# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Absorption
[43] found that water losses can be reduced by increasing the NaOH concentration to 7 M. However, by increasing the NaOH concentration, the solution becomes more viscous and more aggressive, and it is more difficult to handle even if the capture reaction is favored [31]. An alternative to NaOH and Ca(OH)\({}_{2}\) solutions is KOH solution, which has lower levels of regeneration, though it is more expensive, so that few studies involving it are present in the literature [18; 44]. The choice of a particular solution is a critical point: the solution should be strong enough to capture a significant amount of CO\({}_{2}\) from the dilute air but at the same time weak enough to release CO\({}_{2}\) during the regeneration stage. Enzymes can be added to the solution as catalysts in order to increase the absorption rate of CO\({}_{2}\)[45]. However, problems related to pH, temperature limits and organic decay can occur [46]. This DAC capture technology has important design characteristics. In fact, absorption columns are short and have a large cross section due to the dilute nature of CO\({}_{2}\); they are different from traditional packed towers [47]. Moreover, as a large volume of air is processed, pressure drop should be limited [18]. In addition to packed towers, spray towers or open stagnant pools can be used [44; 47]. The spray tower provides a large surface area for air-liquid contact, with a low pressure drop. In this configuration, the costs of large, packed towers are also avoided but the use of a spray causes energy losses [43]. Another problem for this kind of absorber is related to coalescence, which should be reduced by reducing the flow rate and the CO\({}_{2}\) capture rate. In order to improve the process and overcome its limitations for the regeneration of NaOH, alternative causticization reactions have been suggested, such as autocausticization and direct causticization [18]. In the first case, sodium metaborate (NaBO\({}_{2}\)) is used, involving the following reactions [18] (see Equations (9)-(12)):  
\[2 NaBO_{2}+Na_{2}CO_{3} \to Na_{4}B_{2}O_{5}+CO_{2} \tag{9}\] \[Na_{4}B_{2}O_{5}+H_{2}O \to 2 NaOH+2 NaBO_{2}\] (10) \[NaBO_{2}+Na_{2}CO_{3} \to Na_{3}BO_{3}+CO_{2}\] (11) \[CaCO_{3}\ \to CaO+CO_{2}\ \ \ \ \Delta H_{900\ ^{\circ}{\rm C}}=+179.2\ \rm{kJ/mol} \tag{3}\]0  
\[CaCO_{3}\ \to CaO+CO_{2}\ \ \ \ \Delta H_{900\ ^{\circ}{\rm C}}=+179.2\ \rm{kJ/mol} \tag{3}\]1 Process scheme for the DAC absorption process with NaOH aqueous solution. Reproduced under the Standard ACS AuthorChoice/Editors’ Choice Usage Agreement [18]. In this case, the reaction product is water-soluble, removing the need for drying or the movement of solids [48]. However, this requires a lot of energy, as temperatures are higher than 900 \({}^{\circ}\)C.  
In direct causticization, the reaction product is not water-soluble and these reactions involve titanium dioxide (see Equations (13)-(16)) [18]:  
\[CaCO_{3}\ \to CaO+CO_{2}\ \ \ \ \Delta H_{900\ ^{\circ}{\rm C}}=+179.2\ \rm{kJ/mol} \tag{3}\]2 \[CaCO_{3}\ \to CaO+CO_{2}\ \ \ \ \Delta H_{900\ ^{\circ}{\rm C}}=+179.2\ \rm{kJ/mol} \tag{3}\]3 (14) \[CaCO_{3}\ \to CaO+CO_{2}\ \ \ \ \Delta H_{900\ ^{\circ}{\rm C}}=+179.2\ \rm{kJ/mol} \tag{3}\]4 (15) \[CaCO_{3}\ \to CaO+CO_{2}\ \ \ \ \Delta H_{900\ ^{\circ}{\rm C}}=+179.2\ \rm{kJ/mol} \tag{3}\]5  
The disadvantages are that the heat requirement is comparable to that of an aqueous amine-based liquid post-combustion CO\({}_{2}\) capture process (130 kJ/molCO\({}_{2}\)) and more processing steps are needed in comparison with the traditional causticizer process [48]. Studies in the literature are mostly concerned with economic and energetic analysis and process design. Regarding energetic analysis, it needs to be underlined that, in addition to heat, electrical energy is required for fans to blow air through the contactor, to spray the aqueous solution and to move the solution between each piece of equipment. [PERSON] et al.