Method for separating carbon dioxide from an air stream and regenerating the absorbent in a three-chamber electrolytic cell

Abstract

The invention relates to a method for isolating carbon dioxide from an air stream, wherein the method comprises at least the following steps: a) providing an aqueous solution of a carbon dioxide absorbent agent, wherein the carbon dioxide absorbent agent comprises cations of the first main group of the periodic table; b) passing an air stream containing carbon dioxide through the solution provided in step a), wherein at least a portion of the carbon dioxide in the air stream is bound to the carbon dioxide absorbent agent and the air stream is enriched with carbon dioxide;c) introducing the aqueous solution from step b) or an aqueous solution comprising carbon dioxide bound to the carbon dioxide absorbent agent into a central chamber of an electrolysis cell of at least three chambers formed by an anodic chamber, a cathodic chamber and at least one central chamber disposed between the anodic chamber and the cathodic chamber, wherein the three-chamber electrolysis cell has a membrane that is selective for monovalent cations; and electrolyzing the aqueous solution, releasing at least some of the carbon dioxide.

Description

[0001] The present invention relates to a method for isolating carbon dioxide from an air stream, wherein the method comprises at least the steps of: a) Providing an aqueous solution of a carbon dioxide absorbent, wherein the carbon dioxide absorbent contains cations from the 1sta) Main group of the periodic table; b) Passing an air stream containing carbon dioxide through the solution provided in process step a), wherein at least a part 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 a middle chamber of an electrolysis cell comprising at least three chambers, an anode chamber, a cathode chamber, and at least one middle chamber arranged between the anode chamber and the cathode chamber, wherein the three-chamber electrolysis cell has a membrane selective for monovalent cations, and electrolyzing the aqueous solution with the release of at least a part of the carbon dioxide.

[0002] Document US 2019 / 085472 A1 describes a method for the controlled removal of bicarbonate from alkaline water and its replacement by a strong base capable of chemically absorbing CO2 from the atmosphere as a carbonate and bicarbonate solution.

[0003] Publication US 4,197,421 A deals with synthetic carbon-based fuels and feedstocks.

[0004] Document US 2007 / 045125 A1 describes an electrochemical cell for the production of synthesis gas using atmospheric air and water.

[0005] Publication WO 2022 / 023387 A1 describes a process for the binding, transport, reaction activation, conversion, storage and release of water-soluble gases.

[0006] Document US 2011 / 277474 A1 describes methods and systems for the use of natural gas power plants.

[0007] Publication WO 2020 / 163513 A1 describes a system and a process for the production of synthetic fuel by CO2 capture and water splitting.

[0008] Document US 2017 / 209826 A1 describes an arrangement and method for recovering carbon dioxide from gas using an absorption tank housing and a stirrer.

[0009] The publication EP 2 163 294 B1 describes a system and a method for the recovery of CO 2 by the absorption of aqueous carbonate flue gas and highly efficient bipolar membrane electrodialysis.

[0010] The publication lizuka et al., Carbon dioxide recovery from carbonate solutions using bipolar membrane electrodialysis, Separation and Purification Technology, 2012, 101, 49-59 describes carbon dioxide recovery from carbonate solutions using bipolar membrane electrodialysis.

[0011] Publication EP 2 737 937 A1 describes an electrolytic reduction of carbon deposition solutions.

[0012] One of the greatest challenges of the 21st century will be to reduce the gas composition of the atmosphere to levels that prevent excessive global warming. It is scientifically proven that the warming of recent decades, in particular, is due to an increase in so-called greenhouse gases. Carbon dioxide plays a key role among these greenhouse gases, and the sharp rise in its concentration is attributable to the increased burning of fossil fuels. Therefore, in the near future, it will be crucial not only to reduce new carbon dioxide emissions but also to develop technical solutions for capturing 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.

[0013] One technical method for removing carbon dioxide from the air involves passing the carbon dioxide over or through an adsorbent, which then selectively removes it from the airflow. This reduces the carbon dioxide concentration in the air, while the concentration within the adsorbent increases during the process. This method is known for a variety of different absorbents, and the systems are sufficiently complex because both the carbon dioxide concentration and the ambient conditions during absorption can vary so much that different absorbents achieve different absorption efficiencies. Furthermore, environmental considerations naturally play a significant role, as efficient compositions are not always particularly environmentally friendly.Another important aspect is that the absorption medium must be cost-effective and not impede further processing of the captured carbon dioxide. The latter is particularly important because energy- and cost-efficient isolation of the captured carbon dioxide, ideally with unrestricted reusability of the absorbent, significantly contributes to the energy balance and thus the attractiveness of the entire process. Therefore, the economic viability and environmental friendliness of the process depend not only on the absorption itself, but also on the subsequent processing steps, which should ultimately contribute to the provision of the purest possible carbon dioxide streams. These streams can then be converted in various further steps into valuable materials, such as hydrocarbons or alcohols, which can make an additional contribution to the overall environmental impact.

[0014] 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 patent literature.

[0015] For example, WO 2008 072 979 A1 discloses a process 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 comprises an inorganic alkaline carbonate, bicarbonate and at least one 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 absorption accelerator and catalyst, together with the remaining aqueous phase, is returned to the absorber.

[0016] In another patent document, WO 2020 152 330 A1, a method for separating and obtaining carbon dioxide from ambient air is disclosed. The method comprises the continuous execution of the following steps: a) bringing ambient air into contact with an aqueous solution of at least one alkali or alkaline earth metal cation for absorption of the carbon dioxide into the solution, forming hydrogen carbonate or...Carbonate of at least one metal; b) Electrodialysis of the solution obtained 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 in these ions, 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 CO₂-depleted solution, 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 carbon dioxide-water vapor mixture obtained by means of cooling to condense the water vapor and, if necessary, further drying of the carbon dioxide.

