Electrolysis system comprising a pressure electrolyser, and method for operating an electrolysis system

Abstract

The invention relates to an electrolysis system (1) comprising a pressure electrolyser (3) for generating hydrogen (H2) and oxygen (O2) as product gases at a high nominal pressure (PN), with an oxygen product gas line (7) which leads from the pressure electrolyser (3) and is connected to an expansion device (9). The invention also relates to a method for operating such an electrolysis system (1), in which by means of pressure electrolysis a pressurised oxygen product gas (O2) is produced at a high nominal pressure (pN) and is then expanded. The oxygen product gas (O2) is cooled and at least partially liquefied to form liquid oxygen (LO2).

Description

Electrolysis system comprising a pressure electrolyser and method for operating an electrolysis system The invention relates to an electrolysis plant comprising a pressure electrolyzer. The invention further relates to a method of operating an electrolysis plant comprising a pressure electrolyzer. Known electrolysis plants have an electrolyzer having a multitude of electrochemical cells, called the electrolysis cells. These are used in many cases in the conversion of chemical substances to other chemical substances under the action of electricity. In general, an electrical current is used to bring about a chemical reaction, i.e. a conversion of matter. In the course of the transition toward renewable energies, the focus of development of electrolyzers is increasingly on electrochemical converters that generate hydrogen  (H2) and oxygen (O2) as product gases and low-temperature heat from electrical energy and water. It is usually the case that hydrogen gas alone is the target product, since it represents the highest value of the three products from an economic point of view and is also a preferred chemical energy carrier. In many cases, the oxygen gas is simply discharged unused into the environment. This is not appropriate for a variety of reasons. In terms of technology, hydrogen is nowadays produced, for example, by means of proton exchange membrane (PEM) electrolysis or an alkaline electrolysis in electrolysis plants. These electrolysis plants then produce hydrogen and oxygen from the water supplied with the aid of electrical energy. An electrolysis plant has a multitude of electrochemical cells arranged adjacent to one another. The water electrolysis in the electrolysis cells breaks down water to hydrogen and oxygen, for example. In the case of a PEM electrolyzer, distilled water as reactant is typically supplied on the anode side and split into hydrogen and oxygen at a proton-exchange membrane (PEM). The water is oxidized to oxygen at the anode. The protons pass through the proton-exchange membrane. Hydrogen is produced on the cathode side. The water is generally conveyed from a bottom side into the anode space and / or cathode space. For future applications, there is increasing attention on electrolysis plants that work under high or very high operating pressure in the electrolysis cell, known as pressure electrolyzers or high-pressure electrolyzers. This development poses challenging technical constraints on mechanical design and manufacture even at the level of the electrolysis cell for high-pressure operation, and also for a high-pressure electrolyzer and the reliable operation thereof from various points of view. Moreover, pressure electrolysis is a specific technique of very particular interest for large-scale industrial applications, and so there is a clear trend of development toward elevated operating pressures. For example, the water electrolyzer is the main component in what are called power-to-gas systems. One of the most important operating parameters in this context is the operating pressure of the electrolyzer. One reason for pressure operation is the demands of the application to be implemented. Secondly, elevated pressure is essential for efficient storage of hydrogen owing to its small 2024228235   05 Aug 2026 specific volume. This becomes clear on consideration of available PEM electrolyzers or alkaline electrolyzers, and also the demonstration projects implemented in the context of power-to-gas in recent years. A further benefit is the reduction in the water absorption capacity of the gas as the pressure level rises, which leads to lower energy expenditure for the drying of gas. It is therefore apparent that there have been extensive development activities with regard to electrolysis plants having pressure electrolyzers, and there is a need for operating concepts for pressure electrolyzers on an industrial scale. It is an object of this invention to substantially overcome, or at least ameliorate, one of more of the above disadvantages. A special form of electrolysis is pressure electrolysis based on PEM technology, which provides the two product gases under a high system pressure or working pressure in a pressure electrolyzer, for example at 5 to 35 bar or