Modeling CO2 solubility in an ionic liquid: A comparison between a cubic and a group contribution EoS

Abstract

Two equations of state (EoS) have been used to describe the vapor–liquid equilibria of carbon dioxide (CO2) in the low viscosity ionic liquid (IL) 1-hexyl-3-methylimidazolium tetracyanoborate ([hmim][TCB]) at pressures up to 12MPa. The first model is the cubic Peng–Robinson (PR) EoS using both the quadratic van der Waals mixing rules and the free energy model-based Wong–Sandler mixing rules. The second model is the group contribution (GC) EoS from Skjold-Jørgensen, which combines the advantages of an EoS with those of an activity coefficient model using a group contribution approach. In both models the parameterization of the IL is very challenging. The main reason is the lack of experimental data on pure component vapor pressure and critical properties due to the negligible volatility and the decomposition of the molecule before reaching the critical point. This work analyzes the different alternatives available for calculating the pure component parameters of [hmim][TCB] of both PR EoS and GC EoS and discusses how the accuracy of the models changes with the number of fitted binary interaction parameters. The results of both EoS are compared and the suitability of both models for different applications is discussed.