Method for stopping gas production device, and method for producing oxygen gas and hydrogen gas

Abstract

This method for stopping a gas production device is a method for electrolyzing an electrolytic alkali solution under pressurized conditions in which the electrolytic solutions that have flowed out of an anode chamber and a cathode chamber are circulated so as to flow again into the anode chamber and the cathode chamber, and comprises stopping the operation of the device through a procedure including given steps. This method makes it possible to prevent a gas composition from reaching an explosion limit.

Description

Method for shutting down gas production equipment and method for producing oxygen gas and hydrogen gas

[0001] The present invention relates to a method for shutting down a gas production apparatus and a method for producing oxygen gas and hydrogen gas.

[0002] Alkaline water electrolysis is known as a method for producing hydrogen gas and oxygen gas. In alkaline water electrolysis, water is electrolyzed using a basic aqueous solution (alkaline water) containing an alkali metal hydroxide (e.g., NaOH, KOH, etc.) dissolved therein as an electrolyte, thereby generating hydrogen gas from the cathode and oxygen gas from the anode. Typically, an electrolytic cell for alkaline water electrolysis includes an anode chamber and a cathode chamber separated by an ion-permeable diaphragm, and electrolysis is performed while circulating the electrolyte through the anode chamber and the cathode chamber, respectively. The electrolyte recovered from each electrode chamber is temporarily collected and stored in a circulation tank, and the electrolyte stored in the circulation tank is then supplied again to each electrode chamber.

[0003] In the alkaline water electrolysis process, the anodic reaction is 2OH － → (1 / 2) O ２ ↑+H ２ O + 2e － ...(1) and the cathode reaction is 2H ２ O + 2e － →H ２ ↑+2OH － ...(2) Therefore, in the alkaline water electrolysis process, although water is consumed overall, water is consumed in the cathode reaction while water is produced in the anode reaction, so as the electrolysis reaction progresses, a difference in liquid level occurs between the anode side circulation tank and the cathode side circulation tank. － ions are consumed, and in the cathode reaction, OH － Ions are generated, and they move through the diaphragm to maintain charge neutrality between the anode and cathode chambers. － Not all ions migrate from the cathode chamber to the anode chamber. That is, usually, OH produced in the cathode reaction －Only a portion of the ions passes through the diaphragm and moves from the cathode chamber to the anode chamber, and the excess negative charge in the cathode chamber corresponding to the remainder is converted into cations (NaOH if the solute in alkaline water is NaOH). ＋ If the solute in alkaline water is KOH, then ＋ This is resolved by the movement of ions (electrolytes) from the anode chamber to the cathode chamber through the diaphragm. As a result, a difference in the concentration of the electrolyte occurs between the anode side circulation tank and the cathode side circulation tank as the electrolysis reaction progresses.

[0004] As a method for solving the above problems, Patent Document 1 proposes a method in which an electrolyte exchange device is provided between the anode-side circulation tank and the cathode-side circulation tank, and a portion of the electrolyte in one tank is transferred to the other tank, thereby eliminating the difference in electrolyte concentration and the difference in liquid level between the anode-side circulation tank and the cathode-side circulation tank.

[0005] WO2020 / 022190 publication

[0006] However, if oxygen gas is dissolved in the electrolyte on the anode side and this electrolyte is moved to the cathode side, oxygen gas will be dissolved in the electrolyte on the cathode side together with hydrogen gas, which is the main gas.The same is also true in reverse: if hydrogen gas is dissolved in the electrolyte on the cathode side and this electrolyte is moved to the anode side, hydrogen gas will be dissolved in the electrolyte on the anode side together with oxygen gas, which is the main gas.

[0007] According to Henry's law, the amount of gas dissolved in the electrolyte increases with increasing pressure. Therefore, the amount of gas dissolved in the electrolyte increases, particularly in high-pressure alkaline water electrolysis devices that perform electrolysis under pressurized conditions. When a high-pressure alkaline water electrolysis device is operated by controlling the pressure and temperature at constant values, the amount of gas dissolved in the electrolyte is maintained at a constant level. However, for example, if the power source of the water electrolysis device is solar power, hydrogen production is stopped in the evening because there is no power generation at night, and the device is restarted the next morning, repeating this cycle. In this case, the following changes (A) and (B) are expected to occur between the time the water electrolysis device is stopped in the evening and the next time it is restarted the next morning, resulting in a gradual decrease in pressure within the water electrolysis device. (A) Heat generation due to the electrolysis reaction ceases, and / or the temperature of the electrolyte decreases due to a drop in the outside air temperature. (B) A slight gas leak occurs through the pressure control valve, causing a gradual decrease in pressure within the water electrolysis device.

[0008] When the pressure inside the water electrolysis device drops, the gas dissolved in the electrolyte gradually releases into the gas layer. Hydrogen gas and oxygen gas dissolved in the electrolyte exist in the electrolyte at different solubilities depending on the temperature and pressure conditions. However, the solubilities of hydrogen gas and oxygen gas in the electrolyte at high pressures are approximately equal. Therefore, when the pressure inside the water electrolysis device drops, it is assumed that approximately equal amounts of hydrogen gas and oxygen gas are released into the gas layer in each circulation tank, which may increase the flammability of the gas layer in each circulation tank and enter the explosive gas range. Furthermore, mixing of hydrogen gas and oxygen gas in the gas layer in each circulation tank during shutdown of the water electrolysis device may result in a decrease in the purity of the produced gas when the next operation is started.