higher. With the product gases produced under this high pressure level, any possible utilization of oxygen is also gaining increasing interest from a technical and economic point of view. There is therefore an increasing need for future solutions in order also to separate off the oxygen from an electrolysis process at high system pressure and send it to further use. Aspects of the present disclosure provide an electrolysis plant which has a pressure electrolyzer and enables more comprehensive utilization of the product gases. A further aspect provides a method for operating such an electrolysis plant having a pressure electrolyzer. One aspect provides an electrolysis plant comprising a pressure electrolyzer for production of hydrogen(H2) and oxygen (O2) as product gases at a design pressure of at least 5 bar, having an oxygen product gas conduit which leaves the pressure electrolyzer and is connected to an expansion device, wherein the expansion device has a phase separator, such that liquid oxygen (LO2) and gaseous oxygen (O2) are spatially separable, wherein a product conduit downstream of the 2024228235   05 Aug 2026 expansion device for the separated liquid oxygen (LO2) and a return conduit which branches off from the product conduit and is connected to the oxygen product gas conduit is provided, and in which the product conduit and the oxygen product gas conduit are coupled to one another via a heat exchanger, such that preliminary cooling of the gaseous oxygen (O2) with the liquid oxygen (LO2) can be brought about An aspect provides an electrolysis plant comprising a pressure electrolyzer for generation of hydrogen and oxygen as product gases at a high design pressure, having an oxygen product gas conduit which leaves the pressure electrolyzer and is connected to an expansion device. The invention is based on the finding that, in an electrolysis plant, a high pressure level (system pressure) of the product gases generated makes it possible in a pressure electrolyzer, with a low level of energy expenditure, for the pressurized oxygen gas to cool down the gaseous phase by controlled expansion and hence at the same time to at least partly liquefy and utilize it. Therefore, an expansion device is advantageously connected downstream of the pressure electrolyzer on the oxygen side. Ideally, the expansion process possible thereby in operation is characterized by isentropic expansion. Estimates have shown that this alone can result in liquefaction of about 50% of the oxygen, which opens up numerous benefits and fields of application. Liquid oxygen in turn has a much higher commercial and technical value than gaseous oxygen; it can be logistically handled much more easily than high-pressure gaseous oxygen. The latter still requires inconvenient and costly compression. The oxygen from pressure electrolysis is already of very high quality and purity, since it contains no impurities, and at worst still contains small traces of water or hydrogen. The liquid oxygen from the electrolysis plant can therefore be used for high-quality and demanding applications, for example for medical purposes, or for other oxygen-consuming processes, for instance fish farming. This is particularly valuable in particular in remote areas, such as islands, since the transport of oxygen is very energy-intensive owing to its weight. The market price for liquid oxygen (LO2) is about €1 / l. The potential revenue of a 1.25 MW pressure electrolyzer in an electrolysis plant, at 8000 h / a, is therefore estimated at about €500 000 per annum. The hydrogen produced sells at about €5 per kg, thus earning about €850 000 per annum. Liquid oxygen can thus generate relevant additional revenue and significantly improve the economic viability of power-togas plants. Possible customers are available in applications in the refinery sector, where problematic residues are often burnt under a pure oxygen atmosphere. The synthesis gas produced can be integrated in further chemical process steps. This increases the efficiency of the refinery. Especially in applications of power-to-gas technologies, the liquid oxygen thus provided can be used efficiently for thermal conversion of the biomass to synthesis gas, since it is easier and more cost-efficient to transport than pressurized oxygen. Users of industrial gases in relatively large volumes are also customers for liquid oxygen. These include, for example, metal processing companies, the glass industry, welding plants, metal recycling companies and many others. This oxygen from electrolyzers, powered by renewable energies, also has no adverse CO2 footprint by comparison with air fractionation plants with conventional power supply. Expanded and cryogenic oxygen, shortly before liquefaction or already in partly liquefied form, can also be used to cool and reduce the water load of the H2 product stream, i.e. assume the function of a cold trap. In a particularly advantageous configuration of the electrolysis plant, an expansion device is designed