Method and apparatus for the electrochemical reduction of carbon dioxide

Abstract

The invention relates to a method for the electrochemical reduction of carbon dioxide and to an apparatus for carrying out said method.

Description

[0001] Method and apparatus for the electrochemical reduction of carbon dioxide

[0002] The present invention relates to the technical field of the electrolysis of carbon dioxide.

[0003] In particular, the present invention relates to a process for the electrochemical reduction of carbon dioxide.

[0004] Furthermore, the present invention relates to a device for the electrochemical reduction of carbon dioxide.

[0005] The electrochemical reduction of carbon dioxide is increasingly becoming the focus of research and development, as the use of fossil carbon sources is becoming increasingly restricted and will be further limited in the future, and the electrolysis of carbon dioxide offers the possibility of removing carbon dioxide from the atmosphere on the one hand and producing valuable raw materials for the chemical industry on the other.

[0006] The electrolysis of carbon dioxide can be carried out either in closed systems, i.e. autoclaves, or in flow systems in which a carbon dioxide stream is passed over or through the cathode of the electrolysis cell.

[0007] Electrolysis in closed systems, particularly autoclaves, offers the advantage of achieving comparatively high yields and conversion rates of carbon dioxide. However, the fact that the electrolysis cell can only be operated in batches slows down and increases the cost of the process. Furthermore, carbon dioxide has low solubility in water, leading to parasitic hydrogen formation in aqueous systems. Performing electrolysis in organic solvents results in low current densities due to the low conductivity of these media. Both of these factors significantly reduce the efficiency of carbon dioxide electrolysis.

[0008] In contrast, flow-through electrolyzers offer a significantly higher throughput, but have the disadvantage that the material stream leaving the electrolysis cell contains only a very small proportion of the desired products and consists mostly of unreacted carbon dioxide. Thus, the proportion of electrolysis products in the material stream leaving the cathode compartment of a flow-through electrolysis cell is often only 2 mol% or less. Only at very low flow rates can the product fraction be somewhat higher, but this is associated with disadvantages, as explained below.

[0009] In flow-through electrolyzers, the electrochemical reduction of carbon dioxide mainly produces carbon monoxide and / or synthesis gas.

[0010] However, the electrochemical conversion of CO2 and H2O to synthesis gases, preferably CO and H2, under ambient conditions is associated with low efficiencies and current densities.

[0011] Increasing the pressure can increase current densities and Faraday efficiencies, as well as reduce the necessary cell voltages and thus the energy requirement.

[0012] Thus, WO 2023 / 042043 A1 shows an improvement in current density and Faraday efficiency for the target product formate in the range of 100 mA / cm² respectively. 2 and 40% at 270 mA / cm 2 and 85% is achieved by increasing the pressure from 5 to 40 bar. Furthermore, a cryogenic separation process is used to provide carbon dioxide at the necessary process pressures.

[0013] EP 4 249 639 A1 describes the electrochemical conversion of CO2 to CO with subsequent cryogenic gas separation. However, the cryogenic gas separation does not address the separation of the product gases from the CO2 reduction, but rather the separation of O2 and CO2 on the anode side.

[0014] High-pressure electrolysis up to approximately 200 bar can improve the performance of CO2 electrolysis in terms of cell voltages, current densities and CO Faraday efficiencies.

[0015] However, high-pressure electrolysis has the following disadvantages in particular: 1. The compression of carbon dioxide is energy-intensive, which significantly reduces the efficiency of the overall process.

[0016] 2. The gas stream after the electrolyzer consists mainly of CO2, which makes the separation of the synthesis gases more difficult.

[0017] Various methods are used to separate mixed gases. Pressure swing adsorption and cryogenic gas separation, also known as cryogenic distillation, are the most relevant of these.

[0018] Pressure swing adsorption (PSA) is a method for separating gas mixtures based on the different adsorption properties of their components. The process is frequently used for purifying or extracting gases such as oxygen, nitrogen, or hydrogen. The separation of CO2 from gas mixtures using pressure swing adsorption is also well-established.

[0019] Separating CO and CO2 using PSA is possible in principle, but challenging because commercial adsorbents often exhibit similar binding properties for these gases. This means it is difficult to find an adsorbent material that selectively adsorbs either CO or CO2. This similarity in binding properties complicates the selective separation of the two gases. It requires specialized adsorbents or a combination of adsorption and other separation methods to achieve effective separation of CO and CO2. Typically, mixtures of CO2 and CO cannot be practically separated using PSA at low concentrations of either component.

[0020] In cryogenic distillation, the different boiling points of the gas components are used to separate them and decompose them into their constituent parts. Cryogenic distillation requires very low temperatures, depending on the gases being separated. To achieve these temperatures, the Joule-Thomson effect is often used. In this process, the gas mixture is compressed and liquefied. Alternatively, supercritical mixtures can be used. The mixture is then depressurized, resulting in a cooling effect. Due to the simultaneous pressure reduction, certain gas components boil while others remain in their liquid state. This allows the substances to be separated and isolated from one another.

[0021] In principle, cryogenic distillation is also suitable for separating carbon dioxide from gas mixtures or supercritical mixtures, especially from volatile gases.

[0022] Furthermore, carbon dioxide refrigeration machines are also known, in which carbon dioxide is used as a refrigerant.

[0023] The industrial application of CCh refrigeration systems using CO2 as a refrigerant, also known as R-744, is steadily increasing, particularly in the supercritical range. This is due to the environmental advantages of CO2 compared to other refrigerants. A disadvantage of CCh refrigeration systems, however, is the higher system cost resulting from the high process pressures.

[0024] Carbon dioxide chillers therefore have the disadvantage that they are process-technically complex and costly compared to other chillers, but their importance will probably increase in the future due to the ecological advantages.