[0017] EP 3 384 973 A1 describes a process for recovering carbon dioxide to enrich the gas streams used in the production of sodium carbonate and sodium bicarbonate via the ammonia-soda process. The process comprises: contacting the process gas and / or exhaust gas streams occurring in the sodium carbonate and sodium bicarbonate production process via the ammonia-soda process in the CO₂ absorption column, comprising: - a partial or complete gas stream originating from lime burning in shaft lime kilns with air bursts, and / or - a partial or complete exhaust gas stream from the soda carbonation plant and / or a part or all of the exhaust gas stream from the baking soda carbonation plant, and optionally one or more streams of flue gases or other carbon dioxide-containing gases resulting from the combustion of solid, liquid or gaseous fuels to generate heat or electrical energy to meet the production requirements of the ammonia-soda process;and optionally one or more flue gas streams or other carbon dioxide-containing gases supplied by an external provider with a stream of aqueous absorbing solution to form a carbon dioxide-enriched aqueous absorbing solution, heating of a carbon dioxide-enriched aqueous absorbing solution in the evaporator, desorption of gaseous carbon dioxide with regeneration of an aqueous absorbing solution in a desorption column, cooling of the regenerated aqueous absorbing solution and return to the CO₂ absorption column, and removal from the desorption column and cooling of the stream with a high carbon dioxide content for use in the process for the production of sodium carbonate and sodium bicarbonate by the ammonia-soda process.

[0018] Such solutions known from the prior art can offer further potential for improvement, especially with regard to the efficiency of carbon dioxide uptake under variable environmental conditions and especially with regard to the possibility of easily and quickly desorbing once bound carbon dioxide from the absorption liquid with low energy expenditure.

[0019] It is therefore the object of the present invention to at least partially overcome the disadvantages known from the prior art. In particular, it is the object of the present invention to provide a method which is capable of desorbing carbon dioxide, once bound in an absorption liquid, in the purest possible form and as energy-efficiently and quickly as possible.

[0020] The problem is solved by the features of the independent claim, which relates to the inventive method for absorbing and releasing carbon dioxide. Preferred embodiments of the invention are described in the dependent claims, the description, or the figures, wherein further features described or shown in the dependent claims, the description, or the figures, individually or in any combination, may constitute subject matter of the invention unless the context clearly indicates otherwise.

[0021] According to the invention, a method for isolating carbon dioxide from an air stream is therefore a method comprising at least the following steps: a) Providing an aqueous solution of a carbon dioxide absorbent, wherein the carbon dioxide absorbent contains cations from the 1sta) Main group of the periodic table; b) Passing an air stream containing carbon dioxide through the solution provided in process step a), wherein at least a part 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 a middle chamber of an electrolysis cell comprising at least three chambers, an anode chamber, a cathode chamber, and at least one middle chamber arranged between the anode chamber and the cathode chamber, wherein the three-chamber electrolysis cell has a membrane selective for monovalent cations, and electrolyzing the aqueous solution with the release of at least a part of the carbon dioxide.

[0022] Surprisingly, it was found that the above-mentioned process can very efficiently absorb carbon dioxide from air streams and selectively release it from the absorbent via electrolysis. This overall process allows for the effective treatment of very different air streams with widely varying carbon dioxide concentrations. For example, carbon dioxide can be absorbed from normal ambient air with a relatively low carbon dioxide content, or from exhaust gas streams with a very high carbon dioxide content. The carbon dioxide-laden liquids can be processed particularly energy-efficiently via electrolysis, allowing the carbon dioxide to be desorbed from the liquid without significant energy input. A further advantage of this process is that the carbon dioxide can be obtained in pure form.For example, during electrolysis, it is possible to control the release of carbon dioxide from the liquid in such a way that this gas is produced in essentially pure form. In these cases, the carbon dioxide gas stream does not need to be further purified of other gas components. The process can be used to coordinate the two individual steps of carbon dioxide uptake and release so that the liquid is circulated for carbon dioxide uptake. This results in a flexibly adaptable overall process that can be operated continuously. Furthermore, the process according to the invention is significantly more advantageous than the electrolytic processing of aqueous KOH solutions containing carbon dioxide known from the prior art. The latter achieve efficiencies of only 70% to 80%. These efficiency losses result primarily from the overvoltage at the anode.If the aqueous electrolyte, consisting of a KOH solution, is replaced by an aqueous KHCO3-3 mixture, which is formed during CO2 absorption from the air, and the anode and cathode chambers are separated by a cation-selective membrane, CO2 is released at the anode in addition to O2. If the separated CO2 is to be used as a building block in a downstream process, this method is only partially suitable, as removing the oxygen from the carbon dioxide-oxygen gas mixture is technically very complex and energy-intensive. This problem can be circumvented by carrying out 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, thus eliminating the need for component separation.

[0023] The process according to the invention is a method for isolating carbon dioxide from an air stream. The process is thus capable of selectively removing carbon dioxide from an air stream and subsequently, in a further process step, making it available again in essentially pure form. The carbon dioxide is therefore separated from other gaseous components by absorption and desorption. Various gaseous mixtures are suitable as air streams for this purpose. For example, the air stream can be ambient air. However, it is also possible to use the exhaust gas stream from a combustion process. The different air streams can therefore differ in composition due to the other gaseous components. Furthermore, the process according to the invention is suitable for treating air streams with significantly different carbon dioxide concentrations.The airflow can, for example, contain carbon dioxide at a concentration greater than or equal to 100 ppm, more preferably at a concentration greater than or equal to 500 ppm, and more preferably at a concentration greater than or equal to 1000 ppm.

[0024] In process step a), an aqueous solution of a carbon dioxide absorbent is provided, wherein the carbon dioxide absorbent comprises cations from group 1 of the periodic table. The prerequisite for the absorption of carbon dioxide from the air stream is the presence of a suitable absorbent in an aqueous solution. The process according to the invention can be carried out with a variety of different absorbents. However, a key requirement is that the absorbent comprises cations from group 1. Therefore, the absorbent can, for example, contain potassium or sodium ions. The absorbent does not necessarily have to be in the form of a salt.It is also possible that an uncharged absorbent is present in solution, in which case, in addition to the actual absorbent, further components in the form of a salt-like compound with the corresponding cations from Group 1 are also present. The proportion of absorbent can be adjusted according to the separation task at hand and the carbon dioxide concentrations in the air stream. Thus, the proportion of absorbent can preferably be greater than or equal to 2.5 wt%, more preferably greater than or equal to 7.5 wt%, and further preferably greater than or equal to 15 wt% based on the aqueous solution. The cations from Group 1 are therefore 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%, and further preferably greater than or equal to 80 wt%.-% and preferably contain more than or equal to 85% by weight of water.

