Gas separation system and method for producing enriched gas

Abstract

A unit (gas separation membrane unit, the same applies hereinafter) (11) and a unit (12) are connected by a line (14), a unit (11) and a unit (13) are connected by a permeate gas line (15), a raw material mixed gas supply line (16) is connected to the unit (11), and a compression means (21) is interposed in the line (16), a permeate gas line (70) is connected to the unit (13), a gas separation membrane unit (12) and a line (16) are connected by a permeate gas recycle line (17), and a gas separation membrane unit (13) and a line (16) are connected by a non-permeate gas recycle line (18), and the permeate gas line (70) has a flow path that allows a portion of the permeate gas discharged from the unit (13) to be returned to the line (16), and a concentrated and enriched gas is taken out from the unit (12).

Description

[Technical Field]

[0001] The present invention relates to a gas separation system that separates a mixed gas using a plurality of gas separation membrane units, and a method for producing an enriched gas using the gas separation system. [Background technology]

[0002] Patent Document 1 describes a three-stage gas separation system in which a product gas is extracted as a non-permeate gas in the second stage. The system described in this document states that the membrane area of ​​the system can be reduced while obtaining a product gas with a high recovery rate and high purity. Patent Document 2 describes a two-stage gas separation system in which a portion of the permeated gas from the first stage is recycled to a common line for the raw material mixed gas. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2015 / 0273388 [Patent Document 2] International Publication No. 2024 / 014493 Summary of the Invention

[0004] In the gas separation system described in Patent Document 1, if the flow rate of the raw material mixed gas decreases, continuing operation without changing the configuration of each gas separation membrane unit will result in a decrease in the recovery rate of the enriched gas (product gas). To maintain the recovery rate of the enriched gas (product gas) within a certain range, it is necessary to reduce the number of operating modules in the gas separation membrane unit or to lower the operating pressure. Reducing the number of operating modules requires shut-off valves for each module, and over time, differences (variations) in contamination levels (gas permeability) may occur between modules that continue to operate and modules that are closed, potentially leading to a decrease in system performance. While lowering the operating pressure is an option, it is limited by the possibility of exceeding the compressor's operating range. Furthermore, the product gas pressure may fall below the standard value, requiring recompression of the product gas. On the other hand, the present inventors have found that in a three-stage gas separation system in which the non-permeable gas from the second stage is used as the product gas, when a technology is applied in which a portion of the permeable gas from the first stage is returned to the raw material mixed gas supply line, as in Patent Document 2, the effect of maintaining the recovery rate is not sufficient.

[0005] Therefore, an object of the present invention is to provide a gas separation system and a method for producing an enriched gas that can overcome the drawbacks of the prior art.

[0006] As a result of intensive research conducted by the present inventors to solve the above-mentioned problems, they have surprisingly found that in a three-stage gas separation system in which the non-permeable gas from the second stage is extracted as the product gas, by returning a portion of the permeable gas from the third stage, rather than the permeable gas from the first stage, to the raw material mixed gas supply line, the recovery rate of the product gas in the gas separation system can be effectively maintained.

[0007] The present invention was made based on the above findings and provides the following [1] to [9]. [1] A gas separation system that supplies a raw material mixed gas to a gas separation membrane unit and concentrates and enriches at least one gas contained in the raw material mixed gas, the gas separation system comprises a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit; Each gas separation membrane unit has at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a non-permeate gas outlet of the first gas separation membrane unit and a gas inlet of the second gas separation membrane unit are connected by a first non-permeate gas line; a permeate gas outlet of the first gas separation membrane unit and a gas inlet of the third gas separation membrane unit are connected by a first permeate gas line; a raw material mixed gas supply line is connected to the gas inlet of the first gas separation membrane unit, and a compression means is interposed in the raw material mixed gas supply line; a third permeate gas line is connected to the permeate gas outlet of the third gas separation membrane unit; a permeate gas outlet of the second gas separation membrane unit is connected to a position on the suction side of a compression means in the raw mixed gas supply line by a second permeate gas recycle line; a non-permeate gas outlet of the third gas separation membrane unit is connected to a position on the suction side of a compression means in the raw mixed gas supply line by a third non-permeate gas recycle line; the third permeate gas line has a flow path that allows a portion of the permeate gas discharged from the third gas separation membrane unit to be returned to a position on the suction side of a compression means in the raw material mixed gas supply line, A gas separation system in which concentrated and enriched gas is extracted from the non-permeate gas outlet of the second gas separation membrane unit. [2] The gas separation system described in [1], wherein the third permeate gas line has a flow path branching section and is provided with the following first flow path, second flow path, and third flow path, and has a flow rate adjusting means for adjusting the flow rate of the permeate gas flowing into the second flow path. First flow path: A flow path connecting the flow path branching portion and the permeate gas outlet of the third gas separation membrane unit. Second flow path: A flow path that connects the flow path branching portion and a position on the suction side of the compression means in the raw material mixed gas supply line. Third flow path: A flow path that connects the flow path branching portion with the outside of the system. [3] a flow rate detecting means for detecting a flow rate of the raw material mixed gas flowing into the raw material mixed gas supply line; The gas separation system according to [2], further comprising: a control means for instructing the flow rate adjustment means to increase the flow rate of the permeable gas flowing into the second flow path when the flow rate of the raw material mixed gas detected by the flow rate detection means decreases. [4] The raw material mixed gas contains gas A and gas B, and gas A is highly permeable to gas B in the first gas separation membrane unit, the second gas separation membrane unit, and the third gas separation membrane unit. The gas separation selectivity (P1') of the gas separation membrane of the first gas separation membrane unit at 40°C is A (40) / P1' B (40) ) is 30 or more and 150 or less, and the gas separation selectivity at 40°C of the gas separation membrane of the third gas separation membrane unit (P3' A (40) / P3' B (40) ) is 30 or more and 150 or less. [5] The gas separation system according to any one of [1] to [4], wherein the raw material mixed gas is biogas, and a methane-enriched gas is taken out from a non-permeate gas outlet of the second gas separation membrane unit. [6] A gas separation system described in any one of [1] to [5], wherein the ratio of the flow rate PR3 of the permeate gas flow rate P3 of the third gas separation membrane unit that is returned to the suction side position of the compression means in the raw mixed gas supply line is 1% or more and 80% or less. [7] The gas separation system according to any one of [1] to [6], wherein the amount of the raw mixed gas flowing into the gas separation system fluctuates by 5% or more during operation. [8] The gas separation system according to any one of [1] to [7], wherein the first permeate gas line has a flow path that allows a portion of the permeate gas discharged from the first gas separation membrane unit to be returned to a position on the suction side of a compression means in the raw mixed gas supply line. [9] A method for producing an enriched gas by supplying a raw material mixed gas to a gas separation system and concentrating and enriching at least one gas contained in the raw material mixed gas, comprising: the gas separation system comprises a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit; Each gas separation membrane unit has at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a non-permeate gas outlet of the first gas separation membrane unit and a gas inlet of the second gas separation membrane unit are connected by a first non-permeate gas line; a permeate gas outlet of the first gas separation membrane unit and a gas inlet of the third gas separation membrane unit are connected by a first permeate gas line; a raw material mixed gas supply line is connected to the gas inlet of the first gas separation membrane unit, and a compression means is interposed in the raw material mixed gas supply line; a third permeate gas line is connected to the permeate gas outlet of the third gas separation membrane unit; a permeate gas outlet of the second gas separation membrane unit is connected to a position on the suction side of a compression means in the raw mixed gas supply line by a second permeate gas recycle line; a non-permeate gas outlet of the third gas separation membrane unit is connected to a position on the suction side of a compression means in the raw mixed gas supply line by a third non-permeate gas recycle line; the third permeate gas line has a flow path for returning at least a portion of the permeate gas discharged from the third gas separation membrane unit to a position on the suction side of a compression means in the raw material mixed gas supply line, A method for producing an enriched gas, comprising withdrawing concentrated and enriched gas from the non-permeate gas outlet of the second gas separation membrane unit. [Effects of the Invention]

