Gas separation system and gas separation plant

Abstract

Provided are a gas separation system and a gas separation plant which, in a configuration including a plurality of separation membrane modules, make it possible to control, by an easy operation, a specific gas concentration of a gas grid so that such concentration is in a concentration range designated by a user even when said gas concentration varies across a concentration designated by the user. A gas separation system (1) remixes non-permeable gas and permeated gas discharged from a plurality of separation membrane modules, and controls the concentration of a specific gas to within a specified range. The gas separation system includes a control device that receives signals for the opening of a supply gas adjustment valve (10), a supply-side specific gas concentration meter (52), a first pressure gauge (55), and a second pressure gauge (56), and generates signals for adjusting opening degrees of a first flow rate adjustment valve (11), a second flow rate adjustment valve (12), a first pressure adjustment valve (13), and a second pressure adjustment valve (14).

Description

Gas Separation Systems and Plants

[0001] The present invention relates to a gas separation system and a gas separation plant.

[0002] One method for separating a specific gas from a mixed gas containing multiple components uses a separation membrane that selectively permeates the gas. Examples include molecular sieve membranes, such as ceramic membranes, which separate gases based on differences in molecular diameter, and polymer membranes that utilize differences in gas solubility in the membrane. These separation membranes also allow a certain amount of gases other than the specific gas to permeate. The side before permeation through the separation membrane is called the non-permeation side, and the side after permeation is called the permeation side. The amount of gas permeated through a separation membrane is proportional to the difference between the gas partial pressure on the non-permeation side and the gas partial pressure on the permeation side multiplied by the membrane area. Polymer materials are relatively easy to process, and modules in which hollow fibers or sheets are sealed in a container have been put to practical use. Patent Document 1 describes a method for controlling a specific gas to a required concentration using a separation membrane.

[0003] Patent No. 5111829

[0004] In Patent Document 1, the recovery rate is increased while maintaining a constant purity of the permeable gas by mixing the permeable gas with a bypassed feed gas. The flow rate of the feed gas supplied to the separation membrane module is cited as a factor that can change the purity of the permeable gas. For example, when the feed gas flow rate decreases, the partial pressure of the difficult-to-permeate gas increases near the outlet of the separation membrane module, increasing the amount of the difficult-to-permeate gas that permeates and decreasing the purity of the permeable gas. In this case, the flow rate of the low-purity feed gas that is bypassed is reduced to maintain the purity of the permeable gas at a constant level or higher.

[0005] When hydrogen is mixed as a specific gas into an existing gas grid and transported, the hydrogen concentration required by different users varies, and the hydrogen concentration in the gas grid fluctuates depending on the usage situation. For example, for a user requiring a hydrogen concentration of 20%, the configuration of Patent Document 1 can control the hydrogen concentration to 20% if the hydrogen concentration in the gas grid is less than 20%. However, if the hydrogen concentration in the gas grid exceeds 20%, the hydrogen concentration of the gas permeating the separation membrane will be 20% or higher, and the hydrogen concentration of the bypass gas will also exceed 20%, so the configuration of Patent Document 1 cannot control the hydrogen concentration in the permeation gas piping to 20%.

[0006] In the configuration of Patent Document 1, it is possible to control the hydrogen concentration in the non-permeate gas pipe to 20% and extract it as product gas, but it is necessary to connect the non-permeate gas pipe to the product gas pipe and attach valves to the non-permeate gas pipe and the permeate gas pipe. If the hydrogen concentration in the gas grid fluctuates around 20%, gas is extracted from the permeate gas pipe when it is below 20%, and gas is extracted from the non-permeate gas pipe when it exceeds 20%, requiring discontinuous switching of the valves on each pipe. When switching pipes, the hydrogen concentration of the gas supplied to the user may fluctuate significantly. For this reason, there has been a demand for easy operation even when the concentration of a specific gas in the gas grid fluctuates around a concentration specified by the user.

[0007] Furthermore, gas separation systems are configured with multiple separation membrane modules depending on the required product gas flow rate. Mixed gas is supplied to each separation membrane module by branching from the supply gas piping, but the distance from the branching point to the inlet of each separation membrane module varies depending on the arrangement of the separation membrane modules. When controlling the concentration of a specific gas in the product gas using the value of a gas concentration meter installed in the supply gas piping, the time from when the gas concentration meter detects a change in the specific gas concentration to when the gas with changed concentration reaches the separation membrane module inlet varies for each separation membrane module. If this time difference is not taken into account when performing control, it may not be possible to control the concentration of the specific gas in the product gas within the concentration range specified by the user.

[0008] An object of the present invention is to provide a gas separation system and a gas separation plant that, in a configuration equipped with multiple separation membrane modules, can be easily controlled to a concentration range specified by a user even when the concentration of a specific gas in a gas grid fluctuates around the concentration specified by the user.

[0009] In order to solve the above-mentioned problems, the gas separation system of the present invention is a gas separation system that remixes non-permeate gas and permeate gas discharged from a plurality of separation membrane modules and controls the concentration of a specific gas within a specified range, and includes: a supply gas pipe that supplies gas to the gas separation system; supply gas branch pipes that branch from the supply gas pipe and supply gas to each of the plurality of separation membrane modules; a non-permeate gas junction pipe that joins non-permeate gas pipes through which the non-permeate gas discharged from the plurality of separation membrane modules flows; a permeate gas junction pipe that joins permeate gas pipes through which the permeate gas discharged from the plurality of separation membrane modules flows; a non-permeate gas supply pipe through which the non-permeate gas flows from the non-permeate gas junction pipe and that has a first flow control valve; a permeate gas supply pipe through which the permeate gas flows from the permeate gas junction pipe and that has a second flow control valve; The system includes a product gas pipe connected to the permeate gas supply pipe and the permeate gas supply pipe, a non-permeate gas return pipe branching from the non-permeate gas junction pipe upstream of the first flow control valve and having a first pressure control valve, a permeate gas return pipe branching from the permeate gas junction pipe upstream of the second flow control valve and having a second pressure control valve, a supply gas control valve in the supply gas pipe for adjusting the flow rate or pressure of the supply gas, a supply-side specified gas concentration meter in the supply gas pipe for measuring the concentration of a specified gas, a first pressure gauge in the non-permeate gas junction pipe for measuring the pressure on the non-permeate side, and a second pressure gauge in the permeate gas junction pipe for measuring the pressure on the permeate side, and a control device that receives signals from the supply gas control valve aperture and the supply-side specified gas concentration meter, the first pressure gauge, and the second pressure gauge, and generates signals to adjust the apertures of the first flow control valve, the second flow control valve, the first pressure control valve, and the second pressure control valve. Other aspects of the present invention will be described in the embodiments described below.