[0025] In principle, all CO₂ absorbents are suitable, provided they exhibit sufficient solubility in aqueous solution. 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 when the dissolved substance is in contact with ambient air. This group advantageously exhibits a virtually negligible vapor pressure and no potential health or environmental hazards. Suitable amines include primary, secondary, and tertiary amines with a molecular weight of 1000 g / mol or less.Polyethylene glycolamines, diaminopolyethylene glycols, and carboxylic acid derivatives of polyethylene glycolamines can be substances in which one (polyethylene glycolamine) or both (diaminopolyethylene glycol) OH groups of the PEG have been replaced by amine groups. The carboxylic acid derivatives are structured similarly, with one or both OH groups being replaced by carboxylic acid groups (-COOH). Suitable sugar derivatives include, for example, N-methyl-D-glucamine (meglumine) and N-ethyl-D-glucamine (eglumin). Both exhibit very good solubility and a negligible vapor pressure in solution.

[0026] Polyethyleneimines are also suitable as absorbents according to the following formula: The properties regarding viscosity, vapor pressure, and carbon dioxide uptake can be fine-tuned by adjusting the number m. Preferably, the molecular weight of the branched polyethyleneimines can be less than or equal to 800 g / mol.

[0027] In addition to the absorbent, the aqueous solution may 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. For example, the carbon dioxide absorbent may contain polyethylene glycols or polyols with a molecular weight of 1000 g / mol or less in a proportion of 2% or more by weight and 93% or less by weight. When exposed to ambient air, polyethylene glycol (PEG) or polyols with a specifically defined vapor pressure and no adverse environmental or health effects are suitable. Polyethylene glycols with the general formula C₂₆H₄₂O₆n₁₀ have proven particularly suitable. 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*10⁻⁸ Pa for n=8. PEGs are considered non-volatile for n > 4 and above. The polyethylene glycols also exhibit exceptionally low toxicity values ​​and are completely biodegradable for n < 20. The organic compounds are miscible with water in any proportion to adjust the water vapor partial pressure. Polyols according to the invention are aliphatic substances that contain at least two -OH groups. Preferably, the polyols contain at least 3, and more preferably 4, OH groups. A preferred representative of this group is, for example, glycerol. More preferably, the second component b) can be present in the carbon dioxide absorber in a weight fraction of greater than or equal to 30 wt% and less than or equal to 90 wt%, and more preferably in a weight fraction of greater than or equal to 35 wt% and less than or equal to 85 wt%.

[0028] In process step b), an air stream containing carbon dioxide is passed through the solution provided in process step a), whereby at least some of the carbon dioxide from the air stream binds to the carbon dioxide absorbent, and the air stream is thus depleted of carbon dioxide. The air stream is then brought into contact with the provided solution. This contact between the air stream and the liquid can be achieved in various ways. For example, the air stream can be atomized through the liquid in the form of dispersed droplets. Alternatively, the liquid can be provided as a flowing film, which then contacts the air stream. In all cases, it is advantageous to provide the largest possible exchange surface between the air stream and the liquid.Depending on the specific properties of the liquid, the temperature, the pressure, and the contact time, some of the carbon dioxide from the air stream will transfer into the liquid and be bound within it. The specific interactions between carbon dioxide and the absorbent can be covalent or physical in nature. 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 reducing the carbon dioxide content of the air stream. It is also possible, for example, to pass the air stream through the same liquid one or more times.

[0029] 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 a central chamber of an electrolysis cell consisting of at least three chambers: an anode chamber, a cathode chamber, and at least one intermediate chamber arranged between the anode and cathode chambers. The three-chamber electrolysis cell has a membrane selective for monovalent cations. The aqueous solution is then electrolyzed, releasing at least some of the carbon dioxide. After the absorption step, the solution containing the absorbent and the bound carbon dioxide is processed by electrolysis. In the electrolysis cell, the aqueous solution is subjected to an electrical voltage, resulting in a current flow within the electrolysis cell as a function of the voltage.In addition to the individual compartments separated by the membrane, the electrolysis cell also has two electrodes. These electrodes are usually referred to as the anode and cathode. The migration of cations through the cation-selective membrane leads to a pH difference between the middle chamber and the cathode chamber.

[0030] Lowering the pH in the intermediate chamber leads to the release of CO₂. The electrical cell voltages used to release the carbon dioxide can be, depending on the pH difference between the intermediate chamber and the cathode chamber, for example, greater than or equal to 1 V, more preferably greater than or equal to 1.5 V, and even more preferably greater than or equal to 1.75 V. In addition to the release of carbon dioxide, water can also be electrolyzed during the electrolysis process, producing hydrogen and oxygen at the electrodes.With this embodiment of the process, the electrolysis cell can be a three-chamber electrolysis cell consisting of an anode chamber, a cathode chamber, and at least one intermediate chamber arranged between the anode and cathode chambers. The anode chamber can be separated from the intermediate chamber, for example, by a bipolar membrane, and the cathode chamber by a membrane selective for monovalent cations. The aqueous solution from process step c) is fed into the intermediate chamber of the electrolysis. Regardless of the CO₂ loading of the air stream and largely independent of the composition of the absorbant, CO₂-loaded absorbents can be recycled particularly advantageously using a three-chamber electrolysis system. This can be described as particularly advantageous in the case of an absorbant made from a carbonate solution and optionally with additional absorption-accelerating promoters.The CO₂, intermediately bound as bicarbonate, can be released as a parallel reaction during the electrolytic production of hydrogen and oxygen. The resulting carbonate solution can then be used to absorb CO₂ from the air. The goal of separating CO₂ from the air is its use as a building block for synthesis, for example, in the production of hydrocarbons. In addition to CO₂ separation, the production of hydrogen is both necessary and desirable. In the future, this hydrogen will be produced primarily electrolytically as part of decarbonization efforts.

[0031] In a preferred embodiment of the process, the molar saturation of the absorbent with carbon dioxide at the beginning of process step c) can be greater than or equal to 90 mol% and less than or equal to 100 mol%, based on the concentration of the absorbent in the aqueous solution. To carry out a particularly energy-efficient process, it has proven advantageous for the absorbent to be saturated with carbon dioxide to a particularly high degree before the electrolytic treatment. In this embodiment, a particularly low electron load ensures that a large proportion of the carbon dioxide is released from the aqueous solution without further conversion losses. These high loadings can be achieved, for example, by long contact times between the air stream and the aqueous solution.Preferably, the molar saturation can be greater than or equal to 95 mol%, and even more preferably greater than or equal to 98 mol%. The molar saturation indicates the proportion of the absorbent that is saturated with carbon dioxide. This proportion is determined in comparison to the total amount of absorbent present.