[0008] According to the present invention, in a three-stage gas separation system in which a product gas is extracted as a second-stage non-permeate gas, a high product gas recovery rate can be maintained even if the raw mixed gas flow rate decreases. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a gas separation system according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the configuration of a gas separation system according to a second embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram showing the configuration of the gas separation system of Comparative Example 1. As shown in FIG. [Figure 4] FIG. 4 is a schematic diagram showing the configuration of a gas separation system of Comparative Example 2. As shown in FIG. [Figure 5] FIG. 5 is a schematic diagram showing the structure of an example of a gas separation membrane module used in the gas separation system of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will now be described based on its preferred embodiments and implementations with reference to the drawings. In this specification, the expressions "A and B are connected by line C" or "line C connecting A and B" include both cases where A and B and line C are separate members, and cases where A and / or C are one continuous member. Similarly, the expression "line E is connected to D" includes both cases where D and line E are separate members and cases where D and line E are one continuous member. First, referring to FIG. 1, a gas separation system 10 according to a first embodiment of the present invention and a method for producing an enriched gas using the same according to a first embodiment of the present invention will be described. The gas separation system 10 shown in FIG. 1 includes three gas separation membrane units: a first gas separation membrane unit 11, a second gas separation membrane unit 12, and a third gas separation membrane unit 13. As shown in FIG. 5, each of the gas separation membrane units 11, 12, and 13 can be a module 40 formed by housing a gas separation membrane 20, which is made of a hollow fiber membrane or the like and has selective gas permeability, in a casing 31. Each of the gas separation membrane units 11, 12, and 13 of this embodiment may be formed by using a single gas separation membrane module 40 as shown in FIG. 5, or by arranging multiple such modules 40 in parallel. The casing 31 in the module 40 has openings 32 on two opposing sides. It should be noted that the openings 32 are for inserting the gas separation membrane 20 into the casing 31, and are not openings in the gas separation membrane 20. The gas separation membrane 20 is housed in the casing 31 through the openings 32. When the gas separation membrane 20 is made of a bundle of hollow fiber membranes, the gas separation membrane 20 is housed in the casing 31 so that, in the housed state, each end of the hollow fiber membrane is open near each opening 32 of the casing 31.

[0011] When the gas separation membrane 20 is housed in the casing 31, the gas separation membrane 20 is fixed to the inner wall of the casing 31 by tube plates 33 and 34 at both ends in the Y direction, which is the extension direction of the hollow fiber membrane. Each opening 32 of the casing 31 is closed by a lid 35 or 36. The lid 35 is provided with a gas inlet 37. Meanwhile, the lid 36 is provided with a non-permeate gas outlet 38. The mixed gas to be separated is introduced into the module through the gas inlet 37 of the lid 35. Of the introduced gases, the gas that permeates the gas separation membrane 20 is discharged to the outside of the module through a permeate gas outlet 39 provided in the casing 31. Meanwhile, the non-permeate gas that does not permeate the gas separation membrane 20 is discharged to the outside of the module through the non-permeate gas outlet 38 of the lid 36. In some cases, a purge gas supply port (not shown) may be provided in the casing 31. Although the separation membrane module of FIG. 5 has been described as an example, the present invention can naturally be applied to separation membrane modules of other configurations, for example, to shell-feed type modules.

[0012] 1, as shown, a first gas separation membrane unit 11 and a second gas separation membrane unit 12 are connected in series. Specifically, the first gas separation membrane unit 11 and the second gas separation membrane unit 12 are connected by a first non-permeate gas line 14 connecting a non-permeate gas outlet 11b of the first gas separation membrane unit 11 to a gas inlet 12a of the second gas separation membrane unit 12.

[0013] 1, the first gas separation membrane unit 11 and the third gas separation membrane unit 13 are connected in series. Specifically, the first gas separation membrane unit 11 and the third gas separation membrane unit 13 are connected by connecting the permeate gas outlet 11c of the first gas separation membrane unit 11 to the gas inlet 13a of the third gas separation membrane unit 13 via a first permeate gas line 15.

[0014] A raw material mixed gas supply line 16 is connected to the gas inlet 11a of the first gas separation membrane unit 11, for supplying a raw material mixed gas from a raw material mixed gas source (not shown) to the first gas separation membrane unit 11. A compression means 21 is interposed in the raw material mixed gas supply line 16. The compression means 21 is installed for the purpose of pressurizing the mixed gas supplied from the mixed gas source. The compression means 21 is also installed for the purposes of compressing the permeable gas discharged from the second gas separation membrane unit 12 when the permeable gas is returned to the first gas separation membrane unit 11, and for the purposes of compressing the non-permeable gas discharged from the third gas separation membrane unit 13 when the non-permeable gas is returned to the first gas separation membrane unit 11.

[0015] In the second gas separation membrane unit 12, its permeate gas outlet 12c is connected to a position on the suction side of the compression means 21 in the raw mixed gas supply line 16 via a second permeate gas recycle line 17. Meanwhile, in the third gas separation membrane unit 13, its non-permeate gas outlet 13b is connected to a position on the suction side of the compression means 21 in the raw mixed gas supply line 16 via a third non-permeate gas recycle line 18. A third permeate gas line 70 is connected to the permeate gas outlet 13c of the third gas separation membrane unit 13. A second non-permeate gas line 80 is connected to the non-permeate gas outlet 12 b of the second gas separation membrane unit 12 .

[0016] As shown in Figure 1, the third permeate gas line 70 has a flow path 72 that allows a portion of the permeate gas discharged from the third gas separation membrane unit 13 to be returned to a position on the suction side of the compression means 21 in the raw material mixed gas supply line 16. Specifically, the third permeate gas line 70 includes a flow path branch portion 74, and includes a first flow path 71, a second flow path 72, and a third flow path 73 as follows. First flow path 71: A flow path connecting the flow path branching portion 74 and the permeated gas outlet 13c of the third gas separation membrane unit 13. Second flow path 72: A flow path that connects the flow path branching portion 74 and a position on the suction side of the compression means 21 in the raw material mixed gas supply line 16. Third flow path 73: A flow path connecting the flow path branching portion 74 to the outside of the system. Note that connecting to the outside of the system means that the permeable gas in the third flow path is not connected to return to the first to third gas separation membrane units 11 to 13. Connecting to the outside of the system also includes cases where the permeable gas in the third flow path is introduced into a concentration device (not shown), a storage container, a compressor or a blower, an oxidation device or a combustion device, released into the atmosphere, or introduced into the fourth gas separation membrane unit.

[0017] The third permeate gas line 70 preferably has a flow rate adjusting means for adjusting the flow rate of the permeate gas flowing into the second flow path 72. The flow rate adjusting means may be provided at the flow path branching point 74, or may be provided on the flow path 72 at a position away from the branching point 74 and close to the raw material mixed gas supply line 16, or may be provided on the flow path 73. As the flow rate adjusting means, for example, a flow rate adjustable three-way valve 77 (see FIG. 2) may be used, or two two-way valves may be provided at the branching point. In the example of FIG. 1, flow rate adjustable two-way valves 75 and / or 76 are provided, and by controlling these with the control unit 5, the flow rate of the permeate gas (unit: Nm 3 / h), and the ratio (PR3 / P3) of the reflux amount PR3 through the second flow path 72 to the permeation gas flow rate P3 of the third gas separation membrane unit 13 is controlled. Examples of flow-adjustable two-way valves include needle valves, ball valves, and butterfly valves. P3 is the flow rate (unit: Nm) of the gas discharged from the permeate gas outlet of the third gas separation membrane unit 13 (hereinafter also referred to as the "third permeate gas." In addition, similar abbreviations may be used for other gases hereinafter). 3 / h). PR3 is the flow rate (Nm m 3 / h).

