Blending the Boundaries : Enhancing Ion Transport with Electrokinetics
Arputha Paul is a PhD student in the department Soft matter, Fluidics and Interfaces. (Co)Supervisors are prof.dr.ir. R.G.H. Lammertink and dr. J.A. Wood from the faculty of Science & Technology.
Process optimization is crucial in any industry as it allows for time, cost, and effort savings. It is an ongoing endeavor that involves overcoming process limitations using cost-effective and innovative ideas, ensuring a smooth and efficient operation. In chemical processes that involve solid-fluid interfaces, the formation of a boundary layer is inevitable, which can pose mass transfer limitations to the process. Numerous techniques to overcome this barrier of boundary layer and to minimize its influence on mass transfer has been extensively studied so far. This thesis proposes a novel approach to address this issue of boundary layers formed at solid-fluid interfaces. As opposed to the conventional way of mechanically mixing the whole bulk in order to reduce mass transfer limitation caused at the boundary layer, this work presents an innovative idea of creating a mixing effect within the boundary layer that can improve the energy efficiency of the process. This concept utilizes the electrokinetic effect induced by a charged surface in presence of voltage gradient that can be a source of creating the mixing effect ( i.e. blending) within the boundary layer.
Chapter 1 provides a comprehensive background on the formation and properties of boundary layers at solid-fluid interfaces. It also offers a brief introduction to an electro-chemical process used for desalination known as electrodialysis (ED) and describes its working principle along with the mass transfer limitations caused due to formation of boundary layer formed close to ion exchange membrane surface as a result of so-called concentration polarization. It also gives an overview of various elements of ED process that includes spacers and also explains the technique used to modify surfaces of spacers which would be used in the rest of succeeding chapters. Furthermore, it briefly explains the electrokinetic effects that can be expected within an ED stack.
Chapter 2 introduces the hypothesis regarding the electrokinetic effects of surface-modified spacers in the ED process using polyelectrolytes that can be used to create mixing effects in the boundary layer. It explains the experiments carried out in order to test this hypothesis, wherein a lab scale ED stack was used to perform desalination using single monovalent salt (NaCl) as feed solution. The results from these experiments are thoroughly discussed, and a numerical model is used to verify the findings. Based on the obtained results, the chapter confirms that using our technique a 20% increase in process efficiency can be observed.
Expanding the study to different scenarios, Chapter 3 examines the electrokinetic effects of surface-modified spacers in feed solutions containing bivalent cations and mono-bivalent ion mixtures. Our experimental results obtained from the same set-up used in the previous chapter, showed that the surface-modified spacer may not exhibit equal effectiveness for bivalent salts due to specific ion adsorption. However, the study finds that mixtures with mono and divalent cations still experience similar ion transport enhancement as observed in the case of only monovalent ions.
Chapter 4 focuses on investigating surface-modified spacers with different flow attack angles. By conducting experiments using spacers with three different flow attack angles and comparing it with conventional and surface modified spacers, the results demonstrate how the flow field can influence the electrokinetic effects of spacers. The chapter clearly shows which spacer geometries exhibit the maximum mixing effect thereby increasing the process efficiency and identifies cases where the enhancement is not significant enough.
To gain a deeper understanding of the mass transport limitation in ED process due to boundary layer formed along the length of the stack, chapter 5 describes an investigation carried out using a segmented electrode stack to study the transport of ions locally along the length of 1 m long ED stack. The experimental results showed severe ion depletion from the middle towards the end of the stack. The comparison of surface-modified spacers and uncoated conventional spacers gave significant differences between mass transport along the length of the stack, especially in the severely ion depleted zones. This highlights the impact of surface modification on the electrokinetic phenomena in ion depleted region within the ED stack.
Finally, chapter 6 discusses the implications of the research conducted in the thesis and provides further directions for future studies. The chapter aims to reflect on the significance of the thesis work and suggest potential avenues for future research in this area. It also covers several areas that require further research for practical real-life applications, encompassing the associated challenges and limitations. Moreover, it provides a synopsis of a comparable study on surface modification carried out within a micro-channel and discusses the alterations in transport kinetics, drawing analogies to stirring boundary layers and challenges involved, thereby inviting future research opportunities.