[0032] 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 reacts according to the reaction equation below for sodium carbonate: Na₂CO₃(aq) + H₂O + CO₂(aq) → 2 NaHCO₃(s) ↓<

[0033] The solubility of this absorbent changes as a function of the carbon dioxide loading, so that at atmospheric CO₂ concentrations below or around 400 ppm, a continuous uptake of CO₂ and precipitation as bicarbonate or a bicarbonate-containing compound takes place. This effect can be enhanced, for example, by adding further absorption promoters. For instance, Na₂CO₃ and MEG (monoethylene glycol) / H₂O and PEG150 / H₂O can be used. Suitable combinations for K₂CO₃ are, for example, MEG / H₂O and PEG150 / H₂O. In these configurations, particularly large synergistic effects result. For K₂CO₃, the precipitation product in the carbon dioxide absorbent according to the invention is KHCO₃. In addition, the bath composition is also robust enough that the precipitation of sodium or potassium bicarbonate can be increased by adding similar 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 alkali carbonates or amino acid salts alone as absorbents, can also be advantageous.

[0034] In a preferred embodiment of the process, the carbon dioxide absorbing agent can comprise bicarbonate after CO₂ uptake. The uptake of carbon dioxide followed by the reaction of a bicarbonate solution offers significant advantages over the electrolysis of carbonate solutions. The stoichiometry of the CO₂ reaction during the electrolysis of a carbonate solution is known. The following relationship applies to carbonate solutions: K₂CO₃ + 2 H₂O → CO₂ + H₂ + ½ O₂ + 2 KOH

[0035] The release of one CO₂ molecule from a carbonate requires two electrons. The release of CO₂ from a hydrogen carbonate solution can be used as a parallel reaction in the electrolytic production of H₂ and O₂ and can be expressed by the following equation: 4 KHCO₃ → 2 CO₂ + H₂ + ½ O₂ + 2 K₂CO₃ + H₂O

[0036] A key advantage of using a hydrogen carbonate solution is that only one electron is required to release a CO₂ molecule. The simultaneous release of CO₂ from a hydrogen carbonate solution during electrolytic hydrogen and oxygen production is therefore extremely energy-efficient. From a thermodynamic perspective, the potential difference to be overcome results from the water splitting voltage of 1.23 V and the combined overpotentials at the anode and cathode of approximately 0.5 V. The potential difference required for CO₂ release also depends on the concentration ratios in the intermediate and cathode chambers. These ratios generate a potential difference of approximately 0.2 V across the cation-selective membrane. In addition to the electrochemically favorable stoichiometry, a three-chamber electrolyzer induces 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, ½ O₂, ½ H₂, and 2 CO₂ are obtained as nearly pure gaseous components in separate volumes of the three chambers. Therefore, a complex separation of the individual gas components is unnecessary. 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.

[0037] In a further preferred embodiment of the process, the carbon dioxide absorbent in process step a) can comprise potassium carbonate with a concentration greater than or equal to 200 g / L and less than or equal to 1200 g / L. These high concentrations of carbonates in the absorbent surprisingly enable the reliable and rapid absorption of carbon dioxide even from air streams with only low carbon dioxide concentrations. This could, for example, be 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 forming are kept close to their saturation concentration, so that they advantageously precipitate from the liquid even under the conditions in the absorber. This allows for rapid and almost complete absorption of carbon dioxide even at overall low carbon dioxide concentrations.Advantageously, the potassium carbonate concentration can be greater than or equal to 300 g / L and less than or equal to 1000 g / L, more preferably greater than or equal to 400 g / L and less than or equal to 900 g / L, and more preferably greater than or equal to 500 g / L and less than or equal to 850 g / L.

[0038] Within one embodiment, the carbon dioxide absorbent after CO2 uptake comprises hydrogen carbonate, and the carbon dioxide absorbent in process step a) comprises potassium carbonate with a concentration greater than or equal to 200 g / L and less than or equal to 1200 g / L.

[0039] In a further preferred aspect of the process, at least two chambers of the three-chamber electrolysis cell can be separated from each other by a bipolar membrane. In particular, the use of a bipolar membrane in a three-chamber electrolysis cell can contribute to a particularly energy-efficient process for removing carbon dioxide from the solution. Preferably, the bipolar membrane can separate at least one of the middle chambers from the anode chamber. In this configuration with a bipolar membrane, a membrane selective for monovalent cations can also separate one of the middle chambers from the cathode chamber.

[0040] In a further preferred embodiment of the process, in a second process step b'), the carbon dioxide absorbent with absorbed carbon dioxide can be precipitated and separated from the solution of process step b). In process step c), an aqueous solution of the precipitated carbon dioxide absorbent with absorbed carbon dioxide is fed in, and the mother solution, depleted of carbon dioxide absorbent with absorbed carbon dioxide, is at least partially directed into the cathode chamber of the electrolysis cell. To achieve particularly electrically efficient desorption of the carbon dioxide from the absorbent, it has proven advantageous that, in total, only absorbents that are also loaded with carbon dioxide are subjected to electrolysis. In the case of pure absorption of carbon dioxide on an absorbent, an equilibrium will be established as a function of the ambient conditions.Depending on the position of the equilibrium, the absorbent will be more or less loaded with carbon dioxide. If only loaded absorbents are desired for electrolysis, these carbon dioxide-loaded absorbents can be separated from the solution and then passed on to electrolysis in a new solution. In these cases, it is ensured that the electrolyzed solution contains no unloaded absorbents. This is particularly advantageous in cases where the unloaded absorbents are converted during electrolysis, consuming electrons. By providing loaded absorbents, this contribution is eliminated, and the desorption of carbon dioxide can occur with a very low electron loss through further conversion.This method is particularly suitable when using carbonate as an absorbent. The carbonates can be converted into hydrogen carbonates by absorbing carbon dioxide, and the hydrogen carbonates can then be precipitated from the aqueous solutions. This prevents carbonate reactions during electrolysis.

[0041] In a further preferred embodiment of the process, the pH value in the middle chamber can be greater than or equal to pH 7.8 and less than or equal to pH 8.8, the pH value in the cathode chamber greater than or equal to pH 11 and less than or equal to pH 13.5, and the difference in pH values ​​between the middle chamber and the cathode chamber during electrolysis can be greater than or equal to 2.2 and less than 5.7. Within these pH limits, the system can be operated thermodynamically advantageously. The required reaction equilibria are established rapidly, and these differences are also advantageous from an electrochemical perspective. These limits can be particularly useful when using hydrogen carbonates as absorbents.

