Zeolites as Membranes: The Role of the Gas-Crystal Interface

Abstract Steady flow through a porous cystal membrane is associated with concentration distributions across the gas/crystal interfaces which cannot represent true equilibrium. Steady state distributions have been modelled from mass balance equations for a membrane where a fraction of the flow enters the crystal directly from the gas phase and the remainder enters via an externally adsorbed layer. The same fractions are assumed for the nett flow through the exit surface. Special cases of the general equations investigated numerically or otherwise include: the extent of departures from equilibrium across interfaces; the ideal flow when these departures tend to zero; the ratios of ideal to measured flows; and ratios of true intracrystalline diffusivities to apparent diffusivities obtained from steady flows by assuming equilibrium across interfaces. In fast sorptions and desorptions non-equilibrium distributions across the gas/solid interface may also result in differences between true intracrystalline diffusivities and those obtained by incorrectly assuming equilibrium.