METHOD AND SYSTEM FOR CAPTURING CARBON DIOXIDE FROM THE AIR

Abstract

A method for separating and recovering carbon dioxide from ambient air includes continuously bringing ambient air into contact with a basic aqueous solution; electrodialysis of the solution using bipolar and anion-selective ion exchange membranes as well as recycling the depleted solution; separating CO.sub.2 from the enriched solution and recycling the solution depleted of CO.sub.2. The absorption is performed in an absorber, open basin, or a combination thereof. Separation is achieved by thermal desorption of CO.sub.2 by steam stripping to obtain a carbon dioxide/steam mixture; and/or by chemical reaction of the (hydrogen-) carbonate ions, in which the CO.sub.2 contained is converted into a water-insoluble salt or a gas and simultaneously removed from the solution. The pH of either obtained solution is measured before the recycling or before the separation, and is adjusted to a predetermined value. pH is measured and adjusted based on how absorption and separation are performed.

Claims

1. A method for separating and recovering carbon dioxide from ambient air, comprising continuously performing the following steps: a) bringing ambient air into contact with a basic aqueous solution of at least one alkaline metal or alkaline earth metal cation to absorb the carbon dioxide into the solution to form the hydrogencarbonate or carbonate of the at least one metal; b) electrodialysis of the resulting solution using a combination of bipolar and anion-selective ion exchange membranes to obtain one solution enriched in (hydrogen-) carbonate ions and one solution depleted thereof, each having a pH >7, the solution depleted in (hydrogen-) carbonate ions being recycled to step a); c) separating carbon dioxide from the solution depleted of (hydrogen-) carbonate ions to obtain a CO.sub.2-depleted solution which is recycled to step b); and (d) optionally drying and purifying the separated carbon dioxide; wherein: A) the absorption of step a) is performed Ai) in at least one absorber (01a); or Aii) in at least one open basin (01c); or Aiii) in a combination of at least one absorber (01a) and at least one open basin (01c); and/or B) separating the carbon dioxide from the solution enriched in (hydrogen-) carbonate ions in step c) is performed Bi) by thermal desorption of the carbon dioxide by steam stripping to obtain a carbon dioxide/steam mixture; and/or Bii) by chemical reaction of the (hydrogen-) carbonate ions, in which the CO.sub.2 contained is converted into a water-insoluble salt or gas and thereby removed from solution; and/or C) the pH of either solution obtained in step b) by means of electrodialysis is measured prior to recycling to step a) or prior to separating carbon dioxide in step c), respectively, and is in each case adjusted to a predetermined value; with the proviso that, if the absorption is not carried out according to Aiii) in a combination of at least one absorber (01a) and at least one basin (01c), but the separation of carbon dioxide is carried out according to Bi) by thermal desorption of the carbon dioxide by means of steam stripping, the pH is measured and adjusted according to C).

2. The method according to claim 1, wherein the absorption in step a) is performed according to Aiii), first in at least one absorber (01a) and then in at least one open basin (01c).

3. The method according to claim 1, wherein the absorption in step a) is performed in at least one absorber (01a) having a sump or collection tank (01b) at its bottom, in which the solution enriched with (hydrogen-) carbonate ions by absorption is intermittently stored before it is passed on to the next absorber or into an open basin or to electrodialysis.

4. The method according to claim 1, wherein the absorption in step a) is performed in at least one spray scrubber or spray tower or in at least one packed column (01a).

5. The method according to claim 1, wherein the absorption in step a) is performed in at least one open basin (01c), above which at least one cover (51) is arranged at a distance suitable to limit the vaporization of water from the basic aqueous solution of the at least one alkaline metal or alkaline earth metal cation.

6. The method according to claim 5, wherein the cover (51) comprises a photovoltaic system producing at least a part of the electric energy required for performing the method from sunlight.