[0018] The operation of the gas separation system 10 of this embodiment having the above configuration will now be described. The raw mixed gas to be separated is supplied from a mixed gas source (not shown) through the raw mixed gas supply line 16 to the first gas separation membrane unit 11. Prior to supply, the raw mixed gas is pressurized by the compression means 21, and its pressure increases. The compression means 21 can be the same as means that have been used in the relevant technical field. For example, a compressor can be used.

[0019] The raw mixed gas contains at least two different gases, Gas A and Gas B, to be separated. When the mixed gas pressurized by the compression means 21 is supplied to the first gas separation membrane unit 11, it is separated into a permeable gas, which is a gas that has permeated the gas separation membrane, and a non-permeable gas, which is a gas that has not permeated the gas separation membrane, due to differences in permeation rate through the gas separation membrane. Gas A has a higher permeation rate through the gas separation membranes constituting each of the units 11 to 13 than Gas B, i.e., it is a highly permeable gas. Gas B has a lower permeation rate through the gas separation membranes constituting each of the units 11 to 13 than Gas A, i.e., it is a low-permeable gas.

[0020] The non-permeate gas discharged from the first gas separation membrane unit 11 is a concentrated version of the gas B compared to the raw mixed gas. The non-permeate gas is discharged from the non-permeate gas outlet 11b of the first gas separation membrane unit 11 and supplied to the second gas separation membrane unit 12 through the first non-permeate gas line 14. On the other hand, the permeate gas from the first gas separation membrane unit 11 is a concentrated version of Gas A compared to the raw mixed gas. The permeate gas is discharged from the permeate gas outlet 11c of the first gas separation membrane unit 11 and supplied to the third gas separation membrane unit 13 through the first permeate gas line 15.

[0021] The non-permeate gas from the first gas separation membrane unit introduced into the second gas separation membrane unit 12 is separated into a permeate gas and a non-permeate gas by the second gas separation membrane unit 12. The non-permeate gas is more concentrated and enriched in Gas B than the gas introduced into the second gas separation membrane unit 12, and is extracted as a product gas (Gas B-enriched gas) from the non-permeate gas outlet 12b of the second gas separation membrane unit 12. Meanwhile, the permeate gas is discharged from the permeate gas outlet 12c of the second gas separation membrane unit 12 and returned to the suction side of the compression means 21 in the raw material mixed gas supply line 16 via the second permeate gas recycle line 17 connected to the outlet 12c. The returned permeate gas is mixed with the raw material mixed gas and then pressurized by the compression means 21.

[0022] The gas introduced into the third gas separation membrane unit 13 (this gas is enriched in Gas A) is separated into a permeate gas and a non-permeate gas by the unit 13. The non-permeate gas is discharged from a non-permeate gas outlet 13b of the third gas separation membrane unit 13 and returned to the suction side of the compression means 21 in the raw material mixed gas supply line 16 via a third non-permeate gas recycle line 18 connected to the outlet 13b. On the other hand, in the system 10 of this embodiment, the permeated gas from the third gas separation membrane unit 13 is more concentrated and enriched in gas A than the gas introduced into the third gas separation membrane unit 13, and a portion of this gas passes through the first flow path 71, the flow path branch 74, and the second flow path 72 and flows back to the suction side of the compression means 21 in the raw mixed gas supply line 16. On the other hand, the remaining portion passes through the first flow path 71, the flow path branch 74, and the third flow path 73 and is discharged to the outside of the system.

[0023] Conventionally, when the amount of raw material mixed gas flowing into the system (hereinafter also referred to simply as the "flow rate of raw material mixed gas") decreases, the membrane area in the first gas separation membrane unit 11 and the third gas separation membrane unit 13 becomes excessive relative to the amount of raw material mixed gas, making it easier for gas B to be contained in the permeate gas discharged from units 11 and 13, resulting in problems such as a decrease in the recovery rate of gas B and an increase in the amount of gas B discharged outside the system. The only solutions to this problem were to reduce the membrane area or the operating pressure, but these posed problems such as a decrease in system performance and the possibility that the product gas pressure would fall below the standard value, requiring the product gas to be recompressed.

[0024] In contrast, the present invention has the flow path 72, and can return a portion of the third permeable gas to the suction side of the compression means 21 in the raw mixed gas supply line 16 (hereinafter, also simply referred to as "return"). As a result, even if the flow rate of the raw material mixed gas decreases, the first permeable gas and the third permeable gas can be circulated at gas amounts appropriate for the membrane area of ​​the first gas separation membrane unit 11 and the third gas separation membrane unit 13 without reducing the membrane area of ​​the first gas separation membrane unit 11 and the third gas separation membrane unit 13. Furthermore, the third permeable gas usually contains a very high concentration of gas A. By returning a gas containing a high concentration of gas A to the raw material mixed gas supply line 16, it becomes difficult for gas B to be contained in the permeable gas discharged from units 11 and 13, and the recovery rate of gas B can be effectively improved.

[0025] Regarding the above point, as mentioned above, the third permeable gas contains a very high concentration of gas A, and therefore, returning this permeable gas to the first gas separation membrane unit 11 would reduce the operating efficiency of the system, and so this was previously unthinkable.

[0026] As will be apparent from a comparison between Comparative Example 2 and Example 1 described later, the present invention has the advantage that increasing the reflux ratio (PR3 / P3) of the third permeable gas makes it easier to improve the recovery rate of the product gas (Gas B). In contrast, when the first permeable gas is refluxed, the recovery rate of Gas B tends to plateau no matter how much the reflux ratio (PR1 / P1) is increased. Here, P1 is the flow rate of the first permeable gas (the flow rate of the permeable gas discharged from the permeable gas outlet of the first gas separation membrane unit 11, Nm 3 / h), PR1 is the flow rate (Nm m ) of the first permeable gas returned to the suction side of the compression means 21 in the raw material mixed gas supply line 16. 3 / h).

[0027] As will be shown in Example 5 below, in the present invention, both a part of the first permeable gas and a part of the third permeable gas may be refluxed. In order to enhance the advantage of improving the recovery rate of gas B described above, the reflux amount PR1 (Nm 3 / h) and the reflux rate of the third permeable gas PR3 (Nm 3 / h), the reflux rate of the third permeable gas PR3 (Nm 3 / h) (PR3 / (PR1+PR3)(%)) is preferably 1% or more, more preferably 5% or more, and even more preferably 10% or more.

[0028] Furthermore, in the present invention, since a third permeable gas having a high concentration of Gas A is used, even if the flow rate of the raw material supply gas is significantly reduced, the recovery rate of the product gas can be increased without increasing the reflux rate PR3 of the third permeable gas so much (all Examples). In other words, the present invention has the advantage of easily balancing the required power for operating the system with the recovery rate.

[0029] Furthermore, in the present invention, in order to reflux the third permeable gas having a high concentration of gas A, even if the concentration of gas B in the raw material feed gas becomes relatively high, the recovery rate of the product gas can be increased without requiring a large increase in the reflux amount C (140 Nm in Examples 1 and 2). 3 / h, 100Nm 3 / h and 60Nm 3 / h) where the reflux volume C (Nm 3 / h) is the total of the second permeate gas flow rate P2, the third non-permeate gas flow rate R3, the first permeate gas reflux rate PR1, and the third permeate gas reflux rate PR3. The second permeate gas flow rate P2 is the gas flow rate discharged from the permeate gas outlet of the second gas separation membrane unit 12, and the third non-permeate gas flow rate R3 is the gas flow rate discharged from the non-permeate gas outlet of the third gas separation membrane unit 13.

