Thermodynamic and kinetic evaluation of ionic liquids + tetraglyme mixtures on CO2 capture

Abstract

The mixture of ionic liquids (ILs) with a low viscous solvent such as tetraglyme (TGM) may be interesting to reach new absorbent formulations with improved performance in CO2 capture technologies. In this work, CO2 absorption by IL/TGM mixtures was analyzed by equilibrium and kinetic gravimetric measurements and packed column simulations in Aspen Plus. Four ILs with remarkable different absorbent properties were employed. The ILs 1-butyl-methylimidazolium tricyanomethanide ([Bmim][TCM]) and 1-butyl-methylimidazolium methylsulfate ([Bmim][MeSO4]) present CO2 physical absorption, with remarkably different viscosity values. On the other hand, the ILs trihexyltetradecylphosphonium 2-cyanopyrrolide ([P66614][CNPyr]) and1-butyl-methylimidazolium acetate ([Bmim][Acetate]) were selected as CO2 chemical absorbents, being the former significantly less viscous. CO2 absorption experiments were carried out in a magnetic suspension microbalance using a wide composition range of IL/TGM mixtures, measured at 303 K and at different pressures, from 1 up to 20 bar. The results were analyzed considering thermodynamics (CO2 solubility in IL/TGM) and kinetics (CO2 diffusivity in IL/TGM) features. ILs with physical absorption presented higher CO2 solubilities and diffusivities when increasing TGM composition. In fact, the neat TGM shows higher CO2 physical absorption capacity and rate than IL/TGM mixtures at any studied opeating conditions. In contrast, ILs with CO2 chemical absorption generally present higher CO2 absorption capacity than TGM/IL mixtures, but a certain amount of TGM improves drastically the CO2 absortion kinetics with a slight decrease of the solubility. The simulation analysis of IL/TGM performance in the absorption column reveals that: i) TGM is better physical absorbent for CO2 capture than IL/TGM mixtures and neat ILs; and ii) it is possible to select finest IL/TGM mixtures which minimize the solvent and energy expenses in CO2 capture by chemical absorption.