EXPERIMENTAL-VERIFICATION OF THE MAXWELL-STEFAN THEORY FOR MICROPORE DIFFUSION

The main objective of this paper is to test the capability of the Maxw ell-Stefan theory for predicting the diffusion behaviour of multicompo nent mixtures within micropores on the basis of the diffusion behaviou r of single components. Diffusion within micropores involves movement of sorbed species. It is an activated process. The proper driving forc e is the gradient of the surface chemical potential. In the Maxwell-St efan theory for micropore diffusion, the adsorption sites on the surfa ce are viewed as pseudo-species, analogous to craters on the surface o f the moon. The surface coverage has a significant influence on the ma ss transfer fluxes. The Maxwell-Stefan theory yields alternative appro aches to micropore diffusion as special cases and is consistent with t he theory of irreversible thermodynamics. The Maxwell-Stefan diffusivi ty for signal-component diffusion D is usually referred to as the ''co rrected'' diffusivity in the literature. For binary diffusion, the Max well-Stefan equations, when combined with Langmuir adsorption equilibr ium, coincide with the formulation derived by Habgood in 1958. To test the predictive capability of the Maxwell-Stefan theory we performed b reakthrough experiments with single components and mixtures containing methane, carbon dioxide, propane and propene. These experiments were carried out in a packed bed of microporous activated carbon and with c arbon molecular sieves. It is clearly demonstrated that the mixture be haviour can be predicted by the Maxwell-Stefan theory extremely well u nder a wide range of conditions: co-adsorption, co-desorption and coun ter-sorption. A model in which the matrix of Fick diffusivities is ass umed to be constant is shown to be less successful in this regard.