High pressure phase equilibria of binary mixtures of light hydrocarbons in the ionic liquid 1-hexyl-3-methylimidazolium tetracyanoborate

Ionic liquids have been proposed as alternative solvents for several chemical processes, including organic reactions and separations. Their well-known negligible volatility makes them perfect candidates for gas separations as no solvent losses are expected. 1-Hexyl-3-methylimidazolium tetracyanoborate ([hmim][TCB]) is a low viscosity ionic liquid with high capacity for carbon dioxide (CO2) absorption. In order to study whether the selected ionic liquid [hmim][TCB] is an appealing solvent for CO2 absorption, it is equally important that other constituents in the gas mixture will have a low absorption in [hmim][TCB]. In case we are dealing with CO2 removal from natural gas, low absorption in the ionic liquid of the light hydrocarbons like methane (CH4), ethane (C2H6) and propane (C3H8) is a prerequisite. In this paper, the solubility of CH4, C2H6 and C3 has been determined at temperatures ranging from 280 K to 370 K and pressures up to 12 MPa using a synthetic method. The CO2/CH4 selectivity has been found to be very high and a liquid–liquid partial immiscibility has been found for ethane and propane in [hmim][TCB]. The phase behavior of the system has been discussed.