A novel modelling approach to surface and Knudsen multicomponent diffusion through NaY zeolite membranes

A novel pairing between Knudsen and surface diffusion is developed for multicomponent mass transport in NaY zeolite membranes considering the actual conditions inside the pores owing to competitive adsorption of the species. The motivation behind this study starts from the consideration that the adsorption of the species, in particular of the most strongly adsorbed ones (like CO2), can significantly influence the Knudsen permeation. This occurs because the pore geometrical parameters affecting permeation, i.e., porosity, tortuosity and mean diameter, are modified by the steric effect owing to the presence of the adsorbed molecules, whose size is close to pore diameter. Therefore, in the present work, all these geometrical parameters are expressed as functions of the adsorption loading. This leads to a model that correctly predicts the blocking effect on the Knudsen diffusion mechanism at a low tem-perature, where the adsorption strength is sufficiently high to cause a severe occupation of the channel cross-section. As a consequence, such a model allows membrane selectivity to be predicted in a wide range of temperature and pressure. For instance, the CO2/H2 selectivity is observed to change from 25 to 0.3 when the temperature goes from 303 to 673 K. Hence, this novel approach, which is well-validated in both binary- and ternary-mixture conditions, is a useful tool for the analysis of the mass transport of multicomponent mixtures through NaY zeolite membranes, particularly at lower temperatures, where the adsorption strength provides a higher selectivity.