(189c) Separation of Azeotropic 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) and hydrochlorofluorocarbons (HCFCs), which were linked to the depletion of the Earth’s ozone layer. HFCs have zero ozone depletion potential (ODP), but some HFCs have a high Global Warming Potential (GWP). The next-generation refrigerants, hydrofluoroolefins (HFOs) are being developed to replace HFCs in certain applications. The Kigali Amendment to the Montreal Protocol became effective in January 2019 and limits the production of HFCs over the next two decades; therefore, 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 HFC mixtures are azeotropic or near azeotropic (i.e., R-410A, R-404A, R-407C) making the separation of the components nearly impossible using current distillation methods.

Project EARTH (Environmentally Applied Research Towards Hydrofluorocarbons) aims to develop environmentally responsible materials and techniques to separate azeotropic HFC mixtures and recycle the pure component refrigerants.

Extractive distillation is the most widely used form of technology in the separation of homogeneous azeotropic mixtures or close boiling point fluids with the use of an additional solvent, an entrainer, to alter the liquid phase properties and modify the volatility of each component. Organic liquid solvents have been the primary choice as entrainers, but ionic liquids (ILs) have been shown to have high selectivity for some separation processes. There are only three proposed designs that have been modeled to separate a binary mixture of low boiling compounds with an IL entrainer (e.g., carbon dioxide/ethane, tetrafluoroethylene/carbon dioxide, and difluoromethane (HFC-32) /pentafluoroethane (HFC-125)). Furthermore, there has not been any work on separating ternary mixtures.

A rate-based and equilibrium model have been developed using Aspen Plus (v10) to separate binary (R-410A) and ternary refrigerant mixtures (R-404A, R-407C, and R-410A mixed with 10 wt% chlorodifluoromethane (HCFC-22)). Each refrigerant mixture contains at least one azeotrope and extractive distillation will be required to break these azeotropes and separate these binary and ternary mixtures.

This presentation will discuss the optimization procedure to develop an ASPEN Plus process flow sheet for separating these refrigerant mixtures while achieving a minimum purity of 99.5 wt% for each component. Two ILs ([C₂C₁im][Tf₂N] and [C₄C₁im][PF₆]) will be evaluated and discussed as entrainers for these separation processes.