The effect of electro-osmotic water transport on current efficiency and cell performance in chlor-alkali membrane electrolysis

Abstract

A theoretical study is made of the transport processes taking place in the ion-exchange membrane serving as the “separator” in a chlor-alkali electrolysis cell. Special attention is paid to the effects of electro-osmotic water transport (EOWT). Starting from the extended Nernst—Planck flux equations, we derive expressions for characteristic quantities such as the current efficiency of alkali production, n OH, and the voltage drop over the membrane as functions of current density, external concentrations of sodium chloride and sodium hydroxide, and electro-osmotic water transport number, t o. It appears that EOWT has a favourable effect on n OH and that the effect on ohmic resistance and voltage drop is slight, whereas the transport of chloride ions to the catholyte increases. The effect of EOWT on n OH can be represented by a single parameter k o, which is proportional to t o (0≦ k o ≦1). Three regions of k o values can be distinguished, the boundaries of which depend on the external concentrations; in each region the maximum current efficiency depends in a different way on the external concentrations: in the lowest region of k o values η OH, increases linearly with k o and decreases with increasing values of C OH (concentration of NaOH in the membrane on the cathodic side); in the highest region up to η OH = 1, η OH increases linearly with k o and increases with increasing values of C Cl (concentration of NaCl in the membrane on the anodic side), whereas in the intermediate region η OH depends on k only. The concentration of NaOH in the membrane, which at maximum efficiency is constant through-out the membrane, also differs in the three regions of k o values (see Fig. 6). Making use of non-published experimental results on the uptake of water, alkali and sodium chloride by Nafion 170 and 427 membranes, we calculate values of η OH as a function of the external alkali concentration for different values of t o and compare with experimental results obtained in an electrolysis cell. It appears that t o ≌ 1 gives the best agreement between theory and experiment. The same result is deduced from experimental values of the total water transport number, by applying corrections for water of hydration carried along by the migrating ions. Calculation shows that with Nafion 170 membranes under practical conditions, i.e., concentrated alkali (≧ 10 M) and 5 M NaCl, EOWT caused the maximum current efficiency to increase from 0.45 at t o = 0 to about 0.75 or 0.80 at t o = 1; the value of k o lies in the “Intermediate region” and the (constant) concentration of NaOH in the membrane has a value about halfway between C Cl and C OH. At a current density of 3 kA/m 2 the maximum value of η OH is attained within about 2%.