7. The method according to claim 1, wherein the absorption in step a) is performed in at least one open basin (01c) having integrated partitioning walls (01d), between which the basic aqueous solution of the at least one alkaline metal or alkaline earth metal cation is guided for absorption in a meandering manner which increases the residence time thereof within the basin.

8. The method according to claim 1, wherein the separation of carbon dioxide in step c) is performed according to Bi) by thermal desorption of carbon dioxide by means of steam stripping, optionally supported by a vacuum, and that the carbon dioxide from the resulting carbon dioxide/steam mixture is recovered by means of cooling for condensation of steam and optionally by means of subsequent further drying.

9. The method according to claim 1, wherein within the solution of the at least one alkaline metal or alkaline earth metal cation, a pH of at least 7.5, at least 8.0 or at least 10.0 is set.

10. The method according to claim 1, wherein the separation of carbon dioxide in step c) is performed according to Bii) by chemical reaction of the (hydrogen-) carbonate ions.

11. The method according to claim 10, wherein, in step a), an aqueous solution of at least one alkaline metal cation for absorption of carbon dioxide is used and that as a chemical reaction of the (hydrogen-) carbonate ions, a reaction of alkaline earth metal cations for forming not readily soluble carbonates is performed forming a removable precipitate.

12. The method according to claim 11, wherein, in the aqueous solution of the at least one alkaline metal cation, a pH of at least 11 is set, having the carbon dioxide absorbed therein substantially in the form of CO.sub.3.sup.2− ions.

13. The method according to claim 10, wherein electro-chemical reactions into gaseous hydrocarbons C.sub.xH.sub.y escaping from the solution are performed as chemical reaction of the (hydrogen-) carbonate ions according to the following General Reaction Equation:
XHCO.sub.3.sup.−+(5X+Y)H.sup.++(4X+Y)e.sup.−.fwdarw.C.sub.XH.sub.Y↑+3XH.sub.2O
XCO.sub.3.sup.2−+(6X+Y)H.sup.++(4X+Y)e.sup.−.fwdarw.C.sub.XH.sub.Y↑+3XH.sub.2O

14. The method according to claim 13, wherein methane is produced as the gaseous hydrocarbon.

15. The method according to claim 13, wherein the gaseous hydrocarbons are suctioned off by creating a negative pressure within the reactor.

16. The method according to claim 8, wherein the relatively cold solution enriched with (hydrogen-) carbonate ions obtained in step b) is subjected to a heat exchange prior to steam stripping with i) a relatively hot solution already subjected to steam stripping before its recycling into step b), in order to heat said first solution and to cool the second solution; and/or ii) the carbon dioxide/steam mixture obtained during steam stripping, in order to heat it and to cool said carbon dioxide/steam mixture for condensation.

17. The method according to claim 8, wherein i) waste heat of a power plant or factory is used for producing the steam in step c) and/or for heating the solution enriched with (hydrogen-) carbonate ions obtained in step b) prior to steam stripping in step c); and/or ii) direct current from renewable energy sources is used for electrodialysis in step b).

