Scale-Up of Electrochemical Reactors

As in conventional chemical reactors, similarity criteria are also employed in the scale-up of electrochemical reactors. Apart from similarities such as geometric similarity, kinematic similarity, and chemical similarity, electrical similarity is a unique criterion in the scale-up of electrochemical cells. It is defined as the condition where geometrically, kinematically, and chemically similar cells have identical cell voltages and current distributions inside the cells. It is also important to note that, although the principles of similarity criteria are the same for chemical and electrochemical reactors, suitable modifications have to be made to obtain electrochemical similarity. 

The following criteria hold for batch cells operated at constant current conditions  

The criterion for electrical similarity can be stated as follows two similar cells will have identical current distributions if one of the following three parameters is held constant. 

Dimensionless limiting current density A lcd (for rnass transfer controlled reactions) 

Dimensionless exchange current density A ecd (for reactions that are activation controlled in the linear region) 

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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). 

The phenomenon of charge transport, which is unique to all electrochemical processes, must be considered along with mass, heat, and momentum transport. The charge transport determines the current distribution in an electrochemical cell, and has far-reaching implications on the current efficiency, space-time yield, specific energy consumption, and the scale-up of electrochemical reactors. 

Current and potential field distributions, which determine the flow of current between electrodes, the variation of potential within the cell, and the distribution of reaction rates along the electrode surfaces. Knowledge of these phenomena is essential for the rational design and scale-up of electrochemical reactors. 

Electrochemical reactors, unlike their catalytic counterparts, require electrical similarity. This is often the most important consideration in the scale-up of electrochemical reactors. Considerable attention will be devoted to this... 

This electrochemical promotion study was novel in three respects a) The catalyst-electrode was a fully promoted industrial catalyst, (b) The study was carried out at high pressure (50 atm), (c) This was the first attempt for the scale-up of an electrochemically promoted reactor since 24 CaZro.9Irio.1O3m cell-pellets, electrically connected in parallel, were placed in the high pressure reactor (Fig. 9.32).43... 

H Marzouk. EDF, France. The Modified Grignard Reactor for Electrosynthesis Scale-Up of the Indirect Oxidation of Galacturonic Acid Electrochemical Processing, The Versatile Solution, Conference, ICI and EA-Technology. Barcelona, Spain, April 14-18, 1997. 

The ideal cell in order to scale up an electrochemical reaction can depend on the reaction, the electroactivity of the substrate to convert, the concentration of the substrate, as well as the current density at the working electrode. The use of a separator is necessary when the electrode can affect the whole process negatively. With anodic oxidations, the reaction at the counter electrode is most frequently the cathodic formation of hydrogen. In these cases, a separator does not seem indispensable a tank cell (kind of Grignard type reactor equipped with cylindrical electrodes) or a capillary-gap cell (piling of bipolar electrodes in a cylinder-shaped vessel connected to an anodes and a cathode located at the top and the bottom of the cell) can be considered as suitable devices for anodic conversions. More generally, the so-called plate-and-frame cells (Fig. 4) are used in a battery. 

This book on "Environmental Oriented Electrochemistry" concentrates on the Electrochemistry/Environment relationship including, among others, chapters on design and operation of electrochemical reactors and separators, process simulation, development and scale-up, optimization and control of electrochemical processes applied to environmental problems, also including economic analysis, description of unique current and future applications, in addition to basic research into developing new technologies. 

Classical electrochemical reactor designs invariably evolved from direct scale-up of simple laboratory electrolysis experiments. The most common example of this concept is the tank cell where an array of electrodes is immersed in a plastic or metal tank. More sophisticated versions involve a variety of approaches to enhancing convection, by rapid stirring, rotating or moving electrodes or improving geometry with plate and frame or filter-press-type cells. 

Various aspects of RFB development should never be underestimated, including electrochemical engineering, novel RFB design, battery scale-up, and the increase of power density and energy output in a stack. The integration of electrochemical reactors with other devices or unit processes should also be investigated for new RFB systems. 

For the scale up of a chemical reactor, inadequate mixing may result in spatial variations in, for example, reactant composition or temperature. An electrochemical reactor (cell) is a chemical reactor where the reduction and oxidation reactions are spatially separated on cathodes and anodes. The flow of ionic current through the electrolyte results in an electric field through the electrolyte. Since charged species move in response to an electrical field [1-3] and since the potential difference across the double layer impacts reaction rate, electrical field effects can significantly impact current distribution. Thus, in contrast to a chemical reactor, perfect mixing to eliminate all concentration fields does not necessarily result in uniform reaction rates. 

Electro-reduction and -oxidation processes are easy to operate and control remotely. Unlike the use of redox chemicals, they do not give rise to waste salts. Convenient remote control and operation and avoidance of waste salts are especially attractive features for commercial processing of any type of power reactor fuel. According]y, the electrode reactions were introduced quite early as intermediate steps in reprocessing. The electrochemical decladding of spent fuels was the first process in this field to be advanced up to the technical scale in the USA (J, 2 3, ... 

There are numerous ways of quantifying the energy efficiency and product selectivity of an electrochemical reactor, for both scale-up calculations and capitaFoperating cost analyses. Although products are formed at both the anode and cathode in such reactors, the cell performance is normally characterized in terms of the electrode where the desired product is generated. 

The largest exchange current density, j0, of the reaction has to be selected, if possible, since economic limitations are always prevalent in scaled-up engineering. However, with the development of nanodispersed substrates and carbon-supported metal catalysts, this limitation becomes a secondary consideration. At this point, it is important to say that most of the reported values of j usually refer to simple reactions on pure metal substrates using different shapes of electrode designs in a certain and single electrolyte. Thus, the measurement of the real j0 value at select industrial conditions of the electrochemical reactor has to be performed that is, experimental measurements cannot be avoided [4,5]. 

Sometimes it is not possible or convenient to lower the values of Wa, so we have to play with all the adjustable parameters of Equation 13.37. In the case of the electrocatalyst and for an electrochemical reactor scale-up problem, Wa has to be of an appreciable magnitude that is, we can change the electrode size (not geometry) but Wa should remain constant. The problem with real electrochemical reactors is that concentration gradients (especially for long-time uses) are inevitable, and thus the shape of the /, // vs. x/L plot is useful (Figure 13.6). 

Improvements in membrane reactor performance may also be obtained by further developments on new processes and methods for scaling up. Electrochemical desulfurization of gases by a membrane reactor is a promising and proven technology. 

In recent years electro-organic synthesis has been gaining in importance[9]. The first to be carried out on an industrial scale was that of adiponitrile, the raw material for large scale production of nylon. However, the production of many more substances, in particular those of pharmaceutical interest, has been set up at many points around the world in limited volumes achievable with small-scale electrochemical reactors. A review of these products is given in Table 1. 

The parallel plate geometry [1,3] offers uniform current density and potential distribution. The incorporation of this electrode geometry into a plate and frame cell body, particularly in a modular filter-press format, provides a versatile workhorse for many electrochemical reactors. Many developments start with a small, single cell before being scaled-up by increasing the electrode area and then by designing a multiple cell stack in a filter press stmcture. Parallel plate cells have many advantages ... 

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