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

Abstract

In a method for electrolyzing an alkaline electrolyte under pressurized conditions, the electrolyte flowing out of an anode chamber and a cathode chamber is circulated so as to flow back into the anode chamber and the cathode chamber, and the operation of the gas production equipment is stopped by a procedure including predetermined steps, thereby making it possible to prevent the gas composition from reaching the explosion limit.

Description

[Technical field]

[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. [Background technology]

[0002] Alkaline water electrolysis is known as a method for producing hydrogen gas and oxygen gas. In alkaline water electrolysis, a basic aqueous solution (alkaline water) in which an alkali metal hydroxide (e.g., NaOH, KOH, etc.) is dissolved is used as an electrolyte to electrolyze water, generating hydrogen gas from the cathode and oxygen gas from the anode. An electrolytic cell for alkaline water electrolysis typically includes an anode chamber and a cathode chamber separated by an ion-permeable diaphragm, and electrolysis is performed while circulating the electrolyte in each of the anode chamber and the cathode chamber. The electrolyte collected 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 anode reaction is 2OH - →(1 / 2)O2↑+H2O+2e - …(1) The cathode reaction is 2H2O+2e - →H2↑+2OH - …(2) Therefore, in the alkaline water electrolysis process, water is consumed overall, but because water is consumed in the cathode reaction and water is produced in the anode reaction, a difference in liquid level occurs between the anode side circulation tank and the cathode side circulation tank as the electrolysis reaction progresses. - ions are consumed, and in the cathode reaction, OH - Ions are generated, and they move through a membrane 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 only OH ions produced in the cathode reaction -Only a portion of the ions pass through the diaphragm and move from the cathode chamber to the anode chamber, and the excess of negative charges in the cathode chamber that corresponds to the remainder are converted into cations (NaOH if the solute in alkaline water is NaOH). + Ions, if the solute in alkaline water is KOH, then K + This is resolved by the movement of ions from the anode chamber to the cathode chamber through the diaphragm. As a result, a concentration difference in 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 from one tank is moved 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. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] WO2020 / 022190 publication Summary of the Invention [Problem to be solved by the invention]

[0006] However, when oxygen gas is dissolved in the electrolyte on the anode side and this electrolyte is moved to the cathode side, oxygen gas is dissolved in the electrolyte on the cathode side together with hydrogen gas, which is the main gas. The reverse is also true, when hydrogen gas is dissolved in the electrolyte on the cathode side and this electrolyte is moved to the anode side, hydrogen gas is 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 as the pressure increases. Therefore, the amount of gas dissolved in the electrolyte increases particularly in a high-pressure alkaline water electrolysis device that performs 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 generation, hydrogen production is stopped in the evening because there is no power generation at night, and the device is restarted the following morning, and hydrogen production is started again, 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 time it is restarted the next morning, causing a gradual decrease in the pressure inside the water electrolysis device. (A) The heat generated by the electrolytic reaction disappears, and / or the temperature of the electrolyte drops due to a drop in the outside air temperature. (B) A small amount of gas escapes from the pressure control valve, and the pressure inside the water electrolysis device gradually decreases.

[0008] When the pressure in the water electrolysis device drops, the gas dissolved in the electrolyte is gradually released into the gas layer. Hydrogen gas and oxygen gas dissolved in the electrolyte are present in the electrolyte at their respective solubilities depending on the temperature and pressure conditions, but the solubilities of hydrogen gas and oxygen gas in the electrolyte in the high pressure region are approximately the same. Therefore, when the pressure in the water electrolysis device drops, it is assumed that approximately the same amount of hydrogen gas and oxygen gas are released into the gas layer in each circulation tank, and the flammability of the gas layer in each circulation tank may increase and enter the range of explosive gas. In addition, the mixture of hydrogen gas and oxygen gas in the gas layer in each circulation tank during the stop of the water electrolysis device may cause a decrease in the purity of the generated gas at the start of the next operation.

