Extraction of carbon dioxide by absorption in alkali solution and regeneration of the alkali solution by ion exchange

Abstract

A process for the recovery of carbon dioxide (CO2) from a carbon dioxide-containing gas, in particular air and / or exhaust gas, comprises the steps: a) contacting the carbon dioxide-containing gas with an absorbent in the form of a solution of aqueous alkali metal hydroxide solution, in particular sodium hydroxide solution and / or potassium hydroxide solution, so that carbon dioxide is absorbed from the carbon dioxide-containing gas and alkali metal carbonate, in particular sodium carbonate (Na2CO3) and / or potassium carbonate (K2CO3), is formed in the absorbent; b) carrying out an ion exchange step, wherein the alkali carbonate formed in step a) is subjected in dissolved form to a cation exchange process so that the alkali ions of the alkali carbonate are bound to a cation exchanger; c) eluting the alkali ions bound to the cation exchanger in step b) with a solution containing ammonia and / or at least one amine, whereby alkali metal hydroxide solution is recovered, which is preferably used for the renewed absorption of carbon dioxide in step a)

Description

Technical FieldThe invention relates to a method for recovering carbon dioxide (CO 2) from a carbon dioxide-containing gas, in particular air and / or exhaust gas, and to a device, in particular a carbon dioxide sorption unit or absorption unit, for carrying out the method.Prior ArtIn view of the global problems of carbon dioxide or CO 2- processes for recovering carbon dioxide from air or exhaust gas have become increasingly increasingly important in recent years. In this case, the applications referred to as "direct air capture" (DAC), in which ambient air or exhaust gases are used as carbon dioxide source, are of particular interest in particular.Numerous methods for achieving this object have been developed and published in recent years. Various absorbents or adsorbents have been proposed. In principle, many basic substances are suitable for selectively enriching carbon dioxide from the air or exhaust gases to a greater or lesser extent. The classical absorbents for the binding of carbon dioxide from air or off-gases are solutions of alkali metal or alkaline earth metal hydroxides, it having been shown that the higher the concentration and basicity of the solution, the faster and more complete the absorption from the gas stream.WO 2022 / 192408 A2 (Susteon Inc.) discloses, for example, an arrangement for removing and desorption of CO 2- gas. Sodium hydroxide, among others, is described as a sorbent for CO 2. However, this approach requires special carrier substrates.When sodium hydroxide solution (NaOH solution) is used to bind carbon dioxide, sodium carbonate (Na 2 CO 3). is formed in a first stage. This can absorb further carbon dioxide and react further in the second stage to give sodium bicarbonate (NaHCO 3). This second reaction step, however, is an equilibrium reaction proportional to the carbon dioxide partial pressure of the gas stream contacted with sodium carbonate. For the use of ambient air containing 400 ppm of carbon dioxide, the equilibrium is predominantly established in the starting product sodium carbonate. Therefore, in the second stage, only small amounts of carbon dioxide are absorbed, slowly and with poor efficiency.In many processes for the direct recovery of carbon dioxide from air or exhaust gases with a low carbon dioxide content, therefore, a regeneration of the sodium carbonate to sodium hydroxide is proposed. Sodium carbonate can be thermally decomposed to sodium hydroxide (NaOH) and carbon dioxide, but a temperature above 1600°C is necessary. Because of the energy requirement and the costs, direct thermal decomposition is therefore not used.A known indirect method is the regeneration of sodium carbonate with calcium hydroxide [Ca(OH) 2; Kraft Method]. Due to the poor solubility of calcium carbonate in water at high pH, practically all carbon dioxide bound in the sodium carbonate can be transferred to calcium hydroxide, whereby sodium hydroxide solution is released.In a further step, the precipitated calcium carbonate is then calcined, carbon dioxide being liberated as gas. The calcination is carried out at temperatures above 900° C. Although this is a significantly lower temperature than in the case of sodium carbonate, it is still too high a temperature that it would be directly available from waste heat or regenerative energy sources such as solar heat.Various variants of the electrochemical regeneration of sodium carbonate have also been proposed. An overview is provided by US 2007 / 0187247 A1 (Lackner et al.), which describes the use of electrochemically generated acid for releasing carbon dioxide. The resulting salt is separated into acid and sodium hydroxide solution via bipolar membranes. However, these methods are quite complicated and energy intensive.For thermodynamic reasons, the liberation of carbon dioxide in the above-mentioned processes is very energy-intensive. In addition, the known processes for the decomposition of sodium carbonate to carbon dioxide and sodium hydroxide are carried out at very high temperatures and are therefore particularly energy-consuming.There is therefore still a need for improved solutions which at least partially overcome the above-mentioned disadvantages.SUMMARY OF THE INVENTIONIt is the object of the invention to provide an improved method belonging to the technical field mentioned at the beginning, with which the recovery of carbon dioxide from a carbon dioxide-containing gas, in particular air and / or exhaust gas, can be carried out at the lowest possible temperatures and at low cost.The solution of the invention is indicated by the features of the independent claims.The core of the invention consists in a method for the process for recovering carbon dioxide (CO 2) from a carbon dioxide-containing gas, in particular air and / or exhaust gas, comprising the steps: a) contacting the carbon dioxide-containing gas with an absorbent in the form of a solution of aqueous alkali metal hydroxide solution, in particular sodium hydroxide solution