Electrochemical process for recovering carbon

Abstract

The present invention relates to an electrochemical process for recovering carbon dioxide, comprising the following steps of: (a) electrochemically producing an acidic aqueous solution; (b) electrochemically producing a first alkaline aqueous solution comprising an alkali metal hydroxide; (c) reacting a gas containing carbon dioxide, wherein the gas is in particular air or a point source, with an alkaline absorbent suspension comprising the first alkaline aqueous solution with the alkali metal hydroxide and also solid alkali metal carbonate and / or solid alkali metal hydrogencarbonate, to obtain a product suspension containing a solid comprising additional solid alkali metal carbonate and / or solid alkali metal hydrogencarbonate, which is formed by the reaction between carbon dioxide and the alkali metal hydroxide, and a second alkaline aqueous solution comprising an alkali metal hydroxide, the concentration of which is reduced corresponding to the reaction compared to a concentration of alkali metal hydroxide in the first aqueous solution; (d) separating a first partial suspension from the product suspension, wherein the first partial suspension has a higher solids mass fraction based on the total mass of the first partial suspension than the solids mass fraction in the product suspension based on the total mass of the product suspension; (e) reacting the first partial suspension with the acidic aqueous solution in order to obtain dissolved and gaseous carbon dioxide gas; wherein the alkali metal hydroxide in particular comprises sodium hydroxide and / or potassium hydroxide.

Description

[0001] Electrochemical process for carbon extraction

[0002] TECHNICAL AREA OF INVENTION

[0003] The present invention relates to an electrochemical process for carbon dioxide production, in particular from air or point sources, and to a plant for electrochemical carbon dioxide production, in particular from air or point sources.

[0004] DESCRIPTION

[0005] background

[0006] The rise in greenhouse gas emissions has led to a global temperature increase, causing climate change and its devastating effects. Carbon dioxide (CO2) is the most important greenhouse gas contributing to this phenomenon. Reducing CO2 emissions is crucial to limiting global warming; however, this may not be enough to meet the Paris climate goals. Direct air capture (DAC) is a promising contribution to achieving these goals.

[0007] "CO2 capture," which includes DAC, is a process in which CO2 is removed from the atmosphere using various technologies. Electrochemical DAC typically comprises a setup including an electrochemical cell (e.g., an electrolysis cell) that generates an acidic solution (e.g., by oxidation of hydrogen; see equation (1) below) and an alkaline solution (e.g., by reduction of water; see equation (2) below) containing hydroxide ions; an absorber in which carbon dioxide from the air reacts with the hydroxide ions to form a solid containing carbonate; and a mixing device in which the solid reacts with the acidic solution to form dissolved carbon dioxide gas (e.g., equation (3)). The dissolved carbon dioxide gas can be extracted from the solution in gaseous form using a carbon dioxide extraction device.

[0008] H2-> 2 H + + 2 e~ E° = OK (1 )

[0009] H2O + 2 Na + + 2e~ -> 2 NaOH + H2,E° = -0.83 V @ pH = 14 (2)

[0010] CO3 2 “+2 H T = HC03+ H + H2CO3CO2+ H2O (3)

[0011] There is particular room for improvement in absorption. For example, the hydroxide concentration in the cathode must currently be at least as high as that present in the absorption step. This necessitates a reduction in the volumetric flow rate, which increases the percentage of hydrogen gas (according to equation (2)). Furthermore, electrochemical DAC suffers from efficiency disadvantages due to absorption: Remaining electrochemically generated hydroxide ions, i.e., those not converted in absorption, are neutralized by protons, which are also generated in the electrochemical cell. Therefore, especially with high absorption power (i.e., when using a high hydroxide ion concentration), a high hydroxide ion loss is to be expected, as a large number of hydroxide ions remain in solution and are subsequently neutralized by the acidic solution.

[0012] Accordingly, the present invention aims to address the above disadvantages and achieve corresponding improvements.

[0013] Summary of the invention

[0014] The above problem is solved by the invention as specified in the independent claims.

[0015] According to a first aspect, the present invention relates to an electrochemical process for carbon dioxide production comprising the following steps:

[0016] (a) electrochemical generation of an acidic aqueous solution;

[0017] (b) electrochemical generation of a first alkaline aqueous solution comprising an alkali metal hydroxide;

[0018] (c) Reacting a gas containing carbon dioxide, wherein the gas is in particular air or a point source, with an alkaline absorbent suspension comprising the first alkaline aqueous solution containing the alkali metal hydroxide, as well as solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate, yielding a product suspension comprising a solid comprising additional solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate formed by the reaction between carbon dioxide and the alkali metal hydroxide, as well as a second alkaline aqueous solution comprising an alkali metal hydroxide, the concentration of which is reduced according to the reaction compared to a concentration of alkali metal hydroxide in the first aqueous solution;

[0019] (d) Separating a first partial suspension from the product suspension, wherein the first partial suspension has a higher solids mass fraction relative to the total mass of the first partial suspension than the solids mass fraction in the product suspension relative to the total mass of the product suspension;

[0020] (e) Reacting the first partial suspension with the acidic aqueous solution to obtain dissolved and gaseous carbon dioxide gas; wherein the alkali metal hydroxide comprises in particular sodium hydroxide and / or potassium hydroxide.

[0021] According to a second aspect, the present invention relates to an apparatus for the electrochemical production of carbon dioxide from a gas, in particular air or a point source, wherein the apparatus comprises: (a) an electrochemical cell or an arrangement of several electrochemical cells, wherein the electrochemical cell or the arrangement of several electrochemical cells is configured to produce an acidic aqueous solution and a first alkaline aqueous solution, wherein the first alkaline aqueous solution comprises an alkali metal hydroxide;

[0022] (b) an absorption device, wherein the absorption device is designed to react a gas containing carbon dioxide, wherein the gas is in particular air or a point source, with an alkaline absorbent suspension comprising solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate and the first alkaline aqueous solution with the alkali metal hydroxide, thereby obtaining a product suspension comprising a solid comprising additional solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate formed by the reaction between carbon dioxide and the alkali metal hydroxide, and a second alkaline aqueous solution comprising an alkali metal hydroxide, the concentration of which is reduced according to the reaction compared to a concentration in the first aqueous solution, and wherein the absorption device is fluidically connected to the electrochemical cell or electrochemical cell arrangement;

[0023] (c) a separation device, wherein the separation device is designed to separate a first partial suspension from the product suspension, wherein the first partial suspension has a higher solids mass fraction based on the total mass of the first partial suspension than the solids mass fraction in the product suspension based on the total mass of the product suspension, and wherein the separation device is fluidically connected to the absorption device; wherein the apparatus is designed to react the first partial suspension with the acidic aqueous solution to obtain dissolved and gaseous carbon dioxide gas.

[0024] Technical effects of the invention

[0025] Some effects of the present invention are listed here, without being exhaustive or limiting.

[0026] In particular, it was surprisingly found that by using an alkaline absorbent suspension with dissolved alkali metal hydroxide, saturated with alkali metal carbonate and / or alkali metal bicarbonate, such that the alkali metal bicarbonate and / or the alkali metal carbonate are also present in solid form in the absorbent suspension, and reacting this absorbent suspension with carbon dioxide to obtain a product suspension with further solid alkali metal carbonate and / or solid alkali metal bicarbonate formed as a result of the reaction with carbon dioxide, and finally separating a first partial suspension which has a higher solid mass fraction of the solid alkali metal carbonate and / or the solid alkali metal hydroxide, based on the total mass of the partial suspension, than the product suspension, based on the total mass of the product suspension,and the implementation of this first partial suspension with an acidic aqueous solution, efficiency advantages are generated.

[0027] This is largely due to the fact that predominantly the solid alkali metal carbonate and / or the solid alkali metal bicarbonate are removed and converted from the absorption process – but not hydroxide ions, thus reducing efficiency-reducing neutralization with the acidic solution. A further partial suspension obtained from the separation, containing a second alkaline aqueous solution (whose hydroxide concentration is reduced according to the reaction between hydroxide and carbon dioxide), can be recycled back into the electrolysis process, for example, to enrich the cathode, thereby significantly reducing neutralization with protons. This increases the overall efficiency of the process. Furthermore, a high absorption capacity (i.e.,A high hydroxide concentration in the first alkaline solution can be achieved independently of loss minimization (which in known methods requires a low hydroxide concentration). In other words, despite a high hydroxide concentration in the first alkaline solution (e.g., pH > 13), a reduction in hydroxide losses can be achieved. Furthermore, this reduces the operating requirements of the electrochemical cell by eliminating the need for a minimum concentration within the cathode.

[0028] Furthermore, the freezing point and vapor pressure can be lowered by higher salt contents, thus enabling a wider temperature range with less water loss.

[0029] Further details regarding the technical effects achieved by the invention can be found in the detailed description and examples.

[0030] Definitions

[0031] Unless otherwise stated, all technical terms used herein correspond to common professional understanding.

