Cation exchange membranes chlor-alkali process

Their availability has greatly expanded the potential for electrolytic processes in synthesis and fuel cells as well as in environmental control. Perfluorinated cation exchange membranes such as Nafion outlast the material that preceeded them by up to four and a half years [48], Unfortunately very little has been published on their behaviour outside their use in chlor-alkali electrolysis. 

Membrane electrolyzers for water decomposition use cation exchange membranes of the type used in the chlor-alkali membrane process, in this case made H+-ion conducting by different pretreatment. So, the ions generated at the anode (H2O -> 2H++ I/2O2 + 2e ) pass through the membrane and form at the cathode hydrogen (2H+ + 2e -> H2). 

Cation exchange membranes are used in the membrane chlor-alkali cell process and must have good chemical stability. This requirement is satisfied by the perf luoropolymers. The... 

Cation Exchange Membranes for the Chlor-Alkali Membrane Process... 

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. 

Whereas both caiion-cxchance and anion-exchance membranes arc needed in some applieauons. only one type of tonic membrane is required in the production of chlorine and caustic soda via the chlor-alkali process (Figure VI - 67). In this process only cation-exchange membranes are used, with the cell containing only two compartments separated by a negatively charged membrane. 

The ideal chlor-alkali process is one that is energy-efficient and does not use mercury. A type of cell offering these advantages is the membrane cell, in which the porous diaphragm of Figme 19-24 is replaced by a cation-exchange membrane, normally made of a fluorocarbon polymer. The membrane permits hydrated cations (Na" " and H3O ) to pass between the anode and cathode compartments but severely restricts the backflow of Cl and OH ions. As a result, the sodium hydroxide solution produced contains less than 50 ppm chloride ion contaminant. 

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