[0009] Furthermore, the above-mentioned problem of gas coexistence in the gas phase can occur not only when the electrolytes on the anode side and the cathode side are exchanged mutually, but also when only one side is exchanged (anode side → cathode side only, or cathode side → anode side only). Alternatively, instead of exchanging the electrolytes on the anode side and cathode side, both electrolytes can be mixed in a mixing tank and the mixed electrolyte can be returned to the anode chamber and cathode chamber again. However, even in such a case, the above-mentioned problem of gas coexistence in the gas phase can occur.

[0010] Furthermore, even in a configuration that does not fall under the above configuration and in which the electrolyte is circulated independently between the anode side and the cathode side, the electrolytic cell originally separates the anode chamber and the cathode chamber with a diaphragm, and therefore some movement of the electrolyte may occur between the anode chamber and the cathode chamber. As described above, oxygen gas is dissolved in the electrolyte on the anode side, and hydrogen gas is dissolved in the electrolyte on the cathode side, so even in such a configuration in which the electrolyte is circulated independently, the problem of gas coexistence in the gas phase described above may occur.

[0011] An object of the present invention is to provide a method for shutting down a gas production apparatus that prevents the problem of gas coexistence in the gas phase region of each circulation tank, etc., and prevents the gas composition from reaching an explosion limit, even when the water electrolysis apparatus is temporarily shut down in a case where an electrolytic cell provided with a diaphragm is used while electrolytic solution exchange is performed between the anode side and the cathode side, or in a case where alkaline water electrolysis is performed under pressurized conditions even in a case where electrolytic solution exchange is not performed.

[0012] The present invention is as follows: [1] A method for producing oxygen gas and hydrogen gas by electrolyzing an alkaline aqueous solution electrolyte under pressurized conditions using a gas production apparatus having an electrolytic cell equipped with an anode chamber that accommodates an anode and generates oxygen gas, a cathode chamber that accommodates a cathode and generates hydrogen gas, and an ion-permeable diaphragm that separates the anode chamber and the cathode chamber, wherein the electrolyte that flows out of the anode chamber and the cathode chamber is circulated so as to flow back into the anode chamber and the cathode chamber, and the operation of the gas production apparatus is stopped by a procedure comprising the following steps (a) to (c): (a) a step of reducing the pressure inside the apparatus, (b) a step of continuing gas production in a state where the pressure is reduced, and (c) a step of stopping the gas production apparatus.

[0013] [2] The method for shutting down a gas manufacturing apparatus according to [1], wherein an anode-side electrolyte circulation system is formed so that the electrolytic solution flowing out of the anode chamber flows back into the anode chamber during operation of the gas manufacturing apparatus, and a cathode-side electrolyte circulation system is formed so that the electrolytic solution flowing out of the cathode chamber flows back into the cathode chamber, and a portion of the electrolytic solution flowing out of the anode chamber is transferred to the cathode-side electrolyte circulation system and / or a portion of the electrolytic solution flowing out of the cathode chamber is transferred to the anode-side electrolyte circulation system, and the method further comprises a step of stopping the transfer in the step (a).

[0014] [3] The method for shutting down a gas production apparatus according to [1] or [2], wherein the pressurized conditions are conditions in which the pressure inside the anode chamber, the cathode chamber, or both of them is maintained at a pressure higher than atmospheric pressure by 20 kPa or more.

[0015] [4] The method for shutting down a gas production apparatus according to any one of [1] to [3], wherein the anode-side circulation system comprises an anode-side circulation tank that stores the electrolytic solution circulating through the anode-side circulation system, and the cathode-side circulation system comprises a cathode-side circulation tank that stores the electrolytic solution circulating through the cathode-side circulation system, and the step (b) further comprises a step of introducing a part of the oxygen gas generated on the anode side into the electrolytic solution in the anode-side circulation tank to release the dissolved gas in the electrolytic solution, and / or a step of introducing a part of the hydrogen gas generated on the cathode side into the electrolytic solution in the cathode-side circulation tank to release the dissolved gas in the electrolytic solution.

[0016] [5] A method for producing oxygen gas and hydrogen gas, comprising a step of stopping the operation of a gas production apparatus by the method for stopping a gas production apparatus according to any one of [1] to [4].

[0017] According to the present invention, in a method for producing hydrogen gas and oxygen gas under pressurized conditions using an electrolytic cell equipped with a diaphragm while exchanging the electrolyte in the anode side with the electrolyte in the cathode side, or even without exchanging the electrolyte, the pressure inside the gas production apparatus is reduced and gas production is continued for a predetermined period of time, thereby reducing the amount of dissolved gas in the electrolyte before shutting down the apparatus. This prevents the problem of gas coexistence in the gas phase region of each circulation tank, etc. while the gas production apparatus is shut down, and prevents a decrease in the purity of the produced gas at the start of the next operation. Furthermore, it is possible to prevent the flammability of the gas composition in the gas phase region of each circulation tank, etc. from increasing and reaching the explosion limit.

[0018] Fig. 1 is a schematic diagram showing an embodiment 100a of a gas manufacturing apparatus to which the shutdown method of the present invention can be applied. Fig. 2 is a schematic diagram showing an embodiment 100b of a gas manufacturing apparatus to which the shutdown method of the present invention can be applied. Fig. 3 is a schematic diagram showing an embodiment 100c of a gas manufacturing apparatus to which the shutdown method of the present invention can be applied. Fig. 4 is a schematic diagram showing an embodiment 100d of a gas manufacturing apparatus to which the shutdown method of the present invention can be applied. Fig. 5 is a flow chart showing each step of the shutdown method of the present invention. Fig. 6 is a flow chart showing a control example of the shutdown method of the present invention.

