Method of separating carbon dioxide from an air stream, absorbent for absorption of carbon dioxide from a gas stream, and method of separating off carbon dioxide in an electrochemical desorption unit

Abstract

A method of separating off carbon dioxide in an electrochemical desorption unit is specified. In one embodiment, the method of separating off carbon dioxide in an electrochemical desorption unit, where the electrochemical desorption unit comprises at least a first chamber and a second chamber separated by a cation-selective membrane, comprises the steps of: - introducing a first solution into the first chamber, where a molar concentration of hydrogencarbonate in the first solution is at least twice as high as a molar concentration of carbonate, - electrochemically desorbing carbon dioxide from the first solution to release carbon dioxide and form a second solution from the first solution. Additionally specified are a method of separating carbon dioxide from an air stream and an absorbent for absorption of carbon dioxide from a gas stream.

Description

[0001] Description

[0002] Process for separating carbon dioxide from an air stream, absorbent for absorbing carbon dioxide from a gas stream and process for separating carbon dioxide in an electrochemical desorption unit

[0003] A method for separating carbon dioxide from an air stream is specified. Furthermore, an absorbent for absorbing carbon dioxide from a gas stream and a method for separating carbon dioxide in an electrochemical desorption unit are specified.

[0004] One of the greatest challenges of the 21st century will be to return the gas composition of the atmosphere to levels that prevent excessive global warming. It has been scientifically proven that the warming in recent decades, in particular, has been caused by an increase in so-called greenhouse gases. Among the greenhouse gases, carbon dioxide in particular plays a key role, with the sharp rise in carbon dioxide concentrations being due to increased combustion of fossil fuels. In this respect, it will be important for the near future to not only reduce new carbon dioxide emissions but also to provide technical solutions for absorbing and storing carbon dioxide from the environment. Only by largely avoiding new greenhouse gases and efficiently managing existing ones can unwanted global warming be kept within reasonably tolerable limits.

[0005] One technical approach to removing carbon dioxide from the air involves passing the carbon dioxide over or through an absorbent, which selectively removes it from the air stream. This reduces the carbon dioxide concentration in the air, while the carbon dioxide concentration in the absorbent increases over the course of the process. This process is known for a variety of different absorbents, but the systems are sufficiently complex, as both the carbon dioxide concentration and the ambient conditions during absorption can vary so much that different absorbents achieve different absorption efficiencies. Furthermore, environmental aspects naturally play a major role, as efficient compositions are not always particularly environmentally friendly.A further aspect arises from the fact that the absorption medium must be cost-effective and not hinder the further processing of the absorbed carbon dioxide. This is particularly important because energy- and cost-saving isolation of the absorbed carbon dioxide, ideally with unrestricted reusability of the absorption medium, contributes significantly to the energy balance and thus to the attractiveness of the entire process. Thus, the economic viability and environmental friendliness of the process depend not only on the absorption, but also on the subsequent process steps, which ultimately contribute to the provision of carbon dioxide streams that are as pure as possible. These streams can then be converted in various further steps into desirable valuable materials, such as hydrocarbons or alcohols, which can make an additional contribution to the environmental balance.

[0006] Based on these considerations, a large number of carbon dioxide absorbents suitable for absorbing carbon dioxide and fundamental possibilities for further processing them have found their way into the patent literature.

[0007] For example, WO 2008 072 979 A1 discloses a method for capturing CO2 from exhaust gas in an absorber, wherein the CO2-containing gas is passed through an aqueous absorbent sludge, characterized in that the aqueous absorbent sludge contains an inorganic alkaline carbonate, bicarbonate and at least one of absorption accelerator and catalyst, and the CO2 is converted into solids by precipitation in the absorber, wherein the sludge with the precipitated solids is conveyed to a separation device in which the solids are separated, and substantially all of the at least one of absorption accelerator and catalyst is returned to the absorber together with the remaining aqueous phase.

[0008] Another patent document, WO 2020 152 330 A1, discloses a process for separating and recovering carbon dioxide from ambient air. The process comprises the continuous implementation of the following steps: a) contacting ambient air with an aqueous solution of at least one alkali or alkaline earth metal cation to absorb the carbon dioxide into the solution to form the bicarbonate or hydrogen carbonate.Carbonate of the at least one metal; b) electrodialysis of the resulting solution using a combination of bipolar ion exchange membranes and ion exchange membranes selective for mono- or polyvalent anions to obtain a solution enriched in (hydrogen) carbonate ions and a solution depleted therein, wherein the solution depleted in (hydrogen) carbonate ions is recycled to step a); c) thermal desorption of the carbon dioxide from the solution enriched in (hydrogen) carbonate ions obtained in step b) by means of steam stripping to obtain a carbon dioxide-water vapor mixture and a solution depleted in CO2, which is recycled to step b), wherein a pH between 7 and 8.5 or between 8 and 9.5 is adjusted therein; and d) separation of water from the resulting carbon dioxide-water vapor mixture by means of cooling to condense the water vapor and optionally further drying of the carbon dioxide.

[0009] EP 3 384 973 A1 describes a process for recovering carbon dioxide to enrich the gas streams used to produce sodium carbonate and sodium bicarbonate by the ammonia-soda process. The process comprises: bringing the streams of process gases and / or outlet gases occurring in the process for producing sodium carbonate and sodium bicarbonate by the ammonia-soda process into contact in the CO2 absorption column, comprising: - a partial or complete gas stream originating from lime combustion in shaft lime kilns with air blast, and / or - a partial or complete outlet gas stream from the plant for carbonating soda and / or

[0010] - part or all of the outlet gas stream from the baking powder carbonization plant,

[0011] - and, where appropriate, one or more streams of flue gases or other gases containing carbon dioxide resulting from the combustion of solid, liquid or gaseous fuels to generate heat or electrical energy to meet the production requirements by the soda-ammonium process;

[0012] - and optionally one or more flue gas streams or other carbon dioxide-containing gases originating from an external supplier with a stream of an aqueous absorbing solution to form a carbon dioxide-enriched aqueous absorbing solution, heating a carbon dioxide-enriched aqueous absorbing solution in the evaporator, desorbing gaseous carbon dioxide with regeneration of an aqueous absorbing solution in a desorption column, cooling the regenerated aqueous absorbing solution and recycling it to the CO2 absorption column and removing it from the desorption column and cooling the stream with a high carbon dioxide content for use in the process for producing sodium carbonate and sodium bicarbonate by the ammonia-soda process.

[0013] Such solutions known from the state of the art can offer further potential for improvement, in particular with regard to the efficiency of carbon dioxide absorption under variable ambient conditions and in particular with regard to the possibility of easily and quickly desorbing carbon dioxide once bound from the absorption liquid with little energy expenditure.

[0014] The object is to provide a process capable of desorbing carbon dioxide, once bound in an absorption liquid, in the purest form possible, as energy-efficiently and quickly as possible. A further object is to provide an absorbent for absorbing carbon dioxide from a gas stream with increased efficiency. A further object is to provide a process for the energy-efficient separation of carbon dioxide in an electrochemical desorption unit.

[0015] A process for isolating carbon dioxide from an air stream is specified, the process comprising at least the steps of: a) providing an aqueous solution of a carbon dioxide absorbent, wherein the carbon dioxide absorbent comprises cations from the 1st main group of the periodic table; b) passing a carbon dioxide-containing air stream through the solution provided in process step a), wherein at least a portion of the carbon dioxide from the air stream is bound to the carbon dioxide absorbent and the air stream is depleted of carbon dioxide; c) introducing the aqueous solution from process step b) or an aqueous solution comprising the carbon dioxide bound to the carbon dioxide absorbent into an electrolysis cell having a membrane selective for monovalent cations and electrolyzing the aqueous solution to release at least a portion of the carbon dioxide.

[0016] Here and in the following, an electrodialysis cell can be used instead of an electrolysis cell, and the aqueous solution can be electrodialyzed. Here and in the following, electrodialysis can therefore take place instead of electrolysis.

[0017] Surprisingly, it was discovered that the above-described process can be used to very efficiently absorb carbon dioxide from air streams and very selectively free it from the absorption medium via electrolysis. This overall process allows very different air streams with widely varying carbon dioxide contents to be effectively treated. For example, carbon dioxide can be absorbed from normal ambient air with a relatively low carbon dioxide content, or from exhaust streams with a very high carbon dioxide content, particularly from exhaust streams with a high carbon dioxide content that cannot be freed of carbon dioxide using other processes, or can only be freed with great effort, such as exhaust streams from natural gas combustion.Liquids laden with carbon dioxide can be processed particularly efficiently using electrolysis, so that the carbon dioxide can be desorbed from the liquid without a large amount of energy being used. A further advantage of this process is that the carbon dioxide can be produced in its pure form. For example, it is possible to control the release of carbon dioxide from the liquid during electrolysis so that this gas is essentially produced in its pure form. In these cases, the carbon dioxide gas stream does not need to be further freed from other gas components. In these cases, the process can be used to coordinate the two individual steps of carbon dioxide absorption and release in such a way that the liquid is circulated to absorb the carbon dioxide. This can result in a flexibly adaptable overall process that can be operated continuously.

[0018] The process described here is a process for isolating carbon dioxide from an air stream. The process is therefore capable of selectively removing carbon dioxide from an air stream and then, in a further process step, making it available again in essentially pure form. The carbon dioxide is separated from other gas components by absorption and desorption. Various gaseous mixtures are suitable as air streams. For example, the air stream can be ambient air. However, it is also possible to use the exhaust gas stream from a combustion process as the air stream. The different air streams can therefore differ in composition due to the other gas components. In addition, the process described here is suitable for treating air streams with significantly different carbon dioxide contents.The air stream may, for example, contain carbon dioxide in a concentration of greater than or equal to 100 ppm, further preferably greater than or equal to 500 ppm and even more preferably greater than or equal to 1000 ppm.

[0019] In process step a), an aqueous solution of a carbon dioxide absorbent is provided, wherein the carbon dioxide absorbent comprises cations from the first main group of the periodic table. The prerequisite for absorbing carbon dioxide from the air stream is the presence of a suitable absorbent in an aqueous solution. The process described here can be carried out using a variety of different absorbents. However, an essential prerequisite is that the absorbent comprises cations from the first main group. Therefore, the absorbent can contain, for example, potassium or sodium ions as components. The absorbent does not necessarily have to be in the form of a salt.It is also possible to have an absorbent that is uncharged in solution, wherein, in addition to the actual absorbent, further components in the form of a salt-like compound with the corresponding cations from the first main group are added. The proportion of absorbent can be adapted to suit the separation task at hand and the carbon dioxide concentrations in the air stream. For example, the proportion of absorbent can be greater than or equal to 2.5 wt.%, more preferably greater than or equal to 7.5 wt.% and furthermore preferably greater than or equal to 15 wt.%, based on the aqueous solution. The cations from the first main group therefore originate from the group of alkali ions, such as lithium, sodium, potassium or rubidium. Depending on the proportion of absorbent, the aqueous solution can preferably be greater than or equal to 75 wt.%, furthermore preferably greater than or equal to 80 wt.-% and further preferably greater than or equal to 85 wt.% of water.

[0020] In principle, all CCh absorbents are suitable, provided they demonstrate sufficient solubility in the aqueous solution. In contact of the dissolved substance with the ambient air, for example, inorganic carbonates, amines, polyethylene glycolamines, diaminopolyethylene glycols, carboxylic acid derivatives of polyethylene glycolamines, polyethyleneimines, amine-containing sugar derivatives, amino acids, or mixtures of at least two of these components exhibit suitable properties. This group selection advantageously exhibits a virtually negligible vapor pressure and no potential health or environmental hazard. Suitable amines include primary, secondary, or tertiary amines with a molecular weight range of less than or equal to 1000 g / mol.Polyethylene glycolamines, diaminopolyethylene glycols, and carboxylic acid derivatives of polyethylene glycolamines can be substances in which one (polyethylene glycolamine) or both OH groups (diaminopolyethylene glycol) of the PEG have been replaced by amine groups. The carboxylic acid derivatives are structured accordingly, with one or both OH groups replaced by carboxylic acid groups (-COOH). Suitable sugar derivatives include N-methyl-D-glucamine (meglumine) and N-ethyl-D-glucamine (eglumine). Both exhibit very good solubility and a negligible vapor pressure in solution.

[0021] Polyethylenimines according to the following formula are also suitable as absorbents: where the properties with regard to viscosity, vapor pressure and carbon dioxide absorption can be finely adjusted via the number m. The molecular weight of the branched polyethyleneimines can preferably be less than or equal to 800 g / mol. In addition to the absorbent, the aqueous solution can contain further components in the form of polyols, which can be used in particular to regulate the viscosity and water vapor pressure of the liquid. As further components, the carbon dioxide absorbent can contain, for example, polyethylene glycols or polyols with a molecular weight of less than or equal to 1000 g / mol in a proportion of greater than or equal to 2 wt% and less than or equal to 93 wt%. In contact with the ambient air, polyethylene glycol (PEG) or polyols which have a specifically defined vapor pressure and do not exhibit any environmental or health hazards are suitable for this purpose.Polyethylene glycols with the general formula C2nH4n+2O have proven to be particularly suitable. n +i highlighted: where n can be chosen, for example, from 1 to 10. The partial pressure of the substances at 298 K decreases from 5 Pa for n=1 to 5.47* 10' 8Pa for n = 8. PEGs from n > 4 are considered non-volatile. The polyethylene glycols also have exceptionally low toxicity values ​​and are fully biodegradable down to n < 20. To adjust the water vapor partial pressure, the organic compounds are miscible with water in any proportion. Here and below, polyols are aliphatic substances which carry at least two -OH groups. The polyols preferably carry at least 3, further preferably 4 OH groups. A preferred representative from this group is, for example, glycerin. More preferably, the second component b) can be present in the carbon dioxide absorbent in a weight fraction of greater than or equal to 30 wt.% and less than or equal to 90 wt.%, further preferably greater than or equal to 35 wt.% and less than or equal to 85 wt.%.

