(43c) Azeotropic Refrigerant Mixture Separation Using Extractive Distillation with Ionic Liquids Entrainers

The new generation of refrigerants for replacing high-global warming potential (GWP) hydrofluorocarbons (HFCs) are hydrofluoroolefins (HFOs) and hydrofluorocarbons (HFCs) blended with HFOs. According to the American Innovation and Manufacturing (AIM) Act, the production of HFCs with high-GWP will be phased down by 85% over the next two decades. HFOs and HFO/HFC blends provide replacements for HFCs with low-GWP and zero ozone depletion potential (ODP). Blending HFOs and HFCs provides thermophysical properties that are suitable for use in replacing HFCs in existing equipment. Project EARTH (Environmentally Applied Research Towards Hydrofluorocarbons), a multi-university program led by the University of Kansas, is developing three technologies for the separation of HFC and HFC/HFO mixtures so that low-GWP components can be recycled and high-GWP components can be repurposed into environmentally safe products. These separation technologies overcome the challenges of azeotropic mixtures that are not possible to separate using conventional separation technology (i.e., fractional distillation). Extractive distillation using an ionic liquid (IL) as an entrainer can separate the azeotropic mixtures and offers a highly efficient separation.

Ionic liquids are a class of solvents that have the advantage of tunability, negligible vapor pressure, and high thermal and chemical stability. In addition, ILs have differential solubility for HFCs and HFOs and can achieve high selectivity, which makes them ideal entrainers for extractive distillation to separate azeotropic refrigerant mixtures. Refrigerant R-513A is an HFC/HFO azeotropic mixture that consists of 44 wt.% 1,1,1,2-tetrafluoroethane (HFC-134a) and 56 wt.% 2,3,3,3-tetrafluoropropene (HFO-1234yf). Vapor-liquid equilibrium data for HFC-134a and HFO-1234yf in nine different ILs were fit with the Peng-Robinson equation of state with standard mixing rule and Boston-Mathias modifications to simulate the separation of R-513A using equilibrium model in ASPEN plus. The process flow diagram was optimized with a set of physical and chemical constraints to achieve high purity of refrigerant (≥99.5 wt.%) under optimal operating conditions. The process was optimized based on each IL. The ILs used in this work are 1-ethyl-3-methylimidazolium acetate [C₂C₁im][AC], 1-ethyl-3-methylimidazolium tetrafluoroborate [C₂C₁im][BF₄], 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C₂C₁im][OTF], 1-ethyl-3-methyl-imidazolium-thiocyanate[C₂C₁im][SCN], 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C₂C₁im][Tf₂N], 1-butyl-3-methylimidazolium hexafluorophosphate [C₄C₁im] [PF₆], 1-hexyl-3-methyl-imidazolium-tetrafluoroborate [C₆C₁im][BF₄], 1-hexyl-3-methyl-imidazolium-hexafluorophosphate[C₆C₁im][PF₆], and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₆C₁im][Tf₂N].These ionic liquids were found to be highly effective solvents that facilitate the separation.