Pencil sharpener cells

Pencil Sharpener cell for electrolyses with sacrificial anodes (SNPE). 

One of the most intriguing developments in the field of industrial electrochemistry is electrochemical couplings with sacrificial anodes. In the 1980s, the French company SNPE developed the prototype of a specific cell for this purpose, the so-called pencil-sharpener cell [81]. 

Ultimately, this combination of requirements led to the pencil sharpener cell, in which the magnesium anode was a rod (diameter 40 cm) with one end shaped as the point of a pencil. The shaped part of the anode sat within a conforming, conical cathode (Pb-plated stainless steel), with the electrodes separated by a thin polymer spacer. The weight of the anode ensured that the anode pressed down on the spacer and that the shape was maintained as metal was lost from the anode. The production unit had two such cells, each with an active electrode area of 0.6 m, and was operated with a cell current of 600 A. The operating pressure was 5 bar, and electrolyte was circulated though the cell from a 400 liter reservoir. For the manufacture of fenoprofen the unit was operated in batches, whereby a batch of 60 kg chloroethyldiphenylether was dissolved in 340 kg dimethylformamide that also contained 3 kg tetrabutylammonium bromide. Each batch, when processed for 28 h, led to 50 kg of isolated fenoprofen. The good yield ( 80%) showed the electrolytic route to fenoprofen to be very competitive with more traditional syntheses. 

Fig. 15. Pencil sharpener electrolysis cell using consumable electrodes [115]...

In order to pilot this process, SNPE has developed an annular ceU having a cone-shaped electrode section. The magnesium anode is placed into the conic section and fed like a pencil into a sharpener as it is consumed during electrolysis. The carbon dioxide is introduced into the electrolyte externally to the cell. 

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