Phase Transition of n-Heptane/Ethanol Blends from Subcritical to Supercritical Conditions

Abstract

In this study, the non-equilibrium evaporation and transcritical transition processes of the n-heptane/ethanol blends under the pure nitrogen condition are firstly analyzed, and a new transcritical criterion for the multi-component mixtures is suggested based on the molecular dynamics simulation. Firstly, the employed force fields for the n-heptane/ethanol blends are validated against the measured vapor-liquid equilibrium phase diagram and density. Then, the azeotropic phenomena are captured by the macroscopic distillation experiments for the n-heptane/ethanol blends. The MD simulation reveals that the near-azeotrope can occur in the thermodynamic non-equilibrium period for the 50%EtOH mixture in the nanometer scale, but not for the 12.5/25%EtOH mixtures. Because the lifetime of the small amount of ethanol molecules is smaller than the timescale of thermodynamic equilibrium in the 12.5/25%EtOH mixtures, which suppresses the near-azeotropic occurrence. This indicates that the non-equilibrium has a large effect on the near-azeotropic occurrence possibility. Besides, it is found that the increased ethanol concentration accelerates the evaporation rate of the n-heptane/ethanol blends in nanometer scale at low pressure, but shows little effect under high pressure conditions. Finally, the classical criterion with the large density gradient and little surface tension, is found to be insufficient to identify the transcritical transition. The molecular clusters with various sizes in the vicinity of high-density fluid and the short isothermal period are suggested to couple with the classical phenomenological criterion to determine the transcritical transition.