[0010] According to the present invention, in a gas separation device and a gas separation plant equipped with multiple separation membrane modules, even when the concentration of a specific gas in a gas grid fluctuates around the concentration specified by the user, it is possible to control the concentration within the concentration range specified by the user with simple operations.

[0011] It is a schematic diagram showing a gas separation system according to a first embodiment. It is a schematic diagram showing a gas separation system according to a second embodiment. It is a schematic diagram showing a gas separation plant according to a third embodiment. It is a schematic diagram showing a gas separation plant according to a fourth embodiment.

[0012] This embodiment relates to a system for controlling the concentration of a specific gas required by a specific gas user at the usage location when a specific gas other than natural gas, such as natural gas containing methane as the main component, is mixed with natural gas and transported using an existing gas grid, and a system for controlling the concentration of a specific gas in gas production facilities, chemical plants, etc.

[0013] Hereinafter, an embodiment of a gas separation system and a gas separation plant according to the present invention will be described with reference to the drawings, assuming that the specific gas is hydrogen.

[0014] <First embodiment> Fig. 1 is a schematic diagram showing a gas separation system 1 according to a first embodiment. The first embodiment will be described using Fig. 1. In this embodiment, a gas separation system and a control method will be described that targets a supply gas in which hydrogen and natural gas are mixed and controls the hydrogen concentration of a product gas to, for example, 20% (vol%) even if the hydrogen concentration of the supply gas fluctuates beyond a concentration range specified by a user.

[0015] The gas separation system 1 according to the first embodiment includes, as an example, four separation membrane modules 2a, 2b, 2c, and 2d. The number of separation membrane modules 2 does not have to be four, as long as there are multiple (two or more) separation membrane modules, and is determined based on the gas processing volume required for the gas separation system 1. A mixed gas of hydrogen and natural gas is supplied as a supply gas to the gas separation system 1 from a supply gas pipe 20 branched from a main pipe (not shown). The supply gas pipe 20 is equipped with a supply gas flow rate control valve 10 (supply gas control valve 10) for controlling the flow rate of the supply gas, a second hydrogen concentration meter 52 (supply-side specified gas concentration meter 52), and a thermometer 58. Supply gas branch pipes 21a, 21b, 21c, and 21d branched from the supply gas pipe 20 are connected to the separation membrane modules 2a, 2b, 2c, and 2d, respectively. A gas that easily permeates selectively moves to the permeation side through the separation membrane 3 in the separation membrane module 2, separating the supply gas into a non-permeate gas and a permeate gas.

[0016] The non-permeate gas that does not move to the permeate side and remains on the non-permeate side is discharged from the separation membrane module 2 through the non-permeate gas pipes 22a, 22b, 22c, and 22d of the separation membrane modules 2a, 2b, 2c, and 2d. The permeate gas that has moved to the permeate side is discharged from the separation membrane module 2 through the permeate gas pipes 23a, 23b, 23c, and 23d. The non-permeate gas pipes 22a, 22b, 22c, and 22d and the permeate gas pipes 23a, 23b, 23c, and 23d of each separation membrane module merge into a non-permeate gas junction pipe 24 and a permeate gas junction pipe 25, respectively. A first pressure gauge 55 and a second pressure gauge 56 are attached to the non-permeate gas junction pipe 24 and the permeate gas junction pipe 25, respectively.

[0017] The non-permeate gas junction pipe 24 is connected to a non-permeate gas supply pipe 26 via a first flow control valve 11. A portion of the non-permeate gas is returned to the mother pipe through a non-permeate gas return pipe 27 that branches off before (upstream of) the first flow control valve 11. A first pressure control valve 13 is installed in the non-permeate gas return pipe 27 to adjust the pressure on the non-permeate side.

[0018] The permeate gas junction pipe 25 is connected to a permeate gas supply pipe 28 via a second flow control valve 12. A portion of the permeate gas is returned to the mother pipe through a permeate gas return pipe 29 that branches off before (upstream of) the second flow control valve 12. A second pressure control valve 14 that adjusts the pressure on the permeation side is installed in the permeate gas return pipe 29.

[0019] The non-permeable gas, the flow rate of which is adjusted by the first flow rate adjustment valve 11, and the permeable gas, the flow rate of which is adjusted by the second flow rate adjustment valve 12, are mixed in the product gas pipe 30 to adjust the hydrogen concentration of the product gas. The hydrogen concentration of the product gas is monitored by the first hydrogen concentration meter 51.

[0020] The gas separation system 1 is equipped with a control device 60 that controls the opening degrees of the first flow control valve 11, the second flow control valve 12, the first pressure control valve 13, and the second pressure control valve 14 based on the opening degree of the supply gas flow control valve 10 and the signals from the second hydrogen concentration meter 52, the thermometer 58, the first pressure gauge 55, and the second pressure gauge 56 so that the hydrogen concentration of the product gas falls within the concentration range specified by the user.

