## 4 Solvents for CO\({}_{2}\) Capture
### Chemical Solvents
#### 4.2.3 Phase Change Solvents  
Phase change solvents are mixtures that under certain thermodynamic conditions can undergo reversible phase separation forming two distinct liquid phases of different concentrations. When such mixtures are used in CO\({}_{2}\) capture, the resulting two phases, namely an organic and an aqueous one, or a solvent rich and a solvent lean, respectively, can be mechanically separated resulting in significantly reduced thermal regeneration costs due to the avoidance of water vaporization [72]. A phase change solvent can comprise one base component (e.g., aqueous solution of an amine) or a mixture of components (e.g., amine blend). This way, the thermodynamic conditions of phase splitting can also be controlled using the initial concentration or the molar ratio as a parameter [73]. [PERSON] et al. [74] present a comprehensive review on benchmark and novel phase change solvents analyzing their potential as well as drawbacks. Lipophilic amines were considered initially by [PERSON] [75] and subsequently received great attention in the years after. Such solvents exhibit liquid-liquid (LL) phase split within the temperature range of 60-90 \({}^{\circ}\)C, considering aqueous solutions with concentrations between 10-30% \(w/w\). In a capture process where the reaction of CO\({}_{2}\) is favored by low temperatures, the importance of the LL split temperature plays a crucial part. However, as the reaction temperature is outside the LL split temperature range, a better control of the process can be achieved as the phase splitting can be induced after the reaction takes place without compromising the concentration of the reacting mixture. Mixtures of DEEA (N,N-diethylethanolamine) and MAPA (3-(methylamino)propylamine) have been extensively studied as a candidate solvent for efficient CO\({}_{2}\) capture and simultaneous energy reduction. This mixture exhibits LL split at low temperatures around 40 \({}^{\circ}\)C, which coincide with the absorption temperature [76]. As such, CO\({}_{2}\), water and MAPA remain in the organic, CO\({}_{2}\)-rich phase, while DEEA is mostly found in the aqueous phase [77]. [PERSON] et al. [78] studied a novel mixture of S1N (cyclohexylpropane-1,3-diamine) and DMCA (dimethylcyclohexylamine) as well the known phase change solvent MCA (methylcyclohexylamine). Both S1N-DMCA and MCA solvents showed significantly reduced thermal requirements compared to the base case CO\({}_{2}\) absorption with MEA. Regeneration of both solvents was also performed at lower temperatures in the range of 85-105 \({}^{\circ}\)C. The impact of MCA was studied usingan environmental, health, and safety (EHS) analysis by [PERSON] et al. [79], showing that additional hazard-countering measures could be needed for its use; however, the environmental impact is expected to be lower than that of conventional systems. [PERSON] et al. [80] experimentally investigated a ternary phase change solvent, an aqueous solution of DMCA-MCA-AMP. Such solvent exhibits phase splitting at temperatures greater than 40 \({}^{\circ}\)C where the absorption of CO\({}_{2}\) takes place and is able to achieve a rich loading of more than 0.7, far surpassing the thermodynamic limitations of MEA. [PERSON] et al. [81] used an optimal design framework to design an absorption-desorption process using a mixture of S1N-DMCA. They reported a minimized thermal requirement in the order of 2.1. GJ/t CO\({}_{2}\) as well as a CO\({}_{2}\) loading in the organic-rich phase far surpassing 1 (i.e., \(\alpha=1.35\)).