Gas separation system and enriched gas production method

Abstract

The unit (11) and the unit (12) are connected via the line (14); the unit (11) and the unit (13) are connected via the permeate gas line (15); the raw material mixed gas supply line (16) is connected to the unit (11) and the compression means (21) is interposed in the line (16); the permeate gas line (70) is connected to the unit (13); the gas separation membrane unit (12) and the line (16) are connected via the permeate gas recycle line (17); the gas separation membrane unit (13) and the line (16) are connected via the retentate gas recycle line (18); the permeate gas line (70) includes a flow path that allows a part of the permeate gas discharged from the unit (13) to be recycled to the line (16); and the enriched gas is recovered from the unit (12).

Description

Gas separation system and method for producing enriched gas

[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.

[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. It also describes that the system described in this document can obtain a product gas with a high recovery rate and high purity while reducing the membrane area of ​​the system. Patent Document 2 describes a two-stage gas separation system in which a portion of the permeate gas in the first stage is recycled to a common raw material mixed gas line.

[0003] US Patent Application Publication No. 2015 / 0273388 International Patent Application Publication No. 2024 / 014493

[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 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 permeation performance) 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 also an option, it has limitations as it may fall outside the compressor's proper operating range. Furthermore, the product gas pressure may fall below the standard value, requiring recompression of the product gas. Meanwhile, the present inventors have discovered that in a three-stage gas separation system using the non-permeated gas from the second stage as the product gas, applying a technique for returning a portion of the permeated gas from the first stage to the raw material mixed gas supply line, as described in Patent Document 2, does not sufficiently maintain the recovery rate.

[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 a 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 has been made based on the above findings and provides the following [1] to [9]: [1] A gas separation system for supplying a raw material mixed gas to a gas separation membrane unit and concentrating and enriching at least one gas contained in the raw material mixed gas, the gas separation system comprising a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit, each gas separation membrane unit having at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, the non-permeate gas outlet of the first gas separation membrane unit is connected to the gas inlet of the second gas separation membrane unit by a first non-permeate gas line, the permeate gas outlet of the first gas separation membrane unit is connected to the gas inlet of the third gas separation membrane unit 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, and 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 and a position on the suction side of the compressor 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 and a position on the suction side of the compressor in the raw mixed gas supply line by a third non-permeate gas recycle line, the third permeate gas line having a flow path that allows a portion of the permeate gas discharged from the third gas separation membrane unit to be returned to the position on the suction side of the compressor in the raw mixed gas supply line, and a concentrated and enriched gas is extracted from the non-permeate gas outlet of the second gas separation membrane unit. [2] The gas separation system according to [1], wherein the third permeate gas line has a flow path branch and comprises the following first, second, and third flow paths, and has a flow rate adjustment means that adjusts the flow rate of the permeate gas flowing into the second flow path. First flow path: a flow path that connects the flow path branch 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 with 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] The gas separation system according to [2], further comprising: a flow rate detecting means for detecting the flow rate of the raw material mixed gas flowing into the raw material mixed gas supply line; and a control means for instructing the flow rate adjusting 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 detecting means decreases. [4] The raw material mixed gas contains Gas A and Gas B, and Gas A is highly permeable relative 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) at 40°C of the gas separation membrane of the first gas separation membrane unit is 0.05 to 0.1. １ ' Ａ （４０） / P １ ' Ｂ （４０） ) is 30 or more and 150 or less, and the gas separation selectivity (P ３ ' Ａ （４０） / P ３ ' Ｂ （４０）

[0013] The gas separation system according to any one of [1] to [3], wherein the methane-enriched gas is extracted from a non-permeate gas outlet of the second gas separation membrane unit.

[0014] The gas separation system according to any one of [1] to [4], wherein the ratio of a flow rate PR3 of the permeate gas from the third gas separation membrane unit to a flow rate PR3 of the feed mixed gas supply line at the suction side of the compression means is 1% or more and 80% or less.

