Patent Application
20220097004
Because of the low CO2 content of air and the high energy use of separation methods, filtering the global warming gas CO2 from the atmosphere has been regarded as inefficient and hardly implementable in an industrial scale. Nevertheless, corresponding methods have been developed because the CO2 obtained thereby does not necessarily have to be stored, but could be used, for example, in the beverage industry, in greenhouses or for chemical syntheses, e.g for producing methane by reaction with hydrogen, which can sometimes be obtained in a sustainable manner by water electrolysis using solar and/or wind electrical energy, or for producing synthetic fuels. For the latter purpose, however, there are to be high requirements regarding the purity of the separated CO2, and it requires continuous large amounts of pure gas, which results in an enormous increase of production costs.
A modern method for separating carbon dioxide, which is, for example, being used by the Swiss company Climeworks in Switzerland and in Iceland, consists in blowing ambient air via numerous enormous fans over filters, the surface of which is impregnated with amines to which the acidic CO2 is adsorbed. Subsequent desorption is achieved by heating the “loaded” filter, which requires large amounts of thermal energy. While waste heat of power plants or incinerators can (at least largely) be used therefor, the energy requirements of the method per se remain high. In particular because during desorption via simply heating the filter which is done in the presence of air—a product gas is obtained, which also contains relatively large amounts of N2 (typically at least 5% by volume) in addition to CO2, the separation of which is extremely complicated. But also because the absorbent impregnated with amines is relatively instable and has to be replaced from time to time. US 2010/059377 A1 discloses a method for recovering CO2 from exhaust gases from factories, in particular from plants generating electric power by burning fossil fuels and from coal gasification plants. This includes the absorption of the CO2 and further components of the exhaust gas into a 10 to 50% aqueous K2CO3/KHCO3 solution in a spray tower, followed by an ion exchange for removing multivalent cations, where-after the solution is pressurized to 2 to 200 atm, preferably to >30 atm. During a subsequent electrodialysis using a bipolar dialysis membrane, part of the hydrogen carbonate ions is removed. During dialysis, these move to a process stream buffered at a constant pH value of 3 to 4, which is the reason for the previous pressurization, in order to inhibit escape of the CO2 during dialysis. Then, this process stream enriched with CO2 is simply ventilated, i.e. the pressure is released, so that the CO2 escapes from the strongly acidic solution.
A primary disadvantage of this method is the strongly acidic pH value of the process stream enriched with CO2, which on the one hand requires the application of high pressures during dialysis, which increases energy as well as equipment requirements during the execution of the method, and on the other hand leads to a much higher energy requirement for the dialysis, which increases proportionally with the pH difference between the solutions on the concentrate and the diluate sides, which is also described in US 2010/059377 A1. Therefore, this method is suitable for separating CO2 from gas mixtures containing high concentrations of CO2, e.g. 10-25% by volume or more in combustion exhaust gases, but not for recovering CO2 from ambient air, which usually has a CO2 content of only approximately 400 ppm.
Against this background, it was an object of the present invention to develop a new method and a corresponding facility that allow for recovering from air CO2 with high purity and with less energy input than before.
In a first aspect, the present invention achieves this object by providing a method for separating and recovering carbon dioxide from ambient air, comprising the continuous execution of the following steps:
This sequence of steps, being individually known per se, allows for recovering CO2 in high purity and in a relatively energy-efficient manner from air. Due to the enrichment step b) via electrodialysis the volume of the further process stream can be substantially reduced compared to comparable absorption methods according to prior art, which significantly reduces equipment and energy requirements for transport and further treatment. And contrary to the filter according to the state of the art mentioned in the introductory section, which are only superficially provided with amino groups, the present invention, in certain cases, allows for using solutions having high concentrations of alkali or alkaline earth metal cations, which also reduces the volume of the initial process streams and thus the energy consumption.
Also compared to the method disclosed in US 2010/059377 A1, that of the present invention distinguishes itself through much lower energy and equipment requirements, since no pressure has to be applied to any of the two circulated solutions and the dialysis of the (hydrogen) carbonate ions is conducted from a first to a second alkaline solution. The pH difference therebetween is preferably not more than 2, more preferably ably not more than 1, which significantly reduces the energy consumption of the dialysis step compared to the state of the art.
