Abstract
In an embodiment a method includes loading an absorbent with a carbon dioxide in a first device, wherein a hydrogen carbonate-containing solution is formed, feeding the hydrogen carbonate-containing solution from the first device into an electrolysis unit, electrolyzing the hydrogen carbonate-containing solution in the electrolysis unit to release carbon dioxide, wherein a carbonate-rich solution is formed in the electrolysis unit, forming oxygen, hydrogen and carbon dioxide in the electrolysis unit, and recycling the carbonate-rich solution from the electrolysis unit to the first device.
Claims
1-14. (canceled)
15. A method for absorbing carbon dioxide from an air stream, the method comprising: loading an absorbent with the carbon dioxide in a first device, wherein a hydrogen carbonate-containing solution is formed; feeding the hydrogen carbonate-containing solution from the first device into an electrolysis unit; electrolyzing the hydrogen carbonate-containing solution in the electrolysis unit to release carbon dioxide, wherein a carbonate-rich solution is formed in the electrolysis unit; forming oxygen, hydrogen and carbon dioxide in the electrolysis unit; and recycling the carbonate-rich solution from the electrolysis unit to the first device.
16. The method according to claim 15, wherein the electrolysis unit comprises at least a first chamber and a second chamber, wherein the hydrogen carbonate-containing solution is fed to at least the first chamber, wherein, in the first chamber, hydrogen carbonate is consumed and at least the carbon dioxide is formed, and wherein, in the second chamber, the hydrogen carbonate is converted to carbonate to form the carbonate-rich solution.
17. The method according to claim 15, wherein the absorbent comprises an alkali carbonate component which is at least partially converted to hydrogen carbonate in the first device, wherein the hydrogen carbonate is precipitated after the conversion, wherein the precipitated hydrogen carbonate is separated, and wherein the precipitated hydrogen carbonate is used to form the hydrogen carbonate-containing solution.
18. The method according to claim 17, wherein the carbonate-rich solution has a higher carbonate content and a lower hydrogen carbonate content than the hydrogen carbonate-containing solution.
19. The method according to claim 17, wherein the precipitated hydrogen carbonate has a purity of 99?1%.
20. The method according to claim 17, wherein the absorbent is fed to the electrolysis unit after separation of the hydrogen carbonate.
21. The method according to claim 15, wherein the electrolysis unit is a two-chamber electrolysis unit with an anode chamber and a cathode chamber and the hydrogen carbonate-containing solution is fed to the anode chamber, and wherein the anode chamber and the cathode chamber are separated by a cation-selective membrane.
22. The method according to claim 15, wherein the electrolysis unit is an at least three-chamber electrolysis unit with an anode chamber, a cathode chamber, at least one middle bridge chamber, a cation-selective membrane adjacent to the cathode chamber and a bipolar membrane adjacent to the anode chamber, and wherein the hydrogen carbonate-containing solution is fed to at least one of the middle bridge chambers.
23. The method according to claim 22, wherein the hydrogen carbonate-containing solution is a pure hydrogen carbonate solution.
24. The method according to claim 15, wherein the air stream comprises a carbon dioxide concentration of greater than or equal to 100 ppm and less than or equal to 650 ppm.
25. A device for absorbing carbon dioxide from an air stream, the device comprising: a first device configured to load an absorbent with the carbon dioxide and to form a hydrogen carbonate-containing solution; and an electrolysis unit configured to electrolyze the hydrogen carbonate-containing solution from the first device and to release carbon dioxide, wherein a carbonate-rich solution is formed in the electrolysis unit and wherein the device is configured to recycle the carbonate-rich solution from the electrolysis unit to the first device.
26. The device according to claim 25, wherein the electrolysis unit is a two-chamber electrolysis unit with an anode chamber and a cathode chamber, wherein the hydrogen carbonate-containing solution is feedable to the anode chamber, and wherein the anode chamber and the cathode chamber are separated by a cation-selective membrane.
27. The device according to claim 25, wherein the electrolysis unit is an at least three-chamber electrolysis unit with an anode chamber, a cathode chamber, at least one middle bridge chamber, a cation-selective membrane adjacent to the cathode chamber, and a bipolar membrane adjacent to the anode chamber, and wherein the hydrogen carbonate-containing solution is feedable to at least one of the middle bridge chambers.