[0019] <Gas production apparatus applicable to the shutdown method of the present invention> First, a gas production apparatus 100a used in the shutdown method of the present invention will be described with reference to Fig. 1. Note that the gas production apparatus 100a shown in Fig. 1 is merely one embodiment. The gas production apparatus 100a uses an alkaline aqueous solution as an electrolyte and produces oxygen gas and hydrogen gas by electrolysis of the alkaline aqueous solution.

[0020] The gas production apparatus 100a includes an electrolytic cell 10, a first electrolyte circulation system 20, a second electrolyte circulation system 30, a pure water supply system 40, a first gas recovery line 60, and a second gas recovery line 70. In Fig. 1, arrows indicate the direction of material flow.

[0021] The electrolytic cell 10 includes an anode chamber 11 that accommodates an anode and generates oxygen gas, a cathode chamber 12 that accommodates a cathode and generates hydrogen gas, and an ion-permeable diaphragm 13 that separates the anode chamber 11 from the cathode chamber 12. As the electrolytic cell 10, any electrolytic cell having a configuration that is conventionally used in alkaline water electrolysis devices can be used without any particular limitation.

[0022] The first electrolyte circulation system 20 includes a first circulation tank 21 that receives and stores the first electrolyte flowing out from the anode chamber 11, and a first circulation pump 22 that supplies the first electrolyte stored in the first circulation tank 21 to the anode chamber 11. Inside the first circulation tank 21, there is a liquid phase region 21a occupied by the stored first electrolyte, and a gas phase region 21b that is the space above the liquid phase region 21a. The first electrolyte circulation system 20 further includes a pipe 23 that guides the first electrolyte and anode gas flowing out from the anode chamber 11 to the first circulation tank 21, a pipe 24 that guides the first electrolyte from the liquid phase region 21a of the first circulation tank 21 to the first circulation pump 22, and a pipe 25 that guides the first electrolyte delivered from the first circulation pump 22 to the anode chamber 11.

[0023] A first gas-liquid mixture containing the first electrolytic solution and the gas generated in the anode chamber 11 flows out from the anode chamber 11. The first gas-liquid mixture flowing out from the anode chamber 11 is introduced to the first circulation tank 21 through piping 23, and separated (gas-liquid separation) inside the first circulation tank 21, with the first electrolytic solution flowing into a liquid phase region 21a and the gas (first gas flow) flowing into a gas phase region 21b. In this way, the electrolytic solution (first electrolytic solution) flowing out from the anode chamber 11 is circulated so as to flow back into the anode chamber 11.

[0024] The second electrolyte circulation system 30 includes a second circulation tank 31 that receives and stores the second electrolyte flowing out from the cathode chamber 12, and a second circulation pump 32 that supplies the second electrolyte stored in the second circulation tank 31 to the cathode chamber 12. The second circulation tank 31 contains a liquid phase region 31a occupied by the stored second electrolyte, and a gas phase region 31b that is the space above the liquid phase region 31a. The second electrolyte circulation system 30 further includes a pipe 33 that guides the second electrolyte and cathode gas flowing out from the cathode chamber 12 to the second circulation tank 31, a pipe 34 that guides the second electrolyte from the liquid phase region 31a of the second circulation tank 31 to the second circulation pump 32, and a pipe 35 that guides the second electrolyte delivered from the second circulation pump 32 to the cathode chamber 12.

[0025] A second gas-liquid mixture containing the second electrolytic solution and the gas generated in the cathode chamber 12 flows out from the cathode chamber 12. The second gas-liquid mixture flowing out from the cathode chamber 12 is introduced into the second circulation tank 31 through the pipe 33, and separated (gas-liquid separation) inside the second circulation tank 31, with the second electrolytic solution flowing into a liquid phase region 31a and the gas (second gas flow) flowing into a gas phase region 31b. In this way, the electrolytic solution (second electrolytic solution) flowing out from the cathode chamber 12 is circulated so as to flow back into the cathode chamber 12.

[0026] The pure water supply system 40 includes a pure water tank 41 that stores pure water, and a water supply pump 42 that sends the pure water stored in the pure water tank 41 to the second circulation tank 31. The water supply pump 42 sends pure water from the pure water tank 41 to the second circulation tank 31, thereby replenishing the water consumed by the water electrolysis reaction in the electrolytic cell 10.

[0027] The gas production apparatus 100a having the above-described configuration does not include an electrolyte exchange device, which will be described later. Even in this configuration, the electrolytic cell 10 originally separates the anode chamber 11 and the cathode chamber 12 by the diaphragm 13, and therefore some movement of the electrolyte may occur between the anode chamber 11 and the cathode chamber 12. As described above, oxygen gas is dissolved in the electrolyte on the anode side, and hydrogen gas is dissolved in the electrolyte on the cathode side. Therefore, even in this configuration in which the electrolytes are circulated independently, the problem of gas coexistence in the gas phase, which is an issue of the present invention, may occur.

[0028] 2 shows another embodiment of a gas production system 100b to which the shutdown method of the present invention can be applied. The gas production system 100b further includes electrolyte exchange devices 50a and 50b for exchanging the electrolyte between the anode side and the cathode side in addition to the configuration of the gas production system 100a described above.

