Demonstrating the key role of kinetics over thermodynamics in the selection of ionic liquids for CO2 physical absorption

A comprehensive study of over 50 ionic liquids (ILs) -integrating molecular simulation, gravimetric experiments and process analysis- has been conducted to evaluate the role of thermodynamics and kinetics on the physical absorption of CO2 by these widely researched solvents. Despite the common view, CO2 molar gas solubility is found as a misleading criterion to select the adequate IL for being used as CO2 absorbent in commercial separation columns, due to the strong kinetic control of the operation. In contrast, low viscosity and molar weight are demonstrated to be key parameters to minimize solvent consumption, energy duty and equipment size. Short-chain imidazolium-based ILs with tetracyanoborate, tricyanomethanide, and dicyanamide anions are proposed as adequate CO2 absorbents with favorable transport and thermodynamic properties. Current results indicate that ILs do not exhibit better absorbent performance in the CO2 capture unit than conventional organic solvents (such as glymes, components of Selexol) already used in the industry. IL regeneration stage need to be considered in future studies in order to state the promising advantages of IL absorbents in terms of thermal stability, energy consume and economy.