[0025] The current state of the art therefore lacks a method to provide economically viable carbon dioxide electrolysis in a flow process and to separate the resulting product mixture.

[0026] One object of the present invention is therefore to avoid, or at least mitigate, the disadvantages described above that are associated with the prior art.

[0027] In particular, one object of the present invention is to provide an economically and technically sensible method for the electrochemical reduction of carbon dioxides.

[0028] A further object of the present invention is to improve the economic efficiency of carbon dioxide refrigeration machines. The subject matter of the present invention, according to a first aspect of the present invention, is therefore a method for the electrochemical reduction of carbon dioxide according to claim 1; further advantageous embodiments of this aspect of the invention are the subject matter of the corresponding dependent claims.

[0029] A further subject matter of the present invention according to a second aspect of the present invention is a device for generating cold and for electrochemically reducing carbon dioxide according to claim 19; further, advantageous embodiments of this aspect of the invention are the subject matter of the related dependent claims.

[0030] It goes without saying that any special features, characteristics, designs and embodiments, as well as advantages or the like, which are subsequently described in relation to only one aspect of the invention – for the purpose of avoiding unnecessary repetition – shall of course apply accordingly to the other aspects of the invention, without the need for any express mention.

[0031] Furthermore, it should be noted that for all relative or percentage quantities mentioned below, especially those based on weight, it must be ensured that, within the scope of the present invention, these quantities are selected by a person skilled in the art in such a way that the sum of the ingredients, additives, excipients, or the like always results in 100% or 100% by weight. This is self-evident to a person skilled in the art.

[0032] Furthermore, it is assumed that all subsequent parameter specifications or the like can, in principle, be determined or ascertained using standardized or explicitly specified determination methods, or using determination procedures that are generally familiar to the person skilled in the art.

[0033] Having said that, the subject matter of the present invention will now be explained in more detail.

[0034] The subject matter of the present invention - according to a first aspect of the present invention - is thus a method for the electrochemical reduction of carbon dioxide, wherein the electrochemical reduction is carried out in the form of a high-pressure electrolysis and wherein the reaction products of the electrolysis are cryogenically separated from carbon dioxide.

[0035] Preferably, the reaction products are separated from carbon dioxide by means of cryogenic distillation, in particular by means of cryogenic gas separation.

[0036] The inventive method makes it possible in particular to integrate high-pressure electrolysis into a carbon dioxide refrigeration process, thereby enabling, on the one hand, an economically and technically sensible implementation of the electrolysis and a subsequent separation of the products, and on the other hand, increasing the economic efficiency of the CCh refrigeration machine, since valuable raw materials or starting materials for the chemical industry are produced by the electrolysis.

[0037] In the electrolysis process of the present invention, a mixture of substances is often obtained which consists predominantly, in particular to 90 mol% or more, of carbon dioxide. By integrating or coupling the electrolysis into or with a carbon dioxide refrigeration process, it is possible to separate the carbon dioxide from the reaction products within the process steps already occurring in the carbon dioxide refrigeration process, namely compression of carbon dioxide, removal of the heat generated, expansion and liquefaction of the compressed carbon dioxide under cooling, and subsequent evaporation of the liquid carbon dioxide by absorbing energy from an external source. Virtually no additional equipment is required for this.

[0038] The low throughput of flow electrolysis is also not a disadvantage here, since the process costs for the compression and evaporation of the carbon dioxide are incurred anyway in the carbon dioxide refrigeration cycle process.

[0039] Within the scope of the present invention, high-pressure electrolysis can be operated particularly well in one or more bypasses to the actual carbon dioxide refrigeration cycle, since electrolysis generally does not require such high material or mass flows as carbon dioxide refrigeration processes. Furthermore, electrolysis also increases the economic efficiency of the refrigeration process, as no additional energy is required for separating carbon dioxide and the electrolysis reaction products, and the electrolysis process also yields feedstocks or raw materials that can subsequently be marketed.

[0040] Within the scope of the present invention, it is preferably provided that the high-pressure electrolysis is carried out as flow-through electrolysis.

[0041] Preferably, within the scope of the present invention, the high-pressure electrolysis is coupled with a carbon dioxide refrigeration process. Preferably, the high-pressure electrolysis is integrated into the carbon dioxide refrigeration process.

[0042] Within the scope of the present invention, it is preferably provided that the high-pressure electrolysis is arranged between the process steps of the compression of the carbon dioxide and the subsequent expansion or decompression of the carbon dioxide, either in such a way that the mass flow, in particular the carbon dioxide flow, is completely directed into one or more electrolysis cells or stacks, in particular electrolysis cells or stacks arranged in parallel to each other, or that the electrolysis cell(s) is / are arranged in parallel to a connection between a compression device or compressor and an expansion device or expansion valve, i.e., that only a part of the mass flow, in particular the carbon dioxide flow, is directed through the electrolyzer(s).

[0043] Within the scope of the present invention, it is typically provided that the carbon dioxide refrigeration process is carried out transcritically or subcritically, preferably transcritically. Transcritical means that carbon dioxide is temporarily, particularly for certain process steps, brought into the supercritical range, i.e., above a temperature of 31 °C and above a pressure of 73.8 bar. Subcritical means that carbon dioxide is always kept in the subcritical range during the process.

