Electrochemical reactors types

Filtration of Liquids Depending on the specific electrochemical reactor type, the filtration rate of a liqnid electrolyte throngfi tfie separator should be either high (to secure a convective snpply of snbstances) or very low (to prevent mixing of the anolyte and catholyte). The filtration rate that is attained under the effect of an external force Ap depends on porosity. For a separator model with cylindrical pores, the volnme filtration rate can be calcnlated by Poiseuille s law ... 

This type of electrochemical reactor is composed of two bodies by mechanical manufacturing [66, 67]. It contains a two-compartment cell with an anodic and cathodic chamber separated by a membrane as diaphragm. The anodic chamber is equipped with a carbon felt anode made of carbon fibers a platinum wire is inserted in the cathodic chamber (Figure 4.30). 

Reactor type Electrochemical micro flow cell Diaphragm material PTFE... 

Reactor type Electrochemical capillary micro flow reactor Electrode dimensions 30 X 30 X 3 mm ... 

Reactor type Electrochemical sheet micro flow reactor Reaction medium layer depth 600 pm... 

Reactor type Electrochemical plate-to-plate micro flow reactor Reactor volume 35 pi... 

Furan was dimethoxylated to give 2,5-dihydro-2,5-dimethoxyfuran, using electrogenerated bromine molecules generated from bromide salts in electrolyte solutions [71]. This reaction was characterized in classical electrochemical reactors such as pump cells, packed bipolar cells and solid polymer electrolyte cells. In the last type of reactor, no bromide salt or electrolyte was used rather, the furan was oxidized directly at the anode. H owever, high consumption of the order of 5-9 kWh kg (at 8-20 V cell voltage) was needed to reach a current efficiency of 75%. 

Electrochemical reactors (cells, tanks) are used for the practical realization of electrolysis or the electrochemical generation of electrical energy. In developing such reactors one must take into account the purpose of the reactor as well as the special features of the reactions employed in it. Most common is the classical reactor type with plane-parallel electrodes in which positive and negative electrodes alternate and all electrodes having the same polarity are connected in parallel. Reactors in which the electrodes are concentric cylinders and convection of the liquid electrolyte can be realized by rotation of one of the electrodes are less common. In batteries, occasionally the electrodes are in the form of two long ribbons with a separator in between which are wound up as a double spiral. 

The investment for equipment is somewhat higher than for chemical reactors, but an electrochemical reactor can be applied to many more reaction types than a chemical one. Technical electrolyses are mostly run in continuous flow reactors, sometimes for years without change of the electrodes, which saves personnel, maintenance, and solvent costs as compared to chemical conversions, which are often performed in batch reactors. 

Wen and Fan [6] have provided a comprehensive listing of various tracers and experimental techniques for determining the RTD in flow systems. Recent studies [10,11,12] have been performed employing an impulse tracer to determine the RTD in bubble columns and an oscillatory flow electrochemical reactor. The author [13,14] has employed both step-change and an impulse to determine the RTD of nozzle type reactors analysis of the RTD involves an atomic absorption spectrophotometer (AAS), a cine-projector, and a chart recorder. Figures 8-7 and 8-8 show the nozzle-type reactors and the AAS, respectively. Figure 8-9 gives a typical response curve from the AAS. 

Classification by End Use Chemical reactors are typically used for the synthesis of chemical intermediates for a variety of specialty (e.g., agricultural, pharmaceutical) or commodity (e.g., raw materials for polymers) applications. Polymerization reactors convert raw materials to polymers having a specific molecular weight and functionality. The difference between polymerization and chemical reactors is artificially based on the size of the molecule produced. Bioreactors utilize (often genetically manipulated) organisms to catalyze biotransformations either aerobically (in the presence of air) or anaerobically (without air present). Electrochemical reactors use electricity to drive desired reactions. Examples include synthesis of Na metal from NaCl and Al from bauxite ore. A variety of reactor types are employed for specialty materials synthesis applications (e.g., electronic, defense, and other). 

There are various types of electrochemical reactor5,6 the classification is similar to that used for other chemical processes. The three basic types of electrochemical reactor are shown in Fig. 15.1 ... 

