Direct observation of flow and bubble behavior in flat sheet modules with a distributor

Abstract

The air sparging method, which involves the introduction of air bubbles, is widely used in membrane separation to enhance performance. However, in flat sheet modules with distributors, the bubble behavior is complicated, which can have a critical impact on the air sparging efficiency. In this work, industrial-scale transparent modules were fabricated to visualize the two-phase flow with a distributor in different air sparging methods. Initially, the pressure drop in the channels induced by the inclusion of a distributor and spacer was evaluated, and the flow regime of the fluid was determined in visualization test. As for two-phase flows, five different solutions were investigated, categorized into two groups according to the coalescence behavior, namely the coalescence group and the non-coalescence group. Then, the bubble behaviors in a tubular flow module with different air sparging devices were studied for the two coalescence groups. In the pre-mixing method, the anti-coalescence phenomenon of bubbles clearly occurred at a high gas flow rate in the non-coalescence system. When an aerator head was used, smaller air bubbles were obtained, especially for the non-coalescence system, where the bubbles were small enough to ignore the resistance of the orifice. The flow patterns were subsequently investigated with empty flat sheet modules with different air sparging devices. Three flow regions were depicted for different solutions and air sparging devices. In the pre-mixing method, the coalescence effect led to a higher liquid flow rate being required to reach region (III) for water. The introduction of an aerator nozzle had the advantage of uniformly distributing the bubbles, while a higher gas flow rate was required to achieve a bubbly flow. With the incorporation of a spacer, the liquid flow rates needed to be higher than 0.32 m s⁻¹ and 0.29 m s⁻¹ to eliminate stagnant bubbles in the pre-mixing and air nozzle methods, respectively, in the coalescence system, and it was found the phenomenon was similar as for ethanol solution. According to our results, optimal operating conditions of air sparging devices for two-phase flow are proposed.