(537j) Phosphonium- and Imidazolium-Based Ionic Liquids for High-GWP Refrigerant Mixture Separation

Hydrofluorocarbon (HFC) refrigerants are undergoing an 85% phase-down over the next two decades due to their high global warming potential (GWP), as mandated by the American Innovation and Manufacturing (AIM) Act and the U.S. Environmental Protection Agency (EPA). The AIM Act regulations require a transition from HFCs to the next generation of refrigerants, hydrofluoroolefins (HFOs), which have significantly lower GWP and zero ozone-depletion potential (ODP). HFOs are being commercialized as replacements for HFCs, and blends of HFO and HFC refrigerants have been identified as promising drop-in replacements for existing equipment, maintaining performance while reducing environmental impact. Several azeotropic refrigerant mixtures, such as R-450A, R-456A, R-515B, and R-516A, offer lower GWP and similar thermophysical properties to replace HFC-134a in existing systems. However, transitioning to HFO refrigerants poses challenges, particularly in recycling due to the azeotropic nature of certain HFO/HFC mixtures. Conventional distillation methods are ineffective for separating these mixtures into pure components. Extractive distillation with an ionic liquid (IL) as the entrainer has been proposed as a solution, but its efficacy depends on the selectivity and affinity of the IL towards the mixture components.

This work identified phosphonium- and imidazolium-based ILs for the separation of commercial HFO/HFC refrigerant mixtures based on ASPEN Plus simulations. In some cases, excessive operating conditions, such as 60 equilibrium stages, high operation pressure, and higher solvent to feed ratio, were required in the extractive distillation column due to the low IL selectivity. This presentation underscores the need for measuring solubility data for HFOs, the next generation of refrigerants, in ILs that can achieve high absorption capacity, selectivity, and thermal stability as this information can significantly impact the economic viability of separation processes.