Modelling the solubility of H₂S and CO₂ in ionic liquids using PC-SAFT equation of state

The Perturbed Chain Statistical Association Fluid Theory (PC-SAFT) is used to investigate the solubility of carbon dioxide (CO2) and hydrogen sulfide (H2S) in several methylimidazolium bis (trifluoromethylsulfonyl) imide ionic liquids (ILs) or [Cnmim][NTf2] where n = 2; 4; 6 and 8. The pure component parameters of the studied ILs are estimated by fitting to experimental density data and binary solubility data of acid gases in ILs reported in literature.
Two strategies are examined to model the studied ILs. In the first strategy, ILs are modelled as neutral molecules. As for the second strategy, ILs are modelled as charged ions: imidazolium cation [Cnmim]+ and bis (trifluoromethylsulfonyl) imide anion [NTf2]-. For each strategy, four different self association schemes are examined. The schemes are: non associating, 2-sites, 3-sites and 4-sites scheme. Results indicated that, the inclusion of the association term improves the solubility fit. The 4-sites association scheme with two donors and two acceptors provided the best results for almost all investigated acid gases-IL binary systems with AARD% of (7.48%–2.76%) for H2S-ILs systems and (4.98%–1.45%) for CO2-IL systems. The solubility of acid gases in the ILs is successfully represented using PC-SAFT without the need for any binary interaction parameters if the proper association scheme is selected. The inclusion of the electrolyte term in the second strategy improves the predictive capability of the model by allowing for the examination of the effect of using different cation-anion combinations.