[0152] (Calculation examples 1~19) Next, a simulation was performed using separation membranes A to C to simulate the operation of the gas separation system shown in Figure 1. Specifically, the operating conditions of the separation membrane unit, the type of separation membrane used in the separation membrane unit, the composition of the mixed gas, the composition of the second permeate gas, and other factors were determined in advance, and the energy and membrane area of ​​the separation membrane required to separate the mixed gas using the gas separation system were calculated. More specifically, the power required for pressure reduction devices 50 and 52 in Figure 1 and the membrane area of ​​separation membranes 11 and 21 were calculated using Symmetry, a process modeling software program manufactured by Schlumberger.

[0153] In calculation examples 1 to 19, the pressure in the supply space of the separation membrane unit and the type of separation membrane used were changed to confirm their effects. For example, in calculation example 1, the performance of separation membranes 11 and 21 was set to the values ​​of separation membrane A shown in Table 1 (permeation rate T1: 130.3 GPU, selectivity α: 37.2). For each of separation membrane units 10 and 20, the pressure of the gas supplied (pressure in the supply space) was set to atmospheric pressure (101.33 kPa), and the pressure in the permeation space was set to a near-vacuum value (1 kPa). The gas flow rate of carbon dioxide contained in the mixed gas supplied to the first separation membrane unit 10 was set to 300 t / year (34.25 kg / hr), the carbon dioxide content was set to 10 wt%, and the temperature of the mixed gas was set to 25°C. Furthermore, the temperatures of the finally obtained first non-permeate gas and second permeate gas were set to 25°C, and their pressures were set to 101.33 kPa. The carbon dioxide content of the second permeate gas was set to 95 wt%, and its recovery rate was set to 90% (30.82 kg / hr). The adiabatic efficiency of the pressure reducing devices 50 and 52 was set to a typical value of 70%. The results are shown in Tables 2 and 3.

[0154] [Table 2]

[0155] [Table 3]

[0156] (Calculation examples 20~25) Next, a simulation was performed when the gas separation system disclosed in Patent Document 1 was operated. Specifically, it was assumed that a gas separation system 200 shown in Fig. 10 was operated to separate a mixed gas.

[0157] The gas separation system 200 includes a first separation membrane unit 210, a second separation membrane unit 220, and a third separation membrane unit 225. The first separation membrane unit 210 and the second separation membrane unit 220 correspond to the first separation membrane unit 10 and the second separation membrane unit 20, respectively, of the gas separation system 100 in FIG. 1. The third separation membrane unit 225 in FIG. 10 is a membrane separation device that uses a third separation membrane to perform membrane separation on the first non-permeate gas discharged from the first separation membrane unit 210. The third separation membrane can separate the first non-permeate gas into a third permeate gas and a third non-permeate gas.

[0158] The gas separation system 200 includes a mixed gas supply path 230, a permeate gas supply path 232, a first discharge path 234, a second discharge path 236, a third discharge path 238, a fourth discharge path 237, and a fifth discharge path 239. The mixed gas supply path 230 is a path for supplying the mixed gas to the first separation membrane unit 210. A compressor CP1 and a heat exchanger C1 are disposed in the mixed gas supply path 230. The mixed gas can be pressurized by the compressor CP1.

[0159] The permeate gas supply path 232 is a path for supplying the first permeate gas from the first separation membrane unit 210 to the second separation membrane unit 220. No pressure reducing device is provided in the permeate gas supply path 232. The first discharge path 234 is a path for sending the first non-permeate gas from the first separation membrane unit 210 to the third separation membrane unit 225.

[0160] The second discharge path 236 is a path for discharging the second permeate gas from the second separation membrane unit 220. The second discharge path 236 is provided with a pressure reducing device CP2 and a heat exchanger C2.

[0161] The third discharge path 238 is a path for discharging the second non-permeate gas from the second separation membrane unit 220. The third discharge path 238 merges with the mixed gas supply path 230 at a junction position 240, and can send the second non-permeate gas to the mixed gas supply path 230.

[0162] The fourth discharge path 237 is a path for discharging the third non-permeate gas from the third separation membrane unit 225. The fifth discharge path 239 is a path for discharging the third permeate gas from the third separation membrane unit 225. The fifth discharge path 239 merges with the mixed gas supply path 230 at a junction position 240, and can send the third permeate gas to the mixed gas supply path 230.

[0163] In the gas separation system 200, the operating conditions of the separation membrane unit, the type of separation membrane used in the separation membrane unit, the composition of the mixed gas, the composition of the second permeate gas, etc. were determined in advance, and the energy and membrane area of ​​the separation membrane required to separate the mixed gas using the gas separation system 200 were calculated. For the calculations, Symmetry, a process modeling software manufactured by Schlumberger, was used. In calculation examples 20 to 25, the ratio of the membrane areas of the separation membranes in the first separation membrane unit 210 and the third separation membrane unit 225 and the type of separation membrane used were changed to confirm the effects. The results are shown in Table 4.