as a multistage expansion device, which accordingly has a plurality of expansion stages connected in series. As a result, a gradual and iterative means of cooling and liquefaction of the oxygen product gas from the pressure electrolyzer is implemented in the system concept of the electrolysis plant. The expansion device may thus be of single-stage or multistage design. The specific choice and configuration is thus flexibly adaptable to the high pressure level of the design pressure in the pressure electrolyzer, with preferable design pressures of up to 80 bar, typically about 35 bar. The pressure electrolyzer is advantageously designed for a mode of operation at a low pressure differential across the electrolysis cell, i.e. in particular for operation at constant pressure. In the electrolysis plant, the expansion device preferably includes an expansion turbine. An expander, also called a turboexpander or a gas expansion turbine, belongs to the family of turbines or jet engines, in which a pressurized gas expands and performs work as it does so. In contrast to a gas turbine in a broader sense, an expander consists solely of the actual turbine, i.e. does not have a compressor or a combustion chamber as an integral part of the machine. The gas to be expanded - i.e. pressurized oxygen in this case - is thus not generated by the machine itself, e.g. by a combustion process, but is already produced in the present context from the upstream electrolysis process in the pressure electrolyzer at a high design pressure. The expansion turbine may be of single-stage or multistage, and axial or radial design. The work performed can be advantageously used, if required, to drive a pump, a compressor or a generator in the electrolysis plant. In a further-preferred configuration of the electrolysis plant, the expansion device has a throttle valve. In contrast to pressure reduction by means of a simple throttle, the expander can also utilize the energy of the expanding gas, as set out above. Combinations of expansion turbine and throttle valve are also possible in the expansion device, such that a multistage expansion device can advantageously be connected in the electrolysis plant to the oxygen product gas conduit leaving the pressure electrolyzer. In a particularly preferred configuration of the electrolysis plant, the expansion device has a phase separator, such that liquid oxygen and gaseous oxygen are spatially separable. Thus, effective spatial separation of liquid phase and gaseous phase is already ensured within the expansion device. In general, during the - ideally adiabatic - expansion of the pressurized oxygen product gas in the expansion device, distinct cooling of the oxygen is brought about, which also brings about condensation, such that the oxygen is at least partly liquefied and correspondingly condensed out. There is thus a phase mixture which can be at least partly separated by the phase separator, such that high-quality liquid oxygen is separable and obtainable as product. In the simplest case, the phase separator can be designed on the basis of a gravimetric principle, in that the liquid phase simply condenses out and, because of the differences in density, the liquid oxygen runs off via a collecting device and can be sent to further treatment or utilization. Preferably, in the electrolysis plant, the expansion device is followed downstream by a product conduit for the separated liquid oxygen, and a return conduit that branches off from the product conduit and is connected to the oxygen product gas conduit. At a branch point directly downstream of the expansion device in flow direction of the fluid, a separation of valuable liquid oxygen resulting from the phase separation is provided via the product conduit. The valuable liquid oxygen is then available for further purposes - at first still within the plant. Via the return conduit, for cooling purposes, cold oxygen gas, or optionally a phase mixture of liquid and gaseous oxygen from the product gas conduit, can be supplied via a valve, for example, such that very effective preliminary cooling of the oxygen from the pressure electrolyzer can be brought about in the product gas conduit. This preliminary cooling can in principle be achieved via recycling of oxygen gas via the return conduit - after an initially employed compression - to at least the design pressure, and introduction into the product gas conduit, which brings about mixing. A simpler solution for preliminary cooling compared to admixing is achieved by coupling via a heat exchanger. In a particularly preferred configuration, the preliminary cooling of the oxygen product gas in the product gas conduit can also be brought about by means of a heat exchanger. For this purpose, the return conduit is preferably coupled via a heat exchanger to the oxygen product gas conduit, such that a heat exchange can already bring about cooling of the gaseous oxygen. This is an alternative or additional measure compared to an injection. The oxygen