(18i) Electrokinetic Transport in Porous Media for Energy and Environmental Applications
AIChE Annual Meeting
Sunday, November 13, 2016 - 1:00pm to 3:30pm
Teaching Interests: Fluid Mechanics, Thermodynamics, Numerical Analysis, Mathematical Physics
Understanding the transport phenomena and electrochemical processes in porous media is of great importance in many applications. Examples include energy applications such as in batteries and supercapacitors, electrochemical processes such as electro-catalysis, as well as industrial scale water desalination or geological scale fluid flow problems in underground reservoirs such as in oil recovery. Although these examples span several orders of magnitude in both length and time scales, the fundamental picture of electrokinetic phenomena is remarkably relevant in almost all of them. Here, we present some recent works on two aspects of electrokinetic transport in porous media.
First, we discuss the surface conduction phenomena and its effect on shortening the charging kinetics in porous electrodes. In most models of electrokinetic transport in porous media, the role of pore microstructure is often neglected and the transport is descried via homogenized, spatially averaged quantities. Inspired by detailed Direct Numerical Simulation (DNS) of the Poisson-Nernst-Planck (PNP) equations at the pore-scale, we show that the pore microstructure can have significant effect on the charging kinetics. Specifically, we observe that in the presence of large applied potentials, the surface conductivity of the Electric Double Layer (EDL) becomes considerably large, competing with that of the bulk of electrolyte. This allows for passage of large surface current through the EDL, which effectively acts to â??short-circuitâ? the bulk and significantly enhance the charging kinetics. We believe this discovery could be utilized to engineer better preforming electrodes for high power density applications.
Second, we describe the effects of electric field on the viscous fingering instability in Hele-Shaw cells. Large scale flow problems in porous media, such as those encountered in underground oil reservoirs, are typically described via Darcyâ??s law. However, it is well known that many underground rock formations contain surface groups and minerals that dissociate in the presence of water. Convection of these charges by the pressure driven flow can then set up streaming current and streaming potential that affects the flow. Furthermore, electric fields are sometimes used to enhance the oil recovery, e.g. by reducing the oil viscosity through electro-thermal heating or enhancing flow rate via electro-osmotic flow. Thus, a full description of fluid flow in the presence of electric field must also include the electrokinetic phenomena.
It is well known that the moving interface between two fluids in a porous medium becomes unstable if pushed by the less viscous fluid. Here, we report on the role of electrokinetic phenomena on stability of these viscous fronts in Hele-Shaw cells using analytic as well as numerical approaches. Interestingly, we find that instability could be suppressed if the right physical conditions are met or otherwise enhanced, leading to greater mixing of two fluids. These findings might be of interest to researchers working in the area of Electrically Enhanced Oil Recovery.
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