[0029] The first electrolyte solution exchange device 50a transfers a portion of the first electrolyte solution present in the first electrolyte solution circulation system 20 to the second electrolyte solution circulation system 30. In the embodiment shown in FIG. 2, a first circulation transfer pump 51a is provided as a means for transferring a portion of the first electrolyte solution to the second electrolyte solution circulation system 30.

[0030] The second electrolyte solution exchange device 50b transfers a portion of the second electrolyte solution present in the second electrolyte solution circulation system 30 to the first electrolyte solution circulation system 20. In the embodiment shown in FIG. 2, a second circulation transfer pump 51b is provided as a means for transferring a portion of the second electrolyte solution to the first electrolyte solution circulation system 20.

[0031] In the embodiment shown in FIG. 2, the gas production apparatus 100b includes a pair of electrolyte exchangers, the first electrolyte exchanger 50a and the second electrolyte exchanger 50b. However, the effect of eliminating the difference in electrolyte concentration and the difference in liquid level between the anode-side circulation tank and the cathode-side circulation tank does not necessarily need to be achieved by providing both of these; only one of them can be sufficiently effective. Therefore, the gas production apparatus used in the present invention may include only the first electrolyte exchanger 50a or only the second electrolyte exchanger 50b. Furthermore, even if only one of the electrolyte exchangers is provided, the problem of gas coexistence in the gas phase of the circulation tank may occur.

[0032] In addition, in the gas production apparatus 100b, the first electrolyte solution exchanger 50a is connected to the first circulation tank 21 and the second circulation tank 31, and the second electrolyte solution exchanger 50b is connected to the piping 25 in the first electrolyte solution circulation system and the piping 35 in the second electrolyte solution circulation system, but these can be interchanged. That is, as in the configuration shown in FIG. 3 (gas production apparatus 100c), the first electrolyte solution exchanger 50a (transferring the first electrolyte solution to the second electrolyte solution circulation system 30) may be connected to the piping 25, 35, and the second electrolyte solution exchanger 50b (transferring the second electrolyte solution to the first electrolyte solution circulation system 20) may be connected to the first circulation tank 21 and the second circulation tank 31.

[0033] In the gas production apparatus usable in the present invention, known pumps such as positive displacement pumps and non-positive displacement pumps can be used as the circulation pump. Examples of positive displacement pumps include plunger pumps, piston pumps, diaphragm pumps, and gear pumps. Examples of non-positive displacement pumps include centrifugal pumps, turbine pumps, magnetic pumps, and canned pumps. Even when a non-positive displacement pump is used, it is possible to transfer the electrolyte in a predetermined direction at a predetermined flow rate by combining the non-positive displacement pump with a controller that controls the flow rate. Similar pumps can also be used as circulation pumps.

[0034] 4 shows another embodiment of a gas production apparatus to which the shutdown method of the present invention can be applied. In this embodiment, the first electrolytic solution flowing out of the first circulation tank 21 is introduced into the circulation pump 22 via the pipe 24, and the first electrolytic solution delivered from the circulation pump 22 is sent to the mixing tank 80. Furthermore, the second electrolytic solution flowing out of the second circulation tank 31 is introduced into the circulation pump 32 via the pipe 34, and the second electrolytic solution delivered from the circulation pump 32 is sent to the mixing tank 80, where the first electrolytic solution and the second electrolytic solution are mixed.

[0035] The electrolytic solution mixed in mixing tank 80 is introduced to circulation pump 37 via pipe 36, and is sent by circulation pump 37 to cathode chamber 12 via pipe 38. Similarly, the electrolytic solution mixed in mixing tank 80 is introduced to circulation pump 27 via pipe 26, and is sent by circulation pump 27 to anode chamber 11 via pipe 28.

[0036] In this configuration, the electrolyte on the anode side, in which oxygen is dissolved, and the electrolyte on the cathode side, in which hydrogen is dissolved, are constantly mixed in the mixing tank 80, which causes a problem of gas coexistence in the gas phase of the first circulation tank 21, the second circulation tank 31, and the mixing tank 80.

[0037] <Method for Shutdown of Gas Manufacturing Apparatus of the Present Invention> The method for shutting down a gas manufacturing apparatus of the present invention is a method for producing oxygen gas and hydrogen gas by electrolyzing an alkaline aqueous solution electrolyte under pressurized conditions using a gas manufacturing apparatus having an electrolytic cell including an anode chamber that accommodates an anode and generates oxygen gas, a cathode chamber that accommodates a cathode and generates hydrogen gas, and an ion-permeable diaphragm that separates the anode chamber and the cathode chamber, wherein the electrolyte that flows out of the anode chamber and the cathode chamber is circulated so that it flows back into the anode chamber and the cathode chamber, and the operation of the gas manufacturing apparatus is stopped by a procedure including the following steps (a) to (c): (a) a step of reducing the pressure inside the apparatus, (b) a step of continuing gas production in a state where the pressure is reduced, and (c) a step of shutting down the gas manufacturing apparatus.

[0038] A process flow diagram of the shutdown method of the present invention is shown in Figure 5. The present invention solves problems that may occur between the process of shutting down the apparatus and the next process of starting gas production.

[0039] (Gas Production Starting Step) In the gas production starting step, when the gas production apparatus is started, the inside of the gas production apparatus is pressurized, and oxygen gas and hydrogen gas are produced as follows.