28. A method for absorbing carbon dioxide from an air stream, the method comprising: contacting the air stream with a carbon dioxide absorbent, wherein the carbon dioxide absorbent comprises at least: a) water in a proportion of greater than or equal to 2 wt % and less than or equal to 93 wt %; b) polyethylene glycols or polyols with a molecular weight of less than or equal to 1000 g/mol in a proportion of greater than or equal to 2 wt % and less than or equal to 93 wt %; and c) carbon dioxide absorbing agent in a proportion of greater than or equal to 5 wt % and less than or equal to 60 wt %, wherein the carbon dioxide absorbing agent comprises inorganic carbonate.
29. The method according to claim 28, wherein the carbon dioxide absorbing agent is inorganic carbonate.
30. The method according to claim 28, wherein the carbon dioxide absorbing agent is a mixture of an inorganic carbonate and at least one component selected from the group consisting of amines, polyethylene glycol amines, diaminopolyethylene glycols, carboxylic acid derivatives of polyethylene glycol amines, polyethylene imines, amine-containing sugar derivatives and amino acids.
31. The method according to claim 28, wherein the carbon dioxide absorbing agent is selected from the group consisting of potassium carbonate, sodium carbonate and mixtures of these components with a weight proportion of greater than or equal to 10 wt % and less than or equal to 50 wt % based on a total weight of the carbon dioxide absorbent, and wherein the carbon dioxide absorbent comprises an inorganic, organic or enzymatic promoter to accelerate a CO.sub.2 absorption rate.
32. The method according to claim 28, further comprising: subjecting the carbon dioxide absorbent, after contact with the air stream, to electrolysis, wherein the electrolysis is at least a three-chamber electrolysis with anode chamber, cathode chamber and middle bridge chamber with a bipolar membrane adjacent to the anode chamber, and wherein a carbon dioxide-loaded absorbent is fed at least into the middle bridge chamber.
33. The method according to claim 28, further comprising subjecting the carbon dioxide absorbent to electrolysis after contact with the air stream, wherein the electrolysis is a two-chamber electrolysis, and wherein a carbon dioxide-loaded absorbent is fed into an anode chamber.
34. Carbon dioxide absorbent for absorbing carbon dioxide from an air stream, the carbon dioxide absorbent comprising: a) water in a proportion of greater than or equal to 2 wt % and less than or equal to 93 wt %; b) polyethylene glycols or polyols with a molecular weight of less than or equal to 1000 g/mol in a proportion of greater than or equal to 2 wt % and less than or equal to 93 wt %; and c) carbon dioxide absorbing agent in a proportion of greater than or equal to 5 wt % and less than or equal to 60 wt %, wherein the carbon dioxide absorbing agent comprises inorganic carbonate.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0055] Further advantages and advantageous embodiments of the subject-matter according to the invention are illustrated by the drawings and explained in the following description. It should be noted that the drawings are descriptive only and are not intended to limit the invention.
[0056] It shows the:
[0057] FIG. 1 a schematic setup of a test apparatus for determining carbon dioxide uptake in carbon dioxide absorbents according to the invention;
[0058] FIG. 2 the carbon dioxide absorption and desorption isotherms of a carbon dioxide absorbent according to the invention with TEG and DGA;
[0059] FIG. 3 the determination of the loading limit of a TEG/DGA/water system;
[0060] FIG. 4 the carbon dioxide absorption and desorption isotherms of a carbon dioxide absorbent according to the invention with PEI/TEG/water system;
[0061] FIG. 5 the carbon dioxide absorption and desorption isotherms of a carbon dioxide absorbent according to the invention with meglumine/PEG200/water system;
[0062] FIG. 6 the influence of the solubility of potassium carbonate as a function of the TEG content;