[0032] The term "gas containing carbon dioxide" is to be understood broadly in this context. In particular, such a gas can include air. A person skilled in the art knows the composition of the air at the respective location or knows methods for measuring this composition. Furthermore, such a gas containing carbon dioxide can also be a point source. A point source is generally an industrial source of CO2, in which, due to the process, more carbon dioxide gas is produced than is normally present in the air. Examples include flue gases from industrial processes, such as those produced in cement production and / or coal combustion. A point source can contain carbon dioxide with a carbon dioxide content of approximately 600 ppm to approximately 25 mol%, for example, approximately 1000 ppm to approximately 12000 ppm, or approximately 10 mol% to approximately 25 mol%, in particular approximately 14 mol% to approximately 21 mol%, based on a total quantity of the moist gas.Such a gas can also have a carbon dioxide content of approximately 10 vol.% to approximately 20 vol.%. Overall, the mass fraction / mass fraction / molar fraction of carbon dioxide in the gas containing carbon dioxide is fundamentally uncritical for the functioning of the present invention and its aspects.

[0033] The term "carbon dioxide recovery" is to be understood broadly and is understood here as the entire process of removing carbon dioxide in its gaseous form from a carbonate-containing aqueous solution or a gas mixture, such as air or a point source, formally using H +The carbonate / hydrogen carbonate cations are converted into dihydrogen carbonate, which decomposes into water and carbon dioxide gas, as shown, for example, in equation (3) above, based on an aqueous solution. In the case of removing carbon dioxide from a gas mixture, the carbon dioxide is first converted into a suspension containing a solid comprising alkali metal carbonate. In the present case, the carbon dioxide recovery process preferably involves electrolysis, as described below.

[0034] The term "acidic" is to be understood broadly, with an "acidic aqueous solution" having a pH value of < 7, in particular from about -1 to about 5. Similarly, "pH neutral" means a pH value of 7.

[0035] The terms "alkaline" or "basic" are to be understood equally broadly, whereby an "alkaline aqueous solution" has a pH value of > 7.

[0036] The concentrations of selected salts in aqueous solutions can be determined by titration. For this purpose, the “916 Ti-Touch” instrument from Deutsche METROHM GmbH & Co. KG is used. The titrants used were obtained with high precision from Carl Roth GmbH + Co. KG and comprise aqueous standard solutions of KOH and HCl at concentrations of 1 mol / L, 0.1 mol / L, and 0.01 mol / L, respectively. Thus, the concentrations of acids and bases, among other substances, can be determined precisely at any concentration.

[0037] The term "acidic oxidation product", or subsequently equivalently "proton", i.e. H + The term "in aqueous solution" is to be understood broadly and, without being bound to a specific theory, relates to the formal proton product of the oxidation of diatomic hydrogen, especially hydrogen gas, where this formal proton product, for example, solvated in aqueous solution as HsO₂+ (aq) can be present, where Cl-( aqThe proton product can formally exist as a counter-anion and / or as part of a solid electrode and / or membrane material, through which the formal proton product is transported via known mechanisms. In addition to chloride, other counter-anions derived from the acids H₂SO₄, HCO₃⁻, H₂CO₃, HNO₃, H₃PO₄, HClIO₄, H₃BO₃, and HBr can also be formally present in all the aforementioned cases. A proton can also be generated via an electrodialysis cell with a bipolar membrane, as described below. The term "carbonate-containing aqueous solution," or also "dissolved alkali metal hydrogen carbonate and / or dissolved alkali metal carbonate," is to be understood broadly here and includes alkali metal carbonate in any chemical, especially inorganic, form. Such a solution or a suspension containing such a solution is typically alkaline and has a pH value of >7 to 14, e.g., >7 to 9.This means that the aqueous solution contains dissolved carbonate CC>3. 2 '(aq) and / or hydrogen carbonate HCO₃⁻' (aq). Furthermore, the carbonate-containing aqueous solution may contain monovalent cations, such as sodium. A carbonate-containing aqueous solution may also contain, in particular, exclusively or non-exclusively monovalent cations, such as sodium or potassium. These monovalent cations may, for example, include counter-anions selected from the group consisting of chloride, sulfate, perchlorate, nitrate, phosphate, monohydrogen phosphate, dihydrogen phosphate, or combinations thereof. In particular, dissolved salts may be selected from the group consisting of NaCl, KCl, Na₂SO₄, K₂SO₄, K₃PO₄, Na₃PO₄, K₂HPO₄, Na₂HPO₄, KH₂PO₄, NaCIO₄, KCIO₄, NaNO₃, KNO₃. The term "alkaline carbonate-containing aqueous solution" is used equivalently to "carbonate-containing aqueous solution".

[0038] The term "acidic components" usually refers to protons in their aqueous form, i.e., H3O. + ( aq ).

[0039] The term "fluidically connected" is to be understood broadly in this context and refers in particular to a connection, such as a pipeline, between two components of a system, where the pipeline is designed to transport a fluid, e.g., a liquid or a gas, such as hydrogen gas, from a first to a second system component. Such a system component may include, for example, an electrochemical cell or an absorption device. Furthermore, "fluidically connected" can mean that a mixing device may be present, for example, in addition to a pipeline or pipelines. The term "fluidically connected" also includes the possibility that pipelines may be branched and / or diverted.

[0040] In particular, the process according to the first aspect of the invention and the plant according to the second aspect of the invention are each operated continuously. Therefore, any solution mentioned in this context refers specifically to a mass flow rate of the respective solution. For example, the acidic aqueous solution in continuous operation can be understood as a mass flow rate of an input of an "acidic aqueous solution". Similarly, mass percentages in this context can refer to the respective mass flow rates of the solution.

[0041] The terms "electrochemical cell" and "electrochemical process" are to be understood broadly in this context. This includes all electrochemical cells and processes capable of generating an acidic component, and thus an acidic aqueous solution, as well as an alkaline aqueous solution. Examples include electrolysis cells with three compartments and corresponding electrolysis processes, such as those described below and, for example, in DE 10 2022 122 837 B3 or DE 10 2023 109 225 B3. Such an electrochemical cell can also include an electrodialysis cell with a bipolar membrane (BPM). Furthermore, an electrochemical cell can comprise an electrolyzer or an electrolysis cell, as described in WO 2023 / 044474 A1, WO 2023 / 166188 A2, and WO 2022 / 184840 A1. An electrochemical cell as described in AP is also relevant in this context. MUROY AMA and L.GUBLER: Carbonate Regeneration Using a Membrane Electrochemical Cell for Efficient CO2 Capture, ACS Sustainable Chem. Eng, 10, 2022, 49, 16113-16117; or in R. SHARIFAN, RM WAGTERVELD,.

[0042] IA DIGDAYA, C. XIANG, and DA VERMAAS: Electrochemical carbon dioxide capture to close the carbon cycle, Energy Environ Sci., 14, 2021, 781 -814. - ISSN: 1754-5706; or DIGDAYA, I. SULLIVAN, M. LIN, L. HAN, W.-H. CHENG, HA ATWATER, and C. XIANG: A direct coupled electrochemical system for capture and conversion of CO2 from oceanwater, Nature Communications,

[0043] II , 2020, 4412, 1-11 - ISSN: 2041-1723 is disclosed. Combinations of the aforementioned electrochemical cells are also possible. The aforementioned electrochemical cells can also be functionally connected to each other for scaling in the form of stacking, thereby obtaining an arrangement of electrochemical cells.

[0044] The terms "electrolysis cell" (or electrolyzer) and "electrolysis or electrolytic process" are to be understood broadly in this context. In particular, both terms encompass any electrochemical reaction, especially oxidation at the anode and reduction at the cathode, which is forced using a voltage source.

[0045] The term "air" is to be understood broadly and describes air present in the Earth's atmosphere, which is essentially the same in its components; however, it can vary depending on location. For the present invention, it is only critical that this air contains carbon dioxide.

[0046] The term "overpressure" is broadly defined and refers to the difference between the pressure inside the electrolysis cell and the atmospheric pressure at the respective location. To generate overpressure in the electrolysis cell, particularly in the space between the cells, an acid-resistant backpressure regulator can be used. For example, a backpressure regulator from Equilibar is used, which is made entirely of PTFE (Teflon®) and features a diaphragm pneumatically controlled by a gas pressure regulator. Its operation is described here: https: / / www.equilibar.com / back-pressure-regulators / how-it-works / (accessed on March 31, 2023). The pressure inside the electrolysis cell can be measured using conventional pressure measuring devices known to those skilled in the art. The same applies to atmospheric pressure.

[0047] The term "diffusion medium" is broadly defined and refers to a (micro)porous material that facilitates the distribution of liquids and gases. These materials utilize a capillary effect. The term "hydrophobic diffusion medium" is also broadly defined and refers to a material in which capillary depression (capillary descent) occurs when a liquid, such as water or an aqueous solution in neutral, acidic, or basic form, does not wet the pores of the material. Conversely, the term "hydrophilic diffusion medium," which is also broadly defined, refers to a material in which capillary ascent (capillary rise) occurs, where the liquid, such as water or an aqueous solution in neutral, acidic, or basic form, wets the pores of the material.

[0048] Unless otherwise described, electrical voltages can be measured using a potentiostat, e.g., Zahner Zennium Pro, via a measurement method of electrical coupling of the electrodes to the potentiostat.

[0049] The term "width of the gap" is to be understood broadly in this context. In particular, the width of the gap is the distance between the first and second transport membranes when they are arranged parallel to each other.

[0050] In the present text, the terms "aspect of the present invention" and "invention aspect" are used equivalently.

