Transport diffusion and self-diffusion of benzene in NaX and CaX zeolite crystals studied by ZLC and tracer ZLC methods

Transport and self-diffusion of benzene in NaX and CaX zeolite crystals has been studied by zero length column (ZLC) and tracer ZLC methods. In order to investigate the possible intrusion of extracrystalline mass transfer limitations a series of experiments was performed in which the sample quantity, crystal size, sorbate partial pressure and the nature of the purge gas (He or N-2) were changed. At higher sorbate pressures and with larger samples the intrusion of external effects becomes evident but, under properly selected conditions, with sufficiently large crystals, desorption rates are clearly controlled by intracrystalline diffusion. Under these conditions there is good agreement between the limiting transport and tracer exchange diffusivities, confirming the absence of any heat transfer limitation. Diffusivities for benzene-NaX are consistent with earlier reported ZLC values but are substantially smaller than the values obtained from PFG NMR self-diffusion measurements. Diffusivities for benzene-CaX are somewhat smaller than for the NaX system but the trends with concentration are similar. In the presence of water vapour the diffusivity is increased slightly but the effect is not dramatic.