Packed bed electrodes

With the success of the three-dimensional electrodes, it has become commonplace for suppliers of plate and frame cells to offer designs that allow their operation using electrodes with a high surface area. Reilly Tar and Chemicals Corp. and ElectroCell systems AB supply systems that can utilize a packed bed electrode [75,79,254-256],... 

Packed-bed electrodes need not be cylindrical. Takata and Muto reported on a rectangular design some years ago in which a bed of carbon fibers was used for the electrode material [15]. While a number of innovative applications were reported, for the reasons described earlier, cells of this type do not provide detection limits competitive with those that can be achieved using more conventional amperometric detectors. 

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... 

Three-dimensional electrodes may include packed bed electrodes as well as porous electrodes. The most important character of 3-D electrode is that there is a potential or current distribution related to the electrode reaction, coupled with concentration distributions within the whole electrode. The mathematical model for a fixed 3-D electrode will be a set of coupled differential equations. 

Three dimensional packed bed electrodes are generally considered for reactions which operate with low current densities in order to increase localized mass transfer rates and/or increase overall current per unit cell volume. The maximum current density at any position in the electrode structure is limited by the prevailing conditions of mass transfer. The limiting current thus can also have... 

The three-dimensional or packed bed electrode has a thickness L, bounded on one side by a current feeder (x = 0) and on the other side by a membrane or free solution, x = L (Figure 11). Both electrolyte and electrode phases are assumed to be continuous media with uniform effective conductivities and in which the electrical potential obeys Ohm s law. 

The packed bed electrode is one dimensional and both solid and electrolyte phase are continuous media with uniform effective conductivities. 

Electrochemical Reduction of Nitrobenzene in a Packed-Bed Electrode Reactor18... 

The electrochemical reduction of nitrobenzene to produce p-aminophenol has attracted industrial interest for several decades. However, some limitations may be met in this process regarding overall reaction rate, selectivity and current efficiency using a two-dimensional electrode reactor. These restrictions are due to the organic electrode reaction rate being slow and to the low solubility of nitrobenzene in an aqueous solution. In this example, a packed bed electrode reactor (PBER), which has a large surface area and good mass transfer properties, was used in order to achieve a high selectivity and a reasonable reaction rate for the production of p-aminophenol. The reaction mechanism in an acid solution can be simplified as... 

The mathematical model for the PBE reactor should in general be a two-dimensional model describing the potential and concentration distributions within the packed bed electrode. However, the model can be simplified in certain cases. Under a recycle flow operation, for example, the conversation per pass through the packed bed is small, so that the PBER can be treated as a differential reactor, the potential distribution only in the lateral direction is considered. In this case which is similar to the case in 5.4, the two-dimensional model can be written in a onedimensional Poisson equation form as... 

Comparisons of the experimental and theoretical performance with different thickness of the packed bed electrode are shown in Figure 17. Selectivity S generally decreases with an increase in bed thickness, x0, and/or with a decrease in flow rate, Q, due to a greater non-uniformity in electrode potential distribution. 

Direct Electrochemical Oxidation of Propylene in a Sparged Packed-Bed Electrode Reactor19,20... 

In this Section so far, ADM is used to solve theoretical generalized models in the forms of ordinary differential equations). For diffusion-convection problems, the distributions along the axial direction of the packed bed electrode were neglected in certain cases, and mass transfer in the three dimensional electrodes were characterized by an average coefficient kh... 

The model for the three-dimensional or packed bed electrode, which describes the overpotential and concentration distributions, is a set of ordinary different equations with boundary conditions as... 

The principal metals present in low-level nuclear waste nitrates are Hg2+, Ru, in the form of a nitro complex, and CrO . They are contained in a 1.3 MNaOH solution. The laboratory version of the basic device for their separation and removal is the packed-bed electrode shown in Fig. 15.25. 

Fig. 15.25. Schematic for a laboratory-scale packed-bed electrode, a, Bed of particles b, current collector c, Luggin capillary d, thermometer e, purging gas in f, gas out g, bubbler h, gas collector i, Luggin capillary j, reference electrode k, Nation film I, counter-electrode m, septum for gas analysis and n, solution flow-in or flow-out. (Reprinted from J, O M. Bockris and J. Kim, J. Appl. Electrochem. 27 625, copyright 1997.)...

Kaba and Hitchens (1989) found that electrolysis of a mixture of urine and feces produced C02, N2, and H2. Some HOC1 is generated this eliminates the pathogens and bleaches the contents. The anodic reactions at 90 °C consume the biomass the cathode evolves hydrogen and can be assumed to deposit the small metal content. The residuum is sodium chloride from the urine. Most of the electrochemical studies that establish the basis of a practical process for electrochemical sewage treatment have been carried out on packed-bed electrodes as shown in Fig. 15.29. 

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. 

Packed bed electrode — A static three-dimensional - electrode consisting of a restrained bed of electronically conducting particles in continuous intimate contact. Packed Bed Electrodes (PBEs) present high electroactive area per unit electrode volume and moderately high -> mass transport characteristics (the limiting current at a PBE may exceed 100 times the one observed at a two-dimensional electrode of the same volume). 

Three-dimensional electrode — This term is used for electrodes in which the electrode-solution interface is expanded in a three-dimensional way, i.e., the - electrode possesses a significantly increased surface area due to nonplanarity, so that it can be housed in a smaller volume. This can be achieved by constructing corrugated electrodes, reticulated electrodes, -> packed bed electrodes (see also - column electrodes), -> carbon felt electrodes, or fluidized bed electrodes. Three-dimensional electrodes are important for achieving high conversion rates in electrochemical reactions. Therefore they are especially important in technical electrochemistry, wastewater cleaning, and flow-through analytical techniques, e.g., - coulometry in flow systems. However, the - IR-drop within three-dimensional electrodes is an inherent problem. 

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