Monopolar design

EMOS has to date been mostly used in chlorate manufacture, but R2 in Montreal, Canada has recently installed its system on an FM-21 1500-type cell chlor-alkali production facility. This is presently a pilot installation, with only six cells currently being monitored. This installation has led to the monitoring of cell currents rather than cell voltages owing to the monopolar design of these electrolysers. It is too soon to make detailed conclusions about this installation as it has only been fully operational since January 2000. 

The details of the monopolar electrolyzer of Diamond Shamrock are shown in Figure 17. The electrolyzer is based on the monopolar design using DSA structures welded into titanium frames and the cathode structure welded into a steel frame Each cell has an active area of 1.41 m. Expected performance of this cell is shown in Figure 18 (71)(72). 

Clearly electrode spacing is a critical parameter in achieving low energy use, but the design must allow for ready separation of lead and chlorine to ensure high current efficiencies. There have been a number of different designs proposed with this in mind, but it would scan that monopolar design is favoured because of relative simplicity. 

Fig. 13.4 Example of stacks having (a) bipolar and (b) monopolar design (ii)...

Modem electrodes are similar to PEFC electrode stmctures with a porous carbon cloth DM, consisting of a carbon support material with fine metal catalysts, interdispersed with PTFE for hydrophobicity and pressed onto a nickel mesh to improve in-plane conductivity. As discussed, to prevent electrolyte weeping loss into the channels, a thin layer of PTFE is used to coat the electrode, and a monopolar design must be used. This is not a problem, however, considering the nickel mesh used on the electrode surface permits the high in-plane conductivity required to enable efficient current collection around the electrode current-conducting frame. 

Oy Tech MonopolarE,lectroly rs. OxyTech Systems (a joint venture company of Occidental Chemical and Eltech Systems) suppHes monopolar diaphragm electrolyzers of two designs the OxyTech "Hooker" H-Type (27,28) shown in Figure 11 and the "Diamond" MDC-Type (28,29) in Figure 12. 

The latitude that titanium affords the cell designer has made a wide variety of monopolar and bipolar membrane cell designs possible. 

Cell geometry, such as tab/terminal positioning and battery configuration, strongly influence primary current distribution. The monopolar constmction is most common. Several electrodes of the same polarity may be connected in parallel to increase capacity. The current production concentrates near the tab connections unless special care is exercised in designing the current collector. Bipolar constmction, wherein the terminal or collector of one cell serves as the anode and cathode of the next cell in pile formation, leads to gready improved uniformity of current distribution. Several representations are available to calculate the current distribution across the geometric electrode surface (46—50). 

Two-Dimensional Electrode Flow Cells. The simplest and least expensive cell design is the undivided parallel plate cell with electrolyte flow by some form of manifold. Electrical power is monopolar to the cell pack (72). An exploded view of the Foreman and Veatch cell is shown in Figure 7. Note that electrolyte flow is in series and that it is not easily adapted for divided cell operation. 

Kazimir, E.S. (1999) Monopolar Cathode Design Improvements and Other Diaphragm Cell Component Advances. ELTECH Systems Corporation, Chlorine/Chlorate Seminar. 

Two distinct classes of cell design exist the monopolar and the bipolar. Most commercial stacks have the bipolar design, which means that the single cells are connected in series both electrically and geometrically. The bipolar cell design has the advantages of compactness and shorter current paths with lower voltage losses. 

Either parallel, called monopolar, or series, called bipolar, electrical connections can be made to the pairs of electrodes in a complete cell. The monopolar types have individual connections to each electrode and thus require only individual pair potential to be applied to the cell assembly. The bipolar mode has electrical connections only to the terminal electrodes. One design such as Figure 19.16(f) has 48 pairs of electrodes in series and requires 600 V. The equipment of Figure 19.19(a) also has bipolar connections. The voltage profile in such equipment is indicated schematically in Figure 19.17(d). Bipolar equipment is favored because of its compactness and, of course, the simplicity of the electrical connections. No adverse comments appear to be made about the high voltages needed. 

The most widely used brine electrolytic cells are the Hooker and Diamond Shamrock which are both monopolar, but bipolar designs like that of Figure 19.19(a) also are popular. That figure does not indicate the presence of a diaphragm but one must be used. 

Figure 19.16. Basic designs of electrolytic cells, (a) Basic type of two-compartment cell used when mixing of anolyte and catholyte is to be minimized the partition may be a porous diaphragm or an ion exchange membrane that allows only selected ions to pass, (b) Mercury cell for brine electrolysis. The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water, (c) Monopolar electrical connections each cell is connected separately to the power supply so they are in parallel at low voltage, (d) Bipolar electrical connections 50 or more cells may be series and may require supply at several hundred volts, (e) Bipolar-connected cells for the Monsanto adiponitrile process. Spacings between electrodes and membrane are 0.8-3.2 mm. (f) New type of cell for the Monsanto adiponitrile process, without partitions the stack consists of 50-200 steel plates with 0.0-0.2 ram coating of Cd. Electrolyte velocity of l-2 m/sec sweeps out generated Oz.

The dimensionally slahle characteristic of the metal anode made the development of the membrane chlorine cell possible. These cells arc typically arranged in ail electrolyzer assembly which docs not allow for anodc-ro-cathode gap adjustment alter assembly. Also, very close tolerances are required. The latitude that titanium affords the cell designer has made a wide variety of monopolar and bipolar membrane cell designs possible. 

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