Electrolysis system and method for rinsing an electrolyzer

Abstract

Electrolysis system comprising an electrolyzer (1) having an inlet (2) through which a liquid can be introduced and an outlet (3) through which liquid or gas can be discharged, wherein the outlet (3) is connected via an outlet line (4) to a gas-liquid separator (5) in which the gas exiting the electrolyzer (1) is separated from the discharged liquid. The inlet (2) is connectable to a pressure tank (10) in which liquid is held under a purging pressure.

Description

[0001] The invention relates to an electrolysis system such as can be used for the electrolytic splitting of water into hydrogen and oxygen using electrical energy, and a method for rinsing an electrolyzer. State of the art

[0002] Electrical energy can be converted into chemical energy in the form of hydrogen. This is achieved using an electrolyzer, which comprises an electrochemical cell containing an anode compartment and a cathode compartment. The anode and cathode compartments are separated by a semipermeable membrane. The membrane is coated with an anode electrode on the anode side and a cathode electrode on the cathode side, between which a direct current voltage can be applied. To carry out the electrolysis, the anode compartment and—depending on the type of electrolyzer—also the cathode compartment are filled with water or an electrolytic aqueous solution. By applying the electrical voltage between the anode and cathode electrodes, the water on the anode side is catalytically split, producing hydrogen. + Hydrogen ions diffuse through the membrane into the cathode compartment. There, the hydrogen ions recombine.+ -ions react with the electrons in the cathode electrode to form hydrogen gas. Electrolyzers with this operating principle are of the so-called PEM type, meaning that the semipermeable membrane allows protons – i.e., H – to pass through. + -ions - is permeable (proton exchange membrane), while the membrane is largely impermeable to other substances. Other electrolyzers are also known, for example, those in which the membrane is permeable to OH- ions. - - or O 2- -ions are permeable. An example of an electrolysis system is known from DE 10 2021 214 205 A1.

[0003] In the electrochemical cell, hydrogen is produced in the cathode compartment and oxygen in the anode compartment. These reaction gases are carried away by the water or aqueous solution that is continuously pumped through the anode and cathode compartments and fed to a gas-liquid separator (GLS). There, the hydrogen or oxygen is separated from the water, and the hydrogen is stored for later use. Even in an electrolyzer with a dry cathode, where water does not flow through the cathode compartment, water accumulates over time through drag and diffusion and flows out of the cathode compartment along with the hydrogen. The water collected in the gas-liquid separators is then pumped back into the cycle, continuously replacing the water consumed in the anode compartment.

[0004] In electrolysis systems, it is common practice to inert the stacks and at least parts of the tubing under certain operating conditions, particularly during scheduled maintenance shutdowns, emergency shutdowns, and possibly also during standby operation. This process removes the hydrogen and, if necessary, the oxygen from the cathode and anode compartments, respectively, as otherwise there is a risk that the hydrogen will diffuse into the anode compartment over time and mix with the oxygen present there. This inerting can be carried out using nitrogen as an inert gas, but this leads to contamination of the product gas and thus reduces the H₂ yield. Alternatively, deionized water (DI water) can be used for inerting or purging. Advantages of the invention

[0005] The electrolysis system according to the invention has the advantage of enabling rapid and reliable inerting or purging of the electrolyzer, thereby minimizing the mechanical stress on the electrolyzer. For this purpose, the electrolysis system comprises an electrolyzer with an inlet through which a liquid can be introduced and an outlet through which liquid and / or gas can be discharged. The outlet is connected via a line to a gas-liquid separator, in which the gas exiting the electrolyzer is separated from the liquid exiting the electrolyzer. The inlet can be connected to a pressure tank in which liquid is held under a purging pressure.

[0006] During operation of the electrolyzer, hydrogen is produced in a cathode compartment and oxygen in an anode compartment. The gases are continuously discharged along with the liquid, which can be water or an aqueous solution. The hydrogen produced in the cathode compartment is stored there under an elevated pressure, for example, 40 bar, to reduce the subsequent compression work required for storing the hydrogen at several hundred bar or for other downstream processes. When the electrolyzer is switched off, hydrogen production ceases, but the cathode compartment remains filled with hydrogen gas. This gas can diffuse into the anode compartment, especially if the electrolyzer is left idle for an extended period, and mix with the oxygen there. To prevent this, the cathode compartment must be purged with water after shutdown.This is achieved by using water stored in a pressure tank, which is forced into the cathode chamber via a connecting line as needed, where the water displaces the hydrogen. After rinsing the cathode chamber, the electrolyzer can remain switched off for an extended period. The electrolyzer can also be rinsed in this way to remove other impurities.