[0044] Compared to the prior art, the process according to the invention increases the efficiency of product separation in electrochemical CCh reduction. Various key performance indicators are used to assess the economic viability of electrochemical CO2 conversion. Progress has been made in recent years to improve these indicators. Nevertheless, CCh conversion rates are expected to remain low. This means that a small amount of electrolysis products must be separated from a CCh-rich mixture. This effect is amplified when operating at high pressure, which has a positive impact on electrolysis efficiency, since very low flow rates in the reactor would be required to achieve a high product concentration in the liquid or supercritical medium.At the same time, a low flow velocity in supercritical operation with high CCh density can also lead to a two-phase flow due to separation effects, which could potentially be disadvantageous. For example, gaseous products (e.g., CO or H2) can separate even at high concentrations, or process water, which is transported across the membrane to the cathode via electroosmotic drag, can condense. At low flow velocities, these can accumulate in the cathode compartment and limit the CO2 mass transport. A high flow velocity would normally be targeted to expel condensates or gas bubbles.

[0045] Cryogenic distillation is particularly suitable for separating the mixture from high-pressure electrosynthesis because the mixture is already under elevated pressure, requiring only targeted pressure release. However, to recycle the CO2, it must be recompressed. A large portion of the carbon dioxide is therefore compressed and released multiple times without being converted through electrolysis. Economical operation of pure CO2 electrolysis is thus not possible. However, by integrating the process into a CCh refrigeration unit, where alternating compression and release of CO2 is a core operating principle, economical separation of the product gases can be achieved. Only additional compression work needs to be invested in the portion of CO2 that was previously converted into products, separated, and subsequently replaced.

[0046] As for the reaction products, these are usually selected from the group of methanol, ethanol, n-propanol, formic acid, methane, ethylene, carbon monoxide, synthesis gas and mixtures thereof.

[0047] Preferably, the reaction products are selected from the group consisting of carbon monoxide, synthesis gas, and mixtures thereof. Synthesis gas is understood to be, in particular, a mixture of hydrogen (H₂) and carbon monoxide (CO).

[0048] Carbon monoxide and synthesis gas, in particular, can be separated from carbon dioxide very well and easily using known methods, preferably cryogenic distillation. Preferably, the cooling capacity of conventional carbon dioxide chillers, in which carbon dioxide is liquefied, can be used to separate the reaction products of electrolysis, especially carbon monoxide, synthesis gas, or mixtures thereof, from carbon dioxide. Separation is straightforward because carbon dioxide liquefies at higher temperatures than carbon monoxide and hydrogen, so that carbon monoxide and synthesis gas remain in the gas phase or are converted into the gas phase during expansion from the supercritical region, while carbon dioxide is separated in the form of a liquid phase.

[0049] Furthermore, it is typically provided within the scope of the present invention that carbon dioxide and optionally water are reduced during electrolysis. Preferably, within the scope of the present invention, carbon dioxide and water are reduced during electrolysis. The reduction is, in particular, the cathode reaction of the electrolysis.

[0050] Furthermore, it is typically provided within the scope of the invention that a compound from the group consisting of water, glycerol, methanol, chloride, and mixtures thereof is oxidized during electrolysis. Preferably, water is oxidized. In particular, water is oxidized to oxygen. This oxidation is the anode reaction.

[0051] As previously explained, the electrolysis carried out according to the invention is a high-pressure electrolysis process, in order to significantly increase the Faraday efficiency and the product yield. Within the scope of the present invention, high-pressure electrolysis is understood to mean electrolysis carried out at pressures of at least 20 bar.

[0052] Preferably, the electrolysis is carried out at pressures of at least 30 bar, particularly at least 40 bar, and preferably at least 50 bar. Likewise, within the scope of the present invention, it is preferred if the electrolysis is carried out at pressures in the range of 30 to 150 bar, particularly 30 to 130 bar, preferably 40 to 120 bar, preferably 50 to 100 bar, and most preferably 50 to 90 bar. High pressures, especially under supercritical conditions, considerably increase the efficiency of the carbon dioxide electrolysis, i.e., the carbon dioxide conversion rate.

[0053] According to the invention, the high-pressure electrolysis process is carried out by introducing a mass flow, in particular a carbon dioxide flow, into an electrolysis device, in particular an electrolysis cell or a stack, and in particular into the cathode compartment of the electrolysis device. The reduction of the carbon dioxide and, optionally, the water is then carried out in the electrolysis compartment.

[0054] At the same time, a stream of material, especially water, is directed into an anode compartment of an electrolysis device and oxidized there.

[0055] Regarding the temperatures at which the electrolysis is carried out, it has proven effective to carry out the electrolysis at temperatures of at least 20 °C, in particular at least 30 °C, preferably at least 40 °C.

[0056] Particularly good results are obtained within the scope of the present invention when the electrolysis is carried out at temperatures in the range of 20 to 180 °C, in particular 30 to 150 °C, preferably 40 to 120 °C, preferably 40 to 100 °C, and particularly preferably 40 to 80 °C.

[0057] Furthermore, it has proven advantageous to carry out electrolysis at current densities greater than 0.1 A . cm' 2 , especially greater than 0.2 A . cm . -2 , preferably larger than 0.3 A . cm' 2 , preferably 0.5 A . cm' 2 , is carried out.

[0058] Furthermore, it may be provided that electrolysis takes place at current densities in the range of 0.1 to 4.0 A . cm' 2 , especially 0.2 to 3.0 A . cm' 2 , preferably 0.3 to 2.0 A . cm' 2 preferably 0.5 to 1.5 A . cm' 2 , is carried out.

[0059] Electrolysis is typically carried out at voltages in the range of 1.35 to 4.5 V, particularly 1.6 to 4.0 V, preferably 1.8 to 3.5 V, and more preferably 1.8 to 3.0 V. Within the scope of the present invention, it is further typically provided that the proportion of reaction products after electrolysis in a mixture of substances leaving a cathode compartment of an electrolysis cell, particularly a carbon dioxide stream containing reaction products, is 1 to 40 mol%, particularly 2 to 25 mol%, preferably 3 to 20 mol%, and more preferably 5 to 15 mol%, based on the mixture of substances, particularly the carbon dioxide stream containing reaction products. The mixture of substances can be in a gaseous or supercritical state.

