Enhancing hollow fibre ultrafiltration using slug-flow — a hydrodynamic study

Gas–liquid slug flow is commonly employed as an enhancement technique for cross-flow ultrafiltration. This technique worked well on flat sheet and larger diameter membrane modules, but hydrodynamics and mass transfer, hence optimal operation, differs for the smaller diameter hollow fibre membranes where the technique proved less effective. This study investigates slug-flow hydrodynamics in capillary tubes in order to characterise the flow and predict optimal operation in hollow fibre membranes. Experiments investigated the effect of bubble size, bubblesupply frequency, operating pressure and capillary diameter on bubble shape and rise-velocity in non-porous tubes of 0.89 mm bore. Computational fluid dynamics (CFD) was employed to predict the flow behaviour inside capillaries. The CFD model and experimental results compared well. The CFD model yielded detailed information of the flow parameters and the flow patterns inside the capillaries and this allowed for better understanding of the hydrodynamics of the capillary tube slug-flow process.

The new experimental results compared well with published values and this gives one confidence in the ability of the IR method of measurement as a tool to study capillary slug flow hydrodynamics. The range of capillary numbers published here extended published values as far as bubble radius and rise velocity data are concerned. The CFD model compared well with the new and published experimental data, proving a useful tool in the detailed analysis of the underlying hydrodynamic processes. Investigations examining the effect of physical property changes on the system hydrodynamics are underway, which is to be followed by hollow fibre UF studies and process optimisation analysis.