Method and apparatus for purifying gaseous products from CO2 electrolysis

Abstract

The invention relates to a method for treating products from the electrolysis of CO2, in which carbon dioxide (CO2) and water (H2O) are electrochemically converted in the cathode space of an electrolysis cell, in which conversion a gaseous cathode product (11) is formed which has at least ethylene (C2H4), hydrogen (H2) and carbon monoxide (CO), the gaseous cathode product (11) is treated in a multi-stage separation process. In a first step (T1), the product gas stream (11) of the cathode is desublimated such that CO2 and water (H2O) are frozen and separated from the product gas stream (11). In a second step (T2), the CO2-removed product gas stream (11) is compressed to a certain pressure, and subsequently in a third step (T3), the compressed product gas stream (11) is subjected to gas permeation, in which hydrogen (H2) in the product gas stream (11) is separated by guiding hydrogen through a hydrogen-permeable membrane of a membrane unit (21). In a fourth step (T4), the retentate containing ethylene (C2H4) and carbon monoxide (CO) remaining in the product gas stream (11) is separated by distillation, thereby separating ethylene (C2H4) and carbon monoxide (CO). The invention also relates to a corresponding device for treating a product gas stream (11) from CO2 electrolysis.

Description

Technical Field

[0001] The invention relates to a method for purifying gaseous products from the electrolysis of CO2. The invention also relates to an apparatus for purifying gaseous products from the electrolysis of CO2. Background Art

[0002] Currently, about 80% of the world's energy needs are met by burning fossil fuels. Through these combustion processes and further industrial production processes, about 38.017 million tons of carbon dioxide (CO2) were released into the atmosphere worldwide in 2019 (European Commission, Joint Research Centre, Crippa, M., Guizzardi, D., Muntean, M., et al., Fossil CO2 and GHG emissions of all world countries: 2020 report, Publications Office, 2020, https: / / data.europa.eu / doi / 10.2760 / 56420).

[0003] In addition to the production of energy from fossil fuels, the production and conversion of industrial raw materials also contribute significantly to the continued increase in atmospheric CO2 concentrations, accounting for 9.2% of total emissions (Data for Climate Action - Historical GHG Emissions - Global Historical Emissions. In: Climate Watch. World Resources Institute (WRI)). The discussion of the negative impact of greenhouse gas CO2 on the climate has led people to consider the recycling and reuse of CO2. From a thermodynamic point of view, CO2 has low energy and is therefore difficult to reduce back to usable products.

[0004] In nature, CO2 is converted into carbohydrates by photosynthesis. This process, which is divided into multiple sub-steps in time and space at the molecular level, is difficult to replicate technically on a large scale. Compared with pure photocatalysis, the electrochemical reduction of CO2 is currently a more effective approach. As in photosynthesis, in this process, CO2 is converted into products with higher energy (such as CO, CH4, C2H4, etc.) by inputting electrical energy, which is obtained by renewable energy such as wind energy or solar energy. The energy required in this reduction process is ideally consistent with the combustion energy of the fuel, and should only come from renewable energy, or use electricity that cannot be obtained by the power grid. However, renewable energy cannot be continuously overproduced, but can only be temporarily overproduced when the sun is strong and the wind is strong. With the further development of renewable energy, this will be improved in the near future.

[0005] As with many processes that are carried out and intensified by catalysis, the chemical reactions involved do not proceed selectively, but rather give a product spectrum consisting of the different components formed. Furthermore, the reactions do not proceed quantitatively, but rather give a product stream consisting of a mixture of unreacted consumed reactants and target products as well as by-products. These mixed and non-specific product streams cannot be used industrially and commercially in this form and their components must be separated. The separated reactants can be fed back to the electrolysis process.

[0006] Depending on the requirements, it is usually necessary to provide the chemical raw materials in each case with very high purity (>99.9%). It is important to find and combine suitable separation steps / separation methods so that on the one hand high purity and high yield can be achieved and on the other hand the energy consumption required for this is as low as possible, in order to achieve the separation and specific selection of the desired product, especially on an industrial scale.

[0007] Solutions in the prior art to date

[0008] Until the 1970s, the electrochemical reduction of CO2 was extensively systematically studied. Despite many efforts, no electrochemical system has yet been successfully developed that can be used to reduce CO2 to a competitive energy carrier stably and energetically economically at sufficiently high current densities and acceptable yields over a long period of time. Due to the increasing shortage of fossil fuels and the unstable availability of renewable energy sources, research on the reduction or utilization of CO2 has become a focus of attention. Here, metals are usually used as catalysts for the electrolysis of CO2.

[0009]

[0010]

[0011] Table 1 is based on the literature Y. Hori, Electrochemical CO2 reduction on metal electrodes, in: C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189.