[0030] In the present invention, the raw material mixed gas preferably contains 30 mol % or more of Gas B, and particularly preferably 40 to 95 mol %. Furthermore, in the present invention, the raw material mixed gas preferably contains 3 to 70 mol % of Gas A, and particularly preferably 5 to 60 mol %, in view of the high technical significance of the gas separation system of the present invention. When the gas flow rate fluctuates during operation and / or between operations, it is sufficient that the raw material mixed gas falls within this range at least once.

[0031] Furthermore, the reflux rate PR3 of the permeable gas through the third gas separation membrane unit in the gas separation system of the present invention is preferably set so that the ratio (P3L / FL) of the flow rate decrease (P3L) of the permeable gas flow through the third gas separation membrane unit 13 to the flow rate decrease (FL) of the raw material mixed gas is small, and more preferably set to a negative value. This makes it possible to control the reflux rate PR3 of the permeable gas through the third gas separation membrane unit in the gas separation system of the present invention so that the permeable gas flow rate through the third gas separation membrane unit 13 relative to FL remains unchanged or increases, even if a flow rate decrease of the raw material gas occurs. That is, when the set raw material mixed gas flow rate (hereinafter also referred to as the "reference flow rate of raw material mixed gas" in a state where neither the first permeable gas nor the third permeable gas is refluxed; the state where the operation is performed at this flow rate in a state where neither the first permeable gas nor the third permeable gas is refluxed) is F0a, by decreasing this flow rate to F0b (FL=F0a-F0b) without changing the above-mentioned constant conditions, the third permeable gas flow rate P3 becomes equal to the flow rate P3 in the reference state. 0 The decrease from (P3L = P3 0It is preferable that the load F0 is small or increased from the reference state, and it is particularly preferable that the load F0 is increased from the reference state. In Examples 1, 3, and 5 described later, the reference state is a state in Comparative Example 1 where F0 is 200 Nm 3 / h condition (F0a is 200Nm 3 / h) and P3 0 is 78Nm 3 / h. Here, the set raw material mixed gas flow rate refers to a flow rate set as a standard for the gas separation system, for example, a flow rate set as a standard for the gas separation system in a state where neither the first permeable gas nor the third permeable gas is refluxed. In this specification, the reference state is defined as a state where the modules and compressors used are not changed, except for the reflux rates of the first and third permeable gases, and the system is operated at a constant operating pressure, constant membrane area, constant operating temperature, and constant concentrations of Gas A and Gas B (hereinafter also referred to as "constant conditions"). The set raw mixed gas flow rate is the flow rate that provides a recovery rate and purity appropriate for the performance of the gas separation system under the constant conditions.

[0032] Similarly, when the inlet gas flow rate of the raw material mixed gas decreases from Fa to Fb, the first permeable gas flow rate P1 also decreases to the state (P1 0 , and 95 Nm in Examples 1, 3, and 5 described below. 3 / h) (P1L = P1 0 It is preferable that −P1) is small or is set to increase from the reference state, and it is particularly preferable that it is set to increase from the reference state.

[0033] Here, a small rate of decrease in the third permeable gas flow rate means, for example, that the value of P3L / FL is preferably 12% or less, more preferably 10% or less, even more preferably 7% or less, more preferably 5% or less, preferably 1% or less, particularly preferably 0% or less, and most preferably an increase from the reference state, i.e., -1% or less. Furthermore, a small rate of decrease in the permeation gas flow rate of the first gas separation membrane unit 11 preferably means, for example, that the value of P1L / FL is 12% or less, more preferably 10% or less, even more preferably 7% or less, more preferably 5% or less, preferably 1% or less, particularly preferably 0% or less, and particularly preferably an increase from the reference state, i.e., -2% or less.

[0034] The value of P3L / FL is preferably −70% or more, more preferably −60% or more, more preferably −50% or more, and even more preferably −40% or more. The value of P1L / FL is preferably −100% or more, and more preferably −90% or more.

[0035] Furthermore, P3L / P3 0 The value of P3L / P3 is preferably 12% or less, more preferably 7% or less, even more preferably 5% or less, preferably 1% or less, and particularly preferably 0% or less. 0 The value of is preferably −70% or more, more preferably −60% or more, more preferably −50% or more, and even more preferably −40% or more.

[0036] Also, P1L / P1 0 The value of P1L / P1 is preferably 12% or less, more preferably 7% or less, even more preferably 5% or less, even more preferably 1% or less, and particularly preferably 0% or less. 0 The value is preferably −100% or more, and more preferably −90% or more.

[0037] The values ​​of the first permeable gas flow rate P1 and the third permeable gas flow rate P3 at the time when the flow rate of the raw material mixed gas decreases by a predetermined amount can be measured by, for example, a flow rate sensor (not shown).

[0038] A constant operating pressure (unit: MPaG) means that the fluctuation of the operating pressure is within ±1%, for example. A constant membrane area means that there is no change in the number of modules, or the membrane area (unit: m 2 ) fluctuations within ±1%. A constant composition means that the concentration of inflowing gas A (unit: mol%) fluctuates by ±1% or less, and the concentration of gas B (unit: mol%) fluctuates by ±1% or less. A constant operating temperature means that the operating temperature fluctuates by ±1% or less. The concentration fluctuation of gas A and gas B when changing from concentration c1 (mol%) to concentration c2 (mol%) is expressed by the following formula: Concentration fluctuation = (c2 - c1) / c1 x 100 (%)

[0039] The recovery rate of Gas B may be configured to be higher than the recovery rate under the reference state of the system. For example, the recovery rate of Gas B, which is the product gas, can be improved by setting the third permeable gas reflux rate (PR3 / P3) so that the concentration of Gas A in the first permeable gas and / or the concentration of Gas A in the third permeable gas is higher than the concentration under the reference state of the system.

[0040] The permeation gas flow rate P3 (Nm 3 / h), the ratio PR3 (Nm 3 The ratio of PR3 / P3 (permeable gas reflux rate) to P3 (permeable gas reflux rate) can vary depending on the flow rate of the raw material mixed gas. The system of the present invention enables a third permeable gas reflux rate (PR3 / P3) of more than 0% and less than 100%. There is no particular upper limit to the third permeable gas reflux rate (PR3 / P3) during system operation, and it can be determined appropriately depending on the purpose of the target gas separation. For example, operation at a third permeable gas reflux rate (PR3 / P3) of 80% or less is preferable in terms of increasing the system operating efficiency, and 60% or less is more preferable. Furthermore, in order to exert the effects of the present invention, the third permeable gas reflux rate (PR3 / P3) is preferably 1% or more, and more preferably 5% or more.

[0041] Furthermore, in the present invention, the total reflux amount C (Nm 3 / h) to the flow rate PR3 (Nm 3 / h), the ratio PR / C (%) may be 0.1% or more, 0.5% or more, or 1% or more. From the viewpoint of system operation efficiency, PR / C is preferably 80% or less, more preferably 70% or less, and particularly preferably 60% or less.

[0042] In the system of the present invention, the flow rate of the raw mixed gas (F0, Nm 3 The rate of decrease ((F0a-F0b) / F0a(%)) of the flow rate (F0a-F0b / h) may be, for example, 5% or more, or 10% or more. The rate of decrease in flow rate referred to here occurs, for example, during a single operation. The duration of a single operation may be, for example, 10 to 30,000 hours.

[0043] Furthermore, in the system of the present invention, the reflux ratio of the third permeable gas (PR3 / P3) is set to the above-mentioned reflux amount C (unit: Nm 3 From the viewpoint of the operating efficiency of the system, it is preferable that the reflux ratio (C / F1), which is the ratio of the flow rate of the first feed gas F1 to the flow rate of the first feed gas F1, is 90% or less, and more preferably 80% or less.

