Final Thesis

CFD Simulation Study of vortex formation at modified ion-exchange membranes

Key Info

Basic Information

Unit:
Chemical Process Engineering
Type:
Masterthesis / Bachelorthesis
Focus/Key Topic:
simulativ
Date:
01.10.2019

Contact

Access to clean drinking water will be one of the major challenges in the next decades. There are multiple ways of producing clean water. An important method is purification with ion-selective membranes. For example in electrodialysis, brackish water is purified. In novel applications, ion-selective membranes are used in redox flow batteries and microfluidics.

All membrane based separation processes suffer from concentration polarization during operation. This effect is also prominent in electrodialysis, where ion-exchange membranes are used to separate ions in an electrical field. The resistance of the system notably rises when the process is operated above the so-called limiting current density due to the depletion of ions in the laminar boundary layer. Here, the ion transport towards the membrane is diffusion-limited. This manifests in a plateau, where the current stagnates even though the driving force is increased.

In contrast to non-electrically driven membrane processes, a further increase in driving force leads to a third regime of increasing current. At sufficient high applied potential, electroconvective vortices occur mixing the boundary layer. Thereby, solution of high concentration is transported from the bulk to the membrane.

Much work in literature is dedicated to investigate the influence of membrane surface properties on the introduction of electroconvective vortices. Early appearance of vortices shortens the plateau length by providing efficient mass transfer through the boundary layer. We found a promising procedure for early and strong vortex evolution. Experiments showed that membranes modified with P2VP-microgel patterns by inkjet printing do exhibit a significantly decreased plateau length at a simultaneous increased voltage until depletion occurs [1].

We suppose that the surface charge of the patterns has a big impact on the evolution of electroconvection and on the ion transport close to and above the limiting current, as also indicated in literature [2].

Additional knowledge of the significant phenomena for early and effective electroconvection enables tailor-made membrane surfaces to minimize ion depletion. The task of this study is the simulation of the vortex formation at micro structured patches in OpenFOAM. The influence of heterogeneity in charge, hydrophobicity, conductivity and structure is of major interest.

[1] F. Roghmans, E. Evdochenko, F. Stockmeier, S. Schneider, A. Smailji, R. Tiwari, A. Mikosch, E. Karatay, A. Kühne, A. Walther, A. Mani, M. Wessling, 2D Patterned Ion-Exchange Membranes Induce Electroconvection, Adv. Mater. Interfaces 342 (2018) 1801309.

[2] K.A. Nebavskaya, V.V. Sarapulova, K.G. Sabbatovskiy, V.D. Sobolev, N.D. Pismenskaya, P. Sistat, M. Cretin, V.V. Nikonenko, Impact of ion exchange membrane surface charge and hydrophobicity on electroconvection at underlimiting and overlimiting currents, Journal of Membrane Science 523 (2017) 36–44.

 

Haves:

·         Self-initiative

·         Interest in hydrodynamics and CFD

·         Optional: Knowledge of C++

Wants:

·         Interesting and varied tasks

·         Deep look into current research (Cooperations with Stanford (USA), Twente (Netherlands), KUBSU (Russia))

·         Hands on experience (edit OpenFOAM code)

 

Please contact me via mail (+cv and grades) to arrange a meeting. I’m looking forward to your message.