[0009] In addition, the above-mentioned exchange of the electrolyte between the anode side and the cathode side may cause the problem of coexistence of gases in the gas phase not only in the case of mutual exchange but also in the case of exchange of only one side (only from the anode side to the cathode side, or only from the cathode side to the anode side). Alternatively, instead of exchanging the electrolyte between the anode side and the cathode side, both electrolytes may be mixed in a mixing tank and the mixed electrolyte may be returned to the anode chamber and the cathode chamber again. However, even in such a case, the problem of coexistence of gases in the gas phase may occur.

[0010] Furthermore, even in a configuration that does not fall under the above-mentioned configuration and in which the electrolyte is circulated independently between the anode side and the cathode side, since the electrolytic cell originally separates the anode chamber and the cathode chamber with a diaphragm, 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 above-mentioned problem of gas coexistence in the gas phase may occur.

[0011] An object of the present invention is to provide a method for shutting down a gas production apparatus which prevents the problem of coexistence of gases 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 when an electrolytic cell provided with a diaphragm is used while electrolytic solution exchange is performed between the anode side and the cathode side, or when alkaline water electrolysis is performed under pressurized conditions even when electrolytic solution exchange is not performed. [Means for solving the problem]

[0012] The present invention is as follows. [1] A method for producing oxygen gas and hydrogen gas by electrolyzing an alkaline aqueous solution as an electrolyte under pressurized conditions using a gas production 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 from the cathode chamber, comprising: the electrolytic solution 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, A method for shutting down a gas production apparatus, comprising the steps of: (a) shutting down the gas production apparatus by a procedure including the following steps (a) to (c): (a) reducing the pressure within the apparatus; (b) continuing gas production at reduced pressure; (c) shutting down the gas production equipment;

[0013] [2] An anode-side electrolyte circulation system is formed so that the electrolyte flowing out of the anode chamber flows back into the anode chamber during operation of the gas production apparatus, 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 part of the electrolyte flowing out from the anode chamber is transferred to the cathode-side electrolyte circulation system, and / or a part of the electrolyte flowing out from the cathode chamber is transferred to the anode-side electrolyte circulation system, The method for shutting down a gas production apparatus according to [1], further comprising the step of stopping the liquid 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 is maintained at a pressure 20 kPa or higher than atmospheric pressure.

[0015] [4] The anode side circulation system includes an anode side circulation tank that stores an electrolyte circulating through the anode side circulation system, and the cathode side circulation system includes a cathode side circulation tank that stores an electrolyte circulating through the cathode side circulation system, The method for shutting down a gas production apparatus according to any one of [1] to [3], wherein 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 shutting down the operation of a gas production apparatus by the method for shutting down a gas production apparatus according to any one of [1] to [4]. Effect of the Invention

[0017] According to the present invention, in a method for producing hydrogen gas and oxygen gas under pressurized conditions, while exchanging the electrolyte on the anode side with the electrolyte on the cathode side, or even if the electrolyte is not exchanged, using an electrolytic cell equipped with a diaphragm, when stopping the gas production apparatus, the pressure inside the 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 stopping 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 stopped, and prevents the purity of the generated gas from decreasing when the next operation is started. 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. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram showing an embodiment 100a of a gas production apparatus to which the shutdown method of the present invention can be applied. [Diagram 2] FIG. 1 is a schematic diagram showing an embodiment 100b of a gas production apparatus to which the shutdown method of the present invention can be applied. [Diagram 3] FIG. 1 is a schematic diagram showing an embodiment 100c of a gas production apparatus to which the shutdown method of the present invention can be applied. [Figure 4] FIG. 1 is a schematic diagram showing an embodiment 100d of a gas production apparatus to which the shutdown method of the present invention can be applied. [Diagram 5] FIG. 2 is a flow chart showing each step of the stopping method of the present invention. [Figure 6] FIG. 4 is a flow chart showing a control example of a stopping method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] <Gas manufacturing equipment to which the shutdown method of the present invention can be applied> First, a gas production apparatus 100a used in the method for shutting down a gas production apparatus of the present invention will be described with reference to Fig. 1. 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, the arrows indicate the direction of material flow.