[0012] The table above summarizes the Faradaic efficiency for the conversion of CO2 into various products at different metal electrodes. The Faradaic efficiency (also known as Faradaic efficiency, Faradaic yield, Coulombic efficiency or current efficiency) describes the efficiency with which charge (electrons) are transferred in a system capable of carrying out an electrochemical reaction. The word "Faraday" in this term has two interrelated aspects. First, historically the unit of charge was the farad, although this has been replaced by the coulomb. Second, the associated Faraday constant relates charge to the number of moles of substance and the number of moles of electrons (the amount of substance). This phenomenon was first understood through the work of Michael Faraday and expressed in his law of electrolysis.

[0013] Table 1 shows typical Faradaic efficiencies on different metal cathodes. Thus, CO2 is reduced almost exclusively to CO on Ag, Au, Zn, almost exclusively to CO with certain limitations on Pd, Ga, whereas large amounts of hydrocarbons are observed as reduction products on copper. In addition to the pure metals, metal alloys are also of interest, since metal alloys can increase the selectivity for specific hydrocarbons. However, in the prior art, there are still only a few studies on metal alloys.

[0014] The following reaction formula shows the reactions occurring at the anode and cathode for the reduction at the copper cathode. Here, the production of expensive, i.e. economically valuable ethylene is of particular interest. The reduction on other metals is given in a similar manner.

[0015] Cathode: 2CO2+12e － +12H + →C2H4+4H2O (I)

[0016] Anode: 6H2O→3O2+12H + +12e － (II)

[0017] Overall equation: 2CO2+2H2O→C2H4+3O2 (III)

[0018] In addition, however, there are a range of by-products, such as ethanol, CO, H2 as well as formates and acetates. The electrochemical conversion of CO2 into usable hydrocarbons has been investigated and demonstrated in laboratories and pilot plants.

[0019] On the other hand, efficient purification of the product streams from the conversion reactions, which is necessary for industrial utilization, has not been solved to date.

[0020] Although these so-called downstream processes for the industrial production of ethylene from fossil feedstocks (e.g. hydrocracking) have been known for many years and have been largely optimized, due to the completely different composition of the product streams, these methods cannot be applied or cannot be applied without modifications to the purification of products from the electrochemical conversion of CO2. Summary of the invention

[0021] Against this background, the object of the present invention is to provide a method by means of which an efficient separation from CO 2 electrolysis can be achieved on an industrial scale and the desired products can be selected as specifically as possible.

[0022] Another object of the present invention is to provide a corresponding device configured to effectively separate and select products from CO2 electrolysis.

[0023] According to the present invention, the purpose of the method is achieved by a method for treating products from CO2 electrolysis, in which carbon dioxide CO2 and water are converted electrochemically in the cathode space of the electrolytic cell, and a gaseous cathode product is formed in the conversion, which has at least ethylene, hydrogen and carbon monoxide, wherein the gaseous cathode product is processed in a multi-stage separation process. The method is characterized by the following steps: in the first step, the product gas flow of the cathode is desublimated so that CO2 and water are frozen and separated from the product gas flow. In the second step, the product gas flow from which CO2 is removed is compressed to a certain pressure. In the third step, the compressed product gas flow is subjected to gas permeation, wherein the hydrogen in the product gas flow is separated by guiding hydrogen through a membrane permeable to hydrogen. In the fourth step, the retentate containing ethylene and carbon monoxide remaining in the product gas flow is separated by distillation, thereby separating ethylene and carbon monoxide.

[0024] The invention provides a particularly advantageous and mutually coordinated multi-stage separation sequence which makes particularly effective use of the specific physical properties of the substances involved in order to achieve a process which is as energy-efficient and cost-effective as possible for purifying and extracting ethylene as the preferred target product.

[0025] By means of a multi-stage separation process, high-purity ethylene can be obtained with high selectivity from the gaseous products of the CO 2 electrolysis in a particularly advantageous manner. The sequence of the separation processes used in the process is also advantageously selected.

[0026] Preferably, in the third step, the product gas stream is compressed to a pressure of 10 bar to 50 bar, in particular 45 bar. This achieves an advantageous volume flow reduction, especially since CO 2 and water have previously been separated from the product gas stream by freezing in the first step.

[0027] Further preferably, the compressed retentate from the membrane separation is introduced into a rectification column for distillative separation, wherein ethylene is obtained in the liquid phase and carbon monoxide is obtained in the gas phase. Thus, valuable high-purity ethylene is ultimately obtained from a large number of components in the product gas stream, which is formed as a product of the electrochemical conversion path of the CO2 electrolysis.

