Flux enhancements with gas sparging in downwards crossflow ultrafiltration: performance and mechanism

Abstract

In this study gas-liquid two-phase crossflow ultrafiltration was studied in downwards flow condition. A commercially available tubular membrane module was installed vertically and the feed solution and the injected gas bubbles flow downwards inside the membrane tubes. The permeate flux achieved in such an operation was compared to conventional single phase ultrafiltration and also to gas sparged upwards crossflow operation. The effect of operating parameters, including liquid and gas flow rates, transmembrane pressure (TMP) and feed concentrations, was examined with dextran solutions. Flux increases of up to 320% were achieved with gas sparging in the experimental study compared to single liquid phase crossflow ultrafiltration. This enhancement is much more profound when the liquid phase is in laminar flow, particularly when the concentration polarisation is more severe, for example, at a high TMP, a higher feed concentration, and a low liquid crossflow velocity. The enhancing effect is not very significant when liquid flow itself is turbulent. The flow pattern proved to be an important parameter and the operation could be optimised to achieve maximum enhancement with minimum gas injection.