Because injected fuel is subjected to extreme temperatures and pressures in modern diesel and jet engines, there is much interest in understanding vapor-liquid equilibrium (VLE), interfacial properties, and supercritical behavior of hydrocarbons and their mixtures. Molecular dynamics (MD) simulations can be used to predict relevant thermophysical properties, as well as provide atomic-level details of fuel structure. We have performed VLE simulations of twelve pure hydrocarbons to assess the accuracy of three MD force fields in predicting surface tension as a function of temperature, as well as critical points. Simulations were performed with the all-atom CHARMM and OPLS-AA, and united-atom TraPPE-UA, force fields. Because TraPPE-UA was parameterized using VLE data, it provided the most accurate critical temperature values, with CHARMM and OPLS-AA predicting critical temperatures that were too low compared to experiment. However, all three force fields predict critical densities and pressures with about the same accuracy. In addition, when properties such as liquid and vapor densities, vapor pressures, and surface tensions are plotted versus reduced temperature, the results from the three force fields are nearly identical. Thus, while TraPPE-UA has better quantitative accuracy, CHARMM and OPLS-AA could still be used to give qualitative insights into hydrocarbon behavior as a function of temperature. This could be important in high-pressure regimes, where the united-atom TraPPE-UA might fail to capture aspects of fluid structure that would be modeled by all-atom force fields. In addition to pure hydrocarbons, we have also studied binary systems of a hydrocarbon with nitrogen, to mimic mixing of fuel with air. These simulations were used to examine the accuracy of the traditional mixing rules used to determine interactions between unlike molecules. Binary simulations were used to gain insight into the interfacial behavior of fuel-nitrogen systems as a function of temperature and pressure, as well as to examine the clustering behavior of molecules at supercritical conditions.

Predicting properties of near- and supercritical hydrocarbon mixtures using molecular dynamics

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.