[0042] In a further embodiment of the process, the concentration of carbon dioxide absorbent with absorbed carbon dioxide when fed into the electrolysis unit can be 2 mol / L or equal to 2 mol / L and less than or equal to 8 mol / L. Within these concentration limits, high quantities of carbon dioxide can be released with high activity. Furthermore, these concentration limits can be suitable for operating the cell build-up over extended periods.

[0043] According to a preferred characteristic of the process, the conductivity of the input current into a central part of the electrolysis in process step c) can be greater than or equal to 0.1 S / cm and less than or equal to 0.5 S / cm. These conductivities for the input current into one or more parts of the three-chamber electrolysis unit have proven particularly advantageous for the energy efficiency of the electrolysis. The conductivity of this aqueous input current loaded with carbon dioxide is determined under operating conditions, in particular pressure and temperature, using methods known to those skilled in the art. For example, it is possible to determine the conductivity of the aqueous input current using a conductive conductivity sensor. These conductivities can be particularly useful in connection with the electrolysis of aqueous bicarbonate solutions.This results in very high hydrogen carbonate concentrations, which preferentially maintain a high conductivity of the electrolyte in this chamber.

[0044] In a further preferred embodiment of the process, aqueous KOH solution can be used as the electrolyte in the anode chamber of the at least three-chamber electrolysis system. Potassium hydroxide (KOH) is preferably chosen as the electrolyte for the anode chamber because this electrolyte can exhibit a sufficiently high conductivity under the operating conditions of the electrolysis.

[0045] Within a preferred aspect of the process, the conductivity of the aqueous KOH solution present in the anode chamber can be greater than or equal to 0.5 S / cm and less than or equal to 1.3 S / cm under operating conditions. This conductivity range and the use of KOH as the electrolyte can contribute to a particularly electrically efficient conversion process with the release of carbon dioxide during electrolysis.

[0046] In a further preferred embodiment of the process, the conductivity of the electrolyte solution present in the cathode chamber at the start of electrolysis can be greater than or equal to 0.4 S / cm and less than or equal to 1.15 S / cm under operating conditions. To achieve these conductivities, a carbonate / bicarbonate solution with a bicarbonate concentration close to the solubility limit can, for example, be introduced into the cathode chamber. These high concentrations can improve the electrical efficiency of the overall process. Furthermore, solutions with these conductivities have a sufficient potassium carbonate content. Advantageously, the water vapor pressure of the absorbent can also be controlled via the potassium carbonate content in the solution.Depending on the ambient humidity, the water vapor pressure of the absorbent can be adjusted for a specific range so that neither water evaporates from the absorbent nor water from the atmosphere is absorbed into the absorbent. Furthermore, it is advantageous that in these conductivity ranges, the K+ ions that have migrated through the cation-selective membrane also form K2CO3 from the KHCO3 present in the KHCO3 / K2CO3 solution.

[0047] In a further embodiment of the process, after process step b), bicarbonate can be separated from the absorbent and, with the addition of at least water, fed as an aqueous solution into the at least three-chamber electrolysis cell. In this variant, the need to introduce at least part 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 three-chamber electrolysis process.

[0048] In a further preferred embodiment, the solution of the carbon dioxide absorbent 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 and less than or equal to 0.75 at 20°C and in a concentration of 1 mol / kg. The carbamate equilibrium constant is understood to be the following: K carb = RNH ∗ HCO 3 RNHCOO

[0049] The equilibrium constant K carb (in mol / L) is a quantity that describes the stability of carbamates formed in the absorption fluid during CO₂ absorption with the absorption accelerators. Low carbamate stability is indicated by a high value for KThe carbamate is expressed as carb. This means that the carbamate formed reacts back to hydrogen carbonate and the amine. The latter is particularly desirable in this case and leads to an improved carbon dioxide uptake rate. In particular, these amine additions can help to efficiently remove carbon dioxide from air streams with relatively low carbon dioxide concentrations. Suitable amines for accelerating uptake include amino acids, due to their very low vapor pressure. Pipecolic acid, for example, exhibits suitable carbamate stability. Preferably, amino acids with a carbamate equilibrium constant at 20°C of greater than or equal to 0.02 and less than or equal to 0.5, and even more preferably, those with a constant of greater than or equal to 0.03 and less than or equal to 0.25, can be used.

[0050] Furthermore, the invention relates to the use of the process according to the invention for the absorption of 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. The process according to the invention is particularly suitable for further removing carbon dioxide from air streams with very low carbon dioxide concentrations. In prior art baths, these conditions usually lead to an uncontrollable uptake or release of water from the solution, which requires close process monitoring and corresponding control effort. The process presented here enables reliable and continuous uptake of carbon dioxide from these low-carbon dioxide streams and efficient and energetically advantageous desorption via electrolysis.It is also possible to establish processes that efficiently absorb carbon dioxide from the ambient air and ensure an electrically suitable release within the framework of a 3-chamber electrolysis.

[0051] Furthermore, the respective 3-chamber electrolysis apparatuses also represent embodiments according to the invention.

[0052] Further advantages and advantageous embodiments of the invention are illustrated by the figures and explained in the following examples. It should be noted that the figures are for descriptive purposes only and are not intended to limit the invention in any way.