and / or potassium hydroxide solution, such that carbon dioxide is absorbed from the carbon dioxide-containing gas and alkali metal carbonate, in particular sodium carbonate (Na 2 CO 3) and / or potassium carbonate (K 2 CO 3), is formed in the absorbent; b) carrying out an ion exchange step, wherein the alkali carbonate formed in step a) is subjected to a cation exchange process such that the alkali ions of the alkali carbonate are bound to a cation exchanger; c) eluting the alkali ions bound to the cation exchanger in step b) with a solution comprising ammonia and / or at least one amine, wherein alkali liquor is recovered, which is preferably used for renewed absorption of carbon dioxide in step a)The method according to the invention represents an effective, cost-effective, lasting and robust method for separating carbon dioxide from air or exhaust gases. The present method is particularly suitable for the application known as "direct air capture" (DAC), in which ambient air is used as carbon dioxide source.According to the invention, the method can consume large amounts of energy required only at a relatively low temperature level. Heat at a level below 350° C. is available from solar heat and / or waste heat in a virtually unlimited amount and CO 2- is neutral and can be provided economically. However, the use of other energy sources, for example thermal energy from the conversion of photovoltaics, wind power or other energy sources, is likewise possible.The method according to the invention can be carried out particularly advantageously in the sun belt, along the equator, between the north and south turning circles and / or in deserts. At these locations, almost any desired amount of solar heat energy can be generated by solar radiation.According to the invention, the binding of carbon dioxide from the ambient air and / or from waste gas streams with slightly increased carbon dioxide concentration can be effected by the use of inexpensive alkali hydroxides, such as sodium hydroxide, which is available on the market and / or can be obtained on site from seawater using the brine of water desalting plants and an electrolysis process.Depending on the type of ions to be exchanged, a distinction is made between cation and anion exchangers. In the case of a cation exchanger, the active group is an anionic group, such as a sulfonic acid group or a carboxy group with dissociable cation. A distinction is made between weakly acidic cation exchangers, for example having carboxy groups, and strongly acidic cation exchangers, for example having sulfonic acid groups. In the case of an anion exchanger, the active group is a cationic group such as an ammonium group or amino group with dissociable anion. A distinction is made between strongly basic anion exchangers, for example with ammonium groups, and weakly basic anion exchangers, for example with amino groups.The ion exchanger can be present, for example, in the form of a column charged with particulate ion exchanger material. In the case of a cation exchanger, cation exchanger material is used, while anion exchanger material is used accordingly for an anion exchanger.In the present context, "aqueous alkali metal hydroxide solution" is understood to mean an aqueous alkali metal hydroxide solution, in particular an aqueous sodium hydroxide solution and / or an aqueous aqueous potassium hydroxide solution. Aqueous sodium hydroxide solution is also referred to as sodium hydroxide solution, while aqueous potassium hydroxide solution is also referred to as potassium hydroxide solution.In particular, the aqueous alkali solution used in the process according to the invention is sodium hydroxide solution, potassium hydroxide solution or a mixture thereof. In particular, the aqueous alkali solution is sodium hydroxide solution or aqueous sodium hydroxide solution.In particular, the aqueous alkali liquor is concentrated aqueous alkali liquor, saturated aqueous alkali liquor or supersaturated aqueous alkali liquor.Aqueous alkali solution having an alkali hydroxide concentration of at least 10 mol / l is also referred to as concentrated aqueous alkali solution, while saturated aqueous alkali solution has an alkali hydroxide concentration of about 32 mol / l at room temperature, and aqueous alkali solution having a higher concentration is referred to as supersaturated aqueous alkali solution. At different temperatures, the saturation concentration changes.The aqueous alkali metal hydroxide solution has, for example, an alkali metal hydroxide concentration of 1-32 mol / l, in particular 10-19 mol / l.In particular, step b) is performed at a time interval from step a), e.g. at an interval of at least one minute, at least one hour, at least one day, at least one week or at least one month; and / or step b) is performed at a location other than step a).In particular, the sodium carbonate obtained in step a) is temporarily stored, for example for at least one minute, at least one hour, at least one day, at least one week or at least one month, before being subjected to step b). The sodium carbonate can be stored, for example, in solid form and / or in the form of an aqueous solution.The carbon dioxide can be obtained, for example, by reacting the solution obtained after the ion exchange step from step b). The reaction can be carried out, for example, by thermal decomposition, in particular at a temperature below 350° C. In a preferred embodiment, the solution obtained after the ion exchange step is reacted beforehand to form an intermediate product, in particular alkali hydrogen carbonate, and the intermediate product is then reacted, in particular by thermal decomposition, to obtain the carbon dioxide.In a further advantageous embodiment, in step b) the alkali carbonate formed in step a) is additionally subjected to an anion exchange process in dissolved form, such that the carbonate ions of the alkali carbonate are bound to an anion exchanger. In this case, the carbon dioxide in step c) can be obtained by additionally eluting the carbonate ions bound in the anion exchanger.The carbon dioxide obtained by the process according to the invention is preferably used for the