Electrolytic reactor selection

Section 1.1 mentioned that in an electrolytic reactor or cell design it is necessary to start with the notional selection of a design, determine its performance, and then decide whether it will do or whether an alternative 

current efficiency, depends on good mass transfer characteristics of the cell. Details of how to achieve this have been discussed in Section 2.3.5. Uniformity of electrode potential over the electrode system and the nature of the electrode material can also have considerable impact. 

Second, chemical yield, is like C.E. dependent on good mass transfer, uniform electrode potential, and electrode material. 

In the third, space-time yield, the size of the cell is directly proportional to a, the electrode area. Table 5.4 gives values of a for four typical designs (see Section 5.1). The effective electrode area for three-dimensional electrodes is smaller than what is shown in Table 5.4 nevertheless, it will be at least an order of magnitude larger than that for two-dimensional electrodes. In practical terms of cell selection (particularly for small-tonnage chemicals), small differences in cell volume are largely academic because cells are often dwarfed by separation equipment such as distillation columns. 

Returning to Table 5.3, the fourth and fifth parameters, energy yield and energy consumption, depend on current efficiency (the first criterion) and cell voltage. As discussed in Section 1.4.5, this depends on electrode materials, the electrolyte gap between anode and cathode, the conductivity of the electrolyte, and the nature of the diaphragm if any. 

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The overall rate of chemical change at an electrode process depends on the current density, j (the cell current, /, divided by the electrode area. A) for the desired reaction and the selectivity of the chemical change. The latter is usually discussed in terms of the current efficiency, , for the desired chemical change and is defined as the fraction of the charge passed used in the desired reaction. The space-time yield, Tst for n electrolytic reactor may be written... 

An electrolytic reactor or cell is a device in which one or more chemical species are transformed to alternative states with an associated energy change. There are many kinds of reactors selection of an appropriate one is dealt with in Chapter 5. The form a reactor takes depends largely on the number of phases involved and the energy requirements. The size depends on the rate of reaction we shall see that a reactor model can provide important answers about this rate. The reaction rate of an electrolytic reactor is defined by the possible current density, expressed in terms of reaction models in Chapter 3. 

The original German process used either carbonyl iron or electrolytic iron as hydrogenation catalyst (113). The fixed-bed reactor was maintained at 50—100°C and 20.26 MPa (200 atm) of hydrogen pressure, giving a product containing substantial amounts of both butynediol and butanediol. Newer, more selective processes use more active catalysts at lower pressures. In particular, supported palladium, alone (49) or with promoters (114,115), has been found useful. 

In the heavy-water plants constmcted at Savannah River and at Dana, these considerations led to designs in which the relatively economical GS process was used to concentrate the deuterium content of natural water to about 15 mol %. Vacuum distillation of water was selected (because there is Httle likelihood of product loss) for the additional concentration of the GS product from 15 to 90% D2O, and an electrolytic process was used to produce the final reactor-grade concentrate of 99.75% D2O. 

This new design is sought to overcome the limits of conventional porous fixed-bed reactors using an electrode phase flowing through the pores [65]. The latter systems suffer from the low conductivity of the electrolyte phase. This generates electrical resistance and leads to accumulation of the electrical current in certain reactor zones and hence results in a spatially inhomogeneous reaction. This means poor exploitation of the catalyst and possible reductions in selectivity. 

Selection of Corrosion-Resistant Materials The concentrated sofutions of acids, alkalies, or salts, salt melts, and the like used as electrolytes in reactors as a rule are highly corrosive, particularly so at elevated temperatures. Hence, the design materials, both metallic and nonmetallic, should have a sufficiently high corrosion and chemical resistance. Low-alloy steels are a universal structural material for reactors with alkaline solutions, whereas for reactors with acidic solutions, high-alloy steels and other expensive materials must be used. Polymers, including highly stable fluoropolymers such as PTFE, become more and more common as structural materials for reactors. Corrosion problems are of particular importance, of course, when materials for nonconsumable electrodes (and especially anodes) are selected, which must be sufficiently stable and at the same time catalytically active. 

The relative increase Ar /r Q in the rates of epoxidation (i=l) and combustion (i=2) is proportional to A/S, where A is the electrolyte surface area and S is the surface area of the silver catalyst electrode. Thus with a reactor having a low value of S (reactive oxygen uptake Q =.4 10 7 mol O2) a threefold increase in ethylene oxide yield was observed with a corresponding 20% increase in selectivity. 

Again, points on the curve were the measured acrolein production rates, and the line is the predicted production rate based on the current and the stoichiometry according to eq 9. At higher conversions, we observed significant amounts of CO2 and water, sufficient to explain the difference between the acrolein production and the current. It should be noted that others have also observed the electrochemical production of acrolein in a membrane reactor with molybdena in the anode. The selective oxidation of propylene to acrolein with the Cu—molybdena— YSZ anode can only be explained if molybdena is undergoing a redox reaction, presumably being oxidized by the electrolyte and reduced by the fuel. By inference, ceria is also likely acting as a catalyst, but for total oxidation. 

Despite their intrinsic simplicity, the plate-and-frame reactors require accurate selection of the materials, especially when the electrolytic processes are performed in non-aqueous solvents and/or deal with substrates which are currently used as solvents, as in the case of volatile organic chlorides. Special attention should be paid to the selection of the gaskets, for long-term operational stability. Moreover, in the case of the SPE scheme, porous electrodes should be adopted, to allow for optimal contact between electrode, electrolyte and reactants/products. In this case, mass-transfer limitations into the porous structure should be carefully considered. 

The fuel cell itself contains concentrated phosphoric acid (85-100%) and is operated at 180-200°C. This electrolyte rejects CO, therefore the gas mixture produced in the steam reformer can be pumped directly into the fuel cell, where hydrogen is oxidized at the anode. Actually the reaction does not proceed quantitatively according to Eq. 18M, and some CO is produced as a side product. This is removed witli a so-called shift reactor, in which CO is selectively oxidized to CO. The tolerance of the eleclrocatalysi at the anode toward residual CO in the... 

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. 

We have studied the partial oxidation of hydrocarbons with electrochemical reactor using an oxide ionic conductor, e.g. YSZ, SDC, etc. [2-4, 10]. In these studies, it was found that a ceria-based solid electrolyte is useful for the propene oxidation to acrylaldehyde at relatively low temperature of 350°C [10]. In this case, however, the acrylaldehyde selectivity was lower than that obtained with the electrochemical reactor constructed from YSZ. This may be due to the high activity of ceria surface for the complete oxidation of hydrocarbons [13,14]. 

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]. 

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. 

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