Modelling of breakthrough curves in fixed-bed zeolite columns

A diffusional model for the interpretation of breakthrough curves obtained in fixed–bed ion exchange processes was presented. The model, based on the linear driving force (LDF) approximation, takes into account both fluid–particle and intra–particle resistances to diffusion, and makes no specific assumption on their relative magnitude, or on the form of the ion exchange isotherm. Its only parameters, to be evaluated on the basis of the experimental data, are a solid–liquid and an intra–particle mass transfer coefficient. In order to verify the model, its previsions were checked against some published results, relative to Pb2+ removal from water by means of a Na–exchanged phillipsite–rich tuff. The model was capable of satisfactorily reproducing the dependence of the experimental breakthrough curves on solution composition. An analysis of the model results showed that the external mass transfer coefficient does not depend on the composition of the liquid phase. On the other hand it appeared that, in agreement with previous published experimental results, the internal mass transfer coefficient varies with the total normality of the solution.