Gas solubility and diffusivity of hydrofluorocarbons and hydrofluoroolefins in cyanide-based ionic liquids for the separation of refrigerant mixtures

Abstract

Ionic liquids (ILs) have been extensively studied for their use as solvents in separation processes. In this work, we determine the vapor-liquid equilibria (VLE) and diffusion coefficients at infinite dilution of three hydrofluorocarbons (R32 - difluoromethane, R125 - pentafluoroethane, and R134a - 1,1,1,2-tetrafluoroethane) and two hydrofluoroolefins (R1234yf - 2,3,3,3-tetrafluoropropene, and R1234ze(E) - trans-1,3,3,3-tetrafluoropropene) and the ILs 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium dicyanamide ([C2mim][dca] and [C4mim][dca]). Additionally, we study the VLE of R125 and [C2mim][SCN] (thiocyanate) to complete the previous studies with this liquid. These ILs have been selected because other ILs with cyanide moieties in the anion have shown good selectivity to separate fluorinated refrigerant gases and their low viscosity would improve the mass transfer and reduce the pumping costs in a separation process. The experimental data have been determined at temperatures from 283.15 K to 323.15 K and pressures up to 1 MPa, and the nonrandom two-liquid model (NRTL) has been used to fit the VLE. The non-competitive ideal selectivity and the separation factor achieved in an isothermal flash operating at 303.15 K and 5 bar have been evaluated for target mixtures of interest. The use of dicyanamide based ILs leads to the best selectivity results to date, concluding that the studied ILs are promising for the design of separation processes that selectively recover fluorinated refrigerant gases.