production of further chemical substances, in particular methanol, and / or is further processed to solid carbon.In particular, the carbon dioxide obtained from the process according to the invention is reacted with hydrogen to produce methanol and / or carbon (C). For the production of methane, a Sabataier reaction can be carried out, while carbon can be produced via a Bosch reaction.Preferably, in step a), the aqueous alkali solution, in particular concentrated aqueous alkali solution, is introduced, in particular dripped and / or sprayed, into an absorber in order to come into contact with the carbon dioxide-containing gas. This reaction produces alkali carbonate. The absorber is in particular a gas scrubber.In a preferred embodiment, during and / or after step a), the alkali carbonate is separated as a solid from the aqueous alkali liquor.The separation comprises in particular precipitation, in particular crystallization, of the alkali carbonate in the aqueous alkali solution, wherein the precipitation takes place in particular at ambient temperature, for example by exceeding the solubility product in the aqueous alkali solution. In this case, the alkali carbonate can precipitate out of the gas without further measures by exceeding the solubility product as a result of the CO 2- uptake.However, it is also possible to prevent precipitation in the absorber by precipitation taking place in a separate reaction volume outside the absorption volume. Precipitation can be induced by a temperature change, in particular a temperature increase, of the aqueous alkali solution. By increasing the activity of the alkali ions of the alkali liquor at higher temperatures, the solubility of the alkali carbonate in the absorption solution falls, so that precipitation of alkali carbonate occurs directly during heatingIn particular, a crystallization container is provided in the absorber circuit in which the alkali solution / alkali carbonate solution is heated.Preferably, the separation of the precipitated alkali carbonate comprises a sedimentation, filtration and / or centrifugation of the aqueous alkali liquor with the precipitated alkali carbonate.Depending on the humidity of the gas containing carbon dioxide, alkali liquor may give off or take up water. The higher the concentration of the alkali liquor, the more the tendency to absorb water from the gas phase. As a rule, it is desirable that as little water as possible is lost or accumulates in the absorber. For a given humidity and temperature of the gas containing the carbon dioxide, there is an optimum concentration of alkali liquor. Concentrations between 3 wt. % and 50 wt. % can be realized with this method; at concentrations of 28 wt. % and more, water loss under average conditions can be completely avoided.It should be noted that the absorption slows down at low alkali hydroxide concentrations and, with the same dimensions of the absorber, less carbon dioxide can be reacted per time. In addition, the solubility of alkali carbonate decreases with increasing concentration, promoting precipitation of alkali carbonate. Optimum operation of the plant is therefore effected with the highest possible concentration of alkali metal hydroxide. According to the invention, the alkali metal hydroxide content of the aqueous alkali metal hydroxide solution or washing solution can be regulated as a function of the gas humidity. The precipitated alkali metal carbonate can settle in continuous operation at the bottom of the absorber, where it can be drawn off and filtered and / or centrifuged.Alternatively or additionally, the absorber can also be filled with inert filling material, wherein the alkali metal hydroxide solution is sprayed over it, so that the wetted surface of the filling material serves as a contact surface for the absorption of the carbon dioxide by the alkali metal hydroxide solution. In this process variant, the alkali carbonate partially precipitates onto the contents, where it falls off after a certain time alone and / or can be dissolved by showering the contents with undersaturated alkali and / or water.This process variant generally requires a batchwise procedure. The pressure drop of the gas is somewhat higher than in the unfilled absorber plant. However, the energy consumption for pumping and distributing the alkali liquor is lower. Large droplets are completely sufficient here, which is advantageous in particular in the case of very highly concentrated alkali liquors. Aerosol formation can also be kept low in this embodiment. Combinations of the two embodiments are possible, wherein the alkali carbonate is continuously drawn off and the alkali carbonate is only dissolved back on the filling material from time to time.An advantage of this process is that the alkali metal carbonate formed during the absorption is preferably obtained in solid form. This means that it can be separated cleanly from the sodium hydroxide solution or the sodium hydroxide solution and can be used as product without problems or stored until further processing takes place. This is of interest in particular when the further processing is energy-intensive and the energy is to be used for this purpose from regenerative sources such as solar radiation and / or wind.In step b), the alkali carbonate obtained in step a) is converted by ion exchange on a cation exchanger. The alkali metal carbonate is present in particular in dissolved form, in particular in aqueous solution.During the ion dipping, the cation exchanger changes over to the alkali form, for example to the sodium form or the potassium form. If the cation exchanger is present in the ammonium form before step b), this can release ammonium ions during the ion exchange, which ions go into solution and form ammonium carbonate. "Alkali form" means that the dissociable cations of the ion exchanger are alkali ions, in particular sodium ions or potassium ions. Accordingly, the dissociable ions in the "ammonium form" are ammonium ions.In step b), therefore, in a preferred embodiment ammonium carbonate (NH 4)2 CO S is formed from the alkali metal carbonate.In step c), alkali solution can be released from the alkali form of the