[0060] Furthermore, it is typically provided within the scope of the present invention that the proportion of reaction products in a liquid, gaseous, or supercritical phase is increased by cryogenic separation compared to the mixture of substances leaving a cathode compartment of an electrolysis cell, in particular a carbon dioxide stream containing reaction products. It is preferably provided within the scope of the present invention that the proportion of reaction products in a gaseous or supercritical phase is increased by cryogenic separation compared to the mixture of substances leaving a cathode compartment of an electrolysis cell, in particular a carbon dioxide stream containing reaction products. The two-phase mixture resulting from the cryogenic separation is preferably collected, in particular in at least one collection vessel. When a predetermined quantity of the product-rich gaseous or supercritical phase is reached, it can be removed and further processed.

[0061] In cryogenic separation, the process involves expanding the carbon dioxide-rich mixture, particularly the carbon dioxide stream containing the reaction products, thereby reducing the pressure and temperature of the mixture. This expansion is typically achieved via an expansion valve or throttle.

[0062] The reduction in pressure and temperature causes the carbon dioxide to liquefy, while the reaction products, particularly carbon monoxide and / or synthesis gas, remain in the gas phase or supercritical phase. The gas phase or supercritical phase can thus be continuously enriched with reaction products until it becomes possible to separate them economically and efficiently from any remaining carbon dioxide, for example, by pressure swing adsorption, as described previously.

[0063] In particular, the enrichment of products in a gas phase or supercritical phase after cryogenic separation makes the process according to the invention economically viable, since then only impurities need to be separated from the products, rather than the costly separation of small quantities of product from unreacted reactants and impurities. Cryogenic separation, preferably through carbon dioxide liquefaction in the refrigeration cycle of a carbon dioxide chiller, thus yields a product-rich phase, the further separation or processing of which is economically feasible.

[0064] Within the scope of the present invention, as an alternative to the previously described continuous removal of the liquid phase, the two-phase mixture obtained during cryogenic separation can also be collected until a certain fill level is reached in a collection container or a predetermined proportion of products in the gas phase or supercritical phase is reached. Subsequently, the flow of further mixture into this collection container is stopped and directed into another collection container. Then, preferably, the gas phase is removed from the first collection container, and the liquid carbon dioxide is reheated and expanded after utilizing the resulting cooling effect. In this case, the apparatus preferably comprises 2 to 5, in particular 2 or 3, and more preferably 2, collection containers.

[0065] In cryogenic separation, it is also possible for reaction products to be liquefied first by slowly lowering the temperature and pressure before carbon dioxide is transferred from a gaseous or supercritical phase to a liquid phase. The reaction product(s) can then be separated beforehand.

[0066] However, the separation of highly volatile reaction products, in particular synthesis gas and / or CO, from liquid CO2 is simpler and preferred according to the invention, since this cryogenic separation can be carried out without extensive equipment under conditions under which carbon dioxide refrigeration machines are typically operated. Within the scope of the present invention, it is typically provided that the reaction products, in particular the phase containing the reaction products, are removed from the process after cryogenic separation and, if necessary, enrichment of the electrolysis reaction products. As soon as a sufficiently high concentration or quantity of reaction products has formed in the corresponding phase, it can be removed from the process and purified or further processed.

[0067] The processing of the product-rich phase can be carried out in particular by means of usual methods, such as PSA (Pressure Swing Adsorption), membrane processes or amine scrubbing, preferably PSA or membrane processes.

[0068] Within the scope of the present invention, it is typically further provided that converted carbon dioxide is replaced, in particular by supplying carbon dioxide to the process from the outside.

[0069] According to a preferred embodiment, the inventive method is a method for the electrochemical reduction of carbon dioxide, in particular as described above, wherein

[0070] (a) in a first process step an electrochemical reduction of carbon dioxide and, if applicable, water is carried out in the form of high-pressure electrolysis and

[0071] (b) in a subsequent second process step, the reaction products of electrolysis obtained in the first process step are separated by cryogenic separation of carbon dioxide, the cryogenic separation being part of a carbon dioxide colding process.

[0072] For this particular and preferred embodiment of the present invention, all the aforementioned advantages, features and special characteristics apply accordingly.

[0073] Within the scope of the present invention, it is further preferably provided that at least one electrolysis unit or electrolyzer for carrying out the electrolysis is arranged in a bypass of a carbon dioxide refrigeration process. Within the scope of the present invention, it is usually neither necessary nor possible to pass the quantities or flows of materials required for the operation of a carbon dioxide refrigeration machine through one electrolyzer or even several electrolysis units or electrolyzers connected in parallel.

[0074] The electrolyzer(s) are preferably arranged in parallel in branches, so-called bypasses, to the actual refrigerant circuit, in particular between a compression device and an expansion device.

[0075] Within the scope of the present invention, it is usually further provided that the carbon dioxide is carried out in a cycle process, in particular that after separation of the reaction products of the electrolysis, it is partially returned to the electrolysis, preferably in process step (a).

[0076] Within the scope of the present invention, it is further provided that carbon dioxide converted during electrolysis is returned to the carbon dioxide cycle, in particular for the supply of CO2 from the outside.

[0077] Furthermore, it is advantageously provided within the scope of the present invention that, following electrolysis, the mixture containing the reaction products is separated from any entrained water, particularly in a water separation device, preferably by phase separation. The water is separated from the gaseous or supercritical mixture containing the reaction products by lowering the temperature of the mixture in liquid or solid form. The water is typically separated before cryogenic separation or distillation.

