Phenomenological Models of Diffusivities Based on Local Composition

Abstract

In phenomenological models, diffusivity is at least a function of composition and the diffusivities at infinite dilution. An additional parameter , which can be determined by diffusivity in midpoint, are specially brought forward as token of fractional friction related with the interactions of same molecules in this paper, to extrapolate a new correlative equation for the mutual Maxwell-Stefan diffusivities. Furthermore, the correlative equation can be extended to calculate diffusivities in multicomponent mixtures based on binary data alone. The rate of random motion of molecule i, which determine diffusional behavior, consider to be depended on the local composition (xji), comparatively on the average mole fraction (xi and xj), and local composition is calculated by binary thermodynamic parameters available, such as Wilson and NRTL parameters. The theoretical calculations are evaluated with published experimental data. The total average relative deviation of predicted values with respect to experimental data is 4.43% for 17 binary systems. And the M-S diffusivities in a three-component liquid system are regarded as binary coefficients, the predictive results also agree with the experimental data. Results indicate that the model with additional coefficients is superior to currently used Darken methods, especially for systems of polar organic-water and those containing associative component. Keywords： diffusivity, diffusion, phenomenological models, Maxwell-Stefan’s law