Chemical and Biomolecular Engineering - Theses
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ItemRadial ion exchange processes( 1996)Mathematical models to predict the ion exchange performance in one dimensional, linear (vertical) and non-linear (radial) flow are developed. The models assume that instantaneous pointwise ion exchange equilibrium is established throughout an exchanger bed at any time. The models takes account of non-idealities in both the exchanger and the solution phases and also dispersion effects to provide preconditions over a wide range of solution concentrations and flow rates. The models are solved numerically by finite difference techniques. The Mehablia ion exchange equilibrium model is improved and then incorporated in the developed models to calculate the concentrations in both phases. The ion exchange equilibrium model applies the Pitzer model to calculate the activity coefficients in the solution phase. The Mehablia model also considers that ion-pair formation limits the free ions that are available for ion exchange. Dispersion in the flow direction occurring when a solution flows through a porous bed is modelled by mathematical equations. The simple dispersion models for the vertical flow and the radial flow are used to determine the dispersion coefficients for the respective models. The dispersion coefficients are then applied to the calculation of ion exchange performance models to reflect dispersion effects. Using the experimentally-determined equilibrium parameters and dispersion coefficients, the numerical calculations for the ion exchange performance models are performed. The models predict ion exchange performance for both the exhaustion and the regeneration cycles without any modification to the modes. The models’ predictions are then tested by the experiments conducted in resin beds. The comparisons between the experimental observations and the model predictions validate the developed models to predict ion exchange performance in the linear vertical flow and in the non-linear radial flow. Experiments on both dispersion and ion exchange performance are performed using two packed beds of different shapes. For linear vertical flow, experiments are conducted in a column bed; while a thin wedge-shaped bed is used in experiments for non-linear radial flow. The predictions for ion exchange performance in vertical flow are more accurate than those made by existing column models. The predictions for ion exchange performance in radial flow agree well with experimental results. The radial model is the first model to be developed to accurately predict ion exchange performance within a radial exchanger bed. The factors which influence the ion exchange performance are also studies by the models’ simulations. For a given ion exchange system, dispersion is the only factor that may adversely affect the ion exchange performance, causing an early occurrence of breakthrough. Early breakthrough in turn decrease the efficiency of an ion exchanger bed. Therefore, the lower growth of dispersion in radial flow makes radial ion exchange processes more efficient than conventional ion exchange column processes. The use of industrial scale radial ion exchange units may be a result of significant economic benefit in every application of ion exchange.
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ItemMulticomponent ion exchange equilibria( 1994)A semi-theoretical model is proposed to predict multi-component ion exchange equilibria between cations in an aqueous solution and a resin. The model uses equilibrium data from binary systems to predict behaviour in multi-component systems. Non-idealities in the soution phase are considered by applying Pitzer's model for electrolyte behaviour while the resin phase activity coefficients are calculated by applying Wilson's model. In addition the equilibrium constants are obtained from the Gaines and Thomas approach and are independent of the resin phase activity coefficients. The model also considers the non-availability of some ions for exchange due to the formation of ion pairs in the solution phase. The model is tested by applying it to the two ternary systems Na-H-K and Ca-H-Na and the quaternary Ca-H-Na-K. The predictions made by the model are significantly better than those of other models when compared to experimental data. The equilibrium constant and the two binary interaction parameters are found to be independent of the conditions of the solution phase including concentration and the particular non-exchanging anionic species present in the solution. The effect of temperature is also considered.