Visualizing local transport processes in opaque membrane modules with magnetic resonance imaging

RWTH Publications (RWTH Aachen)

Abstract

Clean water is essential for human life. To purify contaminated water, membrane modules are commonly used because they efficiently remove particles and ions. There has been little effort to understand what happens inside the module. In these cases, researchers often modify modules, for example by adding a window, to make the internal processes visible during measurements, which altered the flow dynamics. This study aims to visualize the transport processes in opaque modules. Magnetic Resonance Imaging (MRI) was used to capture images of the membranes, flow velocities, and ion concentrations within these modules. Silica particles served as fouling agents to demonstrate the modules’ fouling behavior, while copper sulfate was used as a model salt to visualize ion removal from a brackish water stream. In addition to MRI, computational fluid dynamics (CFD) simulations provided further insights into the flow behavior inside the module. Experiments with multichannel membrane modules revealed that particle deposition occurs mainly in the outer channels, reducing the contribution of the inner channels to the filtration process. Backwashing was found to partially remove the filter cake, while in some cases it repositioned the deposits leading to a reversed flow direction in certain channels. This led to different fouling patterns depending on the flow direction in each channel. Channels with reversed flow showed a more homogeneous filter cake compared to those with forward flow, suggesting that module design could influence filter cake formation. In flatsheet membrane modules, the permeate structure was found to have a significant impact on the flow path of the permeate. Structures with open channels tended to trap stagnant water, which is undesirable in membrane modules. Additionally, electrodialysis (ED) was studied using MRI to test the assumption of homogeneous desalination along the channel length. The results showed strong desalination at the beginning of the module, with mostly constant ion concentrations along the channel. However, one diluate channel exhibited a localized peak that requires further investigation. In summary, this thesis demonstrates that combining MRI with CFD simulations can successfully reveal the transport processes at membrane surfaces. The research underscores the importance of understanding the behavior of individual membranes in order to explain overall module performance.

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