[0021] In this embodiment, a polymer membrane is used as the hydrogen separation membrane 3. However, ceramic or carbon-based hydrogen separation membranes may also be used. The amount of gas permeated is calculated by multiplying the permeation rate, which varies for each gas component, and the partial pressure difference for each gas component between the non-permeate side and the permeate side by the membrane area of ​​the separation membrane 3. Because the permeation rate of hydrogen through the separation membrane 3 is higher than that of natural gas (mainly methane), hydrogen permeates the separation membrane 3 more easily. Therefore, the hydrogen concentration on the permeate side is higher than that of the feed gas, while the hydrogen concentration of the non-permeate gas, which is the remaining gas after more hydrogen has permeated than methane, is lower than that of the feed gas. Generally, the separation performance of polymer membranes is temperature-dependent. In this embodiment, a thermometer 58 is installed in the supply gas pipe 20, and temperature information is sent to the control device 60 as part of the information used for control. It is known that underground pipelines experience little temperature change throughout the year. If the range of temperature fluctuations does not affect performance, the thermometer 58 may be omitted.

[0022] First, a method for maximizing the hydrogen recovery rate and controlling the hydrogen concentration of the product gas to 20% in a steady state when the hydrogen concentration of the mixed gas supplied to the gas separation system 1 varies between 5% and 40% will be described. Table 1 shows an example of the analysis results of the inventors on a mixed gas of hydrogen and natural gas (mainly methane). Note that the results in Table 1 are steady-state results when the gas separation system 1 has one separation membrane module 2 (a single separation membrane), but the control details described below refer to the control details for a case where there are four separation membrane modules 2 as shown in FIG. 1, with reference to the results in Table 1.

[0023]

[0024] The flow rates of the non-permeate gas and permeate gas are shown as a percentage of the feed gas flow rate. Under conditions of a non-permeate side pressure of 0.8 MPaG and a permeate side pressure of 0.1 MPaG, the hydrogen concentration of the feed gas is 5%, while the hydrogen concentration of the permeate gas is 21.9%, and the flow rate is 5% of the feed gas. The hydrogen concentration of the non-permeate gas is 4.1%. Because the first pressure regulating valve 13, which regulates the non-permeate side pressure, is connected to the non-permeate side of all separation membrane modules 2, the first pressure regulating valve 13 can regulate the non-permeate side pressure of all separation membrane modules 2 to the same pressure. Similarly, the permeate side pressure of all separation membrane modules 2 can be regulated to the same pressure by the second pressure regulating valve 14. To maximize the amount of product gas, the second pressure regulating valve 14 is completely closed, and the entire amount of permeate gas is sent from the second flow regulating valve 12 to the permeate gas supply pipe 28. To achieve a hydrogen concentration of 20% in the product gas, the aperture of the first flow control valve 11 is adjusted to send non-permeate gas at a flow rate of 0.6% of the feed gas to the non-permeate gas supply pipe 26. The remaining non-permeate gas is returned to the mother pipe through the non-permeate gas return pipe 27. If the pressure downstream of the first pressure control valve 13 is lower than that of the mother pipe, the non-permeate gas is pressurized by a compressor (not shown) and returned to the mother pipe. The aperture of the first pressure control valve 13 is adjusted to adjust the pressure on the non-permeate side. The pressure on the permeate side is adjusted by the aperture of the second flow control valve 12. Under these conditions, the hydrogen recovery rate, defined as the amount of hydrogen in the product gas relative to the amount of hydrogen in the feed gas, is 22.4%.

[0025] Control when the feed gas hydrogen concentration increases to 10% is described below. All conditions other than the hydrogen concentration of the feed gas remain unchanged. When the gas with an increased hydrogen concentration reaches the separation membrane module 2, the hydrogen partial pressure on the non-permeate side increases, increasing the amount of hydrogen permeating through the separation membrane 3 and raising the permeate-side hydrogen concentration to 43.1%. The permeate gas flow rate also increases to 6.8% of the feed gas. The second pressure control valve 14 remains closed, and the second flow control valve 12 is slightly opened to maintain the permeate-side pressure while sending the entire increased flow rate of permeate gas to the permeate gas supply pipe 28. To achieve a hydrogen concentration of 20% in the product gas, the aperture of the first flow control valve 11 is increased to send non-permeate gas at a flow rate of 12.7% of the feed gas to the non-permeate gas supply pipe 26. Since the increased amount of non-permeate gas sent to the non-permeate gas supply pipe 26 reduces the non-permeate gas flow rate returned to the mother pipe, the aperture of the first pressure control valve 13 is reduced to maintain the non-permeate-side pressure. Under these conditions, the hydrogen recovery rate is 39.1%.

[0026] Control will be described when the hydrogen concentration of the feed gas rises to 20%. Since the hydrogen concentration of the feed gas is 20%, if the entire amounts of non-permeate gas and permeate gas separated by the separation membrane module 2 are mixed, the hydrogen concentration of the product gas will be 20%. At this time, the first pressure regulating valve 13 on the non-permeate side and the second pressure regulating valve 14 on the permeate side are completely closed, and the gas flow rate returned to the mother pipe is set to zero. The pressures on the non-permeate side and the permeate side are adjusted by the first flow rate regulating valve 11 on the non-permeate side and the second flow rate regulating valve 12 on the permeate side. Since there is no gas returned to the mother pipe, the hydrogen recovery rate is 100%.

[0027] Control will be described when the hydrogen concentration of the feed gas increases to 40%. The hydrogen concentration of the permeate gas is 89.9%, and the flow rate is 37.2% of the feed gas. The hydrogen concentration of the non-permeate gas is 10.4%. When the hydrogen concentration of the feed gas is higher than that of the product gas, the hydrogen recovery rate can be maximized by using the entire amount of non-permeate gas and adjusting the flow rate of the mixed permeate gas. The first pressure regulating valve 13 on the non-permeate side is fully closed, sending the entire amount of non-permeate gas to the non-permeate gas supply pipe 26, and the pressure on the non-permeate side is adjusted with the first flow regulating valve 11. To achieve a hydrogen concentration of 20% in the product gas, the second flow regulating valve 12 on the permeate side is adjusted to send permeate gas at a flow rate of 8.5% of the feed gas to the permeate gas supply pipe 28. The remaining permeate gas is returned to the mother pipe through the permeate gas return pipe 29. If the pressure downstream of the second pressure regulating valve 14 is lower than that of the mother pipe, it is pressurized by a compressor (not shown) and returned to the mother pipe. The opening of the second pressure regulating valve 14 is adjusted to adjust the pressure on the permeate side. Under these conditions, the hydrogen recovery rate is 35.7%.

