Energy efficiency considerations on monopolar vs bipolar fused salt electrolysis cells
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I.
INTRODUCTION
I N recent years, the light metals industries have shown increasing interest in using bipolar cells for fused salt electrolysis. The outstanding examples are the ALCOA* aluminum chloride process ceils ti} and the *ALCOA is a trademark of Aluminum Company of America, Pittsburgh, PA.
Alcan [2] and Ishizuka t3] magnesium cells. The objective of this article is to analyze the relative merits of utilization of monopolar and bipolar cells for fused salt electrolysis. Information is borrowed from experience in aqueous electrolytic processes in which relevant analyses have been made. The trade-off between the two types of cells depends on the state of the technology, materials available, plant size, and other factors. The competition between monopolar and bipolar cells goes back to the beginning of electrochemistry. Alessandro Volta made batteries of cells in the form of a "pile" (bipolar electrodes) and a "crown of cups" (monopolar electrodes) before 1800. [4] The bipolar electrodes were pairs of copper or silver discs against tin or zinc discs. The electrolyte between the bipolar electrodes was soaked in discs of pasteboard or hide. The crown of cups consisted of glass vessels containing electrolyte and monopolar electrodes connected in series externally. The author~*l stated that the main trade-offs were that the pile was more compact but tended to dry out and that the crown of cups required more space.
H. CONFIGURATION OF MONOPOLAR AND BIPOLAR CELLS AND CELL LINES Basic monopolar cell and bipolar cell battery or stack configurations representative of technological systems are illustrated in Figure 1. To simplify the discussion, the electrodes are considered to be of an inert material T.R. BECK, President, is with Electrochemical Technology Corp., Seattle, WA 98109. I. ROUSAR, Professor, is with the Department of Electrochemistry, Norwegian Institute of Technology, Trondheim, Norway, and permanently with Prague Institute of Chemical Technology, Prague, Czech Republic. J. THONSTAD, Professor, is with the Department of Electrochemistry, Norwegian Institute of Technology, Trondheim, Norway. Manuscript submitted September 10, 1993. METALLURGICALAND MATERIALSTRANSACTIONS B
that can serve as either anode or cathode. The electrodes can be connected electrically either in parallel (Figure l(a)) or in series (Figure l(b)). In the parallel monopolar mode (a), the stack voltage is U , and the stack current is Nit. Perhaps there is an ambiguity whether this is N ceils in a tank or a single cell, but the usual industrial convention is to consider this type of configuration as one cell, because it has the voltage of a single cell. In the series bipolar mode (b), the stack voltage is NUt, and the stack current is It. It is logical to consider (b) a battery of single cells because, like the Voltaic pile, there is no exterior electronic path between cells, and the voltage is that of N cells. The main economic trade-off between bipolar and monopolar electrodes is that of current efficiency vs voltage efficiency. Othe
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