(102a) Porous Materials for Separation of Fluorocarbon Refrigerant Mixtures

The most popular fluorocarbon refrigerants in circulation today are hydrofluorocarbon (HFC) refrigerants. However, HFCs have significantly higher global warming potentials (GWPs) compared with carbon dioxide. As a result, the production and use of HFCs is currently being phased down according to the Kyoto Protocol (2005), F-gas regulations in the European Union (2014), the Kigali Amendment to the Montreal Protocol (2016), and most recently the coronavirus relief bill passed by the U.S. Congress and signed by the president (2020). Therefore, there is a pressing need to replace existing HFC refrigerants with more environmentally sustainable options. The next generation of fluorocarbon refrigerants are based on hydrofluoroolefins (HFOs). While many HFCs have a GWP of over 1000, HFOs have a GWP of 1-10. To replace the existing HFCs with HFOs, this will require the removal of an estimated 846 million kilograms of refrigerant currently in use worldwide. Rather than venting or incinerating the HFCs, a more environmentally sustainable option is required. However, several HFC refrigerants exist as mixtures which form azeotropic compositions; the separation of these refrigerants is more challenging and new technologies will be required to efficiently recycle these gases.

Porous materials such as zeolites, activated carbons, and metal organic frameworks (MOFs) have the potential to separate fluorocarbons. Not only have they been proven in the literature to successfully separate zeotropic, azeotropic, and isomeric fluorocarbon mixtures, but also the energy input required is less than that needed for conventional distillation methods. This presentation discusses ongoing work regarding the use of porous materials for fluorocarbon separation. Work has been conducted using a Hiden Isochema XEMIS gravimetric microbalance to measure the pure gas sorption isotherms of HFC-125 (pentafluoroethane) and HFC-32 (difluoromethane) with zeolites 3A, 4A, and 5A. These sorption measurements have revealed the potential for the use of zeolites in separating the commercial refrigerant R-410A (50/50 wt% HFC-125/HFC-32). Heat of adsorption measurements have been performed directly using a Setaram calorimeter and compared with predictions based on the pure gas sorption measurements. Selectivity values for the separation of HFC-32 and HFC-125 are predicted using pure gas sorption isotherms. The predicted selectivities are compared to those measured using a XEMIS microbalance capable of directly measuring the adsorption of mixtures using the integral mass balance (IMB) method.