S. Brandani et al., TRANSPORT DIFFUSION AND SELF-DIFFUSION OF BENZENE IN NAX AND CAX ZEOLITE CRYSTALS STUDIED BY ZLC AND TRACER ZLC METHODS, Microporous materials, 7(6), 1996, pp. 323-331
Transport and self-diffusion of benzene in NaX and CaX zeolite crystal
s has been studied by zero length column (ZLC) and tracer ZLC methods.
In order to investigate the possible intrusion of extracrystalline ma
ss transfer limitations a series of experiments was performed in which
the sample quantity, crystal size, sorbate partial pressure and the n
ature of the purge gas (He or N-2) were changed. At higher sorbate pre
ssures and with larger samples the intrusion of external effects becom
es evident but, under properly selected conditions, with sufficiently
large crystals, desorption rates are clearly controlled by intracrysta
lline diffusion. Under these conditions there is good agreement betwee
n the limiting transport and tracer exchange diffusivities, confirming
the absence of any heat transfer limitation. Diffusivities for benzen
e-NaX are consistent with earlier reported ZLC values but are substant
ially smaller than the values obtained from PFG NMR self-diffusion mea
surements. 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.