[0022] In a further preferred embodiment of the process, the carbon dioxide absorption medium in process step a) can comprise potassium carbonate with a concentration of greater than or equal to 1.5 mol / L and less than or equal to 6.1 mol / L. These high concentrations of carbonates in the absorption medium surprisingly result in carbon dioxide being able to be absorbed reliably and quickly even from air streams which have only low carbon dioxide concentrations. This can be, for example, ambient air with a carbon dioxide concentration of less than or equal to 400 ppm. Due to the very high concentration of carbonates, the hydrogen carbonates formed are operated close to their saturation concentration, so that they advantageously precipitate from the liquid even under the conditions in the absorber. This makes it possible to achieve rapid and significant absorption of carbon dioxide even at overall low carbon dioxide concentrations.Advantageously, the potassium carbonate concentration may be greater than or equal to 2 mol / L and less than or equal to 5.5 mol / L, further preferably greater than or equal to 2.7 mol / L and less than or equal to 5.1 mol / L and further preferably greater than or equal to 3.2 mol / L and less than or equal to 4.9 mol / L.

[0023] In process step b), a carbon dioxide-containing air stream is passed through the solution prepared in process step a), at least some of the carbon dioxide from the air stream being bound to the carbon dioxide absorbent and the air stream being depleted of carbon dioxide. The air stream is brought into contact with the provided solution. The air stream can be brought into contact with the liquid in various ways. For example, it is possible for the air stream to be passed through the liquid in the form of finely distributed beads in an atomized manner. However, it is also possible for the liquid to be provided in the form of a flowing film, which then comes into contact with the air stream. In all cases, it makes sense to provide the largest possible exchange surface between the air stream and the liquid.Depending on the specific design of the liquid, the temperature, the pressure, and the contact time, a portion of the carbon dioxide will pass from the air stream into the liquid and be bound there. The specific interactions between carbon dioxide and the absorbent can be chemical, for example, covalent, and / or physical. For example, the chemical interaction is a covalent interaction. At the very least, the chemical interaction of the carbon dioxide with the absorbent ensures that some of the carbon dioxide does not leave the liquid, thus depleting the air stream of carbon dioxide. It is also possible, for example, to pass the air stream through the same liquid one or more times.

[0024] In process step c), the aqueous solution from process step b) or an aqueous solution comprising the carbon dioxide bound to the carbon dioxide absorbent is introduced into an electrolysis cell having a membrane selective for monovalent cations and the aqueous solution is electrolyzed to release at least a portion of the carbon dioxide. After the absorption step, the solution containing the absorbent and the carbon dioxide bound to it is processed by electrolysis. In the electrolysis cell, the aqueous solution is exposed to an electrical voltage so that a current flows within the electrolysis cell as a function of the voltage. For this purpose, the electrolysis cell has two electrodes in addition to the individual compartments separated by the membrane. These electrodes are usually referred to as the anode and the cathode.The electrical cell voltages that can be used to release carbon dioxide can be greater than or equal to 0.25 V, for example greater than or equal to 1 V, further preferably greater than or equal to 1.5 V and even more preferably greater than or equal to 1.75 V. The energy required for carbon dioxide release in addition to the electrolysis of water results from the pH difference required for carbon dioxide expulsion and the membrane potential between adjacent chambers of the electrolysis cell and the resulting potential difference. This is substance-specific. For example, the electrical cell voltage is between 0.25 V inclusive and 0.8 V inclusive. In addition to the release of carbon dioxide, water can also be electrolyzed during electrolysis, so that hydrogen and oxygen are produced at the electrodes.

[0025] Within a further preferred embodiment of the process, the electrolysis cell can be a three-chamber electrolysis cell comprising an anode chamber, a cathode chamber, and at least one central chamber arranged between the anode chamber and the cathode chamber. The anode chamber is separated from the central chamber by a bipolar membrane, and the cathode chamber is separated from the central chamber by a membrane selective for monovalent cations. The aqueous solution of process step c) is fed into the central chamber of the electrolysis. Irrespective of the CO2 loading of the air stream and largely independent of the composition of the absorbent, CO2-loaded absorbents can be recycled particularly advantageously using a three-chamber electrolysis. This can be described as particularly advantageous in the case of an absorbent consisting of a carbonate solution and, if appropriate, additional absorption-accelerating promoters.The CO2 bound as bicarbonate in the intermediate form can be released in a parallel reaction during the electrolytic production of hydrogen and oxygen. The resulting carbonate solution can be reused for CCL absorption from the air. The goal of capturing CO2 from the air is its use as a synthetic building block, for example, in the production of hydrocarbons. In addition to capturing CO2, the production of hydrogen is both necessary and desirable. As part of the decarbonization process, hydrogen will be primarily produced electrolytically in the future.

[0026] Furthermore, the use of a three-chamber electrolysis is significantly more advantageous than the state-of-the-art electrolytic processing of aqueous KOH solutions. In the latter, efficiencies of only 70% to 80% are achieved. The efficiency losses result primarily from the overpotential at the anode. If the aqueous electrolyte consisting of a KOH solution is replaced by an aqueous KHCCl mixture, which is produced during CO2 absorption from the air, and if the anode and cathode chambers are separated by a cation-selective membrane, both O2 and CO2 are released at the anode. If the separated CO2 is to be used as a synthesis building block in a downstream process, this process is only suitable to a limited extent, since the oxygen must be removed from the carbon dioxide-oxygen gas mixture in a very complex and energy-intensive process.This problem can be circumvented by conducting the electrolysis in a three-chamber setup. In this process step, the different gases—oxygen, carbon dioxide, and hydrogen—can be collected in separate chambers, eliminating the need to separate the components.

[0027] Within a further preferred aspect of the process, at least two chambers of the three-chamber electrolysis cell can be separated from one another by a bipolar membrane. In particular, the use of a bipolar membrane in a three-chamber electrolysis cell can contribute to a particularly energetically efficient process for removing carbon dioxide from the solution. The bipolar membrane can preferably separate at least one of the middle chambers from the anode chamber. In this setup with a bipolar membrane, a membrane selective for monovalent cations can also separate one of the middle chambers from the cathode chamber. In a preferred embodiment of the process, the carbon dioxide absorption medium after the CCL uptake can comprise bicarbonate. The uptake of carbon dioxide and the subsequent conversion of a bicarbonate solution has significant advantages over the electrolysis of carbonate solutions.The stoichiometry of CO2 conversion during the electrolysis of a carbonate solution is known. The following relationship applies to carbonate solutions:

[0028] K2CO3 + 2 H2O CO2? + H2? + ¥2 O2? + 2 KOH

[0029] Two electrons are required for the release of one CO2 molecule from carbonates. The release of CO2 from a bicarbonate solution can be used as a parallel reaction in the electrolytic production of H2 and O2 and can be expressed by the following equation:

[0030] 4KHCO3 2CO2? + H2? + ¥2 0$ + 2 K2CO3 + H2O

[0031] The advantage of using a bicarbonate solution is that only one electron is required to release a CO2 molecule. The simultaneous release of CO2 from a bicarbonate solution during electrolytic hydrogen and oxygen production is therefore extremely energy-efficient.

[0032] From a thermodynamic perspective, the potential difference to be overcome results from the water decomposition at 1.23 V and the overvoltages at the anode and cathode totaling approximately 0.5 V. The potential difference to be overcome for the release of CO2 also depends on the concentration ratios in the intermediate chamber and the cathode chamber. These generate a potential difference of approximately 0.2 V at the cation-selective membrane. In addition to the electrochemically favorable stoichiometry, a three-chamber electrolyzer induced the formation of O2 at the anode, the formation of H2 at the cathode, and simultaneously the formation of CO2 in the intermediate chamber. Stoichiometrically, 2 O2, H2, and 2 CO2 result as almost pure gaseous components in separate volumes in the three chambers. This eliminates the need for complex separation of the individual gas components. This principle can generally be extended to 3 + 2n chambers with n = 0, 1, 2, 3 etc.The solution is then fed into the middle chambers. Electrolysis can preferably be carried out using a bicarbonate solution or an amino acid solution, with each solution being charged with carbon dioxide using the process described here. However, other solutions can also be used, as long as they form bicarbonates or amino acid carbon dioxide compounds with carbon dioxide, so-called carbamates. These solutions can also be converted in an electrolyzer.

[0033] Within a further preferred aspect of the process, the electrolysis cell can be a two-chamber electrolysis cell consisting of an anode chamber and a cathode chamber, wherein the aqueous solution of process step c) is fed into the anode chamber and the anode is a Ni(OH)2 anode. The process described here is particularly suitable for delivering carbon dioxide-laden solutions, which can also be electrolyzed within the framework of a two-chamber process. In this case, hydrogen and a mixture of carbon dioxide and oxygen are produced separately. This variant can be useful in cases where a mixture of oxygen and carbon dioxide is to be further processed. Alternatively, the gas mixture can also be separated into its individual components later. Within this embodiment of the process, the anode is a Ni(OH)2 anode.In particular, the combination of a two-chamber electrolysis with a special nickel electrode offers particular advantages in the electrolytic conversion of the charged solutions described here. This process variant uses a porous nickel anode whose surface has been coated and functionalized with Ni(OH)2. This anode makes it possible to significantly reduce the required overpotential during the electrolysis of a charged solution described here, as well as to sequentially release H2 and O2 by binding O2 to the electrode. By using this or another electrode capable of binding oxygen to the surface, the separation of the individual gas components in a two-chamber electrolysis becomes very cost-effective.If a solution described here, such as a KHCO3 solution described here, is advantageously used, and the anode and cathode chambers are separated by a cation-selective membrane, this configuration allows CO2 to be released at the anode and pure hydrogen to be generated simultaneously at the cathode. The CO2 released and H2 generated in a stoichiometric ratio of 2:1 can be used directly, for example, for the synthesis of hydrocarbons. The overall reaction for this configuration is:

[0034] 4 KHCO3 + 2 Ni(OH)2— > 2 CO2$ + H 2 $ + 2 NiOOH + 2 K2CO3+ 2 H2O.

[0035] The oxygen bound to the surface of the electrode can, for example, be desorbed in a downstream process at higher temperatures:

[0036] 2 NiOOH + H2O 2 Ni(OH)2+ ¥2 O2$

[0037] This process variant is extremely energy-efficient because the excess voltage at the anode is significantly reduced, allowing electrolysis to operate at a voltage of 1.5 V, close to the thermodynamically required minimum of 1.23 V. At the same time, the CO2 previously absorbed from the air is released from the electrolyte in pure form. However, if CO2 is extracted in a separate process step, as is currently the case, this is associated with enormous energy consumption. This process variant can be coupled particularly well with the process described here, as the absorbents described here are very well suited for absorbing carbon dioxide and for electrolysis.

[0038] According to a preferred characteristic of the process, the solution obtained in process step b) can be fed into the electrolysis after carbon dioxide absorption without further processing and returned to process step b) after electrolysis. The electrolyzed solution is fed into the cathode chamber of the electrolysis cell after electrolysis and before being returned to process step b). To achieve the most energy-efficient process possible, it has proven particularly suitable to reuse the solution freed from carbon dioxide as the absorption solution. However, after electrolysis, the solution is not returned to the absorption step as such, but is fed into the cathode chamber of the electrolysis beforehand.In the context of a two-chamber electrolysis, this means that the carbon dioxide-laden solution is first fed into the anode chamber, where electrolysis and thus the desorption of carbon dioxide take place. After a sufficient amount of carbon dioxide has been desorbed from the aqueous solution, the carbon dioxide-depleted solution is fed into the cathode chamber and mixed with the solution present in the cathode chamber. This step can, in particular, ensure that the alkali ion content and the pH value of the solution are rebalanced after electrolysis. Therefore, no further measures are required for the subsequent use of the electrolyzed liquid as an absorption liquid.

[0039] In a further preferred embodiment of the process, in a second process step b'), the carbon dioxide absorbent can be precipitated with absorbed carbon dioxide and separated from the solution of process step b), wherein in process step c) an aqueous solution of the precipitated carbon dioxide absorbent with absorbed carbon dioxide is fed in, wherein the mother liquor depleted in carbon dioxide absorbent with absorbed carbon dioxide is at least partially passed into the cathode chamber of the electrolysis cell. In order to achieve particularly electrically efficient desorption of the carbon dioxide from the absorption liquid, it has been found to be advantageous that only absorbents which are also loaded with carbon dioxide are fed to the electrolysis. In the case of pure absorption of carbon dioxide by an absorbant, an equilibrium will be established as a function of the ambient conditions.Depending on the position of the equilibrium, the absorption medium will sometimes be more or less loaded with carbon dioxide. If one wishes to use only loaded absorption media during electrolysis, one can, for example, separate these carbon dioxide-loaded absorption media from the solution and then pass these separated media on to the electrolysis as part of a new solution. In these cases, it is ensured that the electrolyzed solution does not contain any unloaded absorption media. This is particularly advantageous in cases where the unloaded absorption media are converted during electrolysis with the consumption of electrons. By providing loaded absorption media, this contribution is eliminated, and the desorption of carbon dioxide can take place with a very low loss of electrons via further conversion.This process can be particularly suitable when using carbonate as an absorbent. The carbonates can be converted into bicarbonates by absorbing carbon dioxide, and the bicarbonates can be precipitated from the aqueous solutions.

[0040] This prevents carbonate reactions during electrolysis.