heated thereby in the heat exchanger is returned to the process and, if necessary, processed beforehand. In a particularly preferred configuration of the electrolysis plant, the product conduit and the oxygen product gas conduit are coupled to one another via a heat exchanger, such that preliminary cooling of the gaseous oxygen (O2) with liquid oxygen (LO2) can be brought about. The preliminary cooling of the oxygen can advantageously achieve a reduction in temperature and higher density of the oxygen, and hence a more favorable operating point for the subsequent expansion in the expansion device. This is accompanied by a higher yield of liquid oxygen because of the Joule-Thomson effect owing to the isenthalpic reduction in pressure. The Joule-Thomson effect defines the change in temperature of a gas in the event of an isenthalpic reduction in pressure. The direction and strength of the effect is determined by the strength of the attractive and repulsive forces between the gas molecules. Under normal conditions, most gases and gas mixtures, for example air, are subject to a drop in temperature on expansion. By contrast, there is an increase, for example, in the case of hydrogen, helium, neon. In an ideal gas there are no molecular forces, as a result of which it does not show a Joule-Thomson effect. Preferably, in the electrolysis plant, a compressor has been inserted into the oxygen product gas conduit, with which preliminary compression of the oxygen product gas beyond the design pressure can be achieved. It has been found that an advantageous thermodynamic state of the oxygen product gas can be established by a preliminary compression in the preliminary compressor, in particular in association with a preliminary cooling or intermediate cooling described above. This has a particularly favorable effect on the downstream expansion process in the expander, where the yield of liquid oxygen can be increased to up to 50% as a result of isentropic expansion, i.e. 50% of the originally gaseous pressurized oxygen gas is liquefied in one expansion step. In this case, the return conduit allows the process of compression and intermediate cooling with subsequent isentropic expansion in the expander to be run iteratively or cyclically several times, such that the overall yield of liquefied oxygen gas is increased further for a given gas volume - ideally by a further 50% per cycle in relation to a specific input volume flow rate. In the electrolysis plant, the pressure electrolyzer is preferably configured as a PEM electrolyzer, where the PEM electrolyzer has a proton-permeable membrane and is designed for a high design pressure of 5 to 50 bar, in particular of 35 bar. For example, a typical operating pressure may also be a design pressure in the range of 20 to 25 bar. The pressure electrolyzer generally comprises a multitude of electrolysis cells arranged adjacent to one another. The water electrolysis in the electrolysis cells breaks down water to hydrogen and oxygen. In the case of a PEM electrolyzer, distilled or demineralized water as reactant is typically supplied on the anode side and split into hydrogen and oxygen at a proton-exchange membrane (PEM). The water is oxidized to oxygen at the anode. The protons pass through the protonexchange membrane. Hydrogen is produced on the cathode side. The water is generally conveyed from a bottom side into the anode space and / or cathode space. In the case of alkaline electrolysis too as a possible alternative method, a kind of membrane is provided, in the form of a semipermeable membrane or diaphragm which selectively allows passage of certain ions. The electrolyte used here is potassium hydroxide solution (KOH) with a concentration of typically 20%-40%. The gastight membrane, called the diaphragm, does allow the transport of OH-ions, but simultaneously prevents mixing of the product gases formed. A further aspect of the invention relates to a method of operating a corresponding electrolysis plant with a pressure electrolyzer. In the method, it is envisaged in accordance with the invention that a pressurized oxygen product gas is generated at a high design pressure by a pressure electrolysis and then expanded, wherein the oxygen product gas is cooled and at least partly liquefied to liquid oxygen. The expansion is preferably preceded by preconditioning of the oxygen product gas taken from the pressure electrolyzer at the design pressure, for instance drying of the oxygen product gas, removal of residual gas contamination and preliminary cooling. In a particularly preferred execution of the method, the pressurized oxygen product gas from the pressure electrolysis, prior to expansion, is first subjected to a gas conditioning operation wherein hydrogen as extrinsic gas constituent in the oxygen product gas and water are removed. In spite of the comparatively high gastightness of the ion-permeable membrane, for example the ionomer of the protonconducting membrane in a PEM electrolysis, permeation of oxygen, from