18. A facility for continuously performing a method of separating and recovering carbon dioxide from ambient air according to claim 1, comprising the following facility devices or sections: a) at least one absorber (01a) or at least one open basin (01c) for bringing ambient air into contact with an aqueous solution of at least one alkaline metal or alkaline earth metal cation for absorbing carbon dioxide while forming the hydrogencarbonate or carbonate, respectively, of the at least one metal; b) an electrodialysis separator (05) comprising a combination of bipolar ion exchange membranes and anion-selective ion exchange membranes (A, AK) for performing an exchange of ions to obtain one solution enriched with (hydrogen-) carbonate ions and another solution depleted thereof, as well as a line (105) for recycling the solution depleted of (hydrogen-) carbonate ions to a); c) means (17) for separating the carbon dioxide from the solution enriched with (hydrogen) carbonate ions, as well as a line (201) for recycling the solution thus depleted of (hydrogen-) carbonate ions to a); and d) optionally, means for drying and/or purifying the carbon dioxide separated at c); wherein A) the facility comprises: Ai) at least one absorber (01a); or Aii) at least one open basin (01c); or Aiii) a combination of at least one absorber (01a) and at least one open basin (01c); and/or B) the means (17) for separating the carbon dioxide from the solution enriched with (hydrogen)carbonate ions comprise: Bi) a desorption column (17) for performing steam stripping on the solution enriched with (hydrogen-) carbonate ions to obtain a carbon dioxide/water mixture; and/or Bii) a reactor (17) for performing a chemical reaction of the (hydrogen-) carbonate ions by converting the CO.sub.2 contained into a water-insoluble salt or to a gas and, optionally, means for removing the water-insoluble salt or gas from the reactor (17); and/or C) the facility comprises pH regulators (06, 16) for measuring and adjusting the pH of either solution obtained in the electrodialysis separator (05) prior to recycling to a) or prior to separating the carbon dioxide, respectively; with the provision that, if the facility does not comprise a combination of at least one absorber (01a) and at least one open basin (01c) according to Aiii), but a desorption column for performing steam stripping, the facility will comprise the pH regulators (06, 16) for measuring and adjusting the pH of the solutions contained in the electrodialysis separator (05) according to C).

19. The facility according to claim 18, wherein it comprises a combination of at least one absorber (01a) and at least one open basin (01c) according to Aiii).

20. The facility according to claim 18, wherein it comprises at least one absorber (01a) having a bottom part comprising a sump or collection tank (01b) for intermittently storing the solution enriched with (hydrogen-) carbonate ions by absorption.

21. The facility according to claim 18, wherein the absorber (01) is a spray scrubber, spray tower or at least one packed column.

22. The facility according to claim 18, wherein it comprises at least one open basin (01c), above which at least one cover (51) is arranged within a distance suitable to limit vaporization of water from the basic aqueous solution of the at least one alkaline metal or alkaline earth metal cation.

23. The facility according to claim 22, wherein the cover (51) comprises a photovoltaic system for producing electrical energy from sunlight.

24. The facility according to claim 18, wherein it comprises at least one open basin (01c) having partition walls (01d) arranged to guide the basic aqueous solution of the at least one alkaline metal or alkaline earth metal cation in a meandering manner.

25. The facility according to claim 18, wherein it comprises a desorption column (17) for performing steam stripping on the solution enriched with (hydrogen-) carbonate ions to obtain a carbon dioxide/water mixture according to Bi), was well as a condenser for separating water from the carbon dioxide/water mixture by condensation, and that it optionally further comprises a dryer for the obtained carbon dioxide.

26. The facility according to claim 18, wherein it comprises a reactor (17) for performing a chemical reaction of the (hydrogen-) carbonate ions by converting the CO.sub.2 contained into a water-insoluble salt or a gas according to Bii), and that it optionally further comprises a filter for removing the water-insoluble salt or a device for suctioning off or sonication of the reaction solution for removing the gas from the reactor.

27. The facility according to claim 26, wherein the reactor (17) comprises a storage tank for an aqueous solution of alkaline earth metal ions, or electrodes for a chemical reduction of the (hydrogen-) carbonate ions.

28. The facility according to claim 18, wherein it further comprises one or more of the following components, selected from further pH controllers, (vacuum) pumps, condensers, heat exchangers, heating and cooling devices, filter and membrane separators, storage tanks for fresh water and alkaline (earth) metal ions, metering pumps and a computed controller.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0088] The present invention will now be described in more detail with reference to preferred embodiments that are, of course, only provided for illustrative purposes and are not meant to limit the invention, a calculation example for the energy consumption of the inventive method and the associated facility, as well as with reference to the enclosed drawings, wherein:

[0089] FIG. 1 is a graph showing the pH-dependent equilibrium between carbonic acid, hydrogencarbonate, and carbonate in aqueous solutions.