[0041] Within a preferred aspect of the process, the carbon dioxide absorbent can be selected from the group of alkali carbonates, alkali salts of amino acids, or mixtures of at least two components from this group. In a water-based carbon dioxide absorbent, potassium or sodium carbonate react according to the reaction equation set forth below for sodium carbonate:

[0042] Na2CC>3(aq) + H2O + CC>2(aq) — 2 NaHCO3(see

[0043] The solubility of this absorbent changes as a function of carbon dioxide loading, so that at atmospheric CCL concentrations below or around 400 ppm, a continuous CCL uptake and precipitation as bicarbonate or a bicarbonate-containing compound occurs. This effect can be enhanced, for example, by adding additional absorption promoters. For example, Na2CO3 and MEG (monoethylene glycol) / H2O and PEGI5O / H2O can be used. Suitable combinations for K2CO3 are, for example, MEG / H2O and PEGI5O / H2O. These configurations result in particularly large synergistic effects. For K2CO3, the precipitation product in the context of the carbon dioxide absorbent described here is KHCO3. In addition, the bath composition is so robust that the precipitation of sodium or potassium bicarbonate can be enhanced by adding similarly ionic additives, such as NaCl.This is particularly possible for carbonate / PEG / water solutions. Alternatively, mixtures of alkali carbonates and amino acid salts, or the use of only alkali carbonates or amino acid salts as absorbents, may also be advantageous.

[0044] In a further preferred embodiment of the process, the pH in the anode chamber or the middle chamber can be greater than or equal to pH 7.2 and less than or equal to pH 9.5, the pH in the cathode chamber can be greater than or equal to pH 10.5, in particular greater than or equal to pH 12 and less than or equal to pH 13.5, and the difference in the pH values ​​of the anode and cathode chambers or the middle chamber and the cathode chamber during electrolysis can be greater than or equal to 2.2, in particular greater than or equal to 3.5, and less than 7, in particular less than 5.5. Within these pH limits, the system can be operated thermodynamically advantageously. The required reaction equilibria are established quickly, and these differences are also advantageous from an electrochemical point of view. These limits can be exploited in particular when using bicarbonates as absorbent.

[0045] In a further embodiment of the process, the concentration of carbon dioxide absorbent with absorbed carbon dioxide when fed into the electrolysis can be greater than or equal to 0.5 mol / L and less than or equal to 5.5 mol / L, in particular greater than or equal to 1 mol / L and less than or equal to 5.5 mol / L, for example greater than or equal to 2 mol / L and less than or equal to 5.5 mol / L. These concentration limits allow high amounts of carbon dioxide to be released with high activity. Furthermore, these concentration limits can be suitable for operating the cell structure over long periods of time.

[0046] In a further preferred embodiment of the process, an aqueous KOH solution can be used as the electrolyte in the anode chamber of the at least three-chamber electrolysis unit. Potassium hydroxide (KOH) can preferably be chosen as the electrolyte for the anode chamber, since this electrolyte can exhibit a correspondingly high conductivity under the operating conditions of the electrolysis.

[0047] In a further embodiment of the process, after process step b), bicarbonate can be separated from the absorbent and added as an aqueous solution to the at least three-chamber electrolysis unit with at least the addition of water. In this process variant, the at least partial feeding of the carbon dioxide-depleted mother solution into the cathode chamber of the electrolysis cell can be omitted. The precipitation and concentration of the carbon dioxide absorbent with absorbed carbon dioxide, for example in the form of bicarbonate, can lead to the previously described energy advantages within the framework of the three-chamber electrolysis unit.In a further preferred embodiment, the solution of the carbon dioxide absorption agent in process step a) can comprise a compound with an amine function as a carbon dioxide absorption accelerator, wherein the compound with an amine function has a carbamate equilibrium constant, determined from carbon dioxide partial pressure measurements, of greater than or equal to 0.01 at 20°C and in a concentration of 1 mol / kg. The carbamate equilibrium constant is understood to mean the following equilibrium constant:

[0048] The equilibrium constant K carb(in mol / L) is a physical quantity that describes the stability of carbamates formed during CO2 absorption with the absorption accelerators in the absorption liquid. Low carbamate stability is expressed by a high value for ALu-b. This means that the carbamate formed reacts back to form bicarbonate and the amine. The latter, in particular, is desired in this case and leads to an improved carbon dioxide uptake rate. In particular, these amine additives can contribute to the efficient removal of carbon dioxide even from air streams with relatively low carbon dioxide contents. Suitable amines for accelerating uptake are, for example, amino acids due to their very low vapor pressure. The amino acid pipecolic acid, for example, exhibits suitable carbamate stability.Further preferably, amino acids having a carbamate equilibrium constant at 20°C of greater than or equal to 0.02 and further preferably of greater than or equal to 0.03 can be used.

[0049] Furthermore, the use of the process described here for the absorption of carbon dioxide from an air stream is specified, wherein the air stream has a carbon dioxide concentration of greater than or equal to 100 ppm and less than or equal to 650 ppm. The process described here can be particularly suitable for further removing carbon dioxide from air streams with very low carbon dioxide concentrations. In state-of-the-art baths, these conditions usually lead to an uncontrollable absorption or release of water from the solution, which requires close process monitoring with corresponding control effort. The process presented here enables a reliable and continuous absorption of carbon dioxide from these low-carbon dioxide streams and an efficient and energetically advantageous desorption during electrolysis.Processes can also be established that efficiently absorb carbon dioxide from the ambient air and ensure electrically suitable release during electrolysis.

[0050] Furthermore, the respective electrolysis apparatuses also represent designs. The following describes the design features of the process and the absorbent based on aspects:

[0051] 1. A process for isolating carbon dioxide from an air stream, characterized in that the process comprises at least the following steps: a) providing an aqueous solution of a carbon dioxide absorbent, the carbon dioxide absorbent comprising cations from main group 1 of the Periodic Table; b) passing a carbon dioxide-containing air stream through the solution provided in process step a), at least a portion of the carbon dioxide from the air stream being bound to the carbon dioxide absorbent and the air stream being depleted of carbon dioxide; c) introducing the aqueous solution from process step b) or an aqueous solution comprising the carbon dioxide bound to the carbon dioxide absorbent into an electrolysis cell having a membrane selective for monovalent cations and electrolyzing the aqueous solution to release at least a portion of the carbon dioxide.

[0052] 2. Process according to the preceding aspect, wherein the electrolysis cell is an at least three-chamber electrolysis cell comprising an anode chamber, a cathode chamber and at least one middle chamber arranged between the anode chamber and the cathode chamber, wherein the anode chamber is separated from the middle chamber by a bipolar membrane and the cathode chamber is separated from the middle chamber by the membrane selective for monovalent cations, wherein the aqueous solution of process step c) is fed into the middle chamber of the electrolysis.

[0053] 3. The process according to aspect 1, wherein the electrolysis cell is a two-chamber electrolysis cell comprising an anode chamber and a cathode chamber, wherein the aqueous solution of process step c) is fed into the anode chamber and the anode is a Ni(OH)2 anode.

[0054] 4. Process according to one of the preceding aspects, wherein the solution obtained in process step b) is fed into the electrolysis after the carbon dioxide absorption without processing and is returned to process step b) after the electrolysis, wherein the electrolyzed solution is passed into the cathode chamber of the electrolysis cell after the electrolysis and before being returned to process step b).

[0055] 5. Process according to one of aspects 1-3, wherein in a second process sub-step b') the carbon dioxide absorbent with absorbed carbon dioxide is precipitated and separated from the solution of process step b), wherein in process step c) an aqueous solution of the precipitated carbon dioxide absorbent with absorbed carbon dioxide is fed in, wherein the mother solution depleted in carbon dioxide absorbent with absorbed carbon dioxide is at least partially passed into the cathode chamber of the electrolysis cell.

[0056] 6. Process according to one of the preceding aspects, wherein the carbon dioxide absorption agent is selected from the group of alkali carbonates, alkali salts of amino acids or mixtures of at least two components from this group.

[0057] 7. The method according to any one of the preceding aspects, wherein the pH in the anode chamber is greater than or equal to pH 7.2 and less than or equal to pH 9.5, the pH in the cathode chamber is greater than or equal to pH 10.5 and less than or equal to pH 13.5, and the difference in the pH values ​​of the anode and cathode chambers during electrolysis is greater than or equal to 3.5 and less than 5.5. 8. The method according to any one of the preceding aspects, wherein the concentration of carbon dioxide absorbent with absorbed carbon dioxide when fed into the electrolysis is equal to or equal to 2 mol / L and less than or equal to 8 mol / L.

[0058] 9. Use of a method according to any one of the preceding aspects for absorbing carbon dioxide from an air stream, wherein the air stream has a carbon dioxide concentration of greater than or equal to 100 ppm and less than or equal to 650 ppm.

[0059] Furthermore, an absorbent for absorbing carbon dioxide from a gas stream is specified. Preferably, the absorbent described here is suitable and intended for use in the above-described process for separating carbon dioxide from an air stream. Features and embodiments described in connection with the process for separating carbon dioxide from an air stream also apply to the absorbent, and vice versa.

[0060] According to at least one embodiment, the absorbent comprises an aqueous solution of a carbonate and a compound having an amine function. In other words, the absorbent comprises water, carbonate, and a compound having an amine function, wherein the carbonate and the compound having an amine function are dissolved in the water. In particular, the absorbent consists of an aqueous solution of a carbonate and a compound having an amine function.

[0061] Here and in the following, a carbonate refers to an alkali carbonate. The carbonate can also be referred to as a carbon dioxide absorber. For example, the carbonate is sodium carbonate, potassium carbonate, or a combination of sodium carbonate and potassium carbonate.

[0062] The compound with an amine function is, in particular, a carbon dioxide absorption accelerator. The carbon dioxide absorption accelerator can also be referred to as a promoter. In particular, the compound with an amine function is an amino acid or an amino acid salt, for example, an alkali salt of an amino acid. For example, the compound with an amine function is pipecolic acid or the salt of pipecolic acid.

[0063] In particular, the absorbent is designed to convert carbon dioxide from a gas stream into a dissolved phase by reacting the carbonate to form bicarbonate in the aqueous solution of the absorbent, thereby removing it from the gas stream. Here and in the following, a bicarbonate is understood to mean an alkali bicarbonate.

[0064] According to at least one embodiment, the compound having an amine function is configured to form a carbamate with carbon dioxide. In particular, the compound having an amine function is configured to form an unstable carbamate with carbon dioxide, which decomposes in the presence of carbonate to form bicarbonate, the compound having an amine function being regenerated. In this case, the compound having an amine function is the carbon dioxide absorption accelerator, and the carbonate is the carbon dioxide absorbent that binds the carbon dioxide in the absorbent. In particular, the compound having an amine function has a carbamate equilibrium constant, determined from carbon dioxide partial pressure measurements, of greater than or equal to 0.01, for example greater than or equal to 0.02 or greater than or equal to 0.03, at 20°C and at a concentration of 1 mol / kg.For example, the carbamate equilibrium constant is less than or equal to 0.75, less than or equal to 0.5, or less than or equal to 0.25.

[0065] According to at least one embodiment, the absorbent for absorbing carbon dioxide from a gas stream comprises an aqueous solution of a carbonate and a compound having an amine function, wherein the compound having an amine function is adapted to form a carbamate with carbon dioxide.

[0066] The idea behind such an absorbent is to absorb carbon dioxide from gas streams such as air particularly efficiently and bind it within the absorbent. By using a compound with an amine function in the absorbent, carbon dioxide can be dissolved from a gas stream particularly quickly in the form of a carbamate. As the carbamate reacts further with the presence of the carbonate in the absorbent to form bicarbonate and the compound with an amine function, the carbon dioxide can be bound in the absorbent in the form of bicarbonate. The compound with an amine function is regenerated and is available for further absorption.With an absorbent comprising a compound having an amine function which can form a carbamate with carbon dioxide, the absorption of carbon dioxide from a gas stream, in particular a gas stream with a carbon dioxide content of between 100 ppm inclusive and 650 ppm inclusive, can advantageously be accelerated.

[0067] According to at least one embodiment, the water is selected from deionized water, tap water, groundwater, river water, seawater, desalinated seawater, filtered water, or a combination thereof.

[0068] According to at least one embodiment, the molar concentration of the compound having an amine function in the absorbent is at least 0.5 mol / L and at most 5 mol / L. In particular, the molar concentration of the compound having an amine function in the absorbent is at least 0.5 mol / L and at most 4 mol / L, for example at least 0.5 mol / L and at most 3 mol / L or at least 0.5 mol / L and at most 2 mol / L. Advantageously, a molar concentration of the compound having an amine function in the absorbent of at least 0.5 mol / L and at most 5 mol / L can enable the absorption of carbon dioxide from gas streams with a carbon dioxide content of less than 10% particularly efficiently.

[0069] According to at least one embodiment, the compound with an amine function is present in the absorbent in the form of a salt. Here and below, the presence of the compound with an amine function in the form of a salt is to be understood as meaning that the compound with an amine function is present in the aqueous solution of the absorbent in ionic form. In particular, the compound with an amine function is present in the aqueous solution at least partially dissociated into its corresponding ions. In particular, the compound with an amine function is present in the form of an alkali salt. For example, the absorbent comprises an amino acid, which is present in the absorbent in the form of the amino acid salt. Alternatively, the absorbent can comprise an amino acid salt, which is present in the absorbent in the form of the amino acid salt. According to at least one embodiment, the absorbent is free of polyethylene glycols and / or polyols. In particular, the absorbent is free of polyethylene glycols and polyols.In other words, the absorbent contains neither a polyethylene glycol nor a polyol. Other carbon dioxide absorbents utilize polyethylene glycols and polyols to reduce the water vapor partial pressure of a carbon dioxide absorbent. When carbonates are used in an absorbent to absorb carbon dioxide, the water vapor partial pressure can instead be reduced by varying the molar concentration of carbonate. A high molar concentration of carbonate in the absorbent and the presence of a compound with an amine function in the absorbent can advantageously reduce the water vapor partial pressure in the absorbent to such an extent that further substances for reducing the water vapor partial pressure are no longer necessary. In particular, the solubility of the carbonate in the absorbent is significantly increased when the absorbent is free of polyethylene glycols and / or polyols.In other words, the addition of polyethylene glycols and / or polyols leads to reduced carbonate solubility. Furthermore, the absorption of carbon dioxide from an air stream can be advantageously increased by keeping the number of components in the absorbent low.