In preferred embodiments of the inventive method, in step a), the water of a natural or artificial lake having a sufficiently high concentration of alkali or alkaline earth metal ions, e.g. a flooded gravel pit or open pit lake, is used as the solution of the at least one alkali or alkaline earth metal cation, wherein a sufficiently high ion concentration is meant to be one resulting in a pH of the water of at least 7.5 and which is optionally pre-set by adding a base. In general, according to the present invention, due to the higher solubility of hydroxides and carbonates, a solution of alkali metal ions is preferred to one of alkaline earth metal ions, more preferably a solution of Na+ or K+ ions, in particular of Na+ ions, due to cost reasons. Such electrolyte systems are stable, i.e. there is no exchange or loss of electrolyte ions with or to the atmosphere. By using additional chemicals such as methanol or formaldehyde the sorption capacity of the absorption solution or the sorption rate may be increased; however, this is not necessary for operating the disclosed system and thus not preferred due to cost and environmental protection reasons.
However, it is preferred according to the present invention that the solution has a pH of at least 8.0 so that the absorbed CO2 is not present in the form of relatively instable carbonic acid, but completely in the form of hydrogen carbonate or carbonate ions.
By using a natural or artificial standing water, there are enormous amounts of absorption solution available in step a), so that large amounts of CO2 can be absorbed into the alkaline solution within a relatively short time—and that without having to provide for mixing the solution with the air. According to the present invention, however, the aqueous solution of the at least one alkaline or alkaline earth metal cation is preferably brought into contact with the ambient air by using any device for promoting the absorption of CO2 gas into the alkaline solution, in order to shorten the period of time necessary for absorbing a certain amount of CO2 gas and to increase the yield of the inventive method per time unit. hi addition to absorbers and gas scrubbers, e. g. packed or plate columns, jet, dip, vortex, rotation or venturi washers, a spray washer or spray tower is particularly preferred according to the present invention because it can be operated in a very energy-efficient manner.
Due to the good solubility of, for example, the hydroxides of Na or K, the CO2 sorption capacity of the water can be substantially increased. However, further positive properties of such an electrolyte system based on alkali metals or alkaline earth metals are also its non-toxicity, stability and lowering of the water freezing point, which further increases the sorption capacity into a natural or artificial standing water body at low ambient temperatures.
The electrodialysis in step b) of the inventive method is conducted in an electrodialysis separator, in which a combination of bipolar ion-exchange membranes and ion-exchange membranes selective for mono- or multivalent anions are used because these membranes are able to very efficiently conduct the dialysis of (hydrogen) carbonate ions. In addition, it is to be generally noted that any occurrence of the term “(hydrogen) carbonate ions” as used herein is meant to indicate “hydrogen carbonate and/or carbonate ions”. For the dialysis step, for example, this means that, depending on the desired course of the method, primarily or substantially only monovalent hydrogen carbonate ions or primarily or substantially only bivalent carbonate ions or simultaneously both of them are enriched on the concentrate side of the electrodialysis separator. Which variation is selected depends inter alia on the concentration of the solution of the at least one alkali metal/alkaline earth metal ion as well as on its pH value. In the case of relatively strongly diluted solutions, for example, when natural or artificial standing water bodies are used as the absorption solution in step a), i.e. solutions having a pH between 7.5 and 8.5, the absorbed CO2 is primarily present as hydrogen carbonate, as can also be seen in
In addition, absorption, i.e. the phase transition of CO2 from the gaseous to the liquid phase, can be accelerated by setting even higher pH values because the presence of larger base amounts shifts the equilibrium of chemical reactions towards the product sides:
NaOH+CO2→NaHCO3
2NaOH+CO2→Na2CO3+H2O.
