Membrane cells monopolar design

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

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. 

More than a dozen companies throughout the world oficr membrane cell technology and only two types of cell will be used to illustrate here the important features of the designs. Most membrane cells are based on a filterpress containing a series of plate and frame cells. Both monopolar and bipolar electrical connection is used. The basic structure of this type of cell is illustrated in Fig. 3.12 (note that the relative dimensions are not correct - membrane areas may be up to 2 x 2 m, although more normally I x I m, while the thickness of the electrolyte chambers is usually much less than 1 cm). The electrodes are vertical and constructed with louvres or from expanded metal so that the gases formed at the electrodes are directed to the back of the electrodes and do not stay in the... 

Electrolyzers for the production of chlorine and sodium hydroxide, including both diaphragm and membrane cells, are classified as either monopolar or bipolar. The designation does not refer to the electrochemical reactions that take place, which of course require two poles or electrodes for all cells, but to the electrolyzer construction or assembly. There are many more chlor-alkali production facilities with monopolar cells than with bipolar cells. 

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 The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water, (c) Monopolar electrical connections each cell is connected separately to the power supply so they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 voltage drop in each monopolar electrolyzer is equal to the voltage drop of a single cell. This depends primarily on the type of cell, the current density, and the choice of membrane or type of diaphragm. The number of electrolyzers contained in a cell line then fixes its total voltage. The design voltage must be sufficient to allow for losses in buswork and interceU connectors, the maximum current density to be used, and the deterioration with time of the components of the circuit. 

A commercial membrane plant has multiple cell elements combined into a single unit, called the electrolyzer. The electrolyzers follow two basic designs monopolar and bipolar [148]. 

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