[0063] FIG. 7 the carbon dioxide absorption and desorption isotherms of a carbon dioxide absorbent according to the invention with K.sub.2CO.sub.3/TEG/water system;
[0064] FIG. 8 a simplified reaction scheme for incorporation of carbon dioxide into carbon dioxide absorbents according to the invention;
[0065] FIG. 9 a method variant of an industrial-scale embodiment for using a carbon dioxide absorbent according to the invention;
[0066] FIG. 10 a further method variant of an industrial-scale embodiment for using a carbon dioxide absorbent according to the invention;
[0067] FIG. 11 a further method variant of an industrial-scale embodiment for using a carbon dioxide absorbent according to the invention;
[0068] FIG. 12 a partial method step according to the invention in the form of a three-chamber electrolysis of an air stream charged with carbon dioxide;
[0069] FIG. 13 a partial method step according to the invention in the form of a two-chamber electrolysis of an air stream charged with carbon dioxide with a nickel hydroxide anode;
[0070] FIG. 14 the current profile and the carbon dioxide evolution of a hydrogen carbonate-based absorbent in a three-chamber electrolysis system;
[0071] FIG. 15 the current profile and carbon dioxide evolution of an amino acid-based absorbent in a three-chamber electrolysis;
[0072] FIG. 16 the combination of three-chamber electrolysis with precipitation of KHCO.sub.3 after CO.sub.2 absorption.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0073] FIG. 1 shows an apparatus for detecting CO.sub.2 absorption and determining the maximum loading capacity of the absorbent at a CO.sub.2 concentration of 400 ppm. The setup allows a qualitative assessment of the possible CO.sub.2 absorption rates. A CO.sub.2-nitrogen mixture is brought under atmospheric conditions in contact with the absorbent solution 8 in a circular process via a frit. The time-varying CO.sub.2 concentration is measured by a CO.sub.2 analyzer 5 in the ppm range and recorded 7. A time-constant flow of 50 sccm per minute is maintained by pump 2 during the measurement. The tempering of the absorbing solution is carried out by a thermostat 9. The initial concentrations of 500 ppm and 0 ppm CO.sub.2 are set by the flow controller 1. The rate of CO.sub.2 absorption and setting of the equilibrium partial pressure is essentially given by the carbamate formation reactions when amines are added, for example:
CO.sub.2+RNH.sub.2.Math.RN.sup.+H.sub.2COO.sup.?(1)
RN.sup.+H.sub.2COO.sup.?+RNH.sub.2.Math.RNHCOO.sup.?+RN.sup.+H.sub.3 (2)
[0074] With carbonate addition, reaction 4 determines CO.sub.2 absorption kinetics and equilibrium setting:
K.sub.2CO.sub.2+H.sub.2O.Math.2K.sup.++HCO.sub.3.sup.?+OH.sup.?(3)
CO.sub.2+OH.sup.?.Math.HCO.sub.3.sup.?(4)
[0075] If the amine is used as a catalyst to accelerate CO.sub.2 uptake into a PEG/H.sub.2O/K.sub.2CO.sub.3 solution, the equilibrium of carbamate hydrolysis (5) should be considered in addition to the system of equations (1-4):
RNHCOO.sup.?H.sub.2O.Math.RNH.sub.2+HCO.sub.3.sup.?(5)
[0076] The amine used in reaction 1 is recovered to form hydrogen carbonate.
[0077] FIG. 2 shows the carbon dioxide absorption and desorption isotherms of a carbon dioxide absorbent according to the invention with TEG as component b) and DGA as component c). The composition of the carbon dioxide absorbent was 4.8 wt % DGA, 47.6 wt % TEG, and 47.6 wt % water. The pH was 11.9 and the measurement was performed at 20? C. and a relative humidity of 80%. Starting from a CO.sub.2 concentration of 400 ppm in N.sub.2, CO.sub.2 absorption into a freshly prepared solution occurs up to a CO.sub.2 concentration of 32 ppm within the observation time window of 642 s. Starting from a CO.sub.2 concentration of 0 ppm in a N.sub.2 atmosphere, a comparable CO.sub.2 concentration equilibration between the liquid and gas phases occurs by CO.sub.2 desorption. Comparable final CO.sub.2 partial pressures, starting from 400 ppm or 0 ppm, show the adjustment of the CO.sub.2 absorption/desorption equilibrium under the given experimental conditions.
