Electrochemical cells reactors

Figure 9.23. Schematic diagram of the apparatus (a, left) and of the electrochemical cell-reactor (b, right) used for H2 oxidation on Pt/Nafion.35 Reproduced by permission of The Electrochemical Society, Inc.

Solvents and nonaqueous or aprotic electrolytes are often used to increase the solubility of organic reactants in aqueous solutions or to affect product selectivity. Such electrolytes, however, have low conductivities, which cause high ohmic losses of voltage due to current flow through the solution. To minimize ohmic losses, industrial electrochemical cells (reactors) should be designed with minimal spacing and resistance between the anodic and cathodic electrodes. Conductivities as high as 10 cm are desirable. 

Electrochemical cells (reactors) electrochemical techniques electrolysis wastewater treatment recycling of metals electrodeposition cementation electrodialysis metal ion removal metal ion concentration. 

Other applications of zirconium tetrafluoride are in molten salt reactor experiments as a catalyst for the fluorination of chloroacetone to chlorofluoroacetone (17,18) as a catalyst for olefin polymerization (19) as a catalyst for the conversion of a mixture of formaldehyde, acetaldehyde, and ammonia (in the ratio of 1 1 3 3) to pyridine (20) as an inhibitor for the combustion of NH CIO (21) in rechargeable electrochemical cells (22) and in dental applications (23) (see Dentalmaterials). 

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). 

Product Recovery. Comparison of the electrochemical cell to a chemical reactor shows the electrochemical cell to have two general features that impact product recovery. CeU product is usuaUy Uquid, can be aqueous, and is likely to contain electrolyte. In addition, there is a second product from the counter electrode, even if this is only a gas. Electrolyte conservation and purity are usual requirements. Because product separation from the starting material may be difficult, use of reaction to completion is desirable ceUs would be mn batch or plug flow. The water balance over the whole flow sheet needs to be considered, especiaUy for divided ceUs where membranes transport a number of moles of water per Earaday. At the inception of a proposed electroorganic process, the product recovery and refining should be included in the evaluation to determine tme viabUity. Thus early ceU work needs to be carried out with the preferred electrolyte/solvent and conversion. The economic aspects of product recovery strategies have been discussed (89). Some process flow sheets are also available (61). 

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... 

Two types of continuous flow solid oxide cell reactors are typically used in electrochemical promotion experiments. The single chamber reactor depicted in Fig. B.l is made of a quartz tube closed at one end. The open end of the tube is mounted on a stainless steel cap, which has provisions for the introduction of reactants and removal of products as well as for the insertion of a thermocouple and connecting wires to the electrodes of the cell. A solid electrolyte disk, with three porous electrodes deposited on it, is appropriately clamped inside the reactor. Au wires are normally used to connect the catalyst-working electrode as well as the two Au auxiliary electrodes with the external circuit. These wires are mechanically pressed onto the corresponding electrodes, using an appropriate ceramic holder. A thermocouple, inserted in a closed-end quartz tube is used to measure the temperature of the solid electrolyte pellet. 

Electrochemical cells have also been proposed for carbon that employ a eutectic molten salt mixture of Li2C03 Na2C03or LiCl-CaCl2-CaC2 as the electrolyte. A diffusion-type meter has also been developed for both small-scale and reactor sodium . 

Figure 4.30 Electrochemical micro reactor, a diaphragm micro flow cell, applied to perform the cation flow method. Assembled device (left). Disassembled device showing the two compartments of the cell within the housings and the diaphragm (right) [67. ...

The first study on the oxidation of arylmethanes used this reaction as a model to show the general advantages of electrochemical micro processing and to prove the feasibility of an at this time newly developed reactor concept [69]. Several limits of current electrochemical process technology hindered its widespread use in synthetic chemistry [69]. As one major drawback, electrochemical cells stiU suffer from inhomogeneities of the electric field. In addition, heat is released and large contents of electrolyte are needed that have to be separated from the product. 

Micro reaction systems may help to overcome or at least reduce some of the above-mentioned limitations [69]. Electrochemical micro reactors with miniature flow cells where electrodes approach to micrometer distances should have much improved field homogeneity. As a second result of confined space processing, the addition of a conducting salt may be substantially reduced. In addition, benefits from a uniform flow distribution and efficient heat transfer may be utilized. 

The maximum values of electric power and unit output of electrochemical cells vary within wide limits. The total current load admitted by individual electrolyzers for the electrochemical production of various materials in plant or pilot installations (their capacity) is between 10 A and 200 kA, while the current loads that can be sustained by different types of battery (their current ratings) are between 10 A and 20 kA. Corresponding differences exist in the linear dimensions of the electrodes (between 5 mm and 3 m) as well as in the overall mass and size of the reactors. 

Transport phenomena. See also Chemical reactor transport phenomena for chemical reactor design, 25 269—322 Transport phenomena, electrochemical cell, 9 657-660... 

This reaction has a very small equilibrium constant, but by dipping carbon electrodes in concentrated solutions and withdrawing the gaseous CI2 and purifying the NaOH, two valuable products could be made rather cheaply because of the large AG between anode and cathode in an electrochemical cell. Electrochemical reactors will be discussed in Chapter 9. 

However, there are many other options to combine electricity with chemistry. One that has been studied intensively for a variety of different applications is plasma chemistry (see Fridman, 2008 for a recent overview). A plasma is a partially ionized gas, in which a certain percentage of the electrons is free instead of bound to an atom or molecule. Because the charge neutrality of a plasma requires that plasma currents close on themselves in electric circuits, a plasma reactor shows resemblance to an electrochemical cell, although due to the much lower ionization degree and conductivity, a plasma discharge will typically be operated in the range of hundreds of volts, compared to a few volts in the case of an aqueous electrochemical cell. 

---