Expression for the Viscosity and Diffusivity Product Applicable for Supercritical Fluids

Abstract

This study presents an expression for the viscosity−diffusivity product, derived using the kinetic gas theory and an expansion term into supercritical densities by applying exponential Boltzmann factors. This leads to a transport relation, which describes the diffusivity and viscosity at any fluid density.The equation can be combined with an equation of state such as the Peng−Robinson equation of state (Peng, D. Y.; Robinson, D. B. Ind. Eng. Chem. Fundam. 1976, 15, 59−64). Using the transport relation and viscosity data, binary Maxwell-Stefan diffusion coefficients at supercritical conditions can be estimated. It is shown that, when increasing the density, the model predicts diffusion coefficients that subsequently follow the kinetic theory for gas densities, the hard sphere relation for dense fluids and the Stokes−Einstein equation for liquids. Data are applied to illustrate the predicting quality of the expression. It is shown that accuracies for diffusion coefficients of 25% is obtained. This is in most cases enough for engineering purposes.