[0021] The electrolytic cell 10 comprises 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 form 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 are a liquid phase region 21a occupied by the stored first electrolyte, and a gas phase region 21b that is a 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 guided to the first circulation tank 21 through piping 23, and inside the first circulation tank 21, the first electrolytic solution separates into a liquid phase region 21a, and the gas (first gas flow) separates into a gas phase region 21b (gas-liquid separation). In this manner, the electrolyte (first electrolyte) flowing out of the anode chamber 11 is circulated so as to flow into the anode chamber 11 again.

[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. Inside the second circulation tank 31, there are a liquid phase region 31a occupied by the stored second electrolyte, and a gas phase region 31b that is a 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 guided to the second circulation tank 31 through piping 33, and inside the second circulation tank 31, the second electrolytic solution separates into a liquid phase region 31a, and the gas (second gas flow) separates into a gas phase region 31b (gas-liquid separation). In this manner, the electrolytic solution (second electrolytic solution) flowing out of 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 the 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 in the above-described embodiment does not include an electrolyte exchange device, which will be described later. Even in this embodiment, the electrolytic cell 10 separates the anode chamber 11 and the cathode chamber 12 with the diaphragm 13, so some electrolyte movement 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 embodiment 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 apparatus 100b that can be used with the shutdown method of the present invention. The gas production apparatus 100b further includes electrolyte exchangers 50a and 50b for exchanging electrolyte between the anode side and the cathode side in addition to the configuration of the gas production apparatus 100a described above.

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

[0030] The second electrolyte exchange device 50 b transfers a portion of the second electrolyte present in the second electrolyte circulation system 30 to the first electrolyte 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 electrolytic solution to the first electrolytic solution circulation system 20. As shown in FIG.

[0031] In the embodiment shown in FIG. 2, the gas production apparatus 100b is provided with a pair of the first electrolyte exchanger 50a and the second electrolyte exchanger 50b as electrolyte exchangers, but the effect of eliminating the electrolyte concentration difference and the liquid level difference between the anode side circulation tank and the cathode side circulation tank does not necessarily have to be provided with both of them, and only one of them can be used to sufficiently achieve an excellent effect. Therefore, the gas production apparatus used in the present invention may be provided with only the first electrolyte exchanger 50a, or may be provided with only the second electrolyte exchanger 50b. Even if only one of the electrolyte exchangers is provided, the problem of gas coexistence in the gas phase in the circulation tank may occur.

[0032] In addition, in the gas manufacturing apparatus 100b, the first electrolyte exchange device 50a is connected to the first circulation tank 21 and the second circulation tank 31, and the second electrolyte exchange device 50b is connected to the piping 25 in the first electrolyte circulation system and the piping 35 in the second electrolyte circulation system, but these can be interchanged. In other words, as shown in FIG. 3 (gas manufacturing apparatus 100c), a first electrolyte exchange device 50a (transferring the first electrolyte to the second electrolyte circulation system 30) may be connected to the pipes 25 and 35, and a second electrolyte exchange device 50b (transferring the second electrolyte to the first electrolyte 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, magnet 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. A similar pump can also be used as the circulation transfer pump.

[0034] 4 shows another embodiment of a gas production apparatus to which the stopping method of the present invention can be applied. In this embodiment, the first electrolytic solution flowing out of the first circulation tank 21 is guided to the circulation pump 22 via the pipe 24, and the first electrolytic solution sent 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 guided to the circulation pump 32 via the pipe 34, and the second electrolytic solution sent 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 electrolyte mixed in the mixing tank 80 is guided to the circulation pump 37 via the pipe 36, and is sent to the cathode chamber 12 via the pipe 38 by the circulation pump 37. Similarly, the electrolyte mixed in the mixing tank 80 is guided to the circulation pump 27 via the pipe 26, and is sent to the anode chamber 11 via the pipe 28 by the circulation pump 27.

