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

[0001] Description

[0002] Electrolysis plant with a pressure electrolyzer and method for operating an electrolysis plant

[0003] The invention relates to an electrolysis plant with a pressure electrolyzer. The invention also relates to a method for operating an electrolysis plant with a pressure electrolyzer.

[0004] Known electrolysis systems feature an electrolyzer with a multitude of electrochemical cells, the so-called electrolysis cells. These are often used to convert chemical substances into other chemical substances under the influence of electricity. Typically, a chemical reaction, i.e., a substance transformation, is induced with the help of an electric current.

[0005] In the course of the energy transition, development is increasingly focusing on electrochemical converters such as electrolyzers that generate hydrogen (H2) and oxygen (O2) as product gases, as well as low-temperature heat, from electrical energy and water. Hydrogen gas is usually the only target product, as it represents the highest economic value of the three products and is also a preferred chemical energy carrier. In many cases, the oxygen gas is simply released unused into the environment. This is impractical for many reasons.

[0006] Technologically, hydrogen, for example, is now produced using proton exchange membrane (PEM) electrolysis or alkaline electrolysis in electrolysis plants. These electrolysis plants then use electrical energy to produce hydrogen and oxygen from the supplied water. An electrolysis plant has a large number of electrochemical cells arranged next to one another. By means of water electrolysis, for example, water is split into hydrogen and oxygen in the electrolysis cells. In a PEM electrolyzer, distilled water is typically added as a reactant on the anode side and split into hydrogen and oxygen at a proton-permeable membrane (PEM). The water is oxidized to oxygen at the anode. The protons pass through the proton-permeable membrane. Hydrogen is produced on the cathode side.The water is usually pumped from the bottom into the anode chamber and / or cathode chamber.

[0007] For future applications, electrolysis systems that operate at high or very high operating pressure in the electrolysis cell, so-called pressure electrolyzers or high-pressure electrolyzers, are increasingly being considered. This development places challenging technical constraints on the mechanical design and manufacturing already at the electrolysis cell level for high-pressure operation, as well as on a high-pressure electrolyzer and its safe operation from various perspectives.

[0008] On the other hand, pressure electrolysis is of particular interest for large-scale industrial applications, and a development trend towards increased operating pressures is therefore clearly evident. For example, the water electrolyzer is the main component in so-called power-to-gas plants. One of the most important operating parameters in this context is the operating pressure of the electrolyzer. Pressurized operation must be justified, on the one hand, by the requirements of the application to be served. On the other hand, an increase in pressure is essential for efficient hydrogen storage due to the small specific volume. This becomes clear when considering available PEM electrolyzers or alkaline electrolyzers, as well as the demonstration projects carried out in recent years in the context of power-to-gas.A further advantage is the reduced water absorption capacity of the gas with increasing pressure level, which leads to lower costs for gas drying. Therefore, extensive development activities are underway regarding electrolysis plants with pressure electrolyzers, and there is a need for operating concepts for pressure electrolyzers on an industrial scale.

[0009] A special form of electrolysis is pressure electrolysis based on PEM technology, which provides the two product gases under a high system or working pressure in a pressure electrolyzer, for example at 5 to 35 bar and above. From a technical and economic perspective, the potential for oxygen utilization in the product gases generated at this high pressure level is also gaining increasing interest. There is therefore a growing need for future solutions that also separate the oxygen from an electrolysis process at high system pressure and feed it for further use.

[0010] In light of this need, the object of the invention is to provide an electrolysis plant with a pressure electrolyzer that enables more comprehensive utilization of the product gases. A further object is a method for operating such an electrolysis plant with a pressure electrolyzer.

[0011] The object directed to an electrolysis plant is achieved according to the invention by an electrolysis plant comprising a pressure electrolyzer for producing hydrogen and oxygen as product gases at a high nominal pressure, with an oxygen product gas line leading from the pressure electrolyzer and connected to an expansion device.

[0012] The invention is based on the knowledge that in an electrolysis plant, a high pressure stage (system pressure) of the product gases generated in a pressure electrolyzer makes it possible to cool the pressurised oxygen gas in the gaseous phase with little effort through targeted expansion and thus to at least partially liquefy and use it at the same time. For this reason, an expansion device is advantageously connected downstream of the pressure electrolyzer on the oxygen side. Ideally, the expansion process made possible during operation is characterized by isentropic expansion. Estimates have shown that this alone can liquefy around 50% of the oxygen, which opens up numerous advantages and fields of application.

