Porous and packed-bed electrodes

There are two types of multielectrode reactor monopolar and bipolar cells, as shown in Fig. 15.2. The bipolar configuration has the advantage that the electrical circuit has only to be linked at the ends of the electrode pile the disadvantage is limitation to certain electrode materials when the anode and cathode are of the same material or when they can be easily glued to each other. 

Another way to obtain high degrees of conversion, besides the close positioning of anode and cathode, is through the use of porous and packed bed electrodes. Since these electrodes have great importance in the laboratory as well as at an industrial level, they are described separately. 

It is possible to show that, if the size and distribution of the pores is uniform, then the fraction, R, of species electrolysed is 

These expressions assume mass transport control kinetic limitations complicate the analysis. However, it is reasonable to assume that a potential corresponding to the mass-transport-limited current can almost always be chosen. 

Principal applications are in electrosynthesis, metal recovery and, sometimes, electroanalysis. 

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It is remarkably difficult in practice to obtain a high As value whilst maintauitaif a uniform reaction rate over the entire electrode surface, e.g. porous- and packed-bed electrodes often suffer from poor potential, and current, distributions (section 2.6.3). Often, the adoption of a three-dimensional electrode involves a trade-off between increased electroactivc area and dhtiliii ied... 

Three-dimensional electrodes generally give rise to more problems with potential and current distributions than their two-dimensional counterparts this is primarily due to the anisotropy of porous- and packed-bed electrodes, with respect to electrode conductivity, electrolyte flow and concentration of the... 

The vast majority of porous- and packed-bed electrodes in practice, utilize variations of configuration shown in Pig. 2.19(b) the flow-by configuzattem. 

In this chapter generalized mathematical models of three dimensional electrodes are developed. The models describe the coupled potential and concentration distributions in porous or packed bed electrodes. Four dimensionless variables that characterize the systems have been derived from modeling a dimensionless conduction modulus ju, a dimensionless diffusion (or lateral dispersion) modulus 5, a dimensionless transfer coefficient a and a dimensionless limiting current density y. The first three are... 

In order to achieve a high electrolyte velocity through a porous or packed-bed electrode, it is often desirable to seal the cell. Indeed, one of the present trends in cell design is the incorporation of 3-dimensional electrodes into the plate-and-frame geometry. The electrode material may be, for example, foam, a mesh or a... 

Three-dimensional electrodes are configured as static or solid electrodes (porous or packed bed), or as dynamic or fluid bed electrodes (fluidized bed and moving bed), cf. Table 6. 

The limiting-current method has been used widely for studies in packed and fluidized beds (see Table VII, Part H). Limiting current measurements in these systems overlap in part with the design and analysis of packed-bed and fluidized-bed electrochemical reactors in particular the potential distribution in, and the effectiveness of, such reactors (for example, for metal removal from waste streams) is an extensive area of research, which cannot be covered in this review. For a complete discussion of porous flow-through electrodes the reader is referred to Newman and Tiedemann (N8d). 

Cells with three-dimensional electrodes providing enlarged specific electrode area and improved mass transport due to the specific fluid dynamics inside the three-dimensional structure are, for example, the porous flow-through cell [68], the RETEC (RETEC is a trademark of ELTECH Systems Inc., Cardon, Ohio) cell [15], the packed-bed cell [69-71],... 

Fig.4. (top) Reaction scheme for the enzymatic determination of starch as executed in the FIA manifold shown below, where AMG refers to amyloglucosidase and GDH to glucose dehdrogenase, both of which enzymes are immobilized on porous glass in separate packed-bed reactors. The detection of the generated NADH is facilitated amperometrically by means of a modified electrode at 0 mV vs. Ag/AgCl, 0.1 M KCl. Flow rates are given in ml/min injected volume 30... 

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