[0164] [Table 4]

[0165] (Calculation examples 26-28) Next, a simulation was performed when the gas separation system disclosed in Patent Document 2 was operated. Specifically, it was assumed that a mixed gas was separated by operating a gas separation system 250 shown in Fig. 11. The gas separation system 250 has the same configuration as the gas separation system 200 shown in Fig. 10, except that it does not include the second separation membrane unit 220, the second discharge path 236, and the third discharge path 238.

[0166] In the gas separation system 250, the operating conditions of the separation membrane unit, the type of separation membrane used in the separation membrane unit, the composition of the mixed gas, the composition of the first permeate gas, etc. were determined in advance, and the energy and membrane area of ​​the separation membrane required to separate the mixed gas using the gas separation system 250 were calculated. For the calculations, Symmetry, a process modeling software manufactured by Schlumberger, was used. In calculation examples 26 to 28, the type of separation membrane used was changed to confirm the effect. The results are shown in Table 5.

[0167] [Table 5]

[0168] As can be seen from Tables 2 to 5, the gas separation system of this embodiment, which has a configuration that allows the permeate side spaces of the first separation membrane unit and the second separation membrane unit to be depressurized, can reduce the energy required to separate the mixed gas compared to conventional gas separation systems 200 and 250. In particular, as can be seen from Calculation Examples 1 to 10, the gas separation system of this embodiment can reduce the energy required to separate the mixed gas while also reducing the required membrane area of ​​the separation membrane by pressurizing the supply side space of the first separation membrane unit within a range not exceeding 700 kPa.

[0169] (Calculation examples 29~39) As in Calculation Examples 1 to 19, a simulation was performed when the gas separation system shown in Figure 1 was operated. In Calculation Examples 29 to 39, the composition of the second permeable gas and the composition of the mixed gas were changed to confirm their effects. The results are shown in Table 6.

[0170] [Table 6]

[0171] (Calculation example 40~45) As in Calculation Examples 20 to 25, a simulation was performed when the gas separation system 200 shown in Fig. 10 was operated. In Calculation Examples 40 to 45, the composition of the second permeable gas and the composition of the mixed gas were changed to confirm the effects. The results are shown in Table 7.

[0172] [Table 7]

[0173] (Calculation examples 46~50) As in Calculation Examples 26 to 28, a simulation was performed when the gas separation system 250 shown in Fig. 11 was operated. In Calculation Examples 46 to 50, the composition of the first permeable gas and the composition of the mixed gas were changed to confirm the effects. The results are shown in Table 8.

[0174] [Table 8]

[0175] In a gas separation system, the energy required to separate a mixed gas depends greatly on the composition of the permeate gas and the composition of the mixed gas, particularly the composition of the mixed gas. When the calculation examples shown in Tables 6 to 8 are compared under the same gas composition, it is clear that the gas separation system of this embodiment, which has a configuration that allows the permeate side spaces of the first separation membrane unit and the second separation membrane unit to be decompressed, can reduce the energy required to separate the mixed gas compared to the conventional gas separation systems 200 and 210.

[0176] (Calculation examples 51-58) Similar to Calculation Example 1, a simulation was performed when the gas separation system shown in Figure 1 was operated. In Calculation Examples 51 to 58, the pressure in the permeate side space of the first separation membrane unit or the second separation membrane unit was changed from the conditions of Calculation Example 1, and the effect was confirmed. The results are shown in Table 9.

[0177] [Table 9]

[0178] (Calculation examples 59~72) As in Calculation Example 33, Calculation Examples 59 to 72 were simulated when the gas separation system shown in Figure 1 was operated. In Calculation Examples 59 to 72, the pressure in the permeate side space of the first separation membrane unit or the second separation membrane unit was changed from the conditions in Calculation Example 33, and the effect was confirmed. The results are shown in Tables 10 and 11.

[0179] [Table 10]

[0180] [Table 11]

[0181] As can be seen from Tables 9 to 11, by appropriately setting the pressure in the permeate side space of the first or second separation membrane unit, it is possible to reduce the energy required to separate the mixed gas without significantly increasing the membrane area of ​​the separation membrane. This effect is particularly noticeable in the calculation examples in which the pressure P in the permeate side space of the separation membrane unit satisfies the above-mentioned relational expression (A).

[0182] (Calculation examples 73~77) As in Calculation Example 1, for Calculation Example 73, a simulation was performed when the gas separation system shown in Figure 1 was operated. Furthermore, for Calculation Examples 74 to 77, a simulation was performed when the gas separation system shown in Figure 8 was operated. The simulations for Calculation Examples 74 to 77 were performed in the same manner as for Calculation Example 1. Note that in Calculation Examples 74 to 77, the composition of the third permeable gas was determined in advance, instead of the composition of the second permeable gas. In Calculation Examples 73 to 77, the composition of the mixed gas was changed, and the effect was confirmed. The results are shown in Table 12.

[0183] [Table 12]

[0184] As can be seen from Table 12, the gas separation system of this embodiment can separate even mixed gases with an extremely low carbon dioxide content. [Industrial Applicability]

[0185] The gas separation system of this embodiment is suitable for separating a mixed gas, particularly an exhaust gas containing carbon dioxide and nitrogen.