[0090] FIG. 2 is a flow diagram of an embodiment of the inventive method or the inventive facility, respectively.

[0091] FIG. 3 is a flow diagram of a particularly preferred embodiment of the inventive method or the inventive facility, respectively.

[0092] FIG. 4 is a schematic representation of a preferred arrangement of anion-selective (“A”) and bipolar (“AK”) membranes in an electrodialysis separator in step b) of the method.

[0093] FIG. 5 is a schematic representation of a preferred embodiment of an open basin being a part of the inventive facility for adsorbing CO.sub.2 in step a) of the method.

EXAMPLES

[0094] As previously mentioned, in preferred embodiments of the invention, a pH of the alkaline solution of the at least one alkaline (earth) metal ion is adjusted in step a) to a value of at least 7.7, at least 8.0, or even higher, e.g., >11.0. However, the preferred pH range will also depend on the fact if one or more open basins or absorber, such as one or more spray scrubbers or spray towers, or combinations thereof are used for absorption.

[0095] As has also been mentioned previously, the presence of large quantities of a base will shift the balances of the following chemical reactions to the product sides:

NaOH+CO.sub.2.fwdarw.NaHCO.sub.3

2NaOH+CO.sub.2.fwdarw.Na.sub.2CO.sub.3+H.sub.2O

which is why, according to the present invention, in which brick-built, shallow basins are preferred instead of natural bodies of water as open basins, a pH of the absorption solution of at least 10.0, especially of at least 11.0, is set. Further it is preferred to use a solution of alkaline metal ions because, on the one hand, alkaline earth metal ions could damage the electrodialysis membranes and, on the other hand, a precipitation of poorly soluble alkaline earth metal carbonates, thus separating the CO.sub.2 absorbed, can be performed in this way by adding alkaline earth metal ions.

[0096] A simple embodiment of the facility for carrying out the method according to the invention can be designed as shown schematically in FIG. 2. The reference signs contained therein—and in FIG. 3—have the following meanings, the three-digit reference signs for lines simultaneously indicating the fluid flows transported therein.

KEY TO FIG. 2 AND FIG. 3

[0097] 01a Absorber: packed column or spray scrubber [0098] 01b Absorber: sump or buffer tank [0099] 01c Absorber: open basin [0100] 03 Circulating pump of the 1st liquid circuit [0101] 04 Filtration and/or conditioning in the 1st liquid circuit [0102] 05 Electrodialyzer [0103] 06 pH value determination and adjustment in the 1st liquid circuit [0104] 07 Feed pump between absorbers [0105] 08 Circulation pump for refrigeration drying [0106] 09 Condenser and cooling dryer [0107] 13 Circulation pump of the 2nd liquid circuit [0108] 14 Filtration and/or conditioning in the 2nd liquid circuit [0109] 16 pH value determination and adjustment in the 2nd liquid circuit [0110] 17 CO.sub.2 separation: desorption column and/or reactor [0111] 31 Pump for water level control [0112] 32 Reverse osmosis [0113] 33 Control valve for water level control [0114] 34 Pump for water level control [0115] 35 Water tank [0116] 37 Water level measurement [0117] 41 Control valve or pressure maintaining valve in the 1st circuit [0118] 42 Control valve or pressure maintaining valve in the 2nd circuit [0119] 43 Water conditioning/metal ion treatment [0120] 44 Water and/or metal ion source [0121] 45 Container with metal ion solution [0122] 46 Dosing pump for the metal ion solution [0123] 48 Electrodialysis bypass valve [0124] 58 Bidirectional pump for metal ion balancing [0125] 101 Diluate after CO.sub.2 absorption [0126] 103 Diluate after pressure increase [0127] 104 Diluate after conditioning (diluate inlet to electrodialysis) [0128] 105 Diluate recirculated for absorption (diluate outlet from electrodialysis) [0129] 107 Connection between the absorbers [0130] 108 Supply of cold water to the cooling-drying unit [0131] 109 Recirculation from cooling drying [0132] 405 Bypass of the electrodialysis [0133] 201 Concentrate recycling after CO.sub.2 separation [0134] 203 Concentrate after pressure increase [0135] 204 Concentrate after conditioning (concentrate inlet to electrodialysis) [0136] 205 Concentrate from electrodialysis for CO.sub.2 separation [0137] 301 Inlet into the reverse osmosis system [0138] 302 Inlet of water into the water storage tank [0139] 303 Recycling of the absorbent solution from reverse osmosis [0140] 304 Water supply line to absorbers [0141] 407 Feed line from metal ion or water source to water treatment plant [0142] 408 Fresh water or metal ion supply line [0143] 409 Feed line for metal ion solution

