Flow electrolyzers

In studying residence time distribution in a tank-flow electrolyzer, a tracer injected into it is recovered in a mixing tank placed downstream. If both tanks are perfectly stirred, then with a tracer pulse, the mole balance for the tracer can be written as... 

In a complex reaction system occurring in a flow electrolyzer, one of the anode reaction products reacts with the principal constituent of the composite electrolyte, yielding an anionic species whose parasitic reaction at the cathode reduces the cathode efficiency. 

We mentioned in Section 1.3 some important industrial applications of electrolysis—in the chloralkali industry, metal winning and refining, and organic electrosynthesis. As indicated in Section 1.2, we do not intend to describe electrochemical processes in detail, since there are many books on electrochemical technology. We will discuss the design of individual reactors, with emphasis on modularized, general purpose flow electrolyzers. We will classify reactors by their mode of operation. 

FIGURE 5.7. General purpose flow electrolyzers (a) DEM cell section of divided cell section of undivided cell, (b) SU cell, (c) FM21 cell. Top FMOl laboratory reactor bottom FM21 electrolyzer. Reproduced with permission. 

Mathematical solution depends on the boundary conditions governed by the geometry of the reactor system. We will confine our attention to plate electrodes. Electrochemical reactors can be divided into two categories tank electrolyzers and channel flow electrolyzers. 

FIGURE 5.22. Primary current distribution for a flow electrolyzer. 

Although in theory only parallel plate flow electrolyzers with electrodes whose lengths tend toward infinity, or that fill the channel completely have the ideal geometry that leads to uniform primary current distribution, in practice these effects are not important. It is different, however, for laboratory cells with very small electrodes. 

FIGURE 14.24 In the Hall process, aluminum oxide is dissolved in molten cryolite and the mixture is electrolyzed in a cell with carbon anodes and a steel cathode. The molten aluminum flows out of the bottom ot the cell. 

The potential supplied to an electrolytic cell determines whether or not electrolysis can occur, but the current flow and the time of electrolysis determine the amount of material electrolyzed. Recall Equation from Section 19-1 ... 

Two directions of current flow in galvanic cells are possible a spontaneous direction and an imposed direction. When the cell circuit is closed with the aid of electronic conductors, current will flow from the cell s positive electrode to its negative electrode in the external part of the circuit, and from the negative to the positive electrode within the cell (Fig. 2.2a). In this case the current arises from the cell s own voltage, and the cell acts as a chemical source of electric current or battery. But when a power source of higher voltage, connected so as to oppose the cell, is present in the external circuit, it will cause current to flow in the opposite direction (Fig. 2.2b), and the cell works as an electrolyzer. 

It follows that in batteries, the negative electrode is the anode and the positive electrode is the cathode. In an electrolyzer, to the contrary, the negative electrode is the cathode and the positive electrode is the anode. Therefore, attention must be paid to the fact that the concepts of anode and cathode are related only to the direction of current flow, not to the polarity of the electrodes in galvanic cells. 

An appreciable increase in working area of the electrodes can be attained with porous electrodes (Section 18.4). Such electrodes are widely used in batteries, and in recent years they are also found in electrolyzers. Attempts are made to use particulate electrodes which consist of a rather thick bed of particulate electrode material into which the auxiliary electrode is immersed together with a separator. Other efforts concern fiuidized-bed reactors, where a finely divided electrode material is distributed over the full electrolyte volume by an ascending liquid or gas flow and collides continuously with special current collector electrodes (Section 18.5). 

The electrolyzer and the decomposer are the two main sections of a typical mercury cell. The electrolyzer is slightly inclined steel trough through which a thin layer of mercury flows over the... 

Lla. Landau. U., LBL-2702 Ph.D. thesis. University of California, Berkeley, January 1976. Lib. Landau, U., and Tobias, C. W., Mass Transport and Current Distribution in Channel Type Electrolyzers in the Laminar and Turbulent Flow Regimes, Ext. Abstr., No. 266, Electrochemical Society Meeting, Washington D.C., May 1976, 663. 

The continuous circulation of flow elements inside the electrolyzer is due to the donor-acceptor nature of each zone. If, however, at least one of the zones is an absorbing Markov state,... 

More importantly, in the SPE technology gaseous H2 and O2 are liberated on the electrode surface on the side of the solution, thus solving the problem of the solution resistance due to the presence of bubbles. The membrane acts as an electrolyte. At the anode H2O is oxidized to O2 with liberation of H, which migrates through the membrane to the cathode, where it is reduced to H2. In practice, a flow of solution is needed only at the anode to replace water molecules oxidized to O2. However, the solution no longer needs to be conductive since no current passes through it. Actually, SPE electrolyzers are fed with plain water [20]. 

Dutta S, Morehouse JH, Khan JA (1977) Numerical analysis of laminar flow and heat transfer in a high temperature electrolyzer. Int J Hydrogen Energy 22 883-895... 

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