[0070] According to at least one embodiment, the absorbent is configured to exhibit a net water loss of less than 1 wt.% / 24 h under constant conditions in a range of relative humidity greater than or equal to 50% after equilibrium with the environment is established. In particular, the absorbent is configured to concentrate as the air humidity decreases. This can cause the absorbent to become more hygroscopic, and the water vapor partial pressure of the absorbent can decrease. An equilibrium state can arise in which the absorbent releases and / or absorbs no or almost no water to the environment.

[0071] Furthermore, a method for separating carbon dioxide in an electrochemical desorption unit is specified. Preferably, the absorbent described above for absorbing carbon dioxide from a gas stream is suitable and intended for use in the method described here for separating carbon dioxide in an electrochemical desorption unit. Features and embodiments described in connection with the absorbent also apply to the method for separating carbon dioxide in an electrochemical desorption unit, and vice versa. Features or embodiments described in connection with the method for separating carbon dioxide from an air stream also apply to the method for separating carbon dioxide in an electrochemical desorption unit, and vice versa.

[0072] According to at least one embodiment of the method for separating carbon dioxide in an electrochemical desorption unit, the electrochemical desorption unit comprises at least a first chamber and a second chamber separated by a cation-selective membrane. In other words, the first chamber and the second chamber of the electrochemical desorption unit are separated by a selective, semipermeable membrane that only allows cations to pass through. In particular, the cation-selective membrane is configured so that cations migrate from the first chamber through the cation-selective membrane into the second chamber of the electrochemical desorption unit. For example, the cation-selective membrane is selective for alkali cations such as sodium ions, potassium ions, or combinations thereof. In particular, the electrochemical desorption unit is an electrolysis unit or an electrodialysis unit.In particular, the first chamber is an anode chamber of a two-chamber electrochemical desorption unit or a bridge chamber of an electrochemical desorption unit with at least three chambers. In particular, the second chamber is a cathode chamber of a two-chamber electrochemical desorption unit or an electrochemical desorption unit with at least three chambers.

[0073] According to at least one embodiment, the method comprises the step of introducing a first solution into the first chamber, wherein a molar concentration of bicarbonate in the first solution is at least twice as high as a molar concentration of carbonate. Here and below, a bicarbonate is understood to mean an alkali bicarbonate and a carbonate is understood to mean an alkali carbonate. Here and below, the molar concentration of a solid in a solution is understood to mean the amount of the solid relative to the total volume of the solution. A twice as high molar concentration of bicarbonate means that the molar fraction of bicarbonate in the total volume of the first solution is at least twice as high as the molar fraction of carbonate.In particular, the molar concentration of bicarbonate in the first solution is at least 2.5 times, for example at least three times, as high as the molar concentration of carbonate.

[0074] In particular, the first solution is formed by dissolving a solid in water, the solid comprising at least 90 wt.% bicarbonate. For example, the bicarbonate is an alkali bicarbonate such as sodium bicarbonate, potassium bicarbonate, or combinations thereof. Upon dissolving the solid, which comprises at least 90 wt.% bicarbonate, in water, an equilibrium is established between bicarbonate and carbonate in the dissolved phase. In particular, the equilibrium lies on the side of the bicarbonate.

[0075] According to at least one embodiment, the method comprises the step of electrochemically desorbing carbon dioxide from the first solution, whereby carbon dioxide is released and a second solution is formed from the first solution. In other words, the first solution is reacted in the electrochemical desorption unit to form at least carbon dioxide and the second solution. The second solution is, in particular, depleted of carbon dioxide, which is bound in the form of bicarbonate. In particular, the electrochemical desorption comprises electrolyzing the first solution and / or electrodialyzing the first solution. For example, water is electrolyzed from the first solution in the first chamber, whereby the pH value of the first solution is lowered to such an extent that carbon dioxide is formed from the bicarbonate, whereby the carbon dioxide is subsequently released.

[0076] In particular, during the electrochemical desorption of carbon dioxide from the first solution, fewer than four electrons, in particular fewer than three electrons, are used to form two molecules of carbon dioxide. For example, during the electrochemical desorption of carbon dioxide from the first solution, two molecules of carbon dioxide are formed using two electrons. In contrast, during the electrochemical desorption of carbon dioxide from a carbonate solution, only one molecule of carbon dioxide is formed using two electrons. According to at least one embodiment, the method for separating carbon dioxide in an electrochemical desorption unit, wherein the electrochemical desorption unit comprises at least a first chamber and a second chamber separated by a cation-selective membrane, comprises the steps:

[0077] - introducing a first solution into the first chamber, wherein a molar concentration of hydrogen carbonate in the first solution is at least twice as high as a molar concentration of carbonate,

[0078] - electrochemically desorbing carbon dioxide from the first solution, whereby carbon dioxide is released and a second solution is formed from the first solution.

[0079] Such a process is based on the idea of ​​electrochemically desorbing carbon dioxide from a solution containing a high concentration of bicarbonate. By introducing a solution with a high concentration of bicarbonate into the first chamber of an electrochemical desorption unit, carbon dioxide can be released in a particularly energy-efficient manner.

[0080] According to at least one embodiment, the molar concentration of bicarbonate in the first solution is greater than or equal to 0.5 mol / L and less than or equal to 5.5 mol / L. In particular, the molar concentration of bicarbonate in the first solution is greater than or equal to 1 mol / L and less than 5.5 mol / L, for example greater than or equal to 2 mol / L and less than 5.5 mol / L. In particular, the solubility of bicarbonate depends on the ambient temperature. At a higher temperature, a larger amount of bicarbonate may be present in the first solution than at lower temperatures. In other words, the first solution may have a higher molar concentration of bicarbonate at higher temperatures than at lower temperatures.For example, the molar concentration of bicarbonate in the first solution is at least 2 mol / L at a temperature of 20 °C, at least 2.5 mol / L at a temperature of 30 °C, at least 3 mol / L at a temperature of 40 °C, at least 3.5 mol / L at a temperature of 50 °C, at least 4 mol / L at a temperature of 60 °C, or at least 4.5 mol / L at a temperature of more than 70 °C. Advantageously, carbon dioxide can be released particularly efficiently in the electrochemical desorption unit at a molar concentration of bicarbonate in the first solution of more than 0.5 mol / L inclusive and less than 5.5 mol / L inclusive.

[0081] According to at least one embodiment, the saturation of the first solution with bicarbonate at a temperature of less than or equal to 30°C is above 60%. In particular, the saturation of the first solution with bicarbonate at a temperature of less than or equal to 30°C is above 70%, for example above 80% or above 90% or above 95%. In particular, the electrochemical desorption takes place at a temperature of less than or equal to 30°C and the saturation of the first solution with bicarbonate is above 60%. Here and below, the saturation of a solution is specified with reference to the saturation limit of the solution. The saturation limit is understood to be the point at which the solution of a solid no longer dissolves any further components and this solid precipitates. In other words, saturation is the point of maximum concentration of the dissolved solid.Saturation can depend on the temperature and the substance-specific properties of the components involved. Here and in the following, saturation is therefore always related to the overall system of the respective solution. The saturation limit of a solution can be measured as follows: Differential weighing and stepwise addition of bicarbonate to the solution until the solid is no longer completely dissolved. Care should be taken to ensure that the solubility limit is not distorted by exchange with the ambient air. Alternatively, the saturation limit of a solution can be measured as follows: Preparing a supersaturated solution, taking a sample and separating it from the solids, followed by titration to determine the solution's concentration. The saturation of a solution can be measured as follows: Determining the solution's concentration by titration and calculating the quotient of the solution's concentration and the solution's saturation limit.The electrochemical desorption of carbon dioxide from a first solution with a bicarbonate saturation of over 80% at a temperature of less than or equal to 30°C can advantageously release large amounts of carbon dioxide in a particularly energy-efficient manner. According to at least one embodiment, the bicarbonate saturation of the first solution at a temperature of greater than 30°C and less than or equal to 60°C is over 50%. In particular, the bicarbonate saturation of the first solution at a temperature of greater than 30°C and less than or equal to 60°C is over 55%, for example over 60% or over 70% or over 80% or over 90% or over 95%. In particular, the electrochemical desorption takes place at a temperature of greater than 30°C and less than or equal to 60°C, and the bicarbonate saturation of the first solution is over 50%.The electrochemical desorption of carbon dioxide from a first solution with a bicarbonate saturation of more than 50% at a temperature greater than 30 °C and less than or equal to 60 °C can advantageously release large amounts of carbon dioxide in a particularly energy-efficient manner.

[0082] According to at least one embodiment, a molar concentration of bicarbonate in the first solution at a temperature greater than 60°C is at least 3 mol / L. In particular, the molar concentration of the first solution at a temperature greater than 60°C is at least 3.5 mol / L. For example, the molar concentration of the first solution at a temperature greater than 70°C is at least 4 mol / L or 4.5 mol / L. In particular, the electrochemical desorption takes place at a temperature greater than 60°C and the molar concentration of bicarbonate in the first solution is at least 3 mol / L. The electrochemical desorption of carbon dioxide from a first solution with a molar concentration of bicarbonate of at least 3 mol / L at a temperature greater than 60°C can advantageously release large amounts of carbon dioxide in a particularly energy-efficient manner.

[0083] According to at least one embodiment, the first solution consists of water, bicarbonate, and other components, wherein the sum of all other components amounts to less than 10 wt.%. In particular, the sum of all other components amounts to less than 5 wt.%, for example, less than 1 wt.%. For example, the other components are carbonate, as well as compounds or residues from previous process steps, impurities, or decomposition products. Advantageously, electrochemical desorption can be less susceptible to failure due to the low proportion of the other components and can be operated with a high concentration of bicarbonate.

[0084] According to at least one embodiment, the first solution has a pH of at least 7.2 to at most 9.5, in particular of at least 7.8 and at most 8.7. In particular, the first solution has the pH of an aqueous bicarbonate solution. For example, the pH of the first solution is pressure-dependent. If the pressure increases, the pH of the solution can shift to lower pH values, and if the pressure decreases, for example, under negative pressure, it can shift to higher pH values. In a pH range of at least 7.2 and at most 9.5, the equilibrium between bicarbonate and carbonate in the first solution is advantageously so far on the side of bicarbonate that the molar concentration of bicarbonate in the first solution is at least twice as high as the molar concentration of carbonate. As a result, electrochemical desorption of carbon dioxide from the first solution can take place in a particularly energy-efficient manner.

[0085] According to at least one embodiment, the first solution is formed as follows:

[0086] - contacting an absorbent with a gas stream comprising carbon dioxide, wherein the absorbent comprises at least one carbonate,

[0087] - producing a loaded absorbent, wherein the carbonate is at least partially converted into bicarbonate,

[0088] - Precipitation of a solid comprising at least part of the bicarbonate from the loaded absorbent,

[0089] - Separation of the precipitated solid from the aqueous phase of the loaded absorbent,

[0090] - Producing the first solution by dissolving the precipitated and separated solid in water.

[0091] In particular, the steps of contacting an absorbent with a gas stream comprising carbon dioxide and generating a loaded absorbent take place simultaneously. In particular, the absorbent is the absorbent described above. In particular, the gas stream comprising carbon dioxide is air, for example ambient air with a carbon dioxide concentration of at most 10% or of greater than or equal to 100 ppm and less than or equal to 650 ppm. In particular, the solid comprises at least 90 wt.% bicarbonate. In particular, the separation of the precipitated solid from the aqueous phase of the loaded absorbent takes place using a centrifuge, a hydrocyclone, a belt filter, or combinations thereof. With such a process, carbon dioxide can advantageously be removed particularly efficiently from a gas stream and separated in an electrochemical desorption unit.

[0092] According to at least one embodiment, the method further comprises the steps:

[0093] - Introducing the second solution into the second chamber, and

[0094] - electrochemically converting the second solution, whereby a third solution is formed from the second solution. In particular, the second solution is introduced into the second chamber after the second solution has been formed in the first chamber and discharged from the first chamber. In other words, the first solution is partially converted in the first chamber and then further converted as a second solution in the second chamber. In particular, the electrochemical desorption unit is operated sequentially. Here and in the following, sequential operation of the electrochemical desorption unit is understood to mean that the first solution is first introduced into the first chamber and the second solution formed there is then introduced into the second chamber.Advantageously, the electrochemical desorption can be operated with an initial solution which can initially be used as electrolyte in the first chamber and, after partial conversion in the first chamber, as electrolyte in the second chamber.

[0095] According to at least one embodiment, a molar concentration of bicarbonate in the second solution is lower than the molar concentration of bicarbonate in the first solution. In other words, the molar concentration of bicarbonate at the entrance to the second chamber is lower than at the entrance to the first chamber of the electrochemical desorption unit. In particular, bicarbonate from the first solution is consumed in the first chamber, whereby the second solution is poorer in bicarbonate than the first solution. According to at least one embodiment, a molar concentration of bicarbonate in the third solution is lower than the molar concentration of bicarbonate in the second solution. In other words, the molar concentration of bicarbonate at the entrance to the second chamber is lower than at the exit from the second chamber.In particular, bicarbonate from the second solution is consumed in the second chamber, making the third solution poorer in bicarbonate than the second solution.

