Vapor-Liquid Equilibrium of Trioxane-Formaldehyde-Water Mixtures——New Division of Components in Unifac Groups

A thermodynamic model was revised to describe vapor –liquid and chemical equilibrium of trioxane-formaldehyde-water mixtures. In the model, division of components into UNIFAC groups was developed, according to different hygroscopicity of the CH₂O-groups with different positions in the poly (oxymethylene) glycol molecule. The CH2O groups are classified into the following three types: first, CH2O in formaldehyde monomers, and Formaldehyde is highly active and interacts greatly with water; second, CH2O with a hydroxyl group in the poly(oxymethylene) glycols molecule. Compared to the first type, the interaction of this type of CH2O group with water decreases. Trioxane is divided into three CH2O groups of this type; the third type is CH2O in the poly(oxymethylene) glycols molecule, in which new group-interaction parameters were calculated using experimental data in literatures. This leads to a distinctly improved agreement of model predictions with experimental results as well as the literature data while largely keeping the simplicity of the functional group approach. Especially, the interaction parameters of UNIFAC groups in the new model represent the dependence upon temperature, which avoids the situation that the prediction accuracy of model is bad at the lower temperature and on the contrary is good at the higher temperature. In other words, the application range of temperature of new model is extended, which provides reliable basis for the process design.