[0028] In the process, ethanol is also formed as a cathode product, wherein the ethanol condenses and is extracted in the liquid phase. Thus, another valuable substance, namely ethanol, is obtained, which is selectively separated.

[0029] Preferably, formates and / or acetates are also formed as cathode products in liquid or dissolved form, which are separated from the product gas stream in a separation process.

[0030] According to the present invention, the purpose of the device is achieved by a device for treating a product gas stream from CO2 electrolysis, wherein the product gas stream contains at least ethylene, hydrogen and carbon monoxide as gaseous cathode products, and the device comprises: a desublimation unit for freezing CO2 and water from the product gas stream; a compressor unit connected downstream of the desublimation unit and comprising a compressor; and a hydrogen-permeable membrane unit connected downstream of the compressor unit and for separating hydrogen from the product gas stream.

[0031] In a particularly preferred embodiment, the plant is equipped with a cryogenic distillation unit connected downstream of the hydrogen-permeable membrane unit.

[0032] It is further preferred that the compressor unit has a cooling device connected downstream of the compressor, by means of which the compressed product gas stream can be cooled and the heat of compression can be dissipated.

[0033] Further preferably, the cryogenic distillation unit has a rectification column, at the top of which a condenser is arranged, the condenser being designed to condense out ethylene.

[0034] Preferably, trays or packings are introduced inside the distillation column, which allow the ascending gas and the dripping liquid to contact intensively in countercurrent, thereby enabling a stepwise enrichment of ethylene in the liquid phase and a corresponding enrichment of carbon monoxide in the gas phase.

[0035] In a particularly preferred embodiment, the distillation column has a bottom which is designed so that high-purity liquid ethylene can be extracted at the bottom. Thus, ethylene can be selectively removed from the equipment forming the entire device and can be used as a valuable substance for other purposes.

[0036] In this case, it is preferred to arrange a heater unit at the column bottom, by means of which a part of the liquid stream arriving in the column bottom can be evaporated, so that a continuous countercurrent of gas and liquid can be achieved in the rectification column.

[0037] In a particularly preferred embodiment, the device can be connected to a CO2 electrolysis plant via a connection unit so that a product gas stream from the CO2 electrolysis can be fed to the device. In this case, a connection unit on the cathode side for the cathode product and a connection unit on the anode side for the anode product can be provided.

[0038] The invention is based on the recognition that, due to the specific functional principle of CO2 electrolysis, different electrochemical conversion reactions occur in the spatially separated cathode and anode spaces of a CO2 electrolysis cell or in an electrolyzer with a plurality of CO2 electrolysis cells, which must be taken into account for an efficient separation and purification method. Under the usual reaction conditions, gaseous and condensed products are generated, wherein the product streams leaving the electrolysis cell or electrolyzer can be distinguished essentially into four product streams:

[0039] Gaseous products from the cathode space;

[0040] The catholyte and the products dissolved therein;

[0041] Gaseous products from the anode space;

[0042] • Anolyte and products dissolved therein.

[0043] In this regard, various possible solutions for more detailed functional design and structural design of electrolysis are described in the literature. Thus, a variety of possible solutions are available for the construction and operation of CO2 electrolysis cells. The method and device of the present invention can be applied particularly flexibly to different electrolysis cell structures and operating modes of CO2 electrolysis.

[0044] Therefore, the CO2 electrolyzer can include various designs and operating modes, wherein combinations and variants are also possible. Therefore, they should not be understood as being restrictive and respectively exclusive, but rather different sub-variants are also allowed. From the perspective of their basic functions, different variants of CO2 electrolyzers and their operation are known in principle. Here, the structure of the CO2 electrolyzer can adopt a so-called flow-cell architecture (flow-cell-architecture) (two gaps (Two Gaps)), a single-gap architecture (one-gap-architecture) or a zero-gap architecture (zero-gap-architecture), and these electrolyzer structures are advantageously used as upstream processes of purification within the scope of the present invention.

[0045] The separation and treatment method of the present invention consists of these basic processes, which, when utilized, can be flexibly applied and adapted to various electrolytic cell designs and operating modes of the electrolytic cell, respectively. Thus, when used for its application, the method can be specifically different in terms of treating the electrolyte in the CO2 electrolytic cell and the gases generated on the cathode side and the anode side, respectively.

[0046] Common to all variants of the CO2 electrolysis is the so-called flow-by-flow operation, i.e. the CO2 sweeps over the side of the gas diffusion electrode facing away from the electrolyte. In this process, product gas is generated in the gas space and discharged together with the excess CO2. The product is then fed to the separation method and the processing method according to the invention.

