Enhanced absorption separation of hydrofluorocarbon/hydrofluoroolefin refrigerant blends using ionic liquids

Abstract

The separation of hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) from novel refrigerant blends will become essential to boost the recycling of these compounds and drastically reduce the emission of HFCs, which are powerful greenhouse gases. In this work, the use of ionic liquids (ILs) as solvent media is explored to perform the selective separation of HFC/HFO refrigerant mixtures composed of the HFCs R32 (difluoromethane) and R134a (1,1,1,2-tetrafluoroethane) and the HFO R1234yf (2,3,3,3-tetrafluoropropene). The low-viscosity IL 1-ethyl-3-methylimidazolium thiocyanate, [C2mim][SCN], is selected as separation agent to prove that small and non-fluorinated ILs, with lower hydrogen bonding capability than other ILs, can provide enhanced solubility selectivity and mass transport properties for the selective separation of HFC/HFO refrigerant blends. The phase behavior of IL-refrigerant systems is determined at temperatures between 283.15 and 313.15 K and pressures up to 0.7 MPa. Results are parametrized according to the NRTL activity model and the separation selectivity is assessed in terms of Henry’s law constants, solvation enthalpies and entropies, infinite dilution coefficients, Gibbs free energy of mixing, and ideal and noncompetitive selectivity. Results show superior HFC/HFO solubility selectivity in [C2mim][SCN] compared to other ILs due to unfavorable entropic effects that difficult the solvation of large molecules such as R1234yf.