Quantifying the shear at the surface of submerged hollow fiber membranes

Abstract

The electrochemical shear probe that was developed enabled the surface shear forces on a fiber in a bench-scale submerged hollow fiber membrane system to be quantified for different membrane configurations and hydrodynamic conditions. Using the probe, it was possible to establish that the shear force that resulted from dual-phase flow along the surface of a fiber was not predominantly due to the bulk liquid movement, but rather to the localized high liquid velocities and eddies that were induced by a rising gas bubble. For all bulk cross-flow velocities investigated, the average and peak surface shear forces observed for dual-phase flow were, respectively, approximately three and seven times greater than that observed for single-phase flow. In addition, for dual-phase flow, the surface shear force increased along the length of the fiber in the direction of the bulk flow. As a result, the surface shear force was higher at the end of the fiber that was closest to the liquid surface. The lateral movement induced by a swaying fiber under dual-phase flow did not substantially contribute to the magnitude of shear force at the fiber surface. In general, the average surface shear force measured under dual-phase flow for single- and multi-fiber modules were relatively similar. However, substantially higher peak surface shear forces were periodically observed under dual-phase flow for multi-fiber modules. Depending on the bulk cross-flow velocity and the location along the membrane module, the peak surface shear force was periodically 45% greater for multi-fiber modules, when compared to single-fiber modules. The results suggest that the periodic peak surface shear forces were likely responsible for the significantly higher permeate flux that can be maintained under dual-phase flow for multi-fiber modules. It should be noted that the close proximity of other fibers in a tightly configured multi-fiber module could potentially shield certain areas of a fiber from both the bulk liquid flow and sparged gas bubbles. As a result, the shear force that is generated over the entire length of a fiber surface could be lower for tightly configured multi-fiber modules. Although further research is required, the results collected to date suggest that it should be possible to optimize the design of a full-scale submerged follow fiber membrane system (e.g. fiber length, looseness, density, etc.) using the electrochemical shear probe that was developed as part of the present study, rather than having to resort to a time and capital intensive trial-and-error approach.