ANALYSIS OF SUBCRITICAL TO SUPERCRITICAL TRANSITION OF N-HEPTANE/ETHANOL BLENDS BY MOLECULAR DYNAMICS SIMULATION

Abstract

The transcritical transition of the hydrocarbon/alcohol blends from subcritical to supercritical regime has not been yet well understood. In the present paper, based on the molecular dynamics simulation, the non-equilibrium evaporation and transcritical transition processes of the n-heptane/ethanol mixtures are comprehensively investigated under various ambient conditions, and the development of the vapor-liquid interface is deeply analyzed. Under the subcritical condition, with the increased ethanol concentration, the evaporation rate of liquid film is enhanced because of the decreased vapor pressure and increased thermal conductivity. However, under the supercritical condition, due to the comparable thermal conductivity and diffusion rate of ethanol and n-heptane, the effect of ethanol addition on the mixture temperature becomes slight. In addition, at the moderately high pressure, the increased addition of ethanol leads to the broadened vapor-liquid interface, because of the higher volatility and thermal conductivity of ethanol. Since the interfacial thickness affects the interfacial resistivities and evaporation characteristics in the hydrodynamic framework, the gradually thickened interface indicates the more significant non-equilibrium effect in the vicinity of interface for the mixtures with large ethanol addition. Finally, the transcritical transition time is observed to rapidly decrease with the increased ambient pressure, but the descent gradient gradually decreases with the further increased pressure, because the thermal conductivity becomes less sensitive to the pressure.