[0015] The gas separation system according to any one of [1] to [6], wherein the inflow rate of the feed mixed gas to 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 the compression means in the raw mixed gas supply line.[9] A method for producing an enriched gas, which comprises 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, the gas separation system comprising a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit, each gas separation membrane unit comprising at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, the non-permeate gas outlet of the first gas separation membrane unit and the gas inlet of the second gas separation membrane unit are connected by a first non-permeate gas line, the permeate gas outlet of the first gas separation membrane unit and the 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, and 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 and a position on the suction side of a compression means in the raw mixed gas supply line are connected by a second permeate gas recycle line; a non-permeate gas outlet of the third gas separation membrane unit and a position on the suction side of a compression means in the raw mixed gas supply line are connected by a third non-permeate gas recycle line; the third permeate gas line has a flow path that returns at least a portion of the permeate gas discharged from the third gas separation membrane unit to a position on the suction side of the compression means in the raw mixed gas supply line; and a concentrated and enriched gas is extracted from the non-permeate gas outlet of the second gas separation membrane unit.

[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 flow rate of the raw mixed gas decreases.

[0009] Fig. 1 is a schematic diagram showing the configuration of a gas separation system in a first embodiment of the present invention. Fig. 2 is a schematic diagram showing the configuration of a gas separation system in a second embodiment of the present invention. Fig. 3 is a schematic diagram showing the configuration of a gas separation system of Comparative Example 1. Fig. 4 is a schematic diagram showing the configuration of a gas separation system of Comparative Example 2. 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.

[0010] The present invention will be described below with reference to the drawings based on preferred embodiments and implementations. In this specification, the expressions "connecting A and B by line C" or "line C connecting A and B" include both cases where A and B and line C are separate components, and cases where line C is a single component continuous with A and / or C. Similarly, the expression "connecting line E to D" includes both cases where D and line E are separate components, and cases where D and line E are a single component. First, 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 with reference to FIG. 1 . 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. Each of the gas separation membrane units 11, 12, and 13 can be, for example, a module 40, as shown in FIG. 5, in which a gas separation membrane 20 made of a hollow fiber membrane or the like and having selective gas permeability is housed in a casing 31. Each of the gas separation membrane units 11, 12, and 13 of this embodiment uses one gas separation membrane module 40 shown in FIG. 5 , or is made up of a plurality of such modules 40 arranged in parallel. The casing 31 in the module 40 has openings 32 on two opposing sides. It should be noted that these openings 32 are for inserting the gas separation membrane 20 into the casing 31, and are not openings in the gas separation membrane 20 itself. The gas separation membrane 20 is housed in the casing 31 through these openings 32. When the gas separation membrane 20 is made up 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 in 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 direction in which the hollow fiber membranes extend. Each opening 32 of the casing 31 is closed by a lid 35 or 36. The lid 35 is provided with a gas inlet 37. Meanwhile, the lid 36 is provided with a non-permeate gas outlet 38. The mixed gas to be separated is introduced into the module through the gas inlet 37 of the lid 35. Of the introduced gases, the gas that permeates the gas separation membrane 20 is discharged outside the module through a permeate gas outlet 39 provided in the casing 31. Meanwhile, the non-permeate gas that does not permeate the gas separation membrane 20 is discharged outside the module through the non-permeate gas outlet 38 of the lid 36. In some cases, a purge gas supply port (not shown) may be provided in the casing 31. Although the separation membrane module of FIG. 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 the non-permeate gas outlet 11b of the first gas separation membrane unit 11 to the gas inlet 12a of the second gas separation membrane unit 12.

[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 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 permeable 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 by a second permeable gas recycle line 17. Meanwhile, in the third gas separation membrane unit 13, its non-permeable 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 by a third non-permeable gas recycle line 18. A third permeable gas line 70 is connected to the permeable gas outlet 13c of the third gas separation membrane unit 13. In addition, a second non-permeable gas line 80 is connected to the non-permeable gas outlet 12b of the second gas separation membrane unit 12.