As mentioned above, in preferred embodiments of the invention, a pH value of 8 is set for the solution of the at least one alkali metal/alkaline earth metal ions in step a). However, the particularly preferred pH range also depends on whether a natural or artificial standing water body or another type of absorber(s) is used for absorption, e.g. one or more spray washers or spray towers or the like. Especially in natural water bodies a pH between 8 and 9, in particular 8 and 8.5, is particularly preferred according to the invention. At these values, damage to the environment is largely avoided and the dissolved CO2 is almost exclusively present in the form of HCO3− ions so that 1 mole of CO2 can be bound per mole of alkali metal cations, and even 2 moles of CO2 can be bound per mole of alkaline earth metal cations, which provides for an efficient use of the amount of base present. However, as mentioned above, the absorption process thus takes longer than at pH values of more than 9 or even more than 10. Especially when using absorber devices such as spray washers or the like, a pH of 10 to 11 may absolutely be set.
The type of the carbonate being enriched in the solution, i.e. whether it is primarily a hydrogen carbonate or a carbonate, also determines the choice of the dialysis membranes. When using a standing water boxy as the absorber and setting the pH value between 8 and 9 at the diluate-side solution, which is preferred according to the invention, ion exchanger membranes being selective for monovalent anions are consequently preferably used in step b), and a solution enriched with hydrogen carbonate ions and a solution depleted thereof are obtained during dialysis.
Independently of which ion species are enriched on the concentrate side and depleted on the diluate side in the dialysis step b), the solution depleted of (hydrogen) carbonate ions, preferably hydrogen carbonate ions, obtained thereby is recycled to step a) and thus circulates in the continuous method of the present invention in order to provide for a substantially constant liquid volume in this (first) cycle.
The concentrate-side solution enriched with (hydrogen) carbonate, preferably hydrogen carbonate ions, is subjected to steam stripping in step c), for which any suitable equipment or device may be used, such as falling-film evaporators or other desorbers, herein collectively referred to as “desorption columns”, preferably having a steam supply line discharging into the bottom thereof. In preferred embodiments of the invention, steam stripping is conducted in a packed column and optionally at underpressure due to efficiency reasons, in order to support the phase transition of the carbon dioxide from the liquid into the steam phase. Thereby, a carbon dioxide/water steam mixture as well as a CO2-depleted solution are obtained.
Contrary to the solution of the at least one alkali metal/alkaline earth metal cation in the absorber, in which, as mentioned above, sometimes high pH values of more than 9 or even more than 10 may be set, it is advantageous for the solution which is obtained as a concentrate in the dialysis step and depleted of (hydrogen) carbonate ions, preferably hydrogen carbonate ions, to set a low pH value in order to promote desorption. As shown in
According to the present invention, the solution depleted of CO2 and subjected to steam stripping in step c) is then recycled to step b) in order to enrich it again with (hydrogen) carbonate ions in the dialysis separator, which again provides a substantially constant liquid volume in this second cycle.
Due to the above reasons, preferably a pH between 7 and 8.5 or, in case of high pH values in the solution circulating in the first cycle, also between 8 and 9.5, more preferably between 7.5 and 8.5, is set in the second cycle in order to suppress desorption before steam stripping and to not interfere with the electrodialysis. To promote the decarboxylation of carbonic acid in the desorption column, the solution enriched with (hydrogen) carbonate ions may be heated and/or a vacuum may applied thereto before or during steam stripping instead of—or in addition to—choosing an acidic pH value, which will be explained in more detail further below.
According to the invention, the separation of water from the carbon dioxide/water mixture obtained in step c) is achieved in step d) by simply cooling the mixture to condensate the water steam, which results in liquid water and more or less still damp carbon dioxide gas, which—depending on the intended use—may be further dried.
In preferred embodiments of the inventive method, the relatively cold solution obtained in step b) and enriched with (hydrogen) carbonate ions is, as mentioned above, heated before steam stripping because higher temperatures also promote the desorption of CO2. This is in particular done by heat exchange with
In this way, a large part of the thermal energy used in the method is recycled just like the liquid streams, which substantially increases energy efficiency. This may further increased in preferred embodiments by recycling also the condensate obtained by cooling the carbon dioxide/water steam mixture to step b) and/or to step c), in order to again produce water steam for steam stripping therefrom.