[0053] The show Fig. 1 a two-chamber electrolysis not according to the invention with recycling of the electrolyzed solution from the anode chamber to the cathode chamber; Fig. 2 a two-chamber electrolysis not according to the invention with recycling of the electrolyzed solution from the anode chamber to the cathode chamber and a functionalized Ni anode; Fig. 3 a three-chamber electrolysis not according to the invention with recycling of the electrolyzed solution from the middle chamber to the cathode chamber; Fig. 4 a five-chamber electrolysis not according to the invention with recycling of the electrolyzed solution from the middle chamber to the cathode chamber; Fig. 5 a three-chamber electrolysis according to the invention with a separate precipitation step of the CO₂-saturated absorbent; Fig. 6 a two-chamber electrolysis not according to the invention with separate release of the CO₂ in a further chamber; Fig.7. A two-chamber electrolysis unit not according to the invention, with a separate precipitation step of the CO₂-saturated absorbent and release of CO₂ from KHCO₃ by KHSO₄; Fig. 8. A two-chamber electrolysis unit not according to the invention, with a separate precipitation step of the CO₂-saturated absorbent; Fig. 9. The current profile and the carbon dioxide evolution of a hydrogen carbonate-based absorbent in a three-chamber electrolysis unit; Fig. 10. The current profile and the carbon dioxide evolution of an amino acid-based absorbent in a three-chamber electrolysis unit; Fig. 11. The solubility of KHCO₃ in K₂CO₃ and the CO₂ partial pressure at saturation with KHCO₃; Fig. 12. The mass transfer rates for the uptake of CO₂ into a KHCO₃ / K₂CO₃ solution at 323 K; Fig. 13 shows the current / voltage characteristic of the electrolysis of a 1 molal KOH solution; Fig.14The current / voltage characteristic of the electrolysis of a 1 molal KHCO3 solution in the anode compartment and a 0.5 molal K2CO3 solution in the cathode compartment compared to that in the . Fig.13 Figure 15 shows the electrolysis with a 1 molal KOH solution in the anode and cathode compartments; 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 K₂CO₃ solution, pH = 11.8, in the cathode chamber; Figure 16 shows the current / voltage characteristic of an electrolysis according to the invention in a three-chamber system.

[0054] The Figure 1Figure 1 shows a two-chamber electrolysis unit (not according to the invention) with recirculation of the electrolyzed solution from the anode chamber to the cathode chamber. This is a simple setup for an electrolysis unit for the electrolysis of H₂O and the release of CO₂ from a carbon dioxide absorption liquid. It is also shown that the electrolyzed solution is recirculated from the anode chamber to the cathode chamber. This electrolysis can be carried out, for example, with an absorbent consisting of a salt that decomposes into cations and anions in aqueous solution. The anode and cathode compartments are separated by a cation-selective membrane. During the electrolysis, the pH at the anode decreases while the pH at the cathode increases.Since the absorption of CO₂ into an aqueous solution is always associated with a decrease in pH for known CO₂ absorbents—bases, carbonates, amino acid salts, and amines (primary, secondary, tertiary)—CO₂ can always be driven off the solution by reducing the pH. It is particularly advantageous if the CO₂ absorbant contains the cation itself. Examples include the bases of all alkali metals, the carbonates of all alkali metals, and the amino acid salts of all alkali metals. For CO₂ 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 large number of known CO₂ absorbents.The energy required for CO₂ release, in addition to the electrolysis of H₂O, results from the pH difference between the middle chamber and the cathode chamber required for CO₂ expulsion and the resulting potential difference. This potential difference is substance-specific and can vary between 0.8 V and 0.25 V. Furthermore, the number of electrons required to release a CO₂ molecule is also substance-specific and depends on the achievable charging capacity, which corresponds to the CO₂ partial pressure in the absorber. The number of electrons required varies between 1 and 2. A general difference from known processes in this configuration is that the solution, depleted of CO₂ in the anode chamber, is passed into the cathode chamber before CO₂ reabsorption. Neutralization takes place there, which lowers the pH value in the cathode chamber.The resulting reduction in potential difference significantly reduces the energy required for CO₂ release. The advantages of this process lie particularly in its general applicability; its simple process engineering design, as it can be operated directly with a CO₂ absorber column; and the stacked design of the electrolysis cell achieved through the use of bipolar membranes.

[0055] The Figure 2Figure 1 shows a partial process step not according to the invention, in the form of a two-chamber electrolysis of a carbon dioxide-laden air stream with a nickel hydroxide anode. An absorbent containing an alkali carbonate component is loaded with carbon dioxide from an air stream. The carbonate is, at least partially, converted into hydrogen carbonate, which is introduced into the anode compartment of a two-chamber electrolysis unit. The electrolysis cell has an alkali-permeable membrane that separates the anode compartment from the cathode compartment. The anode is a porous anode capable of binding oxygen. Therefore, only the carbon dioxide produced leaves the anode compartment. Hydrogen is produced in the cathode compartment. Thus, the different gases are produced in different locations and do not need to be separated from each other in a complex process. The anode can then be thermally regenerated from time to time, releasing oxygen. The following reactions occur: Anode: 2 KHCO3 → 2 K+ + 2 HCO3- 2 HCO3- → 2 CO2 + 2 OH- 2 Ni(OH)2 + 2 OH- → 2 NiOOH + 2 H2O + 2 e- The overall reaction is: (KHCO3 + Ni(OH)2 → K+ + CO2 + NiOOH + e-) x 2 At the cathode, the following reactions take place: 4 H2O → 2 H3O+ + 2 OH- 2 H3O+ + 2 e- → H2 + 2 H2O 2 KHCO3 + 2 K+ + 2 OH- → 2 K 2 CO3 + 2 H2O Overall, the reaction in the cathode compartment is: 2 KHCO3 + 2 K+ + 2 e- → 2 K 2 CO 3 + H 2

[0056] The Figure 3Figure 1 shows a partial process step not according to the invention, in the form of a three-chamber electrolysis of a carbon dioxide-laden absorbent. A carbonate-based absorbent can be loaded with carbon dioxide from an air stream, for example, ambient air or industrial exhaust air. The absorption process results in at least a partial conversion of the carbonate to 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, more generally, alkali ions. Applying a voltage generates oxygen 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 carbon dioxide-depleted solution in the middle chamber now has a higher carbonate and a lower bicarbonate content. This recycled solution can be used again as an absorbene for a carbon dioxide-containing air stream. The following reactions occur at the different reaction sites, expressed as reaction equations: At the anode: 2 OH⁻ → H₂O + ½ O₂ + 2e⁻ At the bipolar membrane: 2 H₂O → 2 H⁺ + 2 OH⁻ In the intercellular space: 2 HCO₃⁻ + 2 H⁺ → 2 CO₂ + 2 H₂O The overall reaction in the intercellular space is: 2 KHCO₃⁻ + 2 H⁺ → 2 K⁺ + 2 CO₂ + 2 H₂O At the cathode, the following reactions take place: 4 H₂O → 2 H₃O⁺ + 2 OH⁻ 2 H₃O⁺ + 2 e⁻ → H₂ + 2 H₂O 2 KHCO₃⁻ + 2 K⁺ + 2 OH⁻ → 2 K₂CO₃ + 2 H₂O The overall reaction in the cathode space is: 2 KHCO₃⁻ + 2 K⁺ + 2 e -< → 2 K 2 CO 3 + H 2

[0057] The three-chamber structure can be extended as desired with respect to the central unit. Therefore, with only slightly modified electrochemical properties, 5-, 7-, 9- or, more generally, 3+2n-chamber structures can also be used with the absorbent according to the invention or, for example, with pure hydrogen carbonate or amino acid solutions.

