A modified UNIFAC model for the calculation of thermodynamic properties of aqueous and non-aqueous solutions containing sugars

A UNIFAC based model is proposed to describe experimental thermodynamic properties of aqueous and non-aqueous solutions containing common sugars, e.g., D-glucose, D-fructose and sucrose. A new group 'OHring' is introduced due to proximity effects. New temperature-independent UNIFAC interaction parameters are estimated based on the binary data available for both D-glucose and D-fructose and only on the solubility data for sucrose. The UNIFAC model used in this work is able to accurately predict the water activity, boiling temperature and freezing point of aqueous sucrose solutions, However, the predictions of the osmotic coefficients for the sucrose/water system present relative deviations up to 9%. Satisfactory predictions of vapor-liquid and solid-liquid equilibrium data for ternary and quaternary mixtures of D-glucose, D-fructose, sucrose and water components are also obtained. This approach uses the symmetric convention for all the components, i.e. the pure fused sugar at solution pressure and temperature is its reference state. Therefore, it allows to carry out in a straightforward way, calculations of solid-liquid equilibria of sugars in mixed solvents, It is shown that the new UNIFAC model is able to qualitatively predict the ternary solubility data of D-glucose D-xylose, D-mannose and sucrose in mixed solvent mixtures of ethanol/water, methanol/water and ethanol/methanol. A comparison with a modified UNIQUAC-essentially correlative model-and a UNIFAC -correlative and predictive model-approaches available in the literature is also given: the performance of the model proposed in this work is in general inferior to that of the UNIQUAC model, however it gives always a better agreement with the experimental data when compared with another UNIFAC based model available in the literature. (C) 1997 Elsevier Science B.V.