[0047] As mentioned above, the variants differ mainly in the number of gaps or reaction zones in the electrolytic cell and in the use of different membrane types. At the cathode the following reactions mainly occur:

[0048] 2CO2+8H2O+12e － →C2H4+12OH －

[0049] 2CO2+9H2O+12e － →C2H5OH+12OH －

[0050] 2CO2+5H2O+8e － →CH3COO － +7OH －

[0051] CO2+H2O+2e － →HCOO － +OH －

[0052] 2H2O+2e －→H2+2OH －

[0053] CO2+H2O+2e － →CO+2OH －

[0054] For neutral molecules on the product side, such as ethylene, ethanol, or CO, a corresponding number of hydroxide ions are formed according to the required electrons. For species with a unit negative charge, the number of hydroxide ions is reduced by one, that is, the charge is compensated by the generated anions.

[0055] The hydroxide ions react with excess CO2 to form carbonate or bicarbonate which is released into the electrolyte according to the following reaction:

[0056] CO2+OH － →HCO3 －

[0057] CO2+2OH － →CO3 2－ +H2O

[0058] Formate and acetate are accordingly enriched and must be separated from the carbonate-containing electrolyte. For this purpose, a salt separation method is used. According to the invention, distillable products such as ethanol or ethylene are separated in a particularly advantageous manner.

[0059] In the two-gap structure, CO2 and O2 are formed in the anode gap.

[0060] H2O→1 / 2O2+2H + +2e －

[0061] The released protons release CO2 which is chemically absorbed by the cathode side reaction.

[0062] HCO3 － +H + →H2O+CO2

[0063] CO3 2－ +2H + →H2O+CO2

[0064] Since O2 and CO2 are released at the same location, a two-gap electrolyser design does not appear to be very favorable or economical for commercial operation due to the additional separation issues.

[0065] This problem is avoided in the particularly preferred single-gap design, because CO2 is released in the gap, and O2 is correspondingly released on the anode side. The O2 released on the anode side contains only a small percentage of CO2. The CO2 released in the gap can be recycled. Therefore, commercial operation preferably uses a single-gap design. An additional electrolyte loop can be optionally connected to the back side of the anode, which is not shown in the accompanying drawings. The electrolyte loop can be composed of water or acid. Salts require additional electrolyte management to control the cation balance in the electrolyte. Therefore, in the method of the present invention, the electrochemical conversion of carbon dioxide and water in the cathode space is preferably carried out in a single-gap configuration.

[0066] The zero-gap architecture of the CO2 electrolyzer comprises a membrane that conducts anions. In addition to the hydroxide ions shown, bicarbonate, carbonate, formates or acetates are also ionic charge carriers that move to the anode. There they either release CO2 again or are neutralized to form formic acid or acetic acid. Alternatively, formates, formic acid, acetates or acetic acid can be reoxidized to CO2, which is quite detrimental to the overall efficiency.

[0067] In the particularly preferred design of the separation method and the treatment method of the present invention, the electrolytic cell design of the single gap structure is preferably adopted. This is advantageous, mainly because of its inherent CO2 separation in the gap. However, according to the application, other or combined electrolytic cell devices, such as electrolytic cell devices with a combination of anion-conducting membranes and cation-conducting membranes (i.e., bipolar membranes), may also be used alternatively. However, these electrolytic cell devices are generally less preferred because of their complexity or, in some cases, lower energy efficiency.

[0068] In practice, liquid or dissolved products and gaseous products are preferably separated from the electrolysis in corresponding gas-liquid phase separators, into which the two-phase cathode electrolyte stream and anolyte stream are introduced. In this case, the cathode electrolyte stream is introduced into a first gas-liquid phase separator and the anolyte stream is introduced into a second gas-liquid phase separator. During the electrolysis, gaseous anode products and liquid anode products are generated on the anode side. Correspondingly, gaseous cathode products and liquid cathode products are generated on the cathode side. The gaseous cathode product is of particular interest for processing and separating ethylene, especially as a high-value product in the cathode electrolyte stream, according to the method of the present invention.

[0069] The degassed catholyte and the degassed anolyte are preferably recirculated to the electrolysis cell. This circuit also serves to remove the generated process heat via a correspondingly designed heat exchanger. In the case of electrochemical conversion of CO2 into hydrocarbons, CO2 is introduced into the cathode space as a reactant.

[0070] As can be seen from Table 1, a product spectrum is formed during the electrochemical conversion process, which essentially comprises methane (CH4), ethylene (C2H4), hydrogen (H2), carbon monoxide (CO), ethanol (C2H5OH), propanol (C3H7OH) and formate (HCOO － ), but is also known to contain acetate (H3CCOO － ).

