Thermodynamic Modelling of Electrolyte Solutions with Application to the Pulp and Paper Industry

A thermodynamic model for electrolyte solutions has been derived for binary and multicomponent solutions up to very high molalities. The model was extensively tested on 163 binary aqueous electrolyte systems up to very high molalities, and the results show that the model represents mean ionic activity coefficients and osmotic coefficients very accurately. The model parameters vary significantly between different electrolyte systems, which is probably due to strongly correlated parameters and possibly also due to ion association effects. In the extension of the model to multicomponent systems, the identification of the model parameters was somewhat modified, and an assumption reduced the number of model parameters from four to two per electrolyte–solvent system. It was found that the regressed model parameters vary much less between different electrolyte systems. The multicomponent model predicts osmotic coefficients very accurately and salt solubilities with reasonable accuracy for multicomponent electrolyte systems, without using ternary parameters. Although very good results have been obtained, there is still room for improvement of the model, either by including ternary data in the parameter regression or by introducing ternary parameters.

An electrolyte model available in the commercial simulation program Aspen Plus was used to simulate a cooling crystallization process for the removal of non-process elements from a pulp and paper mill, and the simulation results were compared to experimental results. The comparison shows a fair agreement between the results although improvements still can be made. Leaching of electrostatic precipitator dust as a means of non-process element removal was also investigated experimentally.