[0144] The method according to the invention and the inventive facility start with at least one absorber 01 for absorbing CO.sub.2 from ambient air, as shown in the center of the above FIG. 2, which preferably consist of at least one spray scrubber or spray tower 01a, especially preferably each including a sump or buffer tank 01b, or at least one open basin 01c, or particularly a combination thereof.

[0145] As is particularly preferred and due to the abovementioned reasons, the cations are alkaline metal cations, more preferably Na.sup.+ or K.sup.+ ions or a mixture thereof. Furthermore, the solution within the absorbers may contain additives for increasing the sorption capacity of the absorbing solution or of the absorbing rate, e.g., lower alcohols or formaldehyde, however, these are not required for the efficacy of the invention and are not preferred due to environmental concerns and reasons of cost efficiency.

[0146] Through the absorption of CO.sub.2 from ambient air, dependent on the solution pH, a formation of hydrogencarbonate and/or carbonate anions will take place in the alkaline solution. Preferably, a high pH will be set for supporting absorption, e.g., a pH of >10 or >11.

[0147] The alkaline solution of the absorbed CO.sub.2 is subsequently subjected to a dialysis step and for this purpose fed to an electrodialysis separator 05. This can be done either directly or, as shown in FIG. 2, preferably after a preceding filtration or conditioning of the solution, e.g., by heating, in order to accelerate the mass transfer during the dialysis process, for which the heat of a liquid stream occurring later in the process but recycled to a previous stage can also be used by means of a heat exchange, which would otherwise be lost as waste heat, e.g., the heat of a steam stripping process. For example, the heat of a solution subjected to steam stripping from the second liquid circuit or that of the solution depleted of (hydrogen-) carbonate ions by electrodialysis before it is recycled to the absorption step. The optional filtration allows for impurities to be largely removed from the solution prior to dialysis, and conditioning is understood to mean not only heating but also the optional addition of pH regulators and/or other additives to promote the absorption and desorption processes. In preferred embodiments, filtration and conditioning are carried out essentially simultaneously in a filter/conditioner 04, to which the alkaline solution of absorbed CO.sub.2 in FIG. 2 is fed via line 101, pump 03 and line 103.

[0148] From there, the solution, which may have been heated and/or filtered/conditioned, passes via line 104 into the electrodialysis separator 05, where a solution is obtained which is depleted in (hydrogen-) carbonate ions, and a solution which is enriched in these ions. With the preferred choice of a pH value of the alkaline solution in the desorption step being >10 or >11, a combination of anion-selective ion exchange membranes “A” and bipolar membranes “AK” is used in the electrodialysis separator 05 to carry out the dialysis as shown in FIG. 4. Via the three-way valve still provided upstream of the electrodialysis separator 05, the electrodialysis bypass valve 48, the solution enriched by absorption of CO.sub.2 can, if necessary, e.g., from time to time, also be led past the electrodialyzer 05 and, after prior readjustment of the water quantity and/or the concentration of the alkaline (earth) metal ions via water tank 35 and pump 34 or a (sea) water reservoir, line 407 and water conditioner 43 and line 408, or metal ion reservoir 45, pump 46 and line 409 can be recycled in the first or (with regard to electrodialysis) “diluate” circuit, without being subjected to electrodialysis in this cycle.