[0058] The Figure 4 Figure 1 shows a non-inventive five-chamber electrolysis system with recirculation of the electrolyzed solution from the middle chamber to the cathode chamber. Due to the inventive design, several "middle" chambers can be used for the release of carbon dioxide. Accordingly, several bipolar membranes and cation-selective membranes are employed.

[0059] The Figure 5Figure 1 shows a three-chamber electrolysis process according to the invention with a separate precipitation step of the CO₂-saturated absorbent. After the absorption of the carbon dioxide, the carbon dioxide-saturated absorbent is precipitated together. The precipitated agent can then be added to the electrolysis in 100% pure form. This process ensures that 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 process.

[0060] The Figure 6Figure 1 shows a two-chamber electrolysis system not according to the invention, with separate CO₂ release in a further chamber. To separate the individual gas streams of the electrolysis, the release of carbon dioxide can, in principle, also take place in a compartment arranged outside the electrolysis cell. The release is effected by pressure and / or temperature changes. After the carbon dioxide release, the solution can be directed into the cathode chamber. A separate precipitation step can be carried out before the electrolysis. The solution enriched with KHCO₃ is fed into the anode chamber and the depleted solution into the cathode chamber. Figure 5 ).

[0061] The Figure 7Figure 1 shows a non-inventive two-chamber electrolysis with a separate precipitation step of the CO₂-saturated absorbent. 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: KHCO₃ + KHSO₄ → K₂SO₄ + H₂O + CO₂ ↑

[0062] CO2 is released.

[0063] 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 / K₂SO₄ solution enriched with K₂SO₄ is passed into the anode chamber, where, according to the overall reaction in the anode compartment: 2 K₂SO₄ + H₂O → 2 K⁺ + 2 KHSO₄ + 1 / 2 O₂ + 2e⁻, O₂ is released, forming KHSO₄.

[0064] The KHSO₄ / K₂SO₄ solution, now enriched with KHSO₄, is returned to the reaction vessel to release CO₂. H₂ is released in the cathode compartment, and simultaneously the KHCO₃ / K₂CO₃ solution is enriched with K₂CO₃ for further CO₂ absorption: 2 KHCO₃ + 2 K⁺ + 2e⁻ → 2 K₂CO₃ + H₂

[0065] In comparison to the electrolysis of KHCO3 ( Figure 14 For the electrolysis of KHSO₄, a voltage drop of approximately 0.2 V higher is observed across the cation-selective membrane due to the lower pH value. The process variant is shown in Figure 7 This is therefore an energy-efficient method for the release of CO2 and O2 in separate volumes.

[0066] The Figure 8 as an excerpt from Figure 4An expandable stack, separated by a bipolar membrane, is used. A separate precipitation step of the CO₂-saturated absorbent is also included upstream of the electrolysis. This separate precipitation and dissolution step can increase the electrical efficiency of the electrolysis, as only bicarbonate, and no carbonate, enters the electrolysis cell.

[0067] The Figure 9This figure shows the current profile and carbon dioxide evolution of a bicarbonate-based absorbent in a three-chamber electrolysis setup. The absorbent is based on a 10 wt% KHCO₃ solution, and the measured current and the determined CO₂ volumetric flow rate are plotted as a function of the measurement cycles. The data for measurement cycles from 10,000 to 11,000 are shown, with a time interval of 1 second per cycle. The measurements were performed at a temperature of 20°C. Electrolyte solutions with the following compositions 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 at the surface of the bipolar membrane. For a similar reason, the CO₂ gas flow rate also varies slightly. The stoichiometric ratio of the released gas quantities was approximately 2:1:1 / 2 for CO 2 , H 2 , O 2 .The Faraday efficiency achieved in the simple experimental setup with respect to CO2 was approximately 80%.

[0068] The Figure 10This figure shows the current profile and carbon dioxide evolution of an amino acid-based absorber in a three-chamber electrolyzer. The plot depicts the applied current and the resulting CO₂ gas flow rate during the electrolysis of a CO₂-loaded amino acid salt solution in the three-chamber electrolyzer. The measurement was also performed at a temperature of 20°C. The electrolyte solutions used had the following compositions: Anode: KOH, 5.4 wt%; Intermediate chamber: Amino acid salt solution loaded with proline, 10 wt%; Cathode: Amino acid salt solution loaded with proline, 10 wt%. The CO₂ was loaded prior to electrolysis in a bubble column reactor by passing a 14 vol% CO₂ gas stream through it. The measured fluctuating current flow is again due to bubble formation and detachment at the surface of the bipolar membrane. The same applies to the fluctuations in the CO₂ gas flow rate.The stoichiometric ratio of the released gas quantities was approximately 1:1:1 / 2 for CO₂, H₂, O₂. The Faraday efficiency achieved in this simple experimental setup, based on CO₂, was approximately 85%.

[0069] The Figure 11This graph shows the solubility of KHCO3 in K2CO3 and the CO2 partial pressure at saturation with KHCO3. The plotted line represents 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). The graph shows that with a K2CO3 concentration > 40 g / L, KHCO3 precipitates from the 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 above a K2CO3 concentration > 40 g / L. 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 / l.

[0070] Figure 12This figure 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 at a concentration of 2 mol / L as a CO₂ absorption promoter. 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 within a concentration range of 0 to 440 g / kg. The CO₂ uptake rate was thus determined 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, with an average value of 0.08 mol / sm2 < bar, is sufficient for large-scale industrial use of a packed column.

[0071] Figure 13 shows the current / voltage characteristic of the electrolysis of a 1 molal KOH solution using the in Figure 1 The setup is as outlined. 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 across the cation-selective membrane is observed.

[0072] The Figure 14 enables the direct comparison of the current / voltage characteristics measured in the laboratory for a 1 molal KOH in the anode and cathode compartments (see 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 for CO2 release, is due to the pH difference between the anode and cathode compartments, resulting from the loaded and unloaded absorbers. Depending on the concentration, this varies between pH 7.8 and 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 results in an additional voltage drop of theoretically Ψ = 0.22 V. This corresponds to a ΔG = 22 kJ per mol CO2. This is approximately the thermodynamic minimum.The electrolytic release of CO₂ from a KHCO₃ solution to form K₂CO₃ thus represents the most energetically efficient form of releasing absorbed CO₂. Laboratory measurements confirm the above theoretical considerations with a cell voltage of slightly over 2 V.