[0036] In this embodiment, 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, so there is a problem of gas coexistence in the first circulation tank 21, the second circulation tank 31, and the gas phase of the mixing tank 80.

[0037] <Method of Shutdown of Gas Production Apparatus of the Present Invention> The method for shutting down a gas production apparatus of the present invention is a method for producing oxygen gas and hydrogen gas by electrolyzing an electrolytic solution, which is an alkaline aqueous solution, under pressurized conditions using a gas production 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, in which the electrolytic solution 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 including the following steps (a) to (c): (a) reducing the pressure within the apparatus; (b) continuing gas production at reduced pressure; (c) shutting down the gas production equipment;

[0038] A process 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 process of starting the next gas production.

[0039] (Gas production start process) In the gas production start 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 in Figure 1 A first electrolytic solution is supplied to the anode chamber 11 of the electrolytic cell 10, and a second electrolytic solution is supplied to the cathode chamber 12. At the same time, a current is passed between the anode contained in the anode chamber 11 and the cathode contained in the cathode chamber 12. As a result, 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 flow containing oxygen gas generated in the anode chamber 11 and a first electrolytic solution are recovered from the anode chamber 11 through piping 23 as a gas-liquid mixture and introduced into the first circulation tank 21, where the first gas flow and the first electrolytic solution are separated into gas and liquid. The first electrolytic solution recovered from the anode chamber 11 to the first circulation tank 21 and separated into gas and liquid 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 through piping 33 as a gas-liquid mixture and guided to the second circulation tank 31, where the gas-liquid mixture is separated. The second electrolytic solution recovered from the cathode chamber 12 to the second circulation tank 31 and subjected to 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 flow 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 flow is controlled to a predetermined value by a first pressure control valve 61 provided in the flow path of the first gas flow (first gas recovery line 60). The second gas flow 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 flow is controlled to a predetermined value by a second pressure control valve 71 provided in the flow path of the second gas flow (second gas recovery line 70).

[0044] The above-mentioned 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 the first circulation tank 21 and the second circulation tank 31, and are circulated and resupplied to the anode chamber 11 and the cathode chamber 12. In this embodiment, 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 in which oxygen is dissolved moves to the cathode side, and the electrolytic solution on the cathode side in which hydrogen is dissolved moves to the anode side. Therefore, even in the gas production apparatus of the embodiment of FIG. 1, the problem of gas coexistence when the gas production apparatus is stopped can be solved by applying the stopping method of the present invention.

[0046] Procedure for the apparatus in Figure 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 electrolyte solution exchanger 50a. As a result, a part of the first electrolytic solution is introduced into the second electrolytic solution (a part of the first electrolytic solution → second electrolyte solution circulation system 30). Also, a part of the second electrolyte solution sent out from the second circulation pump 32 is branched by the second electrolyte solution exchanger 50b and merges with the first electrolyte solution sent out from the first circulation pump 22. As a result, a part of the second electrolyte solution is introduced into the first electrolyte solution (a part of the second electrolyte solution → first electrolyte solution circulation system 20). 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.

[0047] In the gas production apparatus 100b shown in FIG. 2, 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 the first circulation tank 21 and the second circulation tank 31. However, the electrolyte exchange devices 50a, 50b introduce a portion of the first electrolytic solution into the second electrolyte circulation system 30 and introduce a portion of the second electrolytic solution into the first electrolyte circulation system 20, thereby eliminating imbalances in the amounts and concentrations of the electrolytes that occur between the anode side and the cathode side due to electrolysis.

