Ionic liquids as key enabling drivers for the separation of close-boiling hydrofluorocarbon refrigerant mixtures
Gabriel ZARCA1, Salvador ASENSIO-DELGADO1, Miguel VIAR1, Fernando PARDO1, Ane URTIAGA1
1Universidad de Cantabria, Santander, Spain
Most refrigerants employed nowadays in refrigeration and air-conditioning systems consist of mixtures of fluorinated hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) that usually exhibit a close-boiling or azeotropic behavior. However, current regulations and international agreements (e.g., the Kigali Amendment to the Montreal Protocol) call for a drastic phase-down of the production of virgin high-global warming HFCs. In the absence of clear environmentally-friendly, efficient and safe alternatives, the possibility of recycling these working fluids to extend their lifespan is gaining strength. To that end, the development of advanced separation processes that enable separating common HFC blends into their main constituents is required.
Considering that the phase behavior of HFCs (or HFOs) binary systems with ionic liquids (ILs) has long been studied, the use of ILs as entrainers in extractive distillation processes have attracted the most research attention. In this field, our research group has contributed to clear the selection of adequate ILs for target separations of hydrofluorocarbons by assessing the phase behavior of more than 50 novel HFC/HFO-IL systems, which together with the available literature data were collected in the Refrigerant Absorption in Ionic Liquids Database (UC-RAIL) . This allowed to perform an in-depth analysis on the influence of the IL and refrigerant properties (e.g., cation and anion type, degree of fluorination, critical properties) on the resulting gas solubility and solubility-selectivity.
Moreover, this database was used to optimize an artificial neural network that can be used as a prescreening tool (i.e., freely available spreadsheet) to predict the solubility of hydrofluorocarbons in ILs from easily accessible properties of the pure compounds . In addition, the solvation environments of some valuable refrigerants showing little solubility differences in ILs were assessed using a computational approach. Molecular dynamic simulations performed with the polarizable CL&Pol forcefield, developed by Prof. Pádua and co-workers, helped to discern the main effects influencing the solubility of target refrigerants in ILs, thus setting the basis for a rational selection and design of ILs for the intended separations .
With this knowledge, we have performed the design of IL-based extractive distillation processes using a rate-based model that considers the influence of the IL on the mass and energy transfer rates. The main operating variables (i.e., pressure, temperature, S/F ratio, HETP, feed and solvent inlet stages) were optimized to obtain high purity HFCs and HFOs from conventional refrigerant mixtures that must be phased-out.
Acknowledgments. LIFE Programme of the EU (LIFE-4-Fgases, LIFE20-CCM/ES/001748) and Agencia Estatal de Investigación (PID2019-105827RB-I00, MCIN/AEI/10.13039/501100011033, Spain).
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