In this work, we use molecular simulations to discover new ionic liquid (IL) separating agents for azeotropic separation of hydrofluorocarbon (HFC) mixtures. Mandated by the 1987 Montreal Protocol, chlorofluorocarbon (CFC) refrigerants have been gradually replaced by HFCs to prevent ozone depletion. Many of these second-generation HFC mixtures, however, have a high global warming potential (GWP) and the 2016 Kigali agreement ordered their gradual phase-out [1]. Due to the often azeotropic compositions of these HFC refrigerants, existing separation methods for removing the high GWP components are currently infeasible or not practical, but it is wasteful to incinerate HFC mixtures as some HFC components have low GWPs and can be recycled. We hypothesize that custom ILs can be designed to remove low GWP HFC components from specific HFC mixtures [2][3]. However, because millions of potential ILs exist [4], prediction of IL properties via molecular simulations is necessary to screen for process-specific ILs prior to their synthesis. To correctly simulate IL-HFC systems, molecular simulations require accurate force field parameters for the molecules being simulated. To this end, molecular simulations were performed to determine thermophysical properties, including phase equilibria, of HFCs using the Cassandra Monte Carlo modeling package. Separate simulations were run using parameters from three force fields, Generalized Amber Force Field (GAFF), Optimized Potentials for Liquid Simulations (OPLS), and a customized force field. The critical temperatures and pressures of the HFCs were calculated using the traditional method of rectilinear diameters. The resulting critical properties for each force field were compared to experimental thermodynamic data to determine optimal force field parameters for the HFC molecules. We then quantified the uncertainty in the critical property predictions to study how the uncertainty propagates to single-component phase equilibria calculations [5]. This uncertainty analysis and comparison with experimental data offers quantitative insights to the suitability of molecular simulations to screen ILs for further study as HFC separating agents.
References
[1]United Nations Environment Programme. Ozone Secretariat. (2006). Handbook for the Montreal protocol on substances that deplete the ozone layer. UNEP/Earthprint.
[2]Plechkova, N. V., & Seddon, K. R. (2008). Applications of ionic liquids in the chemical industry. Chemical Society Reviews, 37(1), 123-150.
[3]Chávez-Islas, L. M., Vasquez-Medrano, R., & Flores-Tlacuahuac, A. (2011). Optimal molecular design of ionic liquids for high-purity bioethanol production. Industrial & Engineering Chemistry Research, 50(9), 5153-5168.
[4]Holbrey, J. D., & Seddon, K. R. (1999). Ionic liquids. Clean products and processes, 1(4), 223-236.
[5] Hajipour, S., & Satyro, M. A. (2011). Uncertainty analysis applied to thermodynamic models and process design–1. Pure components. Fluid Phase Equilibria, 307(1), 78-94.