[0028] When the hydrogen concentration of the feed gas is 20%, the first and second pressure regulating valves 13, 14 are fully closed. When the hydrogen concentration of the feed gas increases beyond 20%, the first pressure regulating valve 13 remains fully closed, the second pressure regulating valve 14 is opened to increase its aperture, and the aperture of the second flow rate regulating valve 12 is reduced to adjust the pressure on the permeate side. As the hydrogen concentration of the feed gas increases, the amount of gas permeating the separation membrane 3 increases and the non-permeate gas flow rate decreases, so the aperture of the first flow rate regulating valve 11 is reduced to adjust the pressure on the non-permeate side. When the hydrogen concentration of the feed gas decreases toward 20%, the aperture of the second pressure regulating valve 14 is reduced and the aperture of the second flow rate regulating valve 12 is increased. As the amount of gas permeating the separation membrane 3 decreases and the non-permeate gas flow rate increases, the aperture of the first flow rate regulating valve 11 is increased.

[0029] When the feed gas hydrogen concentration further decreases and falls below 20%, the second pressure regulating valve 14 is fully closed. Because the amount of gas permeating the separation membrane 3 decreases due to the decrease in feed gas hydrogen concentration, the aperture of the second flow rate regulating valve 12 is gradually decreased when the feed gas hydrogen concentration is 20%. Because the hydrogen concentration and flow rate of the permeate gas decrease, the non-permeate gas flow rate for adjusting the hydrogen concentration of the product gas to 20% also decreases, and the aperture of the first flow rate regulating valve 11 is gradually decreased. Because the non-permeate gas flow rate discharged from the non-permeate side of the separation membrane module 2 increases while the non-permeate gas flow rate sent to the non-permeate gas supply pipe 26 decreases, the aperture of the first pressure regulating valve 13 is gradually increased to increase the flow rate of non-permeate gas returned to the mother pipe and maintain a constant non-permeate side pressure.

[0030] Note that pressure regulating valves 13, 14 and flow regulating valves 11, 12 have play between when the valve body is operated from fully closed and when fluid starts to flow, which can cause a time delay corresponding to the play in response to the control signal to open the valve, potentially resulting in a large fluctuation in the hydrogen concentration of the product gas. To eliminate this time delay, when the hydrogen concentration of the feed gas is 20% or higher, the first pressure regulating valve 13 can be opened slightly instead of fully closed to allow a small amount of non-permeable gas to flow, and when the hydrogen concentration of the feed gas is 20% or lower, the second pressure regulating valve 14 can be opened slightly instead of fully closed to allow a small amount of permeable gas to flow. In this case, the hydrogen recovery rate will be slightly reduced, but the time delay when the pressure regulating valves 13, 14 are operated from fully closed will be eliminated, and the fluctuation in the hydrogen concentration of the product gas can be suppressed.

[0031] In this way, even if the hydrogen concentration of the supply gas fluctuates around the set hydrogen concentration of the product gas, the hydrogen concentration of the product gas can be controlled by changing the opening of the pressure regulating valves 13, 14 and the flow rate regulating valves 11, 12.

[0032] Next, a control method further required for the gas separation system 1 composed of a plurality of separation membrane modules 2 in this embodiment will be described. The gas separation system 1 according to this embodiment includes a control device 60 that receives state quantities of the system and the opening state of the supply gas flow control valve 10 from the supply gas second hydrogen concentration meter 52, thermometer 58, non-permeate side first pressure gauge 55, and permeate side second pressure gauge 56, calculates a valve opening degree based on the state quantities to bring the hydrogen concentration of the product gas into a concentration range specified by a user, and generates a control signal. Furthermore, the control device 60 generates control signals and sends them to the flow control valves 11 and 12 and the pressure control valves 13 and 14. In the figure, signal lines used by the control device 60 to send and receive signals are indicated by dashed lines.

[0033] The flow rate of each gas permeating through the separation membrane can be calculated by multiplying the permeation rate of each gas by the partial pressure difference between the non-permeate side and permeate side of each gas through separation membrane 3 and the membrane area of ​​separation membrane 3. However, in the case of separation membrane module 2, the gas component ratio changes from the inlet to the outlet of separation membrane module 2 due to differences in gas permeation rate, and the partial pressure of each gas also changes, so the permeation rate must be determined by analysis that takes into account the changes in partial pressure of each gas within separation membrane module 2. The control device is provided with a database of the relationship between state quantities determined in advance by analysis and valve opening, and by referencing the database from the measured state quantities, it sends control signals to flow control valves 11 and 12 and pressure control valves 13 and 14 to control the hydrogen concentration of the product gas within a set range.

[0034] In this embodiment, non-permeate gas pipes 22a, 22b, 22c, 22d and permeate gas pipes 23a, 23b, 23c, 23d of each separation membrane module 2a, 2b, 2c, 2d are connected to a non-permeate gas junction pipe 24 and a permeate gas junction pipe 25, respectively. The pressure is controlled by flow control valves 11 and 12 installed in a non-permeate gas supply pipe 26 and a permeate gas supply pipe 28, and pressure control valves 13 and 14 installed in a non-permeate gas return pipe 27 and a permeate gas return pipe 29. The non-permeate side and permeate side of each separation membrane module 2 are joined and connected in parallel, so the non-permeate side pressure and permeate side pressure of each separation membrane module 2 are approximately the same. Although differences in piping length cause differences in pressure loss, the pressure loss is negligible, being 1 / 100 or less of the non-permeate side pressure and the permeate side pressure. Therefore, the non-permeate side and the permeate side of the multiple separation membrane modules 2 can be simultaneously controlled to the same pressure conditions by using four regulating valves, namely, pressure regulating valves 13 and 14 and flow rate regulating valves 11 and 12.