[0096] According to at least one embodiment, the molar concentration of bicarbonate in the second solution is lower than the molar concentration of bicarbonate in the first solution, and the molar concentration of bicarbonate in the third solution is lower than the molar concentration of bicarbonate in the second solution. In other words, a depletion of bicarbonate takes place in the electrochemical desorption unit. In particular, the depletion of bicarbonate takes place in the order of inlet to the first chamber, outlet to the first chamber, and inlet to the second chamber, outlet to the second chamber. In other words, a first portion of the bicarbonate from the first solution is consumed in the first chamber, and a second portion of the bicarbonate is consumed in the second chamber.

[0097] According to at least one embodiment, the molar concentration of bicarbonate in the second solution is greater than or equal to 1 / 3 of the molar concentration of bicarbonate in the first solution and less than or equal to 2 / 3 of the molar concentration of bicarbonate in the first solution. In other words, at least 1 / 3 and at most 2 / 3 of a molar amount of bicarbonate in the first solution is converted in the first chamber. In particular, the molar concentration of bicarbonate in the second solution is greater than or equal to 2 / 5, for example greater than or equal to half, and less than or equal to 3 / 5, for example less than or equal to 11 / 20, of the molar concentration of bicarbonate in the first solution. For example, the molar concentration of bicarbonate in the second solution is equal to half the molar concentration of bicarbonate in the first solution.A conversion of at most half the molar amount of bicarbonate in the first chamber can advantageously enable particularly efficient operation of the electrochemical desorption unit. If at least half the molar concentration of bicarbonate of the first solution is present in the second solution, the bicarbonate is converted to carbonate in the second chamber, and the formation of alkali hydroxide as a reaction product in the second chamber can be prevented. The formation of carbonate from bicarbonate requires fewer electrons than the formation of alkali hydroxide as a reaction product, which allows the process described here to be more energy-efficient.

[0098] According to at least one embodiment, a molar concentration of carbonate in the third solution is higher than a molar concentration of bicarbonate. In other words, there is more carbonate than bicarbonate in solution in the third solution. In particular, the molar concentration of carbonate in the third solution is at least twice as high, for example at least three times as high, as the molar concentration of bicarbonate. In particular, carbonate is produced using the method described here. For example, bicarbonate is consumed from the first solution and from the second solution, and a third solution enriched in carbonate is formed. The third solution can advantageously be used for processes such as the absorption of carbon dioxide from a gas stream, which require a solution with a higher molar concentration of carbonate than bicarbonate.

[0099] According to at least one embodiment, a ratio of a first difference and a second difference is greater than 0.8 and less than 1.2, wherein the first difference is a difference between a molar concentration of bicarbonate in the third solution and a molar concentration of bicarbonate in the second solution, and wherein the second difference is a difference between a molar concentration of bicarbonate in the second solution and the molar concentration of bicarbonate in the first solution. In other words, 0.8 < [(molar concentration of bicarbonate in the third solution) - (molar concentration of bicarbonate in the second solution)] / [(molar concentration of bicarbonate in the second solution) - (molar concentration of bicarbonate in the first solution)] < 1.2.In other words, the decrease in the molar concentration of bicarbonate in the first chamber is approximately equal to the decrease in the molar concentration of bicarbonate in the second chamber. In particular, the ratio of the first difference to the second difference is greater than 0.9 and less than 1.1, for example, greater than 0.95 and less than 1.05. With a ratio of the first difference to the second difference of greater than 0.8 and less than 1.2, sequential operation of the electrochemical desorption unit can advantageously release carbon dioxide in the first chamber and simultaneously neutralize the alkali hydroxide formed therein in the second chamber.

[0100] According to at least one embodiment, the method further comprises the steps:

[0101] - Introducing a fourth solution into the second chamber, and

[0102] - electrochemically converting the fourth solution, whereby a fifth solution is formed from the fourth solution.

[0103] In particular, the fourth solution comprises water, carbonate, and bicarbonate. The fourth solution can be provided independently of the first and / or second solution. The electrochemical desorption unit can be operated in parallel with the first solution in the first chamber and the fourth solution in the second chamber.

[0104] According to at least one embodiment, a molar concentration of bicarbonate in the second solution is greater than 80% of the molar concentration of bicarbonate in the first solution. In particular, the molar concentration of bicarbonate in the second solution is greater than 80% of the molar concentration of bicarbonate in the first solution when the fourth solution is introduced into the second chamber. In particular, the molar concentration of bicarbonate in the second solution is between 80% and 100%, for example between 90% and 100%, of the molar concentration of bicarbonate in the first solution. In other words, the molar concentration of bicarbonate in the first chamber is adjusted so that it is close to the solubility limit of bicarbonate.This advantageously ensures that there is always a molar concentration of bicarbonate in the first chamber that is sufficient to release carbon dioxide.

[0105] According to at least one embodiment, the second solution formed in the first chamber is introduced into a storage vessel after being discharged from the first chamber. In the storage vessel, a solid comprising at least 90% by weight of bicarbonate is added to the second solution, forming the first solution. The solid can be added to the second solution in solid form or in dissolved form, for example as an aqueous solution of the solid. The first solution formed in the storage vessel is then introduced into the first chamber. In other words, a cyclic process can be realized in which the first solution is reacted in the first chamber and the second solution formed is subsequently regenerated in the storage vessel to form the first solution before the first solution is reintroduced into the first chamber. Such a cyclic process with respect to the first chamber can advantageously be operated simply and cost-effectively.

[0106] According to at least one embodiment, the molar concentration of bicarbonate in the fourth solution is greater than zero. In other words, bicarbonate is present in the fourth solution. In particular, the bicarbonate in the fourth solution is configured to neutralize alkali hydroxide formed in the second chamber, forming carbonate and water.

[0107] According to at least one embodiment, a molar concentration of bicarbonate in the fifth solution is lower than a molar concentration of bicarbonate in the fourth solution. In other words, the molar concentration of bicarbonate at the entrance to the second chamber is lower than at the exit from the second chamber. In particular, bicarbonate from the fourth solution is consumed in the second chamber, whereby the fifth solution is poorer in bicarbonate than the fourth solution. For example, the molar concentration of bicarbonate decreases due to the neutralization of alkali hydroxide in the second chamber.

[0108] According to at least one embodiment, a molar concentration of carbonate in the fourth solution is greater than zero. In other words, carbonate is present in the fourth solution.

[0109] According to at least one embodiment, a molar concentration of carbonate in the fifth solution is higher than a molar concentration of carbonate in the fourth solution. In other words, carbonate is formed in the second chamber, making the fifth solution richer in carbonate than the fourth solution. The fifth solution can advantageously be used for processes such as the absorption of carbon dioxide from a gas stream, which require a solution with a high molar concentration of carbonate.

[0110] According to at least one embodiment, the conductivity of the fourth solution under operating conditions in the second chamber is greater than 100 mS / cm. A conductivity of the fourth solution greater than 100 mS / cm can advantageously improve the electrical efficiency of the overall process.

[0111] According to at least one embodiment, the fourth solution is formed as follows:

[0112] - contacting an absorbent with a gas stream comprising carbon dioxide, wherein the absorbent comprises at least one carbonate,

[0113] - producing a loaded absorbent, wherein the carbonate is at least partially converted into bicarbonate,

[0114] - Precipitation of a solid comprising at least part of the bicarbonate from the loaded absorbent,

[0115] - Separating the precipitated solid from the aqueous phase of the loaded absorbent, whereby the aqueous phase of the loaded absorbent forms the fourth solution.

[0116] In particular, the steps of contacting an absorbent with a gas stream comprising carbon dioxide and generating a loaded absorbent take place simultaneously. In particular, the absorbent is the absorbent described above. In particular, the gas stream comprising carbon dioxide is air, for example ambient air with a carbon dioxide concentration of at most 10% or of greater than or equal to 100 ppm and less than or equal to 650 ppm. In particular, the solid comprises at least 90 wt.% bicarbonate. In particular, the separation of the precipitated solid from the aqueous phase of the loaded absorbent takes place using a centrifuge, a hydrocyclone, a belt filter or combinations thereof. Advantageously, the aqueous phase of the loaded absorbent can be used as electrolyte in the second chamber of the electrochemical

[0117] The aqueous phase of the loaded absorbent can thus be used in subsequent process steps of electrochemical desorption after absorption.

[0118] According to at least one embodiment, the fifth solution formed in the second chamber is used to absorb carbon dioxide after being discharged from the second chamber, thereby forming the fourth solution. In particular, the fourth solution can be formed according to the preceding embodiment. The fourth solution is subsequently introduced into the second chamber. In other words, a cyclic process can be realized in which the fourth solution is reacted in the second chamber and the fifth solution formed thereby is subsequently regenerated to the fourth solution by absorbing carbon dioxide before the fourth solution is reintroduced into the second chamber. Such a cyclic process with respect to the second chamber can advantageously be operated simply and cost-effectively.

[0119] According to at least one embodiment, the electrochemical desorption unit is an n-chamber electrochemical desorption unit, where n is a natural number greater than or equal to 2. In particular, an n-chamber electrochemical desorption unit has an anode chamber, a cathode chamber, and n-2 bridge chambers. For n equal to 2, the n-chamber electrochemical desorption unit consequently has no bridge chamber. For n greater than 2, the n-chamber electrochemical desorption unit has n-2 bridge chambers, wherein the n-2 bridge chambers are arranged between the anode chamber and the cathode chamber. In particular, n is greater than or equal to 2 and less than or equal to 1000. For example, n is selected from the natural numbers 2, 3, 4, 5, 6, 7, 8, 9 or 10. In other words, the electrochemical desorption unit may comprise two chambers, three chambers, four chambers, five chambers, six chambers, seven chambers, eight chambers, nine chambers or ten chambers.Alternatively, n is greater than 100. For example, n is 500 and the electrochemical desorption unit comprises 500 chambers.

[0120] According to at least one embodiment, the electrochemical desorption unit is a two-chamber electrochemical desorption unit. The first chamber of the electrochemical desorption unit is an anode chamber, and the second chamber is a cathode chamber. In particular, the first solution is introduced into the anode chamber, and the second solution or the fourth solution is introduced into the cathode chamber. Carbon dioxide and oxygen are formed in the first chamber, and hydrogen is formed in the second chamber.

[0121] According to at least one embodiment, a gas mixture of carbon dioxide and oxygen is formed in the first chamber, and the gas mixture has a molar ratio of carbon dioxide to oxygen of more than 7:3, in particular more than 3:1, for example 4:1. In other words, during the electrochemical desorption in the first chamber, a gas mixture is formed with more than 70 mol% carbon dioxide and less than 30 mol% oxygen, in particular with more than 75 mol% carbon dioxide and less than 25 mol% oxygen, for example with 80 mol% carbon dioxide and 20 mol% oxygen. In contrast, during the electrochemical desorption of carbon dioxide from carbonate, a gas mixture with 66 mol% carbon dioxide and 33 mol% oxygen is formed. The method described here can advantageously be used to release increased amounts of carbon dioxide.

[0122] According to at least one embodiment, the electrochemical desorption unit is a three-chamber electrochemical desorption unit. The three-chamber electrochemical desorption unit comprises an anode chamber, a cathode chamber, and a bridge chamber arranged between the anode chamber and the cathode chamber. The first chamber of the electrochemical desorption unit is the bridge chamber, and the second chamber of the electrochemical desorption unit is the cathode chamber. A bipolar membrane can be arranged between the anode chamber and the adjacent bridge chamber. In particular, the first solution is introduced into the bridge chamber, and the second solution or the fourth solution is introduced into the cathode chamber. An aqueous alkali hydroxide solution can be introduced into the anode chamber as anolyte. Oxygen is formed in the anode chamber, carbon dioxide is formed in the first chamber, and hydrogen is formed in the second chamber.

[0123] According to at least one embodiment, the electrochemical desorption unit is a four-chamber electrochemical desorption unit. In particular, a four-chamber electrochemical desorption unit is an electrodialysis unit. The four-chamber electrochemical desorption unit comprises an anode chamber, a cathode chamber, and two bridge chambers arranged between the anode chamber and the cathode chamber. The bridge chamber adjacent to the anode chamber can also be referred to as the acid chamber. The bridge chamber adjacent to the cathode chamber can also be referred to as the base chamber. The first chamber of the electrochemical desorption unit is the bridge chamber adjacent to the anode chamber, and the second chamber of the electrochemical desorption unit is the bridge chamber adjacent to the cathode chamber. A bipolar membrane can be arranged between the anode chamber and the adjacent bridge chamber.A bipolar membrane can be arranged between the cathode chamber and the adjacent bridge chamber. In particular, the first solution is introduced into the bridge chamber adjacent to the anode chamber, and the second solution or the fourth solution is introduced into the bridge chamber adjacent to the cathode chamber. In particular, the bridge chambers can be operated in parallel or sequentially. An aqueous alkali hydroxide solution can be introduced into the anode chamber as the anolyte. An aqueous alkali hydroxide solution can be introduced into the cathode chamber as the catholyte. Oxygen is formed in the anode chamber, carbon dioxide is formed in the first chamber, and hydrogen is formed in the cathode chamber.

[0124] According to at least one embodiment, the electrochemical desorption unit is an electrochemical desorption unit with more than four chambers. In particular, electrochemical desorption units with more than four chambers are electrodialysis units. The electrochemical desorption unit with more than four chambers comprises an anode chamber, a cathode chamber, and at least three bridge chambers arranged between the anode chamber and the cathode chamber. The electrochemical desorption unit with more than four chambers has at least two acid chambers and at least two base chambers. Acid chambers are, in particular, bridge chambers and / or the anode chamber. Base chambers are, in particular, bridge chambers and / or the cathode chamber. The acid chambers and base chambers are arranged alternately. One acid chamber and one base chamber each form an acid-base space.Within an acid-base compartment, the acid compartment is the one located on the anode side. A cation-selective membrane is located between the acid compartment and the base compartment. Each acid compartment acts as the first compartment, and each base compartment acts as the second compartment. A bipolar membrane is located between adjacent acid-base compartments.

