THE MAXWELL-STEFAN DESCRIPTION OF MASS-TRANSPORT ACROSS ZEOLITE MEMBRANES

Published experimental data on transient permeation fluxes of light hydrocarbons through a silicalite-1 membrane show curious behaviour. For a mixture of methane and n-butane, for example, the transient experiments showed that initially the permeation flux of methane is higher than for n-butane but that this methane flux reduces to a lower steady state value. The n-butane flux is initially low but under steady state conditions is much higher than that for methane. A mathematical model for transient membrane mass transport has been developed using the Maxwell-Stefan formulation. This model is able to predict the observed maximum in the flux of methane, and if we assume that the Fick diffusivity is constant the transient permeation fluxes are predicted to reach their steady state values monotonically. Thus the Pick formulation fails even at the qualitative level to explain the experimental results. It is concluded that the Maxwell-Stefan formulation is indispensable for the description of mass transport across zeolite membranes.