[0048] The liquid delivery rate of the first circulation pump 51a and the second circulation pump 51b in the gas manufacturing apparatus 100b, as well as the pure water delivery rate in the pure water supply system 40, are adjusted so as to maintain the liquid volume (liquid level) and concentration of the first electrolytic solution stored in the first circulation tank 21, and the liquid volume (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 volume %, and more preferably within a range of 30 to 70 volume %, with respect 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 volume %, and more preferably within a range of 30 to 70 volume %, with respect to the total volume of the second circulation tank 31. By keeping the amount of the electrolytic solution stored in each of the circulation tanks 21, 31 within the above range, the gas production apparatus 100 can be operated more stably.

[0050] Procedure for the apparatus in Figure 3 In the device of FIG. 3, the direction of transfer is reversed compared to the device of FIG. 2. A part of the second electrolytic solution stored in the second circulation tank 31 is transferred to the first circulation tank 21 by the second electrolytic solution exchange device 50b. As a result, 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). In addition, a part of the first electrolytic solution sent out from the first circulation pump 22 is branched by the first electrolytic solution exchange device 50a and merges with the second electrolytic solution sent out from the second circulation pump 32. As a result, 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). As described above, either one of "a part of the first electrolytic solution → second electrolytic solution circulation system 30" and "a part of the second electrolytic solution → first electrolytic solution circulation system 20" may be implemented, or both of them may be implemented, as in the embodiment of FIG. 2.

[0051] In the embodiment of Figs. 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 having dissolved oxygen exists in the second electrolyte having dissolved hydrogen, or a situation in which the second electrolyte having dissolved hydrogen exists in the first electrolyte having dissolved oxygen, or both of these situations occur regularly. Therefore, by applying the stopping method of the present invention to the gas production apparatus of the embodiment of Figs. 2 and 3, the problem of gas coexistence when the gas production apparatus is stopped can be solved.

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

[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 become 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 oxygen dissolved therein and the second electrolytic solution (electrolytic solution on the cathode side) having hydrogen dissolved therein coexist. Therefore, by applying the shutdown method of the present invention to the gas production apparatus in the form of 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, in the present invention, "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" means 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 circulate independently as in Figs. 2 and 3, in which a part of the electrolyte is exchanged while the electrolyte on the anode side and the electrolyte on the cathode side are circulated independently as in Figs. 4, 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 then the 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 at a pressure higher than atmospheric pressure by 3 MPa to 100 kPa, more preferably at a pressure higher than atmospheric pressure by 950 kPa to 200 kPa, and particularly preferably at a pressure higher than atmospheric pressure by 880 kPa to 600 kPa. When 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 maintained at or above the lower limit, the problem of dissolved gas generally tends to become apparent, and the effect of the present invention becomes prominent. That is, according to the method for stopping the gas production apparatus of the present invention, even when electrolysis of alkaline water is performed under such pressurized conditions, as described below, the apparatus is stopped after a predetermined process, so that the gas dissolved in the electrolytic solution is released, and when the apparatus is stopped, the flammability of the gas composition in the gas phase region of each circulation tank, etc. is increased and the explosion limit is prevented from being reached, and both hydrogen gas and oxygen gas can be produced while reducing the adverse effect of the dissolved gas in the electrolytic solution on the gas purity. In addition, since 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 at or below the upper limit, pressure control becomes easy, and the selection of the members constituting the electrolytic cell 10 becomes easy.

[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. The pressure inside the apparatus specifically means the pressure inside the apparatus, such as the electrolytic cell 10, the circulation tank, the mixing tank, various pumps, and various pipes.

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

[0059] In addition, when a part of the electrolyte is exchanged between the electrolyte on the anode side and the electrolyte on the cathode side as in the gas production apparatus of Figs. 2 and 3 described above, it is preferable to stop / block this electrolyte transfer in the pressure reduction step. It is further preferable to stop / block the electrolyte transfer in a stage prior to the pressure reduction step (step (a)). This makes it possible to reliably stop the mixing of the electrolytes due to electrolyte transfer before the degassing step (step (a) + step (b)).