cation exchanger by feeding the cation exchanger with aqueous ammonia solution and / or a solution with at least one amine, wherein the cation exchanger in particular changes over again to the ammonium form. In step c), therefore, a solution comprising ammonia is preferably used.The amine used can advantageously be a methylamine, for example monomethylamine (MMA) and / or dimethylamine (DMA), and / or the amine used is an ethylamine, for example monoethylamine (MEA) and / or diethylamine (DEA).Alkylation of the ammonia greatly increases basicity and thus facilitates exchange for sodium on the strongly acidic cation exchanger.Trimethylamine and / or triethylamine are also suitable, although these exhibit increased basicity, although not as strong as those mentioned above. These may be combined with the amines mentioned above.It is also possible to use other amino derivatives such as longer-chain homologues of alkylamines, and mixed alkylamines. The advantage of higher basicity is opposed by the disadvantage of higher costs of the compounds and a limited long-term stability compared to ammonia, so that the economics have to be determined in the individual case, it being possible in particular for mixtures of ammonia with alkylamines to be of interest.Aromatic amines (aniline and homologs) would also be suitable in principle, but because of their toxicity or the human cancerogenity of the compounds, use is less future.The alkali liquor recovered in step c) may optionally be concentrated to a predefined concentration or to dryness. This can thus be concentrated and preferably used for renewed absorption of carbon dioxide in step a).In a particularly preferred embodiment of the method according to the invention, step c) is combined directly with the concentration. As a result, only small amounts of water have to be circulated for release.In this embodiment, the cation exchanger can be positioned, for example, in a gas space above the heated solution comprising ammonia and / or at least one amine or above the distillation bottoms. The ammonia-containing and / or amine-containing water vapor can then be condensed via (as in the case of a Soxhlet extractor) and / or directly into the cation exchanger, for example a bed of the cation exchanger.The condensation on the ion exchanger can be promoted by cooling devices such as cooled pipes and / or plates, etc.The condensate reacts with the cation exchanger, with ammonium ions being absorbed and at the same time the released, thin alkali metal hydroxide solution, for example NaOH, dripping off into the sump under the cation exchanger. The alkali metal solution in the bottom is concentrated continuously, while fresh ammonia solution in the resin condenses and releases alkali metal solution. In this way, highly concentrated alkali metal hydroxide solution can be produced directly and a virtually complete conversion of the cation exchanger can be achieved.Depending on the heat level available for heating and cooling, the boiling point can be adjusted by lowering the pressure in the extraction apparatus. Thus, the method can be carried out, for example, already by reducing the atmospheric pressure to half with a heating temperature of 80° C. instead of 100° C., in the case of a third with 70° C.According to an advantageous embodiment, therefore, in step c) of the process, the solution comprising ammonia and / or at least one amine is condensed from a heated bottom with an overlying gas space comprising the cation exchanger above and / or in the cation exchanger, wherein the alkali metal hydroxide solution liberated in this process drips off and concentrates in the bottom, wherein these process steps are carried out in particular under reduced pressure and / or the cation exchanger is cooled.The process according to the invention can be carried out in various variants. These different variants relate in particular to the route of the carbon dioxide separated from the alkali ions. Various exemplary embodiments are described below, wherein further and in particular combinations of these embodiments are conceivable. All embodiments are based in particular on alkali ions from the alkali carbonate being bound to a cation exchanger in order later to release alkali liquor therefrom with ammonia.In a first particularly preferred embodiment, in step b) the alkali metal carbonate formed during the absorption of carbon dioxide is contacted with a strongly acidic cation exchanger, which is preferably a cation exchanger present in the ammonium form.The cation exchanger is present, for example, in the form of a column charged with particulate cation exchanger material.In step b), the alkali metal carbonate formed during the absorption of carbon dioxide is preferably separated from the alkali metal hydroxide solution and dissolved in water, and the solution is brought into contact with the cation exchanger which is present in the ammonium form, with the result that ammonium carbonate (NH 4)2 CO 3 is formed from the alkali metal carbonate and the cation exchanger changes over into the sodium form.Specifically, in step c), alkali hydroxide is then recovered by eluting with ammonia.The ammonium carbonate formed is decomposed thermally, for example, in particular at a temperature >58° C., to ammonia, carbon dioxide and water. The carbon dioxide, and preferably also the ammonia, can be separated off and collected. The separation can be effected, for example, by steam distillation, in particular at elevated pressure, by extraction and / or by separation via selective membranes.However, in a further advantageous embodiment it is also possible to convert the solution obtained after the ion exchange step via an intermediate product, in particular alkali metal bicarbonate, to carbon dioxide.According to a particularly preferred embodiment, further alkali carbonate can be added to the ammonium carbonate formed in step b), so that preferably at least as many or more moles of alkali carbonate are present as ammonium carbonate in the solution, wherein the carbon dioxide formed during the decomposition of ammonium carbonate is reacted with the alkali carbonate to form alkali hydrogen carbonate. The