CHEMICAL ENGINEERING TRANSACTIONS VOL. 32, 2013 A publication of The Italian Association of Chemical Engineering Online at: www.aidic.it/cet Chief Editors: Sauro Pierucci, Jiří J. Klemeš Copyright © 2013, AIDIC Servizi S.r.l., I SBN 978-88-95608-23-5; I SSN 1974-9791 Fluid Dynamic Optimization of Flat Sheet Membrane Modules – Movement of Bubbles in Vertical Channels Lutz Böhm*, Helmut Prieske, Matthias Kraume Technische Universität Berlin, Chair of Chemical and Process Engineering, FH6-1, Fraunhoferstrasse 33-36, 10587 Berlin lutz.boehm@tu-berlin.de Fouling is one of the key challenges in the operation of membrane bioreactors. Aeration is a simple but cost-intensive measure to clean the membranes. The aim of this project is to deepen the understanding of the behavior of the bubbles in such geometries to be able to optimize the cleaning of membrane systems and make them more cost-effective. Therefore, with respect to flat sheet membrane modules, the rise of a single bubble in a rectangular channel was investigated experimentally and numerically. The experimental part was performed with a high speed camera and the images were analyzed with the help of NI LabView, Vision Development Module and MS Excel. The software was used to determine the centre of mass of the bubble on each image which provides the rising path when combined. This rising path is determined for different parameter combinations varied bubble sizes and superimposed liquid velocities. The same parameter combinations were investigated numerically with the help of Ansys Fluent. Based on the rising paths, a comparison of the simulation and experiments is possible. Specifically the terminal rise velocity, the oscillation frequency and the amplitude are used for comparison. 1. Introduction Aeration is an operational tool widely used in process engineering. Its function is reaching from e.g. mass transport between the phases to enhancement of heat and mass transfer in the liquid phase and – the motivation for this project – the generation of shear forces on surfaces which are used in membrane bioreactors (MBR) to clean the surfaces from deposition layers (Böhm et al., 2012). Especially in MBRs flat sheet membranes are often used. In this case two plane membrane plates are glued together at the edges to form a cushion. Several of such cushions are arranged in modules. Using the static head a high pressure on the outside of the cushions or, alternatively, a low pressure on the inside of the cushions is used to get an outside-in filtration. During filtration a fouling layer builds up on the membrane surfaces. The space between the cushions has a rectangular shape and is aerated to generate flows for mixing purposes and the generation of shear forces which are responsible for the cleaning of the membranes. The system is often also constructed in a way that the air lift loop effect can be used to generate additional liquid cross flows. This makes the system with its multiphase flow in multiple rectangular channels fairly complex. In this project single bubbles and bubble swarms are investigated experimentally in a single gap with the electrodiffusion method for the determination of shear stresses, with particle image velocimetry for the determination of the flow surrounding the bubble and with a high speed camera for the determination of the bubble movement. Additionally the system is investigated numerically with computational fluid dynamics (CFD). The rather academic single bubble approach is chosen to be able to determine the influence of specific parameters on the bubble rise. Bubble swarms are such complicated systems that altering one parameter will most likely have several effects on the behaviour of the entire bubble swarm. Starting from the ‘simplified’ system with single bubbles the complexity can be increased and a deeper understanding of more complex systems can be gained. This article focusses on the investigation of single bubbles with a high speed camera and CFD. The unique feature of this project is the investigation of single bubbles rising in rectangular channels which 1501