[0060] (Continued gas production process under reduced pressure (process (b))) After the pressure reduction step, a step (step (b)) of continuing gas production with the pressure reduced is performed. In this step, the 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 phase 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 beginning of the gas production continuation step (step (b)), the oxygen in the gas phase of the first circulation tank contains hydrogen at a concentration of usually more than 0.1% by volume, and in some cases more than 0.2% by volume, and the hydrogen in the gas phase of the second circulation tank contains oxygen at a concentration of usually more than 0.01% by volume, and in some cases more than 0.02% by volume. In particular, in the case of the gas production apparatus of the form of 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 more than 0.7% by volume, 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 more than 0.07% by volume.

[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 specified time has elapsed, or until the hydrogen concentration in oxygen in the gas phase of the first circulation tank becomes 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 of the second circulation tank becomes 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 electrolyte on the anode side to release the dissolved gas in the electrolyte on the anode side, and / or a step of introducing a part of the hydrogen gas generated on the cathode side into the electrolyte on the cathode side to release the dissolved gas in the electrolyte on the cathode side.

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

[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, since the nitrogen gas dissolves in each electrolyte, there is a possibility that the purity of the generated gas will decrease when the next operation is started, so it is preferable to bubble with oxygen or hydrogen, which is the main gas, 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 Fig. 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 purity of the generated gas in the next operation. Note that a suitable form of bubbling in the mixing tank 80 is to first bubble with nitrogen gas, and then bubble with oxygen gas, which is an excess gas.

[0066] (Equipment stop process (process (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 apparatus is stopped (step (c)).

[0067] (Example of how to stop a gas production unit) FIG. 6 shows an example of a control method for shutting down the gas production apparatus of the present invention. First, the gas production equipment is started, and gas production is commenced under a pressure condition of 800 kPa with a load of 100% gas generation (S1). Gas production continues for a prescribed period of 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 cathode side electrolyte is stopped and shut off (transfer stop) (S2).

[0068] Thereafter, gas production by electrolysis 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). At this time, the dissolved gas present in the electrolyte is released into the gas phase due to a drop in pressure inside the device. Also, in step S3, oxygen, which is 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, which is 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 when 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 when 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 apparatus is shut down (S4, S5). [Industrial Applicability]

[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. [Explanation of symbols]

[0071] 100a, 100b, 100c, 100d Gas production equipment 10 Electrolytic cell 11 Anode chamber 12 Cathode chamber 13 (Ion-permeable) membrane 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 as an electrolyte under pressurized conditions using a gas production apparatus having an electrolytic cell including 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 from the cathode chamber, comprising: the electrolytic solution 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, A method for shutting down a gas production apparatus, comprising the steps of: (a) shutting down the operation of the gas production apparatus by a procedure including the following steps (a) to (c): (a) reducing the pressure within the apparatus; (b) continuing gas production under reduced pressure; (c) shutting down the gas production equipment;

2. an anode-side electrolyte circulation system is formed so that the electrolyte flowing out of the anode chamber flows back into the anode chamber during operation of the gas production apparatus, 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 part of the electrolyte flowing out from the anode chamber is transferred to the cathode-side electrolyte circulation system, and / or a part of the electrolyte flowing out from the cathode chamber is transferred to the anode-side electrolyte circulation system, The method for shutting down a gas production apparatus according to claim 1 , further comprising the step of stopping the liquid transfer in the step (a).

3. 3. The method for shutting down a gas production apparatus according to claim 1, wherein the pressurized condition is a pressure condition 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. the anode-side electrolyte circulation system includes an anode-side circulation tank that stores the electrolyte circulating through the anode-side electrolyte circulation system, and the cathode-side electrolyte circulation system includes a cathode-side circulation tank that stores the electrolyte circulating through the cathode-side electrolyte circulation system, 3. The method for shutting down a gas production apparatus according to claim 2, wherein the step (b) further comprises: 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 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.

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

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