Mass transfer caused by gravitational instability at reactive solid–liquid interfaces

Abstract

Mass transfer in porous media has been investigated experimentally. In this paper, we present a visualization technique and discuss the behavior of a substance which transfers under the influence of gravity and reacts with the surface of porous media. Mass transfer by the reaction with porous media was demonstrated by means of electrochemical deposition experiment on particulate beds with complex structures. A copper plate (anode) and a stainless steel particulate bed (cathode) were respectively placed at the upper and bottom side of a thin vertical cell which was filled with copper sulfate solution. After the application of electricity, cupric ion which is provided from the copper plate to the solution transfers under the influence of gravity and it is consumed by deposition at the particulate bed. The behavior of ions between the electrodes was visualized by utilizing the infrared absorption characteristics of cupric ion. We observed gravitational instability and convection flow due to concentration gradient of ions in opposite direction to that of gravity, which is formed by reaction at solid-liquid interfaces. While downward flow caused by RayleighTaylor instability was observed in the case of flat interfaces, upward flow generated from complex-shaped interfaces was greatly dependent on their geometry. The interaction of these flows resulted in the convection throughout the cell. Consequently, it is found from the results that the gravitational instability significantly varies the transport characteristics and that the reactive interface geometry greatly affects the overall mass transfer.

Mass Transfer from Mobile to Immobile Regions in Irregularly Shaped Micro‐Channels at Low Reynolds Number

“…Moreover, the solute concentration was quantified utilizing the absorption photometry technique based on the Lambert–Beer law. Recently, absorption photometry has been applied to visualize of a two‐dimensional concentration field (Rosso et al 1994; Otomo et al 2014; Tanikoshi et al 2017). As mentioned above, we used a near‐infrared (NIR) light source as irradiating light in the experiment since cupric ions are likely to absorb near‐NIR light with a wavelength of approximately 800 nm.…”

Quantitative evaluation of mass transfer near the edge of porous media by absorption photometry

“…The copper plate dissolves cupric ions into the solution, while the ions shift in reaction to the metallic copper deposits on the surface of the particulate bed. The experimental cell is similar to that utilized for the visualization of mass transfer in our previous study . Since the deposition cell is oriented vertically, the transfer of ions from the copper plate to the particulate bed is affected by gravity.…”

“…Light absorption photometry is based on the relationship between absorbance and medium concentration and represents one of the methods suitable for use in reactive or narrow channels. Although it has been commonly used to measure the concentration of homogeneous solutions, light absorption photometry can be applied to the visualization of inhomogeneous two‐dimensional concentration fields through the use of collimated light irradiation …”

“…Therefore we want to show how these combined effect changes mass transfer into porous media by simple model experiments. The edge of the porous medium is modeled by an assembly of impermeable spherical particles, which is similar to the system used in our previous study . To model the boundary condition (consumption of a substance for reaction) at solid surface, we have adopted electrochemical deposition, which allows the control of total boundary flux.…”