carbon dioxide formed during the decomposition of ammonium carbonate can thereby be taken up by the alkali carbonate even before it goes into the gas phase, with formation of sodium hydrogen carbonate: (NH 4)2 CO 3+ Na 2 CO 3 >2 NaHCO 3+ 2 NH 3 ≅Ammonia is preferably evaporated from the solution, below the decomposition temperature of alkali metal bicarbonate in water, in particular at temperatures of 50-100° C., in particular 70-90° C. Ammonia is preferably condensed from the gas phase and collected, ammonia being condensed and collected together with steam from the gas phase in particular.In order to obtain carbon dioxide, in particular (i) the alkali metal bicarbonate formed in the solution is then directly thermally decomposed in the solution to carbon dioxide and alkali metal carbonate, and / or (ii) the alkali metal bicarbonate is crystallized out and decomposed in dry form to carbon dioxide and alkali metal carbonate. The thermal conversion to carbon dioxide and alkali metal carbonate can take place immediately after the formation of the alkali metal hydrogen carbonate or at a time interval.The decomposition takes place in particular by thermal energy which is and / or has been obtained by solar heat and / or waste heat.The alkali metal carbonate obtained in the decomposition is preferably recirculated in step b) and, in particular together with fresh alkali metal carbonate from step a), used again in the cation exchange step. This procedure uses all intermediate products in a simple manner and thus evaluates the overall process to a closed cycle process.In a second particularly preferred embodiment, in step b) the alkali metal carbonate is converted in the ion exchange step into ammonium hydroxide (NH 4 OH).For this purpose, in particular in step b), the alkali metal carbonate formed during the absorption of carbon dioxide is brought into contact both with a strongly acidic cation exchanger and with a weakly basic anion exchanger, this preferably being a cation exchanger present in the ammonium form and an anion exchanger present in the hydroxide form.In particular, in step b), the alkali ions of the alkali carbonate are bound to the cation exchanger and the carbonate ions of the alkali carbonate are bound to the anion exchanger. In other words, the strongly acidic cation exchanger turns to the sodium form and the weakly basic anion exchanger turns to the carbonate form.The two ion exchangers are preferably in the form of an ion exchanger mixed bed, in particular in the form of a column fed with particulate cation exchanger material and particulate anion exchanger material.After step b), the cation and anion exchangers are preferably separated. This can be done by a mechanical separation, for example by sieving and / or flotation. Sieving is possible with different particle sizes of the ion exchangers, while flotation is possible with different densities of the ion exchangers.In particular, in step c), the alkali ions bound to the cation exchanger are then eluted with a solution containing ammonia and / or at least one amine, wherein the cation exchanger preferably changes to the ammonium form; and the carbonate ions bound to the anion exchanger in the form of carbon dioxide are eluted with water, in particular water vapor, wherein the anion exchanger preferably changes to the hydroxide form. The carbonate ions are preferably eluted with water in the form of steam, for example at a temperature of 100-130° C. and in particular under reduced pressure.The cation exchanger and the anion exchanger can then be recombined and used again in step b).According to a third particularly preferred embodiment, in step b) the alkali metal carbonate formed during the absorption of carbon dioxide is contacted with a zeolite, in particular zeolite in protonated form, as cation exchanger.The zeolite can be artificial and / or natural zeolite. Zeolites are crystalline aluminosilicates. In particular, the zeolite in question is a zeolite with a fraction of clinoptinolite or the zeolite consists of clinoptinolite. These zeolites have been found to be particularly preferred for ion exchange with ammonium ions.Zeolites can also be used as strongly acidic cation exchangers in the above-described particularly preferred first and second embodiments. This is achieved, for example, as a partial or complete replacement for the abovementioned cation exchangers having sulfonic acid groups. Zeolites are particularly advantageous in the present context since they exhibit a higher temperature resistance compared with commercially available cation exchange agents based on polymer.It is particularly preferred that zeolite present in the alkali form, especially in the sodium form, is converted before step b) into zeolite in protonated form, especially by (i) liberation of alkali metal hydroxide, especially sodium hydroxide, from the zeolite in alkali form with ammonia to form the zeolite in ammonium form and (ii) subsequent removal of ammonia from the zeolite in ammonium form, especially by evaporation, to obtain the zeolite in protonated form. The conversion of the zeolite into the protonated form takes place in particular thermally at a temperature of 200-300° C. and preferably under reduced pressure.Particularly preferably, in step b), alkali metal bicarbonate is prepared from the alkali metal carbonate with zeolite in protonated form and the alkali metal bicarbonate is subsequently thermally decomposed to carbon dioxide and alkali metal carbonate, the zeolite being converted into the alkali metal form. Preferably, the zeolite in the alkali form is subsequently recycled in step b), wherein in particular the zeolite present in the alkali form is converted into zeolite in protonated form before step b).In a particularly advantageous embodiment, an excess of zeolite in protonated form is used in step b), such that the alkali metal carbonate is converted directly to carbon dioxide and zeolite in alkali metal form. As a result, the formation of alkali hydrogen carbonate can be reduced or avoided, as a result of which the process is simplified and thermal decomposition of alkali hydrogen carbonate is superfluous.A further aspect of