[0125] In particular, the electrochemical desorption unit with more than four chambers has an even number of bridge chambers. With an even number of bridge chambers, half of the bridge chambers form acid chambers and the other half base chambers. Furthermore, the anode chamber can be an acid chamber and the cathode chamber a base chamber.

[0126] For example, an electrochemical desorption unit with six chambers is constructed as follows: anode, anode chamber, cation-selective membrane, first bridge chamber, bipolar membrane, second bridge chamber, cation-selective membrane, third bridge chamber, bipolar membrane, fourth bridge chamber, cation-selective membrane, cathode chamber, cathode. The anode chamber and the first bridge chamber, the second bridge chamber and the third bridge chamber, and the fourth bridge chamber and the cathode chamber each form an acid-base space. Carbon dioxide is formed in each acid chamber. Oxygen is also formed in the anode chamber. Hydrogen is formed in the cathode chamber.

[0127] Alternatively, an electrochemical desorption unit with six chambers can be constructed as follows: anode, anode chamber, bipolar membrane, first bridge chamber, cation-selective membrane, second bridge chamber, bipolar membrane, third bridge chamber, cation-selective membrane, fourth bridge chamber, bipolar membrane, cathode chamber, cathode. The first bridge chamber and the second bridge chamber, as well as the third bridge chamber and the fourth bridge chamber, each form an acid-base compartment. The anode chamber can be operated with an anolyte, and the cathode chamber with a catholyte. The anolyte and catholyte can be, for example, aqueous alkali hydroxide solutions. Carbon dioxide is formed in each acid compartment, oxygen is formed in the anode compartment, and hydrogen is formed in the cathode compartment.

[0128] In particular, an electrochemical desorption unit with more than four chambers has an odd number of bridge chambers. With an odd number of bridge chambers, n-2 of the bridge chambers are acid chambers and n-3 of the bridge chambers are base chambers. Furthermore, the cathode chamber forms a base chamber. The anode chamber is operated with an anolyte, for example, an aqueous alkali hydroxide solution. For example, an electrochemical desorption unit with five chambers is constructed as follows: anode, anode chamber, bipolar membrane, first bridge chamber, cation-selective membrane, second bridge chamber, bipolar membrane, third bridge chamber, cation-selective membrane, cathode chamber, cathode. The first bridge chamber and the second bridge chamber, as well as the third bridge chamber and the cathode chamber, each form an acid-base compartment. Carbon dioxide is formed in each acid chamber. Oxygen is formed in the anode chamber.Hydrogen is formed in the cathode chamber.

[0129] According to at least one embodiment, two or more electrochemical desorption units are used in the process. In particular, the two or more electrochemical desorption units are arranged in a stack structure. The structure of the individual electrochemical desorption units in the stack structure can correspond to the structures described in connection with the electrochemical two-chamber desorption unit or the electrochemical three-chamber desorption unit. In particular, the respective electrochemical desorption units are operated in parallel or sequentially. In particular, the electrochemical desorption units in the stack structure are separated from one another by bipolar plates. In particular, the bipolar plates are designed to distribute the total potential between the electrochemical desorption units.

[0130] Further advantages and advantageous embodiments of the method for separating carbon dioxide from an air stream, the absorbent for absorbing carbon dioxide from a gas stream, and the method for separating carbon dioxide in an electrochemical desorption unit are illustrated by the figures and explained in the following examples. It should be noted that the figures are merely descriptive and are not intended to limit the invention in any way.

[0131] Figure 1 shows a two-chamber electrolysis with recirculation of the electrolyzed solution from the anode to the cathode chamber according to an embodiment;

[0132] Fig. 2 a two-chamber electrolysis with recirculation of the electrolyzed solution from the anode to the cathode chamber and a functionalized Ni anode according to an embodiment;

[0133] Fig. 3 shows a three-chamber electrolysis with recirculation of the electrolyzed solution from the middle chamber to the cathode chamber according to an embodiment;

[0134] Fig. 4 shows a five-chamber electrolysis with recirculation of the electrolyzed solution from the middle chamber to the cathode chamber according to an embodiment;

[0135] Fig. 5 shows a three-chamber electrolysis with a separate precipitation step of the CO2-saturated absorbent according to an embodiment;

[0136] Fig. 6 shows a two-chamber electrolysis with separate release of CO2 in another chamber according to an embodiment;

[0137] Fig. 7 a two-chamber electrolysis with separate precipitation step of the CO2-saturated absorbent and release of the CO2 from KHCO3 by KHSO4 according to an embodiment;

[0138] Fig. 8 a two-chamber electrolysis unit with separate precipitation step of the CO2-saturated absorbent;

[0139] Fig. 9 shows the current flow and carbon dioxide evolution of a bicarbonate-based absorbent in a three-chamber electrolysis;

[0140] Fig. 1 shows the current flow and carbon dioxide evolution of an amino acid-based absorbent in a three-chamber electrolysis;

[0141] Fig. 11 the solubility of KHCO3 in K2CO3 and the CCL partial pressure at saturation with KHCO3;

[0142] Fig. 12 the mass transfer rates for the uptake of CO2 into a KHCCh / K^CCh solution at 323 K;

[0143] Fig. 13 the current / voltage characteristic of the electrolysis of a 1 molal KOH solution;

[0144] Fig. My comparison of the current / voltage characteristics of a 1 molal KOH solution in the anode and cathode compartments in analogy to the electrolysis of a 1 molal KHCO3 solution in the anode and a 0.5 molal K2CO3 solution in the cathode compartment;

[0145] Fig. 15 shows the current / voltage curve of a 0.5 molal K2SO4 solution, pH = 7.4, in the anode chamber and a 1 molal K2CO3 solution, pH = 11.8, in the cathode chamber; Fig. 16 shows the current / voltage characteristic of an electrolysis described here in a three-chamber system;

[0146] Fig. 17, 18 and 19 each show a schematic representation of a process for separating carbon dioxide in an electrochemical desorption unit according to various embodiments;

[0147] Figs. 20, 22 and 24 each show a schematic representation of an electrochemical desorption unit according to various embodiments;

[0148] Fig. 21 and 23 each show a schematic representation of a stack structure of two electrochemical desorption units according to different embodiments.

[0149] Identical, similar, or functionally identical elements are provided with the same reference numerals in the figures. The figures and the relative sizes of the elements depicted in the figures are not to be considered to scale. Rather, individual elements, particularly distances, dimensions, layer thicknesses, and / or lengths, may be exaggerated for clarity and / or clarity.

[0150] Figure 1 shows a two-chamber electrolysis with recirculation of the electrolyzed solution from the anode to the cathode chamber according to one exemplary embodiment. This is a simple structure of an electrolysis unit for the electrolysis of H2O and the release of CO2 from an absorption liquid for carbon dioxide. It is also shown that the electrolyzed solution is recirculated from the anode to the cathode chamber. This electrolysis can be carried out, for example, using an absorbent consisting of a salt, which decomposes into cations and anions in aqueous solution. The anode and cathode compartments are separated by a cation-selective membrane. During the course of the electrolysis, the pH value at the anode decreases while the pH value at the cathode increases.Since the absorption of CO2 into an aqueous solution is always associated with a decrease in pH for the known CCL absorbents, alkalis, carbonates, amino acid salts, and amines—primary, secondary, and tertiary—CO2 can always be expelled from the solution by reducing the pH. It is particularly advantageous if the CO2 absorbent contains the cation itself. These include, for example, alkalis of all alkali metals, carbonates of all alkali metals, and amino acid salts of all alkali metals. For CCL absorbents that do not contain alkali metals, a neutral salt that is not electrolyzed, such as K2SO4 or similar, can be added to the absorption solution. This results in the fundamental applicability of the electrolysis principle to a wide variety of known CCh absorbents.The energy required for CCE release, in addition to the electrolysis of H2O, results from the pH difference between the anode and cathode chambers required for CCE expulsion and the resulting potential difference. This is substance-specific and can vary between 0.8 V and 0.25 V. Furthermore, the number of electrons required to release one CCE molecule is also substance-specific and depends on the achievable loading capacity, which corresponds to the CCE partial pressure in the absorber. The number of electrons required varies between 1 and 2. A general difference from known processes can be seen in the fact that the solution depleted of CO2 in the anode chamber is passed into the cathode chamber before further CO2 absorption. There, neutralization takes place, which lowers the pH in the cathode chamber.The resulting reduction in the potential difference significantly reduces the energy required to release CCE. The advantages of the process described here are primarily its general applicability, its simple process design, as it can be operated directly on a CCE absorber column, and the stacked structure of the electrolysis cell due to the use of bipolar membranes.

[0151] Figure 2 shows a partial process step in the form of a two-chamber electrolysis of a carbon dioxide-laden air stream with a nickel hydroxide anode according to one exemplary embodiment. An absorbent containing an alkali carbonate component is loaded with carbon dioxide from an air stream. The carbonate is converted, at least partially, into bicarbonate, which is introduced into the anode compartment of a two-chamber electrolysis cell. The electrolysis cell has an alkali-permeable membrane separating the anode from the cathode compartment. The anode is a porous anode capable of binding oxygen. Therefore, only the carbon dioxide formed leaves the anode compartment. Hydrogen is formed in the cathode compartment. Therefore, the different gases are produced at different locations and do not require complex separation. The anode can then be thermally regenerated from time to time, releasing oxygen.The following reactions occur: Anode.

[0152] 2 KHCO3 2 K + + 2 HCO3-

[0153] 2 HCO3- 2 CO2+ 2 OH'

[0154] 2 Ni(OH)2+ 2 OH 2 NiOOH + 2 H2O + 2 e

[0155] The overall reaction is:

[0156] (KHCO3+ Ni(OH)2K + + CO2+ NiOOH + e) ​​x 2

[0157] The following reactions take place at the cathode:

[0158] 2 H2O + 2 e H2+ OH'

[0159] 2 KHCO3+ 2 K + + 2 OH' 2 K2CO3 + 2 H2O

[0160] Overall, the total reaction in the cathode compartment is: 2 KHCO3+ 2 K + + 2 e 2 K2CO3 + H2

[0161] Figure 3 shows a partial process step in the form of a three-chamber electrolysis of a carbon dioxide-laden absorbent according to one exemplary embodiment. A carbonate-based absorbent can be loaded with carbon dioxide, for example, from an air stream, such as ambient air or industrial exhaust air. The absorption results in at least partial conversion of the carbonate into bicarbonate. The bicarbonate-containing solution is introduced into the middle chamber of an electrolysis unit comprising at least three chambers. The middle chamber is separated from the anode compartment by a bipolar membrane and from the cathode compartment by a membrane permeable to potassium, or generally alkali ions. By applying a voltage, oxygen is evolved in the anode compartment, hydrogen in the cathode compartment, and carbon dioxide in the middle compartment. The individual gas streams can be collected separately.The solution formed in the middle chamber then serves as the electrolyte in the cathode compartment. The carbon dioxide-depleted solution formed in the cathode compartment now has a higher carbonate and a lower bicarbonate content. This recycled solution can be reused as an absorbent for a carbon dioxide-containing air stream. The following reactions occur at the various reaction sites in the form of reaction equations:

[0162] At the anode:

[0163] 2 OH' H2O + 2 O2 + 2e

[0164] At the bipolar membrane:

[0165] 2 H2O 2 H + + 2 OH'

[0166] In the intermediate cell:

[0167] 2 HCO3- + 2 H + 2 CO2 + 2 H2O

[0168] In sum, the overall reaction of the intermediate cell is:

[0169] 2 KHCO3 + 2 H + 2 K + + 2 CO2 + 2 H2O

[0170] The following reactions take place at the cathode:

[0171] 2 H2O + 2 e H2+ OH'

[0172] 2 KHCO3 + 2 K + + 2 OH' 2 K2CO3 + 2 H2O

[0173] Overall, the cathode compartment is:

[0174] 2 KHCO3 + 2 K + + 2 e 2 K2CO3 + H2

[0175] The three-chamber structure can be expanded as required with respect to the middle unit.

[0176] Figure 4 shows a five-chamber electrolysis system with recirculation of the electrolyzed solution from a central chamber to a central base chamber or the cathode chamber according to one exemplary embodiment. Due to the design described here, several "central" chambers can be used to release carbon dioxide. Accordingly, several bipolar membranes and cation-selective membranes are used.

[0177] Figure 5 shows a three-chamber electrolysis system with a separate precipitation step for the CO2-saturated absorbent according to one exemplary embodiment. After the carbon dioxide has been absorbed, the carbon dioxide-saturated absorbent is co-precipitated. The precipitated absorbent can then be added to the electrolysis system in, for example, a purity of over 90%. This process ensures that predominantly only one species is electrolyzed during the electrolysis. This can improve the electrical efficiency parameters of the overall process. The separate precipitation step can also be performed upstream of a two-chamber electrolysis system.

[0178] Figure 6 shows a two-chamber electrolysis with separate release of CO2 in a separate chamber according to one exemplary embodiment. To separate the individual gas streams of the electrolysis, the release of carbon dioxide can in principle also occur in a compartment located outside the electrolysis cell. The release occurs through pressure and / or temperature changes. After the carbon dioxide release, the solution can be fed into the cathode chamber. A separate precipitation step can be performed before electrolysis. The KHCO3-enriched solution is fed into the anode chamber, and the depleted solution is fed into the cathode chamber (Figure 5).