[0078] Within the scope of the present invention, it is further typically provided that, following the expansion of the mixture containing the reaction products, and in particular after the liquefaction of the carbon dioxide, the low temperature of the mixture, especially of the liquid carbon dioxide, is used for cooling purposes. Furthermore, it is typically provided that, for the use of the liquid carbon dioxide as a refrigerant, the liquid carbon dioxide is fed to an evaporation device, in which the carbon dioxide is converted back into the gas phase, particularly via heat exchangers. The gaseous carbon dioxide is then preferably fed back to a compression device, which compresses the carbon dioxide so that either a compressed gas or supercritical carbon dioxide is obtained.The heat energy released in this process is preferably dissipated via heat exchangers and can also be used.

[0079] The figure depictions show, according to

[0080] Fig. 1 shows a preferred embodiment of the method according to the invention using a preferred device and

[0081] Fig. 2 shows another preferred embodiment of the method according to the invention using a further preferred device.

[0082] A further object of the present invention – according to a second aspect of the present invention – is a device, in particular a carbon dioxide refrigeration machine, for generating cold for cooling purposes, comprising carbon dioxide as a working medium or refrigerant, and for the electrochemical reduction of carbon dioxide, wherein the device

[0083] (a) an electrolysis apparatus for the electrochemical reduction of carbon dioxide and, where appropriate, water,

[0084] (b) a compression device for compressing carbon dioxide,

[0085] (c) an expansion device for the expansion of a gaseous or supercritical mixture and

[0086] (d) comprises a collection vessel for receiving an expanded, multiphase mixture. In particular, components (b), (c) and (d), i.e. the compression device, the expansion device and the collection vessel, constitute the essential components of a carbon dioxide refrigeration machine coupled to the electrolysis process or at least one electrolysis device.

[0087] The device is preferably designed to carry out the previously described method according to the invention.

[0088] Within the scope of the present invention, it is typically provided that the device is configured to convey carbon dioxide in a closed-loop process. Carbon dioxide is, in particular, the refrigerant in a carbon dioxide refrigeration process.

[0089] According to a preferred embodiment of the present invention, the electrolysis unit is arranged in a bypass of the carbon dioxide refrigeration unit. As previously explained, it is usually neither necessary nor even possible within the scope of the present invention to pass the entire quantity of material from the carbon dioxide refrigeration cycle through the electrolysis unit; rather, preferably a small diverted partial flow of the compressed or supercritical carbon dioxide is directed into a bypass in which the electrolysis unit is arranged. The main part of the carbon dioxide flow is carried out in the normal refrigerant cycle or carbon dioxide cycle process.

[0090] Within the scope of the present invention, it is further possible that several electrolysis devices, in particular 1 to 100, in particular 1 to 50, preferably 2 to 10, electrolysis devices are provided in the device according to the invention.

[0091] It can be provided that the electrolysis units are arranged in one or more bypasses to the carbon dioxide circuit of the carbon dioxide refrigeration unit, and in particular arranged parallel to each other. The carbon dioxide (partial) stream containing the reaction products from the electrolysis unit(s) is then combined with the carbon dioxide stream of the carbon dioxide cycle process in the carbon dioxide refrigeration unit and processed further together. Within the scope of the present invention, it is particularly preferably provided that the expansion unit is connected to the collection tank, in particular wherein the collection tank follows the expansion unit downstream in the carbon dioxide circuit.

[0092] Within the scope of the present invention, it is preferably provided that the carbon dioxide-rich mixture, in particular the mixture containing the reaction product, which is either gaseous or supercritical, is expanded or depressurized in the expansion device, thereby reducing the pressure and temperature of the mixture and resulting in phase separation. During phase separation, either reaction products in liquid form can be separated from gaseous carbon dioxide, but preferably volatile, i.e., gaseous or supercritical, reaction products are separated from a liquid carbon dioxide phase.

[0093] The liquid carbon dioxide and the preferably product-rich gas or supercritical phase are collected in the collection vessel, whereby the electrolysis products accumulate in the gas phase of the collection vessel due to the continuous process. The collection vessel preferably has outlet openings for removing the gas or supercritical phase and for removing the liquid phase, in particular the liquid carbon dioxide.

[0094] In particular, a further device for separating and purifying the gas or supercritical phase, especially the gas or supercritical phase containing the reaction products of electrolysis, may be connected to the collection tank.

[0095] According to a preferred embodiment of the invention, the device has several collection containers, in particular 2 to 4, preferably 2 or 3, preferably 2, collection containers.

[0096] With multiple collection tanks, particularly two, it is possible to perform the separation and processing of the electrolysis products in batches. Depending on the plant configuration and design, continuous or intermittent removal of the liquid carbon dioxide after cryogenic separation may not be feasible or may be detrimental to the process. In this case, it is preferred to first collect the two-phase mixture—namely, the gaseous or supercritical product-rich phase and the liquid carbon dioxide phase—in one collection tank. Once a predetermined product concentration or quantity, or a predetermined fill level, this mixture is extracted from the carbon dioxide cycle and subsequently processed. The two-phase mixture from the cryogenic separation is collected in a second collection tank while the first is emptied.

[0097] Within the scope of the present invention, it is typically provided that the collection container(s) and the compression device are connected. The collection container(s) and the compression device can be connected, in particular, via connecting lines. It is especially possible that the collection container(s) and the compression device are connected indirectly, i.e., that further means or devices are interposed or arranged between the collection container and the compression device.

[0098] According to a preferred embodiment of the present invention, an evaporation device is arranged between the collection container(s) and the compression device. In the evaporation device, liquid carbon dioxide is converted back into the gaseous state.

