Contents Electrochemical Reactors

These few examples grew out of the chest of treasures opened up by Galvani and by Volta. Diabetics will soon be able to check their glucose levels by glancing at a wrist meter that measures sugar content electrochemically. Tritium, an essential component of nuclear weaponry, may be made electrochemically at a fraction of the cost of its production in a nuclear reactor. 

There are many applications in chemical engineering where diffusion of charged species is involved. Examples include ion exchange, metals extraction, electrochemical reactors, and membrane separations. There is an excellent textbook in this area (Newman, 1991). Here we will be content to show that the treatment of electrolyte diffusion follows naturally from the generalized treatment of diffusion given in Section 2.3. 

If we define II as the enthalpy of the reactor and its contents as above, and in the case of an electrochemical reactor of constant volume, we can transform (14.3) into... 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

The first study on the oxidation of arylmethanes used this reaction as a model to show the general advantages of electrochemical micro processing and to prove the feasibility of an at this time newly developed reactor concept [69]. Several limits of current electrochemical process technology hindered its widespread use in synthetic chemistry [69]. As one major drawback, electrochemical cells stiU suffer from inhomogeneities of the electric field. In addition, heat is released and large contents of electrolyte are needed that have to be separated from the product. 

Thus, the current density, flowing across the cell in the direction perpendicular to the electrode surface, is one of the most important factors that influences the performance of the membrane reactor. Second, the activation energy is a function of temperature, and, in a sense, the equflibrium voltage deceases with increasing temperature. Therefore, a proper temperature can promote the electrode reactions. In addition, because the electrolyte concentration has an influence on the equilibrium potential of the electrochemical reaction and resistance in the electrolyte, the concentration of the electrolyte is also a key factor in membrane reactors. Combined with the previous contents, the operating parameters of the membrane reactor are mainly as follows pH, current density, temperature, and electrolyte concentration. 

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