[0071] In the gas stream on the cathode side, during the electrolysis in an electrolyzer having a plurality of electrolysis cells, ethylene, hydrogen and CO are preferably produced as gaseous cathode products. In addition, the product stream also contains a large amount of CO2, which reacts in a superstoichiometric ratio in the electrolysis. Under the conditions in the corresponding gas-liquid phase separators for the cathode electrolyte stream and the anolyte stream, water vapor and other liquid products are contained in the gaseous product stream as further components, depending on their saturated vapor pressure.

[0072] In such an operated CO2 electrolysis, it is particularly preferred to obtain or separate ethylene as a product. In order to obtain commercially usable products, ethylene must be separated from the gas stream in the purest possible form.

[0073] To this end, the present invention proposes an advantageous multi-stage and suitable separation sequence which effectively utilizes the particular physical properties of the substances involved in order to achieve an energy-efficient and cost-efficient method for purifying and extracting ethylene as the preferred target product that is as high as possible. The electrochemical reduction of CO2 to hydrocarbons, in particular to the valuable chemical raw material ethylene C2H4 (about 1000€ / t), has been described in the literature since the 1990s. In recent years, research activities have increased significantly, because the storage / use of this energy seems to make sense in economic terms due to the availability of excess electrical energy from non-fossil energy sources, such as solar or wind power.

[0074] The invention describes a particularly energy-efficient method for generating products that are economically and practically usable in sufficient purity from a CO2 electrolysis process. Such a method was previously unknown. The proposed method management is technically feasible and feasible in terms of equipment technology and has been tested and confirmed with the help of reliable modern process simulation tools. In this regard, a series of close, conventional separation sequences, such as the classical thermal separation of gas components according to the boiling order, have also been studied within the scope of process synthesis. However, due to specific material properties or system properties, it has been shown that this is energetically disadvantageous or technically / physically impractical. For example, due to the small difference in volatility, the separation of CO2 and ethylene by distillation is very complicated, and more precisely, due to the formation of azeotropes predicted in process simulations, it may even be impossible to separate CO2 and ethylene by simple distillation. In this regard, the method and device of the invention are superior.

[0075] In this case, a separation sequence is provided in a particularly advantageous manner, in which a very effective combination of particular method steps is provided, wherein particular advantages result from particular component properties and system properties.

[0076] Furthermore, the proposed method steps solve the problem of components that need to be recycled and components that have to be discarded surprisingly very economically and efficiently, thereby simultaneously providing a very resource-saving process in which only small proportions of unusable waste products or emissions are generated.

[0077] The advantages of the method according to the invention and of the device according to the invention can be appropriately transferred to one another. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Further advantages, features and details of the invention are apparent from the following description of preferred exemplary embodiments and from the drawings. The features and feature combinations mentioned above in the description and the features and feature combinations mentioned below in the description of the drawings and / or shown only in the drawings can be used not only in the respectively given combination but also in other combinations or alone without departing from the scope of the invention.

[0079] Embodiments of the present invention are explained in more detail below with the aid of the accompanying drawings. Here, schematically and greatly simplified:

[0080] Figure 1 In one embodiment, a device for processing products from CO2 electrolysis in different separation steps is shown;

[0081] Figure 2 In one exemplary embodiment, an overall system is shown which has a CO 2 electrolysis system and a processing device for the electrolysis products connected downstream on the product stream side.

[0082] The same reference numerals have the same meaning in the figures. DETAILED DESCRIPTION

[0083] Figure 1An embodiment of the separation step sequence according to the present invention is shown. It shows a device for treating products from CO2 electrolysis in different separation steps. The separation or purification of gaseous products on the cathode side of a CO2 electrolysis cell or electrolysis device is shown in detail in a schematic diagram. The input stream 11 entering the separation sequence includes gaseous cathode products and is composed of CO2 in a large part (about 80m%). As a minor component, there are about 5m% carbon monoxide CO and about 0.5 to 1m% hydrogen H2. The content of ethylene C2H4 as the target product is about 3 to 4m%. The input stream 11 is saturated with water H2O at about 1 bar and 60°C, which corresponds to a content of about 10m%.

[0084] It has proven to be particularly preferred in the method to first separate carbon dioxide CO2 and water H2O in a first separation step T1. In this desublimation step, a selective separation can advantageously be carried out by cooling the gaseous input stream 11 to such an extent that the water H2O and the carbon dioxide CO2 freeze. For this purpose, a process temperature of less than about -80° C. is set in the separation step T1, wherein this is possible even without energy-intensive and cost-intensive compression. In the simplest case, the freezing can be carried out, for example, in a cooled container, in which dry ice and frozen water accumulate during the cooling process.