Claims

1. a first separation membrane unit that separates a mixed gas containing carbon dioxide and nitrogen into a first permeate gas and a first non-permeate gas; a second separation membrane unit that separates the first permeable gas into a second permeable gas and a second non-permeable gas; a first pressure reducing device that reduces the pressure in the permeate side space of the first separation membrane unit; a second pressure reducing device that reduces the pressure in the permeate side space of the second separation membrane unit; Equipped with The carbon dioxide content of the second permeable gas is 80 wt % or more, A gas separation system, wherein the first non-permeate gas has a nitrogen content of 95 wt % or more.

2. 10. The gas separation system of claim 1, which is a continuous system.

3. 3. The gas separation system according to claim 1, wherein at least one of the following is established: (I) the mixed gas is separated into the first permeable gas and the first non-permeable gas while the permeate side space of the first separation membrane unit is depressurized by the first depressurization device; and (II) the first permeable gas is separated into the second permeable gas and the second non-permeable gas while the permeate side space of the second separation membrane unit is depressurized by the second depressurization device.

4. the mixed gas includes a main component gas and a subcomponent gas, the first permeable gas has a higher content of the subcomponent gas than the mixed gas, 4. The gas separation system according to claim 1, wherein the first non-permeating gas has a lower content of the minor component gas than the mixed gas.

5. the second permeable gas has a higher content of the subcomponent gas than the first permeable gas, The gas separation system according to claim 4 , wherein the second non-permeate gas has a lower content of the minor component gas than the first permeate gas.

6. The gas separation system according to claim 4 or 5, wherein the mixed gas contains nitrogen as the main component gas and carbon dioxide as the secondary component gas.

7. The carbon dioxide content C in the mixed gas CO2 (wt%), and the nitrogen content C in the mixed gas N2 The gas separation system according to any one of claims 1 to 6, wherein (wt%) satisfies the following relationship (1): C CO2 <C N2 / 4 (1)

8. The gas separation system according to any one of claims 1 to 7, wherein the mixed gas has a carbon dioxide content of 5 wt% or more.

9. a permeate gas supply path connected to the first separation membrane unit and the second separation membrane unit for supplying the first permeate gas to the second separation membrane unit; 9. The gas separation system according to claim 1, wherein the first pressure reducing device is disposed in the permeate gas supply path.

10. The gas separation system according to claim 9 , wherein a collector for collecting the first permeable gas is not disposed in the permeable gas supply path.

11. 11. The gas separation system according to claim 9, wherein the permeate gas supply path is composed only of the first pressure reducing device and piping.

12. The gas separation system according to any one of claims 1 to 11, comprising a plurality of the first separation membrane units.

13. The gas separation system according to claim 12 , wherein the first pressure reducing device is shared by a plurality of the first separation membrane units.

14. a mixed gas supply path connected to the first separation membrane unit for supplying the mixed gas to the first separation membrane unit; a discharge path connected to the second separation membrane unit for discharging the second non-permeate gas from the second separation membrane unit; Furthermore, The gas separation system according to any one of claims 1 to 13, wherein the discharge path merges with the mixed gas supply path at a merging position.

15. the first separation membrane unit has a first separation membrane that separates the mixed gas, the second separation membrane unit has a second separation membrane that separates the first permeate gas, The gas separation system according to any one of claims 1 to 14, wherein at least one selected from the group consisting of the first separation membrane and the second separation membrane contains a polyether block amide resin or an ionic liquid.

16. a first separation step of supplying a mixed gas containing carbon dioxide and nitrogen to a first separation membrane unit and separating the mixed gas into a first permeate gas and a first non-permeate gas by reducing the pressure in a permeate side space of the first separation membrane unit; a second separation step of supplying the first permeable gas to a second separation membrane unit and reducing the pressure in the permeate side space of the second separation membrane unit, thereby separating the first permeable gas into a second permeable gas and a second non-permeable gas; Including, The carbon dioxide content of the second permeable gas is 80 wt % or more, The method for separating a mixed gas, wherein the first non-permeating gas has a nitrogen content of 95 wt % or more.

17. 17. The separation method according to claim 16, wherein in the first separation step, the differential pressure between the supply-side space and the permeate-side space of the first separation membrane unit is adjusted to 400 kPa or less.

18. 18. The separation method according to claim 16, wherein at least one of the following is true: (i) in the first separation step, when the pressure in the permeate side space of the first separation membrane unit is expressed as P (kPa) and the carbon dioxide content in the mixed gas is expressed as x (wt%), the following relational formula (A) is satisfied; and (ii) in the second separation step, when the pressure in the permeate side space of the second separation membrane unit is expressed as P (kPa) and the carbon dioxide content in the first permeate gas is expressed as x (wt%), the following relational formula (A) is satisfied. 0.2e 0.0536x P0.55e 0.0536x (A)

19. The separation method according to any one of claims 16 to 18, wherein the pressure in the permeate side space of the second separation membrane unit in the second separation step is the same as or higher than the pressure in the permeate side space of the first separation membrane unit in the first separation step.

20. The separation method according to any one of claims 16 to 19, wherein the mixed gas is exhaust gas produced by combustion of fuel.

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