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 280K to 370K and pressures up to 12MPa 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.