Method for electrolytic splitting of water

Abstract

The invention relates to an electrolysis system for the electrolytic splitting of water, having an electrolysis cell (1) which has two reaction chambers (2; 3) which are separated by a semipermeable barrier, wherein a reaction chamber (2; 3) is connected to a discharge line (9) through which water and gas are discharged from the reaction chamber (2; 3). A riser pipe (20) branches off from the discharge line (9), in which riser pipe a gas sensor (17) is arranged which detects the concentration of a gas in the riser pipe (20).

Description

[0001] R. 415138

[0002] - 1 -

[0003] title

[0004] Electrolysis system for the electrolytic splitting of water

[0005] The invention relates to an electrolysis system for the electrolytic splitting of water, wherein the water is electrolytically split into hydrogen and oxygen.

[0006] State of the art

[0007] In known membrane-based electrolysis systems, a stack of electrolysis cells is used, with each cell having a cathode compartment and an anode compartment. An electrode is located in both the cathode and anode compartments. The anode compartment, and—depending on the electrolyzer type—also the cathode compartment, is filled with water or an electrolytic solution. When a direct current voltage is applied between the cathode and anode compartments, the water in the anode compartment is catalytically split, resulting in the accumulation of hydrogen gas in the cathode compartment and oxygen gas in the anode compartment. The anode compartment is continuously flushed with water or an electrolytic solution, thus removing the water and oxygen from the anode compartment(s) of all electrolysis cells and, consequently, from the entire stack.The oxygen is then separated from the water in a gas-water separator, and the water is returned to the anode compartment. The hydrogen produced on the cathode side is also removed along with water or an electrolyte solution and separated from the water in another glass-water separator.

[0008] The anode compartment and the cathode compartment are separated by the aforementioned membrane, which is semipermeable and, for example, in a PEM electrolyzer (proton exchange membrane), is only permeable to IT ions. R. 415138

[0009] - 2 -

[0010] The membrane is not completely gas-tight. Due to the different partial pressures of oxygen and hydrogen on either side of the membrane, small amounts of both gases migrate to the opposite side. This effect is called crossover. The passage of hydrogen to the oxygen side, i.e., from the cathode compartment to the anode compartment, occurs to a greater extent than in the opposite direction due to its smaller molecular size. Since the stack is often operated at an increased pressure on the hydrogen side, typically 30 bar (3 MPa), this significantly amplifies the crossover effect.

[0011] Crossover from hydrogen to the oxygen side has two significant disadvantages: Firstly, it leads to a loss of efficiency, as the hydrogen that enters the anode compartment can no longer be used; secondly, it increases the risk of forming an ignitable mixture of oxygen and hydrogen. It must be taken into account that hydrogen is ignitable at a concentration of just 3.8% in oxygen.

[0012] The membrane of an electrolytic cell can become more permeable to gas over extended periods, for example, due to changes in material properties across its entire surface. Increased permeability can also occur on shorter timescales due to the formation of a hole in the membrane, which enlarges over time. The membrane can also fail suddenly, for example, by tearing or the thermally induced rapid enlargement of a hole. An elevated hydrogen concentration in the oxygen, due to the flammable mixture within the stack, also poses a hazard to adjacent balance-of-plant components. Therefore, monitoring the hydrogen concentration in the oxygen is essential for the safe operation of such electrolysis systems.

[0013] In the gas-water separator, where the oxygen is separated from the water, the hydrogen concentration in the oxygen can be measured. However, this has the disadvantage that the change in concentration when a single cell fails is relatively small and only occurs after a certain time, when the entire path up to the gas-water separator is already enriched with hydrogen. R. 415138

[0014] - 3 -

[0015] Furthermore, gas-water separators are often used for multiple stacks, which further reduces the change in hydrogen concentration and also makes it impossible to determine which stack has an increased leak. As a result, a leak in a relatively large volume can create a potentially ignitable mixture, the hazard potential of which is considerable.