[0007] In an advantageous embodiment of the invention, the volume of the pressure tank is greater than or equal to the volume in the electrolyzer into which the inlet opens. This ensures that there is always enough water or rinsing fluid available to flush the volume in the electrolyzer and thus reliably remove hydrogen or other impurities.

[0008] In a further advantageous embodiment, the pressure tank is designed as a diaphragm pressure accumulator. This allows a high pressure to be maintained on the liquid, ensuring that rinsing water or rinsing fluid is always available at the required rinsing pressure.

[0009] In a further advantageous embodiment, the pressure tank has a movable piston that limits the liquid-filled volume and is subjected to a pressure force. This maintains the pressure in the liquid-filled volume, allowing the electrolyzer to be purged at a constant pressure.

[0010] In a further advantageous embodiment, the pressure tank has two partial volumes, each delimited by a movable piston, with the two pistons being connected. This allows the first partial volume to be emptied while the second partial volume is refilled with water from the gas-liquid separator. This design enables a longer purging process by repeatedly passing the initially held volume through the electrolyzer until the hydrogen concentration in the purge water is sufficiently low, which can also be controlled via a sensor signal. Depending on the design and desired operating mode, this storage tank can be directly connected to the gas-liquid separator, or an additional pressure accumulator can be interposed for better decoupling of the fill level control in the gas-liquid separator and the purge operation.

[0011] In a further advantageous embodiment, the gas-liquid separator has a drain through which liquid can be introduced into the pressure tank via a filling line. This allows the pressure tank to be filled with water generated in the gas-liquid separator. If necessary, a pump can be arranged in the purge line to compress the liquid and build up the required pressure in the pressure tank. If the gas-liquid separator is connected to the cathode side of the electrolyzer, a high pressure already exists there, originating from the cathode chamber, which can also be used to fill the pressure tank. In this case, a pump is often unnecessary.

[0012] In a further advantageous embodiment, the liquid is water or an aqueous solution, in particular an electrolytic solution. Rinsing is usually carried out with pure water, but an electrolytic solution may also be required, especially when the anode compartment of a so-called AEM electrolyzer (anion exchange membrane) is to be rinsed.

[0013] The purging method according to the invention can be applied to an electrolyzer designed to electrolytically split water into hydrogen and oxygen using an electric current. For this purpose, the electrolyzer has an inlet through which a liquid can be introduced and an outlet through which liquid or gas can be discharged. A pressure tank is connected to the inlet via a purging line, the pressure tank containing a liquid at elevated pressure, and the purging line having a shut-off valve. To carry out the method, the electric current in the electrolyzer is first switched off. Then the shut-off valve is opened, allowing the liquid from the pressure tank to flow into the inlet and out through the outlet. This purging is continued until the concentration of a gas in the electrolyzer falls below a predetermined limit.This can be done by measuring the concentration or by purging the system for a predetermined period. The shut-off valve is then closed again. This process safely removes chemical gases present in the electrolyzer, allowing it to be shut down for extended periods without unwanted chemical reactions or mixing of the gases, and enabling safe restarting.

[0014] The rinsing is advantageously carried out with water or an aqueous solution, in particular an electrolytic solution. Preferably, a cathode compartment of the electrolyzer is rinsed, in which hydrogen gas is produced during operation. drawing

[0015] The drawing shows various embodiments of the electrolysis system according to the invention. Fig. 1 a first embodiment in schematic representation, Fig. 2 a schematic representation of an electrolyzer, Fig. 3 another embodiment in the same representation as Fig. 1, Fig. 4 a first embodiment of a pressure tank, Fig. 5 a second embodiment of a pressure tank and Fig. 6 a third embodiment of a pressure tank. Description of the exemplary implementations

