Membrane and Desalination Technologies - TCE Moodle Website

676 K. Mohanty and R. Ghosh
Fig. 16.3. Schematic representation of various membrane/bubbling arrangements (1).
Cui and Wright (37) studied the flux enhancement in downwards crossflow condition using
dextran as the test media. They reported that flux increases up to 320% with gas sparging.
They attributed the significant increase in permeate flux to the disruption of the concentration
polarization layer and improved mass transfer. The best results were obtained at higher
TMPs, lower liquid cross-flow velocities and higher feed concentrations.
Membrane orientation has been found to have a significant effect on flux enhancement
(12, 38). Figure 16.5 compares the flux enhancement for vertical and horizontal membranes.
It can be seen that flux enhancement is higher for vertical upward flow than the horizontal
flow for the same range of operating parameters. Cheng et al. (39) have studied the effect of
the mounting angle of the membrane and reported an optimum inclination angle of around
50 that has resulted in maximum flux enhancement (Fig. 16.6). Table 16.2 summarizes the
effect of inclination angle in two-phase ultrafiltration (40). The flux enhancement due to
membrane inclination increases with increasing gas velocity, feed concentration, and TMP,
while it decreases with increasing liquid velocity.
Effect of liquid Reynolds number on the enhancement of permeate flux by gas sparging
was reported by Cui and Wright (37). The flux enhancement was found to be higher in the
laminar flow region (Fig. 16.7) and not very significant in the turbulent regime. The observed
enhancement was believed to be caused by improved distribution of bubbles over the
membrane surface at lower liquid flow rates.
4.2. Gas-Sparging in Hollow Fibre Membrane Modules
Hollow fibre membrane modules are of great interest due to their compactness (41). Their
application was earlier limited to water and wastewater treatment. Of late they are increasingly