[0044] To further enhance the effect of maintaining the recovery rate of gas B, it is preferable to use a separation membrane module with high separation performance for the first gas separation membrane unit 11. The high separation performance of the first gas separation membrane unit 11 reliably prevents gas B from being trapped on the permeate side together with a large amount of gas A, thereby improving the recovery rate of gas B. From this perspective, the gas separation selectivity (P1'A / P1'B) of the first gas separation membrane unit 11 is preferably 30 or more, more preferably 35 or more, even more preferably 40 or more, and particularly preferably 50 or more. Furthermore, it is preferable that the gas separation selectivity (P1'A / P1'B) of the first gas separation membrane unit be a certain value or less in terms of reducing the required compression power. From this perspective, it is preferably 150 or less, more preferably 130 or less, even more preferably 120 or less, and particularly preferably 110 or less. In this specification, the gas separation selectivity of a unit refers to the gas separation selectivity at 40°C of the gas separation membrane constituting the unit.

[0045] Similarly, the permeation rate P1′A of gas A through the first gas separation membrane unit 11 is set to 1.5×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more is preferable, and 2×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more is more preferable, and 3×10 -5 cm 3 (STP) / cm 2 sec cmHg or more is more preferable, and 7×10 -5 cm 3 (STP) / cm 2 sec cmHg or more. From the viewpoint of improving the recovery rate of gas B, the permeation rate P1'A of gas A is preferably 100×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is preferable, and 45×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is more preferable, and 25×10-5 cm 3 (STP) / cm 2 sec cmHg or less, and more preferably 15×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is particularly preferred. In this specification, the gas permeability of a unit refers to the gas permeability at 40°C of the gas separation membrane that constitutes the unit.

[0046] It is also preferable to use a separation membrane module with high separation performance for the third gas separation membrane unit 13. In addition to the first gas separation membrane unit 11, the high separation performance of the third gas separation membrane unit 13 can further increase the recovery rate of gas B. From this perspective, the gas separation selectivity (P3'A / P3'B) of the third gas separation membrane unit is preferably 30 or more, more preferably 35 or more, even more preferably 40 or more, and particularly preferably 50 or more. Furthermore, in terms of reducing the required compression power, the gas separation selectivity (P3'A / P3'B) of the third gas separation membrane unit is preferably 150 or less, more preferably 130 or less, even more preferably 120 or less, and particularly preferably 110 or less.

[0047] Similarly, the permeation rate P3'A of gas A through the third gas separation membrane unit 13 is set to 1.5 × 10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more is preferable, and 2×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more is more preferable, and 3×10 -5 cm 3 (STP) / cm 2 sec cmHg or more is more preferable, and 7×10 -5 cm 3 (STP) / cm 2 sec cmHg or more. From the viewpoint of improving the recovery rate of gas B, the permeation rate P3'A of gas A is preferably 100×10 -5 cm 3 (STP) / cm2 ·sec·cmHg or less is preferable, and 45×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is more preferable, and 25×10 -5 cm 3 (STP) / cm 2 sec cmHg or less, and more preferably 15×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is particularly preferred.

[0048] In the present invention, since it is easy to obtain a gas separation system that can maintain the recovery rate of gas B without increasing the required compression power even when the flow rate of the raw material mixed gas is reduced, the gas separation selectivity (P2'A / P2'B) of the gas separation membrane of the second gas separation membrane unit is preferably 5 or more, more preferably 10 or more, even more preferably 15 or more, and particularly preferably 20 or more. Furthermore, from the viewpoint of reducing the required compression power, the gas separation selectivity (P2'A / P2'B) of the gas separation membrane of the second gas separation membrane unit is preferably 150 or less, more preferably 90 or less, even more preferably 65 or less, and particularly preferably 55 or less.

[0049] In the present invention, since it is easy to obtain a gas separation system that can maintain the recovery rate of gas B without increasing the required compression power even when the flow rate of the raw material mixed gas is reduced, the permeation rate P2'B of the second gas separation membrane unit is set to 3×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is preferable, and 2.5×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or less is more preferable, and 2.0×10 -5 cm 3 (STP) / cm 2sec cmHg or less, and from the viewpoint of reducing the required membrane area, the permeation rate P2'B of the gas separation membrane of the second gas separation membrane unit is preferably 0.03×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more is preferable, and 0.08×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more is preferable, and 0.3×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more is more preferable, and 0.8×10 -5 cm 3 (STP) / cm 2 ·sec·cmHg or more is particularly preferable.

[0050] In addition, it is preferable that the gas separation selectivity of the second gas separation membrane unit 12 is equal to or lower than that of the first gas separation membrane unit 11 and / or the third gas separation membrane unit 13, in order to reduce the membrane area while maintaining the recovery rate of gas B.

[0051] From the above point of view, it is preferable that the separation selectivity P2'A / P2'B of the gas separation membrane constituting the second gas separation membrane unit is lower than the separation selectivity P1'A / P1'B of the gas separation membrane constituting the first gas separation membrane unit, in order to reduce the membrane area of ​​the second gas separation membrane unit 12. To more effectively reduce the membrane area of ​​the second gas separation membrane unit 12, the separation selectivity P2'A / P2'B of the gas separation membrane constituting the second gas separation membrane unit 12 is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.70 or less, and particularly preferably 0.60 or less, when the separation selectivity P1'A / P1'B of the gas separation membrane constituting the first gas separation membrane unit 11 is taken as 1. Furthermore, from the viewpoint of suppressing an increase in compression power, P2'A / P2'B is preferably 0.2 or more, and more preferably 0.3 or more, when the gas separation selectivity P1'A / P1'B of the first gas separation membrane unit 11 is taken as 1.

[0052] It is preferable that the permeation rate P2'A of the gas separation membrane constituting the second gas separation membrane unit is higher than the permeation rate P1'A of the gas separation membrane constituting the first gas separation membrane unit, in order to reduce the membrane area of ​​the second gas separation membrane unit 12. From this perspective, when the permeation rate P1'A of the gas separation membrane constituting the first gas separation membrane unit 11 at 40°C is taken as 1, the permeation rate P2'A of the gas separation membrane constituting the second gas separation membrane unit 12 is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.4 or more. Furthermore, when the permeation rate P1'A of the gas separation membrane constituting the first gas separation membrane unit 11 at 40°C is taken as 1, P2'A at 40°C is preferably 12 or less, more preferably 7 or less, in order to suppress an increase in compression power.

[0053] The separation selectivity P2'A / P2'B of the second gas separation membrane unit is preferably lower than the separation selectivity P3'A / P2'B of the gas separation membrane constituting the third gas separation membrane unit. By adopting such a configuration, the membrane area of ​​the second gas separation membrane unit 12 can be further reduced. In order to more effectively reduce the membrane area of ​​the second gas separation membrane unit 12, the gas separation selectivity P2'A / P2'B of the second gas separation membrane unit 12 is ) However, when the gas separation selectivity P3'A / P3'B of the third gas separation membrane unit 13 is taken as 1, it is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.70 or less, and particularly preferably 0.60 or less. Furthermore, when the separation selectivity P3'A / P3'B of the gas separation membrane constituting the third gas separation membrane unit 13 is taken as 1, the separation selectivity P2'A / P2'B of the gas separation membrane is preferably 0.2 or more from the viewpoint of ease of availability of gas separation membranes, and is preferably 0.3 or more from the viewpoint of product gas purity.

[0054] The permeation rate P2'A of the gas separation membrane constituting the second gas separation membrane unit is preferably higher than the permeation rate P3'A of the gas separation membrane constituting the third gas separation membrane unit, in order to reduce the membrane area of ​​the second gas separation membrane unit 12. From this perspective, the permeation rate P2'A of the gas separation membrane constituting the second gas separation membrane unit 12 is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.4 or more, when the permeation rate P3'A of the gas separation membrane constituting the third gas separation membrane unit 13 is taken as 1. Furthermore, from the perspective of ease of gas separation membrane availability, it is preferable that P2'A be 12 or less, when the permeation rate P3'A of the gas separation membrane constituting the third gas separation membrane unit 13 is taken as 1, in order to reduce the membrane area of ​​the second gas separation membrane unit 12.

