Taha

CFD modelling of gas-sparged ultrafiltration in tubular membranes

Abstract In ultrafiltration processes, injecting gas to create a gas–liquid two-phase crossflow operation can significantly increase permeate flux and, moreover, can improve the membrane rejection characteristics. It has been shown that controlled pulse injection to generate slug flow is more advantageous than uncontrolled gas sparging, especially when the gas flow rate is low. The slug size and frequency affect the performance of ultrafiltration, and there exits an optimal slug size and frequency to achieve high permeate flux. Previous studies have been based on the analysis of the experimental data and mass-transfer correlations. In this work, an attempt is made to model the slug flow ultrafiltration process using the volume of fluid (VOF) method with the aim of understanding and quantifying the details of the permeate flux enhancement resulting from gas sparging. For this numerical study, the commercial CFD package, FLUENT, is used. The first part of the model uses the VOF method to calculate the shape and velocity of the slug, the velocity distribution and the distribution of local wall shear stress in the membrane tube (neglecting the wall permeation effect). The second part links the local wall shear stress to the local mass-transfer coefficient that is then used to predict the permeate flux. In order to validate the model, experimental data reported in the literature over a wide range of gas and liquid velocities, slug frequencies, and transmembrane pressures are compared with the CFD predictions. Good agreement is obtained between theory and experiment.

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

Abstract 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, bubble-supply 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.

Hydrodynamic analysis of upward slug flow in tubular membranes

Abstract Injecting gas to create a gas—liquid two-phase crossflow operation in ultrafiltration can significantly increase permeate flux and improve the membrane rejection characteristics. Previous study is largely based on the analysis of the experimental data and mass transfer corrections. In this work, an attempt is made to model the slug flow ultrafiltration process using the volume of fluid (VOF) method with the aim of process optimisation by understanding the mechanism behind the permeate flux enhancement. The commercial CFD package, Fluent, is used for this numerical study. Good agreement is obtained between theory and experiment.

Theoretical and experimental representation of a submerged membrane bio-reactor system

Abstract The impact of introducing ‘slug’ flow into a tubular ‘in-to-out’ membrane bio-reactor is discussed in this article. Data are presented from both a theoretical hydrodynamic model of slug flow through a tube, and from a practical experiment based on a hybrid membrane bio-reactor which incorporates submerged and cross-flow membrane elements.

Examination of the Mass–Heat Transfer Analogy for Two-Phase Flows in Narrow Channels: Comparison of Gas Bubble Enhancement of Membrane Separation and Heat Transfer to Vapour Bubbles in Boiling

The analogy between heat and mass transfer is frequently employed in single-phase flows. Here the analogy is examined for two processes involving two-phase flow in meso-channels with bores ≤1 mm: the injection of gas bubbles to enhance the ultrafiltration of large molecules in aqueous solutions using hollow fibre membranes and flow boiling in the confined-bubble regime. This paper considers the local mechanisms in these superficially similar processes, based on information from experiments and numerical simulations and discusses the extent to which they are indeed similar. It is concluded that there is no useful analogy between mass and heat transfer in these processes but they share a requirement for accurate estimation of the thickness of the liquid film between confined bubbles and the channel wall at high Reynolds numbers.