[0016] As shown in FIG. 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 mixed gas supply line 16. Specifically, the third permeate gas line 70 has a flow path branch 74 and includes the following first flow path 71, second flow path 72, and third flow path 73. First flow path 71: A flow path that connects the flow path branch 74 to the permeate gas outlet 13c of the third gas separation membrane unit 13. Second flow path 72: A flow path that connects the flow path branch 74 to a position on the suction side of the compression means 21 in the raw mixed gas supply line 16. Third flow path 73: A flow path that connects the flow path branch 74 to the outside of the system. Note that connecting to the outside of the system means that the permeate gas in the third flow path is not connected to be returned 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 from the third flow path is introduced, for example, into a concentration device (not shown), into a storage container, into a compressor or blower, into an oxidation device or combustion device, released into the atmosphere, or introduced into a 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 portion 74, or may be provided on the flow path 72 at a position away from the branching portion 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 portion. 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 flow rate PR3 through the second flow path 72 to the permeation gas flow rate P3 of the third gas separation membrane unit 13. Examples of flow-rate-adjustable two-way valves include a needle valve, a ball valve, and a butterfly valve. P3 is the flow rate (unit: Nm) of the gas discharged from the permeation gas outlet of the third gas separation membrane unit 13 (hereinafter also referred to as the "third permeation gas"; hereinafter, similar abbreviations may be used for other gases). ３ / h). PR3 is the flow rate (Nm 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 types of 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 the difference 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 concentrated with 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 concentrated with 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 the non-permeate gas outlet 13b of the third gas separation membrane unit 13 and returned to the suction side of the compression means 21 in the raw mixed gas supply line 16 via the third non-permeate gas recycle line 18 connected to the outlet 13b. Meanwhile, in the system 10 of this embodiment, the permeate gas from the third gas separation membrane unit 13 is further enriched in Gas A compared to the gas introduced into the third gas separation membrane unit 13. A portion of this gas is returned to the suction side of the compression means 21 in the raw mixed gas supply line 16 via the first flow path 71, the flow path branch 74, and the second flow path 72. Meanwhile, the remaining portion is discharged to the outside of the system via the first flow path 71, the flow path branch 74, and the third flow path 73.

[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 to the outside of the system. The only solutions to this problem were to reduce the membrane area or the operating pressure, but these had problems such as leading to a decrease in system performance and the possibility that the product gas pressure would fall below the standard value, making it necessary to recompress the product gas.

[0024] In contrast, the present invention has the flow path 72, and can return (hereinafter simply referred to as "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. This allows the first permeable gas and the third permeable gas to flow at amounts appropriate for the membrane areas of the first gas separation membrane unit 11 and the third gas separation membrane unit 13, without reducing the membrane areas of the first gas separation membrane unit 11 and the third gas separation membrane unit 13, even if the flow rate of the raw mixed gas decreases. 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 mixed gas supply line 16, gas B is less likely 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 described 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 the 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 amount 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 feed 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 (see each example). In other words, the present invention has the advantage of being able to easily balance 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 terms, 100 Nm ３ / h terms, and 60 Nm ３ / h). Here, the reflux amount C (Nm ３ / h) is the total amount of the second permeable gas flow rate P2, the third non-permeable gas flow rate R3, the reflux gas amount PR1 of the first permeable gas, and the reflux gas amount PR3 of the third permeable gas. The second permeable gas flow rate P2 is the gas flow rate discharged from the permeable gas outlet of the second gas separation membrane unit 12, and the third non-permeable gas flow rate R3 is the gas flow rate discharged from the non-permeable 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 of the permeable gas through the third gas separation membrane unit 13 (P3L) to the flow rate decrease of the raw material mixed gas (FL) 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 the raw material mixed gas") is F0a 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 referred to as the reference state, when the flow rate is reduced to F0b (FL=F0a-F0b) without changing the above-mentioned constant conditions, the third permeable gas flow rate P3 decreases to the flow rate P3 in the reference state. ０ The decrease from (P3L = P3 ０ It is preferable that the load F0 is set to be small or increased from the reference state, and it is particularly preferable that the load F0 is set to be 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 state (F0a is 200 Nm 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 reference for the gas separation system. For example, it refers to 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 under conditions where, except for the reflux rates of the first permeable gas and the third permeable gas, the modules and compressors used are not changed, and the system is operated at a constant operating pressure, a constant membrane area, a constant operating temperature, and constant concentrations of Gas A and Gas B (hereinafter also referred to as "constant conditions"). The set raw material mixed gas flow rate is a flow rate that provides a recovery rate and purity suitable for the performance of the gas separation system under the constant conditions.