the anode to the cathode, and of hydrogen, from the cathode to the anode, takes place in operation. One reason for this is that complete gas impermeability of the ionomer cannot be achieved. Secondly, the membrane itself absorbs water through direct contact with water. The extrinsic gases that occur as a result of permeation cause unwanted side reactions which can reduce the efficiency of water electrolysis and possibly damage the membrane. Therefore, in practice, small amounts of hydrogen are present in the oxygen gas stream and small amounts of oxygen are present in the hydrogen gas stream. The quantity of the respective extrinsic gas depends on the electrolysis cell design and also varies under the influence of current density, catalyst composition, aging and on the membrane material of the electrolysis cell as well. It is inherent to the system that the gas stream of one product gas contains the other product gas in very small amounts. Later on in the process, even small traces of hydrogen are preferably removed from the oxygen in downstream steps of gas purification, especially when a particularly high product gas quality is required. It may be necessary under some circumstances to reduce the extrinsic gas concentration, and to do so immediately adjacent to or directly downstream of the electrolysis cell or the electrolysis stack, for example in gas separators downstream of the electrolyzer. The gas purification can be conducted, for instance, in a recombination device with a catalytically active zone, such that extrinsic gas residues of hydrogen in the oxygen product gas react by reaction with the oxygen to give water. In a preferred embodiment of the method, pressurized oxygen product gas is subjected to heat exchange with liquid oxygen prior to expansion, and so preliminary cooling of the oxygen product gas is brought about. This is a particularly efficient measure for achieving preliminary cooling, since the liquid oxygen is at a low temperature level of less than -183°C and provides a preferred heat sink such that the oxygen product gas stream is cooled. In order to make the liquefaction process in the process even more efficient, the pressurized oxygen product gas is preferably compressed from the design pressure to a supply pressure. This preliminary compression ensures the provision of a correspondingly high input pressure before the isentropic expansion step, which is set well above the design pressure of the pressure electrolyzer, for example above 100 to 150 bar. This is accompanied by efficiency gains in terms of the yield of liquid oxygen after the expansion step. The oxygen product gas compressed to a supply pressure is preferably intermediately cooled. The combination of the process steps of preliminary compression and intermediate cooling can optionally also be executed repeatedly. It is advantageous that the process of compression and intermediate cooling with subsequent isentropic expansion in the expander is repeatedly performed iteratively or cyclically. The overall yield of liquefied oxygen gas increases for a given gas volume - ideally by a further 50% per cycle in relation to a specific input volume flow rate. Further benefits of the operating method of the invention will correspondingly be apparent from the stated benefits of the electrolysis plant. The invention is elucidated in detail hereinafter by the appended drawings. It should be noted that the working examples illustrated in the drawings serve primarily to elucidate the invention. However, they are not intended to restrict the invention. These show, in schematic and highly simplified form: FIG 1     an electrolysis plant with a pressure electrolyzer and an oxygen liquefaction according to the invention; FIG 2     an electrolysis plant with pressure electrolyzer and oxygen liquefaction with further system components. Identical reference symbols have the same meaning in the figures. FIG 1 shows an electrolysis plant 1 in a greatly simplified detail of plant parts and components. The electrolysis plant 1 has a pressure electrolyzer 3 which is optionally designed as a PEM electrolyzer or an alkali electrolyzer and is designed for a high design pressure pN of at least 5 bar as the working pressure. Higher design pressures pN of 35 bar or higher are therefore likewise possible - depending on the design concept and the requirements of the electrolysis plant 1 in use -within the scope of the execution of the invention. The electrolyzer 3 comprises a cathode chamber 31 and an anode chamber 29, separated by an ion-permeable membrane 27. The anode space 29 and the cathode space 31 are each composed of and formed from a multitude of respective anodic or cathodic half cells (not shown in detail in FIG 1) that are stacked in an axial direction. The cathodic half-cells and the anodic half-cells are assembled to form respective electrolysis cells and separated in each case by an ion-conducting membrane 27. FIG 1 shows a pressure electrolyzer 3 designed for electrochemical