Development of Membrane Technology for the Separation of Azeotropic Refrigerant Mixtures

Rising temperatures across the globe have caused a surge in demand for cooling technologies—the global number of air conditioners will increase from 1.6 billion today to 5.6 billion in 2050, which is equivalent to 5 air conditioners being sold every second over the next 26 years. While access to air-conditioning and refrigeration is imperative for the survival of people in Earth’s hottest regions, the use of refrigerants has an environmental impact. The same compounds that provide relief from increasing global temperatures are some of the most potent contributors to global warming. It is estimated that 20% of global energy consumption results from the demand for refrigeration and air conditioning. Today’s generation of refrigerants, hydrofluorocarbons, can have global warming potentials up to 4000 times higher than CO2. The negative environmental impact of refrigerants has led to recent legislation such as the American Innovation and Manufacturing Act that requires a scheduled phasedown in hydrofluorocarbon refrigerant production over the next two decades. Although the production of these refrigerants is decreasing, new units designed for hydrofluorocarbons are still being sold into the market and existing units require hydrofluorocarbons to be serviced. A process for separating multi-component, azeotropic mixtures is urgently needed to account for the anticipated supply shortages in the refrigerant market and provide a stream of recycled refrigerant. This work investigated membrane technology for the energy-efficient separation of the refrigerant mixture R-410A, a widely used refrigerant for residential cooling and heat pump applications. R-410A is an azeotropic, 50-50 wt% mixture of difluoromethane and pentafluoroethane. Membrane separation offers a more energy efficient alternative to traditional separation processes like distillation since it is not reliant on phase-change or high temperature. Mixtures can be separated with membranes based on differences in both solubility and diffusivity. High performing polymers exhibit high permeability, a measure of polymer throughput for a given mixture component, and selectivity, the ratio of permeabilities for a given mixture. Polymers resistant to physical aging and plasticization are required for real-world utilization. This work has identified fluorinated, amorphous polymers to be promising materials for the separation of refrigerant gases due to high permeability, selectivity, resistance to plasticization, and stable separation performance over time. Investigation methods for polymeric materials included pure-gas permeability and mixed-gas permeability with the pressure-rise method, solubility with the gravimetric method, and diffusivity analysis with Fickian models. The scale-up of promising materials was accomplished through the fabrication of composite hollow fiber membranes. Amorphous copolymer of perfluoro(butenyl vinyl ether) and perfluoro(2,2-dimethyl-1, 3-dioxole) have been identified to provide a tunable, size-driven separation of R-410A. Increasing the amount of the dioxole-containing monomer increases the free volume of the polymer, leading to a sizeable increase in permeability of the smaller difluoromethane refrigerant. The lag in the pentafluoroethane permeability leads to an increase in membrane selectivity until a critical volume is reached. Beyond the critical volume, the permeability of both gases increases and the selectivity decreases. The findings indicate that fluorinated copolymers can be designed with optimal free volume to maximize both permeability and selectivity for refrigerant separations. In addition to amorphous perfluoropolymers, copolymers consisting of a perfluorinated monomer and a non-fluorinated vinyl acetate monomer were identified to further improve the separation of R-410A. The combination of high free volume from the perfluoropolymer and the chain mobility of the vinyl acetate monomer resulted in a highly diffusivity-driven separation. Composite membranes of ionic liquids and perfluoropolymers were also synthesized to assess the impact of ionic liquid on separation performance. The introduction of ionic liquid into the system resulted in an improved solubility selectivity that provides an additional design strategy to separate refrigerants. Amorphous perfluoropolymers were scaled up through the development of composite membranes of porous hollow fiber supports with a selective layer of perfluoropolymer. Predictive correlations to guide hollow fiber coating were developed based on physical properties of the coating solution. A continuous, reel-to-reel coating apparatus was developed capable of forming submicron coatings on porous hollow fiber supports. The fabricated hollow fibers were used to separate R-410A to a purity > 95 mol% for HFC-32 in a single pass. The large-scale separation of R-410A with fluorinated polymers was modeled in ASPEN Plus with the MEMSIC software tool. 2-stage and 3-stage separations were designed to assess achievable purities, energy costs, and area requirements. Results predict that membrane technology can be used to separate refrigerants with low energy usage to purities greater than 99.5 wt%—the purity required for refrigerant resale.

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