Theoretical Study of Maxwell-Stefan Binary Diffusion Coefficients of n-alkanes-N2 at High Densities

Abstract

ABSTRACT In this study, the Maxwell-Stefan binary diffusion coefficients ( D ij ) of C1-C6 n-alkanes-N2 are calculated using the Green-Kubo formula. The ratio of pD ij at different pressures (from 1 to 1000 bar) to that at 1 atm (reference condition) is studied. It is found that the ratio pD ij /(pD ij ) ref first decreases and then increases with increasing density, finally reaching 1.3 for n-C6H14-N2 at 1000 bar, 600 K. The gradual increase in molecular attraction at the beginning of the density increase results in a slower pressure increase compared to that of an ideal gas, contributing to the decreasing trend of pD ij /(pD ij ) ref . As the density continues to increase, the enhanced molecular repulsion and the molecular volume exclusion effect accelerate the pressure increase beyond that of an ideal gas, leading to an increasing trend in pD ij /(pD ij ) ref . Finally, the results obtained by the Green-Kubo method are compared with those obtained from empirical methods, and an equation for the refinement of D ij is formulated.