[0035] An example of a control method when the hydrogen concentration of the supply gas changes will be described. When a concentration-changing gas (a supply gas whose hydrogen concentration changes) flows from a main pipe into the supply gas piping 20, a time difference occurs before the concentration-changing gas reaches each separation membrane module 2 due to differences in the lengths of the supply gas piping 20a, 20b, 20c, and 20d and the supply gas branch piping 21a, 21b, 21c, and 21d to each separation membrane module 2. To detect and control changes in the hydrogen concentration of the supply gas, the distance (piping length) from the second hydrogen concentration meter 52 to each separation membrane module 2 is important. In the system shown in FIG. 1 , the concentration-changing gas first flows into the separation membrane module 2a, and then, with a time delay, flows into the downstream separation membrane modules 2b, 2c, and 2d. When the hydrogen concentration of the supply gas increases, the hydrogen partial pressure on the non-permeation side of the separation membrane module 2 increases, increasing the amount of hydrogen permeation and increasing the flow rate and hydrogen concentration on the permeation side. Since the non-permeate side pressure decreases as the permeation rate increases, the aperture of the first pressure regulating valve 13 is reduced to maintain the non-permeate side pressure constant. Meanwhile, since the pressure on the permeate side rises due to the increase in the permeate gas flow rate, the aperture of the second pressure regulating valve 14 is increased to maintain the permeate side pressure constant. Since the hydrogen concentration on the permeate side is increasing, the aperture of the first flow regulating valve 11 is increased to increase the flow rate of the non-permeate gas flowing into the non-permeate gas supply pipe 26, or the aperture of the second flow regulating valve 12 is reduced to decrease the flow rate of the permeate gas flowing into the permeate gas supply pipe 28. If the hydrogen concentration of the supply gas decreases, the regulating valves are controlled in the reverse manner to that described above.

[0036] As described above, when the gas separation system 1 includes multiple separation membrane modules 2, differences in piping length result in differences in the time it takes for the concentration-changing gas to reach each separation membrane module 2. Therefore, when the concentration-changing gas reaches the most upstream separation membrane module 2a and the hydrogen concentration on the permeate side begins to increase, the hydrogen concentration on the permeate side remains unchanged in the downstream separation membrane modules 2b, 2c, and 2d. When the permeated gases from each separation membrane module 2 at this point merge in the permeate gas junction piping 25, the rate of increase in the hydrogen concentration in the permeate gas junction piping 25 is moderated by the permeated gases in the other separation membrane modules whose hydrogen concentrations have not increased, relative to the rate of increase in the permeate gas concentration in the separation membrane module 2a. Therefore, the rate of adjustment of the valve opening of the flow control valves 11 and 12 and the pressure control valves 13 and 14 described above becomes slower as the number of separation membrane modules 2 increases.

[0037] In this embodiment, the hydrogen concentration of the product gas is adjusted by mixing the non-permeate gas and the permeate gas, so the distance from each separation membrane module 2 to the product gas pipe 30 also affects the control of the hydrogen concentration of the product gas. A change in the hydrogen concentration of the supply gas changes the flow rates and hydrogen concentrations of the non-permeate gas and the permeate gas. The non-permeate gas, which has changed in flow rate and hydrogen concentration from the separation membrane module 2a into which the concentration-change gas first flows, reaches the product gas pipe 30 through the length of the non-permeate gas pipe 22a, the non-permeate gas junction pipe 24, and the non-permeate gas supply pipe 26. Furthermore, because the flow rate changes, the time it takes for the supply gas to reach the product gas pipe 30 varies depending on the hydrogen concentration. For this reason, a control logic that takes into account the time difference determined by the distance to the product gas pipe 30 on the non-permeate and permeate sides of each separation membrane module 2 and the gas flow rate is incorporated into the control device's database.

[0038] Control taking into account the difference in piping length from the second hydrogen concentration meter 52 to the separation membrane module 2 and the difference in piping length from the separation membrane module 2 to the non-permeate and permeate side product gas piping 30 between the multiple separation membrane modules 2 in the gas separation system 1 according to this embodiment will be described below in the case where the hydrogen concentration of the supply gas increases. When gas with an increased hydrogen concentration in the mother pipe (hereinafter referred to as "concentration-increased gas") flows into the supply gas piping 20, the change in concentration is detected by the second hydrogen concentration meter 52 installed in the supply gas piping 20. When the increase in concentration is detected by the second hydrogen concentration meter 52, the concentration-increased gas has not yet reached any of the separation membrane modules 2, so the apertures of the regulating valves 11, 12, 13, and 14 are not changed.

[0039] When the concentration-increasing gas reaches the most upstream separation membrane module 2a, the amount of hydrogen permeated increases, the non-permeate gas flow rate in the separation membrane module 2a decreases, and the permeate gas flow rate increases, increasing the hydrogen concentration. Because the non-permeate side pressure decreases, the aperture of the first pressure regulating valve 13 is narrowed to reduce the return gas, thereby maintaining the non-permeate side pressure. Because the permeate side pressure increases, the aperture of the second pressure regulating valve 14 is widened to increase the return gas, thereby maintaining the permeate side pressure. At this point, the hydrogen concentrations in the non-permeate gas supply pipe 26 and the permeate gas supply pipe 28 have not changed, so the apertures of the flow control valves 11 and 12 are not changed. As time passes, when the concentration-increasing gas reaches the next separation membrane module 2b, the non-permeate side pressure decreases, just as in the separation membrane module 2a. Therefore, the aperture of the first pressure regulating valve 13 is further narrowed to reduce the return gas, thereby maintaining the non-permeate side pressure. Because the permeate side pressure increases, the aperture of the second pressure regulating valve 14 is further widened to increase the return gas, thereby maintaining the permeate side pressure.