[0099] Furthermore, the evaporation unit may include a heat exchanger, particularly for extracting cooling power. The heat exchanger utilizes the cooling of the mixture, especially the low temperature of the liquid carbon dioxide phase, which occurs during the expansion of the reaction product, and transfers this cooling energy to the heat exchanger for cooling purposes.

[0100] Within the scope of the present invention, it is further preferably provided that the compression device and the expansion device are connected.

[0101] The compression unit and the expansion unit are connected to each other directly or indirectly, in particular via lines. Preferably, the compression unit and the expansion unit are connected to each other both directly and indirectly. The direct connection typically corresponds to the main circuit of the carbon dioxide cycle process, while a portion of the carbon dioxide is extracted for electrolysis via the indirect connection, in particular by means of a bypass.

[0102] Within the scope of the present invention, it is preferably provided that the electrolysis device is connected to the compression device and the expansion device, in particular arranged in a bypass for the connection between the compression device and the expansion device.

[0103] Within the scope of the present invention, it may further preferably be provided that several bypasses are provided, each containing individual electrolysis devices, or that several electrolysis devices are arranged parallel to each other in a bypass.

[0104] Furthermore, it is preferred if a heat exchanger is arranged between the compression device and the expansion device, in particular between the compression device and the electrolysis device, preferably for heat removal.

[0105] The heat generated during compression must be dissipated in order to prepare the compressed or supercritical carbon dioxide to ideal temperatures for electrolysis and, on the other hand, to enable cooling and phase separation during the expansion of the carbon dioxide-rich mixture containing the reaction products of the electrolysis.

[0106] Within the scope of the present invention, it may further be provided that an oil separator is arranged downstream of the compression device. Preferably, the oil separator is arranged directly downstream of the compression device in order to separate any oil that enters the carbon dioxide stream during compression.

[0107] Preferably, a previously described heat exchanger follows the oil separator. Furthermore, it is preferably provided that the device includes a water separation unit, particularly for phase separation, downstream of the cathode compartment of the electrolysis cell. Water would accumulate in the form of ice during the subsequent expansion of the carbon dioxide-rich mixture containing the electrolysis products and could potentially clog valves.

[0108] The following describes a preferred procedure in a device according to the invention:

[0109] The efficiency of high-pressure electrosynthesis is improved by integrating the electrolyzer into a carbon dioxide (CCh) refrigeration system. In the CCh refrigeration system, carbon dioxide is compressed to the required electrolyzer pressure. This increases the temperature. Due to the compressors used, oil can enter the fluid stream, which is then separated before electrolysis. A heat exchanger reduces the temperature isobarically to the required electrolyzer temperature. The waste heat can be utilized. In the electrolyzer, CO2 is partially converted into synthesis gas. Water, which is produced as a byproduct, is first separated via phase separation. The gas mixture, which after the electrolyzer consists primarily of carbon dioxide, is expanded by a throttle. This pressure reduction lowers the temperature of the gas mixture. The heat can be used, for example, to cool the electrolyzer.Cooling liquefies the CO2, while the synthesis gases remain supercritical or gaseous. This results in phase separation. The product gas-rich gas or supercritical mixture is separated, and the liquefied carbon dioxide remains in the cycle. Fresh carbon dioxide is then added.

[0110] The subject matter of the present invention is explained below in an exemplary and non-limiting manner with reference to the figures.

[0111] Figure 1 shows a schematic representation of a device 1 according to the invention.

[0112] The device 1 according to the invention is a carbon dioxide refrigeration machine with a coupled or integrated electrolysis unit 2. The electrolysis unit 2 is connected via a line 7 to a compression unit 3, in which carbon dioxide is compressed so that either a compressed gas or supercritical carbon dioxide is obtained. The compressed or supercritical carbon dioxide is then fed via the line 7 into the cathode compartment of the electrolysis unit 2.

[0113] A carbon-rich mixture containing electrolysis products leaves the electrolysis unit 2 via a further line 7. The reaction products are preferably carbon monoxide and / or synthesis gas, preferably synthesis gas. The electrolysis unit 2 is connected, in particular via line 7, to an expansion device 4, especially an expansion valve or a throttle.

[0114] The expansion device 4 relaxes or expands the reaction product-containing, carbon dioxide-rich mixture, thereby reducing its pressure and simultaneously causing a significant decrease in temperature.

[0115] In the expansion unit 4, phase separation takes place, forming a liquid carbon dioxide phase on the one hand and a gas phase or supercritical phase rich in electrolysis products on the other. The two phases are then collected in a collection vessel 5. Preferably, the liquid carbon dioxide phase is fed to an evaporator 6, in which the low temperature of the liquid carbon dioxide is used for cooling purposes by means of a heat exchanger. Here, the carbon dioxide heats up and eventually evaporates again. Through the continuous inflow of carbon dioxide or mixture containing electrolysis products, the gas phase in the collection vessel 5 becomes enriched with the gaseous electrolysis products. When the concentration of electrolysis products is sufficiently high, the gas phase can be removed via an outlet and separated into its individual components.

[0116] The carbon dioxide converted into the gas phase in the evaporator 6 is then fed to the compression unit 3 via a further line 7, where it is compressed again. According to the present invention, it is preferably provided that the carbon dioxide reacted during electrolysis is mixed with further carbon dioxide before the carbon dioxide stream is fed into the compression unit 3 and after the evaporator 6, so that the total amount of carbon dioxide in the refrigeration cycle remains constant. According to a preferred embodiment of the present invention, a cooling device, in particular a precooler 13, is arranged between the electrolysis unit 2 and the expansion unit 4, in particular between the separation device for separating H2O 12 and the expansion unit 4.The cooling device, in particular the precooler 13, adjusts the carbon dioxide flow containing the electrolysis product to optimal temperatures for the subsequent expansion.

