Mixed electrochemical reactor

Mixed electrochemical reactor, 30 310 selectivity function, 30 315-316 Mixed ensembles in alloy catalysis, 32 198-201... 

Fig. 32. Typical mixed electrochemical reactors (MER). A CER with infinite recycling (c) behaves like a MER. Symbols as in Fig. 31 (67). (Reprinted by permission of the publisher. The Electrochemical Society, Inc.)...

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 ... 

The macroscopic approach, in which it is not taken into account what happens inside the cell in detail, but only an overall view of the system is described. In fact, the system is considered as a black box from the fluid dynamic point of view and then, it is assumed that the cell behaves a mixed tank reactor (the values of the variables only depend on time and not on the position since only one value of every variable describes all positions). This assumption allows simplifying directly all the set of partial differential equations to an easier set of differential equations, one for each model species. For the case of a continuous-operation electrochemical cell, the mass balances take the form shown in (4.5), where [.S, ]... 

Different classifications of electrochemical reactors can be made depending on their configuration (divided, undivided cathodic, and anodic compartments) electrode geometry (bi- and tri-dimensional), the fluid flow through the reactor (mixing, plug-flow, fluidized baths), among others. There are several demands for electrochemical reactors (Pletcher and Walsh, 1990 Molina et al., 2004) ... 

Ceramic electrochemical reactors are currently undergoing intense investigation, the aim being not only to generate electricity but also to produce chemicals. Typically, ceramic dense membranes are either pure ionic (solid electrolyte SE) conductors or mixed ionic-electronic conductors (MIECs). In this chapter we review the developments of cells that involve a dense solid electrolyte (oxide-ion or proton conductor), where the electrical transfer of matter requires an external circuitry. When a dense ceramic membrane exhibits a mixed ionic-electronic conduction, the driving force for mass transport is a differential partial pressure applied across the membrane (this point is not considered in this chapter, although relevant information is available in specific reviews). 

The fluid hydrodynamics and geometric configuration of the electrochemical reactor are key to understand the mixed processes that occur in a system. Though the specific geometry of the electrocatalysts is important, the mass transfer can be determined solely by fluid hydrodynamics [1],... 

Electrochemical reactors are heterogeneous by their very nature. They always involve a solid electrode, a liquid electrolyte, and an evolving gas at an electrode. Electrodes come in many forms, from large-sized plates fixed in the cell to fluidizable shapes and sizes. Further, the total reaction system consists of a reaction (or a set of reactions) at one electrode and another reaction (or set of reactions) at the other electrode. The two reactions (or sets of reactions) are necessary to complete the electrical circuit. Thus, although these reactors can, in principle, be treated in the same manner as conventional catalytic reactors, detailed analysis of their behavior is considerably more complex. We adopt the same classification for these reactors as for conventional reactors, batch, plug-flow, mixed-flow (continuous stirred tank), and their extensions. 

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. 

Although oxides have a wide range of catalytic applications their transport properties are most obviously critical when they are used in the form of a membrane within a chemical or electrochemical reactor. As such their ionic conductivity must be high if they are going to support a reasonable ion flux. Such materials fall broadly into two classes those materials that exhibit a very low electronic conductivity and, if the electronic transport number is <0.01, are generally termed solid electrolytes (solid electrolytes are covered in a separate chapter) and those materials that exhibit an appreciable or high electronic conductivity as well as ionic conductivity and are hence termed mixed conductors. In the rest of this chapter we will focus on such mixed ionic and electronic conducting (MIEC) materials. First, we will address transport in MIEC membranes from a theoretical perspective... 

A typical apparatus for electrochemical promotion experiments consists of three parts (a) The gas feed and mixing system (b) the reactor and (c) the analysis and electrochemical measurements system. A detailed schematic of the experimental apparatus is shown in Figure B.l, where the three parts are clearly shown. 

Enzyme linked electrochemical techniques can be carried out in two basic manners. In the first approach the enzyme is immobilized at the electrode. A second approach is to use a hydrodynamic technique, such as flow injection analysis (FIAEC) or liquid chromatography (LCEC), with the enzyme reaction being either off-line or on-line in a reactor prior to the amperometric detector. Hydrodynamic techniques provide a convenient and efficient method for transporting and mixing the substrate and enzyme, subsequent transport of product to the electrode, and rapid sample turnaround. The kinetics of the enzyme system can also be readily studied using hydrodynamic techniques. Immobilizing the enzyme at the electrode provides a simple system which is amenable to in vivo analysis. 

Electrochemical reactions proceed, in principle, heterogeneously at the electrode surfaces. Hence, the mass transfer has a major influence, especially on the selectivity of the electrode reactions. Therefore, the mixing conditions in the cell have to be optimized, considering also the operation mode as batch or as flow-through reactor. 

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