[0051] The term "dissolved alkali metal hydrogen carbonate and / or dissolved alkali metal carbonate" is to be understood broadly and refers in particular to the aforementioned species in solution, whereby it is known that an equilibrium exists between carbonate and hydrogen carbonate depending on external conditions, such as the pH value.

[0052] The term "solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate" or "solid alkali metal carbonate and / or alkali metal hydrogen carbonate" is to be understood broadly and does not, of course, exclude the possibility that the solid may contain other species (besides carbonate / hydrogen carbonate), and that alkali metal carbonate and alkali metal hydrogen carbonate may coexist within the solid in a chemical equilibrium or as a mixture or alternatively in their pure forms, whereby in the alkaline range (e.g. pH > 13) alkali metal carbonate is primarily present.

[0053] The term "alkali metal hydroxide," when stated as being contained in an alkaline aqueous solution, refers to the monovalent cation of the alkali metal and its hydroxide in dissolved form. Such an alkali metal hydroxide, as described herein, may be selected from the group consisting of hydroxides of lithium, sodium, potassium, cesium, rubidium, or combinations thereof, wherein the alkali metal is preferably sodium and / or potassium. The aforementioned cations may also be considered when alkali metals in solution are mentioned as cations without the term hydroxide being used.

[0054] In this context, "suspension" is understood to mean a solid dispersed in a liquid, particularly water. The term "suspension" is to be interpreted broadly here and also includes the possibility that the liquid is a solution for non-solid, i.e., dissolved, substances. Typically, and in the case of solid alkali metal carbonate and / or solid alkali metal bicarbonate as the solid, a saturated solution of dissolved alkali metal bicarbonate and / or alkali metal carbonate is present alongside the solid, since, as is known to those skilled in the art, the solid would otherwise dissolve. Thus, any suspension can also be understood as a saturated solution of the inorganic solid that is dispersed as a solid.Of course, in addition to the solid components in the form of alkali metal carbonate and / or alkali metal bicarbonate and the dissolved alkali metal bicarbonate and / or dissolved alkali metal carbonate, other dissolved components may be present in the suspension, such as dissolved alkali metal hydroxide. It is also possible that the suspension contains other solid components that are not additionally dissolved, such as activated carbon.

[0055] The term "alkaline absorbent suspension comprising a first alkaline aqueous solution" includes an absorbent in the form of dissolved alkali metal hydroxide having an alkaline pH as defined herein. Specifically, the alkaline absorbent suspension may comprise only the first alkaline aqueous solution, along with solid alkali metal carbonate and / or solid alkali metal bicarbonate and other possible solids. Alternatively, the alkaline absorbent suspension may comprise a combination of the first alkaline solution and the second alkaline aqueous solution. Furthermore, as previously described, the alkaline absorbent suspension may comprise the first alkaline aqueous solution, the second alkaline aqueous solution, and solid and dissolved alkali metal carbonate and / or bicarbonate.

[0056] The carbon dioxide gas obtained by reacting the acidic aqueous solution with the first partial suspension containing solid and / or dissolved alkali metal bicarbonate and / or solid and / or dissolved alkali metal carbonate is typically obtained in an aqueous solution, thereby yielding – as is known to those skilled in the art – typically dissolved and gaseous carbon dioxide gas in equilibrium. This is encompassed by the term "dissolved and gaseous carbon dioxide gas".

[0057] A suspension containing both solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate as well as dissolved alkali metal carbonate and / or dissolved alkali metal carbonate is also referred to here as a "carbonate solution at the saturation limit", since the solution is already saturated by the aforementioned carbonate salts and these salts are also present as solids, thus constituting a suspension.

[0058] Whenever "alkali metal carbonate and / or alkali metal hydrogen carbonate" is mentioned, the alternatives "alkali metal carbonate only", "alkali metal hydrogen carbonate only", or "alkali metal carbonate and alkali metal hydrogen carbonate" are considered disclosed. Preferably, due to the aforementioned equilibrium, alkali metal carbonate and alkali metal hydrogen carbonate are the case.

[0059] The term "mixing device" encompasses a technical device or assembly used to homogeneously blend or mix two or more substances (e.g., liquids, gases, solids, or powders). Such a mixing device could, for example, be a continuous stirred tank reactor. The term "solids mass fraction" refers to the percentage of the mass of solid components in a mixture—especially in suspensions—relative to the total mass of the mixture. The term "solid content" refers to the proportion of solid components in a mixture of substances (e.g., suspension, emulsion, sludge, or solution).

[0060] The term "separation device" refers to a technical device used to remove or separate specific components from a mixture. A separation device can include, for example, a sedimentation device such as a sedimentation hopper, a decanter, or a hydrocyclone.

[0061] Brief description of the characters

[0062] Figure 1 shows a first embodiment of the system according to the invention. This system according to the invention is designed to carry out the method according to the invention.

[0063] Figure 2 shows a second embodiment of the system according to the invention. This system according to the invention is designed to carry out the method according to the invention.

[0064] Detailed description

[0065] The embodiments shown below represent advantageous configurations of the present invention, which are in no way to be understood as limiting the present invention. The embodiments listed within the various aspects of the invention can be freely combined with one another, unless otherwise stated.

[0066] Electrochemical process

[0067] According to a first aspect, the present invention relates to an electrochemical process for carbon dioxide production comprising the following steps:

[0068] (a) electrochemical generation of an acidic aqueous solution;

[0069] (b) electrochemical generation of a first alkaline aqueous solution comprising an alkali metal hydroxide;

[0070] (c) Reacting a gas containing carbon dioxide, wherein the gas is in particular air or a point source, with an alkaline absorbent suspension comprising the first alkaline aqueous solution containing the alkali metal hydroxide, as well as solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate, yielding a product suspension comprising a solid comprising additional solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate formed by the reaction between carbon dioxide and the alkali metal hydroxide, as well as a second alkaline aqueous solution comprising an alkali metal hydroxide, the concentration of which is reduced according to the reaction compared to a concentration of alkali metal hydroxide in the first aqueous solution;(d) Separating a first partial suspension from the product suspension, wherein the first partial suspension has a higher solids mass fraction relative to the total mass of the first partial suspension than the solids mass fraction in the product suspension relative to the total mass of the product suspension;

[0071] (e) Reacting the first partial suspension with the acidic aqueous solution to obtain dissolved and gaseous carbon dioxide gas; wherein the alkali metal hydroxide comprises in particular sodium hydroxide and / or potassium hydroxide.

[0072] The technical effects of the method according to the first aspect of the present invention are explained in the above paragraph "Technical Effects of the Invention".

[0073] Steps (a) and (b) can be carried out within an electrochemical cell or an arrangement of such electrochemical cells. The type of electrochemical cell is not critical, as long as it is suitable for generating an acidic solution and an alkaline aqueous solution. For example, an electrolysis cell, in particular comprising an anode compartment, an intermediate compartment, and a cathode compartment, wherein the intermediate compartment is arranged between the cathode compartment and the anode compartment, can be used. The electrochemical cell can also comprise an electrodialysis cell with at least one bipolar membrane. Such a bipolar membrane can be fused within an electrochemical cell under the influence of an electric field and in the presence of an aqueous salt solution, e.g., an alkali metal salt solution, water in hydroxide, and formally H₂. + ( aqThe electrodialysis cell splits the solution to obtain a first alkaline solution and an acidic aqueous solution. Specifically, such an electrodialysis cell can have an anode and a cathode, with a first bipolar membrane, a cation exchange membrane, and a second bipolar membrane arranged between the cathode and the anode in that order. Of course, the electrodialysis cells can also be arranged as a stacked array.

[0074] Preferably, the electrochemical cell comprises said electrolysis cell with an anode compartment, intermediate compartment, and cathode compartment, as described in more detail below and as shown, for example, in DE 10 2022 122 837 B3 or DE 10 2023 109225 B3. Furthermore, an electrochemical cell can comprise an electrolyzer or an electrolysis cell as described in WO 2023 / 044474 A1, WO 2023 / 166188 A2, and WO 2022 / 184840 A1, respectively. In addition, an electrochemical cell as described in AP MUROYAMA and L. GUBLE, "Carbonate Regeneration Using a Membrane Electrochemical Cell for Efficient CO2 Capture" (ACS Sustainable Chem. Eng., 10, 2022, 49, 16113-16117), is also suitable. - ISSN: 2168-0485; or in R. SHARIFAN, RM WAGTERVELD, IA DIGDAYA, C. XIANG, and DA VERMAAS: Electrochemical carbon dioxide capture to close the carbon cycle, Energy Environ Sci., 14, 2021, 781-814. - ISSN: 1754-5706; or in DIGDAYA, I. SULLIVAN, M. LIN, L. HAN, W.-H. CHENG, HAATWATER, and C. XIANG: A direct coupled electrochemical system for capture and conversion of CO2 from oceanwater, Nature Communications, 11, 2020, 4412, 1-11. - ISSN: 35 2041-1723, 2020. Furthermore, steps (a) and (b) can be carried out in the same or in different electrochemical cells, as long as the acidic aqueous solution and the initial alkaline aqueous solution are available for the process.

