Reactor, electrochemical

Electrochemical reactors are usually batch operated. Obviously energy is supplied in the form of electricity. In some cases, such as uranium production, the product can be prepared by both electrochemical and other methods, but other methods may then prove to be more economic in practice. Uranium, for example, is manufactured by heating a mixture of magnesium and uranium tetrafluoride  

Aluminium production is an intermediate case. It is made by electrolysis of a molten mixture of aluminium oxide and sodium hexafluoro-aluminate(III) (cryolite), but the carbon anodes waste away by reaction with the liberated oxygen and this provides thermal energy to melt the electrolyte, thus lowering the electrical energy input required .  

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  

Batch reactor. The process requires total conversion of the reagents, and therefore includes a time for discharging and recharging. It is not easily adaptable to industrial situations, because of  

This reactor is best for laboratory use in investigations of electrolysis kinetics and mechanism. 

Backmix flow reactor or continuously stirred tank reactor. The conversion rate is lower than for plug-flow reactors because the reagent is immediately diluted on being introduced into the reactor. Many flow reactors, e.g. tubular reactors, and especially in the turbulent regime are in this class. 

Plug-flow and backmix flow reactors can be used as single-pass, with recirculation or in cascade, leading to many possible configurations, but always with the aim of optimizing product yield in space and time. 

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Fig. 2. Schematic models of a plug flow electrochemical reactor (PFER) and a stirred tank electrochemical reactor (STER).

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). 

Newman, J. S., Electrochemical Reactor Design, Prentice-Hall, New Jersey (1973)... 

A fuel cell is an electrochemical reactor with an anodic compartment for the fuel oxidation giving a proton and a cathodic compartment for the reaction of the proton with oxygen. Two scientific problems must be solved finding a low-cost efficient catalyst and finding a membrane for the separation of anodic and cathodic compartments. The membrane is a poly electrolyte allowing the transfer of hydrated proton but being barrier for the gases. 

J.N. Michaels, C.G. Vayenas, and L.L. Hegedus, A Novel Cross-Flow Design for Solid State Electrochemical Reactors, J. Electrochem. Soc. 133, 522-525 (1986). 

Figure 7.5. (a) Solid electrolyte cell consisting of an YSZ disk with working (Pt), reference (Au, Ag) and counter electrodes (Au). (b) Schematic diagram of the electrochemical reactor.21 Reprinted with permission from The Electrochemical Society. 

H. Christensen, J. Dinesen, H.H. Engell, and K.K. Hansen, Electrochemical Reactor for Exhaust Gas Purification, SAE paper 1999-01-0472, Diesel Exhaust Aftertreatment (SP-1414)225-229 (1999). 

Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

Figure 4.29 Electrochemical reactor with alternating conducting and insolating porous sections each connected to separate power supplies [65].

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). 

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%. 

Matlosz, M., Vallieees, C., Micro-sectioned electrochemical reactors for selective partial oxidation, in Eheeeld,... 

Electrochemical reactors are inferior to chemical reactors in their productivity. Hence, electrosynthesis is used when chemical ways to synthesize a given substance... 

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 productivity of modem electrolyzers per unit volume or unit of floor space as a rule is lower than that of chemical reactors with a similar purpose. This is due to the fact that in an electrochemical reactor the reactions occur only at the electrode surfaces, while in a chemical reactor they can occur in practically the full volume. Therefore, recent efforts go in a direction of designing new, more efficient electrochemical reactors. 

The major functions of separators in electrochemical reactors are (1) the mechanical separation of electrodes of different polarity and prevention of their contact (e.g., during vibrations) and of electronic conduction (short-circuiting) between... 

The coefficient s shonld be low for separators in electrochemical reactors. It has valnes between 1.1 and 1.6 for simple separators, but for porous diaphragms and swollen membranes it has valnes between 2 and 10. The total porosity shonld be at least 50%, and the separator s pore space shonld be impregnated completely and sufficiently rapidly with the liqnid electrolyte. 

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 ... 

Fahidy, T. Z., Principles of Electrochemical Reactor Analysis, Elsevier, Amsterdam, 1985. 

The design optimization of an electrolytic cell aims at a high throughput with a low energy consumption at the lowest feasible cost. The throughput of an electrochemical reactor is measured in terms of the space time yield, Yt, defined as the volumetric quantity of the metal produced per unit time per unit volume of the process reactor. This quantity is expressed as ... 

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