Thermodynamic modelling of black liquor-related solutions. Pitzer method for estimating activities, solubilities and boiling point rise

Abstract: Black liquor is considered as being one of the most important process streams as far as the design and optimization of kraft pulp mills are concerned. Many methods have been developed in order to describe and, possibly, predict its physico-chemical properties. Most of them are, however, empirical methods that are difficult to generalize. Attempts to model the properties of black liquor based on rigorous solution thermodynamics are rare. This work uses Pitzer’s semi-empirical model to calculate the activity coefficients and osmotic coefficient in aqueous electrolyte solutions in order to estimate the solubilities of salts and the boiling point elevation in black liquor and related systems. The salts investigated included burkeite and (sodium sulphate) dicarbonate, both double salts of Na2CO3 and Na2SO4. Besides activity coefficients, a thermodynamic definition of the solubility constant was applied to the modelling of the solubilities of these salts. A solubility model for burkeite was consequently developed, based on literature data. The model predicts the solubility and the composition of the solid phase in solid solutions of burkeite and sodium carbonate. Furthermore, the average solubility constant values for dicabonate were calculated for three temperature levels using the experimental values of the solubility obtained in this work. The boiling point elevation for salt solutions was calculated using the Pitzer method for estimating the osmotic coefficient and compared with the literature data. The predictions were excellent in salt solutions at low-to-moderate concentration levels. It was found that neglecting the higher-order interaction coefficients did not affect the predictions at these concentrations significantly. The Pitzer method was also used to estimate the boiling point elevation in black liquor solutions; the results were compared to the experimental values obtained in an earlier project. The solubility models developed earlier were used to account for the precipitation of salt. It was found that the model predicted the boiling point rise reasonably well at low concentration levels; at concentrations above those recommended by Pitzer, the predictions were somewhat poorer. It was also found that a proper description of the organic compounds found in black liquor is important for the accurate estimation of its boiling point.