[0078] FIG. 3 shows the determination of the loading limit of the DGA/TEG/water composition at different carbon dioxide loadings. To quantitatively assess the CO.sub.2 uptake capacity, the measurement method was carried out for different CO.sub.2 loadings of the solution. For rapid loading, a 14 vol % CO.sub.2 stream was introduced between each measurement. CO.sub.2 absorption leads to a decrease in pH, which is proportional to the amount of CO.sub.2 absorbed. FIG. 3 shows the CO.sub.2 concentration in the gas phase that will be set in contact with the liquid surface as a function of pH for 4.8 wt % DGA in an each 47.6 wt % TEG/H.sub.2O solution. The measurement results prove that in a pH interval of 11.8 to 9.8, corresponding to a loading of 0.2 mol CO.sub.2 per mol absorbent, the CO.sub.2 uptake below 400 ppm is possible. The achievable loading is sufficient for the application of the polyethylene glycol amines and the substances listed above in a technical process for CO.sub.2 absorption.
[0079] FIG. 4 shows the carbon dioxide absorption and desorption isotherms of a PEI/TEG/water system. Polyethylene imines are completely soluble in water or polyethylene glycol/water mixtures up to a molecular weight of M.sub.w?10.sup.5 up to 60 wt %. Both linear and branched polyethylene amines are suitable for CO.sub.2 absorption. Starting with a CO.sub.2 concentration of 450 ppm in N.sub.2, a concentration of about 70 ppm is obtained at equilibrium on a 7 wt % polyethyleneimine solution. This is confirmed by CO.sub.2 enrichment in an initially pure nitrogen atmosphere. Experiments with CO.sub.2 loaded solution show CO.sub.2 absorptions below 400 ppm in a pH interval of 11.9 to 10. This corresponds to a CO.sub.2 loading of 0.2 based on the number of nitrogen atoms in the molecule.
[0080] FIG. 5 shows the carbon dioxide absorption and desorption isotherms of a meglumine/PEG200/water system with carbon dioxide. In general, sugar derivatives dissolved in a PEG/H.sub.2O solution, such as N-methyl-D-glucamine (meglumine) or N-ethyl-D-glucamine (eglumine), which exhibit negligible partial pressure in solution, are suitable for CO.sub.2 absorption. Starting from an initial CO.sub.2 concentration of 500 ppm in N.sub.2, a concentration of about 79 ppm is obtained on a 45.6 wt % PEG 200 solution containing 8.8 wt % polyethyleneimine within the observation period of about 21 minutes. Experiments with CO.sub.2 loaded solution showed CO.sub.2 absorptions below 400 ppm in a pH interval of 11.1 to 10.1 for the ternary composition. When PEG is replaced by MEG, CO.sub.2 adsorption occurs in a pH interval of 11.3-10.0. This corresponds to a CO.sub.2 loading of 0.16 (PEG 200) and 0.18 (MEG) based on the number of nitrogen atoms in the molecule. Depending on the mass fraction used for PEG of 1 to 0, for example, additions of sugar derivatives of 1 wt % to 60 wt % are possible.
[0081] FIG. 6 shows the influence of the solubility of potassium carbonate as a function of the TEG content. In a carbonate-containing PEG/water solution, CO.sub.2 is absorbed by the formation of hydrogen carbonate. In this case, PEG not only serves to adjust the water vapor partial pressure according to the ambient conditions, but also decisively lowers the solubility of the hydrogen carbonate, so that at atmospheric CO.sub.2 concentrations below 400 ppm, there is a steady uptake of CO.sub.2 and precipitation as hydrogen carbonate or a compound containing hydrogen carbonate. In principle, sodium and potassium carbonates are suitable. When adding the carbonates to a PEG/water mixture, it should be noted that the solubility changes as a function of the binary composition of the solvent. FIG. 6 shows the solubility of K.sub.2CO.sub.3 in a triethylene glycol/water solution as a function of the binary composition of the solvent.
[0082] FIG. 7 shows the carbon dioxide absorption and desorption isotherms of a K.sub.2CO.sub.3/TEG/water system with carbon dioxide. Using a 38.6 wt % TEG solution with 23.4 wt % K.sub.2CO.sub.3, a CO.sub.2 concentration decrease to 32 ppm is achieved within a time interval of 828 s. Experiments to determine the achievable loading capacity with the above trinary composition show, starting at a pH of 12.6, from a pH of 11.6 the precipitation of a white precipitate which can be identified essentially as potassium hydrogen carbonate. There is thus a steady uptake of CO.sub.2 from the environment and precipitation as hydrogen carbonate. The use of sodium carbonate as CO.sub.2 absorbent showed a comparable absorption behavior, but mixtures of sodium carbonate, sodium carbonate hydrate as well as sodium hydrogen carbonate were identified as precipitation products. Thus, to obtain a specific precipitate product for subsequent CO.sub.2 release, K.sub.2CO.sub.3 is favored as the absorbent. The precipitation of sodium or potassium hydrogen carbonate can also be enhanced by the addition of equionic additives. This is possible in carbonate/water solutions as well as in carbonate/PEG/water solutions.
