The Flow Pattern in Single and Multiple Submerged Channels with Gas Evolution at the Electrodes

We show that the gas-liquid flow pattern in a single gas-evolving electrochemical channel can be remarkably different from the flow pattern in multiple submerged gas-evolving electrochemical channels. This is due to the fact that in a single channel there is a higher accumulation of small bubbles and these can considerably affect the liquid velocity pattern which in turn may affect the performance of a cell. Since experimental work is often carried out in single channels, while industrial applications almost always involve multiple channels, this study provides insight into the factors that affect the flow pattern in each situation and establishes the basis for relating the behavior of single-and multiple-channel devices. 1. Introduction Many industrial processes are based on electrochemistry and it is anticipated that, if their efficiency can be improved, they will find increased use in new green technologies. Often, however, high power consumption is associated with these processes and most of the production cost is due to electrical energy. Thus, every advance in optimization and improvement of these processes can lead to significant energy conservation and cost reduction. This provides the motivation for many experimental [1–3] and theoretical [4–6] studies in the field. In many cases (e.g., chlorate production, hypochlorite generation, or perchlorate electrolysis) the industrial scale electrochemical cell consists of a series of channels immersed in a bath of electrolyte. A gas (e.g., hydrogen or oxygen), which is the byproduct of the reaction, is often generated at the electrode surfaces. A generic representation of such industrial scale process is sketched in Figure 1 and contains a series (generally of the order of hundreds) of parallel channels (gaps) between electrode plates immersed in a large container (reactor) that contains the electrolytes. The electrodes are the walls of the channels in the reactor. Electrochemically evolved gas is produced at the electrodes in the dissolved state; small bubble nucleation starts at the imperfections of the electrode surface and then these bubbles depart into the surrounding, highly supersaturated, electrolyte [7, 8]. The generated bubbles are responsible for the gas-lift effect that promotes global circulation of the liquid in the cell. They also enhance local mixing that helps in continuously refreshing the electrolyte in the vicinity of the electrodes. Figure 1: Schematic representation of an electrochemical process. A series of plates (the electrodes) are immersed in a larger container (the reactor).