[0179] Figure 7 shows a two-chamber electrolysis system with a separate precipitation step for the CO2-saturated absorbent according to one embodiment. In this variant, a pre-precipitation of the saturated absorbent takes place. The precipitate, KHCO3, is transferred as a solid or sludge into a KHSO4 / K2SO4 solution, according to the following reaction equation:

[0180] KHCO3 + KHSO4 K2SO4 + H2O + CO2T

[0181] CO2 is released.

[0182] The driving force here is that KHSO4, pH = 0.7 corresponding to a 10 wt.% solution, is a stronger acid than KHCO3, pH = 7.9 corresponding to a 10 wt.% solution. The KHSO4-enriched KHSO4 / IgSCk solution is fed into the anode chamber, where, according to the overall reaction in the anode chamber, the following occurs:

[0183] 2 K2SO4 + H2O 2 K - 2 KHSO4 + 1 / 2O2 + 2e the release of O2 with formation of KHSO4.

[0184] The KHSO4 / K2SO4 solution, now enriched with KHSO4, is returned to the reaction vessel to release the CO2. In the cathode compartment, H2 is released, while the KHCO3 / K^CCh solution is simultaneously enriched with K2CO3 for further CO2 absorption:

[0185] 2 KHCO3 + 2 K + +2e 2 K2CO3 + H2

[0186] Compared to the electrolysis of KHCO3 (Figure 14), the electrolysis of KHSO4 results in a voltage drop across the cation-selective membrane that is approximately 0.2 V higher due to the lower pH value. The process variant shown in Figure 7 is therefore an energy-efficient process for the release of CO2 and O2 in separate volumes.

[0187] Figure 8, a section of Figure 4, shows a stackable or expandable stack separated by bipolar membranes. In addition, a separate precipitation step of the CO2-saturated absorbent precedes the electrolysis. This separate precipitation and dissolution step can increase the electrical efficiency of the electrolysis, since in these cases only bicarbonate and no carbonate enters the electrolysis cell.

[0188] Figure 9 shows the current profile and carbon dioxide evolution of a bicarbonate-based absorbent in a three-chamber electrolysis system. The absorbent is based on a 10 wt.% KHCO3 solution, and the figure shows the measured current and the determined CO2 flow rate as a function of the measurement cycles. The data from the measurement cycles from 10,000 to 11,000 are plotted, with the time interval of one measurement cycle being 1 second. The measurement was carried out at a temperature of 20°C. Electrolyte solutions containing the following ingredients were used: anode: KOH 5.4 wt.%; intermediate chamber: KHCO3 10 wt.%; cathode: KHCO3 10 wt.%. The fluctuating current flow is due to bubble formation and detachment on the surface of the bipolar membrane. For a similar reason, the CO2 gas flow rate also fluctuates slightly. The stoichiometric ratio of the released gases was approximately 2: 1: 1 / 2 for CO2, H2, O2.The Faraday effectiveness achieved in the simple experimental setup relative to CO2 was about 80%.

[0189] Figure 10 shows the current profile and carbon dioxide evolution of an amino acid-based absorbent in a three-chamber electrolysis system. The current used and the achieved CCL gas flow rate are plotted during the electrolysis of a CO2-loaded amino acid salt solution in the three-chamber electrolyzer. The measurement was also carried out at a temperature of 20°C. Electrolyte solutions of the following composition were used: anode: KOH, 5.4 wt.%; intermediate chamber: loaded amino acid salt solution with proline, 10 wt.%; cathode: loaded amino acid salt solution with proline, 10 wt.%. CO2 loading took place before electrolysis in a bubble column reactor by passing a 14 vol.% CO2 gas stream through it. The measured fluctuating current flow is also attributable to bubble formation and detachment on the surface of the bipolar membrane. The same applies to the fluctuations in the CO2 gas flow rate.The stoichiometric ratio of the released gases was approximately 1:1:1 / 2 for CO2, H2, and O2. The Faradaic efficiency achieved in this simple experimental setup, relative to CO2, was approximately 85% of the previously described stoichiometric ratio.

[0190] Figure 11 shows the solubility of KHCO3 in K2CO3 and the CO2 partial pressure at saturation with KHCO3. The plotted line corresponds to the solubility of KHCO3 (left ordinate) in a K2CO3 solution at 293 K. The calculated and experimentally confirmed (open circles) CO2 partial pressure is shown as a dashed line (right ordinate). From the plot, it can be seen that with a K2CO3 concentration of > 40 g / kg, KHCO3 precipitates from solution below a CO2 concentration of 140,000 Pa. CO2 absorption from flue gases into a K2CO3 solution and precipitation of the absorbed CO2 as KHCO3 is possible from a K2CO3 concentration of > 40 g / kg. The absorption of CO2 from the air, with a CO2 concentration of about 400 ppm, and precipitation as KHCO3 can be observed from a K2CO3 concentration of 560 g / kg.

[0191] Figure 12 shows the experimentally determined mass transfer rates for the uptake of CO2 into a KHCO3 / K2CO3 solution at 323 K. The potassium salt of pipecolic acid was added as a CO2 absorption promoter at a concentration of 2 mol / l. The K2CO3 concentration was gradually increased from 138.25 g / kg to 542.5 g / kg during the measurement period. KHCO3 was also added in portions over a concentration range of 0 to 440 g / kg. The determination of the CO2 uptake rate was carried out along the KHCO3 saturation limit with increasing K2CO3 concentration. The CO2 uptake rate decreases with increasing K2CO3 concentration and with the addition of KHCO3 until the saturation limit is reached. The CO2 mass transfer rate is 0.08 mol / sm 2 bar is sufficient for large-scale use of a packed column.

[0192] Figure 13 shows the current / voltage characteristic of the electrolysis of a 1 molal KOH solution using the setup outlined in Figure 1. The anode and cation compartments are separated by a cation-selective membrane. Electrolysis of the KOH solution requires a cell voltage of approximately 1.8 V. No additional voltage drop is observed across the cation-selective membrane.

[0193] Figure 14 allows a direct comparison of the current / voltage characteristics measured in the laboratory for a 1 molal KOH solution in the anode and cathode compartments (cf. Figure 13) in analogy to the electrolysis of a 1 molal KHCO3 solution in the anode compartment and a 0.5 molal K2CO3 solution in the cathode compartment. The additional voltage drop across the cation-selective membrane required to release the CO2 is determined by the pH difference in the anode and cathode compartments, which results from the loaded and unloaded absorbent. Depending on the concentration, this varies between pH 7.8 - 8 for KHCO3 in the anode compartment and between pH 11.5 and 13.5 for K2CO3. At a concentration of 1 mol / kg KHCO3 in the anode chamber, pH = 7.9 and 0.5 mol / kg K2CO3 in the cathode chamber, pH = 11.7, applying the Nernst equation, an additional voltage drop of theoretically = 0.22 V results.This corresponds to an AG = 22 kJ per mol CO2 This roughly corresponds to the thermodynamic minimum. The electrolytic release of CO2 from a KHCO3 solution with formation of K2CO3 is therefore the energetically most effective form of release of absorbed CO2. The laboratory measurements confirm the above theoretical considerations with a cell voltage of just over 2 V. Figure 15 shows the current / voltage curve of a 0.5 molal K2SO4 solution, pH = 7.4 in the anode chamber and a 1 molal K^CCh solution, pH = 11.8, in the cathode chamber. The direct comparison of the current / voltage curves with the previously discussed electrolysis of a KHCCh / K^CCh solution confirms that the higher pH difference between the anode and cathode chambers results in a higher experimentally observed electrolysis voltage of about 0.2 V. The electrolysis of a K2SO4 solution corresponds to the process presented in Figure 7.

[0194] Figure 16 shows the current / voltage characteristic of electrolysis in a three-chamber system corresponding to Figure 5. The anode chamber contains a 1 molal KOH solution with a pH of 14. The intermediate chamber and the cathode chamber contain the same solution with a composition of: KHCO3 5 wt.% + K2CO3 5 wt.% with a pH of 9.5. Compared to Figure 13, this figure shows that the use of the bipolar membrane requires an additional electrolysis voltage of approximately 0.5 V. Theoretically, this results in a membrane potential of 0.27 V.

[0195] Figure 17 shows a schematic representation of a method for separating carbon dioxide in an electrochemical desorption unit 10 according to an exemplary embodiment. The exemplary embodiment shown in Figure 17 shows a method in which the electrochemical desorption unit 10 is operated sequentially.

[0196] The electrochemical desorption unit 10 comprises a first chamber 11 and a second chamber 12, which are separated from each other by a cation-selective membrane 13. The cation-selective membrane 13 is selective for alkali cations, in particular sodium ions, potassium ions, or combinations thereof. The cation-selective membrane 13 is configured to allow alkali cations to migrate from the first chamber 11 into the second chamber 12. The electrochemical desorption unit 10 can be an n-chamber electrochemical desorption unit, where n is a natural number greater than or equal to 2. For example, the electrochemical desorption unit 10 is a two-chamber electrochemical desorption unit, a three-chamber electrochemical desorption unit, or a four-chamber electrochemical desorption unit.Alternatively, the electrochemical desorption unit 10 may comprise more than four chambers, for example, five chambers, six chambers, seven chambers, eight chambers, nine chambers, ten chambers, or 500 chambers. For reasons of clarity, only the first chamber 11 and the second chamber 12 of the electrochemical desorption unit 10 are shown in the embodiment of Figure 17. The electrochemical desorption unit 10 may comprise further chambers, elements, and components in addition to the chambers, elements, and components shown in Figure 17.

[0197] A first solution 1 is introduced into the first chamber 11. The first solution 1 has a molar concentration of bicarbonate that is at least twice as high as a molar concentration of carbonate. In the first chamber 11, carbon dioxide is electrochemically desorbed from the first solution 1. Carbon dioxide is released in the process. In the first chamber 11, the desorption of carbon dioxide from the first solution 1 forms a second solution 2, which is then discharged from the first chamber 11 and introduced into the second chamber 12. In the second chamber 12, the second solution 2 is electrochemically converted to form a third solution 3, which is then discharged from the second chamber 12. In particular, in the second chamber 12, bicarbonate is consumed to form carbonate, and the third solution 3 has a higher molar concentration of carbonate than of bicarbonate.

[0198] In the sequential operation of the electrochemical desorption unit 10 shown in Figure 17, a depletion of bicarbonate occurs along the chain of introducing the first solution 1 into the first chamber 11, introducing the second solution 2 into the second chamber 12, and discharging the third solution 3 from the second chamber 12. In particular, at least 1 / 3 and at most 2 / 3 of the molar amount of bicarbonate in the first solution 1 is converted in the first chamber 11. For example, the decrease in the molar concentration of bicarbonate in the first chamber 11 corresponds to between 0.8 and 1.2 times the decrease in the molar concentration of bicarbonate in the second chamber 12.

[0199] In particular, the first solution 1 consists of water, bicarbonate and other

[0200] components. The sum of all other components can be less than 10 wt.%. The molar concentration of bicarbonate in the first solution 1 can be greater than or equal to 0.5 mol / L and less than or equal to 5.5 mol / L. For example, the saturation of the first solution 1 with bicarbonate is more than 60% at a temperature of less than or equal to 30 °C, more than 50% at a temperature of greater than 30 °C and less than or equal to 60 °C, or at least 3 mol / L at a temperature of greater than 60 °C. The first solution 1 can also have a pH of at least 7.2 and at most 9.5.

[0201] The first solution 1 can be formed as follows:

[0202] An absorbent comprising at least carbonate is contacted with a gas stream comprising carbon dioxide. The carbonate of the absorbent is partially converted into bicarbonate, thereby producing a loaded absorbent. Subsequently, a solid comprising at least a portion of the bicarbonate is precipitated from the loaded absorbent, and the precipitated solid is separated from the aqueous phase of the loaded absorbent. In particular, the precipitated solid comprises at least 90 wt.% bicarbonate. The first solution 1 is then produced by dissolving the precipitated and separated solid in water.

[0203] Figure 18 shows a schematic representation of a method for separating carbon dioxide in an electrochemical desorption unit 10 according to an embodiment. The method of the embodiment of Figure 18 essentially corresponds to the embodiment shown in Figure 17. In contrast to the embodiment shown in Figure 17, the embodiment of Figure 18 shows a method in which the electrochemical desorption unit 10 is operated in parallel. Instead of the second solution 2, a fourth solution 4 is introduced into the second chamber 12 and electrochemically reacted in the second chamber 12. This forms a fifth solution 5, which is subsequently discharged from the second chamber 12. In particular, in the method shown in Figure 18, the molar concentration of bicarbonate in the second solution 2 is greater than 80% of the molar concentration of bicarbonate in the first solution 1.In other words, in parallel operation of the electrochemical desorption unit 10, the first chamber 11 can be operated with a nearly constant molar concentration of bicarbonate. The molar concentration of bicarbonate in the fourth solution 4 is greater than zero. In the second chamber 12, bicarbonate is consumed and carbonate is formed. In other words, the fifth solution 5 has a lower molar concentration of bicarbonate and a higher molar concentration of carbonate than the fourth solution 4. In particular, the conductivity of the fourth solution under operating conditions in the second chamber is more than 100 mS / cm.

[0204] The fourth solution 4 can be formed as follows:

[0205] An absorbent comprising at least carbonate is contacted with a gas stream comprising carbon dioxide. The carbonate of the absorbent is partially converted into bicarbonate, thereby producing a loaded absorbent. Subsequently, a solid comprising at least a portion of the bicarbonate is precipitated from the loaded absorbent, and the precipitated solid is separated from the aqueous phase of the loaded absorbent. The aqueous phase of the loaded absorbent forms the fourth solution 4.

