Multiphase dynamics of colloidal deposition and resuspension at membranes

  • Mehrphasendynamik kolloidaler Ablagerungen und Resuspendierungen an Membranen

Lohaus, Johannes; Wessling, Matthias (Thesis advisor); Lammertink, Rob G. H. (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Book, Dissertation / PhD Thesis

In: Aachener Verfahrenstechnik Series AVT.CVT - chemical process engineering 22
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Colloidal fouling significantly limits the performance of membrane filtration processes. In particular, the complex interplay between surface potentials and hydrodynamic interactions on the pore-scale level pose a big challenge for science. Numerical simulations accompanied by microfluidic experiments enable to visualize the mechanisms occurring during fouling and thus helping to decode the fouling processes. This thesis used coupled computational fluid dynamics - discrete element methods (CFD-DEM) simulations to examine pore-scale mechanisms during membrane fouling and membrane backwashing. The simulations were accompanied by microfluidic experiments. The clogging behavior was analyzed inside varying pore structures with CFD-DEM simulations and compared to experimental findings from literature. The simulations revealed a dominant role of the inner pore structure, which is consistent with experimental observations from literature. The origin of clogging relied on a variety of parallel occurring processes: (1) adsorption on the constriction entrance; (2) interparticle interaction leading to adsorption; (3) gliding of adsorbed and loosely bond particles and, (4) particle resuspension. In membrane backwashing, particle resuspension plays an important role and was therefore investigated in more detail. Simulations and experiments both revealed that the backwash is decisively controlled by particle clusters instead of single-particle effects. This clarifies that single-particle models are not representative of describing mechanisms occurring during backwash. Two dominant events of backwash were identified from the simulations and experiments: (1) partial resuspension of particle clusters and, (2) orientation of attached particle clusters in the region of lower drag. Another important point in membrane fouling is temperature dependency which is only minorly addressed in literature. This thesis presents a systematic analysis of experimental and numerical study for particle-laden flow through a membrane mimicking pore-like channel under the influence of temperature. The effect of temperature on colloidal aggregation was incorporated by attractive and repulsive temperature-dependent potentials between particles and membrane. This study showed that are still gaps between theory and experimental observation, which have to be resolved in future studies. Another property which is of decisive interest in understanding colloidal fouling is the softness of the particle. The role of softness during filtration was studied with the help of CFD-DEM simulations. The simulations demonstrated that soft particles arrange in highly-ordered and compact filter cake structures due to hydrodynamic stress. Therefore, the soft filter cake caused significantly higher resistance in comparison to hard filter cakes. Furthermore, the simulations revealed that cake relaxation of soft filter cakes is fully reversible, whereas a short-term flux increase can lead to irreversible changes in cake structure. In summary, this thesis highlights and illustrates microscopic events of particle deposition and resuspension, which will ultimately help to understand colloidal fouling and make membrane backwashing more efficient.

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