(636d) Thermal Conduction in Ionic Liquids, Hydrofluorocarbons, and Their Mixtures: Molecular Simulation, and Elucidation of Heat Conduction Mechanisms

Hydrofluorocarbons (HFCs) are the predominant working fluids in heating, ventilation, air-conditioning, and refrigeration (HVACR) systems. Several currently used HFCs in HVACR systems have high global warming potential (GWP), causing severe climate change concerns that have led to ongoing efforts globally to phase them out. One requirement for the sustainable phase-out of these refrigerant fluids is to separate these refrigerant mixtures into pure components for reuse and recycling instead of flaring or incinerating them. The challenge is that these mixtures are azeotropic or near-azeotropic and thus require unconventional separation technologies. Ionic liquids (ILs) in extractive distillation (ED) systems have been proposed for the sustainable separation of these high GWP refrigerant fluids. Several key thermophysical properties are required to develop the needed extractive distillation technologies. One of them is thermal conductivity (λ). Currently, there is minimal experimental λ data for IL/HFC systems. Understanding the thermal conduction of ILs and IL/HFC mixtures could provide valuable information to aid the selection or design of the optimal ILs for separating HFC mixtures. Molecular dynamics (MD) simulation is an excellent tool for predicting λ for complex systems such as pure ILs and IL/HFC systems. Furthermore, MD offers a route to gaining a fundamental understanding of thermal conduction for these complex systems.

This talk presents results from the first MD study on thermal conduction in IL/HFC mixtures. We applied non-equilibrium MD to predict λ for a range of HFC-IL systems. The predicted values of λ for the pure ILs and the IL/HFC mixtures for the range of HFC mole fractions studied showed excellent qualitative agreement with experiments but overestimated the experimental values by 30 – 40 %. However, the pure HFC λ predictions showed excellent qualitative and quantitative agreement with experiments. To better understand these results, we performed a heat flux decomposition (HFD) analysis to investigate the dominant molecular modes of heat transfer in the studied systems. We found that one of the dominant molecular mechanisms of heat transfer for ILs and IL/HFC mixtures with low to moderate HFC mole fractions, was through bonded interactions comprised of heat transfer through bond stretching, angle bending, and proper & improper torsional barriers. The molecular convection contribution to the observed heat flux was small for pure ILs but was a dominant mode of heat transfer for the pure HFCs with bonded interactions making only a small contribution for the pure HFCs. This explains the overestimation of λ using classical force fields for ILs and IL/HFC mixtures. Systems with high bonded interaction contributions to the heat flux will have their thermal conductivity overestimated when classical all-atom force fields (FFs) are used. This insight provides a potential route for correcting the predictions of λ using classical FFs.

The results of this study have provided a better understanding of thermal conduction in IL/HFC systems with potential application for high throughput and accurate prediction of thermal conductivity for selecting ILs to use in ED technologies for refrigerant recovery and recycling.