[0040] Procedure for the apparatus of FIG. 1 While supplying a first electrolytic solution to the anode chamber 11 of the electrolytic cell 10 and a second electrolytic solution to the cathode chamber 12, a current is passed between the anode housed in the anode chamber 11 and the cathode housed in the cathode chamber 12, whereby oxygen gas is generated from the anode in the anode chamber 11 and hydrogen gas is generated from the cathode in the cathode chamber 12.

[0041] A first gas stream containing oxygen gas generated in the anode chamber 11 and a first electrolytic solution are recovered from the anode chamber 11. The first gas stream and the first electrolytic solution are recovered as a gas-liquid mixture from the anode chamber 11 through piping 23 and introduced to the first circulation tank 21, where they are separated into gas and liquid. The first electrolytic solution recovered from the anode chamber 11 to the first circulation tank 21 and after gas-liquid separation is stored in the first circulation tank 21. The first electrolytic solution stored in the first circulation tank 21 is circulated by the first circulation pump 22 and sent to the anode chamber 11 again.

[0042] A second gas stream containing hydrogen gas generated in the cathode chamber 12 and the second electrolytic solution are recovered from the cathode chamber 12. The second gas stream and the second electrolytic solution are recovered as a gas-liquid mixture from the cathode chamber 12 through piping 33 and introduced to the second circulation tank 31, where they are separated into gas and liquid. The second electrolytic solution recovered from the cathode chamber 12 to the second circulation tank 31 and after gas-liquid separation is stored in the second circulation tank 31. The second electrolytic solution stored in the second circulation tank 31 is circulated by the second circulation pump 32 and sent again to the cathode chamber 12.

[0043] The first gas stream recovered from the anode chamber 11 is taken out from the gas phase region 21b of the first circulation tank 21 through a first gas recovery line 60. The pressure of the first gas stream is controlled to a predetermined value by a first pressure control valve 61 provided in the first gas stream flow path (first gas recovery line 60). The second gas stream recovered from the cathode chamber 12 is taken out from the gas phase region 31b of the second circulation tank 31 through a second gas recovery line 70. The pressure of the second gas stream is controlled to a predetermined value by a second pressure control valve 71 provided in the second gas stream flow path (second gas recovery line 70).

[0044] The above steps are carried out simultaneously and continuously to produce oxygen gas and hydrogen gas.

[0045] In the gas production apparatus 100a, the first electrolytic solution circulated to the anode chamber 11 and the second electrolytic solution circulated to the cathode chamber 12 are stored separately in a first circulation tank 21 and a second circulation tank 31, and are circulated and resupplied to the anode chamber 11 and the cathode chamber 12, respectively. In this configuration, the electrolytic solution is not exchanged. However, in the electrolytic cell 10, the anode chamber 11 and the cathode chamber 12 are separated by a diaphragm. Therefore, the electrolytic solution moves through the diaphragm, and the electrolytic solution on the anode side containing dissolved oxygen moves to the cathode side, and the electrolytic solution on the cathode side containing dissolved hydrogen moves to the anode side. Therefore, by applying the shutdown method of the present invention to the gas production apparatus of the configuration shown in FIG. 1, the problem of gas coexistence during shutdown of the gas production apparatus can be solved.

[0046] ・Procedure in the apparatus of FIG. 2 A part of the first electrolytic solution stored in the first circulation tank 21 is transferred to the second circulation tank 31 by the first electrolytic solution exchanger 50a. Thereby, a part of the first electrolytic solution is introduced into the second electrolytic solution (a part of the first electrolytic solution → the second electrolytic solution circulation system 30). Also, a part of the second electrolytic solution sent out from the second circulation pump 32 is branched by the second electrolytic solution exchanger 50b and merges with the first electrolytic solution sent out from the first circulation pump 22. Thereby, a part of the second electrolytic solution is introduced into the first electrolytic solution (a part of the second electrolytic solution → the first electrolytic solution circulation system 20). Either one of the above-mentioned "a part of the first electrolytic solution → the second electrolytic solution circulation system 30" and "a part of the second electrolytic solution → the first electrolytic solution circulation system 20" may be implemented, or both of them may be implemented.

[0047] In the gas production apparatus 100b shown in FIG. 2, the first electrolytic solution circulated and supplied to the anode chamber 11 and the second electrolytic solution circulated and supplied to the cathode chamber 12 are separately stored in the first circulation tank 21 and the second circulation tank 31. However, by introducing a part of the first electrolytic solution into the second electrolytic solution circulation system 30 and a part of the second electrolytic solution into the first electrolytic solution circulation system 20 by the electrolytic solution exchangers 50a and 50b, the imbalance in the amount and concentration of the electrolytic solution generated between the anode side and the cathode side by electrolysis is eliminated.

[0048] The liquid feed amounts of the first circulation pump 51a and the second circulation pump 51b in the gas production apparatus 100b, and the liquid feed amount of pure water in the pure water supply system 40 are adjusted so as to maintain the liquid amount (liquid level) and concentration of the first electrolytic solution stored in the first circulation tank 21, and the liquid amount (liquid level) and concentration of the second electrolytic solution stored in the second circulation tank 31 at predetermined levels.