[0040] In this way, the opening degrees of the pressure regulating valves 13 and 14 are controlled each time the concentration-increasing gas reaches the separation membrane module 2 in sequence. The timing of the adjustment can be calculated from the distance from the second hydrogen concentration meter 52 to each separation membrane module 2 and the flow rate in the supply gas pipe. For example, if the distance from the second hydrogen concentration meter 52 to the branching point of the supply gas pipe 20a is 10 m and the gas flow rate in the pipe is 2 m / s, it takes 5 seconds for the concentration-increasing gas to reach the branching point after the second hydrogen concentration meter 52 detects the increase in concentration. If the length of the supply gas branching pipe 21a from the branching point to the separation membrane module 2a is 5 m and the concentration-increasing gas is evenly distributed to the four separation membrane modules at the branching point, resulting in a gas flow rate of 0.5 m / s in the pipe to the separation membrane module 2a, it takes 10 seconds for the concentration-increasing gas to reach the separation membrane module 2a from the branching point. In this case, control of the pressure regulating valves 13 and 14 begins 15 seconds after the second hydrogen concentration meter 52 detects the increase in hydrogen concentration.

[0041] Next, the length of the supply gas pipe 20b from the branch of the supply gas pipe 20a to the separation membrane module 2b is 6 m, and the length of the supply gas branch pipe 21b from the branch to the separation membrane module 2b is 5 m. Because the gas flow velocity in the supply gas pipe 20b is 1.5 m / s, it takes 4 seconds for the concentration-increasing gas to reach the inlet of the supply gas branch pipe 21b from the end of the supply gas pipe 20a. Because the flow velocity in the supply gas branch pipe 21b is 0.5 m / s, it takes 10 seconds to pass through the supply gas branch pipe 21b. Therefore, 19 seconds after the second hydrogen concentration meter 52 detects the increase in hydrogen concentration, the concentration-increasing gas reaches the separation membrane module 2b, and control by the pressure regulating valves 13 and 14 begins. In this way, by storing a control method that takes into account the distance from the second hydrogen concentration meter 52 to each separation membrane module 2 in the database of the control device 60, the pressure on the non-permeate side and the permeate side of each separation membrane module 2 can be maintained.

[0042] Next, control downstream of the separation membrane module 2 will be described. When the concentration-increasing gas begins to be separated in the separation membrane module 2, the hydrogen concentration and flow rate of the non-permeate gas and permeate gas change. Even if the flow rates of the non-permeate gas and permeate gas change, the return gas flow rate is adjusted by the aperture of the pressure regulating valves 13 and 14, so that the flow rate sent to the product gas pipe 30 does not change. Therefore, the aperture of the flow regulating valves 11 and 12 may be maintained until the concentration-increased non-permeate gas and permeate gas reach the product gas pipe 30. For example, if the distance from the inlet of the non-permeate gas pipe 22a of the separation membrane module 2a to the product gas pipe 30 is 10 m and the flow velocity in the pipe is 1 m / s, the aperture control of the first flow regulating valve 11 begins 10 seconds after the concentration-increasing gas flows into the separation membrane module 2a. If the distance to the product gas inlet on the permeate side is 10 m and the flow velocity in the pipe is 1 m / s, the second flow control valve 12 should begin controlling its aperture 10 seconds after the concentration-increasing gas flows into the separation membrane module 2a. However, if the distance to the product gas pipe 30 on the permeate side is 5 m and the flow velocity in the pipe is 1 m / s, the permeate gas with a different hydrogen concentration will arrive at the product gas pipe 30 5 seconds later, increasing the hydrogen concentration of the product gas. Therefore, the non-permeate gas flow control valve 11 is controlled to increase its aperture at 5 seconds to keep the hydrogen concentration of the product gas within the concentration range specified by the user. In this way, by storing a control method that takes into account the distance from each separation membrane module 2 to the product gas pipe 30 in the control device's database, the hydrogen concentration of the product gas can be maintained within a constant range. As the hydrogen concentrations and flow rates of the non-permeate gas and permeate gas in the separation membrane module 2 change sequentially, the hydrogen concentrations in the non-permeate gas supply pipe 26 and the permeate gas supply pipe 28 also change sequentially with a time delay. This change can be calculated from the distance from each separation membrane module 2 to the product gas pipe 30 and the flow rate within the pipe, and by entering the timing for controlling the flow control valves 11, 12 and the pressure control valves 13, 14 into a database in advance, the hydrogen concentration of the product gas can be controlled within a certain range.

[0043] The above description has focused on the case where the hydrogen concentration of the supply gas increases. However, if the hydrogen concentration decreases, the hydrogen concentration of the product gas can be controlled within a certain range by operating the control valves in the opposite direction. Furthermore, the operational principle has been described assuming that the supply gas flow rate to each separation membrane module 2 is equal. However, as the hydrogen concentration increases, the permeate gas flow rate increases, reducing the flow resistance of the separation membrane modules 2. This results in flow rate differences between the separation membrane modules 2 during periods when there are differences in the hydrogen concentration of the supply gas. Even in this case, the hydrogen concentration of the product gas can be controlled within a certain range by storing the difference in separation performance of the separation membrane modules 2 due to flow rate differences in the database of the control device 60. The above-described control method is merely an example. With four control valves, the hydrogen concentration of the product gas can be controlled within a user-specified concentration range by operating the other control valves. According to this embodiment, the hydrogen concentration of the product gas can be controlled within a certain range by controlling the four control valves, even if the hydrogen concentration of the supply gas fluctuates above or below the specified hydrogen concentration of the product gas.