Fig. 15.1. Types of electrochemical reactor (a) Batch reactor (b) Plug flow reactor (c) Backmix flow reactor.

The effectiveness of RE and PC electrochemical reactors for methoxylation of furan was examined by Thomas et al. (1988). The performance of various types of reactors is compared in Table XXIX. As shown, both RE and PC with cathode spinning gave better performance than a capillary gap cell. For the pump cell, the results differ depending on whether the cathode is spinning or... 

Some filter-press type electrochemical reactors can be used both in a divided and undivided mode. This is illustrated in Ref. 51. The ICI filter-press cell system FM 21 is discussed in the context of several pilot-plant applications, most of them organic. 

Molina, V.M., Frias, A., Gonzalez, J., Montiel, V., Gonzalez. D.. Dommguez. M.. and Aldaz, A. 2004. Design and development of filter-press type electrochemical reactors for their application in the resolution of environmental problems. In Trends in Electrochemistry and Corrosion at the Beginning of the 21st Century. Brillas. E.. and Cabot. P.-L.. Eds. Universitat dc Barcelona, Barcelona, pp. 383-399. 

Corrosion of the material used is another factor that limits the selection of the electrocatalyst. The electrochemical corrosion of pure noble metals is not as important as in the case of binary or ternary alloys in strong acid or alkaline solutions, since these catalysts are widely used in electrochemical reactors. In the case of anodic bulk electrolysis, noble metal alloys used in electrocatalysis mainly contain noble metal oxides to make the oxidation mechanism more favorable for complete electron transfer. The corrosion problem that occurs from this type of catalyst is the auto-corrosion of the electrode surface instead of the electrode/electrolyte solution interface degradation. The problem of corrosion is considered in detail in Chapter 22. 

On the other hand, the selectivity of the electrochemical deposition of the metal on the substrate must be 100% of the current efficiency, with no interference from the other metal deposition processes. Therefore, the potential distribution needs to be presented for any serious electrochemical reactor study and the electrocatalyst selection problem. The major problem of current distribution depends on the type of the process that controls the entire reaction rate, such as charge transfer, ohmic contributions, or mass transport to or from the electrode. Many parameters have to be evaluated in the course of an electrochemical process to obtain the desired uniform potential and current distributions. One of the conditions that has to be fulfilled is the continuity equation for the current density vector, j ... 

Many factors can affect the rate of the electrocatalytic reaction, but the use of large-area rough electrodes makes almost no changes in the conceivable electrochemical reactor. Surface areas of more than 104 cm2 cm 3 of the catalyst are attainable. This type of electrode is commonly used in fuel cells and in industrial gas-evolving electrodes. In this case, the matrix is electronically conductive but the electrode reactions take place on a highly dispersed catalyst area all over the substrate. There is a particular distribution of reaction rates due to the diffusion of reactants into the pores and the changes in the electrolyte resistances therein. 

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]. 

Accordingly, the concentration profile of the processes changes with respect to the type of mechanism and to the rate determining specific constant, k (from 10 5 to 1010 s-1). In the case of industrial electrochemistry, the optimized conditions of work imply the minimization of loss, according to the side reactions. This is a consequence of the selectivity condition needed in the case of an electrochemical reactor. In a general treatment the theoretical model of the reactor is based on mass conservation laws with the corresponding electrochemical kinetics (coupled or not to side reactions). For example, the EC mechanism can be treated as follows ... 

Using a similar type of the microflow cell, Haswell s group reported the electroreductive coupling of activated olefins and benzyl bromide derivatives (Equation 12.8) [40]. The microflow electrochemical reactor can be easily multiplexed to generate a number of parallel flow cells, which offer the performance of a single cell while increasing the volumetric throughput of the system. 

Electrochemical reactor used for wastewater treatment A new undivided "bipolar electrochemical reactor" of channel type with flat parallel electrodes has been developed in our laboratory. Its design is very simple, it consists of a bipolar stack of 5 - 30 metallic sheets (electrodes) separated from each other by insulating plastic spacers at a fixed distance. 

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