Electrolytic reactor batch

Electrochemical reactors are heterogeneous by their very nature. They always involve a solid electrode, a liquid electrolyte, and an evolving gas at an electrode. Electrodes come in many forms, from large-sized plates fixed in the cell to fluidizable shapes and sizes. Further, the total reaction system consists of a reaction (or a set of reactions) at one electrode and another reaction (or set of reactions) at the other electrode. The two reactions (or sets of reactions) are necessary to complete the electrical circuit. Thus, although these reactors can, in principle, be treated in the same manner as conventional catalytic reactors, detailed analysis of their behavior is considerably more complex. We adopt the same classification for these reactors as for conventional reactors, batch, plug-flow, mixed-flow (continuous stirred tank), and their extensions. 

Product Recovery. Comparison of the electrochemical cell to a chemical reactor shows the electrochemical cell to have two general features that impact product recovery. CeU product is usuaUy Uquid, can be aqueous, and is likely to contain electrolyte. In addition, there is a second product from the counter electrode, even if this is only a gas. Electrolyte conservation and purity are usual requirements. Because product separation from the starting material may be difficult, use of reaction to completion is desirable ceUs would be mn batch or plug flow. The water balance over the whole flow sheet needs to be considered, especiaUy for divided ceUs where membranes transport a number of moles of water per Earaday. At the inception of a proposed electroorganic process, the product recovery and refining should be included in the evaluation to determine tme viabUity. Thus early ceU work needs to be carried out with the preferred electrolyte/solvent and conversion. The economic aspects of product recovery strategies have been discussed (89). Some process flow sheets are also available (61). 

Recent advances on the Ca-Br cycle were presented in an ANL paper. The original concept for this cycle involved solid phase reactions in a semi-continuous batch operation. The ANL paper reported on experiments that used a direct sparging reactor in the hydrolysis reaction to allow continuous production of HBr which is then electrolytically decomposed to produce hydrogen. The sparging steam was introduced into the molten bath of CaBr2 which yielded HBr in a stable and continuous operation. 

The reactor can be operated in a batch recycle mode. One of the advantages of SPE HDH technology is that separation and recycling of a supporting electrolyte are unnecessary, which can greatly reduce process cost. 

The simple batch reactor was considered in Chapter 4, A modified version in which the electrolyte is recirculated, however, is the preferred mode of operation in the electrochemical industry because it provides flexible batch volume and also enhances the mass transfer characteristics of the cell due to circulation. Further, with recirculation, the reactor can be operated either in the plug-flow or mixed-flow mode. We consider all three cases here along with a few other common modes of operation. 

Equation 21.51a or b is the design equation for a stirred tank batch reactor with recirculation of electrolyte. It can be solved by combining it with the reaction model given by Equation 21.41 or 21.42 to obtain A as a function of conversion, current density, and other system parameters. The current efficiency can be calculated from and (as in Example 21.1). 

In case of batch reactor experiment, described earlier, the concentration of the solution decreases continuously as the structure grows. To maintain the constant electrolyte concentration, lead acetate solution was influxed continuously throughout the experiment using a continuously stirred tank reactor as shown in Fig. 13.24. 

The tank cell is the classical batch or semi-batch reactor of electrochemical technology. In most tank cells, the electrodes are vertical and made from sheet, gauze or expanded material. The cell is arranged with parallel lines of alternate anodes and cathodes, the electrodes extending across and to the full depth of the tank. The anode-cathode gap is made as small as possible to maximize the space-time yield and to reduce the energy consumption. It is unusual in tank cells to induce convection by mechanical means, but electrolyte stirring is in generally promoted... 

A batch reactor is charged with reactant, the required conversion takes place, and the reactor is emptied. One consequence is that the concentration of the reactant and products in the reactor are a function of time. There are two types of electrochemical batch reactors those without electrolyte recirculation (Fig. 4.1a), and those with recirculation (Fig. 4.1b). The former are suitable as laboratory devices for obtaining performance data as a function of electrode potential, reactant concentration, and hence conversion as described in Section 3.2.2.2. The reactor is often run in a potentiostatic... 

THE PROBLEM A batch laboratory reactor with an electrolyte volume of 700 cm and an electrode area of 30 cm is used to deposit a divalent metal from an aqueous solution in a potentiostatic mode. Initial concentration of the metal is O.lkmol/m. The reactor mass transfer coefficient has been measured as 3.3 x 10" m/s. Hydrogen evolution occurs as a parallel reaction according to the equation % = H p [ — ], where kn = 1.30 X 10" A/m and = 12 If the metal deposition is operated at its limiting current density at an electrode potential of —0.9 V (SCE), determine how conversion, total current density, and current efficiency vary with time, in a potentiostatic mode. What will be the current efficiency at the final... 

Electrolyte from a well-mixed reservoir of volume is rapidly recirculated through a reactor of volume (Fig. 4.1b). Analysis of this system is more complicated than that for the stirred batch reactor considered in Section 4.2.1, since the reactor may be assumed to have a plug-flow or stirred-tank configuration. 

THE PROBLEM A batch recirculating system is used to recover metal from an aqueous electrolyte under limiting current conditions. Compare the conversions achieved after 2 x 10" seconds with a simple batch reactor, a recirculating plug-flow reactor and a recirculating stirred-tank reactor, assuming that the total electrolyte volume remains the same. The data in Table 4.3 can be used. 

In building up multiple units (Sections 5.1.2 and 5.1.3), alternative systems of electrical and hydraulic connections are possible, each of which have advantages. The choice often depends on the scale of the process and whether batch or continuous operation is to be used. In batch processes with flow electrolysis, the conversions per cell pass are usually small and the type of hydraulic connection will depend on mechanical aspects of reactor design and operation. With parallel plate cells, parallel electrolyte flow is certainly the most common option. In continuous processes the type of hydraulic connections—series or parallel—may affect the production capacity of a unit. Further information can be found in Picket s book" on reactor design. 

Our examples start with the optimization of the performance of an electrolytic batch reactor with respect to its operating current density. 

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