Channel flow electrochemical reactor

Fig. 31. Typical channel flow electrochemical reactors (CER) with flow-by (a, b) and packed-bed (c) working electrodes. The control volume (d) includes the active electrocatalyst area (67). A, reactant B, C, products E, electrolyte ce, counter electrode we, working electrode. (Reprinted by permission of the publisher. The Electrochemical Society, Inc.)...

The plane electrodes are separated by isolating spacers, which may lead to the formation of parallel flow channels. In any case, the electrodes are plane sheets which can be replaced and thus made out of any plain material, e.g. nickel, lead, glassy carbon or graphite. Recent technolo cal developments made at the Institute of Microtechniques, Mainz [6, 7], have led to the construction of versatile microchannel electrochemical reactors. Indeed, the pressure can be elevated to up to 35 bar and the electrodes can be stacked in order to increase the overall electrode area. Moreover, polymer electrolyte membranes can be inserted, separating anodic and cathodic compartments if necessary, and finally heat exchangers may be integrated. 

The modification of hydrodynamic aspects is exploited in the falling-film cell [12], where the electrolyte flows as a thin fllm in the channel between an inclined plane plate and a sheet of expanded metal which work as electrodes. Other proposal is to include turbulence promoters in the interelectrode gap in conventional parallel plate electrochemical reactors [13-16], or the use of expanded metal electrodes immersed in a fluidized bed of small glass beads, called Qiemelec cell [17]. Likewise, the Metelec cell [18] incorporates a cylindrical foil cathode concentric arranged around an inner anode, with a helical turbulent electrolyte flow between the electrodes. The electrochemical hydrocyclone cell [19] makes use of the good mass-transfer conditions due to the helical downward accelerated flow in a modified conventional hydrocyclone. 

An electrochemical reactor containing two concentric cylindrical electrodes (Fig. 2.7), with or without a diaphragm, represents a practical and attractive geometry since it offers a uniform primary current distribution (see Chapter 5). Axial flow through the annular space between the two electrodes, or an electrode and a diaphragm, has characteristics between those of a pipe and a rectangular channel. A theoretical analysis for the condition of laminar or turbulent flow is approached in the same way as that for pipes and channels hence, we merely identify appropriate experimental correlations. 

Mathematical solution depends on the boundary conditions governed by the geometry of the reactor system. We will confine our attention to plate electrodes. Electrochemical reactors can be divided into two categories tank electrolyzers and channel flow electrolyzers. 

There are numerous applications that depend on chemically reacting flow in a channel, many of which can be represented accurately using boundary-layer approximations. One important set of applications is chemical vapor deposition in a channel reactor (e.g., Figs. 1.5, 5.1, or 5.6), where both gas-phase and surface chemistry are usually important. Fuel cells often have channels that distribute the fuel and air to the electrochemically active surfaces (e.g., Fig. 1.6). While the flow rates and channel dimensions may be sufficiently small to justify plug-flow models, large systems may require boundary-layer models to represent spatial variations across the channel width. A great variety of catalyst systems use... 

In this reactor, the feed solution enters via a central channel between the anode and cathode beds and then flows in the upward and downward vertical directions (where the majority of the solution passes through the porous cathode). When the cathode bed is filled to capacity with deposited metal, the polarity of the electrode beds is reversed and the metal is electrochemically etched into a small liquid volume to create a concentrated solution. The longer the contact time of the metal-laden solution in the porous cathode, the greater the extent of metal removal (where the contact time is inversely proportional to the catholyte flow rate and directly proportional to the cathode bed thickness). To maximize the energy efficiency for metal removal, the entire bed should operate at or near the metal reduction limiting current density, but this is difficult to achieve because of unwanted hydrogen gas evolution. The relevant differential equations are solved to obtain the metal ion concentration, electric potential, and current density distributions in the cathode bed are [125]... 

FIGURE 27.2 General flow configuration using enzyme packed reactor with an optical/ electrochemical detection. Dashed lines represent channels that could be added to the system. I = indicator electrode, R = reference electrode. 

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