[0083] FIG. 8 shows a simplified reaction scheme for the uptake of carbon dioxide into carbon dioxide absorbents according to the invention, wherein carbonates or amines as component c) are applied. During CO.sub.2 absorption into a PEG/H.sub.2O/K.sub.2CO.sub.3 solution plus an amine-containing additive, carbamate as well as hydrogen carbonate are formed depending on the ternary composition and carbamate stability. The relative ratio is given by the equilibrium constant of reaction equation 5. A potential precipitation results from the solubility products of the carbamate ions or hydrogen carbonate ions with the cations in solution (Na.sup.?, K.sup.+). The precipitation reduces the CO.sub.2 concentration in the solution and thus the corresponding CO.sub.2 partial pressure. The addition of amines to a PEG/H.sub.2O/K.sub.2CO.sub.3 solution leads to an initial substantial increase in the absorption kinetics of CO.sub.2, which are dependent on the amine used and proportional to the amine concentration. The dissolved salts of proline and pipecolic acid, as amino acid salts with a heterocycle, for example, show high CO.sub.2 mass transfer rates. The transfer rates are similar to those of the heterocyclic amines. The absorption kinetics of the aminohexanoic acid salts correspond approximately to those of the alkanolamines, while the kinetics using aminoisobutyric acid correspond to those of sterically hindered amines. The extent to which the amine used has a catalytic effect depends not only on the speed of the carbamate formation reactions but also on the hydrolysis equilibrium and the associated equilibrium constants. Due to the low carbamate stability, secondary amines and sterically hindered amines, such as amino isobutyric acid and pipecolic acid, are advantageous. The rate of the carbamate formation reaction as well as the hydrolysis equilibrium determine the catalytic activity. Sterically hindered heterocycles are particularly suitable as catalytically active substances due to their fast CO.sub.2 absorption kinetics and low carbamate stability. This was demonstrated using pipecolic acid.
[0084] FIG. 9 shows a method variant of an industrial-scale design for using a carbon dioxide absorbent according to the invention. In this and the other variants, the air is brought into contact with the absorbent solution. In the simplest case, an open liquid vessel is sufficient. To optimize the CO.sub.2 absorption in the technical process, an absorber column can be used. Given the low CO.sub.2 concentration in the air and the required flow rates, it is energetically necessary to keep the pressure loss in the air flow as low as possible. Possible technical absorber designs are: packing columns, spray absorbers, bubble column reactors. Packing columns should be designed to handle slurries. Gaining of the CO.sub.2 and recycling of the absorbent may be accomplished by various methods. In this design, it is shown that the loaded detergent may be in the form of a liquid or slurry. If appropriately configured, this can be fed directly to a desorber via a heat exchanger and regenerated there by heating, flashing to a lower pressure or stripping. PEG/H.sub.2O/amine solutions, for example, are suitable for use in this method. The overall composition may result from adjustment to relative humidity, with mole fraction of water around 0.5. The amine concentration can be between 10 wt % and 60 wt %. PEG/H.sub.2O/K.sub.2CO.sub.3 solutions are also suitable. The ternary composition n.sub.H.sub.2.sub.O, n.sub.PEG and n.sub.k.sub.2.sub.CO.sub.3 results from the adjustment to the relative humidity taking into account the additional water vapor partial pressure reduction due to the amount of carbonate added. To achieve high CO.sub.2 uptake and precipitation as hydrogen carbonate, the maximum carbonate concentration is aimed for at the given PEG solubility. Thus, for K.sub.2CO.sub.3 concentrations between 10 wt % and 50 wt % and Na.sub.2CO.sub.3 concentrations between 2 wt % and 20 wt % are obtained. Amine-containing substances, for example, can be used to accelerate CO.sub.2 uptake. The concentration can be adapted to the particular method and can, for example, be between 10% wt % and 30% wt %.