[0032] Similarly, when the inflow 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 later. ３ / h) (P1L = P1 ０ It is preferable that the value of −P1) is small or is set to increase from the reference state, and it is particularly preferable that the value of −P1) is set to increase from the reference state.

[0033] Here, a small decrease in the third permeable gas flow rate preferably refers to a P3L / FL value of 12% or less, more preferably 10% or less, even more preferably 7% or less, more preferably 5% or less, preferably 1% or less, and particularly preferably 0% or less, and most preferably an increase from the reference state, i.e., -1% or less. Furthermore, a small decrease in the permeable gas flow rate of the first gas separation membrane unit 11 preferably refers to a P1L / FL value of 12% or less, more preferably 10% or less, even more preferably 7% or less, more preferably 5% or less, preferably 1% or less, and particularly preferably 0% or less, and most 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 (unit: mol%) of the inflowing gas A fluctuates by ±1% or less, and the concentration (unit: mol%) of gas B fluctuates by ±1% or less. A constant operating temperature means that the operating temperature fluctuates by ±1% or less. The concentration fluctuations of gas A and gas B can be expressed by the following formula when changing from concentration c1 (mol%) to concentration c2 (mol%): 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 are higher than the concentrations under the reference state of the system.

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

[0041] Furthermore, in the present invention, the total reflux amount C (Nm ３ / h) with respect to the flow rate PR3 (Nm ３ / h), PR / C (%) may be 0.1% or more, 0.5% or more, or 1% or more. In terms 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 The reflux rate (C / F1), which is the ratio of the flow rate F1 of the first feed gas to the flow rate F1 of the first supply gas, is preferably 90% or less, and more preferably 80% or less, from the viewpoint of the operating efficiency of the system.

[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 can reliably prevent gas B from being contained in 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 (P １ 'A / P １ The gas separation selectivity (P １ 'A / P １ 'B) is preferably a certain value or less in terms of reducing the required compression power, and from this viewpoint, 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 that constitutes the unit.

[0045] Similarly, the permeation rate P of gas A through the first gas separation membrane unit 11 １ 'A is 1.5 x 10 in order to reduce the required compression power. －５ cm ３ (STP) / cm ２ sec cmHg or more, preferably 2 x 10 －５ cm ３ (STP) / cm ２ sec cmHg or more, and more preferably 3 x 10 －５ cm ３ (STP) / cm ２sec cmHg or more, and more preferably 7 x 10 －５ cm ３ (STP) / cm ２ In order to improve the recovery rate of gas B, the permeation rate P １ 'A is 100 x 10 －５ cm ３ (STP) / cm ２ sec cmHg or less, and 45 x 10 －５ cm ３ (STP) / cm ２ sec cmHg or less, and more preferably 25 x 10 －５ cm ３ (STP) / cm ２ sec cmHg or less, and more preferably 15 x 10 －５ cm ３ (STP) / cm ２ 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. By providing high separation performance for the third gas separation membrane unit 13 in addition to the first gas separation membrane unit 11, the recovery rate of gas B can be further increased. From this perspective, the gas separation selectivity (P ３ 'A / P ３ The gas separation selectivity (P ３ 'A / P ３ 'B) is preferably 150 or less, more preferably 130 or less, even more preferably 120 or less, and particularly preferably 110 or less, in view of reducing the required compression power.

