Gas transport and separation with ceramic membranes. Part I. Multilayer diffusion and capillary condensation

Multilayer diffusion and capillary condensation of propylene on supported y-alumina films greatly improved the permeability and selectivity. Multilayer diffusion, occurring at relative pressures of 0.4 to 0.8 strongly increased the permeability to 6 times the Knudsen permeability, yielding permeabilities of 3.2 x 10m5 mol/m’-see-Pa. The occurrence of a maximum in the permeability coincides with blocking of the pore by adsorbate (capillary condensation). This point could be predicted, employing adsorption data and the slit shape form of the pore. Separation factors of 27 were obtained with a Na-CsHs mixture and a supported y-alumina film, with CsHs the preferentially permeating component. This very effective separation is due to pore blocking by adsorbate. The separation factor increased to 85 after modification of the system with magnesia by the reservoir method. However, the permeability of propylene decreased by a factor of 20 to 1.6 x 10e6 mol/m’-see-Pa.