GENERALIZED EXPRESSIONS FOR ESTIMATING THE CURRENT EFFICIENCY OF ION-EXCHANGE MEMBRANE AND DIAPHRAGM CHLORALKALI CELLS

Comprehensive chlorine and caustic current efficiency equations were d erived for ion-exchange membrane and diaphragm chlor-alkali cells usin g material balances that account for all common and significant reacti ve brine impurities and their respective interactions with Cl-2 and/or NaOH. These generalized equations, formulated in terms of molar flow rates of the species entering, exiting, generated in or consumed in th e cells, require a knowledge of the feed and depleted brine compositio ns to calculate the current efficiency of either an individual electro lyser or an entire circuit. It was shown that the global current effic iency expression can be degenerated to the various currently employed equations in the chlor-alkali industry for estimating the current effi ciency with certain assumptions or approximations. These simplified as sumptions and approximations, usually involving omission of some brine components or anolyte reactions, were shown to result in differences in the published current efficiency expressions. Disparities between t he caustic and chlorine current efficiency of membrane and diaphragm c ells were quantified and approaches to achieve either high eta(Cl2) Or eta(OH) were also outlined. Current efficiency equations, based on an analysis of anode gases and anolyte composition, require an accurate measure of all the components in the feed and depleted brine. A simpli fied method, which is analytically less cumbersome, is also presented to estimate the caustic current efficiency of ion-exchange membrane ch lor-alkali cells.