[0047] Similarly, the permeation rate P of gas A through the third gas separation membrane unit 13 ３ 'A is 1.5 x 10 in terms of required compression power. -5 cm 3 (STP) / cm2 sec cmHg or more, preferably 2 x 10 －５ cm ３ (STP) / cm ２ sec cmHg or more, and more preferably 3 x 10 －５ cm ３ (STP) / cm ２ sec cmHg or more, and more preferably 7 x 10 －５ cm ３ (STP) / cm ２ In order to improve the recovery rate of gas B, the permeation rate P ３ 'A is 100 x 10 －５ cm ３ (STP) / cm ２ sec cmHg or less, and 45 x 10 －５ cm ３ (STP) / cm ２ sec cmHg or less, and more preferably 25 x 10 －５ cm ３ (STP) / cm ２ sec cmHg or less, and more preferably 15 x 10 －５ cm ３ (STP) / cm ２ sec·cmHg or 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 (P ２ 'A / P ２ 'B) is preferably 5 or more, more preferably 10 or more, even more preferably 15 or more, and particularly preferably 20 or more. In order to reduce the required compression power, the gas separation selectivity (P ２ 'A / P ２ 'B) 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 P ２ 'B is 3 x 10 －５ cm ３ (STP) / cm ２ sec cmHg or less, preferably 2.5 x 10 -5 cm 3 (STP) / cm 2 sec cmHg or less, more preferably 2.0 x 10 -5 cm 3 (STP) / cm 2 sec cmHg or less is more preferable, and from the viewpoint of reducing the required membrane area, the permeation rate P ２ 'B is 0.03 x 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 more 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 preferred.

[0050] Furthermore, 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 viewpoint, the separation selectivity P of the gas separation membrane constituting the second gas separation membrane unit ２ 'A / P ２ 'B is the separation selectivity P of the gas separation membrane constituting the first gas separation membrane unit; １ 'A / P １It is preferable that the separation selectivity P′ of the gas separation membrane constituting the second gas separation membrane unit 12 is lower than the separation selectivity P′ of the gas separation membrane constituting the second gas separation membrane unit 12 in order to reduce the membrane area of ​​the second gas separation membrane unit 12 more effectively. ２ 'A / P ２ 'B is the separation selectivity P of the gas separation membrane constituting the first gas separation membrane unit 11; １ 'A / P １ When B 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. ２ 'A / P ２ 'B is the gas separation selectivity P of the first gas separation membrane unit 11 １ 'A / P １ When 'B' is taken as 1, it is preferable that 'B' is 0.2 or more from the viewpoint of suppressing an increase in compression power, and it is more preferable that 'B' is 0.3 or more.

[0052] The permeation rate P of the gas separation membrane constituting the second gas separation membrane unit ２ 'A is the permeation rate P of the gas separation membrane constituting the first gas separation membrane unit. １ It is preferable that the permeation rate P ′ is higher than the permeation rate A ′ in order to reduce the membrane area of ​​the second gas separation membrane unit 12. ２ 'A is the permeation rate P of the gas separation membrane constituting the first gas separation membrane unit 11 at 40°C. １ When A is taken as 1, it is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.4 or more. ２ 'A is the permeation rate P of the gas separation membrane constituting the first gas separation membrane unit 11. １ When 'A is 1, it is preferably 12 or less from the viewpoint of suppressing an increase in compression power, and more preferably 7 or less.

[0053] Separation selectivity P of the second gas separation membrane unit ２ 'A / P ２ 'B is the separation selectivity P of the gas separation membrane constituting the third gas separation membrane unit ３ 'A / P２ It is preferable that the gas separation selectivity P′ of the second gas separation membrane unit 12 is lower than that of the second gas separation membrane unit 12. By adopting such a configuration, the membrane area of ​​the second gas separation membrane unit 12 can be further reduced. ２ 'A / P ２ 'B ） is the gas separation selectivity P of the third gas separation membrane unit 13 ３ 'A / P ３ When B 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. ２ 'A / P ２ 'B is the separation selectivity P of the gas separation membrane constituting the third gas separation membrane unit 13. ３ 'A / P ３ When 'B' is taken as 1, it is preferable that the ratio is 0.2 or more in terms of availability of gas separation membranes, and it is preferable that the ratio is 0.3 or more in terms of product gas purity.

[0054] The permeation rate P of the gas separation membrane constituting the second gas separation membrane unit ２ 'A is the permeation rate P of the gas separation membrane constituting the third gas separation membrane unit. ３ It is preferable that the permeation rate P ′ is higher than the permeation rate A ′ in order to reduce the membrane area of ​​the second gas separation membrane unit 12. ２ 'A is the permeation rate P of the gas separation membrane constituting the third gas separation membrane unit 13. ３ When A is 1, it is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.4 or more. ２ 'A is the permeation rate P of the gas separation membrane constituting the third gas separation membrane unit 13. ３ When 'A' is 1, it is preferably 12 or less in terms of availability of gas separation membranes, and it is preferably 5 or less in terms of product gas purity.