[0117] Within the scope of the present invention, it is particularly provided that an oil separation device 8 is connected directly downstream of the compression device 3, since the carbon dioxide can become contaminated with oil during compression. Oil must be separated before electrolysis and also before the expansion of the compressed carbon dioxide.

[0118] Furthermore, it is particularly provided within the scope of the present invention that heat generated during compression is dissipated in a heat exchange device 9, in particular via heat lines 10.

[0119] The cooled and compressed carbon dioxide, which is gaseous or supercritical, is then fed back to the cathode side of the electrolysis device 2.

[0120] Water is preferably supplied to the anode side of the electrolysis device 2 and oxidized to oxygen. The water-oxygen mixture leaving the anode side of the electrolysis device 2 is then preferably separated in a separation device 11 to separate the oxygen and water, in particular by phase separation. The water is then reheated to temperature ranges preferred for electrolysis, in particular by means of a further heat exchanger 9, which is connected to the heat exchanger 9 for heat dissipation from the compression device 3 via heat conductors 10. An ion filter, typically containing a mixed-bed resin, is usually provided downstream of the heat exchanger 9 and removes any ions from the water. However, it is also possible for a compound from the group consisting of glycerol, methanol, chloride, and mixtures thereof to be oxidized at the anode.Preferably, water is oxidized. The carbon dioxide-rich mixture containing electrolysis products leaving the cathode side of the electrolysis device 2 is preferably directed into a separation device for the separation of water 12. Here, the water is preferably separated in liquid or solid form and can be supplied to the anode side of the electrolysis device 2.

[0121] The electrolysis device 2 is preferably a so-called stack, i.e., a series of electrolysis cells connected in series, with which the conversion of carbon dioxide can be increased. Preferably, the electrolysis cells are zero-gap electrolyzers.

[0122] Figure 2 shows another preferred embodiment of the present invention. In particular, Figure 2 shows a device 1 according to the invention in which only a portion of the carbon dioxide is supplied to the electrolysis device 2.

[0123] Within the scope of the present invention, it is preferably provided that only a portion of the carbon dioxide from the carbon dioxide refrigeration circuit is supplied to the electrolysis unit 2. The main portion of the carbon dioxide flow is routed directly back to the expansion unit 4 after the compression unit 3 or the heat exchange unit 9 to generate cooling. Thus, according to an embodiment not shown in the figures, it is preferably provided that the electrolysis unit is arranged in a bypass to the carbon dioxide refrigeration circuit within the scope of the present invention.

[0124] After the carbon dioxide is compressed in the compression unit 3 and subsequently cooled in the heat exchange unit 9, a portion of the carbon dioxide is fed via a bypass line 7A to one or more, in particular several, electrolysis units. The product-containing carbon dioxide stream exiting the electrolysis unit(s) 2 is then separated from water in a separator 12 and fed back into the general carbon dioxide cycle via another bypass line 7A. The product-containing carbon dioxide is then expanded via a throttle 4, resulting in pyrogenic separation. The two-phase mixture is collected in a collection tank 5. According to the equally preferred embodiment shown in Fig. 2, the device has several, in particular two, collection tanks 5.Multiple collection containers 5 enable, in particular, batch processing of the two-phase mixture, especially improved separation of the product-containing gas phase. Specifically, multiple collection containers 5 allow for the continuous continuation of the carbon dioxide cycle process, especially the cryogenic separation, while the product-rich gas phase is removed in the collection container 5 and the liquid carbon dioxide phase is subsequently fed to the evaporator 6.

[0125] The subject matter of the present invention is further and without limitation clarified by the exemplary embodiments.

[0126] Example implementation:

[0127] Example 1: Subcritical process coupling

[0128] High-pressure electrolysis tests conducted at Fraunhofer UMSICHT have shown that the Faraday efficiencies for CO production using gaseous CO2 increase significantly with increasing pressure. At ambient pressure, the efficiency is approximately 30%, while at 30 bar it reaches over 70%. However, even at 30 bar, the CO concentration in the product gas is only about 5 mol%. Due to this low concentration, cryogenic gas separation, separation by PSA, or other separation processes would not be economically feasible. Furthermore, the overall efficiency decreases because the CO2 must be compressed before entering the electrolyzer, a process that requires significant energy. Integrating the electrolyzer between the compressor stage and the cooling system of a CO2 refrigeration unit would minimize the energy expenditure for compression and cryogenic gas separation.

[0129] In a subcritical CO2 refrigeration system, the pressure before the compressor is approximately 10 bar and the temperature is -20 °C (1). Compression to 30 bar raises the temperature to 95 °C (2). Subsequently, heat is released, cooling the CO2 to a temperature of 40 to 80 °C, which is the required temperature for electrolysis. In this state, the CO2 is partially converted to CO in the electrolyzer at 30 bar, with water being separated. Further heat release cools the CO2 to -5 °C. Phase separation occurs at this stage, resulting in a phase rich in CO. Expansion reduces the temperature to -40 °C. At this stage, a two-phase mixture exists, with the product gas becoming more concentrated in the gas phase.The product gas can be extracted and fed to a conventional gas purification process such as PSA (Pressure Swing Adsorption), membrane processes, or amine scrubbing to obtain high-purity synthesis gas. Reference numeral list: Device 8 Oil separator Electrolysis unit 9 Heat exchanger Compression unit 10 Heat transfer line Expansion unit 11 Separation unit O2 and H2O collection tank Evaporator 12 Separation unit H2O line A Bypass line 13 Precooler.