splitting of water H2O or an electrolyte as reactant to hydrogen H2 and oxygen O2 as product gases by means of electric current. The respective reactant is supplied via the reactant supply conduit 33 to the pressure electrolyzer 3. In the case of acidic electrolysis, demineralised water H2O is used as the reactant. In the case of alkaline electrolysis, an alkali is used as reactant, for example potassium hydroxide KOH in an aqueous solution with a concentration typically of 20% to 40%. Several such electrolysis cells may be connected in series in horizontally stacked “electrolysis stacks”. In the working example shown in FIG 1, the pressure electrolyzer 3 in the electrolysis plant 1 is designed by way of example as a PEM electrolyzer for high design pressures pN of at least 5 bar; in the present context the design pressure is typically pN = 35 bar. Each electrolysis cell in PEM electrolysis has a proton-permeable membrane 5 based on a fluoropolymer, adjoined by a respective electrode - an anode and a cathode - on each side, via which an external DC voltage is applied in operation. On the anode side, a reactant supply conduit 33 for supplying water H2O is connected to the anode space 29. As a result, a water circuit is implemented in the pressure electrolyzer 3, configured as a PEM electrolyzer. It is also possible that water H2O is fed not only through the anode space 29 but also through the cathode space 31, such that two circuits are implemented. In the present context, a pressure electrolyzer 3 is shown, which is implemented with only an anode-side circuit. In operation of the electrolysis plant 1, the oxygen O2 generated from the anode space 7 in the electrolysis cell is discharged via an oxygen product gas conduit 7. On the cathode side, a hydrogen product gas conduit 25 is provided for discharging the hydrogen H2 generated from a cathode space 31. The product gases are generated at a high design pressure pN and discharged from the pressure electrolyzer 3 via the respective product gas conduit 7, 25 and provided for further purposes. In order to reduce pressure differential stress across the membrane 27 at the high design pressure pN, the pressure differential between the anode space 29 and the cathode space 31 is set at a very low level, for example between 0 mbar and 100 mbar. The mode of operation is therefore ideally constant pressure operation. For recovery of the oxygen O2 under the high design pressure from the pressure electrolysis, the oxygen product gas conduit 7 is connected to an expansion device 9. The expansion device 9 has an expansion turbine 11 with an electric generator 37 and a throttle valve 13 connected downstream of the expansion turbine 11. With the generator 37, the mechanical energy resulting from the expansion of the oxygen product gas O2 in the expansion turbine 11 is utilizable. In addition, a phase separator 5 for spatial separation of liquid and gaseous phase in the partly liquefied oxygen O2 is functionally integrated into the expansion device 9. The expansion turbine 9 may be of multistage design. A product conduit 15 is led out of the phase separator 5 for conveying of the liquid oxygen LO2 obtained in the isentropic expansion process and separated out in the phase separator 5. In addition, a return conduit 17 is conducted out of the phase separator 5 and opens into the product gas conduit 7. A control valve 35 is connected to the return conduit 17. This makes it possible to meter recirculated cold gaseous oxygen O2, or else in a phase mixture of liquid and gaseous oxygen O2 if required, into the oxygen product gas conduit 7 for preliminary cooling and to set a preliminary cooling temperature via the amount fed in. The mixture of the streams of matter involved at different temperature levels leads to preliminary cooling of the oxygen product gas O2 in the product gas conduit 7. It is also possible that only a portion of the partly liquefied oxygen O2, i.e. in a phase mixture in the present context, is returned to the product gas conduit 7 via the return conduit 17 for operational reasons. The other portion is discarded or discharged. The product gas conduit 7 and the product conduit 15 are thermally coupled via a heat exchanger 21. The heat exchanger 21 is designed as a gaseous-liquid heat exchanger 21. This implements heat exchange, and further cooling of the oxygen product gas O2 by the very cold liquid oxygen LO2 conducted in the product conduit 15 in operation. A removal conduit 39 finally leads out of the heat exchanger 21, such that the valuable and very pure liquid oxygen LO2 from the pressure electrolyzer 3 is available for further purposes and applications. For example, the liquefied oxygen LO2 can be collected in thermally highly insulated vessels and transported for use. In operation of the electrolysis plant thus equipped, the pressurized oxygen product gas O2 is generated at a high design pressure  pN  by pressure electrolysis in the pressure electrolyzer 3 and then expanded in the