Bypass current

Compact design No busbars required Low-cost electrical equipment Higher operating voltage Lower operating current Parasitic electrical bypass currents Consequent reduced electrical efficiency Inhomogeneous current distribution Increased corrosion rate of electrodes... 

BASF in Germany operated a number of commercial, electrolytic processes [44, 56, 57] using a strategy based on the availability of a reliable and simple cell design and then noting the chemistry that can be carried out within this cell. In this cell, a series of horizontal carbon disks (diameter 1 m) were stacked with a separation of 1 mm (maintained by polymer spacers) and the electrolyte was pumped outward from the center of the disks. The cell is operated undivided and as a bipolar stack with bypass currents minimized because the stack is not immersed in electrolyte. The cell is well suited to methoxylation reactions carried out in methanol as the solvent. For example, BASF have carried out the following conversions ... 

Modem bipolar technologies were developed in the 1980s [12, 13]. Figure 2 shows a bipolar magnesium cell from a patent [12] where the electrodes are made from graphite which are coated by steel on the cathode side. Bipolar cells have shorter interelectrode distances, but the current efficiency is much lower due to bypass current, so the energy consumption is a little higher than that of the monopolar cells. 

It is also possible to operate three-dimensional electrodes in a bipolar manner, as in the case of the bipolar trickle tower reactor (Fig. 2.26) where each electrode layer is separated from its neighbour by an insulating mesh. The potential distribution along the height of each layer is non-uniform (Fig. 2.26(b)) and the packing conditions, electrolyte composition and electrolyte flow must be controlled to minimize the bypass current flowing past each layer, within the reactor envelope. Hence, such reactors are best suited to poorly conducting electrolytes. 

The Dished Electrode Membrane cell (Fig. 2.30) uses specially shaped electrodes in order to provide a smalt interelectrode gap over the active electrode, while maintaining adequate peripheral space at the bottom and top of the electrodes for a conventional pipe manifold. When bipolar operation is used, the use of relatively long external manifolds (Fig. 2.30(d)) minimizes bypass currents. 

The bipolar trickle tower reactor has already been mentioned (Fig. 2 26). Here, the electrolyte falls, under gravity, down a packed column containing a bipolar array of electrodes. In order to maintain trickle flow (and hence, minimize bypass currents) the flow rate is restricted High flow rates cause flooding while tow values result in incomplete wetting of the electrodes. 

The electrolyte inlets and outlets must be designed to give a low pressure drop over the reactor and the required flow characteristics (for mixing or mass transport reasons) with due regard for the nature of the reactants and products. In the case of a filterpress, parallel-plate reactor, internal manifolding provides a neat, compact method of distributing the catholyte and anolyte flows but it requires precise sealing and offers little control over bypass currents in bipolar cells. 

The cell is of a very simple design. It consists of a bipolar stack of carbon steel sheets whose cathode faces are smoothly electroplated with cadmium to a thickness of 0.1-0.2 mm. The anode-cathode gap is fixed with spacers at about 2 mm and the steel sheets are surrounded by insulating skirts to reduce the bypass currents. The electrolyte is fed uniformly across the parallel interelectrode gaps to give a flow velocity of 1-2 m s" The emulsion is recycled through the cell from a reservoir and a fraction of the organic phase is removed continuously for extraction of the product. The process is run so that the organic phase in... 

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