[0075] Steps (a) and (b) are subsequently described using the example of an electrolysis cell. First, anodic oxidation of hydrogen gas takes place in step (a). This can be done in an aqueous solution or via a gas diffusion electrode, e.g., a zero-gap electrode. The acidic oxidation product formally corresponds to H₂. + ( aq), i.e., a proton, in equation (1) above. The pH of the acidic aqueous solution may be about < 7. Preferably, the pH is about 3 to about 7. Alternatively, the pH may be about < 3, in particular about 0.

[0076] H + (aq>, i.e., a proton, can exist, for example, in aqueous solution as HsO+faq) or as part of a solid electrode and / or membrane material. The actual form of the acidic oxidation product H + ( aq The hydrogen gas is not critical as long as it is available for the following steps. In certain embodiments, hydrogen gas can be oxidized in step (a) at a diffusion medium in the form of a gas diffusion electrode without using an aqueous solution, or at most, minimal humidification of the hydrogen gas. Oxidation of the hydrogen gas allows for lower energy consumption within the process, as oxygen generation is not necessary.

[0077] In step (b), in the case of electrolysis, an alkali metal hydroxide solution, for example a sodium hydroxide solution, can be produced cathodically according to equation (2) above. However, all other alkali metal hydroxides as described herein, e.g. potassium hydroxide, are also suitable.

[0078] In certain embodiments, in the case of electrolysis, the hydrogen gas produced cathodically in step (b) is transferred from step (b) to step (a) and oxidized. In other words, a hydrogen cycle can take place within the process (oxidation in step (a), reduction in step (b), re-oxidation in step (a)). Thus, in this specific embodiment of an electrolytic process, the process according to the invention can generate hydrogen largely autonomously, and the need for an external hydrogen supply is reduced. Since hydrogen production is typically associated with high energy consumption, this hydrogen cycle represents significant savings.

[0079] In certain embodiments, in the case of electrolysis and the use of an electrolysis cell described above, comprising the anode compartment, intermediate space, and cathode compartment, the acidic oxidation product from step (a) is transported through a first transport membrane to generate the acidic aqueous solution. This transport membrane is in contact with the aqueous electrolyte solution at a point of release of the acidic oxidation product. For example, such a first transport membrane may comprise a gas diffusion electrode, a gas diffusion layer (GDL), and / or a zero-gap membrane electrode (CEM). A gas diffusion layer can also be understood as a first transport membrane. In particular, the first transport membrane may comprise a perfluorosulfonic acid membrane. Preferably, such a transport membrane may be based on a perfluorosulfonic acid / polytetrafluoroethylene copolymer.Materials for transport membranes may further, or alternatively, be selected in particular from the group consisting of: PTFE / PTFE (polytetrafluoroethylene / Teflon)-based membranes, hydrocarbon membranes, sPPS (sulfonated polyphenylene sulfones) membranes. Examples include, in particular, membranes familiar to those skilled in the art, which are marketed under the names Nation, Gore, Fumasep, Fumapem, Aquivion, Lonomr Pemion and / or Xion, with a Nafion membrane or a Gore Select membrane being preferred among them.

[0080] In certain embodiments, in the case of electrolysis and the use of an electrolysis cell described above with said anode compartment, intermediate space, and cathode compartment, step (b) takes place at a cathode comprising a hydrophilic diffusion medium. Preferably, the hydrophilic diffusion medium can comprise porous materials selected from the group consisting of iron, nickel, titanium, and carbon, or combinations thereof. Particularly preferably, the diffusion medium can be selected from the group consisting of titanium meshes, nickel meshes, carbon paper, or combinations thereof. Here, it was also surprisingly found that the use of a hydrophilic diffusion medium significantly reduces overpotentials in the electrolysis cell. Without wishing to be bound to any theory, hydrophobic materials have been used in the prior art to enable the removal of gas.Therefore, the use of a hydrophilic diffusion medium at the cathode is counterintuitive.

[0081] In certain embodiments, in the case of electrolysis and the use of an electrolysis cell described above with said anode compartment, intermediate space, and cathode compartment, step (a) takes place at an anode which comprises a hydrophobic diffusion medium. For example, such a hydrophobic diffusion medium may comprise carbon-based fiber materials that have been pretreated with PTFE.

[0082] In certain embodiments, in the case of electrolysis, step (a) has an oxidation potential of approximately 0 V. In the present case, therefore, the anode can simultaneously serve as the reference electrode, at which hydrogen is oxidized at a platinum electrode at pH = 0.

[0083] In certain embodiments, in the case of electrolysis and the use of an electrolysis cell described above with said anode compartment, intermediate compartment, and cathode compartment, step (b) has a reduction potential of at least about 0.8 V, in particular about 0.8 V to about 1.2 V. The reduction potential is calculated in particular according to the Nernst equation. Overall, the total cell voltage, including overpotentials, can be, for example, about 0.8 to about 3 V, preferably about 0.8 to about 1.8 V, and particularly preferably about 0.8 to about 1.5 V.

[0084] In certain embodiments, in the case of the electrochemical process and an electrochemical cell or arrangement of electrochemical cells, an aqueous electrolyte solution is used. In particular, the aqueous electrolyte solution contains alkali metal cations, and especially cations selected from the group consisting of sodium and potassium, or a combination thereof. Additionally or alternatively, the aqueous electrolyte solution contains anions selected from the group consisting of chloride, nitrate, sulfate, and monohydrogen phosphate (HPO4). 2 ), dihydrogen phosphate (H2PO4'), phosphate (PO4 3-The aqueous electrolyte solution comprises potassium bicarbonate, hydrogen carbonate, and perchlorate, or a combination thereof. In particular, the aqueous electrolyte solution is an aqueous potassium bicarbonate solution, and / or aqueous potassium chloride solution, and / or aqueous sodium chloride solution, and / or aqueous sodium bicarbonate solution, preferably having a molarity of about 1 M to about 6 M, more preferably about 1 M to 4 M, and more preferably about 2 M. Additionally or alternatively, the aqueous electrolyte solution is, in particular, a sodium perchlorate solution, preferably having a molarity of about 4 M to about 6 M, and more preferably about 5 M. Additionally or alternatively, the aqueous electrolyte solution comprises a sodium nitrate solution.

[0085] In certain embodiments, in the case of electrolysis using an electrolysis cell described above with the aforementioned anode compartment, alkali metal cations, e.g., potassium cations, are transferred from the aqueous electrolyte solution to a cathode compartment via a second transport membrane during the reaction. The second transport membrane can, in particular, have the features of the first transport membrane mentioned above. Specifically, sodium cations from the carbonate-containing aqueous solution can be transported from the intermediate compartment to the cathode compartment via a second transport membrane.

[0086] In certain embodiments, e.g., in the case of electrolysis, the process is operated using alternating or direct current. In particular, the electrolyzer or electrolysis cell can be operated with a direct current voltage of up to 3 V, particularly < 1.5 V, preferably < 1.3 V. In particular, the process is operated with a direct current voltage of approximately 0.6 V to approximately 1.5 V, preferably approximately 0.83 V to approximately 1.6 V.

[0087] The first alkaline aqueous solution may, for example, have a pH value of approximately > 9, preferably approximately > 13.

[0088] In step (c), an alkaline absorbent suspension is reacted with a gas containing carbon dioxide. The alkaline absorbent suspension comprises a mixture of: the first alkaline aqueous solution comprising an alkali metal hydroxide, and solid alkali metal hydrogen carbonate and / or solid alkali metal carbonate.

[0089] The solid mass fraction, e.g., of solid alkali metal hydrogen carbonate and / or solid alkali metal carbonate, based on the total weight of the alkaline absorbent suspension, is in particular at least about 0.1 wt.% to about 20 wt.%, preferably about 2 wt.% to about 10 wt.%. The alkaline suspension may optionally comprise further additives such as activated carbon and / or sparingly soluble or insoluble inorganic salts or insoluble organic substances in the present solution to promote the formation of the product suspension described below. Activated carbon is preferred.

[0090] The carbon dioxide reacts – without being bound to any specific theory – with the alkali metal hydroxide from the alkaline absorbent suspension according to equation (4) to form solid alkali metal carbonate, yielding a product suspension. Furthermore, alkali metal hydrogen carbonate can be obtained in the product suspension as a subsequent reaction according to equation (5). The additional solid alkali metal hydrogen carbonate can also be formed, to a lesser extent, from the reaction between the hydroxide and the alkali metal carbonate already present in the alkaline absorbent suspension. This is the origin of the terms "solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate".

[0091] The product suspension comprises a mixture of: additional solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate; and a second alkaline aqueous solution comprising a hydroxide.