[0013] Liquid oxygen, on the other hand, has significantly higher commercial and technical value than gaseous oxygen; it is logistically much easier to handle than gaseous high-pressure oxygen. The latter still requires complex and expensive compression. The oxygen from pressure electrolysis is already of very high quality and purity, as it contains no impurities, except for small traces of water or hydrogen. The liquid oxygen from the electrolysis plant can therefore be used for high-quality and demanding applications, such as medical purposes, or for other oxygen-consuming processes, such as fish farming. This is particularly valuable in remote areas, such as islands, where transporting oxygen is very energy-intensive due to its weight. The market price for liquid oxygen (LO2) is approximately 1G / l.The estimated revenue potential of a 1.25 MW pressure electrolyzer in an electrolysis plant operating at 8,000 hours per year is approximately €500,000 per year. The hydrogen produced sells for approximately €5 per kg, thus generating approximately €850,000 per year.

[0014] Liquid oxygen can thus generate significant additional revenue and significantly improve the economic viability of power-to-gas plants. Purchasing opportunities exist for applications in refineries, where problematic residues are often burned in a pure oxygen atmosphere. The resulting synthesis gas can be integrated into further chemical process steps, thus increasing the refinery's efficiency.

[0015] Especially in power-to-gas applications, the liquid oxygen produced in this way can be used efficiently for the thermal conversion of biomass to synthesis gas, as it is easier and more cost-effective to transport than pressurized oxygen. Users of industrial gases in larger quantities are also customers for liquid oxygen. These include, for example, metalworking companies, the glass industry, welding shops, metal recycling companies, and many others. Furthermore, this oxygen from electrolyzers, powered by renewable energy, has no adverse carbon footprint, unlike air separation plants powered by conventional electricity.

[0016] Expanded and cryogenic oxygen shortly before or already partially liquefied can also be used to cool and reduce the water load of the H2 product stream, thus taking on the function of a cold trap.

[0017] In a particularly advantageous embodiment of the electrolysis plant, an expansion device is designed as a multi-stage expansion device which accordingly has a plurality of expansion stages connected in series.

[0018] As a result, a successive and iterative option for cooling and liquefying the oxygen product gas from the pressure electrolyzer is implemented in the system concept of the electrolysis plant. The expansion device can therefore be designed in one or more stages. The specific selection and design can therefore be flexibly adapted to the high pressure level of the nominal pressure in the pressure electrolyzer, with nominal pressures of up to 80 bar, typically around 35 bar, being preferable in the design. The pressure electrolyzer is advantageously designed for operation with a low differential pressure across the electrolysis cell, i.e. in particular for constant pressure operation. The expansion device in the electrolysis plant preferably has an expansion turbine.

[0019] An expander, also called a turbo expander, gas expansion turbine or gas relaxation turbine, belongs to the family of turbines or fluid machines in which a pressurised gas expands and in the process performs work. In contrast to a gas turbine in the broader sense, an expander only consists of the actual turbine and does not have a compressor or combustion chamber as an integral part of the machine. The gas to be expanded - in this case pressurised oxygen - is therefore not produced by the machine itself, e.g. through a combustion process, but in this case is already produced from the upstream electrolysis process in the pressure electrolyzer at a high nominal pressure.

[0020] The expansion turbine can be single-stage or multi-stage, axial or radial. The dissipated energy can be advantageously used to drive a pump, compressor, or generator in the electrolysis plant, if required.

[0021] In a further preferred embodiment of the electrolysis plant, the expansion device has a throttle valve.

[0022] In contrast to pressure reduction by a simple throttle, the expander also allows the energy of the expanding gas to be utilized, as explained above. Combinations of expansion turbine and throttle valve are also possible in the expansion device, so that a multi-stage expansion device can advantageously be connected to the oxygen product gas line leading from the pressure electrolyzer in the electrolysis plant.