[0044] Second Embodiment A second embodiment of the present invention will be described with reference to FIG. 2 . In the second embodiment, in addition to the configuration of the first embodiment, an orifice is installed as a flow rate limiting mechanism 31 in the supply gas branch pipe 21 of each separation membrane module 2, and a third pressure gauge 57 is installed in the supply gas pipe 20a. In the first embodiment, flow rate distribution (differences in flow rate) may occur between the separation membrane modules 2 due to changes in the hydrogen concentration of the supply gas, and control is performed taking this flow rate distribution into consideration. In the second embodiment, flow rate distribution is prevented between the separation membrane modules 2 by choking the orifice 31 with a narrowed pore size. When choked, the flow rate passing through the orifice 31 is not affected by pressure changes downstream of the orifice 31, and is determined by the pressure upstream of the orifice 31. In FIG. 2 , the upstream side of the orifice 31 installed in the supply gas branch pipe 21 of each separation membrane module 2 is connected by the supply gas pipe 20. Although pressure loss occurs due to the flow of the supply gas, it is negligible, being less than one-hundredth of the pressure upstream of the orifice 31. Therefore, by installing orifices 31 of the same shape, the supply gas flow rate to each separation membrane module 2 can be made the same. Because the supply gas flow rate is determined by the pressure upstream of the orifice 31, a third pressure gauge 57 is installed in the supply gas pipe 20a and the measured pressure value is sent to the control device 60. The control device 60 refers to a database based on the received pressure value and adjusts the opening of the flow control valves 11 and 12 and the pressure control valves 13 and 14. In the first embodiment, control was required that took into account the flow rate distribution of the supply gas to each separation membrane module 2 due to changes in the concentration of the supply gas. However, in the second embodiment, the orifice 31 uniforms the supply gas flow rate between each separation membrane module 2, eliminating the supply gas flow rate distribution from the items required for control and simplifying the control method. The flow rate limiting mechanism 31 does not have to be an orifice, and may be a valve that can adjust the opening inside the pipe to cause choking.

[0045] <Third embodiment> A third embodiment of the present invention will be described using Figure 3. The third embodiment shown in Figure 3 is a gas separation plant 100 in which a plurality of gas separation systems 1 having the configuration of Figure 1 or Figure 2 are installed in parallel. In the gas separation system 1, the more separation membrane modules 2 are added, the more information such as time lag required for control becomes, and therefore the control becomes more complex. In this embodiment, the gas separation system 1 is constructed with a number of separation membrane modules that does not make the control excessively complicated. Although three gas separation systems 1 are arranged in parallel in Figure 3, there are no particular limitations as long as there are multiple (two or more), and the number of gas separation systems 1 to be installed is determined depending on the required product gas flow rate.

[0046] In this embodiment, a third hydrogen concentration meter 151 is installed in the plant product gas piping 130 of the gas separation plant 100 to monitor the hydrogen concentration of the product gas. Although a first hydrogen concentration meter 51 is installed in the product gas piping 30 shown in Figures 1 and 2, it may be left in place to monitor the hydrogen concentration of the product gas of each gas separation system 1, or the first hydrogen concentration meter 51 of each gas separation system 1 may be eliminated since it can be replaced by the third hydrogen concentration meter 151.

[0047] When gas with a changed hydrogen concentration flows from a parent pipe into gas separation plant 100, the gas is distributed at the branching portion of plant gas supply piping 120 and flows into each gas separation system 1a, 1b, 1c. The timing at which the concentration-changed gas flows into each gas separation system 1 differs depending on the length of plant supply gas piping 120a, 120b, 120c and plant supply gas branch piping 121a, 121b, 121c, but as described in the first and second embodiments, each gas separation system 1 controls the hydrogen concentration of the product gas to be within a specified range. Therefore, because the hydrogen concentration of the product gas coming out of each gas separation system 1 is controlled within a specified concentration range, no control using an adjustment valve or the like is required between the product gas piping 30 of each gas separation system 1 and the plant product gas piping 130. Furthermore, the non-permeate side return gas and permeate side return gas discharged from each gas separation system 1 are returned to the parent pipe by connecting the non-permeate side return pipe 27 and permeate side return pipe 29 of each gas separation system 1 to the plant non-permeate side return pipe 127 and plant permeate side return pipe 129 of the gas separation plant 100. In this way, by configuring the gas separation plant 100 by arranging the required number of gas separation systems 1, each configured with a controllable number of separation membrane modules, it is possible to stably supply product gas at the flow rate and hydrogen concentration required by the user.

[0048] <Fourth embodiment> A fourth embodiment of the present invention will be described using Figure 4. The fourth embodiment differs from the third embodiment in that a fourth hydrogen concentration meter 152 (plant specified gas concentration meter 152) is installed in the plant supply gas piping 120, the second hydrogen concentration meter 52 (supply-side specified gas concentration meter 52) installed in the supply gas piping 20 of each gas separation system is eliminated, and a hydrogen concentration transmitting device 153 (signal transmitting device 153) is installed. The hydrogen concentration transmitting device 153 is a device that transmits the measurement value of the fourth hydrogen concentration meter 152 to the control device 60 of each gas separation system 1.

[0049] When a change in the hydrogen concentration of the supply gas is detected by a fourth hydrogen concentration meter 152 installed in the plant supply gas piping 120, information on the hydrogen concentration is sent from a hydrogen concentration transmitting device 153 to the control device 60 of each gas separation system 1. Because the distance from the fourth hydrogen concentration meter 152 to each gas separation system 1 is known, the time when the concentration-changed gas will reach each gas separation system 1 can be determined from the gas flow rate flowing into each gas separation system 1. The control device 60 of each gas separation system 1 determines the time to start controlling the pressure regulating valves 13, 14 and the flow rate regulating valves 11, 12 by referring to the database of the control device 60 and using the calculated arrival time of the concentration-changed gas.

[0050] For example, the length of the plant supply gas pipe 120a from the fourth hydrogen concentration meter 152 to the first branch of the plant supply gas pipe 120 is 3 m, the gas flow rate in the pipe is 0.6 m / s, the length of the plant supply gas branch pipe 121a from the first branch to the gas separation system 1a is 2 m, the gas flow rate in the pipe is 0.2 m / s, the length of the plant supply gas pipe 120b from the first branch to the second branch is 4 m, the gas flow rate in the pipe is 0.4 m / s, the length of the plant supply gas branch pipe 121b from the second branch to the gas separation system 1b is 2 m, the gas flow rate in the pipe is 0.2 m / s, the length of the plant supply gas pipe 120c from the second branch to the third branch is 4 m, the gas flow rate in the pipe is 0.2 m / s, and the length of the plant supply gas branch pipe 121c from the third branch to the gas separation system 1c is 2 m, the gas flow rate in the pipe is 0.2 m / s. In this case, the concentration-change gas reaches gas separation system 1a in 15 seconds, gas separation system 1b in 25 seconds, and gas separation system 1c in 45 seconds after being detected by fourth hydrogen concentration meter 152. Control device 60 of each gas separation system 1 detects changes in the hydrogen concentration signal transmitted from hydrogen concentration transmitter 153 and, taking into account the time it takes for the concentration-change gas to reach gas separation system 1 as described above, adjusts the opening of pressure regulating valves 13, 14 and flow rate regulating valves 11, 12 in each gas separation system 1 to control the hydrogen concentration of the product gas within a specified range. In the fourth embodiment, second hydrogen concentration meter 52 can be eliminated from each gas separation system 1, thereby achieving lower costs than the third embodiment.