[0092] The "additional" solid alkali metal carbonate and / or solid alkali metal carbonate in the product suspension is formed according to equations (4) and (5), respectively, in addition to the solid alkali metal carbonate and / or alkali metal bicarbonate already obtained in the alkaline absorbent suspension. This means that the solid mass fraction in the product suspension, relative to the total mass of the product suspension, is higher than the solid mass fraction in the alkaline absorbent suspension. The solid mass fraction, e.g., of the solid alkali metal bicarbonate and / or the solid alkali metal carbonate, relative to the total weight of the product suspension, is in particular at least about 0.1 wt% to about 20 wt%, preferably about 2 wt% to about 10 wt%.The alkaline absorbent suspension is saturated with dissolved alkali metal bicarbonate and / or dissolved alkali metal carbonate, such that the proportion of solid alkali metal bicarbonate and / or solid alkali metal carbonate within the alkaline absorbent suspension is essentially constant. The same applies to the product suspension and all other suspensions mentioned. Surprisingly, the presence of solid alkali metal bicarbonate and / or solid alkali metal carbonate was found to be beneficial for step (c), which represents an absorption.Without being bound to any specific theory, it is assumed that the presence of solid alkali metal carbonate and / or solid alkali metal bicarbonate in the alkaline absorbent suspension improves wettability within, for example, an absorption device. This allows more carbon dioxide gas to be absorbed and not desorbed as quickly as in a purely liquid phase. Furthermore, saturating the alkaline absorbent suspension with alkali metal carbonate and / or alkali metal bicarbonate can directly result in the formation of a solid (i.e., without prior dissolution as carbonate and / or bicarbonate), which, as explained below, is necessary for subsequent process steps to improve efficiency.

[0093] According to the reaction equation (4), the second alkaline solution in the product suspension has a lower concentration of alkali metal hydroxide than the first alkaline solution.

[0094] CO2(g / aq)+ 2 NaOHaq -> Na2CO3(s) + H2O0) (4)

[0095] Na2CO3(s) + CO2(g,aq) + H2O0) -> 2 NaHCO3(s) (5)

[0096] It should be noted that sodium is shown in equations (4) and (5) only as an example and any other alkali metal cation can be used instead.

[0097] The conversion can take place in an absorption device. The absorption device can, for example, consist of three parts: a device for distributing the absorbent suspension, a mass transfer area, and a device for collecting and combining the absorbent suspension. Any type of absorption device known to those skilled in the art is suitable. An absorption device can, for example, include a spray scrubber, a bubble reactor, and / or a cooling tower. Examples of such a cooling tower are the CMDR or CMC series from Kelvion. The exchange packing of the cooling tower can, for example, be Sanipacking from Pruebas. The absorption can take place over a wide temperature range. The absorber can operate at temperatures from approximately -40°C to approximately 120°C, preferably from approximately -20°C to approximately 70°C, and particularly preferably from approximately -10°C to approximately 30°C or from over 30°C to 70°C.Temperatures from approximately -10°C to approximately 30°C are suitable for atmospheric carbon dioxide concentrations, while temperatures from approximately 30°C to approximately 70°C are suitable for point sources. This is due, not least, to the aforementioned freezing point depression and the increase in vapor pressure, as the alkaline absorbent suspension is nearing its saturation limit.

[0098] In step (d), a first partial suspension is separated from the product suspension. The first partial suspension comprises: solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate, and a portion of the second alkaline aqueous solution reduced according to the separation.

[0099] The solid mass fraction, i.e., that of the alkali metal carbonate and / or alkali metal bicarbonate, based on a total mass of the first partial suspension, is higher than the solid mass fraction in the product suspension based on the total mass of the product suspension. In other words, liquid or solution is separated from the product suspension to obtain a partial suspension with more solids than in the product suspension. The mass fraction of the reduced portion of the second alkaline aqueous solution, based on the total mass of the first partial suspension, is particularly less than 10 wt.%, preferably less than 5 wt.%, while the remaining mass fraction consists of solids, i.e., essentially solid alkali metal carbonate and / or solid alkali metal bicarbonate. The mass fraction of the solids, based on the total mass of the first partial suspension, is particularly about 20 wt.% to about 80 wt.%, preferably about 20 wt.% to about 60 wt.%.

[0100] Step (d) can take place in a separation device. A separation device can include, for example, a sedimentation device, such as a sedimentation hopper, a decanter, or a hydrocyclone. The separation device can also comprise combinations of the aforementioned alternatives with other separation units. A filter or filter press can be used as a separation unit in combination with the sedimentation device, decanter, or hydrocyclone to further reduce the liquid.

[0101] It should be noted here – as is immediately apparent to anyone skilled in the art – that the third solution, upon separation, contains dissolved alkali metal carbonate and / or dissolved alkali metal bicarbonate, since the formation of a solid is also subject to equilibrium. This solution is saturated. Furthermore, after the separation of the first partial suspension, liquid remains which may contain unseparated solids. This liquid is called the second or third partial suspension.

[0102] The first partial suspension can be directly added to step (e) as described below. Furthermore, the partial suspension can be further dispersed. This results in a mixture comprising a dilute first partial suspension. This mixture also includes alkali metal carbonate and / or alkali metal bicarbonate. Additionally, this suspension also contains alkali metal carbonate and / or alkali metal bicarbonate in dissolved form. The dispersion takes place, in particular, in a dispersion device, which includes embodiments known to those skilled in the art. The dispersion device can, for example, comprise a static mixer in a feed pipe and / or a stirred container. Stirring in this stirred container can be achieved using a mechanical stirrer or by introducing a liquid with sufficient momentum.

[0103] In step (e) the first partial suspension (or, if applicable, the first dilute partial suspension) is reacted with the acidic aqueous solution to obtain dissolved and gaseous carbon dioxide gas.

[0104] The solid mass fraction, in particular the solid mass fraction of solid alkali metal hydrogen carbonate and / or solid alkali metal carbonate, relative to the respective suspension, decreases in the following order: first partial suspension > product suspension > alkaline absorbent suspension.

[0105] In certain embodiments, the reaction of the first partial suspension takes place outside an electrochemical cell in a mixing device. Within the mixing device, the pH of the acidic aqueous solution can be approximately < 4, whereas in the electrochemical cell, during its generation according to step (a), it is particularly approximately < 2; in any case, it is lower than in the mixing device. This yields dissolved and gaseous carbon dioxide. Furthermore, an aqueous salt solution is obtained, which contains the dissolved alkali metal as well as the counter-anion to the acid. This salt solution can still be acidic, but its acidity is reduced according to the reaction in step (e). The salt solution can be recycled in step (a) to regenerate the acidic aqueous solution for the process. The resulting acidic aqueous solution can then be recirculated back into the mixing device.

[0106] The mixing device is not critical as long as it is designed to mix the mixture with the acidic aqueous solution to obtain dissolved and gaseous carbon dioxide. Such a mixing device could, for example, be a continuous stirred tank reactor.

[0107] In certain embodiments, the first partial suspension is dissolved in step (f1) in the mixing device outside the electrochemical cell with a first part of the acidic aqueous solution, in particular with a pH of about 4 to about < 7, or an aqueous solution with a pH of 7 to about < 10, preferably 7 to about < 9, thereby yielding dissolved alkali metal bicarbonate and / or dissolved alkali metal carbonate, as well as dissolved and gaseous carbon dioxide. Subsequently, in step (f2), the dissolved alkali metal bicarbonate and / or alkali metal carbonate is reacted in the electrochemical cell with a second part of the acidic aqueous solution, in particular with a pH of about < 1, preferably < 0 or lower than the pH of the first partial suspension dissolved in the solution. In particular, the presence of the dissolved alkali metal bicarbonate or alkali metal carbonate can lead to the formation of a second part of the acidic aqueous solution, in particular with a pH of about < 1, preferably < 0 or lower than the pH of the dissolved first partial suspension.The presence of alkali metal carbonate within the cell ensures that—without being bound to any specific theory—a buffering effect occurs, which—again, without being bound to any specific theory—ensures that the possible recombination between hydroxide ions and hydronium ions takes place within the cell, i.e., neutralization, which is detrimental to efficiency. In the preceding steps of the reaction of the dissolved alkali metal hydrogen carbonate and / or the dissolved alkali metal carbonate within the cell, the solid was completely dissolved, so that, in particular, no solid enters the cell and no solid is present in substep (f2).

[0108] For example, this also increases the efficiency of an electrolysis cell described herein with an anode compartment, cathode compartment, and intermediate space, in which, in this case, the dissolved alkali metal bicarbonate and / or the dissolved alkali metal carbonate react with the second part of the acidic aqueous solution in the intermediate space. This ensures that the aforementioned buffering effect does not result in a high proton concentration in the intermediate space, which would reduce efficiency; even at low flow rates, which can be reduced to approximately one-tenth. A low flow rate also makes it possible to reduce the width of the intermediate space and thus additional electrical losses. Therefore, in certain embodiments, the first part of the acidic aqueous solution can be considered as that which is carried outside the cell, and the second part is that which reacts inside the cell in step (f2).

[0109] In the case of an electrodialysis cell with at least one bipolar membrane, the steps (f1) and (f2) can also be followed. In this way, the cell efficiency can be increased by consuming the acidic components of the acidic aqueous solution, i.e., protons, i.e., H3O. + ( aq ), which are further increased by their reaction with alkali metal hydrogen carbonate or alkali metal carbonate within the cell.

[0110] Alternatively, in the case of such an electrodialysis cell, a conversion can also be carried out outside the electrodialysis cell, as described above, to obtain dissolved and gaseous carbon dioxide.

[0111] In certain embodiments, the electrochemical cell is an electrolysis cell comprising: i. an anode compartment, ii. an intermediate compartment, and iii. a cathode compartment, wherein the intermediate compartment is arranged between the anode compartment and the cathode compartment; the anode compartment is connected to the intermediate compartment via a first transport membrane; and the cathode compartment is connected to the intermediate compartment via a second transport membrane.