the present invention relates to a device, in particular a carbon dioxide sorption unit or absorption unit, for carrying out a method as described above. In particular, the device is designed to obtain at least 200,000 tons of carbon dioxide per year, preferably at least 500,000 tons of carbon dioxide per year.The apparatus preferably comprises an absorber which is designed to carry out step a), an ion exchange unit which is designed to carry out step b) of the method according to the invention, and / or an elution unit which is designed to carry out step c) of the method according to the invention.A particularly advantageous elution unit has (i) a heatable sump receptacle for receiving the solution containing ammonia and / or at least one amine, (ii) a device for positioning the cation exchanger in a gas space above the sump container and (iii) optionally a cooling device for cooling the cation exchanger, and / or wherein (iv) the elution unit is designed such that it can be operated under reduced pressure. The elution unit is preferably designed to carry out the method according to claim 25.Such an elution unit can also be used independently of the other components of the above-described device and / or for methods other than the methods according to the invention.In particular, the device is connected to a solar thermal heat source and / or waste heat and / or the device comprises a solar thermal collector for generating heat which is used in the method according to the invention.Preferably, the apparatus further comprises a methanol synthesis unit configured to produce methanol from the carbon dioxide and hydrogen separated by the method according to the invention.Preferably, the hydrogen is generated by an electrolysis unit operated with solar energy.Further preferably, the apparatus further comprises a carbonization unit for producing carbon (C) from the deposited carbon dioxide and hydrogen, in particular the carbonization unit is a Bosch reaction unit configured for carrying out a Bosch reaction.According to a particularly preferred embodiment, the device is part of a plant, in particular a power plant.A further aspect of the present invention therefore relates to a plant, in particular a power plant, for reducing the carbon dioxide content in the atmospheric air, in particular for reducing the carbon dioxide content in the atmospheric air and proportionally in water, preferably seawater.From the following detailed description and the totality of the patent claims, further advantageous embodiments and combinations of features of the invention result.Brief Description of the DrawingsThe drawings used to explain the exemplary embodiment show: FIG. 1 shows a schematic flow diagram of a method according to the invention, in which carbon dioxide is absorbed with sodium hydroxide solution and converted to sodium carbonate. The sodium carbonate is then subjected to an ion exchange step and a subsequent decomposition step. The ion exchanger is regenerated in an elution step; FIG. 2 shows a variant of the method from FIG. 1, in which additional sodium carbonate is used, so that sodium bicarbonate is formed as intermediate product, which can be thermally decomposed; FIG. 3 shows a further variant of the process from FIG. 1, in which the sodium carbonate is brought into contact both with a strongly acidic cation exchanger and with a weakly basic anion exchanger. FIG. 4 shows an elution unit for carrying out method step c). From below, a bottom is heated with ammonia solution; cation exchangers are located in the gas phase. By cooling directly above the resin, ammonia solution in the resin condenses and ammonium ions in the condensate are exchanged for sodium ions, sodium hydroxide solution being formed, which drips into the bottom and concentrates there. In principle, identical parts are provided with identical reference numerals in the figures.WAYS OF CARRYING OUT THE INVENTIONFIG. 1 shows a schematic flow diagram of a method according to the invention. In a first step 1, a carbon dioxide-containing gas is contacted with an absorbent in the form of a solution of concentrated aqueous sodium hydroxide solution (NaOH), wherein the carbon dioxide is absorbed from the carbon dioxide-containing gas and sodium carbonate (Na 2 CO 3) is formed in the absorbent.The sodium carbonate is then subjected to a cation exchange step 2. The cation exchanger is present, for example, as strongly acidic particulate cation exchanger material in the ammonium form, which is packed in a column. The sodium carbonate from step 1 dissolved in water is passed through the column, the cation exchanger changing to the sodium form and the ammonium ions being released from the cation exchanger into the solution, so that ammonium carbonate [(NH4) 2 CO 3)] is formed there.The cation exchanger is then eluted with ammonia in elution step 3, the cation exchanger changing to the ammonium form and sodium hydroxide solution being recovered. The sodium hydroxide solution can then be recycled into the absorption step 1.The ammonium carbonate formed from step 2 is thermally decomposed in decomposition step 4 at a temperature >58° C. to form ammonia (NH 3), carbon dioxide (CO 2) and water (H 2 O). The carbon dioxide and the ammonia are separated off separately, for example by steam distillation at elevated pressure, and are collected. The ammonia can be reused for the elution step 3.FIG. 2 shows a variant of the process from FIG. 1, in which case the ammonium carbonate formed is treated in the decomposition step 4' with an equimolar amount of sodium carbonate. This has the result that the carbon dioxide formed during the decomposition of ammonium carbonate is reacted with the sodium carbonate to give sodium bicarbonate (NaHCO 3). The ammonia which likewise forms in this process is evaporated from the solution, below the decomposition temperature of the sodium bicarbonate in water, at a temperature of, for example, 70-90° C., and condensed from the gas phase together with steam and collected. The ammonia can thus be recovered and reused.To obtain carbon dioxide, the sodium hydrogen carbonate formed in the solution is thermally decomposed directly in