Bipolar capillary gap cell

Figure 2.16 Bipolar capillary gap cell. The electrodes are a stack of closely spaced discs. From Beck, F. and Guthke, H. (1969) Chem.-Ing.-Tech., 41, 943.

Fig. 2J3 Capillary gap (1 mm) cdls, (a) The bipolar capillary gap cell. The electrodes are a slack of closely spaced discs. Electrolyte flows radially outwards in each inter-electrode gap. [AfteK Beck, F, and Guthke , H. (1 9) Chem. lng. TecL, 41 943)l (b) The monopolar capillary gap cell. The inner cylindrical electrode is concentrkally mounted in a conflrming lube. (Afier, Eberson, Nybcrg, K. and Sternerup, H. (1973) Chemica Scripta, 12).

Fig. 12 Capillary gap cell (From Ref. [4]) 1 bipolar electrode plates,...

Capillary gap cell — The undivided capillary gap (or disc-stack) cell design is frequently used in industrial-scale electroorganic syntheses, but is also applicable for laboratory-scale experiments when a large space-time yield is required. Only the top and bottom electrodes of c.g.c. (see Figure) are electrically connected to - anode and cathode, respectively, whereas the other electrodes are polarized in the electrical field and act as -> bipolar electrodes. This makes c.g.c. s appropriate for dual electrosynthesis, i.e., pro duct-generating on both electrodes. 

Cells with three-dimensional electrodes have bipolar electrodes such electrodes are characterized by the feature that one part of the electrode is anode and another part cathode. This can be realized in different ways, such as the pile capillary gap cell (Beck/Guthke cell. Chapter 32) [9,80], the Swiss roll cell [81], and packed- and fluidized-bell cells (Chapter 32) [82-84]. These cells are developed to meet economic demands, such as high space-time yield and simplicity in construction they are discussed in Chapter 31. 

Figure 5. Thin-film cells, (a) Capillary-gap cell, (b) Bipolar trickle tower.

The anodic methoxylation of ketones is a recent commercial example for an indirect process in an undivided cell [107]. The process (Fig. 18) was developed for BASF s undivided capillary gap cells with bipolar graphite electrodes [108] (Scheme 12). 

The ideal cell in order to scale up an electrochemical reaction can depend on the reaction, the electroactivity of the substrate to convert, the concentration of the substrate, as well as the current density at the working electrode. The use of a separator is necessary when the electrode can affect the whole process negatively. With anodic oxidations, the reaction at the counter electrode is most frequently the cathodic formation of hydrogen. In these cases, a separator does not seem indispensable a tank cell (kind of Grignard type reactor equipped with cylindrical electrodes) or a capillary-gap cell (piling of bipolar electrodes in a cylinder-shaped vessel connected to an anodes and a cathode located at the top and the bottom of the cell) can be considered as suitable devices for anodic conversions. More generally, the so-called plate-and-frame cells (Fig. 4) are used in a battery. 

BASF in Germany considered the use of the capillary gap cell (see Fig. 2.16 in Chapter 2) for this electrolysis. The cell consists of a stack of carbon discs closely spaced by 0.2 mm thick strips of an insulator. The electrolyte is pumped through the inter-electrode gaps and a voltage is applied to endplates so that a bipolar cell is formed. The cell design is compact, simple and cheap compared with a filter press cell and the tetraalkylammonium concentration could be reduced to below 0.5% without loss of yield and with an energy consumption below 3000 kWh ton . ... 

It is carried out in an undivided cell developed by BASF the capillary gap cell [19] (Fig. 2). This cell contains a stack of bipolar round graphite electrodes. The electrodes with a central hole are separated by spacers and connected in series. The electrolyte flows through the middle channel which is generated by the stacking and outwards between the electrodes. By this stacking the capillary gap cell is one answer to the permanent question in electrochemistry on how to realize sufficient electrode area in as little space as possible, respectively, in one cell. 

Many aspects like the optimal current density or the optimization of the anode process by lowering of the anode overpotential have been explored [25]. Together with the development of a special adapted cell, an optimized process in a large tonnage could be achieved. First, a divided cell with a membrane was used. Later on an undivided cell with a stack of vertical bipolar electrodes was employed [18, 25]. As in the BASF capillary gap cell, one achieves a large electrode area by electrode stacking. The vertical assembly has turned out to be advantageous for the reductive process. 

Fig. 17. Bipolar capillary cell (a) 125 fim gaps, (b) glass vessel, (o) electrical leads, (d) stack of graphite plates, (e) solution flow. (Taken from Beck and Guthke, 1969.)...

---