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Abstract
The effect of air-sparging on the performance of cross-flow microfiltration of size-distributed fine particles is studied. The filtration flux and cake properties under various air velocities are measured and discussed. A higher air velocity leads to a lighter cake due to the higher shear stress acting on the membrane surface, especially under a bubble flow regime. However, the decrease in deposited particle size causes higher average specific cake filtration resistance as well as lower pseudo-steady filtration flux. A force balance model is employed to grasp the particle deposition on the membrane surface. The drag forces due to fluid flows play a major role in determining the particle deposition probability. The interparticle forces are dominant for submicron particles, while the inertial lift and gravitational force increase the weights for those particles larger than 10 µm. The occurrences of a minimum deposited probability for 0.2–0.3 µm particles and a maximum value for ca 2 µm particles can be reasonably explained by the force analysis. The particle size distribution in the filter cake, cake mass, average specific cake filtration resistance, and filtration flux can be estimated satisfactorily using theoretical models. In cross-flow microfiltration using a sample including submicron and micron particles, the filtration flux may be increased by sparging a few air bubbles. Air sparging is more effective in enhancing the filtration flux under lower suspension and air velocities due to the particle size effect. However, the flux-enhanced effectiveness decreases with increasing air velocity under bubble flow and becomes negative under a slug flow regime.
ACKNOWLEDGEMENTS
The authors wish to express their sincere gratitude to the National Science Council of the Republic of China for its financial support.