the solution in a further decomposition step 5 to carbon dioxide and sodium carbonate. Alternatively, the sodium hydrogen carbonate can be crystallized out and decomposed in dry form to carbon dioxide and sodium carbonate. The sodium carbonate can be recovered and reused.FIG. 3 shows a further variant of the method from FIG. 1. In this case, in the ion exchange step 2', the sodium carbonate formed during the absorption of carbon dioxide is brought into contact both with a strongly acidic cation exchanger (e.g. based on a material having sulfonic acid groups) and with a weakly basic anion exchanger (e.g. based on a material having amino groups). The cation exchanger is present in the ammonium form, while the anion exchanger is present in the hydroxide form. The two ion exchangers are in the form of a column charged with particulate cation exchange material and particulate anion exchange material. The two ion exchange materials form a mixed bed.The sodium ions of sodium carbonate are bound to the cation exchanger in step 2' and the carbonate ions of sodium carbonate are bound to the anion exchanger. At the same time, ammonium ions and hydroxide ions go into solution, so that ammonium hydroxide (NH 4 OH) is formed in the solution.After step 2', the cation and anion exchangers are separated, for example by sieving and / or flotation.In the elution step 3', the sodium ions bound to the cation exchanger are then eluted with a solution containing ammonia and / or at least one amine, the cation exchanger changing over to the ammonium form. The carbonate ions bound to the anion exchanger are eluted with water vapor in the form of carbon dioxide, the anion exchanger changing to the hydroxide form. The carbon dioxide can thereby be recovered. The sodium hydroxide also formed can be recovered and reused.The above-described exemplary embodiments are merely to be understood as illustrative examples which can be modified as desired within the scope of the invention.Thus, for example, a zeolite can be used instead of the cation exchangers described. The zeolites mentioned in the general description are preferably used here.It is likewise possible to carry out the processes shown in the examples on a potassium basis and / or to use amines instead of or in addition to ammonia. Here, it is possible with advantage to use, in particular, the methylamines monomethylamine (MMA), diemethylamine (DMA) and the ethylamines monoethylamine (MEA), diethylamine (DEA). Alkylation of the ammonia greatly increases basicity and thus facilitates exchange for sodium on the strongly acidic cation exchanger. Trimethylamine and triethylamine are also suitable, although these show an increased basicity, although not as strong as those mentioned above. It is also possible to use other amino derivatives such as longer-chain homologues of alkylamines, and mixed alkylamines. The advantage of higher basicity is opposed by the disadvantage of higher costs of the compounds and a limited long-term stability compared to ammonia, so that the economics have to be determined in the individual case, it being possible in particular for mixtures of ammonia with alkylamines to be of interest.Aromatic amines (aniline and homologs) would also be suitable in principle; owing to their toxicity or the human cancerogenity of the compounds, use is not future.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedWO 2022 / 192408 A2

[0004] US 2007 / 0187247 A1

[0009]

Claims

A process for recovering carbon dioxide (CO 2) from a carbon dioxide-containing gas, in particular air and / or offgas, comprising the steps of: a) contacting the carbon dioxide-containing gas with an absorbent in the form of a solution of aqueous alkali metal hydroxide solution, in particular sodium hydroxide solution and / or potassium hydroxide solution, such that carbon dioxide is absorbed from the carbon dioxide-containing gas and alkali metal carbonate, in particular sodium carbonate (Na 2 COs) and / or potassium carbonate (K 2 CO 3), is formed in the absorbent; b) carrying out an ion exchange step, wherein the alkali metal carbonate formed in step a) is subjected in dissolved form to a cation exchange process, such that the alkali metal ions of the alkali metal carbonate are bound to a cation exchanger; c) Elution of the alkali ions bound to the cation exchanger in step b) with a solution containing ammonia and / or at least one amine, wherein alkali liquor is recovered, which is preferably used for renewed absorption of carbon dioxide in step a).The method according to claim 1, wherein during and / or after step a) the alkali carbonate is separated as a solid from the solution of aqueous alkali liquor, wherein the separation in particular comprises sedimentation, filtration and / or centrifugation of the aqueous alkali liquor with the precipitated alkali carbonate.The method according to claim 2, wherein the separation comprises a precipitation, in particular a crystallization, of the alkali carbonate in the aqueous alkali liquor, wherein the precipitation takes place in particular at ambient temperature, for example by exceeding the solubility product in the aqueous alkali liquor.The method of claim 3, wherein the precipitation occurs in a separate reaction volume outside an absorption range.Method according to one of claims 3 - 4, wherein the precipitation is induced by a temperature change, in particular a temperature increase, of the aqueous alkali liquor.Process according to one of Claims 1-5, sodium hydroxide solution being used as alkali metal hydroxide solution.Process according to any of Claims 1-6, wherein in step b) the alkali metal carbonate formed during the absorption of carbon dioxide is contacted with a strongly acidic cation exchanger, which is preferably a cation exchanger present in the ammonium form.The process according to any of claims 1-7, wherein in step b) ammonium carbonate [(NH 4)2 CO 3] is formed from the alkali carbonate.The process according to claim 8, wherein in step b) the alkali carbonate formed during the absorption of carbon dioxide is separated from the solution and dissolved in water, the solution is brought into contact with the cation exchanger