Peter H. Nelson, Michael Tsapatsis and Scott M. Auerbach,
``Modeling Permeation Through Anisotropic Zeolite Membranes
with Nanoscopic Defects,'' J. Membrane Sci. 184(2), 245-255 (2001).
Abstract
We have modeled permeation through anisotropic zeolite membranes with
nanoscopic defects that create shortcuts perpendicular to the
transmembrane direction (x). We have found that the dimensionless ratio
Dy/(kddy)
can be used to estimate whether the shortcuts contribute significantly
to the overall flux. Here Dy
is the diffusion coefficient for motion in
the plane of the membrane, kd
is the rate of desorbing into defect voids,
and dy is the spacing between adjacent defects. For values of
Dy/(kddy) >>1, we find
that shortcuts increase the flux by significant amounts. The magnitude
of the flux is increased as the imperfection spacing dy is decreased.
For small values of dy, permeation through shortcuts becomes sorption
limited so that decreasing dy further does not increase the flux through
a single shortcut. However, as dy is decreased, the concentration of
shortcuts increases, thereby increasing the total contribution of the
shortcuts to the flux. We have found regimes where increasing dy or
decreasing Dy
decreases the overall flux, showing that permeation can be
diffusion-limited by motion perpendicular to the transmembrane
direction.
Prof SM Auerbach
2 April 2001