[0049] The amount of the first electrolytic solution stored in the first circulation tank 21 is preferably maintained within a range of 1 to 99% by volume, and more preferably within a range of 30 to 70% by volume, relative to the total volume of the first circulation tank 21. Similarly, the amount of the second electrolytic solution stored in the second circulation tank 31 is preferably maintained within a range of 1 to 99% by volume, and more preferably within a range of 30 to 70% by volume, relative to the total volume of the second circulation tank 31. When the amount of the electrolytic solution stored in each circulation tank 21, 31 is within the above range, the gas production apparatus 100 can be operated more stably.

[0050] Procedure in the apparatus of Figure 3 Compared to the apparatus of Figure 2, the direction of transfer in the apparatus of Figure 3 is reversed. A portion of the second electrolyte solution stored in the second circulation tank 31 is transferred to the first circulation tank 21 by the second electrolyte solution exchanger 50b. This introduces a portion of the second electrolyte solution into the first electrolyte solution (a portion of the second electrolyte solution → first electrolyte solution circulation system 20). Also, a portion of the first electrolyte solution delivered from the first circulation pump 22 is branched by the first electrolyte solution exchanger 50a and merges with the second electrolyte solution delivered from the second circulation pump 32. This introduces a portion of the first electrolyte solution into the second electrolyte solution (a portion of the first electrolyte solution → second electrolyte solution circulation system 30). As described above, either one of "a part of the first electrolytic solution → the second electrolytic solution circulation system 30" and "a part of the second electrolytic solution → the first electrolytic solution circulation system 20" may be implemented, or both of them may be implemented, as in the embodiment of FIG. 2.

[0051] 2 and 3, electrolyte exchange is performed between the first electrolyte circulation system 20 and the second electrolyte circulation system 30. Therefore, a situation in which the first electrolyte solution having dissolved oxygen exists in the second electrolyte solution having dissolved hydrogen, a situation in which the second electrolyte solution having dissolved hydrogen exists in the first electrolyte solution having dissolved oxygen, or both of these situations steadily occur. Therefore, by applying the shutdown method of the present invention to the gas production apparatuses having the configurations of Figures 2 and 3, the problem of gas coexistence when the gas production apparatus is shut down can be solved.

[0052] 4, the first electrolytic solution flowing out from the first circulation tank 21 through the pipe 24 is transferred by the first circulation pump 22 through the pipe 25 to the mixing tank 80. Also, the second electrolytic solution flowing out from the second circulation tank 31 through the pipe 34 is transferred by the second circulation pump 32 through the pipe 35 to the mixing tank 80.

[0053] In mixing tank 80, to which the first electrolytic solution and the second electrolytic solution have been transferred, these electrolytic solutions are mixed to form a mixed electrolytic solution, which is then introduced via pipe 26 and pipe 28 by circulation pump 27 into anode chamber 11. Furthermore, after becoming a mixed electrolytic solution, the mixed electrolytic solution is introduced via pipe 36 and pipe 38 by circulation pump 37 into cathode chamber 12.

[0054] As described above, in the gas production apparatus 100d, the first electrolytic solution and the second electrolytic solution are mixed in the mixing tank 80, and the mixed electrolytic solution flows again into the anode chamber 11 and the cathode chamber 12. As a result, the first electrolytic solution (electrolytic solution on the anode side) having dissolved oxygen and the second electrolytic solution (electrolytic solution on the cathode side) having dissolved hydrogen coexist. Therefore, by applying the shutdown method of the present invention to the gas production apparatus of the configuration shown in FIG. 4 as well, it is possible to solve the problem of gas coexistence when the gas production apparatus is shut down.

[0055] As described above with reference to FIGS. 1 to 4 , the phrase "the electrolytes flowing out of the anode chamber and the cathode chamber are circulated so as to flow back into the anode chamber and the cathode chamber" in the present invention means to include a configuration in which the electrolyte on the anode side and the electrolyte on the cathode side circulate independently as in FIG. 1 , a configuration in which the electrolyte on the anode side and the electrolyte on the cathode side are partially exchanged while the electrolyte on the anode side and the electrolyte on the cathode side are circulated independently as in FIGS. 2 and 3 , and a configuration in which the electrolyte on the anode side and the electrolyte on the cathode side are mixed once to form a mixed electrolyte, and this mixed electrolyte is circulated so as to flow back into the anode chamber and the cathode chamber as in FIG. 4 .

[0056] In the gas production apparatus used in the shutdown method of the present invention, the pressure inside one or both of the anode chamber 11 and the cathode chamber 12 is preferably maintained at a pressure higher than atmospheric pressure by 20 kPa or more. For example, the pressure of the first gas flow on the upstream side (primary side) of the first pressure control valve 61 and the pressure of the second gas flow on the upstream side (primary side) of the second pressure control valve 71 can be maintained at a pressure higher than atmospheric pressure by 5 MPa to 50 kPa, preferably 3 MPa to 100 kPa, more preferably 950 kPa to 200 kPa, and particularly preferably 880 kPa to 600 kPa. When the pressures of the first gas flow and the second gas flow on the upstream side of the first pressure control valve 61 and the upstream side of the second pressure control valve 71 are maintained at or above the above-mentioned lower limit, problems with dissolved gases generally become more pronounced, and the effects of the present invention become more pronounced.