which is present in the ammonium form, such that ammonium carbonate is formed from the alkali carbonate and the cation exchanger changes over to the sodium form.The process of claim 9, wherein in step c) alkali hydroxide is recovered by eluting with ammonia.The process according to any of claims 8 to 10, wherein, in order to obtain carbon dioxide, the ammonium carbonate formed is thermally decomposed, in particular at a temperature > 58°C, to ammonia, carbon dioxide and water, wherein the carbon dioxide, and preferably also the ammonia, is or are separated off and collected.The process according to any of claims 8 to 10, wherein further alkali carbonate is added to the ammonium carbonate formed, such that at least as many or more moles of alkali carbonate are present as ammonium carbonate, such that the carbon dioxide formed during the decomposition of ammonium carbonate is reacted with the alkali carbonate to form alkali hydrogen carbonate.The process according to claim 12, wherein ammonia is evaporated from the solution, below the decomposition temperature of alkali hydrogen carbonate in water, in particular at temperatures of 50-100°C, in particular 70-90°C, and wherein ammonia is preferably condensed from the gas phase and collected, wherein in particular ammonia is condensed together with steam from the gas phase and collected.The process according to any of claims 12-13, wherein, for the recovery of carbon dioxide, (i) the alkali metal bicarbonate formed in the solution is thermally decomposed directly in the solution to carbon dioxide and alkali metal carbonate, and / or (ii) the alkali metal bicarbonate is crystallized out and decomposed in dry form to carbon dioxide and alkali metal carbonate.The process according to claim 14, wherein the alkali carbonate obtained by decomposition is recycled in step b) and reused in the cation exchange step.Process according to any of Claims 1-15, wherein in step b) the alkali metal carbonate formed during the absorption of carbon dioxide is brought into contact both with a strongly acidic cation exchanger and with a weakly basic anion exchanger, preferably a cation exchanger present in the ammonium form and an anion exchanger present in the hydroxide form.The method according to claim 16, wherein in step b) the alkali ions of the alkali carbonate are bound to the cation exchanger and the carbonate ions of the alkali carbonate are bound to the anion exchanger and after step b) the cation and anion exchangers are separated, for example by sieving and / or flotation.The method according to any one of claims 16 - 17, wherein in step b) the dissolved alkali carbonate is converted in the ion exchange step into ammonium hydroxide NH 4 OH.The method according to any one of claims 16 to 18, wherein in step c) the alkali ions bound on the cation exchanger are eluted with a solution containing ammonia and / or at least one amine, wherein the cation exchanger preferably transitions into the ammonium form; and the carbonate ions bound on the anion exchanger are eluted in the form of carbon dioxide with water, in particular water vapor, wherein the anion exchanger preferably transitions into the hydroxide form.The process according to any of claims 17-19, wherein the cation exchanger and the anion exchanger are subsequently recombined and reused in step b).Process according to any of Claims 1-20, wherein in step b) the alkali metal carbonate formed during the absorption of carbon dioxide is contacted with a zeolite, in particular zeolite in protonated form, as cation exchanger.The process according to claim 21, wherein prior to step b) zeolite present in the alkali form is converted into zeolite in protonated form, in particular by (i) liberation of alkali metal hydroxide, in particular sodium hydroxide, from the zeolite in the alkali form with ammonia to form the zeolite in ammonium form and (ii) subsequent removal of ammonia from the zeolite in ammonium form, in particular by evaporation, to obtain the zeolite in protonated form.The process according to any of claims 21-22, wherein in step b) alkali hydrogen carbonate is prepared from the alkali carbonate with zeolite in protonated form and subsequently the alkali hydrogen carbonate is thermally decomposed to carbon dioxide and alkali carbonate, wherein the zeolite is converted into the alkali form and is preferably recycled in step b), wherein in particular the zeolite present in the alkali form is converted into zeolite in protonated form before step b).The process according to any of claims 21-23, wherein in step b) an excess of zeolite in protonated form is used, such that the alkali carbonate is directly converted to carbon dioxide and zeolite in sodium form.The process according to any of claims 1-24, wherein in step c) of the process the solution comprising ammonia and / or at least one amine is condensed from a heated bottom with an overlying gas space comprising the cation exchanger above and / or in the cation exchanger, wherein the alkali metal hydroxide solution released in the process drips off and concentrates in the bottom, wherein these process steps are carried out in particular under reduced pressure and / or the cation exchanger is cooled.Device, in particular a carbon dioxide sorption unit or absorption unit, for carrying out a method according to one of Claims 1 to 24, wherein the device preferably has an absorber which is designed to carry out step a), an ion exchange unit which is designed to carry out step b) of the method and / or an elution unit which is designed to carry out step c) of the method.The device according to claim 26, wherein the elution unit is designed to carry out the method according to claim 25, wherein the elution unit preferably comprises (i) a heatable sump receptacle for receiving the solution containing ammonia and / or at least one amine, (ii) a device for positioning the cation exchanger in a gas space above the sump container and (iii) optionally a cooling device for cooling the cation exchanger, and / or wherein preferably (iv) the elution unit is designed such that it can be operated under reduced pressure.

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