[0023] In a particularly preferred embodiment of the electrolysis plant, the expansion device has a phase separator so that liquid oxygen and gaseous oxygen can be spatially separated. This ensures effective spatial separation of the liquid and gaseous phases within the expansion device. In general, the ideally adiabatic expansion of the pressurised oxygen product gas in the expansion device causes the oxygen to cool significantly, which also brings about condensation, so that the oxygen is at least partially liquefied and correspondingly condenses out. This results in a phase mixture which can be at least partially separated by the phase separator so that high-quality liquid oxygen can be separated and obtained as a product.In the simplest case, the phase separator can be designed on the basis of a gravimetric principle, whereby the liquid phase simply condenses out and, due to the density differences, the liquid oxygen flows through a collection device and can be fed for further treatment or recycling.

[0024] Preferably, in the electrolysis plant, a product line for the separated liquid oxygen is connected downstream of the expansion device, as well as a return line branching off from the product line and connected to the oxygen product gas line.

[0025] At a branch point immediately downstream of the expansion device in the direction of flow of the fluid, a separation of valuable liquid oxygen resulting from the phase separation is provided via the product line. The valuable liquid oxygen is then available for further purposes - initially still within the plant. For cooling purposes, cold oxygen gas or, if necessary, a phase mixture of liquid and gaseous oxygen can be fed from the product gas line via the return line, for example via a valve, so that a very effective pre-cooling of the oxygen from the pressure electrolyzer can be brought about in the product gas line. This pre-cooling can in principle be achieved by returning oxygen gas via the return line - after an initial compression - to at least the nominal pressure and introducing it into the product gas line, whereby a mixture is brought about.A simpler solution for pre-cooling than admixing is achieved by coupling via a heat exchanger.

[0026] In a particularly preferred embodiment, the pre-cooling of the oxygen product gas in the product gas line can also be achieved via a heat exchanger. For this purpose, the return line is preferably coupled to the oxygen product gas line via a heat exchanger, so that cooling of the gaseous oxygen can be achieved in a single heat exchange. This represents an alternative or additional measure compared to injection. The oxygen heated in the heat exchanger is fed back into the process and, if necessary, previously treated.

[0027] In a particularly preferred embodiment of the electrolysis plant, the product line and the oxygen-product gas line are coupled to one another via a heat exchanger (21), so that pre-cooling of the gaseous oxygen (O2) with liquid oxygen (LO2) can be effected.

[0028] By pre-cooling the oxygen, a reduction in temperature and a higher density of the oxygen can be achieved, thus creating a more favorable operating point for the subsequent expansion in the expansion device. This is accompanied by a higher yield of liquid oxygen due to the Joule-Thomson effect caused by the isenthalpic reduction in pressure. The Joule-Thomson effect describes the temperature change of a gas when the pressure is reduced isenthalpicly. 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, the temperature of most gases and gas mixtures, e.g. air, drops when the pressure is expanded. In contrast, it rises for substances such as hydrogen, helium and neon. In an ideal gas there are no molecular forces, and as a result it does not exhibit the Joule-Thomson effect.

[0029] Preferably, in the electrolysis plant, a compressor is connected to the oxygen product gas line, with which a pre-compression of the oxygen product gas can be effected above the nominal pressure.

[0030] It has been shown that an advantageous thermodynamic state of the oxygen product gas can be achieved by means of pre-compression in the pre-compressor, particularly in conjunction with pre-cooling or intermediate cooling as described above. This has a particularly beneficial effect on the downstream expansion process in the expander, whereby the yield of liquid oxygen can be increased by up to 50% as a result of the isentropic expansion, i.e. 50% of the originally gaseous, pressurised oxygen gas is liquefied in one expansion step. The provision is made for the process of compression and intermediate cooling with subsequent isentropic expansion in the expander to be repeated iteratively or repeatedly via the return line.can be cycled through so that the total yield of liquefied oxygen gas increases further for a given amount of gas - ideally by a further 50% per cycle in relation to a specific input volume flow.

[0031] In the electrolysis plant, the pressure electrolyzer is preferably designed as a PEM electrolyzer, wherein the PEM electrolyzer has a proton-permeable membrane and is designed for a high nominal pressure of at least 5 to 50 bar, in particular 35 bar. A typical operating pressure can also be provided, for example, by a nominal pressure in the range of 20 to 25 bar.

