# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Electrodialysis  
The electrodialysis methodology using a bipolar membrane electrodialysis (BPMED) stack has been described in the literature for CO\({}_{2}\) capture from the air, although it has not been deployed at industrial or even at pilot scale yet. A process scheme for CO\({}_{2}\) capture based on this technology was proposed by [PERSON] et al. [22], as in Figure 11. CO\({}_{2}\) is captured from the air through an air contact using an aqueous solution of KOH. In this capture reaction, K\({}_{2}\)CO\({}_{3}\) and KHCO\({}_{3}\) are produced, present in waste as K\({}^{+}\) and CO\({}_{3}\)\({}^{2-}\)/HCO\({}_{3}\)\({}^{-}\). The regeneration of KOH solution is carried out using a bipolar membrane electrodialysis stack, the scheme for which is shown in Figure 12.  
Figure 11: Process scheme for CO\({}_{2}\) capture from the air using bipolar membrane electrodialysis. Reproduced under the ACS AuthorChoice/Editors’ Choice via CC-BY-NC-ND Usage Agreement [22]. Figure 10: Scheme of the experimental process (MEA = membrane electrode assembly, CEM = cation exchange membrane). Reproduced under the ACS AuthorChoice/Editors’ Choice via CC-BY-NC-ND Usage Agreement [121]. The cell is divided by an anion-exchange membrane (AEM) into acidic and basic compartments: the stream with K\({}_{2}\)CO\({}_{3}\) and KHCO\({}_{3}\) is sent to the basic compartment, while into the acid compartment a buffer solution flows. At the extremity, the bipolar membrane (BPM) has cation-exchange and anion-exchange layers with ion-selective properties. When an electric current is passed through them, BPMs splits water in H\({}^{+}\) and OH\({}^{-}\), and in the basic compartment K\({}^{+}\) and OH\({}^{-}\) react to form KOH (see Equation (29)) [22]. \[K^{+}+OH^{-}=KOH \tag{29}\]  
The CO\({}_{2}\)-based ions diffuse through the AEM to the acid compartment, where they react with H\({}^{+}\) to produce dissolved carbon dioxide (see Equations (30)-(31)) [22]. \[CO_{3}^{2-}+H^{+}=HCO_{3}^{-} \tag{30}\]  
\[HCO_{3}^{-}+H^{+}=H_{2}CO_{3} \tag{31}\]  
The CO\({}_{2}\) solubility in the acid solution is low, then bubbles are generated so that the biphasic solution is sent to a vessel to separate the gaseous CO\({}_{2}\) from the acid buffer solution. In this way, pure CO\({}_{2}\) is released, while the CO\({}_{2}\)-depleted acid stream is repressurized and sent to the electrodialysis unit. On the other hand, the aqueous solution of KOH, the lean stream, is sent to the air contactor. The authors considered a techno-economic analysis of the process, finding an energy consumption of 236 kJ/molCO\({}_{2}\) and a cost of 773 $/tonCO\({}_{2}\), (the bipolar and ion-exchange membranes have the highest cost). In the economic analysis, a membrane lifetime of 5 years was supposed, with an area of 1.785 m\({}^{2}\) and 2400 cells. The electricity cost was fixed to 0.06 $/kWh, while the stack unit costs 0.75 M$/unit and the anion-exchange membrane costs 70 $/m\({}^{2}\) and the BPM 750 $/m\({}^{2}\). From these results, it is evident that this new proposed regeneration method requires an amount of regeneration energy (that is only electrical energy) lower than that of the Carbon Engineering plant and avoids solid handling but has higher costs (about three time more expensive than the Carbon Engineering plant). The authors suggested that other studies should be focused on optimizing the electrolyte compositions, aiming to decrease the transport losses in the system and provide more insights into the reaction kinetics. Other works in the literature have investigated this technology. An integration of this kind of system in a process for producing methanol by exploiting the electrical energy produced by photovoltaic panels has been suggested in [PERSON] et al. [122]. Bipolar membrane electrodialysis has also been analyzed in the work of [PERSON] et al.