Chemical reactors electrochemical processes

Scale- Up of Electrochemical Reactors. The intermediate scale of the pilot plant is frequendy used in the scale-up of an electrochemical reactor or process to full scale. Dimensional analysis (qv) has been used in chemical engineering scale-up to simplify and generalize a multivariant system, and may be appHed to electrochemical systems, but has shown limitations. It is best used in conjunction with mathematical models. Scale-up often involves seeking a few critical parameters. Eor electrochemical cells, these parameters are generally current distribution and cell resistance. The characteristics of electrolytic process scale-up have been described (63—65). 

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). 

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. 

A more recent review by Fahidy (FI) concerns the chemical engineering approach to electrochemical processes, such as fluidized-bed reactors, bipolar particulate reactors, pulsed electrochemical reactors, gas-phase electrochemical reactors, electrocrystallization and electrodissolution, and the enhancement of heat and mass transfer in electric fields. In this review, the author also discusses dimensionless mass-transfer equations applied in cell design. Such equations are reviewed in greater detail in Section VI. 

Trickle-bed reactors are found in many industries such as the petroleum, petrochemical, and chemical industry, and in various applications, i.e. wastewater treatment and biochemical and electrochemical processing. 

L.L. Raja, R.J. Kee, R. Serban, and L.R. Petzold. A Computational Algorithm for Dynamic Optimization of Chemical Vapor Deposition Processes in Stagnation Flow Reactors. J. Electrochem. Soc., 147 2718-2726,2000. 

However, industrial electrochemical chlorate production is aimed at the chemical path in external reactors because of the higher energy efficiency compared with the summary electrochemical process ... 

Electrochemical reaction engineering deals with modeling, computation, and prediction of production rates of electrochemical processes under real technical conditions in a way that technical processes can reach their optimum performance at the industrial scale. As in chemical engineering, it centers on the appropriate choice of the electrochemical reactor, its size and geometry, mode of operation, and the operation conditions. This includes calculation of performance parameters, such as space-time yield,... 

On the other hand, well engineered manufacturing operations depend on the availability of manipulated variables for real-time feedback control. These variables usually operate at macroscopic length scales (e.g. the power to heat lamps above a wafer, the fractional opening of valves on flows into and out of a chemical reactor, the applied potential across electrodes in an electrochemical process). The combination of a need for product quality at the molecular scale with the economic necessity that feedback control systems utilize macroscopic manipulated variables motivates the creation of methods for the simulation, design and control of multiscale systems. 

Process or device development is intimately linked to the availability of materials suitable as active or passive cell components. Design, even in its conceptual stage, is inseparable from what materials are available for electrodes or for containment, what electrolyte compositions may come into consideration, and what separators (if any) are needed. Electrochemical engineering involves not only the cell or cell process but also the often considerable chemical and physical operations (separations, chemical reactors, heat exchangers, control, etc.) that precede and follow the electrochemical step. 

The usual procedures for the conception of electrochemical reactors arise from the mass conservation laws and the hydrodynamic structure of the device. In fact, four types of balances can be considered energy, charge, mass, and linear movement quantity. Since the reactor must include the anodic and the cathodic reactions, it is possible to make a complete balance for the mass. The temperature also governs the stability of a chemical reactor, but in the case of an electrochemical device, the charge involved in the entire process has to be considered first [3-5]. 

The design of the electrochemical reactor, in the case of a fuel cell, is not yet totally solved as classical heterogeneous chemical reactors do not meet the requirements of the triphasic interface anode and the cathode binary system. Some papers [1-3] have considered the problem at the cathode and at the anode independently. However, the electrocatalytic reactions on both the electrodes produce a single chemical reaction, which is the chemical outlet of the energy conversion process. 

In the next layer of subjects we list the engineering sciences which are needed in various ways for understanding and further developing the core engineering subjects thermodynamics, chemical kinetics, electrochemical phenomena, and transport phenomena. These engineering sciences, which are themselves interrelated, form the basis for the analytical and numerical description of the chemical reactor and its peripheral equipment. For example, the subject of transport phenomena can be used to analyze difiiision-controlled reactions, separation schemes, transient processes in reactors, thermal processes, flow patterns in reacting systems, corrosion, difiusion in porous media, and other problems connected with reactor engineering. 

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