[0085] The containers or reactors of the desublimation unit 15 are preferably designed redundantly so that when the holding capacity of the first container 15A is reached, a switch can be made to the second container 15B and continuous operation can be achieved in this way. The frozen CO2 and water can then be returned to the gaseous state by heating and in this way the previously filled desublimation containers 15A, 15B can be emptied again. The released CO2 and water H2O can be recycled and preferably fed back into the electrolysis process.

[0086] Another advantage of the method is that, since carbon dioxide CO2 and water H2O together here constitute about 90 m% of the input flow 11 to the desublimation unit 15, the product mass flow to be processed is greatly reduced in this step. This is meaningful and advantageous, because in the subsequent treatment step T2, the remaining reduced gaseous product mass flow is compressed to 10 to 50 bar. For this purpose, a compressor unit 17 with a compressor 19 is provided. Since the product mass flow is reduced, the subsequent process step T2 can be carried out with much less equipment technology expenditure and energy consumption in terms of the compressor performance of the compressor 19.

[0087] The heating caused by the compression in the compressor 19 in the separation step T2 is preferably compensated by corresponding cooling, so that the gaseous product mass flow is kept at a temperature of about 60° C. For this purpose, the compressor unit 17 has a cooling device 25 which is connected downstream of the compressor 19 in the flow direction of the product mass flow.

[0088] After compression and cooling in the compressor unit 17, the next separation step T3 in the form of gas permeation is carried out. In this step, the fact that hydrogen is a very small molecule and therefore exhibits a relatively high permeability by corresponding use of a gas permeable membrane is used in a targeted manner. For this purpose, a membrane unit 21 with a membrane that is permeable to hydrogen is connected to the downstream of the compressor unit 17 in a flow-technical manner. Therefore, the following treatment is performed in the gas permeation step T3 of the method: hydrogen H2 is very effectively separated from the product gas flow in the separation step T3. The hydrogen H2 that selectively passes through the membrane that is permeable to hydrogen is preferably also transported back to the upstream CO2 electrolysis process. Alternatively, in particular in the case of undesirable accumulation, the hydrogen gas flow or part of it can also be removed from the device. Hydrogen is an inert gas to the electrochemical reaction itself, which even promotes the diffusion of the product. After accumulation, it is preferably separated and used separately. Since the membrane-based separation in step T3 is not carried out completely selectively and other volatile secondary components can also pass through the hydrogen-permeable membrane to a certain extent (even if to a smaller extent), this material flow can be used as output so that the concentration of secondary components is not undesirably increased in the process.

[0089] The remaining retentate from separation step T3 now contains mainly carbon monoxide CO and ethylene C2H4, the difference in their volatility is large enough to be separated by distillation at a correspondingly low temperature in step T4 by means of a cryogenic distillation unit 23. For this purpose, the already compressed retentate stream from the membrane separation is introduced into a distillation column 27, which in a preferred embodiment of the method is set and operated at low temperatures, i.e. at a pressure of about 50 bar and at about -100° C. at the top and about +6° C. at the bottom.

[0090] A condenser 29 is arranged at the top of the distillation column 27, which cools the rising gas so that the relatively high-boiling ethylene C2H4 condenses and is directed back into the distillation column 27. The condenser 29 is only partially operated, and the carbon monoxide gas stream produced in the condenser 29 is directed back to the electrolysis in gaseous form, or a partial stream can also be conveyed here for thermal utilization. Carbon monoxide is advantageously also reduced to ethylene C2H4 or ethanol C2H5OH on the same catalyst.

[0091] Inside the rectification column 27 there are internal components, such as so-called trays or packings, which bring the rising gas and the dripping liquid into intensive contact according to the countercurrent principle. This process leads to a stepwise enrichment of ethylene C2H4 in the liquid phase and a corresponding enrichment of carbon monoxide CO in the gas phase.

[0092] In this way, high-purity ethylene C2H4 can be obtained at the bottom 31 of the distillation column 27. In order to maintain the distillation process T4 also in the area below the gas feed inlet, part of the liquid stream reaching the bottom 31 is evaporated by means of a heater unit 33, so as to ensure and continuously maintain a continuous countercurrent of gas and liquid during operation.

[0093] As a further valuable product in the electrolysis of carbon dioxide CO2, ethanol C2H5OH is formed, which, under the operating conditions of the electrolysis, enters the multi-component material flow mainly in liquid form after the gas separator 107. When the electrolysis plant is connected to a downstream device for processing the product gas flow, it is possible and advantageous to introduce a partial flow from this material flow into a further distillation column 111. Figure 2 Such an interconnection is shown by way of example in . Here, ethanol C2H5OH in an azeotropic composition can be obtained as a top product in the distillation column 111. The bottom product is led back to the CO2 electrolysis.

