(374s) Separation of Refrigerant Mixtures Using Extractive Distillation with Ionic Liquid Entrainers

Hydrofluorocarbons (HFCs) have been used as refrigerants globally since the 1990’s and replaced chlorofluorocarbons (CFCs), which were linked to the depletion of the Earth’s ozone layer. HFC refrigerants are being phased out over the next two decades due to their high global warming potential. The refrigerant industry is moving toward the production and marketing of HFOs and HFO/HFC refrigerant blends. This will require the recycling and repurposing of HFC mixtures, but many of the HFC mixtures are azeotropic or near azeotropic (i.e., R-410A, R-404A, R-407C, to name a few) making the separation of the components impossible using current methods. Extractive distillation using an ionic liquid entrainers offers a solution.

Ionic liquids (ILs) are being developed as novel entrainers for extractive distillation because of their negligible vapor pressure, thermal and chemical stability, and high and differential solubility with HFC refrigerants. Vapor liquid equilibria data for refrigerants difluoromethane (HFC-32), chlorodifluoromethane (HCFC-22), pentafluoroethane (HFC-125), 1,1,1-trifluoroethane (HFC-143a), and 1,1,1,2-tetrafluoroethane (HFC-134a) in ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂C₁im][Tf₂N]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄C₁im][PF₆]) were fit with the Peng−Robinson equation of state to simulate the separation of multi-component azeotropic refrigerant mixtures and to develop rate-based and equilibrium models in ASPEN Plus. Process flow diagrams (PFD) were developed and optimized based on a set of physical and chemical constraints with the goal to optimize parameters and achieve refrigerant grade (>99.5 wt%) purity. The ionic liquids were found to be effective entrainers for separating refrigerant mixtures.