Prediction of Phase Behavior of CO₂ Absorbents Using Conductor-like Screening Model for Real Solvents (COSMO-RS): An Approach to Identify Phase Separation Solvents of Amine/Ether/Water Systems upon CO₂ Absorption

Developing energy-saving absorbents for carbon dioxide (CO₂) is essential for improving carbon capture and storage (CCS) technologies. Recently, we have designed phase separation solvents, which can significantly reduce the regeneration energy for CO₂ capture and separation down to 1.6 GJ/ton-CO₂ [Machida, H.; Int. J. Greenhouse Gas Control2018, 75, 1−7]. For further developing better solvents, this paper studied a theoretical approach with conductor-like screening model for real solvents (COSMO-RS) to screen the amine/ether/water systems to identify phase separation solvents upon CO₂ absorption. In this work, liquid–liquid equilibria of 21 amine/ether/water systems were determined both before and after CO₂ absorption. Experimentally, it has already been demonstrated that the phase behavior of these systems is classified into three categories: phase separation type, miscible type, and immiscible type. This study demonstrates that the octanol/water partition coefficient is empirically able to search for a combination of amine and ether compounds, in which the system exhibits the phase separation behavior by absorbing CO₂. The COSMO-RS calculations successfully reproduced experimental phase behavior with a rate of agreement of more than 80% by accounting the following two factors: (i) ion pairing for the description of the association/dissociation state for ionic species and (ii) relevant low-lying conformations of ether and amine, which are rationalized by experiments such as conductivity and excess enthalpy measurements. Moreover, we also validated the ability of the COSMO-RS calculation to qualitatively describe the compositions of 2-(ethylamino)­ethanol, diethylene glycol diethyl ether, and water in CO₂-rich and CO₂-lean phases at varying CO₂ loading conditions.