[0094] In this case, a partial stream 13 can be removed in order to remove further by-products (eg formates and acetates) in liquid or dissolved form in a targeted manner from the overall process.

[0095] Figure 2 A schematic diagram based on the above-described method steps is shown, thereby realizing a preferred integrated overall method, which includes CO2 electrolysis on the one hand and subsequent purification of the product in the separation steps of desublimation T1, compression T2, membrane-based separation T3 and cryogenic distillation T4 on the other hand.

[0096] Here, Figure 2 A schematic diagram of the overall method is shown in a flow chart of the process according to the present invention for electrochemically producing ethylene C2H4 and ethanol C2H5OH from CO2 and water H2O and separating the gaseous products on the cathode side.

[0097] Here, as already described, water H2O and carbon dioxide CO2 are first converted electrochemically into ethylene, hydrogen, carbon monoxide, ethanol, formate and acetate in the cathode space 101 in the electrolysis device 100 or electrolysis cell. In the anode space 103, oxygen is essentially formed, which is separated from the anolyte with sufficient purity by means of a gas-liquid separator 105.

[0098] The cathode electrolyte with the reaction products is also introduced into the corresponding gas-liquid separator 107, and the generated gas stream and liquid stream are processed separately. The product gas stream 11 of the cathode contains about 80m% carbon dioxide CO2. As a minor component, there are about 5m% carbon monoxide CO and about 0.5 to 1m% hydrogen. As the target product for treatment and separation, it contains ethylene C2H4 with a proportion of about 3 to 4m%. The product gas stream 11 of the cathode is saturated with water at about 1 bar and 60°C, which corresponds to a content of about 10m%.

[0099] This product gas stream 11 is also referred to as the feed stream and is firstly fed into a desublimation unit 15, in which the gas is cooled to at least -80°C, thereby freezing CO2 and water H2O out of the product gas stream 11. The desublimation is designed redundantly with the aid of two containers, namely a first container 15A and a second container 15B. As a result, switching can be performed when the holding capacity of one of the containers 15A, 15B is reached, and continuous operation is thus possible. The already frozen CO2 and water H2O can then be returned to the gaseous state by heating, and thus the previously filled container 15A or container 15B can be emptied. The released CO2 and water H2O can be recycled and fed back into the electrolysis process of the electrolysis device 100.

[0100] In step T1 (see Figure 1 ) and the CO2-free product gas stream 11 is compressed in a compressor 19 to 45 bar and adjusted to a temperature of 60° C. in step T2 and then introduced into a membrane unit 21 for gas permeation, wherein hydrogen preferably passes through the membrane and is thus selectively separated from the product gas stream 11. This hydrogen stream is preferably also conducted back into the electrolysis device 100, or in the case of undesirably high concentrations, this hydrogen stream or a portion thereof can also be removed from the process. Since the membrane separation in the membrane unit 21 is generally not carried out completely selectively and therefore other volatile secondary components 109 can also pass through the membrane to a lesser extent, this material stream 109 can be used as an output so that secondary components do not accumulate to an undesirable extent in the process.

[0101] The remaining product stream, which mainly comprises or consists of carbon monoxide CO and ethylene C2H4, is separated by means of a cryogenic distillation column 27, the separation performance of which is There are about 10 theoretical separation stages in the column and about 20 theoretical stages in the stripping section. A reflux ratio of 2 is required for adequate separation. At a column pressure of 45 bar, condensation takes place at the top of the column at a temperature of -100° C. and the bottom of the column is adjusted to a temperature of +5.6° C. This results in a very high purity of 99.98 vol% ethylene at the bottom 31 of the column.

[0102] The condenser 29 is only partially operational and the carbon monoxide stream produced in the condenser 29 is fed back in gaseous form into the electrolysis plant 100 or, in particular in the case of very high concentrations in the process, also as a partial stream for other uses.

[0103] The liquid catholyte stream leaving the gas separator 107 on the cathode side contains about 10m% of ethanol C2H5OH as another valuable product. The catholyte contains, in addition to other by-products (<1% of acetate and formates), essentially 73m% of water, 11m% of potassium bicarbonate and 7m% of potassium sulfate. The material stream is introduced into another rectification column 111, which comprises a rectification part with about 15 theoretical separation stages and a stripping part with about 30 theoretical separation stages, wherein an ethanol / water mixture with an azeotropic composition is obtained as a top product by distillation. The other rectification column 111 is operated at atmospheric pressure with a reflux ratio of 2. The condensation temperature is 78° C. and the bottom boiler is heated to 100° C. As a bottom product, a catholyte from which ethanol C2H5OH has been released is obtained and is directed back to the electrolysis device.