[0073] Figure 15 The figure 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. A direct comparison of the current / voltage curves with the previously discussed electrolysis of a KHCO₃ / K₂CO₃ solution confirms that the larger pH difference between the anode and cathode chambers results in a higher experimentally observed electrolysis voltage of approximately 0.2 V. The electrolysis of a K₂SO₄ solution corresponds to that described in the figure. Figure 7 presented procedure.

[0074] In de Figure 16The current / voltage characteristic of electrolysis in a three-chamber system corresponds to the 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, corresponding to a composition of: KHCO₃ 5 wt% + K₂CO₃ 5 wt% with a pH of 9.5. The figure can be compared to the Figure 13 It can be deduced 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 with increasing K₂CO₃ concentration. The CO₂ uptake rate decreases with increasing K₂CO₃ concentration and upon addition of KHCO₃ until the saturation limit is reached. With an average value of 0.08 mol / sm² < bar, the CO₂ mass transfer rate is sufficient for large-scale industrial use of a packed column.

[0075] Figure 13shows the current / voltage characteristic of the electrolysis of a 1 molal KOH solution using the in Figure 1 The setup is as outlined. 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 across the cation-selective membrane is observed.

[0076] The Figure 14 enables the direct comparison of the current / voltage characteristics measured in the laboratory for a 1 molal KOH in the anode and cathode compartments (see 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 for CO2 release, is due to the pH difference between the anode and cathode compartments, resulting from the loaded and unloaded absorbers. Depending on the concentration, this varies between pH 7.8 and 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 results in an additional voltage drop of theoretically Ψ = 0.22 V. This corresponds to a ΔG = 22 kJ per mol CO2. This is approximately the thermodynamic minimum.The electrolytic release of CO₂ from a KHCO₃ solution to form K₂CO₃ thus represents the most energetically efficient form of releasing absorbed CO₂. Laboratory measurements confirm the above theoretical considerations with a cell voltage of slightly over 2 V.

[0077] Figure 15 The figure 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. A direct comparison of the current / voltage curves with the previously discussed electrolysis of a KHCO₃ / K₂CO₃ solution confirms that the larger pH difference between the anode and cathode chambers results in a higher experimentally observed electrolysis voltage of approximately 0.2 V. The electrolysis of a K₂SO₄ solution corresponds to that described in the figure. Figure 7 presented procedure.

[0078] In the Figure 16The current / voltage characteristic of electrolysis in a three-chamber system corresponds to the 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, corresponding to a composition of: KHCO₃ 5 wt% + K₂CO₃ 5 wt% with a pH of 9.5. The figure can be compared to the Figure 13 It can be deduced 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.

Claims

1. A method for isolating carbon dioxide from an air stream, characterized in that the method comprises at least the steps of a) providing an aqueous solution of a carbon dioxide absorbent, the carbon dioxide absorbent comprising cations from the 1st main group of the periodic table; b) passing an air stream containing carbon dioxide through the solution provided in method step a), wherein at least some of the carbon dioxide from the air stream is bound to the carbon dioxide absorbent and the air stream is depleted in carbon dioxide, wherein in a second method substep b') the carbon dioxide absorbent with absorbed carbon dioxide is precipitated and separated from the solution of method step b); c) feeding an aqueous solution of the precipitated carbon dioxide absorbent with absorbed carbon dioxide into a middle chamber of an at least three-chamber electrolysis cell of an anode chamber, a cathode chamber, and at least one middle chamber arranged between the anode chamber and the cathode chamber, wherein the mother solution depleted in carbon dioxide absorbent with absorbed carbon dioxide is at least in part fed into the cathode chamber of the electrolysis cell, wherein the three-chamber electrolysis cell comprises a membrane selective for monovalent cations and electrolyzing the aqueous solution to release at least a portion of the carbon dioxide .

2. The method according to claim 1, wherein the molar saturation of the absorbent with carbon dioxide at the beginning of method step c) is greater than or equal to 90 mol% and less than or equal to 100 mol% based on the concentration of the absorbent in the aqueous solution.

3. The method according to any one of the preceding claims, wherein the carbon dioxide absorbent is selected from the group of alkali metal carbonates, alkali metal salts of amino acids or mixtures of at least two components from this group.

4. The method according to any one of the preceding claims, wherein the carbon dioxide absorbent comprises hydrogen carbonate after CO2 uptake.

5. The method according to claim 4, wherein the carbon dioxide absorbent in method step a) comprises potassium carbonate having a concentration of greater than or equal to 200 g / L and less than or equal to 1200 g / L.

6. The method according to any one of the preceding claims, wherein the concentration of carbon dioxide absorbent with absorbed carbon dioxide when fed into the electrolysis is greater than or equal to 2 mol / L and less than or equal to 8 mol / L.

7. The method according to any one of the preceding claims, wherein the conductivity of the input current to a middle part of the electrolysis in method step c) is greater than or equal to 0.1 S / cm and less than or equal to 0.5 S / cm.

8. The method according to any one of the preceding claims, wherein aqueous KOH solution is used as electrolyte in the anode chamber of the at least 3-chamber electrolysis.

9. The method according to claim 8, wherein the conductivity of the aqueous KOH solution present in the anode chamber is greater than or equal to 0.5 S / cm and less than or equal to 1.3 S / cm under operating conditions.

10. The method according to any one of the preceding claims, wherein the conductivity of the electrolyte solution present in the cathode chamber at the beginning of the electrolysis is greater than or equal to 0.4 S / cm and less than or equal to 1.15 S / cm under operating conditions.

11. The method according to any one of claims 4-10, wherein, after method step b), hydrogen carbonate is separated from the absorbent and added to the at least three-chamber electrolysis as an aqueous solution with at least the addition of water.

12. The method according to any one of the preceding claims, wherein the solution of the carbon dioxide absorbent in method step a) comprises a compound having an amine function as carbon dioxide absorption accelerator, wherein the compound having an amine function at 20°C and at a concentration of 1 mol / kg comprises a carbamate equilibrium constant, determined from carbon dioxide partial pressure measurements, of greater than or equal to 0.01 and less than or equal to 0.75.

13. Use of a method according to any one of the preceding claims 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.

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