[0055] Furthermore, the ratio of the membrane area S1 of the first gas separation membrane unit 11 to the membrane area S3 of the third gas separation membrane unit is not particularly limited, but from the viewpoint of increasing the recovery rate of gas B, S1:S3 is preferably 1:0.3 or more, and more preferably 1:0.5 or more. Furthermore, S1:S3 is preferably 1:2.0 or less, and more preferably 1:1.5 or less.

[0056] Furthermore, the ratio of the membrane area S2 of the second gas separation membrane unit 12 to the membrane area S3 of the third gas separation membrane unit is not particularly limited, but from the viewpoint of increasing the recovery rate of gas B, S3:S2 is preferably 1:0.2 or more, and more preferably 1:0.4 or more. Furthermore, S3:S2 is preferably 1:4.0 or less, and more preferably 1:3.0 or less.

[0057] The change in the flow rate of the raw material mixed gas may occur either during operation or between operations, and the present invention can deal with either change by varying the third permeate gas reflux rate. The change in the flow rate of the raw material mixed gas can be detected by a sensor (not shown).

[0058] As described above, the system 10 may also include a control device 5 configured to detect a change in the flow rate of the raw material mixed gas or a change in the concentration of gas in the first permeable gas or the third permeable gas that occurs due to the change in flow rate, and adjust the flow control valves 75 and 76.

[0059] The control means 5 of this embodiment includes a flow rate / composition information acquisition unit 50 and an adjustment instruction unit 55. The control means 5 is electrically connected to flow rate adjustment valves 75, 76, flow rate sensors (not shown) installed in the raw material mixed gas supply line, and gas component concentration sensors provided at the permeate gas outlet of the first gas separation membrane unit 11 and the permeate gas outlet of the third permeate gas, and controls their operation. The control of each unit in the control means 5 is configured to include a CPU, ROM, RAM, etc., and is realized, for example, by the CPU loading a program stored in the ROM, disk, etc. into the RAM and executing it.

[0060] The flow rate / composition information acquiring unit 50 acquires flow rate values ​​of the raw material mixed gas supply line 16 measured by a flow rate sensor (not shown) and stores the data of the flow rate values ​​in chronological order. The flow rate / composition information acquiring unit 50 acquires these flow rate values ​​in real time while the system 10 is operating. The data of the flow rate values ​​stored in chronological order will hereinafter be simply referred to as "flow rate data." The flow rate / composition information acquiring unit 50 stores the flow rate data in a storage unit (not shown). The flow rate / composition information acquiring unit 50 also controls the flow rate data, such as controlling the writing of flow rate data to the storage unit and the reading of flow rate data from the storage unit.

[0061] When the flow rate value of the raw material mixed gas supply line 16 detected by a sensor (not shown) reaches a predetermined value, the adjustment instruction unit 55 controls the flow rate adjustment valves 75 and / or 76 to adjust the reflux rate (PR3 / P3) of the third permeable gas. The control unit 5 may further have the flow rate / composition information acquisition unit 50 read the composition (gas A concentration and gas B concentration) of the raw material mixed gas in the raw material mixed gas supply line with a sensor (flow rate detection means) (not shown), and the adjustment instruction unit 55 may determine the reflux rate (PR3 / P3) of the third permeable gas based on the obtained composition of the raw material mixed gas in addition to the flow rate value. Alternatively, instead of measuring the flow rate of the raw material mixed gas, the reflux ratio (PR3 / P3) of the third permeable gas may be controlled based on the concentration of gas B in the first permeable gas and the third permeable gas, which is detected by a gas component concentration sensor provided at the permeable gas outlet of the first gas separation membrane unit 11 and the permeable gas outlet of the third permeable gas.

[0062] As will be seen from the examples described below, in this system, by adjusting the reflux ratio of the third permeable gas (PR3 / P3) in response to changes in the flow rate of the raw material mixed gas, the product gas recovery rate can be maintained and the purity of the product gas can be improved without increasing or decreasing the number of gas separation membrane modules or reducing the operating pressure. This makes it possible to maintain the operating efficiency of the system and stabilize the number of gas separation membrane modules used in the gas separation system, thereby preventing the above-mentioned variation in module fouling and reducing the overall costs of the gas separation system, including manufacturing costs and maintenance costs.

[0063] Furthermore, the present invention can easily accommodate variations in the composition of the raw material mixed gas. For example, if the ratio of Gas B in the raw material mixed gas increases and the ratio of Gas A decreases during the supply of the raw material mixed gas and / or while the supply is stopped, the present invention can maintain the recovery rate of Gas B by increasing the reflux ratio (PR3 / P3) of the third permeable gas without increasing the number of modules in the third gas separation membrane unit 13.

[0064] As a flow rate sensor for the raw material mixed gas, a thermal mass flow meter, an ultrasonic flow meter, a vortex flow meter, or the like can be used.

[0065] Changes in the composition of the raw material mixed gas (particularly changes in the concentration of Gas A or Gas B) can be detected by a sensor (not shown). The sensor is not particularly limited as long as it can measure the concentration of Gas A or Gas B, and known sensors can be used depending on the usage environment and type of gas. Examples include a CH4 concentration meter, CH4 detector, CO2 concentration meter, and CO2 detector. Analytical instruments that apply gas chromatography analysis technology and infrared spectroscopy technology are also suitable for use.

[0066] In the present invention, the gas separation system can be operated without changing the number of operating modules in the third gas separation membrane unit 13 during and / or between operations. "During operation" refers to the state in which the raw material mixed gas is continuously supplied into the gas separation system from the start of supply until the supply is stopped, during which time gas flows from the raw material mixed gas supply line 16 to each unit 11, 12, and 13. "Between operations" refers to the period in which the flow of gas from the raw material mixed gas supply line 16 to each unit 11, 12, and 13 is stopped and then restarted. An operating module is a module that contributes to gas separation. In some conventional systems, the number of operating modules is adjusted by using an on-off valve or the like installed in each module in response to changes in the composition of the raw material mixed gas or changes in the gas flow rate. However, in such systems, not only are on-off valves required for each module, but the degree of degradation of membrane permeability and other factors can vary between modules that continue to operate and modules that are closed, which can lead to a decrease in system performance. In contrast, the present invention does not require the change in the number of operating modules during and / or between operations, and therefore can avoid the degradation of system performance.

[0067] The gas separation membranes in each gas separation membrane unit 11, 12, and 13 can be appropriately selected depending on the type of mixed gas supplied and the target product gas. Gas separation membranes similar to those conventionally used in the relevant technical field can be used without particular limitation. Examples include rubbery polymer materials such as silicone resin and polybutadiene resin, glassy polymer materials such as polyimide, polyetherimide, polyamide, polyamideimide, polysulfone, polycarbonate, and cellulose, and ceramic materials such as zeolite. The gas separation membrane may be a homogeneous membrane, an asymmetric membrane consisting of a homogeneous layer and a porous layer, or a microporous membrane. The gas separation membrane may be housed in a casing in any of a plate-and-frame type, a spiral type, a hollow fiber type, and the like. A particularly preferred gas separation membrane is an aromatic polyimide hollow fiber gas separation membrane having an asymmetric structure in which the homogeneous layer is 10 nm to 200 nm thick, the porous layer is 20 μm to 200 μm thick, and the inner diameter is approximately 30 μm to 500 μm.

[0068] One gas separation membrane unit may contain one or more gas separation membrane modules. When two or more gas separation membrane modules are contained in one gas separation membrane unit, they are preferably connected in parallel within the unit. When each gas separation membrane unit contains multiple gas separation membrane modules, the membrane area within the unit can be easily adjusted by changing the number of gas separation membrane modules.

