Membrane chlorine alkaline electrolysis

Electromembrane processes such as electrolysis and electrodialysis have experienced a steady growth since they made their first appearance in industrial-scale applications about 50 years ago [1-3], Currently desalination of brackish water and chlorine-alkaline electrolysis are still the dominant applications of these processes. But a number of new applications in the chemical and biochemical industry, in the production of high-quality industrial process water and in the treatment of industrial effluents, have been identified more recently [4]. The development of processes such as continuous electrodeionization and the use of bipolar membranes have further extended the range of application of electromembrane processes far beyond their traditional use in water desalination and chlorine-alkaline production. 

One of the technically and commercially most important cation-exchange membranes developed in recent years is based on perfluorocarbon polymers. Membranes of this type have extreme chemical and thermal stability and they are the key component in the chlorine-alkaline electrolysis as well as in most of today s fuel cells. They are prepared by copolymerization of tetrafluoroethylene with perfluorovinylether having a carboxylic or sulfonic acid group at the end of a side chain. There are several variations of a general basic structure commercially available today [11]. The various preparation techniques are described in detail in the patent literature. 

The fundamental principle of SPE reactors is the coupling of the transport of electrical charges, i.e. an electrical current with a transport of ions (cations or anions), through a SPE membrane due to an externally applied (e.g. electrolysis) or internally generated (e.g. fuel cells) electrical potential gradient. For example, in a chlorine/alkaline SPE reactor (Fig. 13.3), the anode and cathode were separated by a cation-SPE membrane (e.g. Nafion 117) forming two compartments, containing the anolyte (e.g. 25 wt% NaCl solution) and the catholyte (e.g. dilute sodium hydroxide), respectively. 

Industrial chlorate electrolysis takes place in undivided cells, where sodium chlorate and hydrogen gas are formed as described by reaction 1. More detailed, reactions 2 and 3 show the main anode and cathode reactions of chloride oxidation and hydrogen evolution, respectively. Note that these electrode reactions are similar to those in a chlor-alkali cell, though while a chlor-alkali cell has a membrane or diaphragm separating an acidic anolyte from an alkaline catholyte, the chlorate cell is undivided with an electrolyte at close to neutral pH. Chlorine formed therefore dissolves as in reactions 4 and 5 and, chlorate is formed in a disproportionation reaction, number 6 below [3]. 

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