In preferred embodiments, the inventive method is particularly energy efficient and environmentally friendly by
In a second aspect, the present invention also provides a facility or system (herein in the following used synonymously) for continuously conducting a method for separating and recovering carbon dioxide from ambient air according to the first aspect of the invention, wherein this facility comprises the following devices or facility sections in fluid communication with one another via corresponding connecting conduits:
By means of such a facility, the method according to the first aspect of the present invention may be efficiently conducted, wherein according to the invention any of the devices mentioned under a) to d) above, i.e. the absorber, the separator, the desorption column, and the condenser, depending on the respective method implementation multiple units thereof—connected in series or parallel—may be provided. This means that the terms “an absorber” and “the absorber” may also be interpreted as “at least one absorber” or “the at least one absorber”, and the same is true for the separator, the desorption column, and the condenser, which will be explained in further detail in the examples below.
According to the preferred embodiment of the method described above, the inventive facility for obtaining the advantages described above is preferably characterized by one or more—in particular all—of the following eight features:
Further increases of energy efficiency and environmental compatibility of the inventive facility may be achieved by—as has also been already explained regarding the implementation of the method—connecting the facility
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;
Preferred embodiments for implementing the inventive method or the inventive facility may be designed as schematically shown in
The inventive method and the inventive facility start, as shown at the top of
In particular, the cations are—due to better solubility of hydroxides and carbonates compared to alkaline earth metal ions—alkali metal cations, more preferably Na+ or K+ ions or a mixture thereof. In the calculation example below, K+ ions are used. Furthermore, the solution may contain additives in the absorbers to increase the sorption capacity of the absorption solution or the sorption rate, such as low alcohols or formaldehyde: however, these are not required for operation of the invention and not preferred due to environmental and cost reasons.
In the alkaline solution, CO2 is absorbed from ambient air—either due to spontaneous absorption without external interference or through accelerated absorption, e.g. in a spray tower—depending on the pH value of the solution to form hydrogen carbonate and/or carbonate anions. Preferably, a pH between 7.5 and 8.5 is set when using one or more standing water bodies, at which pH the absorbed CO2 is primarily present as hydrogen carbonate, which may be seen in the graphic representation of the equilibrium between carbonic acid, hydrogen carbonate and carbonate in
The alkaline solution of the absorbed CO2 is then subjected to a dialysis step and for this purpose supplied to a electrodialysis separator 05. This may either be done directly or, as shown in
From there, the heated and filtered/conditioned solution reaches, via a conduit 104, the electrodialysis separator 05, where one solution depleted of (hydrogen) carbonate ions and one enriched therewith are obtained. Regarding the preferred choice of a pH value of the alkaline solution in the absorption step between 10 and 11 according to the invention—in the case of using spray washers as absorbers—a combination of anion-selective ion exchanger membranes “A” and bipolar membranes “AK” is used in the electrodialysis separator 05 in order to conduct dialysis as shown in
The concentrate thus obtained in the separator 05 is passed to the next step, steam stripping, via a conduit 205, while the diluate depleted of CO2, which is usually heated during dialysis through the warm solution preferably recycled from steam stripping, is in preferred embodiments recycled via conduit 105 to the heat exchanger 06, where it releases its heat to the alkaline solution of the absorbed CO2 before it reaches the dialysis step, and subsequently recycled back via conduit 106 to the absorbers 01a to 01c. In order to balance any water loss in this absorber solution cycle, preferably a water supply conduit is provided consisting of conduits 110 and 11l with a valve 08 therebetween.
Forwarding the solution enriched with (hydrogen) carbonate ions in the electrodialysis separator for steam stripping via conduit 205 may again be done directly or in preferred embodiments of the present invention subjected to various operations before, particularly preferred to a heat exchange, in particular with one or more process steams arising from steam stripping, in order to heat them before they enter the desorption column 26 and thus increase desorption. In addition, the solution being subject to overpressure during pumping may also be subjected to an expansion step for the same purpose.
