Multicomponent interphase diffusion of carbon dioxide–methanol–water under near-critical conditions

Abstract

Multicomponent diffusion between liquid and vapor phases is analyzed for a closed system at high pressure. The movement of the interface during the diffusion is taken into account. The principle model used is the Fick's law in one-dimensional form, which is a second-order partial differential equation with respect to time and a spatial dimension. The model is solved using fixed grid explicit finite difference method. The Fick diffusivities are estimated at each iteration using the Maxwell–Stefan approach, which decouples the drag effects from the thermodynamic nonideality effects. It is found that the thermodynamic correction factor, due to nonideality effects, enhances the off-diagonal Fick diffusivities for gases at high pressure, particularly near critical conditions. The off-diagonal diffusivities in turn make a significant change on the composition-time trajectories.