[0032] The pressure electrolyzer usually has a large number of electrolysis cells arranged next to one another. By means of water electrolysis, water is split into hydrogen and oxygen in the electrolysis cells. In a PEM electrolyzer, distilled or demineralized water is typically fed in as the reactant on the anode side and split into hydrogen and oxygen on a proton-permeable membrane (PEM). The water is oxidized to oxygen at the anode. The protons pass through the proton-permeable membrane. Hydrogen is produced on the cathode side. The water is usually pumped from the bottom into the anode compartment and / or cathode compartment.Alkaline electrolysis, a possible alternative method, also involves a type of membrane, designed as a semipermeable membrane or diaphragm, which selectively allows the passage of certain ions. The electrolyte used is potassium hydroxide solution (KOH) with a concentration of typically 20-40%. The gas-tight membrane, the so-called diaphragm, allows the transport of OH~ ions but simultaneously prevents the mixing of the resulting product gases.

[0033] A further aspect of the invention relates to a method for operating a corresponding electrolysis plant with a pressure electrolyzer. According to the invention, the method involves generating a pressurized oxygen product gas at a high nominal pressure by means of pressure electrolysis and subsequently expanding it, whereby the oxygen product gas is cooled and at least partially liquefied into liquid oxygen.

[0034] Preferably, before the expansion, the oxygen product gas taken from the pressure electrolyzer at the nominal pressure is preconditioned, such as drying the oxygen product gas, removing residual gas contamination and precooling.

[0035] In a particularly preferred embodiment of the process, the pressurized oxygen product gas from the pressure electrolysis is first subjected to gas conditioning before expansion, whereby hydrogen as a foreign gas component in the oxygen product gas as well as water are removed.

[0036] Despite the comparatively high gas impermeability of the ion-permeable membrane, for example the ionomer of the proton-conducting membrane in PEM electrolysis, permeation of oxygen from the anode to the cathode and of hydrogen from the cathode to the anode occurs during operation. This is partly because complete gas impermeability of the ionomer cannot be achieved. Secondly, the membrane itself absorbs water through direct contact with water. The foreign gases arising from permeation cause undesirable side reactions that reduce the efficiency of water electrolysis and can potentially damage the membrane.

[0037] In practice, therefore, there are small amounts of hydrogen in the oxygen gas stream and small amounts of oxygen in the hydrogen gas stream. The quantity of each foreign gas depends on the electrolysis cell design and also varies under the influence of current density, catalyst composition, aging and also depends on the membrane material of the electrolysis cell. It is inherent in the system that the other product gas is present in very small quantities in the gas stream of one product gas. As the process progresses, even small traces of hydrogen are removed from the oxygen, preferably in downstream gas cleaning steps, especially if a particularly high product gas quality is required. Under certain circumstances, it may be necessary to reduce the foreign gas concentration, even immediately at or directly after the electrolysis cell or the electrolysis stack, e.g.in the gas separators or gas separators downstream of the electrolyzer.

[0038] The gas purification can be carried out, for example, in a recombination device with a catalytically active zone, so that foreign gas residues of hydrogen in the oxygen product gas react with the oxygen to form water. In a preferred embodiment of the process, pressurized oxygen product gas is heat exchanged with liquid oxygen before expansion, thus causing precooling of the oxygen product gas.

[0039] This is a particularly efficient measure to achieve pre-cooling, since the liquid oxygen is at a low temperature level of below -183 °C and provides a preferential heat sink, so that the oxygen product gas stream is cooled.

[0040] In order to make the liquefaction process in the process even more efficient, the pressurized oxygen product gas is preferably compressed from the nominal pressure to a pre-pressure.

[0041] This pre-compression ensures the provision of a correspondingly high inlet pressure before the isentropic expansion step, which is set significantly above the nominal pressure of the pressure electrolyzer, for example, above 100 to 150 bar. This results in efficiency gains in terms of the liquid oxygen yield after the expansion step.

[0042] Preferably, the oxygen product gas compressed to a pre-pressure is intermediately cooled.

[0043] The combination of the process steps of pre-compression and intermediate cooling can optionally be carried out multiple times. It is advantageous if the process of compression and intermediate cooling with subsequent isentropic expansion in the expander is carried out iteratively or cyclically several times. The total yield of liquefied oxygen gas increases for a given gas quantity - ideally by a further 50% per cycle in relation to a specific input volume flow. Further advantages of the operating method according to the invention arise accordingly from the advantages of the electrolysis plant described above.

[0044] The invention is explained in more detail below with reference to the accompanying drawings. It should be noted that the exemplary embodiments shown in the drawings primarily serve to explain the invention. However, they are not intended to limit the invention.