[0016] In Fig. Figure 1 schematically illustrates an electrolysis system according to the invention, showing only the essential components. The electrolysis system comprises an electrolyzer 1, which is configured to produce hydrogen and oxygen from water by electrolytic splitting using an electric current. The electrolyzer 1 typically comprises a plurality of electrolytic cells 101, one of which is arranged in Fig. Figure 2 is shown schematically. Several hundred such cells 101 can be installed in an electrolyzer 1, stacked on top of each other and electrically connected in series. As shown in Fig. Figure 2 shows an electrolytic cell 101 comprising an anode compartment 25 and a cathode compartment 26, which are separated from each other by a semipermeable membrane 27. The membrane 27 is provided with an anode electrode 28 on the side facing the anode compartment 25 and with a cathode electrode 29 on the side facing the cathode compartment 26. During operation, a DC voltage is applied between the anode electrode 28 and the cathode electrode 29. For operation of the electrolyzer 1, the anode compartment 25 is filled or traversed with water or an electrolytic solution. A catalytic coating on the membrane 27 in the region of the anode electrode 28 catalytically converts water into H₂. + - and O 2-ions are split. The resulting H + Due to the electrical voltage, hydrogen ions (protons) diffuse through the membrane 27 and recombine at the cathode electrode 29 to form hydrogen gas, which collects in the cathode compartment 26. Electrolyzers of this type are called PEM electrolyzers (proton exchange membrane). Since the H + Since hydrogen ions are surrounded by a hydration shell when passing through the membrane, water (so-called drag water) always enters the cathode compartment 26 along with the hydrogen ions.

[0017] The cathode chamber 26 has an inlet 2 and an outlet 3. In the electrolysis system shown here, the hydrogen gas and the water produced in the cathode chamber 26 are discharged via the outlet 3 and enter a gas-liquid separator 5 via an outlet line 4. There, the water is separated from the hydrogen. In the gas-liquid separator 5, the water 7 collects at the bottom due to gravity and can be drained via a drain 9 when the fill level reaches a defined height. The hydrogen gas 6 is discharged via a gas outlet 8 for storage or further use. During operation of the electrolyzer 1, the pressure in the cathode chamber 26 is significantly elevated and is usually between 10 and 70 bar (1 to 7 MPa), while the pressure in the anode chamber 25 is only slightly higher than the ambient pressure.The pressure in the cathode chamber 26 is regulated, for example, by a pressure regulator at the gas outlet 8 of the gas-liquid separator 5, ensuring that the gas-liquid separator 5 and the cathode chamber 26 are at the same pressure. In other pressure regulation configurations, the pressure in the gas-liquid separator 5 can be reduced compared to the pressure in the cathode chamber 26.

[0018] The pressure tank 10 can be filled with deionized water at the desired purge pressure via an external water supply. Alternatively, the water can be taken from the already pressurized gas-liquid separator 5, as shown in another embodiment of the Fig. Figure 3 illustrates this. The water 7 separated in the gas-liquid separator 5 is discharged via the outlet 9 of the gas-liquid separator 5 and a valve 17. If necessary, the water is compressed by a pump 16 and finally fed to the pressure tank 10. Since an increased pressure of, for example, 40 bar prevails in the cathode chamber 26 during operation, it is advantageous if the cathode chamber 26 is also flushed with this pressure, as this reduces the mechanical stress on the electrolyzer 1 and, in particular, on the membrane 27. If there is too much water, it can be discharged via a drain 18 by opening a drain valve 19.

[0019] If the electrolyzer 1 is to be switched off, hydrogen gas remains in the cathode chamber 26. This hydrogen gas is highly diffusive and, due to the pressure difference between the cathode chamber 26 and the anode chamber 25, diffuses over time through the membrane 27 into the anode chamber 25, where it mixes with the oxygen, forming a reactive gas mixture (oxyhydrogen gas). To prevent this, the cathode chamber 26 is purged with water in the electrolysis system according to the invention. For this purpose, the inlet 2 of the cathode chamber 26 is connected to a pressure tank 10 in which water is held at a purging pressure. This pressure essentially corresponds to the pressure that prevails in the cathode chamber 26 during normal operation of the electrolyzer 1, in order to minimize the mechanical stress on the electrolyzer 1.

