Solubility of CO2 and CH4 in Ionic Liquids: Ideal CO2/CH4 Selectivity

A synthetic method has been used to measure the bubble-point pressures of carbon dioxide (CO2) and methane (CH4), for a temperature range of 303.15363.15 K and for pressures up to 14 MPa, in the following ionic liquids: 1-ethyl-3-methylimidazolium diethylphosphate [emim][dep], trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate [thtdp][phos], trihexyltetradecylphosphonium dicyanamide [thtdp][dca], 1-allyl-3-methylimidazolium dicyanamide [amim][dca], 1-butyl-1-methylpyrrolidinium dicyanamide [bmpyrr][dca], 1,2,3-tris(diethylamino)cyclopropenylium dicyanamide [cprop][dca], 1,2,3-tris(diethylamino)cyclopropenylium bis(trifluoromethylsulfonyl)imide [cprop][Tf2N], 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide [bmpip][Tf2N], triethylsulfonium bis(trifluoromethylsulfonyl)imide [tes][Tf2N], and methyltrioctylammonium bis(trifluoromethylsulfonyl)imide [toa][Tf2N]. The solubility of CH4 on mole fraction basis is a factor of 10 lower than that of CO2 at similar conditions. Henry constants of CO2 and CH4 in all the ionic liquid (IL) systems are presented, from which the ideal CO2/CH4 selectivities are obtained. The ideal CO2/CH4 selectivities of the investigated ILs are in the range of the conventional solvents like Selexol, Purisol, Rectisol, Fluor Solvent, and sulfolane. The ideal CO2/CH4 selectivity decreases dramatically with increasing temperature and increasing IL molecular weight. Furthermore, the experimental data has been modeled accurately with the PengRobinson equation of state in combination with van der Waals mixing rules.