Application of the Maxwell-Stefan theory to the membrane electrolysis process - Model development and simulations

A model is developed which describes the mass transfer in ion-selective mem branes as used in the chloralkali electrolysis process. The mass transfer m odel is based on the Maxwell-Stefan theory, in which the membrane charged g roups are considered as one of the components in the aqueous mixture. The M axwell-Stefan equations are re-written in such a way that the current densi ty can be used as an input parameter in the model, which circumvents an ext ensive numerical iterative process in the numerical solution of the equatio ns. Because the Maxwell-Stefan theory is in fact a force balance, and the c lamping force needed to keep the membrane charged groups in its place is no t taken into account, the model is basically over-dimensioned: the mole fra ction of the membrane can be calculated by using the equivalent weight (EW) of the membrane or by using the equations of continuity. In this work, the latter method has been chosen. The results of the computer model were veri fied in several ways, which show that the computer model gives reliable res ults. Several exploratory simulations have been carried out for a sulfonic layer membrane and the conditions as encountered in. the chloralkali electr olysis process. As there are no (reliable) Maxwell-Stefan diffusivities ava ilable for a Nafion membrane, in this trend study the diffusivities were al l chosen equal at a more or less arbitrary value of 1.10(-10) m(2) s(-1). D ue to this, the absolute values of several performance parameters are incor rect as compared with industrial chloralkali operation (e.g. an unrealistic ally high current efficiency of 95.7% was found), but the model can still b e used to obtain trends. For example, it is shown that the thickness of the membrane hardly increases the current efficiency (CE), however, the requir ed potential drop proportionally increases with thickness; The pH rapidly i ncreases to values greater than 12 just inside the membrane at the anolyte side. Moreover, for different values of the pH in the anolyte, the pH profi les inside the membrane nearly coincide with each other. A change in the an olyte strength does not have a significant effect on the performance of the membrane. At low values of the current density, a high value of the curren t efficiency is found. However, this is not due to a low OH- counter flux, but to the simultaneous transport of OH- and Cl- towards the catholyte. (C) 2001 Elsevier Science Ltd. All rights reserved.