[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 flow rate of the third permeable gas reflux. The change in the flow rate of the raw material mixed gas can be detected by a sensor (not shown).

[0058] Furthermore, as described above, the system 10 may 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 in conjunction with 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 instructing unit 55 controls the flow rate control valves 75 and / or 76 to adjust the reflux ratio (PR3 / P3) of the third permeable gas. The control unit 5 may further have the flow rate / composition information acquiring 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 using a sensor (flow rate detecting means) (not shown), and the adjustment instructing unit 55 may determine the reflux ratio (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 gas B concentrations in the first permeable gas and the third permeable gas, which are detected by gas component concentration sensors 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 (PR3 / P3) of the third permeable gas 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 contamination and reducing the overall costs, such as the manufacturing costs and maintenance costs, of the gas separation system.

[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 of the raw material mixed gas 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] The change in the composition of the raw material mixed gas (particularly the change 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 a known sensor can be used depending on the environment of use and the type of gas. As an example, a CH ４ Concentration meter, CH ４ Detector, CO ２ Concentration meter, CO ２ Also suitable for use are analytical instruments that apply gas chromatography analysis technology and infrared spectroscopy technology.

[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 gas flows from the raw material mixed gas supply line 16 to each unit 11, 12, and 13. "Between operations" refers to the period from when the flow of gas from the raw material mixed gas supply line 16 to each unit 11, 12, and 13 is stopped until it is restarted. An operating module is a module that contributes to gas separation. In conventional systems, the number of operating modules is adjusted using on-off valves or the like installed in each module in response to changes in the composition of the raw material mixed gas or changes in gas flow rate. However, such systems require on-off valves for each module, and variations in the degree of membrane permeability degradation occur between modules that continue to operate and modules that are closed, which can lead to a deterioration in system performance. In contrast, the present invention does not require changing the number of operating modules during and / or between operations, thereby avoiding the above-mentioned deterioration in 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 previously used in the relevant technical field can be used without particular limitation. Examples include rubber-like 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. Therefore, when Gas B is methane, the methane emission concentration in the system is very 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 low-permeability gas B, and carbon dioxide gas corresponds to high-permeability gas A. 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 the first gas separation membrane unit 11 by the compression means is usually preferably 0.2 MPaG to 3.0 MPaG, more preferably 0.3 MPaG to 2.4 MPaG. The pressure of the gas flowing into the third gas separation membrane unit 13 is usually preferably 0.01 MPaG to 0.7 MPaG, more preferably 0.02 MPaG to 0.5 MPaG.

[0073] The operating temperature in each gas separation membrane unit is preferably 0° C. or higher and 80° C. or lower, and more preferably 5° C. or higher and 60° C. or lower. The operating temperature here 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 Figure 2. In the second embodiment of the present invention, the first permeate gas line 15 connecting 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 Figure 1 is branched to provide a flow path 90. 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 branching from the first permeate gas line 15 to 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 Figure 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 with respect to various conditions such as the recovery rate and purity of gas B, the reflux ratio (C / F1), and the operating pressure. Furthermore, 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, a compressor 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 refluxed). In the embodiment of Fig. 2 in which a portion of the first permeate gas in the first permeate gas line 15 is refluxed to the raw material mixed gas supply line 16, it is advantageous in terms of reducing the compression power to provide the compressor 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 flow to pass through the separation membrane. Such pressure reducing means may include a known vacuum pump.