[0033] That is, according to the method for shutting down a gas production apparatus of the present invention, even when alkaline water is electrolyzed under such pressurized conditions, the apparatus is shut down after going through a predetermined process as described below, so that gas dissolved in the electrolytic solution is released, and when the apparatus is shut down, the flammability of the gas composition in the gas phase region of each circulation tank, etc., increases, preventing it from reaching its explosion limit, and it is possible to produce both hydrogen gas and oxygen gas while reducing the adverse effect of the gas dissolved in the electrolytic solution on the gas purity. Furthermore, because the pressures of the first gas flow and the second gas flow upstream of the first pressure control valve 61 and the second pressure control valve 71 are equal to or less than the above-mentioned upper limit, pressure control is facilitated and the selection of components constituting the electrolytic cell 10 is facilitated.

[0057] (Pressure Reduction Step (Step (a))) In the shutdown method of the present invention, first, a step of reducing the pressure inside the gas production apparatus is carried out (step (a)). As described above, hydrogen gas and oxygen gas are produced under pressurized conditions. The pressure inside the gas production apparatus, which is maintained at a pressure higher than atmospheric pressure by 20 kPa or more, is reduced to preferably 0 kPa or more and 150 kPa or less, more preferably 30 kPa or more and 120 kPa or less. Note that the pressure inside the apparatus specifically refers to the pressure inside the apparatus, such as the electrolytic cell 10, the circulation tank, the mixing tank, various pumps, and various piping.

[0058] Specifically, the output of the main rectifier of the gas production device is controlled so that the pressure inside the device is within the above range. For example, the pressure inside the device is reduced by reducing the output to preferably 30% to 70%, more preferably 40% to 60%, of the main output in gas production.

[0059] 2 and 3, when a portion of the electrolyte is exchanged between the electrolyte on the anode side and the electrolyte on the cathode side, it is preferable to stop or shut off this liquid transfer in the pressure reduction step. It is even more preferable to stop or shut off the liquid transfer before the pressure reduction step (step (a)). This ensures that the mixing of the electrolytes due to the liquid transfer is stopped before the degassing step (step (a) + step (b)).

[0060] (Step of continuing gas production in a reduced pressure state (step (b))) After the pressure reduction step, a step of continuing gas production in a reduced pressure state (step (b)) is performed. In this step, dissolved gas present in the electrolytic solution is released into the gas phase in the circulation tank, and in the embodiment of FIG. 4, into the gas phases in the circulation tank and the mixing tank. The gas released into the gas phase in the circulation tank is released to the outside through pipes 62 and 72. The gas released into the gas phase in the mixing tank is released to the outside via a pressure reducing valve (not shown). At the start of the gas production continuation step (step (b)), the oxygen in the gas phase in the first circulation tank contains hydrogen at a concentration typically exceeding 0.1% by volume, and in some cases, at a concentration exceeding 0.2% by volume, and the oxygen in the hydrogen in the gas phase in the second circulation tank contains oxygen at a concentration typically exceeding 0.01% by volume, and in some cases, at a concentration exceeding 0.02% by volume. In particular, in the case of the gas production apparatus of the form shown in Figures 2 and 3, the oxygen in the gas phase of the first circulation tank contains hydrogen at a concentration of usually 0.3% by volume or more, and in some cases 0.7% by volume or more, and the hydrogen in the gas phase of the second circulation tank contains oxygen at a concentration of usually 0.03% by volume or more, and in some cases 0.07% by volume or more.

[0061] In step (b), the time for which gas production is continued cannot be determined in general because it is affected by electrolysis conditions such as the capacity and voltage of the electrolytic cell, but is usually from 10 minutes to 2 hours, more preferably from 20 minutes to 1 hour. Step (b) is continued after the above-mentioned predetermined time has elapsed, or until the hydrogen concentration in oxygen in the gas phase part of the first circulation tank has become preferably 0.2% by volume or less, more preferably 0.1% by volume or less, or until the oxygen concentration in hydrogen in the gas phase part of the second circulation tank has become preferably 0.02% by volume or less, more preferably 0.01% by volume or less.

[0062] It is preferable that the step (b) further comprises a step of introducing a part of the oxygen gas generated on the anode side into the electrolytic solution on the anode side to release the dissolved gas in the electrolytic solution on the anode side, and / or a step of introducing a part of the hydrogen gas generated on the cathode side into the electrolytic solution on the cathode side to release the dissolved gas in the electrolytic solution on the cathode side.

[0063] 1 to 4, by introducing a portion of the oxygen generated in the anode chamber 11 into the liquid phase portion of the first circulation tank 21 and bubbling the liquid phase portion, it is possible to promote the release of the dissolved gas into the gas phase portion. Also, by introducing a portion of the hydrogen generated in the cathode chamber 12 into the liquid phase portion of the second circulation tank 31 and bubbling the liquid phase portion, it is possible to promote the release of the dissolved gas into the gas phase portion.

[0064] It is also possible to release the dissolved gas by introducing nitrogen gas into the liquid phase parts of the first circulation tank 21 and the second circulation tank 31 and bubbling the liquid phase parts. However, in this case, nitrogen gas will dissolve in the respective electrolytic solutions, which may result in a decrease in the purity of the generated gas at the start of the next operation. Therefore, it is preferable to perform bubbling with oxygen or hydrogen, which are the main gases, as described above.

[0065] Also, oxygen generated in the anode chamber, hydrogen generated in the cathode chamber, or nitrogen can be introduced into the liquid phase of the mixing tank 80 shown in Figure 4 and bubbled to release the dissolved gas. In this case, too, it is preferable to use hydrogen gas or oxygen gas from the viewpoint of suppressing a decrease in the purity of the generated gas during the next operation. Note that a preferred form of bubbling in the mixing tank 80 is to first bubble with nitrogen gas, and then bubble with excess oxygen gas.

