From soft matter filtration processes to microfluidic filter cake visualization

  • Von kolloidalen Filtrationsprozessen zu mikrofluidischen Deckschichtvisualisierungen

Lüken, Arne Can; Wessling, Matthias (Thesis advisor); Ramon, Guy (Thesis advisor)

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

In: Aachener Verfahrenstechnik series - AVT.CVT - chemical process engineering 30
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

Membrane ultra- and microfiltration is a chemical unit operation used to purify and concentrate liquid suspensions of colloids and particles. Typical industrial applications are removing or concentrating colloids, proteins, or cells in water treatment, food processing, and bioprocessing. The filtration performance suffers from the agglomeration of the filtrated matter on the membrane surface as a filter cake. This filter cake adds hydrodynamic resistance on top of the membrane's resistance and decreases the process's overall efficiency. Traditional applications measure the resistances for a specific filtrate at the point of operation and adapt the process design and the operating conditions accordingly. This thesis aims to provide mechanistic insights into the filtration process by developing experimental methods for microscopic filter cake visualization and applying these methods to analyze colloid and particle interactions in the filter cake during filtration. Finally, the thesis presents a novel ultrafiltration-based lab-scale microgel purification process and validates the process compared to state-of-the-art technologies. This thesis developed two devices for visually observing filter cakes: The first is a microfluidic system that withholds the filtrated particles at a membrane-mimicking structure and enables a visual observation of the filter cake's cross-section. Filtering soft spherical polyethyleneglycol particles provoked avalanche-like compaction of the filter cake during cake built up. The filtration of additive manufactured non-isotropic any-shape particles showed the dependency of the hydraulic resistance and the cake morphology from the particle shape and material. These microscopic particle scale results give essential insights into the particle interactions occurring in filter cakes and permeated assemblies. The second observation device developed and applied in this thesis is a flat sheet membrane filtration cell that visually observes the filter cake using Confocal Laser Scanning Microscopy. During cross-flow cleaning procedures, microgel filter cakes unravel instability-driven 3D patterns on the cake surface, accelerating cake removal. Finally, the microscopic filtration results are supplemented with developing a tangential flow microgel lab-scale ultrafiltration device. The thesis validates the applicability of the process for purifying microgels and reports operational procedures, process quantification, and the purification's challenges compared to state-of-the-art technologies. This thesis's findings provide one step towards bridging the gap between microscopic particle interactions and filtration cake layer properties. The presented results need to be incorporated into filtration models to translate the fundamental phenomena into applicable separation processes in future works. Finally, the findings are not only relevant for filter cakes, but additionally give insights in various applications of permeated micro-particle assemblies. The properties of self-assembling microparticle scaffolds, e.g., as tissue in cell culture, depend strongly on the particle-particle interactions, the assembly's morphology, and its hydraulic resistance. This interdisciplinary perspective will transfer the methods presented here to drive future developments.

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