[0045] Here we show schematically and very simplified:

[0046] FIG 1 shows an electrolysis plant with a pressure electrolyzer and an oxygen liquefaction system according to the invention;

[0047] FIG 2 an electrolysis plant with pressure electrolyzer and oxygen liquefaction with further plant components.

[0048] The same reference symbols have the same meaning in the FIGS.

[0049] FIG. 1 shows an electrolysis plant 1 in a highly simplified section of plant parts and components. The electrolysis plant 1 has a pressure electrolyzer 3, which can be designed either as a PEM electrolyzer or as an alkali electrolyzer and is heated to a high nominal pressure p N of at least 5 bar as working pressure. Higher nominal pressures p Nof 35 bar and above are therefore also possible within the scope of the invention, depending on the design concept and requirements of the electrolysis plant 1 in use.

[0050] The electrolyzer 3 comprises a cathode chamber 31 and an anode chamber 29, which are separated by an ion-permeable membrane 27. The anode chamber 29 and the cathode chamber 31 are each composed and formed by a plurality of anodic and cathodic half-cells (not shown in detail in FIG. 1) stacked in an axial direction. The cathodic half-cells and the anodic half-cells are composed to form a respective electrolysis cell and are each separated by an ion-conducting membrane 27. FIG. 1 shows a pressure electrolyzer 3, which is designed for the electrochemical splitting of water H2O or an electrolyte as reactant into hydrogen H2 and oxygen O2 as product gases by means of an electric current. The respective reactant is fed to the pressure electrolyzer 3 via the reactant feed line 33. In the case of acid electrolysis, demineralized water H20 is used as the reactant.In the case of alkaline electrolysis, an alkali is used as the reactant, for example, potassium hydroxide (KOH) in an aqueous solution with a concentration typically of 20% to 40%. Several such electrolysis cells can be connected in series in horizontally stacked so-called electrolysis stacks.

[0051] In the embodiment shown in FIG. 1, the pressure electrolyzer 3 in the electrolysis plant 1 is designed as an example as a PEM electrolyzer for high nominal pressures p N of at least 5 bar, in this case the nominal pressure is typically p N= 35 bar. In PEM electrolysis, each electrolysis cell has a proton-permeable membrane 5 based on a fluoropolymer, to which an electrode - an anode and a cathode - is attached on both sides, via which an external direct voltage is applied during operation. On the anode side, a reactant feed line 33 for feeding water H2O is connected to the anode chamber 29. This creates a water circuit in the pressure electrolyzer 3 designed as a PEM electrolyzer. It is also possible for water H2O to be fed in not only through the anode chamber 29, but also through the cathode chamber 31, so that two circuits are created. In the present case, a pressure electrolyzer 3 is shown which is implemented with only one anode-side circuit. During operation of the electrolysis plant 1, the oxygen O2 produced from the anode chamber 7 in the electrolysis cell is discharged via an oxygen product gas line 7.On the cathode side, a hydrogen product gas line 25 is provided for discharging the produced hydrogen H2 from a cathode chamber 31. The product gases are discharged at a high nominal pressure p. N generated and led out of the pressure electrolyzer 3 via the respective product gas line 7, 25 and made available for further purposes. In order to achieve the high nominal pressure p N To reduce the differential pressure load across the membrane 27, the differential pressure between the anode chamber 29 and the cathode chamber 31 is set very low, for example, between 0 mbar and 100 mbar. Therefore, the ideal operation is constant pressure.

[0052] To utilize the oxygen O2 from the pressure electrolysis, which is subjected to the high nominal pressure, the oxygen product gas line 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, which is connected downstream of the expansion turbine 11. The generator 37 makes it possible to utilize the mechanical energy resulting from the expansion of the oxygen product gas O2 in the expansion turbine 11. In addition, a phase separator 5 for the spatial separation of the liquid and gaseous phases in the partially liquefied oxygen O2 is functionally integrated into the expansion device 9. The expansion turbine 9 can be designed in several stages. A product line 15 is led out of the phase separator 5 to convey the liquid oxygen LO2 obtained in the isentropic expansion process and separated in the phase separator 5.Furthermore, a return line 17 leads out of the phase separator 5 and opens into the product gas line 7. A control valve 35 is connected to the return line 17. This makes it possible to meter returned cold gaseous oxygen O2, or if necessary also in a phase mixture of liquid and gaseous oxygen O2, into the oxygen product gas line 7 for pre-cooling, and to adjust a pre-cooling temperature via the amount fed in. The mixture of the material streams involved at different temperature levels leads to pre-cooling of the oxygen product gas O2 in the product gas line 7. It is also possible that, for operational reasons, only a portion of the partially liquefied oxygen O2, i.e. present in a phase mixture, is returned to the product gas line 7 via the return line 17. The other part is discarded or drained.