[0020] To purge the cathode chamber 26, the power supply between the anode electrode 28 and the cathode electrode 29 is interrupted, with the cathode chamber 26 being filled with hydrogen gas and a certain amount of water. Since the electrolyzer 1, or rather the individual electrolytic cells, represent an electrical capacitance, the electrolysis does not end immediately, but a small amount of hydrogen continues to be produced for a certain period of time, which also accumulates in the cathode chamber 26 until the capacitor formed by the cathode and anode electrodes is discharged. To remove the hydrogen from the cathode chamber 26, the shut-off valve 11 is opened, and water flows from the pressure tank 10 into the cathode chamber 26 and from there via the outlet 3 into the gas-liquid separator 5. This purging process is continued until the hydrogen gas has been removed from the cathode chamber 26. The shut-off valve 11 is then closed again.If there is too much water, it can be drained away via a drainage system 18 by opening a drain valve 19.

[0021] The purging process can be time-controlled, meaning that the cathode chamber 26 is purged for a predetermined period. Volume control is also possible, in which a predetermined quantity of liquid is passed through the cathode chamber 26, for example, the entire liquid volume of the pressure tank 10. Control via a sensor that measures the hydrogen concentration in the liquid during the purging process is also easily implemented.

[0022] After completion of the purging process, the pressure tank 10 is refilled with water at a purging pressure, either from the gas-liquid separator 5 or via an external water source, to prepare it for the next purging process. If only water from the gas-liquid separator 5 is used, a closed system is formed, requiring no additional external supply of purging water. The volume of the pressure tank 10 and the available water volume must be dimensioned such that the cathode chamber 26 can be completely purged at least once. The volume of the purging line 12 and the outlet line 4 must also be taken into account, as hydrogen gas can accumulate there as well. Advantageously, the pressure tank 10 has a water volume that is two to three times greater than the volume of the cathode chamber 26 plus the volume of the purging line 12 and the outlet line 4.Since the water in pressure tank 10 is under operating pressure, the rinsing process can be started simply by opening the shut-off valve 11, without the need for a pump or any other electrically powered unit. Depending on the pressure in the electrolyzer 1, a check valve must be installed in addition to valve 11 to prevent backflow to pressure tank 10.

[0023] To ensure that the water in pressure tank 10 maintains increased pressure throughout the entire rinsing process, the pressure tank 10 must be designed to maintain this pressure even during emptying. Such a pressure tank 10 is located in Fig. Figure 4 shows a schematic representation. The pressure tank 10 has a water volume V1, which is filled either via the gas-liquid separator 5 – as shown in Fig. Figure 3 shows the system – or via an external water supply providing highly purified water. The water volume V1 is limited by a movable piston 20, which is subjected to a force in the direction of the water volume V1 via a rod 21. This force results in a largely constant pressure within the water volume V1, so that the cathode chamber is flushed with a constant flow of water at a pressure that is largely the same as that prevailing in the cathode chamber 26 during normal operation. Since the piston 20 moves forward as the volume V1 empties, the pressure is maintained.

[0024] In Fig. Figure 5 shows another embodiment of a pressure tank 10. Here, the pressure tank 10 is designed as a diaphragm pressure accumulator, in which the water volume V1 is separated from a pressurized gas volume V. D separated by a flexible membrane 22. The pressurized gas volume V DThe water volume V1 is pressurized and thereby compressed. If the water volume V1 empties, the pressure in the pressurized gas volume V decreases. D It decreases somewhat, but sufficient flushing pressure can still be maintained.

[0025] In Fig.Figure 6 shows another embodiment of a pressure tank 10. Here, the pressure tank 10 has two water volumes, V1 and V2, each limited by a piston 20a and 20b, respectively. The two pistons 20a and 20b are connected by a rod 21. If the two pistons 20a and 20b move, for example, towards the water volume V1, volume V1 empties, while volume V2 can simultaneously refill with water. This application has the advantage that water can be constantly supplied to the gas-liquid separator and the electrolyzer 1 can be purged at the same time. Depending on the design and desired operating mode, this storage tank with two volumes V1 and V2 can be directly connected to the gas-liquid separator 5, or an additional pressure accumulator can be interposed for better decoupling of the level control in the gas-liquid separator 5 on the one hand and the purging operation on the other.