[0069] The types of raw material mixed gas and separation membrane module are not limited. Examples include a separation membrane module 40 supplied with the raw material mixed gas that has a higher carbon dioxide (CO) permeation rate than methane (CH), and a gas containing CH and CO; a separation membrane module 40 supplied with the raw material mixed gas that has a higher oxygen gas (O) permeation rate than nitrogen gas (N), and a gas containing N and O. Examples of raw material mixed gases containing CH and CO include biogas, landfill gas, and natural gas. Examples of raw material mixed gases containing N and O include air. Biogas is a gas generated when biomass feedstocks are contacted with microorganisms under anaerobic conditions to undergo microbial fermentation, such as methane fermentation. Biomass feedstocks include organic matter such as food waste, agricultural residues, sewage sludge, and livestock waste. Landfill gas is gas generated by microbial decomposition of organic matter in waste landfills. Biogas and landfill gas are typically composed primarily of methane and carbon dioxide. The present invention is preferably applied to biogas and landfill gas, which may have an unstable flow rate. Even in such cases, the present invention allows the gas separation system to be operated without changing the number of modules in each gas separation membrane unit while maintaining an extremely stable recovery rate of the product gas, methane.

[0070] Furthermore, when Gas B is methane, because methane is a greenhouse gas, an upper limit on its emission concentration is specified, and if this limit is exceeded, direct emission becomes impossible and combustion may be necessary. For this reason, when Gas B is methane, the methane emission concentration in the system is extremely important because it serves as an indicator of environmental regulations for the system. As described above, in the present invention, the concentration of Gas B in the exhaust gas can be controlled in response to changes in the flow rate of the raw material mixed gas, so that an increase in the amount of methane in the third permeate gas discharged outside the system can be prevented, making it even easier to design an environmentally friendly system.

[0071] The gas separation system of this embodiment can be suitably used, for example, in a method for separating and recovering methane gas, a low-permeability gas, from biogas containing mainly methane and carbon dioxide. Methane gas corresponds to Gas B, a low-permeability gas, and carbon dioxide gas corresponds to Gas A, a high-permeability gas. In this case, when the separated and recovered methane gas is to be supplied to a city gas line, a gas compressor serving as compression means may be provided at the non-permeate gas outlet 12b of the second gas separation membrane unit 12 to pressurize the methane gas.

[0072] The pressure of the gas flowing into first gas separation membrane unit 11 by the compression means is preferably 0.2 MPaG or more and 3.0 MPaG or less, and more preferably 0.3 MPaG or more and 2.4 MPaG or less. The pressure of the gas flowing into the third gas separation membrane unit 13 is preferably 0.01 MPaG or more and 0.7 MPaG or less, and more preferably 0.02 MPaG or more and 0.5 MPaG or less.

[0073] The operating temperature in each gas separation membrane unit is preferably from 0° C. to 80° C., more preferably from 5° C. to 60° C. Here, the operating temperature refers to the temperature of the inlet gas of the gas separation membrane unit while the mixed gas is being supplied to the gas separation membrane unit (gas separation membrane module).

[0074] In addition, in order to make the separation selectivity of the second gas separation membrane unit 12 lower than that of the first gas separation membrane unit 11 during operation, the operating temperature of the second gas separation membrane unit 12 may be higher than the operating temperature of the first gas separation membrane unit 11.

[0075] Next, a second embodiment of the present invention will be described with reference to FIG. 2. In the second embodiment of the present invention, a flow path 90 is provided by branching the first permeate gas line 15, which connects the permeate gas outlet 11c of the first gas separation membrane unit 11 and the gas inlet 13a of the third gas separation membrane unit 13 in the first embodiment shown in FIG. 1. The flow path 90 connects the first permeate gas line 15 to the suction side of the compression means 21 of the raw mixed gas supply line 16. A flow path branch 92, where the first permeate gas line 15 branches into the flow path 90, is provided with a flow control valve 91 that can adjust the flow rate of gas flowing from the first permeate gas line 15 into the flow path 90. In FIG. 2, this flow control valve 91 is a three-way valve. The flow path 90 allows a portion of the first permeate gas to be returned to a position on the suction side of the compression means 21 in the raw mixed gas supply line 16. In this embodiment, the control unit 5 controls the flow rate adjustment valve (three-way valve) 91 in addition to the flow rate adjustment valve (three-way valve) 77, thereby controlling both the reflux rate of the first permeable gas (PR1 / P1) and the reflux ratio of the third permeable gas (PR3 / P3), thereby enabling fine control according to various purposes for various conditions such as the recovery rate and purity of gas B, the reflux ratio (C / F1), and the operating pressure. Also, refluxing not only the third permeable gas but also the first permeable gas has the advantage of keeping the compression flow rate within the appropriate operating range of the compressor.

[0076] Furthermore, in the embodiment of Fig. 2, the compression means 22 is disposed in the first permeate gas line 15. This may also be installed in the embodiment of Fig. 1 (an embodiment in which the first permeate gas is not returned). In the embodiment of Fig. 2 in which a portion of the first permeate gas in the first permeate gas line 15 is returned to the raw material mixed gas supply line 16, it is advantageous in terms of reducing the compression power to provide the compression means 22 closer to the third gas separation membrane unit 13 than the branching portion 91 of the first permeate gas line 15.

[0077] Although the present invention has been described above based on its preferred embodiments and implementations, the present invention is not limited to these embodiments and implementations. For example, in the above embodiments and implementations, a unit composed of a gas separation membrane module having hollow fiber membranes was used as an example of each gas separation membrane unit, but gas separation membrane units of other configurations may be used instead.

[0078] In addition to the compression means in the above embodiments and implementations, a pressure reducing means may be provided on the permeation side of one or more of the units to provide power to the mixed gas stream to pass through the separation membrane. Such pressure reducing means may include a known vacuum pump. [Example]

[0079] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to these examples.

[0080] Comparative Examples 1 and 3 A mixed gas containing carbon dioxide and methane (a model biogas gas) was separated using a conventional gas separation system 10'' shown in Figure 3. A compressor was used as the compression means 21 in the system 10''. The flow rate (flow rate flowing into the system) and composition of the raw material mixed gas were as shown in Table 2 below. The raw material mixed gas was heated by compression using the compression means, then cooled in a cooling device (not shown) and supplied to each gas separation membrane unit. The operating conditions were as follows: The operating temperature is the temperature of the gas separation membrane in each gas separation membrane unit. Temperature of raw material mixed gas flowing into the system: 40℃ Pressure of gas flowing into the first gas separation membrane unit 11 (operating pressure) 1.0 MPa·G Pressure of gas flowing into the third gas separation membrane unit 13 (operating pressure) 0.19 MPa·G Operating temperature of the first gas separation membrane unit 11: 40°C Operating temperature of the second gas separation membrane unit 12: 40°C Operating temperature of the third gas separation membrane unit 13: 40°C Number of modules in the first gas separation membrane unit 11: 11 Number of modules in the third gas separation membrane unit 13: 11 Gas separation selectivity of the first gas separation membrane unit 11: Module A was used. Gas separation selectivity of the third gas separation membrane unit 13: Module A was used.

[0081] Module A, which houses a gas separation membrane made of a polyimide hollow fiber membrane in a case, was used as the module constituting the first and third gas separation membrane units 11 and 13. Module A or B, which houses a gas separation membrane made of a polyimide hollow fiber membrane in a case, was used as the module constituting the second gas separation membrane unit 12. As shown in Table 1, gas separation membrane module A has a P' CO2 is 9.9, P' CH4 is 0.18, P' CO2 / P' CH4 The value was 55. These values ​​were obtained at an operating temperature of 40°C. The membrane area of ​​one module was 24 m 2 It was.