A combination of both measures is shown in
Preferably, an evaporator 27 is connected to the desorption column 26, which evaporator generates the water steam required for steam stripping, which steam is fed into the column via conduit 216. Here, the evaporator is preferably operated with waste heat from a power or incineration plant, as is partly shown in
In the desorption column 26, which preferably is a packed column, as indicated in
The alkaline solution depleted of CO2 may now be discarded; preferably, however, it is recycled, either directly—or after previous heat exchange with the solution to be desorbed in heat exchanger 21—to the dialysis step or first again to the desorption column 26, in order to complete desorption, before it is recycled to the electrodialysis separator 05. The latter variation is shown in
On the way back to electrodialysis, the recycled solution 217 is, as mentioned, subjected, in heat exchanger 21, to a heat exchange with the solution 205 from the dialysis step that still has to be subjected to steam stripping, in order to heat the latter before steam stripping. From there, it reaches, via conduit 201 and 203 and pump 13, preferably also, i.e. like the diluate-side cycled solution, a filter/conditioner 14 in order to remove contaminants before dialysis and optionally set the pH value, and from there via conduit 205 the electrodialysis separator 05. Optionally, the water amount of the so circulated solution may be supplemented by a water supply (not shown). Preferably, in the inventive method, however this supplement is added in a further recycled material, namely the water steam that is drained from the column 26 together with the desorbed CO2 as a gas mixture. In preferred embodiments it is, as mentioned above, fed via conduit 231 to a heat exchanger 22, where it gives part of the heat to the solution that was enriched with (hydrogen) carbonate ions in the dialysis step and still has to be subjected to steam stripping, whereafter it is fed to a cooler 31 in order to condense off the water from the mixture and thus obtain CO2 largely freed from water. The latter is drawn from the method via conduits 234 and 235 as well as pump 33, wherein—depending on the intended purpose—it may be subjected to further drying.
The cooler 31 may, for example, be operated with the cold water of a natural flowing water (or also standing water). The condensed water arising and being collected in the condensate container 32 is, according to the present invention, preferably fed back via conduit 239 into the recycled material 201 drawn from the heat exchanger 21 on its way back to dialysis and thus recycled, wherein here the additional water supply is preferably provided via conduits 241 and 232 as well as valve 28 in order to keep the solution volume constant.
Trough these process steps under circulation of water in two cycles on the dilate or condensate side, respectively, of the electrodialysis separator, the method of the present invention may be executed in a highly efficient manner, which will be clearly shown in the calculation example below. However, it is to be understood that the method described in detail above and the associated facility of the present invention may also be put into practice with numerous variations, as long as they are within the scope defined by the attached claims.
For example, the electrodialysis separator 05 may, depending on the technical layout, be provided with various conventional side aggregates, e. g. internal recirculation pumps for intensifying the ion transport, and/or anti-fouling systems (e.g. by alternating electrical polarity). Since during longer operation of the facility, membrane defects in the separator may lead to the enrichment of metal ions in one of the cycles, electro-dialysis may additionally be conducted by using additional pumps for periodically or continuously balancing the metal ions in the diluate or in the concentrate. Nevertheless, all embodiments of the inventive method and the inventive facility comprise two liquid cycles.
In this example, a model of the inventive method and the inventive facility as shown in
The model is based on the following assumptions:
All further assumptions and selected or (automatically) set parameters are shown in Table 1 below.
From the table it can be seen that per standard cubic meter CO2 that is absorbed from the atmosphere and recovered as a 96% pure gas (rest: H2O) at position 235 in
In addition, the heat energy required for steam production in evaporator 27 could, as mentioned above, be obtained from waste heat of a power plant (or factory) close to the inventive facility and the energy required for cooling in the condenser 31 could at least partly be provided by using the already relatively cold water of a nearby river or lake, which would further reduce the costs for recovering CO2.
The present invention thus provides an extraordinarily efficient and economic method and an associated facility by means of which carbon dioxide can be recovered from air continuously and in comparably high purity.
| Number | Date | Country | Kind |
|---|---|---|---|
| 19153598.8 | Jan 2019 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/051747 | 1/24/2020 | WO | 00 |