[0149] If, on the other hand, the solution is subjected to electrodialysis during normal operation in electrodialyzer 05, it is recycled via control or pressure holding valve 41, with pH measurement and control taking place immediately after electrodialysis at position 06 in order to maintain the desired pH value constant in the diluate circuit and thus ensure smooth continuous operation. The solution removed by electrodialysis is then recycled via line 105, although it may still be fed to a reverse osmosis unit 32 via line 301 and pump 31 in order to remove any excess water from the liquid circuit. Subsequently, it can also be subjected to the same conditioning and treatment steps as previously described for the bypass, if necessary.

[0150] The electrodialysis “concentrate” obtained in the electrodialyzer 05, which passes through the second liquid circuit, is discharged from the dialyzer via control or pressure maintaining valve 42. The concentrate obtained in the electrodialyzer 05, which passes through the second liquid circuit, is recycled from the electrodialyzer 05 via the control valve 42, after which a pH measurement and control of this liquid flow also takes place directly at position 16, whereafter the concentrate is passed via line 205 to the next step, the separation of the CO.sub.2 as step c) of the method according to the invention, in the means 17 for separating the carbon dioxide from the solution containing (hydrogen-) carbonate ions.

[0151] This separation at position 17 can be carried out either by means of steam stripping in a desorption column, as in the earlier applications of the present inventor, or by means of chemical reactions of the (hydrogen-) carbonate ions, in which the CO.sub.2 is removed from the solution as a poorly soluble precipitate or as a gas, or also by means of a combination thereof, e.g., in which steam stripping is carried out first and then the residual CO.sub.2 is removed by chemical reaction.

[0152] Preferably, for the separation by chemical reactions, as mentioned above, a solution containing essentially only hydrogencarbonate ions, especially preferably one with a pH of about 8.0 or about 9.0, is mixed in a reactor 17 with alkaline earth metal ions, preferably Ca.sup.2+ cations, to obtain a final CaCO.sub.3 precipitate which can be separated from the solution by means of a simple filter. In alternative preferred embodiments, a concentrate solution—even more preferably one with a higher pH, e.g., about pH 10.0 or about pH 11.0, to promote the absorption process—is subjected to electrochemical reduction, yielding a hydrocarbon gas, e.g., methane or ethylene, which escapes from the solution, which may be supported by suction means.

[0153] The alkaline solution depleted of CO.sub.2 in column and/or reactor 17 is recycled to electrodialyzer 05 via line 201, pump 13, line 203, and filter or conditioner 14 for removal of impurities and, if necessary, for heat exchange, as described previously for 04 in the diluate circuit, as well as line 204.

[0154] FIG. 3 shows an even more preferred embodiment of the method according to the invention and the associated facility, which comprises essentially the same steps and facility components as those in FIG. 2, but differs in particular in that several different types of absorbers 01 are provided—and that the pH measurement and control do not necessarily have to take place at positions 06 and 16.

[0155] The embodiment shown in FIG. 3 comprises an absorber 01a shown as a spray tower, the bottom of which encloses a sump or buffer tank 01b for intermediate storage of the CO.sub.2-enriched absorption solution and as a second absorber, since absorption continues during intermediate storage, as described above. Via pump 07 and line 107, this second absorber is connected to a third element, namely an open basin 01c.

[0156] This embodiment with positioning of a gas scrubber 01a immediately after electrodialysis in the diluate circuit (or after optional prior readjustment of the water quantity and/or the metal ion concentration as previously described) provides the advantage that the solution depleted in (hydrogen-) carbonate ions heated during electrodialysis causes a “stack effect” in the column or the spray tower. In this process, the air in the absorber will heat up through contact with the warm or hot recycled solution and causes buoyancy, which sets the air in an upward motion or at least supports such a motion, which promotes the transfer of CO.sub.2 from the air into the absorbing solution. In the sump tank 01b provided thereafter and in the open basin 01c following it, the solution, already partially enriched in (hydrogen-) carbonate ions and cooled during the passage of the absorber 01a, can then also more easily absorb further CO.sub.2, since higher temperatures impede gas absorption.