[0016] Advantages of the invention

[0017] The electrolysis system according to the invention for the electrolytic splitting of water has the advantage that a leak – whether sudden or gradual – at the electrolytic stack or an electrolysis cell can be detected quickly, so that countermeasures can be initiated and the formation of larger potentially flammable or explosive areas due to the mixing of hydrogen and oxygen is avoided. For this purpose, the electrolysis system comprises an electrolysis cell with two reaction chambers separated by a semipermeable barrier, one reaction chamber being connected to a drain line through which water and gas are discharged from the reaction chamber. A riser pipe branches off from the drain line, in which a gas sensor is arranged that detects the concentration of a specific gas in the riser pipe. Preferably, this system is used to detect hydrogen gas in the oxygen.

[0018] If the drain line originates from the anode compartment, for example, it contains a mixture of water and oxygen. Since the oxygen and water form a two-phase mixture, the oxygen gas rises in the riser tube along with the hydrogen, which is always present in small quantities. The riser tube can be designed to be thin, allowing only a small amount of gas to rise. A gas sensor is installed in the riser tube to detect an increase in hydrogen concentration at an early stage, thus identifying a malfunction of the electrolysis cell or stack. The gas sensor is sensitive to hydrogen; however, a separate gas sensor can also be provided to detect the ratio of hydrogen to oxygen gas, as this is crucial for the ignitability of such a mixture. R. 415138

[0019] - 4 -

[0020] The riser pipe preferably branches off from a cavity formed at the top of the drain pipe. Gas contained in the water collects in this cavity and rises from there into the riser pipe. The riser pipe is oriented essentially vertically upwards. Since the water in the drain pipe has a temperature of, for example, 70 °C due to the hot stack, the gas in the riser pipe contains a large amount of water vapor, which can lead to measurement errors if the water vapor condenses in the sensor. Therefore, the gas in the riser pipe should cool rapidly so that the water vapor condenses before it reaches the gas sensor. To accelerate this process, cooling fins can advantageously be formed on the outside of the riser pipe. The water condensed by the cooling of the gas runs down the outside of the riser pipe back into the drain pipe.To accelerate this process, a heat exchanger can also be advantageously provided in the riser pipe to quickly cool the gas to the desired temperature.

[0021] The gas, which has been sufficiently cooled by the cooling fins, the heat exchanger, or simply by the surrounding environment, still has a relative humidity of 100%. To prevent condensation in the gas sensor during further cooling, the gas flow in the riser tube can, in a further advantageous embodiment, be heated by a heating element before it enters the gas sensor to determine the hydrogen content. Alternatively or additionally, the sensor itself can also be heated.

[0022] If the gas flow, rising solely by convection from the drain pipe into the riser pipe, is insufficient for adequate gas transport, a pump element can, in a further advantageous embodiment, be arranged in the riser pipe. This is preferably located downstream of the gas sensor and generates sufficient negative pressure to maintain a gas flow of the desired magnitude. A control valve or an orifice for pressure reduction can also be arranged in the riser pipe to limit or regulate the volume flow.

[0023] To prevent a backflow into the electrolysis system in the event of ignition of the gas mixture, an R. 415138 can be installed at the riser pipe outlet.

[0024] - 5 -

[0025] A flame arrestor must be installed. It prevents a flame from penetrating into the riser pipe and further into the electrolyzer if an ignitable gas mixture escapes from the end of the riser pipe and ignites outside the riser pipe.

[0026] In a further advantageous embodiment, the riser pipe outlet is connected to a return line that directs the gas exiting the riser pipe back into the discharge line. If no excess hydrogen is detected, the oxygen gas can be routed back to the discharge line in this way so that it can be separated from the water in the gas-water separator and then used like the remaining oxygen or released into the atmosphere in a controlled manner. Alternatively, the riser pipe outlet can also lead directly into the environment to release the oxygen into the ambient air.

[0027] In a further advantageous embodiment, the diameter of the riser pipe is dimensioned such that, at the flow velocities occurring, the rising gas does not carry away any liquid droplets. The excess water should flow back into the drain pipe on the inside of the riser pipe to prevent the sensor from becoming contaminated. The amount of water carried can be determined by the pipe diameter and the pressure. This must be designed so that larger liquid droplets cannot be carried along but instead flow back down into the drain pipe.

[0028] Preferably, the reaction chambers of the electrolytic cell are an anode chamber and a cathode chamber, with the drain line removing water and oxygen from the anode chamber. This main application of the invention thus detects the hydrogen that passes through the membrane between the anode and cathode chambers into the anode chamber and mixes there with the oxygen.

