Permeate pressure was varied from 0.666612×102 to
13.33224×102 Pa (0.5 to 10 mmHg) in case of crosslinked
PVA/CS blend membrane at a constant thickness of 50 m and
at the azeotropic feed composition. At lower pressures (high
vacuum), the influence of driving force on diffusing molecules
inside the membrane was high. This could result in the components
to be swept away immediately from the permeate side,
thereby resulting in high mass transfer rates. From Fig. 7, we
observe that the membrane exhibited considerable lowering of
the flux from 0.3201 to 0.014 kg/m2 h as well as a reduction in
selectivity from 211 to 13, with increasing permeate pressure
from 0.666612×102 to 13.33224×102 Pa (0.5 to 10 mmHg).
Under high vacuum conditions (lower pressures), diffusion
through the hydrophilic membrane of this study becomes the
rate determining step, and therefore, diffusing water molecules
would experience a large driving force, thereby enhancing the
desorption rate at the downstream side. On the other hand, lower vacuum levels reduce the driving force, thus slowing down the desorption of molecules from the membrane matrix. In such cases, vapor pressures of two components of the mixture govern the selectivity of the membrane. 1,4-Dioxane, which has a high vapor pressure, permeates competitively with water molecules, thus lowering the concentration of water in the permeate.