Electrochemical reactors scale

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). 

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). 

Using a fluidized bed electrode, this process was studied by Jircny 1985 [118]. Jircny [119] worked with a laboratory scale cell and subsequently a pilot plant. The pilot plant was designed to produce one ton of D-arabinose per year. The electrochemical reactor was 0.3 x 0.6 x 0.6 m and contained five 225 A cells in series. A major advantage of the electrooxidation over the usual chemical route (oxidation with sodium perchlorate) was the ease of separation of D-arabinose from the reactor outflow. In chemical routes, the separation is made difficult by the presence of large amounts of sodium chloride. 

The electrical potential and/or current required for electroenzymatic treatment have been shown to be lower than those needed in electrochemical treatment, which are not economically viable for large-scale. Electroenzymatic oxidation by peroxidases was proposed for the oxidation of veratryl alcohol by LiP [40], Then, electroenzymatic reactors have been used for the treatment of petrochemical wastewater [91], dyes, and textile wastewater [90, 92, 118] and phenol streams [93] utilizing peroxidase immobilized onto inorganic porous Celite beads or directly onto the electrode. The integration of a second electrochemical reactor, which generated hypochlorite in the presence of sodium chloride, has been used for indirect oxidation of the reaction products of the electroenzymatic treatment [91]. 

Kalu and Oloman [75] studied the simultaneous synthesis of alkaline hydrogen peroxide and sodium chlorate in a bench-scale flow-by single-cell electrochemical reactor. A schematic of the electrode conditions is shown in Fig. 18. Graphite felt was used as the cathode to synthesize peroxide from 0.5 -2.0 M NaOH chlorate was the product at a dimensionally stable anode (DSA). The anodic and cathodic reactions were as follows ... 

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,... 

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. 

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. 

Factors to consider when selecting an electrochemical reactor are listed in Table 26.10 [30]. Batch reactors are chosen for the synthesis of high value-added products in small quantities, whereas continuous reactors are more amenable to large-scale production. When the anolyte and catholyte... 

Different reactors have been employed for various bench-scale and industrial-scale electrochemical processes. A partial listing of typical cell designs follows. More information on electrochemical reactors can be found in [28, 30, 33]. 

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. 

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]. 

At various places throughout the first five chapters in the book we have, when it appeared relevant to the discussion, referenced studies which addressed issues pertaining to the economic/technical feasibility of membrane reactor processes. In this chapter we specifically focus our attention on these issues. In the discussion in this chapter we have, by necessity, drawn our information from published studies and reports. Several proprietary studies reportedly exist, carried out by a number of industrial companies, particularly during the last decade, which have evaluated the potential of membrane reactors for application in large-scale catalytic processes. By all accounts the conclusions reached in these proprietary reports mirror those found in the published literature. In the discussion which follows, we will first discuss catalytic and electrochemical reactors. We will then conclude with a discussion on membrane bioreactors. 

Numerous electrochemical reactor designs have been described in the literature for the removal and recovery of a range of dissolved metals from both synthetic and industrial process streams. Many of these have been developed into successful pilot- and full scale devices as indicated in this paper. In addition to the choice of reactor design (which includes decisions regarding electrode geometry and electrode/electrolyte motion), the following operational factors are seen to be important ... 

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. 

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. 

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. 

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 ... 

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. 

Microbial electrochemical cells (MXCs) represent a new class of technologies, within the realms of wastewater treatment, bioremediation, and bioenergy generation [1-3], MXCs rely on the unique capability of certain bacteria to use electrodes in their metabolism, either as electron acceptors or as electron donors [4-6], MXCs represent one of the most challenging types of bioreactors and electrochemical reactors to scale... 

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. 

Electrode materials and scalable reactors - in different bioelectrochemical systems expensive electrode materials such as carbon nanotubes or precious metal electrodes are used. These materials are unconsolidated for large-scale MES. In terms of maximizing productivity and minimizing costs, cheap and reusable three-dimensional electrodes are needed. In a technical electrochemical reactor, the use of an expensive separator such as a membrane should be avoided. During the lab stage, the scalability of the reactor concept should receive attention as important parameter. 

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