[0029] In a preferred embodiment, the drain line leads into a gas-water separator, in which the gas, particularly the oxygen, is separated from the water. R. 415138

[0030] - 6 -

[0031] drawing

[0032] The drawing shows an electrolysis system according to the invention.

[0033] Fig. 1 is a schematic representation of an electrolysis system and Fig. 2 is an enlarged representation of the riser tube according to the invention.

[0034] Description of the exemplary implementations

[0035] Figure 1 schematically illustrates an electrolysis system according to the invention. The electrolysis system comprises an electrochemical stack 10 containing a plurality of electrolytic cells 1. Each cell 1 comprises an anode compartment 2 and a cathode compartment 3, with all anode compartments 2 and all cathode compartments 3 being supplied via common inlet and outlet lines. A semipermeable membrane, not shown in detail in the drawing, is arranged between each cathode compartment 3 and anode compartment 2. This membrane allows selective mass exchange for certain ions while largely blocking all other substances. To supply the cathode compartment 3, the stack 10 is connected to an inlet line 4, through which the cathode compartments 3 are supplied with water or an electrolyte solution, depending on the type of electrolyzer. The water or electrolyte solution is discharged via an outlet line 5.The electrolyte solution and the hydrogen produced during electrolysis are removed from the cathode compartments 3 and put to further use.

[0036] The anode chambers 2 are filled with water or an electrolyte solution via a feed line 8, driven by a pump 11. In so-called PEM electrolyzers, pure water is supplied, flowing into the stack 10 and thus into all anode chambers 2 via the feed line 8. The water flows through the anode chambers 2 and, together with the oxygen produced by electrolysis, flows out of the stack 10 via a drain line 9. Since there is far more oxygen in the water than is physically soluble in water, the oxygen in the drain line 9 exists as a two-phase mixture. The drain line 9 leads into a gas-water separator 12, where the oxygen gas 14 is separated from the water. The oxygen gas 14 collects in the upper part of the gas-water separator 12, while the R. 415138

[0037] - 7 -

[0038] Water 15 remains in the lower section and is pumped back into the stack 10 via the inlet line 8. This water is typically further treated by passing through filters and / or temperature control devices, which are not shown in the drawing. The oxygen gas 14 is removed from the gas-water separator 12 via an oxygen outlet 16 and either released into the ambient air in a controlled manner or used for another purpose.

[0039] A riser pipe 20 branches off from the drain pipe 9. Its structure and function are explained in more detail below with reference to Fig. 2. The riser pipe 20 serves to detect an increased hydrogen concentration in the drain pipe 9. A cavity 21 is formed at the top of the drain pipe 9, from which the riser pipe 20 extends. Since the oxygen forms a two-phase mixture with the water, the oxygen collects in the cavity 21 and thus enters the riser pipe 20. Cooling fins 24 can be provided in the riser pipe 20 to cool the gas, which is saturated with water vapor and also contains some water droplets, and to accelerate the cooling process. A heat exchanger 22 can also be provided, either additionally or alternatively, to cool the gas to the desired temperature. The water that condenses in this process runs down the wall of the riser pipe 20 back into the cavity 21 and thus into the drain pipe 9.The oxygen, containing a certain amount of hydrogen, rises in the riser tube 20 and eventually enters the gas sensor 17, which measures the hydrogen concentration. Since the oxygen remains saturated with water vapor even after the gas mixture in the riser tube 20 has cooled, and condensation of the water in the gas sensor 17 must be prevented, a heating element 23 can be provided to heat the oxygen sufficiently, if necessary, to prevent water droplets from forming in the gas sensor 17. The gas sensor 17 detects the hydrogen concentration or the ratio of hydrogen to oxygen, allowing a control unit connected to the gas sensor 17 to issue an error or warning message and initiate appropriate measures.

[0040] The gas passing through the gas sensor 17 rises further via a control valve 26 and a flame arrestor 27 to the riser pipe outlet 28. From the riser pipe outlet 28, the oxygen can be simply released into the atmosphere or via a return line 30, which is shown schematically on the right side of the drawing R. 415138.