[0112] The transport membranes have already been defined above. The anode compartment and the cathode compartment can be fluidically connected via a hydrogen gas line.

[0113] In a particular embodiment of the electrolysis cell, it has a pressure generating device which is designed to generate a pressure above atmospheric pressure, at least in the space between the cells.

[0114] The pressure generating device can also be designed to generate pressure above atmospheric pressure predominantly or exclusively in the space between the layers.

[0115] Naturally, the electrolysis cell incorporates the features and technical effects of the electrochemical process. In particular, this electrolysis cell is configured to carry out the electrochemical process according to the first aspect of the present invention. Accordingly, the electrolysis cell according to the first aspect of the present invention can also be used for carbon dioxide production in a plant according to the second aspect of the present invention.

[0116] The pressure generation device is uncritical as long as it is designed to generate a pressure above atmospheric pressure in the space. For example, the pressure generation device is fluidically connected downstream of an inlet to the space. In certain embodiments, the cathode compartment has a cathode active material selected from the group consisting of platinum, iron, carbon, and nickel, or combinations thereof. Additionally or alternatively, the anode compartment has an anode active material selected from the group consisting of platinum and carbon, or combinations thereof.

[0117] In certain embodiments, the width of the gap is approximately 0.05 mm to approximately 8 mm, in particular approximately 0.1 mm to approximately 8 mm or approximately 0.2 mm to approximately 8 mm. Preferably, the width of the gap is approximately 0.05 mm to approximately 5.5 mm, in particular approximately 0.1 mm to approximately 5.5 mm or approximately 0.2 mm to 5.5 mm. Particularly preferably, the width of the gap is approximately 0.2 mm to 2.0 mm, in particular approximately 0.5 mm. It should be emphasized again that the width of the gap can be significantly reduced by the efficiency gains that can be achieved using the method or system according to the invention.

[0118] The electrolysis cell can be operated at temperatures below 100 °C. Specifically, it can be operated at temperatures between 10 °C and 80 °C, or between 20 °C and 80 °C, and optionally between approximately 60 °C and 80 °C. Furthermore, it is possible to operate the electrolyzer at a temperature of at least 95 °C and below 100 °C.

[0119] The space within the electrolysis cell can be defined by a frame that is directly connected to the first and second transport membranes via seals on both sides. Furthermore, the anode and cathode compartments can be defined by a diffusion medium, as described herein, a seal, and a current collector, e.g., made of titanium. In this configuration, the diffusion medium is in direct contact with the first and second transport membranes, followed layer by layer by the seal and the current collector, which also contains a current field.

[0120] As mentioned previously, in certain embodiments the cathode compartment contains a hydrophilic diffusion medium and / or the anode compartment a hydrophobic diffusion medium. The diffusion medium can also include the cathode or anode active material.

[0121] To achieve the aforementioned advantages, it is particularly preferred to allow the substep of the conversion (f2) to take place within the space of the electrolysis cell.

[0122] In certain embodiments, a second partial suspension is obtained in step (d), wherein the second partial suspension has a lower solid mass fraction of the solid, comprising solid alkali metal carbonate and / or solid alkali metal bicarbonate, based on the total mass of the second partial suspension, than the solid mass fraction of the product suspension based on the total mass of the product suspension, and is used as part of the alkaline absorbent suspension. In this case, the alkaline absorbent suspension comprises a mixture of: the first alkaline aqueous solution of the second partial suspension, comprising solid alkali metal carbonate and / or alkali metal bicarbonate, the mass of which is correspondingly reduced by the separation of the first partial suspension, and a portion of the second alkaline aqueous solution, which is reduced correspondingly by the separation.

[0123] In other words, the second partial suspension is fed to step (c), i.e., recirculated. Because the alkaline absorption suspension contains a solid, it flows more slowly through the absorption device than if it were a pure liquid. Therefore, pump cycles for recirculation, which would otherwise be necessary with a pure liquid, are eliminated.

[0124] Thus, in this case, this alkaline absorbent suspension in these embodiments has a lower pH value of about > 9, preferably > 13, than the first alkaline aqueous solution as such; however, it has a higher pH value than the second alkaline aqueous solution as such.

[0125] In certain embodiments, step (e) is followed, in particular immediately, by step (g) of extracting the carbon dioxide gas in gaseous form. This is achieved via a carbon dioxide extraction device. In particular, the carbon dioxide extraction device is configured to perform step (g) of the method according to the first aspect of the present invention. In certain embodiments, the carbon dioxide extraction device is selected from the group consisting of membrane contactors, preferably 3M Liqui-Cel, heat exchangers, and combinations thereof. A membrane contactor is preferably used. In the case of a heat exchanger, it is configured to heat the acidic aqueous solution to enable the outgassing of carbon dioxide gas. Furthermore, any existing carbon dioxide gas can be transported away via suitable pipelines known to those skilled in the art.

[0126] In certain embodiments, a third partial suspension is obtained from the second partial suspension in step (d), wherein the third partial suspension has a lower solids mass fraction relative to its total mass than the solids mass fraction of the product suspension relative to its total mass, and the third partial suspension is recycled to step (b). For example, the third partial suspension may have a low solids mass fraction of < 1 wt% relative to its total mass. In this case, the solids may be dissolved using a heat exchanger before being fed to step (b), for example, into the cathode compartment of the electrolysis cell. For higher solids mass fractions, a filter, such as those known to those skilled in the art, may be used to separate the solids before they enter the electrolysis cell in step (b), for example, the cathode compartment.

[0127] The product suspension can be subjected to separation step (d) such that it separates into the first partial suspension, the second partial suspension, and the third partial suspension. For example, the mass fraction of the second partial suspension, based on the total mass of the second and third partial suspensions, is > 50 wt.%, preferably > 80 wt.%. In certain embodiments, the first alkaline aqueous solution has a pH of about > 9, in particular about > 13. For example, the pH may also be about 14. This corresponds essentially to the pH of the alkaline absorbent, which may be equal to the pH of the first alkaline solution or, as described above, reduced accordingly in the case of recirculation of the second alkaline solution.

[0128] It should also be noted that all named embodiments relate to the first aspect of the invention and thus any embodiments described as "in certain embodiments" can be combined with each other and are not mutually exclusive unless otherwise stated.

[0129] Attachment

[0130] According to a second aspect, the present invention relates to a system for the electrochemical production of carbon dioxide from a gas, in particular air or a point source, wherein the system comprises the following:

[0131] (a) to generate an electrochemical cell or an arrangement of several electrochemical cells, wherein the electrochemical cell or the arrangement of several electrochemical cells is configured to produce an acidic aqueous solution and a first alkaline aqueous solution, the first alkaline aqueous solution comprising an alkali metal hydroxide;

[0132] (b) an absorption device, wherein the absorption device is designed to react a gas containing carbon dioxide, wherein the gas is in particular air or a point source, with an alkaline absorbent suspension comprising solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate and the first alkaline aqueous solution with the alkali metal hydroxide, thereby obtaining a product suspension comprising a solid comprising additional solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate formed by the reaction between carbon dioxide and the alkali metal hydroxide, and a second alkaline aqueous solution comprising an alkali metal hydroxide, the concentration of which is reduced according to the reaction compared to a concentration in the first aqueous solution, and wherein the absorption device is fluidically connected to the electrochemical cell or electrochemical cell arrangement;

[0133] (c) a separation device, wherein the separation device is designed to separate a first partial suspension from the product suspension, wherein the first partial suspension has a higher solids mass fraction relative to the total mass of the first partial suspension than the solids mass fraction in the product suspension relative to the total mass of the product suspension, and wherein the separation device is fluidically connected to the absorption device; wherein the system is designed to react the first partial suspension with the acidic aqueous solution to obtain dissolved and gaseous carbon dioxide gas. For this purpose, for example, the mixture can be reacted with the acidic aqueous solution in a mixing device outside the electrolysis cell.Alternatively, a reaction can be carried out according to substeps (f1) and (f2), whereby the solid suspension is dissolved as described above and dissolved alkali metal hydrogen carbonate and / or dissolved alkali metal carbonate are reacted in the electrochemical cell.

[0134] The resulting carbon dioxide gas can be removed via a carbon dioxide extraction device, as described above.

[0135] An ion exchanger can be arranged between the mixing device and the electrochemical cell in the flow direction from the mixing device to the electrochemical cell. This ion exchanger is designed to remove dissolved divalent cations, such as strontium, magnesium, and / or calcium, while leaving alkali metal cations in the solution. For example, such an ion exchanger can be used in the following applications: The use of the ion exchanger prevents divalent cations from entering the cell, which could otherwise result in a reduced operating time of the electrochemical cell. An ion exchanger is known to those skilled in the art, for example, from chlor-alkali electrolysis and can include Lewatit Monoplus TP 208 or Lewatit MDS TP 208 from Lanxess.

[0136] For example, several electrochemical cells, in particular electrolysis cells, can be used together within the system according to the first aspect of the present invention. These several electrochemical electrolysis cells can be connected to each other in a stacking configuration. For example, two electrolysis cells can be linked together via a common anode compartment. Another electrolysis cell can be linked to these two electrolysis cells via a common cathode. Yet another electrolysis cell can in turn be linked to this further electrolyzer via a common anode compartment, and so on.