[0053] The product gas line 7 and the product line 15 are thermally coupled via a heat exchanger 21. The heat exchanger 21 is designed as a gas-liquid heat exchanger 21. This implements heat exchange and further cooling of the oxygen product gas O2 by the very cold liquid oxygen LO2 carried in the product line 15 during operation. Finally, an extraction line 37 leads out of the heat exchanger 21 so 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 highly thermally insulated vessels and transported for use.

[0054] During operation of the electrolysis plant equipped in this way, the pressurised oxygen product gas O2 is produced by pressure electrolysis in the pressure electrolyser 3 at a high nominal pressure p Ngenerated and then expanded in the expansion device 9. In the process, the oxygen product gas O2 is cooled and at least partially liquefied to liquid oxygen LO2. Before the expansion, the pressurized oxygen product gas O2 is brought into heat exchange with liquid oxygen LO2 by means of the heat exchanger 21. This already brings about a pre-cooling of the oxygen product gas O2.

[0055] Another electrolysis plant 1 with pressure electrolyzer 3 and an integrated oxygen liquefaction for the oxygen product gas O2 is shown in FIG. 2. Compared to the exemplary embodiment in FIG. 1, further functional components are integrated into the electrolysis plant 1 or advantageous modifications have been made. For example, in the electrolysis plant 1, a compressor 23, which is designed in two stages, is connected into the oxygen product gas line 7. This allows the oxygen product gas O2 from the pressure electrolyzer 3 to be liquefied well above the nominal pressure p Nbrought to a higher compression, for example more than 100 bar. A gas conditioning device 41 is connected in the product gas line 7 immediately after the pressure electrolyzer 3 and the compressor 23 upstream in the process. This is used for gas cleaning and gas drying of the oxygen product gas O2. A recombiner with a catalyst is also integrated in the gas conditioning device 41. This allows unwanted foreign gas components to react with hydrogen H2 in the oxygen product gas O2 through a catalytic reaction to form water and the water H2O thus formed can be discharged from the catalytic reactor via a drainage line 45 and the reactor is thereby emptied. In the direction of flow of the oxygen product gas O2, a heat exchanger 19 is connected downstream of the compressor 23 in the product gas line.Alternatively, the gas conditioning device 41 with the catalytic reactor can also be installed downstream of the compressor 23 and upstream of the cooling device in the heat exchanger 19. This has significant efficiency advantages for the recombiner due to the higher temperature level of the compressed oxygen product gas O2. This is not shown in detail in FIG. 2.

[0056] A return line 17 with a control valve 35 is connected to the heat exchanger 19 so that the oxygen product gas O2 compressed and correspondingly heated in the compressor 23 can be cooled. This is achieved by a metered return of cold oxygen via the return line 17 into the heat exchanger 19. Before entering the heat exchanger 19, the oxygen product gas O2 discharged from the compressor 23 and correspondingly compressed can be subjected to intermediate cooling so that a desired low gas temperature of the compressed oxygen product gas O2 is already provided before it enters the heat exchanger 19. For this purpose, a further heat exchanger - not shown in detail - is to be provided in the product gas line 7 after the outlet from the compressor 23 as an intermediate cooler, which can be operated conventionally with air or water as the coolant.During operation, additional pre-cooling or intermediate cooling of the compressed oxygen product gas O2 can thus be effected before the expansion of the pressurized oxygen product gas O2 in the expansion device 9. Compared to the design in FIG. 1, a heat exchanger 19 is provided here in FIG. 2 as an alternative design for the direct feed of cold and additionally compressed gaseous oxygen O2 or a phase mixture of liquid oxygen O2 and gaseous oxygen O2.

[0057] The expansion device 9 is designed in FIG. 2 as a multi-stage expansion device 9, wherein a first expansion turbine 11A and a second expansion turbine 11B are connected in series. An intercooler 23 is provided between the first expansion turbine 11A and the second expansion turbine 11B, so that further cooling of the oxygen product gas O2 can be effected and efficient liquefaction with the highest possible yield of liquid oxygen LO2 at the phase separator 5 in one process cycle. Several intercooling stages and further expansion stages are also possible.

