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

Published experimental data on transient permeation fluxes of light hy drocarbons through a silicalite-1 membrane show curious behaviour. For a mixture of methane and n-butane, for example, the transient experim ents showed that initially the permeation flux of methane is higher th an for n-butane but that this methane flux reduces to a lower steady s tate 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 Max well-Stefan formulation. This model is able to predict the observed ma ximum in the flux of methane, and if we assume that the Fick diffusivi ty is constant the transient permeation fluxes are predicted to reach their steady state values monotonically. Thus the Pick formulation fai ls even at the qualitative level to explain the experimental results. It is concluded that the Maxwell-Stefan formulation is indispensable f or the description of mass transport across zeolite membranes.