expansion device 9. This cools the oxygen product gas O2 and at least partly liquefies it to liquid oxygen LO2. Prior to expansion, the pressurized oxygen product gas O2 here is subjected to heat exchange with liquid oxygen LO2 by means of the heat exchanger 21. This already brings about preliminary cooling of the oxygen product gas O2. A further electrolysis plant 1 with pressure electrolyzer 3 and an integrated oxygen liquefaction for the oxygen product gas O2 is shown in FIG 2. In comparison to the working example of FIG 1, further functional components here are integrated into the electrolysis plant 1 or advantageous modifications are made. For instance, in the electrolysis plant 1, a compressor 23 is connected to the oxygen product gas conduit 7 and is of two-stage design. This means that the oxygen product gas O2 from the pressure electrolyzer 3 can be brought well above the design pressure pN to a higher compression, for instance more than 100 bar. In the product gas conduit 7, a gas conditioning device 41 is connected immediately upstream of the pressure electrolyzer 3 and the compressor 23 in the process. This serves for gas cleaning and gas drying of the oxygen product gas O2. A recombiner with a catalyst is also integrated into the gas conditioning device 41. In this way, undesirable extrinsic gas constituents of hydrogen H2 in the oxygen product gas O2 can react to form water by a catalytic reaction, and the water H2O formed in this way can be discharged from the catalytic reactor via a drainage conduit 45 and the reactor can be emptied thereby. In flow direction of the oxygen product gas O2, a heat exchanger 19 is connected downstream of the compressor 23 in the product gas conduit. The gas conditioning device 41 with the catalytic reactor can alternatively also be connected downstream of the compressor 23 and installed in the heat exchanger 19 upstream of the cooling. This has considerable efficiency benefits for the recombiner because of the higher temperature level provided in the compressed oxygen product gas O2. This is not shown in detail in FIG 2. A return conduit 17 is connected to the heat exchanger 19 by a control valve 35, such that cooling of the compressed and correspondingly heated oxygen product gas O2 in the compressor 23 is performable . This is done by metered recycling of cold oxygen via the return conduit 17 into the heat exchanger 19. Before entering the heat exchanger 19, the oxygen product gas O2 that has been discharged from the compressor 23 and correspondingly compressed can be subjected to intermediate cooling such that a desired low gas temperature of the compressed oxygen product gas O2 is already provided before entry into the heat exchanger 19. For this purpose, a further heat exchanger (not shown in detail) in the product gas conduit 7 should be provided as an intermediate cooler downstream of the exit from the compressor 23, which is operable conventionally with air or water as coolant. In operation, an additional preliminary cooling or intermediate cooling of the compressed oxygen product gas O2 may therefore precede the expansion of the pressurized oxygen product gas O2 in the expansion device 9. In contrast to the configuration in FIG 1, a heat exchanger 19 is provided here in FIG 2 as an alternative configuration for direct feeding of cold and additionally compressed gaseous oxygen O2 or a phase mixture of liquid oxygen LO2 and gaseous oxygen O2. The expansion device 9 is configured in FIG 2 as a multistage expansion device 9, wherein a first expansion turbine 11A and a second expansion turbine 11B are connected in series. An intermediate cooler 23 is provided between the first expansion turbine 11A and the second expansion turbine 11B, and so further cooling of the oxygen product gases O2 can be achieved, and efficient liquefaction with maximum yield of liquid oxygen LO2 at phase separator 5 in a process cycle. Multiple intermediate cooling operations and further expansion stages are also possible. In addition, the expansion device 9 has a throttle valve 13 in order to promote phase separation by a further isentropic expansion and to increase the yield. In an analogous method regime to that described in FIG 1, the expansion of the pressurized and finished oxygen product gas O2 is preceded by heat exchange via the heat exchanger 21 with liquid oxygen LO2, which brings about further preliminary cooling of the oxygen product gas O2. The electrolysis plant 1 with the pressure electrolyzer 3 can produce oxygen LO2 as a high-quality and highly pure “cryogas” in a manner which is efficient and of economic interest. The cryogas in the present context is a compressed O2 gas with very high density and high quality, which is obtained either in liquid form LO2 or in highly compressed gaseous form as what is called GO2. Numerous recycling options are thus opened up, and the oxygen O2 - in addition to the hydrogen H2 - also now becomes commercially available as a valuable product of a pressure electrolysis.

