Chlor-alkali cell, membrane selectivity

Also, discussions of a number of applications of Nafion are not included in this document and are, at most, mentioned within the context of a particular study of fundamental properties. A number of these systems are simply proposed rather than in actual commercial applications. Membranes in fuel cells, electrochemical energy storage systems, chlor-alkali cells, water electrolyzers, Donnan dialysis cells, elec-trochromic devices, and sensors, including ion selective electrodes, and the use of these membranes as a strong acid catalyst can be found in the above-mentioned reviews. 

Membranes can be characterized by their structure and function, that is how they form and how they perform. It is essential that the cation exchange membranes used in chlor-alkali cells have very good chemical stability and good structural properties. The combination of unusual ionic conductivity, high ionic selectivity and resistance to oxidative hydrolysis, make the perfluorinated ionomer materials prime candidates for chlor-alkali membrane cell separators. 

A. General. Preferential transport of selected species is the primary characteristic property of membranes. In a chlor-alkali cell, for example, one equivalent of cation will pass across the cation-exchange membrane for each Faraday of electricity if the selectivity is perfect. In practice, some OH passes through the membrane in the opposite direction, resulting in current inefficiency. The membrane selectivity, therefore, directly determines the caustic current efficiency of the process. 

A.2. Water Transport. The presence of water and its transport across the membrane are vital if the membrane is to function with excellent conductivity and selectivity. Because of their importance, the transport characteristics of water have been studied extensively both theoretically [88] and experimentally. Water transport numbers in chlor-alkali cell environments have been studied by several workers [64,88-92]. 

In the membrane-cell process, highly selective ion-exchange membranes of Du Font s Nation type are used which allow only the sodium ions to pass. Thus, in the anode compartment an alkali solution of high purity is produced. The introduction of Nafion-type membranes in chlor-alkali electrolyzers led to a significant improvement in their efficiency. Today, most new chlor-alkafi installations use the membrane technology. Unfortunately, the cost of Nafion-type membranes is still very high. 

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 chlor-alkali industry. 

E. Pearson, Criteria for the Selection of Membrane Cell Technology. In C. Jackson (ed.). Modem Chlor-Alkali Technology, vol. 2, Ellis Horwood, Chichester (1983), p. 177. 

The availability and stability of performance of electrodes, membranes and other cell components. It should be noted that no organic process is large enough to warrant the development of optimized cell components and the process designer can usually only select from the best available. This is in marked contrast to the chlor-alkali industry. 

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