Hindering effect of the strongly adsorbed species in permeation through zeolite membranes

Membranes are increasingly replacing the traditional processes in the separation field, allowing the selective removal of components from gas or liquid streams. In this work, the separation performance of zeolite membranes for gas mixtures containing a strongly adsorbed species (i.e., CO2 and water vapor) and a weakly adsorbed one (e.g., H2) was analyzed based on a developed model,1, 2 which considers the interaction between the mass transport mechanisms contributing to the overall diffusion (i.e., surface and gas translation diffusion). As a main result, the predicted H2 permeance drops in mixture with H2O compared to the single gas condition, describing very well the literature experimental values 3 (Figure 1). This reduction occurs because water vapor molecules occupy the adsorption sites, impeding the H2 diffusion. The higher temperatures mitigate this hindering effect, causing an increase of H2 permeance since the H2O adsorption decreases. The H2O/H2 selectivity in 4A was estimated to be about 240 at 25°C. A strong hindering effect was also obtained in case of CO2 mixed with H2 through a NaY membrane, with a CO2/H2 selectivity of about 45 at 25°C. Hence, zeolite membranes are suitable materials for separation of gas mixtures. The presence of a strongly adsorbed species hinders the permeation of the other components and membrane selectivity increases.