[0041] - 8 - is indicated, and is routed back into the drain line 9. The control valve 26 serves to regulate the quantity and flow rate of the gas in the riser pipe 20. If the gas flow in the riser pipe 20 is too low, a pump element 25 can also be provided in the riser pipe 20. Should an ignitable hydrogen-oxygen mixture be present in the riser pipe 20, which ignites outside the riser pipe outlet 28, the flame arrestor 27 prevents the flames from spreading to the riser pipe 20 and to the cavity 21.

[0042] If a leak occurs in stack 10, meaning that more hydrogen than tolerable passes through the semipermeable barrier or membrane from cathode compartment 3 into anode compartment 2, the oxygen in the drain line 9 is mixed with a larger quantity of hydrogen. This gas mixture rises in the riser tube 20 and is detected in the gas sensor 17, allowing timely action to be taken; in particular, stack 10 can be shut down. Since the hydrogen is detected close to stack 10, before the majority of the gas reaches the gas-water separator 12, the total volume of oxygen mixed with hydrogen is relatively small, making it relatively easy to control such a leak.

[0043] In addition to detecting hydrogen in oxygen, which can occur in the event of a leak in the anode compartments 2, such a riser tube 20 can also be used to detect oxygen impurities in the hydrogen on the cathode side. In this case, a comparable riser tube 20 is arranged in the drain line 5 of the cathode compartments 3, where hydrogen gas mixed with some oxygen gas rises in the riser tube. A leak that leads to an increase in oxygen in the hydrogen can also be detected on the cathode side with a suitably adapted gas sensor.

Claims

R. 415138 - 9 - Claims 1. Electrolysis system for the electrolytic splitting of water, comprising an electrolysis cell (1) having two reaction chambers (2; 3) separated by a semipermeable barrier, wherein one reaction chamber (2; 3) is connected to a drain line (9) through which water and gas are discharged from the reaction chamber (2; 3), characterized in that a riser pipe 20 branches off from the drain line (9) in which a gas sensor (17) is arranged which detects the concentration of a gas in the riser pipe (20).

2. Electrolysis system according to claim 1, characterized in that the detected gas is hydrogen and the gas sensor (17) is a hydrogen sensor.

3. Electrolysis system according to claim 1 or 2, characterized in that the gas sensor (17) detects the ratio of hydrogen gas to oxygen gas.

4. Electrolysis system according to one of claims 1 to 3, characterized in that the riser pipe (20) branches off from a cavity (21) which is formed on the top of the anode drain line (9).

5. Electrolysis system according to one of claims 1 to 4, characterized in that the riser pipe (20) leads substantially vertically upwards.

6. Electrolysis system according to one of claims 1 to 5, characterized in that the riser pipe (20) has cooling fins (24) on the outside. R. 415138 - 10 - 7. Electrolysis system according to one of claims 1 to 6, characterized in that a heat exchanger (22) is arranged in the riser pipe (20) in the direction of flow upstream of the gas sensor (17).

8. Electrolysis system according to one of claims 1 to 7, characterized in that a heating element (23) is arranged in the riser pipe (20) in the direction of flow upstream of the gas sensor (17).

9. Electrolysis system according to one of claims 1 to 8, characterized in that a pump element (25) is arranged in the riser pipe (20).

10. Electrolysis system according to one of claims 1 to 9, characterized in that a control valve (26) or an orifice for pressure reduction is arranged in the riser pipe (20).

11. Electrolysis system according to one of claims 1 to 10, characterized in that a flame arrestor (27) is arranged at the riser pipe outlet (28).

12. Electrolysis system according to one of claims 1 to 11, characterized in that the outlet of the riser pipe (20) is connected to a return line (30) which directs the gas in the riser pipe (20) back into the drain line (9).

13. Electrolysis system according to one of claims 1 to 12, characterized in that the diameter of the riser pipe (20) is dimensioned such that, at the flow velocities occurring, the rising gas does not carry away any liquid droplets.

14. Electrolysis system according to one of claims 1 to 13, characterized in that an anode compartment (2) and a cathode compartment (3) form the reaction compartments and the drain line (9) carries away water and oxygen from the anode compartment (2).

15. Electrolysis system according to one of claims 1 to 14, characterized in that the discharge line (9) leads into a gas-water separator. R. 415138 - 11 - (12) discharges, in which the gas contained in the water, in particular oxygen, is separated from the water.

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