Flow fields in the overlimiting current regime in electrically-driven membrane processes

Autor: Stockmeier, Felix
Přispěvatelé: Wessling, Matthias, Mani, Ali
Jazyk: angličtina
Rok vydání: 2022
Předmět:
Zdroj: Aachen : RWTH Aachen University, Aachener Verfahrenstechnik Series-AVT.CVT-Chemical Process Engineering 32 (2022), 1 Online-Ressource : Illustrationen, Diagramme (2022). doi:10.18154/RWTH-2022-11194 = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022
DOI: 10.18154/RWTH-2022-11194
Popis: Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022; Aachen : RWTH Aachen University, Aachener Verfahrenstechnik Series - AVT.CVT - Chemical Process Engineering 32 (2022), 1 Online-Ressource : Illustrationen, Diagramme (2022). doi:10.18154/RWTH-2022-11194 = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022
Electroconvection, a hydrodynamic instability which convectively mixes the ion-depleted diffusion boundary layer in electrically-driven membrane processes, enables the operation of such processes beyond a diffusion-limited current density. When operating at overlimiting current densities, industrial processes could be designed with smaller membrane modules and, thus, reduced investment costs. Still, the energy needed to overcome the diffusion-limitation as well as concerns regarding water splitting still stand in the way of their application. Both hindrances can be reduced by effectively triggering and tailoring the 3D electroconvective vortex field. However, the 3D features of electroconvection and their interaction with membrane surface modifications or spacer hydrodynamics are lacking experimental quantification. Until now, experimental studies were limited to 2D measurement techniques and 3D simulations which are restricted to small scales due to computational costs.This thesis surpasses this limitation and presents an experimental method for quantification of the 3D velocity field of electroconvection with high temporal and spatial resolution. Using this method, we quantify the velocity field and its statistics close to a cation-exchange membrane in a steady or pumped electrolyte. We further measure its interaction with modified membrane surfaces and spacer structures.We conducted these measurements in a newly designed electrochemical cell using micro particle tracking velocimetry for 3D velocity reconstruction. The recorded velocity fields were then used to visualize coherent vortex structures and reveal changes in the velocity field and its statistics. During the transition from vortex rolls to vortex rings with increasing current densities, changes in the rotational direction, mean square velocity, and temporal energy spectrum with only little influence on the spatial spectrum were revealed. Additionally, we investigated the impact of membrane surface modifications with two types of microgels varying in zeta potential on the vortex field's build-up. We discovered that a large difference in zeta potential between microgel and membrane material offers full control over the velocity field in terms of structure and rotational direction. Lastly, we quantified the interaction of the electroconvective velocity field with spacer-induced hydrodynamics in a pumped electrolyte. The velocity fields and their statistics revealed that significant interaction only appears at Reynolds numbers below one. However, pilot-scale experiments reported the appearance of overlimiting currents in such systems. Which is why measurements on a smaller scale at higher current densities are expected to provide further insights.This thesis emphasizes the potential of specifically engineered membrane surfaces and spacer structures for overcoming the limitations of electrically driven membrane processes. Tailored interaction of controlled electroconvection and spacer hydrodynamics could reduce the energy to overcome limiting currents in industrial applications to a minimum.
Published by RWTH Aachen University, Aachen
Databáze: OpenAIRE
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