Generalized expressions for estimating the current efficiency of ion-exchange membrane and diaphragm chlor-alkali cells

Comprehensive chlorine and caustic current efficiency equations were derived for ion-exchange membrane and diaphragm chlor-alkali cells using material balances that account for all common and significant reactive brine impurities and their respective interactions with Cl₂ and/or NaOH. These generalized equations, formulated in terms of molar flow rates of the species entering, exiting, generated in or consumed in the cells, require a knowledge of the feed and depleted brine compositions to calculate the current efficiency of either an individual electrolyser or an entire circuit. It was shown that the global current efficiency expression can be degenerated to the various currently employed equations in the chlor-alkali industry for estimating the current efficiency with certain assumptions or approximations. These simplified assumptions 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 the caustic and chlorine current efficiency of membrane and diaphragm cells were quantified and approaches to achieve either high _{Cl 2} or _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 simplified method, which is analytically less cumbersome, is also presented to estimate the caustic current efficiency of ion-exchange membrane chlor-alkali cells.