Factors affecting the performance of a submerged hollow fiber bundle

Abstract

This study examines the performance of submerged hollow fibers with and without bubbling. The mechanisms of fouling were investigated with a model submerged hollow fiber bundle of nine fibers arranged in a 3 × 3 array. Parameters varied included the axial flow velocity, the bubble size and bubble frequency. An experimental setup was devised that could maintain a constant average flux across the bundle, yet allow measurement of the individual flux provided by each fiber. It was found that the standard deviations of the fluxes of the individual fibers tend to rise due to fouling. The standard deviation can be used as a characteristic indicator of uneven fouling inside the bundle and to predict an imminent increase in the rise in trans-membrane pressure (TMP). ‘Hydrodynamic resistance’ is a performance measure which takes into account the effect of the other hollow fibers on the apparent resistance experienced by an individual hollow fiber in a bundle. The ‘hydrodynamic resistance’ has two components, one is the ‘bundle resistance’ caused by lateral flow through the bundle, the other is ‘permeate competition’ from surrounding fibers; the ‘permeate competition’ effect can be dominant. It was found that the standard deviation of the fluxes that preceded TMP rise was close to the standard deviation predicted by incorporating the hydrodynamic resistance. One outcome of these observations is that it is important to arrange the bubbling configuration so that the agitation is homogeneous within the bundle, to ensure that the fibers in the bundle foul evenly. The hydrodynamics within the fiber bundle were investigated by the particle image velocimetry (PIV) technique. To facilitate observation with PIV, optical fibers were used in place of the hollow fibers, and the refractive index of the fluid was matched to that of the optical fibers. It was found that the axial velocities within the hollow fiber module could be up to 10 times lower than the velocities outside the fiber bundle, which indicated a strong sheltering effect. The standard deviation of the velocities within the hollow fiber bundle had a much larger effect on the performance of the bundle compared to the velocity per se. We have also compared the fouling control effects of bubbles of different sizes and report preliminary evidence that many small bubbles can be more effective than a few large bubbles at the same volumetric flow rate.