Application of the Maxwell-Stefan theory to the transport in ion-selectivemembranes used in the chloralkali electrolysis process

The results of a fundamental mass transport model based on the Maxwell-Stef an approach are compared to experimental data obtained by Akzo-Nobel for a Dupont Nafion ion-selective membrane as used in chloralkali electrolysis pr ocesses. The main problem in the application of the Maxwell Stefan based ma ss transfer model to the chloralkali electrolysis process is a lack of avai lable diffusivities for the membrane. Estimation of these diffusivities in the membrane based on a method presented by Wesselingh et al. (1995. Chem. Engng J., 57, 75-89) gave unrealistic high membrane potential drops. Theref ore, another method was followed. First, a sensitivity analysis was carried out which resulted in a reduced set consisting of the dominating Maxwell-S tefan diffusivities. First estimates of these remaining diffusivities were determined for single layer sulfonic and a carboxylic membranes. With a sli ght adjustment of the values of the diffusivities obtained for the separate sulfonic and carboxylic layers, the performance parameters of the DuPont N afion membrane could be predicted well for a reference experiment. These di ffusivities also proved to be suitable for other anolyte strengths. However , for other catholyte strengths and current densities these diffusivities ( even after a correction for the water uptake according to the method of Wes selingh et al. (1995. Chem. Engng. 5., 57, 75-89)) did not result in a good agreement between the simulated and experimentally observed performance pa rameters. Only after a correction of the diffusivities the simulations yiel ded approximately the same performance parameters as experimentally observe d. From this it can be concluded that although a fundamental model is used in order to describe the mass transfer in a membrane, a single set of diffu sivities is not sufficient in order to obtain the experimentally observed p erformance parameters at different process conditions. At this moment there is not enough knowledge on the exact phenomena taking place in the membran e in order to predict the necessary corrections of the diffusivities a prio ri. As long as there are no theoretically founded and reliable relations av ailable to predict the Maxwell-Stefan diffusivities in a membrane (or accur ate experimental data for these diffusivities) only a semi-empirical method as used in this study can serve as a basis for a further progress in the d evelopment of an existing (in this case DuPont Nafion) membrane. (C) 1999 E lsevier Science Ltd. All rights reserved.