[0085] FIG. 10 shows a further method variant of an industrial-scale design for using a carbon dioxide absorbent according to the invention. This is particularly suitable for PEG/H.sub.2O/K.sub.2CO.sub.3 solutions with a further additive. If the product of the CO.sub.2 uptake is present here as a suspended crystalline substance in the solution, then by precipitation, the CO.sub.2-containing compound can be enriched in the suspension. The depleted absorber solution is returned to the absorber via the mixer, M1. The partial stream enriched with the precipitation product enters a desorber via a heat exchanger and, after desorption, can be recycled as a carbonate solution or suspension via the heat exchanger M1 and subsequently into the absorber A1. The presented method is in the sequence of the individual process steps: CO.sub.2 absorption, precipitation of a CO.sub.2-containing compound and desorption.
[0086] FIG. 11 shows a further method variant of an industrial-scale design for using a carbon dioxide absorbent according to the invention. In this variant, the precipitation product containing CO.sub.2 is completely precipitated and dried as sodium or potassium hydrogen carbonate. The main advantage of this method is that calcination takes place at a temperature as low as 160? C. For the most part, the required thermal energy can be provided by subsequent synthesis steps, such as the conversion of the CO.sub.2 with H.sub.2 to methane. Here, it is favorable that rapid CO.sub.2 absorption as well as an equilibrium constant as high as possible for the equilibrium reaction can be achieved. For these cases, for example, primary and secondary amines can be used as sufficiently rapid substances. Low carbamate stability may be due to steric hindrance. In addition, the solubility product for the cations in solution with the hydrogen carbonate ions should be much lower than the solubility product with the carbamate ions. For this purpose, the amine concentration should be as low as possible. For example, the precipitation products can be obtained with a purity of 99?1%.
[0087] FIG. 12 shows a partial method step according to the invention in the form of a three-chamber electrolysis of an absorbent loaded with carbon dioxide. A carbonate-based absorbent can be loaded with carbon dioxide, for example, from an air stream, such as ambient air or industrial exhaust air. Due to the uptake at least partial conversion of the carbonate to hydrogen carbonate occurs. The hydrogen carbonate-containing solution is added to the middle chamber of an electrolysis unit comprising at least three chambers. The middle chamber is separated from the anode compartment by a bipolar membrane and from the cathode compartment by means of a membrane permeable to potassium, or alkali ions in general. By applying a voltage, oxygen is evolved in the anode chamber, hydrogen in the cathode chamber and carbon dioxide in the middle chamber. The individual gas streams can be collected separately. The carbon dioxide-depleted solution in the middle chamber now has a higher carbonate and a lower hydrogen carbonate content. This recycled solution can be used again as an absorbent for an air stream containing carbon dioxide. In terms of reaction equations, the following conversions occur at the different reaction sites:
[0088] At the anode:
2OH.sup.?.fwdarw.H.sub.2O+?O.sub.2+2e
[0089] At the bipolar membrane:
2H.sub.2O.fwdarw.2H.sup.++2OH.sup.?
[0090] In the intermediate cell:
2HCO.sub.3.sup.?+2H.sup.+.fwdarw.2CO.sub.2+2H.sub.2O
[0091] In sum, the total reaction of the intermediate cell is:
2KHCO.sub.3.fwdarw.2K.sup.++2CO.sub.2+2H.sub.2O
[0092] The following reactions take place at the cathode:
4H.sub.2O.fwdarw.2H.sub.3O.sup.++2OH
2H.sub.3O.sup.++2e.fwdarw.H.sub.2+2H.sub.2O
2KHCO.sub.3+2K.sup.++2OH.sup.?.fwdarw.2K.sub.2CO.sub.3+2H.sub.2O
[0093] Overall, the result for the cathode compartment is thus:
2KHCO.sub.3+2K.sup.++2e.fwdarw.2K.sub.2CO.sub.3+H.sub.2
[0094] The three-chamber structure is extendable as desired with respect to the middle unit. In this respect, 5-, 7-, 9- or generally 3+2n-chamber structures can also be used with the absorbent of the invention or, for example, with pure hydrogen carbonate or amino acid solutions with only slightly modified electrochemical properties.