[0082] Table 1 shows the number of modules in the second gas separation membrane unit 12, the total number of modules used in each of the units 11 to 13, the gas flow rate and CO2 concentration flowing into each gas separation membrane unit. Furthermore, the CH4 purity and recovery rate (the proportion of the CH4 component flow rate in the non-permeable gas of the second gas separation membrane unit 12 when the CH4 component flow rate in the raw mixed gas is 100%) in the product gas (non-permeable gas of the second gas separation membrane unit 12) are shown in Table 2.

[0083] Comparative Example 2 The gas separation system 10''' shown in FIG. 4 was used, and the flow rate PR1 of the reflux gas of the first permeable gas was set to the value in Table 2. Other than that, the same procedures as in Comparative Example 1 were carried out.

[0084] Comparative Example 4 The module used in the second gas separation membrane unit 12 was changed to module B in Table 1. Other than that, the comparative example was the same as comparative example 1.

[0085] Examples 1 to 3 1, the system 10 was used, and the flow rate PR3 of the reflux gas of the third permeable gas was set to the value shown in Table 2. Other than that, the system was the same as Comparative Example 1.

[0086] Example 4 The module used in the second gas separation membrane unit 12 was changed to module B in Table 1. Other than that, the procedure was the same as in Example 1.

[0087] Example 5 The gas separation system 10' shown in Figure 2 was used. However, the compression means 22 was not provided. The flow rate PR3 of the third permeate recycle gas was set to the value shown in Table 2, and the flow rate PR1 of the first permeate recycle gas was also set to the value shown in Table 2. Other than this, the system was the same as Comparative Example 1.

[0088] [Table 1]

[0089] [Table 2]

[0090] As shown in Table 2, in each of Examples 1 to 5, which used the system of the present invention in which a portion of the third permeate gas was recycled to the first gas separation membrane unit, the purity of the product gas was maintained and a high CH4 recovery rate of 99.5% or more was maintained without increasing or decreasing the number of modules, even when the flow rate of the raw material mixed gas decreased. In particular, in Example 4, compared to Example 1, the required membrane area was reduced while maintaining the same operating pressure, and high purity and high recovery rate were maintained. On the other hand, the CH recovery rate decreased significantly as the flow rate of the raw mixed gas decreased in Comparative Examples 1, 3, and 4. Furthermore, in Comparative Example 2, in a three-stage system, in which the permeated gas from the first gas separation membrane unit 11, rather than the third gas separation membrane unit 13, was returned to the suction side of the compression means 21 in the raw mixed gas supply line 16 as described in Patent Document 1, the effect of improving the CH recovery rate was not sufficient. [Industrial Applicability]

[0091] According to the present invention, there is provided a three-stage gas separation system that can be operated without changing the number of operating modules in the gas separation membrane unit, even when the flow rate of the raw mixed gas decreases, while suppressing the impact on the purity and recovery rate of the enriched gas (product gas).

Claims

1. A gas separation system that supplies a raw material mixed gas to a gas separation membrane unit and concentrates and enriches at least one gas contained in the raw material mixed gas, the gas separation system comprises a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit; Each gas separation membrane unit has at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a non-permeate gas outlet of the first gas separation membrane unit and a gas inlet of the second gas separation membrane unit are connected by a first non-permeate gas line; a permeate gas outlet of the first gas separation membrane unit and a gas inlet of the third gas separation membrane unit are connected by a first permeate gas line; a raw material mixed gas supply line is connected to the gas inlet of the first gas separation membrane unit, and a compression means is interposed midway along the raw material mixed gas supply line; a third permeate gas line is connected to the permeate gas outlet of the third gas separation membrane unit; a permeate gas outlet of the second gas separation membrane unit is connected to a position on the suction side of the compression means in the raw mixed gas supply line by a second permeate gas recycle line; a non-permeate gas outlet of the third gas separation membrane unit is connected to a position on the suction side of the compression means in the raw mixed gas supply line by a third non-permeate gas recycle line; the third permeate gas line has a flow path that allows a portion of the permeate gas discharged from the third gas separation membrane unit to be returned to a position on the suction side of the compression means in the raw mixed gas supply line, A gas separation system in which concentrated and enriched gas is taken out from the non-permeate gas outlet of the second gas separation membrane unit.

2. 2. The gas separation system according to claim 1, wherein the third permeate gas line has a flow path branching section and includes the following first flow path, second flow path, and third flow path, and further has a flow rate adjusting means for adjusting the flow rate of the permeate gas flowing into the second flow path. First flow path: A flow path connecting the flow path branching portion and the permeate gas outlet of the third gas separation membrane unit. Second flow path: A flow path that connects the flow path branching portion and a position on the suction side of the compression means in the raw material mixed gas supply line. Third flow path: A flow path that connects the flow path branching portion with the outside of the system.

3. a flow rate detecting means for detecting a flow rate of the raw material mixed gas flowing into the raw material mixed gas supply line; 3. The gas separation system according to claim 2, further comprising: a control means for instructing the flow rate adjustment means to increase the flow rate of the permeable gas flowing into the second flow path when the flow rate of the raw material mixed gas detected by the flow rate detection means decreases.

4. The raw material mixed gas contains gas A and gas B, and gas A is highly permeable to gas B in the first gas separation membrane unit, the second gas separation membrane unit, and the third gas separation membrane unit, and the gas separation selectivity (P １ 'A / P １ 'B) is 30 or more, and the gas separation selectivity at 40°C of the gas separation membrane of the third gas separation membrane unit (P ３ 'A / P ３ 3. The gas separation system of claim 1, wherein B) is 30 or greater.

5. 3. The gas separation system according to claim 1, wherein the raw material mixed gas is biogas, and a methane-enriched gas is taken out from a non-permeate gas outlet of the second gas separation membrane unit.

6. A gas separation system as described in claim 1 or 2, wherein the ratio of the flow rate PR3 of the permeable gas flow rate P3 of the third gas separation membrane unit that is returned to the suction side position of the compression means in the raw mixed gas supply line is 1% or more and 80% or less.

7. 3. The gas separation system according to claim 1, wherein the inflow rate of the raw mixed gas into the gas separation system fluctuates by 5% or more during operation.

8. 3. The gas separation system according to claim 1, wherein the first permeate gas line has a flow path that enables a portion of the permeate gas discharged from the first gas separation membrane unit to be returned to a position on the suction side of the compression means in the raw mixed gas supply line.

9. A method for producing an enriched gas by supplying a raw material mixed gas to a gas separation system and concentrating and enriching at least one gas contained in the raw material mixed gas, comprising: the gas separation system comprises a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit; Each gas separation membrane unit has at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, a non-permeate gas outlet of the first gas separation membrane unit and a gas inlet of the second gas separation membrane unit are connected by a first non-permeate gas line; a permeate gas outlet of the first gas separation membrane unit and a gas inlet of the third gas separation membrane unit are connected by a first permeate gas line; a raw material mixed gas supply line is connected to the gas inlet of the first gas separation membrane unit, and a compression means is interposed midway along the raw material mixed gas supply line; a third permeate gas line is connected to the permeate gas outlet of the third gas separation membrane unit; a permeate gas outlet of the second gas separation membrane unit is connected to a position on the suction side of the compression means in the raw mixed gas supply line by a second permeate gas recycle line; a non-permeate gas outlet of the third gas separation membrane unit is connected to a position on the suction side of the compression means in the raw mixed gas supply line by a third non-permeate gas recycle line; the third permeate gas line has a flow path that allows a portion of the permeate gas discharged from the third gas separation membrane unit to be returned to a position on the suction side of the compression means in the raw mixed gas supply line, The method for producing an enriched gas comprises withdrawing concentrated and enriched gas from the non-permeate gas outlet of the second gas separation membrane unit.

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