[0137] Furthermore, this plant can also be operated using the method according to the first aspect of the present invention.

[0138] To avoid repetition, it should be stated that the system exhibits the features, technical advantages and effects according to the first aspect of the present invention.

[0139] It should also be noted that all named embodiments relate to the second aspect of the invention and thus any embodiments described as "in certain embodiments" can be combined with each other and are not mutually exclusive unless otherwise stated.

[0140] Description of the characters

[0141] The following figures show exemplary embodiments of the present invention and are therefore by no means to be interpreted as limiting. Figure 1 shows a first embodiment 100a of the system according to the invention. In absorption device 1, e.g., a cooling tower, an alkaline absorption suspension comprising solid potassium carbonate and / or solid potassium bicarbonate as well as dissolved potassium carbonate and / or dissolved potassium bicarbonate is used. The alkaline absorption suspension therefore contains dissolved carbonate at the saturation limit.This alkaline absorption suspension reacts with air containing carbon dioxide in a countercurrent process according to equations (4) and (5) above to form a product suspension containing solid and dissolved potassium carbonate and / or solid and dissolved potassium bicarbonate, whereby the solid mass fraction in the product suspension, relative to the total mass of the product suspension, is higher than the solid mass fraction in the alkaline absorption suspension relative to the total mass of the alkaline absorption suspension due to the formation of additional solids. In this case, the alkaline absorption suspension contains the following components: a first alkaline aqueous solution, which contains KOH(. aq) and, for example, has a pH of approximately 14 and was generated by electrochemical cell 7, e.g., an electrolysis cell. This alkaline aqueous solution also contains dissolved potassium bicarbonate and / or dissolved potassium carbonate at the saturation limit. Furthermore, this first alkaline aqueous solution is combined with a second alkaline aqueous solution containing solid potassium bicarbonate and / or solid potassium carbonate and KOH( aq) as well as dissolved potassium bicarbonate and / or dissolved potassium carbonate, combined before entering the absorption device with a second partial suspension separated after the absorber (with a solid mass fraction based on the total mass of the second partial suspension that is lower than the solid mass fraction of the product suspension based on the total mass of the product suspension), which is recirculated, ultimately yielding the alkaline absorbent suspension, which reacts with carbon dioxide according to equation (4) to obtain the product suspension. At the outlet of the absorption device 1, a second partial suspension is obtained by means of a separation device 2, e.g., a hydrocyclone, which consists predominantly of liquid, whereas the first partial suspension obtained consists predominantly of solids.The first partial suspension is introduced into the mixing device 5 and mixed with an acidic aqueous solution (pH = approximately 0 to 2) generated by the electrochemical cell 7 to contain dissolved and gaseous carbon dioxide according to equation (5) above, whereby the dissolved carbon dioxide can be removed from the mixing device, e.g., by mechanical stirring. After the removal of carbon dioxide, the potassium chloride-containing solution from the mixing device is passed over the electrochemical cell 7, whereby an exchange between potassium cations and protons takes place within the electrochemical cell 7, resulting in an acidic aqueous solution containing HCl(. aq j is obtained, which is then circulated back into the mixing device 5. The potassium cations are used in the electrochemical generation of hydroxide ions for charge balancing, which in turn produces KOH( aqj is formed, which in turn represents the first alkaline aqueous solution. An ion exchanger 6 is located between the mixing device 5 and the electrochemical cell 7. This exchanger is specifically designed to remove divalent cations from the solution so that they do not potentially damage the electrochemical cell. Figure 2 shows a second embodiment 100b of the system according to the invention. All components as described for Figure 1 are identical, so repetition is omitted. However, the second embodiment 100b of the system according to the invention differs in its operating mode. In the mixing device, a higher pH value of approximately 3-9 of the acidic aqueous solution and / or aqueous solution is used to dissolve the solid potassium carbonate and / or solid potassium bicarbonate only in order to obtain dissolved potassium bicarbonate and / or dissolved potassium carbonate. In this case, no solid remains.This dissolved potassium bicarbonate and / or dissolved potassium carbonate is then passed through the electrochemical cell 8, where it reacts with the acidic aqueous solution, which is generated, for example, at an anode, to obtain dissolved and gaseous carbon dioxide and dissolved potassium bicarbonate. These components are then transferred back to the mixing device 5 to extract carbon dioxide in its gaseous form, for example, by mechanical stirring.

[0142] EXAMPLES

[0143] Example 1

[0144] Experimental setup

[0145] The absorption of carbon dioxide from air was investigated experimentally. For this purpose, A) an aqueous potassium hydroxide solution and B) a saturated solution consisting of potassium carbonate and potassium hydroxide were used. The composition of the liquids was determined by titration before and after absorption. The amount of carbon dioxide absorbed was also determined by measuring the air volume flow rate and the carbon dioxide concentration. The air volume flow rate was measured using the TGFL measuring turbine with the FI2010 transducer from RÖSLER + CIE INSTRUMETS GmbH. The carbon dioxide concentration in the airflow was measured using GMP343 sensors from Vaisala GmbH. Additionally, C) the desorption of potassium carbonate formed by the absorption of carbon dioxide was carried out using an electrochemical cell.For this purpose, an experimental setup analogous to Figures 100a and 100b was used to demonstrate the effects according to the invention.

[0146] Experiment description

[0147] A) Experiment A, as absorption in a purely liquid phase, corresponds to the state of the art. At the beginning of the experiment, the aqueous absorption solution contained a potassium hydroxide concentration of 0.8 mol / L. This solution was continuously brought into contact with an airflow in the absorption apparatus. Air volume flows of 370 m³ / h were used. 3 / h and 110 m 3 The liquid volume flow rate was investigated. The liquid volume flow rate was kept constant during the test period and was identical for both air volume flows.

[0148] B) Experiment B represents the absorption process according to the invention, in which a saturated potassium carbonate solution, i.e., a suspension, containing potassium hydroxide, circulates continuously in the test rig while exposed to air. The CO2 from the air reacts with the potassium hydroxide in solution, producing further potassium carbonate. The consumed potassium hydroxide is replaced over the course of the experiment by adding more potassium hydroxide. Due to the lower solubility of potassium carbonate compared to potassium hydroxide, and the addition of further potassium hydroxide to the already saturated solution, a solid precipitates. This solid can be separated from the liquid by sedimentation, thus allowing the recovery of the reaction product. This product was subsequently identified by titration.

[0149] C) The production of carbon dioxide from the carbonate and / or bicarbonate formed during absorption using an acid generated by the electrochemical cell was investigated in a continuous process. The reaction was carried out once outside and once inside the electrochemical cell. In the reaction outside the cell, an acidic solution generated by the cell is mixed with the carbonate previously dissolved in water, resulting in the formation of carbon dioxide, which could be separated from the liquid as a gas due to the density difference. In the reaction inside the cell according to the invention, the carbonate solution is introduced directly into the cell, where it reacts immediately with the acidic solution generated there, producing carbon dioxide gas.The efficiency of the electrochemical cell in the two process configurations was determined by titrating the solutions at the cell's inlet and outlet. The titration was performed using an aqueous hydrochloric acid solution or an aqueous potassium hydroxide solution, thus measuring the concentration of dissolved carbonate and bicarbonate ions, as well as the pH value. According to equations 4 and 5, the amount of carbon dioxide released corresponds to the reduction in the amount of carbonate or bicarbonate, respectively. By dividing the electrical energy input by the amount of carbon dioxide released, the process variants can be compared with respect to their energy consumption.

[0150] Results

[0151] A) Comparison according to the state of the art: Using the absorption process according to the state of the art, with an airflow of 370 m 3 / h, 32% of the hydroxide ions are converted to carbonates by reaction with carbon dioxide from the air. At a volume flow rate of 110 m³ / h 3 18% of the carbon dioxide was converted per hour. The low utilization rates result in a tripling, or more than a fivefold increase, of the energy required for desorption per unit of absorbed carbon dioxide, since the unused hydroxide ions are also neutralized by the acid generated in the electrochemical cell.

[0152] B) The absorption process according to the invention produces a solid as a product. Titration of the solid showed that it was potassium carbonate, so that this absorption process achieved a utilization of the hydroxide ions used of >97%. Due to the utilization of both the solid and liquid phases in the absorption step, a higher mass transfer coefficient than in experiments A and B could be achieved due to the high hydroxide ion concentration in the solution, without having to accept a reduced hydroxide ion utilization due to this high hydroxide concentration.

[0153] C) According to the invention: The efficiency of the cell – measured by the electrical power consumed in relation to the amount of alkali metal carbonate or bicarbonate converted – shows a strong dependence on the acid concentration at the cell outlet when using external desorption according to the prior art. At acid concentrations of 1 mol / L, this is only 68%. Increasing the flow rate reduces this, so that an efficiency of 74% was achieved at 0.1 mol / L. If, according to the invention, the desorption takes place within the cell, i.e., the acid reacts directly with the potassium carbonate or bicarbonate present during its formation, the acid concentration is several orders of magnitude lower, resulting in an efficiency of up to 95%. This was achieved even at lower flow rates than with external desorption.Both the lower required flow rates and the significantly higher efficiency result in a lower energy requirement for the overall process and exceed the slight observed increase in cell voltage due to the resulting gas content of less than 0.1 V.