[0157] In particularly preferred embodiments of the invention, the at least one open basin 01c is as shown in FIG. 5, i.e., including integrated partition walls 11, between which or around which the basic aqueous absorption solution for absorption of the carbon dioxide is meandered, which increases its residence time in the basin, ensures better mixing of the solution during contact with the air and, above all, suppresses back-mixing with the absorption solution, which is still somewhat poorer in CO.sub.2 and continuously flows in via line 107 (and possibly also 304).

[0158] At position 37, a level meter is provided to determine the water level of the absorption solution in the open basin 01c in order to be able to re-adjust the water quantity, if necessary, as described above, i.e., to reduce it by means of reverse osmosis at 32 or to increase it by means of water supply via lines 304 and/or 408. Furthermore, a cover 51 is provided above the basin 01c to prevent excessive evaporation of water from the solution during its residence time in the basin. Particularly preferably, this cover 51 comprises a photovoltaic system in order to be able to generate at least part of the electricity required for the operation of the facility from sunlight.

[0159] In addition, in the embodiment shown in FIG. 3, a condenser 09 is also provided at the head of the spray tower 01a, which is fed via pump 08 and line 108 with cooled absorption solution enriched with CO.sub.2 from the basin 01c and is intended to cause condensation of any moist air (as indicated by arrows) escaping from the spray tower 01a in order to reduce water losses. At the same time, the portion of enriched absorption solution used for this purpose, which is returned to the diluate circuit via line 109, is preheated before being introduced into electrodialyzer 05, which favors electrodialysis.

[0160] In all other respects, the embodiment shown in FIG. 3 corresponds essentially to that in FIG. 2, i.e., a desorption column, a reactor or even a combination thereof can be used to separate the CO.sub.2 from the absorption solution at position 17.

[0161] With these process steps, using circulation of the water in two circuits on the diluate and the concentrate side, respectively, of the electrodialysis separator and pH measurement and control immediately after electrodialysis and/or using various absorbers and/or separation of the CO.sub.2 by means of chemical reactions instead of steam stripping, in particular in the respective particularly preferred embodiments, the method of the present invention can be carried out even more efficiently than the inventors' earlier method, as the following model example demonstrates. However, it is understood that the method described in detail above, as well as the associated facility of the present invention, can also be put into practice with numerous modifications, provided that they lie within the scope defined by the appended claims.

[0162] For example, depending on the technical design, the electrodialysis separator 05 can be equipped with various conventional ancillary units, such as internal recirculation pumps for intensifying ion transport, and/or antifouling systems (e.g., by alternating the electrical polarity). Furthermore, since membrane defects in the separator can lead to the accumulation of metal ions in one of the circuits during prolonged operation of the facility, electrodialysis can be carried out using additional pumps for periodic or continuous balancing of the metal ions in the diluate or concentrate. Nevertheless, all embodiments of the method and facility according to the invention comprise two separate liquid circuits.

Model Example

[0163] In this example, a model for the inventive method and the inventive facility as shown in FIG. 3 was calculated with computer assistance, including pH measurement and control at positions 06 and 16 of FIG. 2 and using chemical reactions for separating CO.sub.2 in reactor 17, based on empirical data, in order to estimate energy consumption. This model is based on the following parameters and assumptions:

[0164] Spray tower 01a is supplied with 1000 m.sup.3/h of a 20% potassium carbonate/bicarbonate solution via line 105. The temperature at the spray head inlet is ˜55° C. and the pH is >11.7. As can be seen from FIG. 1, the solution thus comprises almost exclusively carbonate anions (˜1.7 mol/l) and only traces of hydrogencarbonate ions (˜0 mol/l).

