Sorption kinetics in zeolite crystals with finite intracrystal mass exchange: Isothermal systems

Abstract

In this paper we present a theoretical analysis of sorption kinetics in zeolite crystals which are assumed to consist of transport channels and capacity channels. The transport channels not only allow sorbate molecules an access into the crystal interior but also provide a capacity sink for these sorbate molecules, while the capacity channels intersecting with the transport channels act only as a capacity sink. The sorbate molecules in these channels are treated in this analysis as two distinct sorbed species or phases. The transport of sorbate molecules in the transport channels is assumed to be driven by the gradient of chemical potential. The interchange of mass between the sorbates in the transport channels and the capacity channels is assumed to follow Langmuir type kinetics at the intersection between the two channel systems. There is assumed to be no diffusional resistance within the capacity channels. A general model is proposed and degenerate models are also identified when one of the mass transfer resistances is negligible. We have shown in this analysis that the apparent intracrystalline diffusivity obtained under differential conditions and defined in accordance with the traditional Fickian equation may exhibit diverse trends with increasing loading. This behaviour pattern depends on the relative affinities and saturation capacities of the transport channels and the capacity channels and also on the ratio of the mass exchange rate at the channel intersection and the diffusion rate in the transport channels.