[0066] (Device Shutdown Step (Step (c))) After performing the above step (b) for a predetermined time, or after the hydrogen concentration in the oxygen in the first circulation tank or the oxygen concentration in the hydrogen in the second circulation tank becomes equal to or lower than a predetermined concentration, the gas production device is shut down (step (c)).

[0067] 6 shows a control example of a method for stopping a gas production apparatus according to the present invention. First, the gas production apparatus is started, and gas production is initiated under a pressure of 800 kPa at a load of 100% gas generation (S1). Gas production continues for a predetermined time, and when it is time to stop gas production, a stop signal is issued, the main rectifier output is reduced to about half of the main output (control value change), and the transfer of electrolyte between the anode side and the cathode side is stopped and shut off (liquid transfer stop) (S2).

[0068] Thereafter, gas production by electrolysis reaction is continued for a certain period of time under conditions of a gas generation rate of 50% load and a pressure of 50 kPa (S3). During this time, the pressure inside the device is reduced, causing dissolved gas present in the electrolyte to be released into the gas phase. Also, in step S3, oxygen, the main gas in the anode chamber, is introduced into the liquid phase of the first circulation tank to release the dissolved gas in the electrolyte. Also, hydrogen, the main gas in the cathode chamber, is introduced into the liquid phase of the second circulation tank to release the dissolved gas in the electrolyte.

[0069] The duration of step S3 is measured by a timer, and after a predetermined time has elapsed, or the hydrogen gas concentration in the oxygen gas in the gas phase of the first circulation tank is measured and the hydrogen gas concentration falls below a predetermined value, or the oxygen gas concentration in the hydrogen gas in the gas phase of the second circulation tank is measured and the oxygen gas concentration falls below a predetermined value, the power supply is stopped and the gas production device is shut down (S4, S5).

[0070] The method for shutting down a gas production apparatus of the present invention is applicable when producing oxygen gas and hydrogen gas by electrolyzing an alkaline aqueous solution under pressurized conditions, and contributes to improving the purity of the hydrogen gas produced in hydrogen production, which is attracting attention as a next-generation energy source.

[0071] 100a, 100b, 100c, 100d Gas production apparatus 10 Electrolytic cell 11 Anode chamber 12 Cathode chamber 13 (Ion-permeable) diaphragm 20 First electrolyte circulation system (anode-side electrolyte circulation system) 21 First circulation tank (anode-side circulation tank) 22 First circulation pump (cathode-side circulation tank) 30 Second electrolyte circulation system (cathode-side electrolyte circulation system) 31 Second circulation tank 32 Second circulation pump 40 Pure water supply system 41 Pure water tank 42 Water supply pump 50a First electrolyte exchange device 50b Second electrolyte exchange device 51a First circulation pump 51b Second circulation pump 80 Mixing tank

Claims

1. A method for producing oxygen gas and hydrogen gas by electrolyzing an alkaline aqueous solution of electrolyte under pressurized conditions using a gas production apparatus having an electrolytic cell equipped with an anode chamber that houses an anode and generates oxygen gas, a cathode chamber that houses a cathode and generates hydrogen gas, and an ion-permeable diaphragm that separates the anode chamber and the cathode chamber, wherein the electrolyte flowing out of the anode chamber and the cathode chamber is circulated so as to flow back into the anode chamber and the cathode chamber, and the operation of the gas production apparatus is stopped by a procedure comprising the following steps (a) to (c): (a) a step of reducing the pressure inside the apparatus, (b) a step of continuing gas production in a state where the pressure is reduced, and (c) a step of shutting down the gas production apparatus.

2. A method for shutting down a gas manufacturing apparatus according to claim 1, wherein, during operation of the gas manufacturing apparatus, an anode side electrolyte circulation system is formed so that the electrolyte flowing out of the anode chamber flows back into the anode chamber, and a cathode side electrolyte circulation system is formed so that the electrolyte flowing out of the cathode chamber flows back into the cathode chamber, a portion of the electrolyte flowing out of the anode chamber is transferred to the cathode side electrolyte circulation system, and / or a portion of the electrolyte flowing out of the cathode chamber is transferred to the anode side electrolyte circulation system, and the method further comprises a step of stopping the transfer in step (a).

3. A method for shutting down a gas production apparatus according to claim 1 or 2, wherein the pressurized conditions are conditions in which the pressure inside the anode chamber, the cathode chamber, or both of them is maintained at a pressure higher than atmospheric pressure by 20 kPa or more.

4. A method for shutting down a gas production apparatus according to claim 1 or 2, wherein the anode side circulation system comprises an anode side circulation tank for storing an electrolyte circulating through the anode side circulation system, and the cathode side circulation system comprises a cathode side circulation tank for storing an electrolyte circulating through the cathode side circulation system, and the step (b) further comprises a step of introducing a portion of the oxygen gas generated on the anode side into the electrolyte in the anode side circulation tank to release the dissolved gas in the electrolyte, and / or a step of introducing a portion of the hydrogen gas generated on the cathode side into the electrolyte in the cathode side circulation tank to release the dissolved gas in the electrolyte.

5. A method for producing oxygen gas and hydrogen gas, comprising a step of stopping the operation of a gas production apparatus by the method for stopping the gas production apparatus according to claim 1 or 2.

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