[0051] REFERENCE SIGNS LIST 1 Gas separation system (hydrogen separation system) 2 Separation membrane module 3 Separation membrane 10 Supply gas flow rate control valve (supply gas control valve) 11 First flow rate control valve 12 Second flow rate control valve 13 First pressure control valve 14 Second pressure control valve 20 Supply gas piping 21 Supply gas branching piping 22 Non-permeate gas piping 23 Permeate gas piping 24 Non-permeate gas junction piping 25 Permeate gas junction piping 26 Non-permeate gas supply piping 27 Non-permeate gas return piping 28 Permeate gas supply piping 29 Permeate gas return piping 30 Product gas piping 31 Flow rate limiting mechanism (orifice) 51 First hydrogen concentration meter (specific gas concentration meter) 52 Second hydrogen concentration meter (supply-side specific gas concentration meter) 55 First pressure gauge 56 Second pressure gauge 57 Third pressure gauge 58 Thermometer 60 Control device 100 Gas separation plant 120 Plant supply gas piping 121 Plant supply gas branch piping 127 Plant non-permeate gas return piping 129 Plant permeate gas return piping 130 Plant product gas piping 151 Third hydrogen concentration meter (specific gas concentration meter) 152 Fourth hydrogen concentration meter (plant specific gas concentration meter) 153 Hydrogen concentration transmitter

Claims

1. A gas separation system that remixes non-permeate gas and permeate gas discharged from a plurality of separation membrane modules and controls the concentration of a specific gas within a specified range, comprising: a supply gas pipe that supplies gas to the gas separation system; supply gas branch pipes that branch off from the supply gas pipe and supply gas to each of the plurality of separation membrane modules; a non-permeate gas junction pipe that joins together the non-permeate gas pipes through which the non-permeate gas discharged from the plurality of separation membrane modules flows; a permeate gas junction pipe that joins together the permeate gas pipes through which the permeate gas discharged from the plurality of separation membrane modules flows; a non-permeate gas supply pipe through which the non-permeate gas flows from the non-permeate gas junction pipe and that has a first flow control valve; a permeate gas supply pipe through which the permeate gas flows from the permeate gas junction pipe and that has a second flow control valve; and product gas pipes connected from the non-permeate gas supply pipe and the permeate gas supply pipe downstream of the first flow control valve and the second flow control valve. a non-permeate gas return pipe branching off from the non-permeate gas junction pipe upstream of the first flow rate control valve and having a first pressure control valve; a permeate gas return pipe branching off from the permeate gas junction pipe upstream of the second flow rate control valve and having a second pressure control valve; a supply gas control valve in the supply gas pipe for adjusting the flow rate or pressure of the supply gas; a supply-side specified gas concentration meter in the supply gas pipe for measuring the concentration of a specified gas; a first pressure gauge in the non-permeate gas junction pipe for measuring the pressure on the non-permeate side; and a second pressure gauge in the permeate gas junction pipe for measuring the pressure on the permeate side; and a control device that receives signals from the aperture of the supply gas control valve and the supply-side specified gas concentration meter, the first pressure gauge, and the second pressure gauge, and generates signals to adjust the apertures of the first flow rate control valve, the second flow rate control valve, the first pressure control valve, and the second pressure control valve.

2. A gas separation system as described in claim 1, characterized in that the control device is equipped with a database that calculates the opening degrees of the first flow rate control valve, the second flow rate control valve, the first pressure control valve and the second pressure control valve based on information on the opening degree of the supply gas control valve, the measurement values ​​of the supply-side specified gas concentration meter and the first pressure gauge, the measurement values ​​of the second pressure gauge and the length of piping from the supply-side specified gas concentration meter to the plurality of separation membrane modules.

3. A gas separation system according to claim 1, characterized in that the supply gas branch pipe is provided with a flow rate restricting mechanism, and the supply gas pipe is provided with a third pressure gauge.

4. A gas separation system as described in claim 3, wherein the control device further includes a database that adds information from the third pressure gauge and calculates the opening degrees of the first flow control valve, the second flow control valve, the first pressure control valve, and the second pressure control valve.

5. A gas separation plant equipped with a plurality of gas separation systems, comprising: a plant supply gas piping for supplying gas to said gas separation plant; plant supply gas branch piping branching from said plant supply gas piping to supply gas to each of said plurality of gas separation systems; a plant non-permeate gas junction piping where plant non-permeate gas piping through which return non-permeate gas discharged from said plurality of gas separation systems flows joins; a plant permeate gas junction piping where plant permeate gas piping through which return permeate gas discharged from said plurality of gas separation systems flows joins; and a plant product gas junction piping where product gas piping through which product gas flows joins; and wherein said plurality of gas separation systems are gas separation systems defined in any one of claims 1 to 4.

6. A gas separation plant as claimed in claim 5, further comprising a plant specified gas concentration meter for measuring the concentration of the specified gas in the plant supply gas piping instead of the supply-side specified gas concentration meter of the gas separation system, and a signal transmitting device for sending a signal from the plant specified gas concentration meter to the control device of the gas separation system, and the control device of the gas separation system further comprising a database for adding information on the specific gas concentration from the signal transmitting device and the length of the piping from the plant specified gas concentration meter to the plurality of separation membrane modules, and for calculating the opening degrees of the first flow control valve, the second flow control valve, the first pressure control valve and the second pressure control valve.

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