[0095] FIG. 13 shows a partial method step according to the invention in the form of a two-chamber electrolysis of a carbon dioxide-loaded air stream with nickel hydroxide anode. An absorbent with an alkali carbonate component is loaded with carbon dioxide from an air stream. The carbonate is converted, at least partially, to hydrogen carbonate, which is introduced into the anode compartment of a two-chamber electrolysis system. The electrolysis cell has an alkali-permeable membrane separating the anode compartment from the cathode compartment. The anode is a porous anode which is able to bind oxygen. In this respect, only the carbon dioxide formed leaves the anode compartment. Hydrogen is formed in the cathode chamber. In this respect, the different gases occur at different locations and do not have to be separated from each other in a complex manner. The anode can then be thermally regenerated from time to time with the release of oxygen. The following reactions result:
Anode
[0096]
2KHCO.sub.3.fwdarw.2K.sup.++2HCO.sub.3.sup.?
2HCO.sub.3.sup.?.fwdarw.2CO.sub.2+2OH.sup.?
2 Ni(OH).sub.2+2OH.sup.?.fwdarw.2NiOOH+2H.sub.2O+2e
[0097] The overall reaction is:
(KHCO.sub.3+Ni(OH).sub.2.fwdarw.K.sup.++CO.sub.2+NiOOH+e)?2
[0098] The following reactions take place at the cathode:
4H.sub.2O.fwdarw.2H.sub.3O.sup.++2OH.sup.?
4H.sub.3O.sup.++2e.fwdarw.H.sub.2+2H.sub.2O
2KHCO.sub.3+2K.sup.++2OH.sup.?.fwdarw.2K.sub.2CO.sub.3+2H.sub.2O
[0099] Overall, the total reaction in the cathode compartment is:
2KHCO.sub.3+2K.sup.++2e.fwdarw.2K.sub.2CO.sub.3+H.sub.2
[0100] FIG. 14 shows the current profile and carbon dioxide evolution of a hydrogen carbonate-based absorbent in a three-chamber electrolysis. The absorbent is based on a 10 wt % KHCO.sub.3 solution and the figure shows the measured current and the determined CO.sub.2 volumetric current as a function of the measurement cycles. Plotted are the data of the measurement cycles from 10000 to 11000, where the time interval of a measurement cycle is 1 second. The measurement was performed at a temperature of 20? C. Electrolyte solutions with the following ingredients were used: anode: KOH 5.4 wt %; intermediate chamber: KHCO.sub.3 10 wt %; cathode: KHCO.sub.3 10 wt %. The fluctuating current flow is due to bubble formation and detachment on the surface of the bipolar membrane. For a similar reason, the CO.sub.2 gas flow rate also fluctuates somewhat. The stoichiometric ratio of the released gas amounts was approximately the ratio: 2:1:1/2 for CO.sub.2, H.sub.2, O.sub.2. The Faraday efficiency obtained in the simple experimental setup related to CO.sub.2 was about 80%.
[0101] FIG. 15 shows the current profile and the carbon dioxide evolution of an amino acid-based absorbent in a three-chamber electrolyzer. The current applied as well as the CO.sub.2 gas flow rate achieved during the electrolysis of an amino acid salt solution loaded with CO.sub.2 in the three-chamber electrolyzer are plotted. The measurement was also performed at a temperature of 20? C. Electrolyte solutions of the following composition were used: anode: KOH, 5.4 wt %; intermediate chamber: loaded amino acid salt solution with proline, 10 wt %; cathode: loaded amino acid salt solution proline 10 wt %. CO.sub.2 loading was performed before electrolysis in a bubble column reactor by passing a 14 vol % CO.sub.2 gas stream. The measured fluctuating current flow is again due to bubble formation and detachment at the surface of the bipolar membrane. The same applies to the fluctuations in the CO.sub.2 gas flow rate. The stoichiometric ratio of the released gas quantities was approximately the ratio: 1:1:1/2 for CO.sub.2, H.sub.2, O.sub.2. The Faraday efficiency obtained in this simple experimental setup related to CO.sub.2 was about 85%.
[0102] FIG. 16 shows one possibility for combining three-chamber electrolysis with precipitation of KHCO.sub.3 after CO.sub.2 absorption. It is therefore also possible that the hydrogen carbonate formed by carbon dioxide uptake does not have to be processed directly in the cycle with electrolysis. For example, the hydrogen carbonate can be stored and then subjected to electrolysis in batches.