[0206] Figure 19 shows a schematic representation of a method for separating carbon dioxide in an electrochemical desorption unit 10 according to an embodiment. The method of the embodiment of Figure 19 essentially corresponds to the embodiment shown in Figure 18. In contrast to the embodiment shown in Figure 18, the embodiment of Figure 18 shows a method in which a cyclic process is implemented with respect to the first chamber 11 and with respect to the second chamber 12.

[0207] The circular process with respect to the first chamber 11 is as follows:

[0208] The second solution 2 formed in the first chamber 11 is introduced into a storage vessel 14 after being discharged from the first chamber 11. In the storage vessel 14, a solid comprising at least 90 wt.% bicarbonate is added to the second solution 2, thereby forming the first solution 1. The solid can be added to the second solution in solid form or in dissolved form, for example, as an aqueous solution of the solid. The solid is, in particular, the precipitated and separated solid comprising at least a portion of bicarbonate from a loaded absorbent. The first solution 1 formed in the storage vessel 14 is then introduced into the first chamber 11. The storage vessel 14 thus serves to regenerate the first solution 1 from the second solution 2.

[0209] The cycle process with respect to the second chamber 12 is as follows:

[0210] The fifth solution 5 formed in the second chamber 12 is used, after being discharged from the second chamber 12, in an absorption device 15 for absorbing carbon dioxide from a gas stream. The absorption device 15 can be, for example, an absorption column. In particular, the fifth solution 5 can be used as an absorbent for contacting the absorbent with a gas stream comprising carbon dioxide and generating the loaded absorbent. For example, the fourth solution 4 is the aqueous phase of the loaded absorbent, from which a precipitated solid comprising a portion of bicarbonate has been separated from the loaded absorbent. The fourth solution 4 is subsequently introduced into the second chamber 12. The absorption device 15 thus serves to regenerate the fourth solution 4 from the fifth solution 5.

[0211] Figure 20 shows a schematic representation of an electrochemical desorption unit 10 according to an embodiment. The starting example in Figure 20 shows a two-chamber desorption unit. The electrochemical desorption unit 10 of Figure 20 comprises, between an anode 16 and a cathode 17, an anode chamber 18, which forms the first chamber 11, and a cathode chamber 19, which forms the second chamber 12. A cation-selective membrane 13 is arranged between the anode chamber 18 and the cathode chamber 19. In Figure 20, the reactions taking place in the respective chambers in the anode chamber 18 and in the cathode chamber 19 are shown in the form of reaction equations using the example of the corresponding potassium compounds. Alternatively, the electrochemical desorption unit 10 can also be operated with the corresponding sodium compounds or a mixture of the corresponding potassium compounds and the corresponding sodium compounds.The reactions shown take place during sequential operation of the electrochemical desorption unit 10 according to the process shown in Figure 17 and during parallel operation of the electrochemical desorption unit 10 according to the processes shown in Figures 18 and 19. In the anode chamber 18, carbon dioxide is electrochemically desorbed from the first solution 1, consuming bicarbonate and forming carbon dioxide and oxygen. In the cathode chamber 19, the second solution 2 or the fourth solution 4 is electrochemically converted, consuming bicarbonate and forming carbonate and hydrogen.

[0212] Figure 21 shows a schematic representation of a stack structure of two electrochemical desorption units 10 according to one embodiment. In the embodiment of Figure 21, the two electrochemical desorption units are two electrochemical two-chamber desorption units according to the embodiment of Figure 20. The two electrochemical desorption units 10 are separated from each other by a bipolar plate 20. The bipolar plate 20 functions as the anode 16 for one electrochemical desorption unit 10 and as the cathode 17 for the other electrochemical desorption unit 10.

[0213] Figure 22 shows a schematic representation of an electrochemical desorption unit 10 according to an embodiment. The starting example in Figure 22 shows a three-chamber desorption unit. The electrochemical desorption unit 10 of Figure 22 comprises, between an anode 16 and a cathode 17, an anode chamber 18, a cathode chamber 19, and a bridge chamber 21 arranged between the anode chamber 18 and the cathode chamber 19. The bridge chamber 21 forms the first chamber 11, and the cathode chamber 19 forms the second chamber 12. A bipolar membrane 22 is arranged between the anode chamber 18 and the bridge chamber 21. A cation-selective membrane 13 is arranged between the cathode chamber 19 and the bridge chamber 21. In Figure 22, the reactions taking place in the anode chamber 18, the bridge chamber 21 and the cathode chamber 19 are shown in the form of reaction equations using the example of the corresponding potassium compounds.Alternatively, the electrochemical desorption unit 10 can also be operated with the corresponding sodium compounds or a mixture of the corresponding potassium compounds and the corresponding sodium compounds. The reactions shown take place during sequential operation of the electrochemical desorption unit 10 according to the method shown in Figure 17 and during parallel operation of the electrochemical desorption unit 10 according to the methods shown in Figures 18 and 19. Oxygen is formed in the anode chamber 18. In the bridge chamber 21, carbon dioxide is electrochemically desorbed from the first solution 1, whereby bicarbonate is consumed and carbon dioxide is formed. In the cathode chamber 19, the second solution 2 or the fourth solution 4 is electrochemically reacted, whereby bicarbonate is consumed and carbonate and hydrogen are formed.

[0214] Figure 23 shows a schematic representation of a stack structure comprising two electrochemical desorption units 10 according to one embodiment. In the embodiment of Figure 23, the two electrochemical desorption units are two electrochemical three-chamber desorption units according to the embodiment of Figure 22. The two electrochemical desorption units 10 are separated from each other by a bipolar plate 20. The bipolar plate 20 functions as the anode 16 for one electrochemical desorption unit 10 and as the cathode 17 for the other electrochemical desorption unit 10.

[0215] Figure 24 shows a schematic representation of an electrochemical desorption unit 10 according to an embodiment. The starting example in Figure 24 shows a four-chamber desorption unit. The electrochemical desorption unit 10 of Figure 24 comprises, between an anode 16 and a cathode 17, an anode chamber 18, a cathode chamber 19, and two bridge chambers 21 arranged between the anode chamber 18 and the cathode chamber 19. The bridge chamber 21 adjacent to the anode chamber 18 forms the first chamber 11, and the bridge chamber 21 adjacent to the cathode chamber 19 forms the second chamber 12. The bridge chamber 21 adjacent to the anode chamber 18 is also referred to as the acid chamber. The bridge chamber 21 adjacent to the cathode chamber 19 is also referred to as the base chamber. A bipolar membrane 22 is arranged between the anode chamber 18 and the bridge chamber 21 adjacent to the anode chamber 18.A bipolar membrane 22 is arranged between the cathode chamber 19 and the bridge chamber 21 adjacent to the cathode chamber 19. A cation-selective membrane 13 is arranged between the bridge chambers 21. Figure 24 shows the reactions taking place in the respective chambers in the anode chamber 18, in the bridge chambers 21, and in the cathode chamber 19 in the form of reaction equations using the example of the corresponding potassium compounds. Alternatively, the electrochemical desorption unit 10 can also be operated with the corresponding sodium compounds or a mixture of the corresponding potassium compounds and the corresponding sodium compounds. The reactions shown take place during sequential operation of the electrochemical desorption unit 10 according to the method shown in Figure 17 and during parallel operation of the electrochemical desorption unit 10 according to the methods shown in Figures 18 and 19.Oxygen is formed in the anode chamber 18. In the bridge chamber 21 adjacent to the anode chamber 18, carbon dioxide is electrochemically desorbed from the first solution 1, consuming bicarbonate and forming carbon dioxide. In the bridge chamber 21 adjacent to the cathode chamber 19, the second solution 2 or the fourth solution 4 is electrochemically converted, consuming bicarbonate and forming carbonate. Hydrogen is formed in the cathode chamber 19.

[0216] The features and exemplary embodiments described in conjunction with the figures can be combined with one another according to further exemplary embodiments, even if not all combinations are explicitly described. Furthermore, the exemplary embodiments described in conjunction with the figures can alternatively or additionally comprise further features according to the description in the general part.

[0217] The invention is not limited to the embodiments by the description. Rather, the invention encompasses any novel feature and any combination of features, including, in particular, any combination of features in the claims, even if this feature or combination itself is not explicitly stated in the claims or embodiments. List of reference symbols

[0218] 1 first solution

[0219] 2 second solution

[0220] 3 third solution

[0221] 4 fourth solution

[0222] 5 fifth solution

[0223] 10 Electrochemical desorption unit

[0224] 11 First Chamber

[0225] 12 second chamber

[0226] 13 cation-selective membrane

[0227] 14 storage vessel

[0228] 15 Absorption device

[0229] 16 Anode

[0230] 17 Cathode

[0231] 18 Anode chamber

[0232] 19 Cathode chamber

[0233] 20 bipolar plate

[0234] 21 Bridge Chamber

[0235] 22 bipolar membrane

Claims

Patent claims 1. A method for separating carbon dioxide in an electrochemical desorption unit (10), wherein the electrochemical desorption unit (10) comprises at least a first chamber (11) and a second chamber (12) separated by a cation-selective membrane (13), the method comprising the steps of: - introducing a first solution (1) into the first chamber (11), wherein a molar concentration of hydrogen carbonate in the first solution (1) is at least twice as high as a molar concentration of carbonate, - Electrochemical desorption of carbon dioxide from the first solution (1), whereby carbon dioxide is released and a second solution (2) is formed from the first solution (1).

2. Process according to the preceding claim, wherein the molar concentration of bicarbonate in the first solution (1) is greater than or equal to 0.5 mol / L and less than or equal to 5.5 mol / L.

3. The method according to any one of the preceding claims, wherein a saturation of the first solution (1) in bicarbonate at a temperature of less than or equal to 30 °C is more than 60%, or wherein a saturation of the first solution (1) in bicarbonate at a temperature of greater than 30 °C and less than or equal to 60 °C is more than 50%, or wherein a molar concentration of bicarbonate in the first solution (1) at a temperature of greater than 60 °C is at least 3 mol / L.

4. Process according to one of the preceding claims, wherein the first solution (1) consists of water, bicarbonate and further components, wherein a proportion of a sum of all further components is less than 10 wt.%.

5. Method according to one of the preceding claims, wherein the first solution (1) has a pH of at least 7.2 and at most 9.

5.

6. A method according to any one of the preceding claims, wherein the first solution (1) is formed as follows: - contacting an absorbent with a gas stream comprising carbon dioxide, wherein the absorbent comprises at least one carbonate, - producing a loaded absorbent, wherein the carbonate is at least partially converted into bicarbonate, - Precipitation of a solid comprising at least part of the bicarbonate from the loaded absorbent, - Separation of the precipitated solid from the aqueous phase of the loaded absorbent, - Producing the first solution (1), whereby the precipitated and separated solid is dissolved in water.

7. Method according to one of the preceding claims, further comprising: - introducing the second solution (2) into the second chamber (12), and - Electrochemical conversion of the second solution (2), whereby a third solution (3) is formed from the second solution (2).

8. The method according to the preceding claim, wherein a molar concentration of bicarbonate in the second solution (2) is lower than the molar concentration of bicarbonate in the first solution (1).

9. The method according to any one of claims 7 or 8, wherein a molar concentration of bicarbonate in the third solution (3) is lower than the molar concentration of bicarbonate in the second solution (2).

10. The method according to any one of claims 7 to 9, wherein a molar concentration of bicarbonate in the second solution (2) is greater than or equal to 1 / 3 of the molar concentration of bicarbonate in the first solution (1) and less than or equal to 2 / 3 of the molar concentration of bicarbonate in the first solution (1).

11. The method according to any one of claims 7 to 10, wherein a molar concentration of carbonate in the third solution (3) is higher than a molar concentration of bicarbonate.

12. The method according to any one of claims 7 to 11, wherein a ratio of a first difference and a second difference is greater than 0.8 and less than 1.2, wherein the first difference is a difference between a molar concentration of Bicarbonate in the third solution (3) and a molar concentration of bicarbonate in the second solution (2), and wherein the second difference is a difference between a molar concentration of bicarbonate in the second solution (2) and the molar concentration of bicarbonate in the first solution (1).

13. The method according to any one of claims 1 to 6, further comprising: - introducing a fourth solution (4) into the second chamber (12), and - Electrochemically converting the fourth solution (4), whereby a fifth solution (5) is formed from the fourth solution (4).

14. The method according to the preceding claim, wherein a molar concentration of bicarbonate in the second solution (2) is greater than 80% of the molar concentration of bicarbonate in the first solution (1).

15. The method according to any one of claims 13 or 14, wherein a molar concentration of bicarbonate in the fourth solution (4) is greater than zero.

16. The method according to any one of claims 13 to 15, wherein a molar concentration of bicarbonate in the fifth solution (5) is lower than a molar concentration of bicarbonate in the fourth solution (4).

17. The method according to any one of claims 13 to 16, wherein a molar concentration of carbonate in the fifth solution (5) is higher than a concentration of carbonate in the fourth solution (4).

18. A method according to any one of claims 13 to 17, wherein the fourth solution (4) is formed as follows: - contacting an absorbent with a gas stream comprising carbon dioxide, wherein the absorbent comprises at least one carbonate, - producing a loaded absorbent, wherein the carbonate is at least partially converted into bicarbonate, - Precipitation of a solid comprising at least part of the bicarbonate from the loaded absorbent, - Separating the precipitated solid from the aqueous phase of the loaded absorbent, wherein the aqueous phase of the loaded absorbent forms the fourth solution (4).

19. An absorbent for absorbing carbon dioxide from a gas stream comprising an aqueous solution of a carbonate and a compound having an amine function, wherein the compound having an amine function is adapted to form a carbamate with carbon dioxide.

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