CO2 separation using ionic liquid-based absorbents : thermodynamics and kinetics

Abstract: Ionic liquids (ILs) have shown great potential to be used as absorbents for CO2 separation owing to their unique properties such as immeasurably low vapour pressure, high thermal stability, high CO2 affinity, and tunable structure. A huge amount of researches have been carried out, and most of them focused on developing novel ILs for CO2 separation. However, high viscosity will impede the IL-based absorbents for the large-scale applications in CO2 separation.The goal of this thesis was to develop IL-based hybrid green absorbents (i.e. choline-2-pyrrolidine-carboxylic acid/polyethylene glycol/H2O ([Cho][Pro]/PEG200/H2O) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/titanium dioxide /PEG200 ([Hmim][Tf2N]/TiO2/PEG200)) for CO2 separation, in which H2O and/or PEG200 were used as co-solvents to adjust the viscosity. A systematic study was conducted from pure to multi-component systems convering experimental measurements and theoretical modeling of thermdynmaics and kinetics, and thus the effects of co-solvents on properties, phase equilibria, and kinetics were also analyzed.To provide reliable and systematic properties and study the effect of H2O, the properties of PEG200 and PEG200/H2O were surveyed, evaluated, and correlated. The density and viscosity of [Cho][Pro]/PEG200/H2O were measured and correlated, and the excess mole volume and excess Gibbs energy of activation were estimated to understand the molecular structure and interactions between [Cho][Pro]/PEG200 and H2O. The results showed that H2O and PEG200 could decrease the viscosity of the hybrid absorbent significantly, and H2O interacted strongly with [Cho][Pro]/PEG200.To obtain reliable and systematic gas solibilities and analyze the effect of H2O, the solubilities of pure CO2, CH4, and N2 in PEG200 were surveyed, measured, evaluated, and described with theoretical models. The CO2, CH4, and N2 solubilities in PEG200/H2O were measured and described with the Redlich-Kwong Nonrandom-Two-Liquid (RK-NRTL) model. The CO2 solubilities in [Cho][Pro]/PEG200 and [Cho][Pro]/H2O were also surveyed, measured, evaluated, and described using the RK-NRTL model. Furthermore, CO2 solubility in the hybrid absorbent was predicted with the model and compared with the new experimental results. The further investigation showed that the formation of carbomate dominated the entire CO2 absorption process by [Cho][Pro]/PEG200/H2O, and the presence of H2O resulted in the formation of bicarbonate.To study the kinetics systematically combined with the effect of H2O, the liquid-side mass-transfer coefficients (kL) of [Cho][Pro]/PEG200/H2O were obtained from the CO2 diffusion coefficients () and the kL values in PEG200/H2O, in which was calculated from the density and viscosity of the hybrid absorbents, while the kL values in PEG200/H2O were measured experimentally. The reaction rate constant of CO2 in the hybrid absorbents was also estimated.Based on the systematic studies from experimental measurement and modeling, it showed that 30 wt%[Cho][Pro]/H2O + PEG200 could be promising for CO2 separation with the consideration of both thermodynamic and kinetic properties. In addition, the hybrid absorbent also showed the good recyclability.To use the confinement as the other strategy for further developing IL-based technology, the CO2 solubility in [Hmim][Tf2N]/TiO2/PEG200 with different ratios of [Hmim][Tf2N]/TiO2 was measured, and the Henry’s constant and absorption enthalpy of CO2 in the hybrid absorbent were calculated. The CO2 solubility in the hybrid absorbent was described with the Henry's law, the contribution of the confinement effect on the CO2 solubility was quantified, and the relationship between the roughness of TiO2 and molecular parameters was established. The results showed that the mass ratio of [Hmim][NTf2]/TiO2 in the hybrid absorbent should be lower than 5.72 %, where the contribution of confinement occupied around 20 % of the total CO2 absorption capacity in [Hmim][NTf2]/T500/PEG200. 2.88 wt%[Hmim][NTf2]/T500 + PEG200 could be promising for CO2 separation on aspects of CO2 capacity and absorption enthplay, and further study will be conducted on the properties and CO2 absorption kinetics.