[0165] Due to the stack effect mentioned above of the hot solution, the spray tower will automatically suck in cold ambient air (˜15° C.), whereby a cooling of the solution to −30° C. and a slight decrease of the solution pH to pH ˜11.3 resulting from the absorption of CO.sub.2 are effected. The solution which is thereby slightly cooled and partially enriched with CO.sub.2 will be collected in the sump container 01b, where further absorption takes place, and will subsequently be pumped into the shallow open basin 01c by means of pump 07 and line 107, which is embodied as shown in FIG. 5 and with slight downwards inclination to the left side so that it is traversed exclusively due to a gravitational effect, where further cooling of the solution to −15° C. and due to further CO.sub.2 absorption further decrease of the pH to pH >˜11 are effected, until the balance between atmospheric CO.sub.2 and hydrogencarbonate ions within the solution is more or less achieved.

[0166] Above the shallow basin 01c a cover 51 with photovoltaic panels having a total peak capacity of ˜7 MW is provided, which may provide up to 30% of the electricity which is needed for electrodialysis on sunny days.

[0167] At the basin 101 outlet, the proportion of hydrogencarbonate ions of the total adsorbed CO.sub.2, i.e., carbonate and hydrogencarbonate ions, is ˜20 mol %. The solution is then pumped through the electrodialyzer 05. At the diluate outlet 105 of the electrodialysis, a pH measurement and control is performed, as is shown in FIG. 2, the pH within the line 105 being re-adjusted to >11.7.

[0168] In the second circuit, i.e., within the concentrate, a pH measurement and control are initially also conducted at 16 after electrodialysis. However, the pH within the concentrate (in line 205) is set to a lower value, in particular to pH ˜9.1. This means that the concentrate solution almost exclusively comprises hydrogencarbonate ions, in particular ˜3 mol/l or ˜94 mol %, and only small amounts of carbonate ions (˜6 mol %).

[0169] CO.sub.2 separation from the concentrate is being performed in reactor 17 by means of chemical reaction with Ca.sup.2+ ions, e.g., by adding a calcium hydroxide solution according to the following equations:

Ca(OH).sub.2+KHCO.sub.3.fwdarw.CaCO.sub.3↓+KOH+H.sub.2O

Ca(OH).sub.2+K.sub.2CO.sub.3.fwdarw.CaCO.sub.3↓+2KOH

[0170] CaCO.sub.3 precipitates from the solution, which shifts the equilibrium of the chemical reactions towards the product sides and thus ensures a more or less quantitative conversion of the absorbed CO.sub.2 with a relatively short residence time in the reactor. The precipitated product is separated by means of simple sedimentation, i.e., the CaCO.sub.3 precipitate is simply allowed to settle, and the depleted solution is pumped out of the reactor and recycled to the electrodialyzer 05. Residual CaCO.sub.3 can be removed from the reactor chamber at intermittent intervals. In this way, almost no energy is required to separate the CO.sub.2 from the alkaline solution.

[0171] Using these assumptions, according to computer calculations, ˜5000 Nm.sup.3/h of pure CO.sub.2 can be obtained, bound as CaCO.sub.3. This is possible due to the triple adsorption (01a, 01b, 01c), the pH control immediately after electrodialysis and the use of chemical reactions instead of steam stripping, i.e., by selecting the combination of the features Aiii), Bii) and C) according to the invention with the additional sump tank 01b and the partial production of the power required for electrodialysis by photovoltaics, only an energy input of ˜12.5 MW is necessary.

[0172] The total specific energy requirement for the extraction of CO.sub.2 from the ambient air by means of the present invention is thus, according to the calculation, only ˜2.5 kWh/Nm.sup.3CO.sub.2, which represents a further considerable improvement compared to the inventors' earlier method, in which a consumption of 3.8 kWh of electrical energy and 1.41 kWh of thermal energy per standard cubic meter CO.sub.2 had been calculated.