Molecular investigation of sub-to-supercritical transition of hydrocarbon mixtures: Multi-component effect

Molecular dynamics (MD) simulations were conducted to better understand the evaporation and mixing behavior of liquid hydrocarbon fuels in ambient gases of varying temperatures and pressures. The emphasis was to understand the multi-component effect of both the liquid fuel and the ambient on the transition from the classical two-phase evaporation regime to the diffusion-controlled mixing regime. Mixture critical point were calculated using the Heidemann and Khalil method to connect the MD observations to the critical locus criterion. Furthermore, vapour-liquid equilibrium (VLE) using the Peng-Robinson equation of state (PR-EOS) were also calculated to understand the phase equilibria. The results show for binary fuel at low ambient temperatures and pressure, evaporation is first dominated by the lighter species and then followed by the heavier species. However, under supercritical conditions, preferential evaporation does not occur and both the lighter and heavier species dominate the evaporation process concurrently. For binary ambient, the results show that the presence of helium has very minimal effect on the mixture critical temperature and pressure and does not change the transition behavior. However, the increasing presence of water in the ambient reduces the mixture critical pressure and promotes sub-to-supercritical transition. These observations support the critical locus criterion suggesting this criterion can be used to determine the exact state of a mixture at given temperatures and pressures. (C) 2019 Elsevier Ltd. All rights reserved.