[0154] Example 2

[0155] Experiment description: Absorption with solid suspension on a pilot plant

[0156] The following absorption data for the described process are from a pilot-scale setup with a capacity of 10 t CO2 / year. The packing volume is 11.5 m³. 3In this setup, ambient air comes into contact with the alkaline absorbent, leading to the absorption of CO2. A carbonate forms, which precipitates as a solid because the absorbent solution is already at its saturation point. The resulting solid is separated by sedimentation and subsequently desorbed. Samples of the two-phase mixture were taken from this setup and analyzed. The absorption capacity was determined by measuring the volumetric flow rate and the CO2 concentrations in the gas. Additionally, the absorbent was analyzed by titration.

[0157] Results

[0158] A) Absorption capacity: During operation of the system, the CO2 concentration of the incoming and outgoing air, as well as the treated air volume flow rate, are continuously measured. From this, the amount of CO2 removed by absorption with the absorbent can be determined. At the time the samples for the following measurements were taken, the following values ​​were measured: • CC>2 concentration of the incoming air: 402 ppm

[0159] • CCh concentration of the outgoing air: 149 ppm

[0160] • Air volume flow: 2300 m³ / h 3 / h

[0161] From these measurements, a separation efficiency of 63% can be determined, thus demonstrating the effectiveness of the selective separation of CO2.

[0162] B) Titration of the absorbent: The sample of alkaline absorbent taken from the setup was analyzed by titration. This yielded a sodium hydroxide concentration of 0.704 mol / L, corresponding to a pH of 13.85.

[0163] C) Solids content in suspension: The total density of this sample is determined by weighing a defined volume of 100 mL. This measurement is also performed for the separated solids suspension with an increased solids content, as well as for the separated suspension with a decreased solids content. The following values ​​were obtained:

[0164] • Pipe solution (absorbent): 1176 kg / m 3

[0165] • Suspension with increased solids content: 1335 kg / m³ 3

[0166] • Reduced solids suspension: 1204 kg / m³ 3

[0167] These results show that the resulting solid can be effectively separated from the absorbene in order to be regenerated in the following process step.

[0168] Example 3

[0169] Experimental setup for the electrochemical desorption of CO2

[0170] The setup described below is used to investigate the electrochemical desorption of CO2. Online measurements and sampling are used to demonstrate the effectiveness of CO2 desorption from the alkali metal (hydrogen) carbonates generated in the previous absorption step, as well as the regeneration of the absorber by supplying alkali metal hydroxides.

[0171] The experimental setup used has an electrochemical cell, which is an electrolysis cell, comprising an anode compartment, an intermediate compartment, and a cathode compartment, wherein the intermediate compartment is arranged between the anode compartment and the cathode compartment; the anode compartment is connected to the intermediate compartment via a first transport membrane; and the cathode compartment is connected to the intermediate compartment via a second transport membrane.

[0172] The intermediate and cathode compartments are supplied with liquid salt solutions to enrich them with protons and hydroxide ions, respectively, according to the invention. The following results are obtained from a steady-state operating point of the electrochemical desorption system at 52 °C and an electric current of 10 A.

[0173] Results

[0174] A) Protons are supplied to the space within the electrochemical cell and react with the supplied alkali metal carbonate or alkali metal bicarbonate solution. The electrochemical system is supplied with a 2 mol / L Na₂CC>3 solution, which reacts with the protons to form, among other things, NaHCO₃⁻ and CO₂. Titration of the resulting solution revealed a concentration of 1.1 mol / L NaHCO₃⁻. Furthermore, a product gas flow of 51 NmL / min with a CO₂ volume concentration of 97% was measured and removed from the process during the experiment. This demonstrates that the described setup is capable of generating CO₂ from the alkali metal carbonate or alkali metal bicarbonate solution obtained by absorption.

[0175] B) In the cathode compartment of the electrochemical cell, hydroxide ions are released into the electrolyte. This part of the electrochemical system is supplied with a Na₂CC>3 solution at a concentration of 2 mol / L. The resulting solution was also analyzed by titration, revealing a NaOH concentration of 2.15 mol / L. The solution is therefore suitable as an absorbent for the CO₂ absorption step of the described process.

[0176] Reference symbol list

[0177] 1 Absorption device

[0178] 2 Separating device

[0179] 3 Heat exchangers 4 Fresh water supply

[0180] 5 Mixing device

[0181] 6 ion exchangers

[0182] 7 Electrochemical cell

[0183] 100a Plant according to the first embodiment 100b Plant according to the second embodiment

Claims

REQUIREMENTS 1. Electrochemical process for carbon dioxide production comprising the following steps: (a) electrochemical generation of an acidic aqueous solution; (b) electrochemical generation of a first alkaline aqueous solution comprising an alkali metal hydroxide; (c) Reacting a gas containing carbon dioxide, wherein the gas is in particular air or a point source, with an alkaline absorbent suspension comprising the first alkaline aqueous solution containing the alkali metal hydroxide, as well as solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate, yielding a product suspension comprising a solid comprising additional solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate formed by the reaction between carbon dioxide and the alkali metal hydroxide, as well as a second alkaline aqueous solution comprising an alkali metal hydroxide, the concentration of which is reduced according to the reaction compared to a concentration of alkali metal hydroxide in the first aqueous solution; (d) Separating a first partial suspension from the product suspension, wherein the first partial suspension has a higher solids mass fraction relative to the total mass of the first partial suspension than the solids mass fraction in the product suspension relative to the total mass of the product suspension; (e) Reacting the first partial suspension with the acidic aqueous solution to obtain dissolved and gaseous carbon dioxide gas; wherein the alkali metal hydroxide comprises in particular sodium hydroxide and / or potassium hydroxide.

2. The method according to claim 1, wherein the conversion of the first partial suspension takes place outside an electrochemical cell in a mixing device, whereby dissolved and gaseous carbon dioxide gas is obtained.

3. The method according to claim 2, wherein the conversion of the first partial suspension (f1) takes place outside the electrochemical cell in a mixing device with a first part of the acidic aqueous solution or an aqueous solution with a pH > 7, yielding dissolved alkali metal hydrogen carbonate and / or dissolved alkali metal carbonate as well as dissolved and gaseous carbon dioxide gas, and (f2) the dissolved alkali metal hydrogen carbonate and / or the dissolved alkali metal carbonate is reacted within the electrochemical cell with a second part of the acidic aqueous solution to obtain dissolved and gaseous carbon dioxide gas.

4. Method according to claim 2 or 3, wherein the electrochemical cell is an electrolysis cell comprising: i. an anode compartment, ii. an intermediate compartment, and iii. a cathode compartment, wherein the intermediate compartment is arranged between the anode compartment and the cathode compartment; the anode compartment is connected to the intermediate compartment via a first transport membrane; and the cathode compartment is connected to the intermediate compartment via a second transport membrane.

5. Method according to claim 4, wherein the conversion (f2) takes place within the electrolysis cell in the space between.

6. A method according to any of the preceding claims, wherein a second partial suspension is obtained in step (d), the second partial suspension having a lower solid mass fraction of the solid, comprising solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate, based on the total mass of the second partial suspension than the solid mass fraction of the product suspension based on the total mass of the product suspension, and being used as part of the alkaline absorbent suspension.

7. A method according to any of the foregoing claims further comprising, following step (e), step (g) Extraction of the carbon dioxide gas in gaseous form.

8. A method according to claim 6 or 7, wherein a third partial suspension is obtained in step (d), wherein the third partial suspension has a lower solids mass fraction based on the total mass of the third partial suspension than the solids mass fraction of the product suspension based on the total mass of the product suspension, and the third partial suspension is recycled in step (b).

9. Method according to one of the preceding claims, wherein the first alkaline aqueous solution has a pH of about > 9, in particular about > 13.

10. Plant for the electrochemical production of carbon dioxide from a gas, in particular air or a point source, wherein the plant comprises the following: (a) to generate an electrochemical cell or an arrangement of several electrochemical cells, wherein the electrochemical cell or the arrangement of several electrochemical cells is configured to produce an acidic aqueous solution and a first alkaline aqueous solution, the first alkaline aqueous solution comprising an alkali metal hydroxide; (b) an absorption device, wherein the absorption device is designed to react a gas containing carbon dioxide, wherein the gas is in particular air or a point source, with an alkaline absorbent suspension comprising solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate and the first alkaline aqueous solution with the alkali metal hydroxide, thereby obtaining a product suspension comprising a solid comprising additional solid alkali metal carbonate and / or solid alkali metal hydrogen carbonate formed by the reaction between carbon dioxide and the alkali metal hydroxide, and a second alkaline aqueous solution comprising an alkali metal hydroxide, the concentration of which is reduced according to the reaction compared to a concentration in the first aqueous solution, and wherein the absorption device is fluidically connected to the electrochemical cell or electrochemical cell arrangement; (c) a separation device, wherein the separation device is designed to separate a first partial suspension from the product suspension, wherein the first partial suspension has a higher solids mass fraction based on the total mass of the first partial suspension than the solids mass fraction in the product suspension based on the total mass of the product suspension, and wherein the separation device is fluidically connected to the absorption device; wherein the apparatus is designed to react the first partial suspension with the acidic aqueous solution to obtain dissolved and gaseous carbon dioxide gas.

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