[0026] The electrolysis system shown here using the example of the cathode compartment 26 for rinsing the cathode compartment 26 can also be used analogously for rinsing the anode compartment 25. For this purpose, either a further pressure tank 10 can be provided, or the same pressure tank 10 can be connected to both the anode compartment 25 and the cathode compartment 26. The rinsing of the anode compartment 25 is then preferably carried out at a low pressure, which is only slightly above the ambient pressure, corresponding to the operating pressure in the anode compartment 25. Instead of rinsing with pure water, an aqueous solution, for example an electrolytic solution, can also be held in the pressure tank 10 for rinsing the anode compartment 25 or the cathode compartment 26, depending on the type of electrolyzer. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2021 214 205 A1

[0002]

Claims

[1] Electrolysis system with an electrolyzer (1) having an inlet (2) through which a liquid can be introduced, and an outlet (3) through which liquid and / or gas can be discharged, wherein the outlet (3) is connected via an outlet line (4) to a gas-liquid separator (5) in which the gas emerging from the electrolyzer (1) is separated from the emerging liquid, characterized by that the inlet (2) can be connected to a pressure tank (10) in which liquid is kept under a flushing pressure. [2] Electrolysis system according to claim 1, characterized by that the volume of the pressure tank (10) is greater than or equal to the volume in the electrolyzer (1) into which the inlet (2) opens. [3] Electrolysis system according to claim 1 or 2, characterized by that the pressure tank (10) is designed as a diaphragm pressure accumulator. [4] Electrolysis system according to claim 1 or 2, characterized bythat the pressure tank (10) has a movable piston (20) which limits the volume (V1) filled with liquid and which is subjected to a pressure force. [5] Electrolysis system according to claim 1, characterized by that the pressure tank (10) has two partial volumes (V1; V2), both of which are delimited by a movable piston (20a; 20b), wherein the two pistons (20a; 20b) are connected and move synchronously. [6] Electrolysis system according to one of claims 1 to 5, characterized by that the gas-liquid separator (5) has an outlet (9) through which liquid can be fed into the pressure tank (10) via a filling line (15). [7] Electrolysis system according to claim 6, characterized by that a pump (16) is arranged in the filling line (15) which compresses the liquid so that an increased pressure prevails in the pressure tank (10). [8] Electrolysis system according to one of claims 1 to 7, characterized bythat the liquid is water or an aqueous solution, in particular an electrolytic solution. [9] Electrolysis system according to one of claims 1 to 8, characterized by that the inlet (2) and the outlet (3) open into an anode chamber (25) or a cathode chamber (26) of the electrolyzer (1). [10] Method for flushing an electrolyzer (1) which has an inlet (2) through which a liquid can be introduced, and an outlet (3) through which liquid and / or gas can be discharged, and the electrolyzer (1) is designed to electrolytically split water into hydrogen and oxygen with the aid of electric current, and with a pressure tank (10) which is connected to the inlet (2) via a flushing line (12) and in which liquid is held under a flushing pressure, wherein a shut-off valve (11) is arranged in the flushing line (12), characterized by - Switching off the electrical current in the electrolyzer (1), - Opening the shut-off valve (11) so that the liquid from the pressure tank (10) flows into the inlet (2) of the electrolyzer (1) and flows out via the outlet (3), - Flushing the electrolyser (1) until the concentration of a gas in the flushed area of ​​the electrolyser (1) falls below a predetermined limit, - Close the shut-off valve (11). [11] Method according to claim 10, characterized by that the flushing of the electrolyzer (1) is time-controlled, quantity-controlled or dependent on a measured gas concentration in the liquid. [12] Method according to claim 10 or 11, characterized by that the liquid is water or an aqueous solution, in particular an electrolytic solution. [13] Method according to claim 10, 11 or 12, characterized by that the inlet (2) opens into a cathode chamber (25) of the electrolyzer (1), in which hydrogen gas is formed during operation of the electrolyzer (1).

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