Claims

1. An electrolysis plant comprising a pressure electrolyzer for production of hydrogen(H2) and oxygen (O2) as product gases at a designpressure of at least 5 bar, having an oxygen product gas conduit whichleaves the pressure electrolyzer and is connected to an expansiondevice, wherein the expansion device has a phase separator, such thatliquid oxygen (LO2) and gaseous oxygen (O2) are spatially separable, wherein a product conduit downstream of the expansion device for theseparated liquid oxygen (LO2) and a return conduit which branches off from the product conduit and is connected to the oxygen product gasconduit is provided, and in which the product conduit and the oxygenproduct gas conduit are coupled to one another via a heat exchanger, such that preliminary cooling of the gaseous oxygen (O2) with the liquid oxygen (LO2) can be brought about.

2. The electrolysis plant as claimed in claim 1, wherein the expansion device is designed as a multistage expansion device having a multitude of expansion stages connected in series.

3. The electrolysis plant as claimed in claim 1 or 2, wherein theexpansion device includes an expansion turbine.

4. The electrolysis plant as claimed in any one of the precedingclaims, wherein the expansion device includes a throttle valve.

5. The electrolysis plant as claimed in any one of claims 1 to 4, inwhich a compressor has been inserted into the oxygen product gasconduit, and downstream of this in flow direction of the oxygen product gas (O2) is a heat exchanger, and wherein a return conduit with a control valve is provided and is connected to the heat exchanger, such that cooling of the oxygen product gas (O2) that hasbeen compressed and correspondingly heated in the compressor is performable in a heat exchange.

6. The electrolysis plant as claimed in any one of the precedingclaims, having a compressor connected to the oxygen product gas2024228235   05 Aug 2026conduit for precompression of the oxygen product gas (O2) beyond thedesign pressure.

7. The electrolysis plant as claimed in any one of the preceding claims, having a pressure electrolyzer which is configured as a PEM electrolyzer, has a proton-permeable membrane and is designed for a high design pressure (pN) of 5 to 50 bar, in particular of 35 bar.

8. A method of operating an electrolysis plant as claimed inany one of the preceding claims, wherein a pressurized oxygen product gas (O2) is generated at a design pressure of at least 5    bar bya pressure electrolysis and then expanded, wherein the oxygen product gas (O2) is cooled and at least partly liquefied to liquid oxygen (LO2), wherein the expansion is preceded by heat exchange of pressurized oxygen product gas (O2) with liquid oxygen  (LO2), whichbrings about preliminary cooling of the oxygen product gas (O2).

9. The method as claimed in claim 8, product gas (O2) is compressed from the pressure.wherein the pressurized oxygendesign pressure (PN) to a supply10. The method as claimed in claim 9, in which the oxygen productgas (O2) compressed to a supply pressure is intermediately cooled.

11. The method as claimed in any of claims 8 to 10, wherein the pressurized oxygen product gas (O2) from the pressure electrolysis, prior to expansion, is first subjected to a gas conditioning operation in which hydrogen (H2) as extrinsic gas constituent in the oxygen product gas (O2) and water (H2O) are removed.

Images