Molecular dynamics simulation study on the transport properties of the supercritical carbon dioxide and alkanes mixture systems

The TraPPE model for carbon dioxide and the TraPPE-UA model for alkanes are used to analyze the pressure, viscosity, and thermal conductivity of supercritical carbon dioxide/alkane mixtures. Simulations are conducted on three binary mixtures of supercritical carbon dioxide with methane, ethane, and propane, having composition ratios of 0.25, 0.5, and 0.75, respectively. These simulations cover a temperature range from 400 to 600 K and pressures ranging from 5 to 80 MPa. The results for pressure are influenced by several factors, including the alkane mole fraction, density, and temperature. The viscosity results for the mixtures demonstrate a high degree of accuracy, with most state points showing relative deviations below 10% compared to the NIST data. However, the simulations of thermal conductivity reveal noticeable uncertainties, underscoring the need for refinements in the potential models to improve prediction accuracy. Notably, the equilibrium molecular dynamics method often yields higher thermal conductivity values at lower pressures compared to the non-equilibrium method. This difference is attributed to variations in heat transfer mechanisms. The discrepancy indicates that the adjustments in the Green-Kubo relations, specifically the convection terms, are necessary to enhance accuracy.