Membrane chloralkali cells

Figure 1. Basic operational process for membrane chloralkali cells.

Bayer AG The Importance of Electrolyte Quality to Membrane Chloralkali Cells, Uhde Chlorine Symposium 1995 (F. R. Minz). 

D. Bergner, Membrane Cells for Chloralkali Cells, J. Appl. Electrochem., Vol. 12, p. 631,1982. 

Chrysotile is a noncombustible fibrous solid that has been widely used as a fireproof thermal insulator, for brake linings, in construction materials, and for filters under the name of asbestos. It decomposes with loss of water at 600-800 °C, eventually forming forsterite and silica at 810-820 °C. Because it is more resistant to attack by alkalis than are the amphibole asbestoses, chrysotile has been used in chloralkali cell membranes and in admixture with Portland cement for making sewer pipes (Chapter 11). 

Since the products of the electrolysis of aqueous NaCl will react if they come in contact with each other, an essential feature of any chloralkali cell is separation of the anode reaction (where chloride ion is oxidized to chlorine) from the cathode reaction (in which OH- and H2 are the end products). The principal types of chloralkali cells currently in use are the diaphragm (or membrane) cell and the mercury cell. 

We will continue platform extensions, taking Nafion fluoropolymer membranes from chloralkali cells and catalysts to advanced fuel cells, and Kapton polyimide film from flat electronic computer circuits to complex flexible circuits in cell... 

Major areas of application are in the field of aqueous electrochemistry. The most important application for perfluorinated ionomers is as a membrane separator in chloralkali cells.86 They are also used in reclamation of heavy metals from plant effluents and in regeneration of the streams in the plating and metals industry.85 The resins containing sulfonic acid groups have been used as powerful acid catalysts.87 Perfluorinated ionomers are widely used in worldwide development efforts in the held of fuel cells mainly for automotive applications as PEFC (polymer electrolyte fuel cells).88-93 The subject of fluorinated ionomers is discussed in much more detail in Reference 85. 

Structure and Function of Membranes for Modem Chloralkali Cells... 

A typical polarization plot for the chloraUcah membrane cell with a DS A anode and a Ni cathode is shown in Figure 15.18 (Dias, 2009). The mathematical model described above, i.e., in Eqn (15.20), may be used with appropriate parameters for the chloralkali cell and with i = —i, i.e., with negative external current in the electrolytic mode and using the identity sinh ( x) = sinh (x), to describe such polarization behavior. 

The transport properties of perfluorinated ionomers are of particular interest due to their use as membrane separators in chloralkali cells. Gierke and Hsu have developed a cluster network model for these systems which suggests that the ionic clusters are inverted micellar structures. In this model, the absorbed water phase is predicted to separate into approximately spherical domains and the ion-exchange groups are near the interface, probably imbedded in the aqueous phase. Based on water... 

Ionomer membranes show good ion selectivity. They are able to distinguish between ions on the basis of size and charge, and show such good selectivity that they have also been used for membranes in experimental ion-selective electrodes. Their main use, though, remains in membrane cells of which numerous examples are currently employed throughout the world s chloralkali industry. 

Other remaining technical concerns with membrane cells relate to somewhat lower current efficiencies and to relatively short membrane lifetimes. At present, this is limited to 2-3 year of operation when coupled to much more careful brine pretreatment than is required for conventional asbestos diaphragm cells. A combination of mercury cell and membrane cell technologies has been recently tested for commercial feasibility [19]. The economics of the three primary chloralkali technologies have also been reviewed [20]. 

T. Sasano and W. Schlegel, W, Technology and Economics of Chloralkali Membrane Cells. SRI International, Zurich, 1985. 

F. Hine and A.J. Acioli Maciel,... the Amalgam Cell and the Membrane Cell Processes for Chloralkali Production,/. Appl. Electrochem. 22, 699-704 (1992). 

Perfluorinated polyethers have also gained importance as actively functional materials. Ionic polymer membranes (e.g. DuPont s Nafion ) based on sulfonic acid-derivatized perfluoropolyethers have been used for nearly 30 years as ion-con-ducting membranes in chloralkali electrolysis cells, replacing the large amounts of toxic mercury used until then in the classic Castner-Kellner cells (Scheme 4.8.). One of the earliest applications of Nafion was as a membrane in the hydrogen-oxygen fuel cells which powered the Apollo spacecraft carrying the first men to the moon. 

In 2000, 45 Mt of CI2 was manufactured by the chloralkali process this represents 95% of the global supply. The main producers are the US, Western Europe and Japan. Whereas the Japanese chloralkali industry operates almost entirely with the membrane cell, the US favours use of the diaphragm cell, and just over half of the Western European industry retains use of the mercury cell. On environmental grounds, the chloralkali industry is being pressured to replace mercury and diaphragm cells by the membrane cell. This is not the only environmental concern facing the industry demand for CI2 has fallen in the pulp and paper industry and in the production of chlorofluorocarbons, the latter being phased out as a result of the Montreal Protocol for the Protection... 

Figure 9.24 The chloralkali process (a) membrane cell (b) the membrane is permeable only to Na" " ions, leading to segregation of Cr and OH in the anode and cathode compartments...

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