Claims

Patent claims:

1. A process for the electrochemical reduction of carbon dioxide, characterized in that the electrochemical reduction is carried out in the form of high-pressure electrolysis and that the reaction products of the electrolysis are separated by cryogenic separation of carbon dioxide.

2. Method according to claim 1, characterized in that the high-pressure electrolysis is coupled with a carbon dioxide refrigeration process.

3. Method according to claim 1 or 2, characterized in that the carbon dioxide refrigeration process is carried out transcritically or subcritically.

4. A method according to any of the preceding claims, characterized in that the reaction products of the electrolysis are selected from the group consisting of methanol, ethanol, n-propanol, formic acid, methane, ethylene, carbon monoxide, synthesis gas and mixtures thereof.

5. The method according to claim 4, characterized in that the reaction products of the electrolysis are selected from the group consisting of carbon monoxide, synthesis gas and mixtures thereof.

6. Method according to one of the preceding claims, characterized in that carbon dioxide and, if applicable, water are reduced during electrolysis.

7. Method according to one of the preceding claims, characterized in that a compound from the group consisting of water, glycerol, methanol, chloride and mixtures thereof, preferably water, is oxidized during electrolysis.

8. Method according to one of the preceding claims, characterized in that the electrolysis is carried out at pressures of at least 30 bar, in particular, pressure is applied at least 40 bar, preferably at least 50 bar.

9. Method according to one of the preceding claims, characterized in that the electrolysis is carried out at pressures in the range of 30 to 150 bar, in particular 30 to 130 bar, preferably 40 to 120 bar, preferably 50 to 100 bar, particularly preferably 50 to 90 bar.

10. Method according to one of the preceding claims, characterized in that the electrolysis is carried out at temperatures of at least 20 °C, in particular at least 30 °C, preferably at least 40 °C.

11. Method according to one of the preceding claims, characterized in that the electrolysis is carried out at temperatures in the range of 20 to 180 °C, in particular 30 to 150 °C, preferably 40 to 120 °C, preferably 40 to 100 °C, particularly preferably 40 to 80 °C.

12. Method according to one of the preceding claims, characterized in that the proportion of the reaction products after electrolysis in a mixture of substances leaving a cathode compartment of an electrolysis cell, in particular a carbon dioxide stream, is 1 to 40 mol%, in particular 2 to 25 mol%, preferably 3 to 20 mol%, preferably 5 to 15 mol%, based on the mixture of substances, in particular the carbon dioxide stream.

13. Method according to one of the preceding claims, characterized in that the proportion of the reaction products in a liquid, gaseous or supercritical phase is increased by means of cryogenic separation compared to the mixture of substances leaving a cathode compartment of an electrolysis cell, in particular a carbon dioxide stream.

14. Method according to claim 13, characterized in that the proportion of reaction products in a gas or supercritical phase is increased by means of cryogenic separation compared to the mixture of substances leaving a cathode compartment of the electrolysis cell, in particular a carbon dioxide stream.

15. A method according to one of the preceding claims, characterized in that the reaction products, in particular the phase containing the reaction products, are removed from the method.

16. A process for the electrochemical reduction of carbon dioxide, in particular according to one of the preceding claims, characterized in that (a) in a first process step an electrochemical reduction of carbon dioxide and, if applicable, water is carried out in the form of high-pressure electrolysis and (b) in a subsequent second process step, the reaction products of electrolysis obtained in the first process step are separated by cryogenic separation of carbon dioxide, the cryogenic separation being part of a carbon dioxide colding process.

17. Method according to one of the preceding claims, characterized in that at least one electrolyzer for carrying out the electrolysis is arranged in a bypass of a carbon dioxide refrigeration process.

18. Method according to one of the preceding claims, characterized in that the carbon dioxide is carried out in a cycle process, in particular at least partially returned to electrolysis, preferably in process step (a).

19. Device (1), in particular a carbon dioxide refrigeration machine, for generating cold for cooling purposes, comprising carbon dioxide as a refrigerant, and for the electrochemical reduction of carbon dioxide, characterized in that the device (1) (a) an electrolysis apparatus (2) for the electrochemical reduction of carbon dioxide and, if applicable, water, (b) a compression device (3) for compressing carbon dioxide, (c) an expansion device (4) for the expansion of a gaseous or supercritical mixture and (d) has a collection container (5) for receiving an expanded, multiphase mixture.

20. Device (1 ) according to claim 19, characterized in that the device (1 ) is designed to convey carbon dioxide in a circulating process.

21. Device (1) according to claim 19 or 20, characterized in that the electrolysis device (2) is arranged in a bypass of the carbon dioxide refrigeration machine.

22. Device (1) according to one of claims 19 to 21, characterized in that the expansion device (4) is connected to the collection container (5), in particular wherein the collection container (5) follows the expansion device (4) downstream in the carbon dioxide cycle.

23. Device (1 ) according to one of claims 19 to 22, characterized in that the collection container (5) and the compression device (3) are connected.

24. Device (1) according to claim 23, characterized in that an evaporation device (6) is arranged between the collection container (5) and the compression device (3).

25. Device (1 ) according to claim 24, characterized in that the evaporation device (6) has a heat exchange device, in particular for extracting cold.

26. Device (1 ) according to one of claims 19 to 25, characterized in that the compression device (3) and the expansion device (4) are connected.

27. Device (1) according to one of claims 19 to 26, characterized in that the electrolysis device (2) is connected to the compression device (3) and the expansion device (4), in particular in a bypass to the connection between the compression device (3) and the expansion device (4).

28. Device (1 ) according to one of claims 26 or 27, characterized in that a heat exchange device (9) is arranged between the compression device (3) and the expansion device (4), in particular between the compression device (3) and the electrolysis device (2), preferably for the removal of heat.

Images