Extractive Distillation for Separating Refrigerants Using Ionic Liquids

Hydrofluorocarbons (HFC) refrigerants are widely used in refrigeration and air conditioning applications. Although HFCs have zero ozone-depleting potential compared to chlorofluorocarbons (CFCs), many HFCs have high global warming potential (GWP). The need to recycle and repurpose these refrigerants are critical for reducing the environmental impact. Traditional distillation methods are inadequate for separating azeotropic refrigerant mixtures, which requires alternative approaches such as extractive distillation using ionic liquids (EDIL). A pilot-scale EDIL for studying the separation of azeotropic refrigerant mixtures is located at the University of Kansas.

This research focuses on optimizing EDIL to separate azeotropic mixtures containing HFC-32 (difluoromethane, CH₂F₂), HFC-125 (pentafluoroethane, CHF₂CF₃), HFC-143a (1,1,1-trifluoroethane, CH₃CF₃), HFC-134a (1,1,1,2-tetrafluoroethane), and HFO-1234yf (2,3,3,3-tetrafluoropropene, CH₂=CFCF₃), with 99.5 wt% purity of each component. The separation of refrigerants such as R-410A (HFC-32/HFC-125 50/50 wt%), R-507 (HFC-143a/HFC-125 50/50 wt%), R-404A (HFC-125/HFC-143a/HFC-134a 44/52/4 wt%), R-407C (HFC-134a/HFC-125/HFC-32 52/25/23 wt%), and R-513A (HFC-134a/HFO-1234yf 56/44 wt%) are being studied using the EDIL. The purity of the distillate and bottom products are analyzed using a Buck Scientific, (model 910) gas chromatograph with a thermal conductivity detector, a HayeSep D packed column, and a Valco 0.5 ml sampling valve.

A process simulation using an ASPEN Plus rate-based model is in good agreement with the experimental results. Binary interaction parameters based on vapor-liquid equilibria for ten ionic liquids (ILs) with HFC-32 and HFC-125 were modeled using the Peng-Robinson equation of state with Boston-Mathias mixing rule (PR-BM). This allowed us to determine the best IL for separating R-410A based on a solubility-selectivity Pareto front. The VLE regression was in good agreement with the experimental data. The best IL for the separation of R-410A was 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide [C₂C₁im][Tf₂N]. . A sensitivity analysis found the optimal operational parameters for achieving the highest distillate and bottom product purities was 20 feed stages, solvent-to-feed ratio of 6, and a reflux-ratio of 3.

Rate-based modeling requires measured transport properties such as density, viscosity, and surface tension for the HFC + IL mixtures. The surface tension for HFC-32 and HFC-125 in mixtures with the IL [C₂C₁im][Tf₂N] under high-pressure and high-temperature conditions were measured using the pendant drop method over a temperature range from 293.15 to 373.15 K and a pressure range from 1 to 9 bar. The experimental data was successfully regressed using Li-Fu-Wang correlation.