[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 refers to the temperature of the gas separation membrane in each gas separation membrane unit. - Temperature of raw material mixed gas flowing into the system: 40°C - Pressure (operating pressure) of gas flowing into first gas separation membrane unit 11: 1.0 MPa·G - Pressure (operating pressure) of gas flowing into third gas separation membrane unit 13: 0.19 MPa·G - Operating temperature of first gas separation membrane unit 11: 40°C - Operating temperature of second gas separation membrane unit 12: 40°C - Operating temperature of third gas separation membrane unit 13: 40°C - Number of modules in first gas separation membrane unit 11: 11 - Number of modules in third gas separation membrane unit 13: 11 - Gas separation selectivity of first gas separation membrane unit 11: Module A was used. - Gas separation selectivity of 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] 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 flow rate of gas flowing into each gas separation membrane unit, CO ２ The concentrations are shown in Table 1. Furthermore, the CH purity and recovery rate (the proportion of the CH component flow rate in the non-permeable gas of the second gas separation membrane unit 12 when the CH component flow rate in the raw mixed gas is taken as 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 shown in Table 2. Other than that, the procedure was the same as in Comparative Example 1.

[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 procedure was the same as Comparative Example 1.

[0085] 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 same procedures as in Comparative Example 1 were carried out.

[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 same procedure as in Comparative Example 1 was followed.

[0088]

[0089]

[0090] As shown in Table 2, in each of Examples 1 to 5 using the system of the present invention in which a portion of the third permeable gas is recycled to the first gas separation membrane unit, even if the flow rate of the raw material mixed gas decreases, the purity of the product gas is maintained without increasing or decreasing the number of modules. ４ In particular, in Example 4, the required membrane area was reduced while maintaining the same operating pressure as in Example 1, and high purity and high recovery rates were maintained. On the other hand, in Comparative Examples 1, 3, and 4, the flow rate of the raw material mixed gas was reduced, and the CH ４ In addition, in Comparative Example 2, in a three-stage system, 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, and the recovery rate of CH ４ The effect of improving the recovery rate was not sufficient.

[0091] According to the present invention, there is provided a three-stage gas separation system that can be operated even when the flow rate of the raw material mixed gas decreases, without changing the number of operating modules in the gas separation membrane unit, 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 comprising a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit, each gas separation membrane unit having at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, the non-permeate gas outlet of the first gas separation membrane unit is connected to the gas inlet of the second gas separation membrane unit by a first non-permeate gas line, the permeate gas outlet of the first gas separation membrane unit is connected to the gas inlet of the third gas separation membrane unit 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, and 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 and a position on the suction side of the compression means in the raw mixed gas supply line are connected by a second permeate gas recycle line; a non-permeate gas outlet of the third gas separation membrane unit and a position on the suction side of the compression means in the raw mixed gas supply line are connected 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; and a concentrated and enriched gas is extracted from the non-permeate gas outlet of the second gas separation membrane unit.

2. A gas separation system as described in claim 1, wherein the third permeate gas line has a flow path branch section and is provided with the following first flow path, second flow path, and third flow path, and has a flow rate adjustment 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 connecting the flow path branching portion and a position on the suction side of the compression means in the raw mixed gas supply line. Third flow path: A flow path that connects the flow path branching portion with the outside of the system.

3. The gas separation system according to claim 2, further comprising: a flow rate detection means for detecting the flow rate of the raw material mixed gas flowing into the raw material mixed gas supply line; and 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. The gas separation system according to claim 1 or 2, wherein the raw 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. The gas separation system according to claim 1 or 2, wherein the inflow rate of the raw mixed gas into the gas separation system fluctuates by 5% or more during operation.

8. A gas separation system as described in claim 1 or 2, 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, which comprises 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, the gas separation system comprising a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit, each gas separation membrane unit comprising at least a gas inlet, a permeate gas outlet, and a non-permeate gas outlet, the non-permeate gas outlet of the first gas separation membrane unit is connected to the gas inlet of the second gas separation membrane unit by a first non-permeate gas line, the permeate gas outlet of the first gas separation membrane unit is connected to the gas inlet of the third gas separation membrane unit 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, and 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 and a position on the suction side of the compression means in the raw mixed gas supply line are connected by a second permeate gas recycle line; a non-permeate gas outlet of the third gas separation membrane unit and a position on the suction side of the compression means in the raw mixed gas supply line are connected 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; and a concentrated and enriched gas is extracted from the non-permeate gas outlet of the second gas separation membrane unit.