[0058] In addition, the expansion device 9 has a throttle valve 13 in order to promote phase separation through further isentropic expansion and to increase the yield. In a process analogous to that described in FIG 1, before the expansion the pressurised and prepared oxygen product gas O2 is brought into heat exchange with liquid oxygen LO2 via the heat exchanger 21, whereby a further pre-cooling of the oxygen product gas O2 is effected. With the electrolysis plant 1 with the pressure electrolyzer 3, oxygen LO2 can be produced as high-quality and ultra-pure so-called cryogas in an efficient and economically interesting way. The cryogas is presently provided as an O2 pressurised gas with a very high density and high quality, which is obtained either in liquid LO2 or in highly compressed gaseous form as so-called GO2.This opens up numerous possibilities for utilization and oxygen 02 - along with hydrogen H2 - is now commercially usable as a valuable product of pressure electrolysis.

Claims

Patent claims 1. Electrolysis plant (1) comprising a pressure electrolyzer (3) for producing hydrogen (H2) and oxygen (O2) as product gases at a high nominal pressure (P N ), with an oxygen product gas line (7) leading from the pressure electrolyzer (3) and connected to an expansion device (9).

2. Electrolysis plant (1) according to claim 1, wherein the expansion device (9) is designed as a multi-stage expansion device (9) having a plurality of expansion stages connected in series.

3. Electrolysis plant (1) according to claim 1 or 2, wherein the expansion device (9) comprises an expansion turbine (11, 11A, 11B).

4. Electrolysis plant (1) according to one of the preceding claims, wherein the expansion device (9) has a throttle valve (13).

5. Electrolysis plant (1) according to one of the preceding claims, in which the expansion device (9) has a phase separator (5) so that liquid oxygen (LO2) and gaseous oxygen (O2) can be spatially separated.

6. Electrolysis plant (1) according to claim 5, comprising a product line (15) for the separated liquid oxygen (LO2) connected downstream of the expansion device (9) and a return line (17) branching off from the product line (15) and connected to the oxygen product gas line (7).

7. Electrolysis plant (1) according to claim 5, wherein the return line (17) is coupled to the oxygen product gas line (7) via a heat exchanger (19), so that in a Cooling of gaseous oxygen (O2) can be achieved through heat exchange.

8. Electrolysis plant (1) according to one of claims 6 or 7, in which the product line (15) and the oxygen product gas line (7) are coupled to one another via a heat exchanger (21), so that a pre-cooling of the gaseous oxygen (O2) with liquid oxygen (LO2) can be effected.

9. Electrolysis plant (1) according to one of the preceding claims, with a compressor (23) connected in the oxygen product gas line (7) for pre-compressing the oxygen product gas (O2) above the nominal pressure (p N ) can be effected beyond .

10. Electrolysis plant (1) according to one of the preceding claims, comprising a pressure electrolyzer (3) designed as a PEM electrolyzer, which has a proton-permeable membrane and which is operated at a high nominal pressure (p N ) from 5 to 50 bar, in particular from 35 bar.

11. A method for operating an electrolysis plant (1) according to one of the preceding claims, wherein a pressure-laden oxygen product gas (O2) is produced by pressure electrolysis at a high nominal pressure (p N ) and then expanded, whereby the oxygen product gas (O2) is cooled and at least partially liquefied to liquid oxygen (LO2).

12. The method according to claim 11, wherein, prior to the expansion, pressurized oxygen product gas (O2) is brought into heat exchange with liquid oxygen (LO2), whereby a pre-cooling of the oxygen product gas (O2) is effected.

13. The method according to claim 11 or 12, wherein the pressurized oxygen product gas (O2) is reduced from the nominal pressure (p N ) is compressed to a pre-pressure.

14. The method according to claim 13, wherein the oxygen product gas (O2) compressed to a pre-pressure is intermediately cooled.

15. The method according to any one of claims 11 to 14, wherein the pressurized oxygen product gas (O2) from the pressure electrolysis is first subjected to gas conditioning before expansion, wherein hydrogen (H2) as a foreign gas component in the oxygen product gas (O2) and water (H2O) are removed.

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