How Molecular Simulations Are Being Used to Phase out High Global Warming Potential Hydrofluorocarbon Refrigerants

Co-authors: Ning Wang, Ryan Smith, Eliseo Marin-Rimoldi, Bridgette Befort, Alexander Dowling, and Edward J. Maginn

Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556

The Montreal Protocol of 1987 phased out chlorofluorocarbon (CFC) refrigerants because of their high ozone depletion potential. The replacements, typically mixtures of hydrofluorocarbons (HFCs), are safe for the Earth's ozone layer, but most have been found to have high global warming potentials (GWPs). As a result, 197 countries signed the Kigali agreement in 2016 to phase out the use of high-GWP HFCs with the goal of reducing emissions by 80% in the next 20 years. The U.S. Environmental Protection Agency is proposing its first rule under the American Innovation and Manufacturing (AIM) Act of 2020 to phase down the production and consumption of hydrofluorocarbons (HFCs). There are currently millions of metric tons of high-GWP refrigerants that will need to be reclaimed, but there are no good methods for separating and recycling the individual components, given that many are azeotropic or near-azeotropic mixtures.

In this talk, we will describe our recent work using molecular simulations to compute key thermophysical properties of HFCs and their mixtures with ionic liquid solvents and adsorbents. The simulations provide important data that can help identify technologies that can be used to recover and repurpose HFCs. Using machine learning and optimization approaches, we show how accurate HFC force fields can be developed and used to make property predictions. We describe ways of rapidly estimating low pressure Henry’s Law constants of HFCs in ionic liquid solvents, enabling us to screen over hundreds of potential ionic liquid solvents. We also demonstrate how advanced free energy sampling methods can be used to compute full isotherms of HFCs and HFC mixtures in solvents and porous solid adsorbents. Finally, we use a range of equilibrium and non-equilibrium molecular dynamics simulations to compute key transport properties of HFCs and their mixtures with ionic liquids.