(377f) Permeability, Solubility, and Diffusivity of HFC-32 and HFC-125 in Polymeric Membranes

Project EARTH (Environmentally Applied Research Towards Hydrofluorocarbons) is focused on identifying sustainable processes for the selective separation of hydrofluorocarbon (HFC) refrigerant mixtures. HFCs and HFC mixtures were developed to replace chlorofluorocarbons (CFCs), which were linked to the depletion of the Earth’s ozone layer. However, some HFCs and HFC mixtures have high global warming potentials (GWPs) and recent legislation, such as the Kigali agreement, are calling for restrictions in certain applications with an eventual phase-out of HFCs. Currently, millions of kilograms of HFCs and HFC mixtures are in use with no efficient method for their disposal. In order to avoid incinerating or venting HFC refrigerants into the atmosphere—processes that are harmful to the environment and would waste millions of kilograms of refrigerants—Project EARTH aims to develop environmentally responsible methods for separating azeotropic refrigerant mixtures so that low-GWP components can be repurposed into next-generation refrigerants. Membranes are an attractive method for separating HFC refrigerant mixtures due to lower energy consumption and smaller capital requirements compared with alternative separation methods. This project focuses on the use of polymeric membranes for the separation of R-410A—a 50-50 weight percent mixture of difluoromethane (HFC-32, CH₂F₂) and pentafluoroethane (HFC-125, CHF₂CF₃). Permeability of HFC-32 and HFC-125 in rubbery polydimethylsiloxane, glassy polyimide, and fluorinated PBVE-co-PDD were measured using a static membrane apparatus and pressure-rise method. Solubility and diffusivity were measured using a Hiden gravimetric microbalance. Permeability experiments indicate a high flux of HFCs through rubbery PDMS with moderate HFC-32 (1)/HFC-125 (2) selectivity at 35 °C (S_1/2 = 3.4). Polyimide membranes showed a similar selectivity for HFC-32 (1)/HFC-125 (2) separation at 35 °C (S_1/2 = 3.1), but exhibited very low flux. Permeability through the PBVE/PDD (5 mol%/95mol%) copolymer showed promising separation potential in regards to both permeability and selectivity as a function of temperature (selectivity of S_1/2 = 3.2 at 35 °C, S_1/2 = 13 at 65 °C).