[0104] In order to remove other by-products present in liquid or dissolved form (eg formates and acetates) from the overall process, a partial stream 13 of the bottom product is removed and conveyed to other uses.

Claims

1. A method for treating products from CO2 electrolysis, wherein: Carbon dioxide CO2 and water are converted electrochemically in the cathode space of an electrolytic cell, in which a gaseous cathode product (11) is formed, which contains at least ethylene (C2H4), hydrogen (H2) and carbon monoxide (CO), wherein the gaseous cathode product (11) is processed in a multi-stage separation process, characterized in that the method comprises the following steps: (T1) in a first step, desubliming the cathode product gas stream (11) so that CO2 and water are frozen and separated from the product gas stream (11), (T2) In a second step, the product gas stream (11) from which CO2 has been removed is compressed to a certain pressure, (T3) in a third step, subjecting the compressed product gas stream (11) to gas permeation, wherein hydrogen (H2) in the product gas stream (11) is separated by passing the hydrogen (H2) through a membrane permeable to hydrogen, and (T4) In the fourth step, the retentate containing ethylene (C2H4) and carbon monoxide (CO) remaining in the product gas stream (11) is separated by distillation, thereby separating ethylene (C2H4) and carbon monoxide (CO).

2. The method according to claim 1, wherein: In a third step T3 , the product gas stream ( 11 ) is adjusted to a pressure of 10 bar to 50 bar, in particular to 45 bar.

3. The method according to claim 1 or 2, wherein: The compressed retentate from the membrane separation in the membrane unit (21) is introduced into a rectification column (27) for distillative separation (T3), wherein ethylene (C2H4) is obtained in the liquid phase and carbon monoxide (CO) is obtained in the gas phase.

4. A method according to any one of the preceding claims, wherein: Ethanol (C2H5OH) is formed as cathode product, wherein the ethanol (C2H5OH) condenses and is extracted in the liquid phase.

5. A method according to any one of the preceding claims, wherein: As cathode product, formates and / or acetates are also formed as by-products ( 13 ) in liquid or dissolved form, wherein these by-products ( 13 ) are separated off.

6. An apparatus for treating a product gas stream (11) from CO2 electrolysis, the product gas stream containing at least ethylene (C2H4), hydrogen (H2) and carbon monoxide (CO) as gaseous cathode products, the apparatus comprising: a desublimation unit (15), the desublimation unit being used to freeze CO2 and water from the product gas stream (11); a compressor unit (17), the compressor unit being connected downstream of the desublimation unit (15) and comprising a compressor (19); and a hydrogen-permeable membrane unit (21), the hydrogen-permeable membrane unit being connected downstream of the compressor unit (17) and being used to separate hydrogen from the product gas stream.

7. The device according to claim 6, wherein: The desublimation unit (15) is designed redundantly, wherein a first desublimation container (15A) and a second desublimation container (15B) connected in parallel to the first desublimation container are provided.

8. The device according to claim 6 or 7, wherein: The apparatus has a cryogenic distillation unit (23) connected downstream of the hydrogen permeable membrane unit (21).

9. The device according to claim 6, 7 or 8, wherein: The compressor unit (17) has a cooling device (25) connected downstream of the compressor (19), by means of which the compressed product gas flow (11) can be cooled and the heat of compression can be dissipated.

10. The device according to claim 8 or 9, wherein: The cryogenic distillation unit (23) has a distillation column (27), at the top of which is arranged a condenser (29), which is designed to condense ethylene (C2H4).

11. The device according to claim 10, wherein: Plates or fillers are introduced into the interior of the distillation tower (27), which allow the rising gas and the dripping liquid to be in intensive contact, thereby enabling the gradual enrichment of ethylene (C2H4) in the liquid phase and the corresponding enrichment of carbon monoxide (CO) in the gas phase.

12. The device according to claim 10 or 11, wherein: The distillation tower (27) has a tower bottom (31) which is designed so that high-purity liquid ethylene (C2H4) can be extracted at the tower bottom (31).

13. The device according to claim 12, wherein: A heater unit (33) is arranged at the column bottom, by means of which a part of the liquid flow reaching the column bottom (31) can be evaporated, thereby achieving a continuous countercurrent of gas and liquid in the distillation column (27).

14. The device according to any one of claims 6 to 13, comprising a connection unit for receiving a product gas stream (11) from CO2 electrolysis.

15. An apparatus comprising a device according to claim 14 and a CO2 electrolysis device (100) connected to the device via the connection unit, so that a product gas stream (11) containing gaseous cathode products from the electrolysis can be conveyed to the device for purification.

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