Vapor–Liquid Equilibrium of 1,3,5-Trioxane + Formaldehyde + Water System Based on Salt Effect

1,3,5-trioxane (TOX), formaldehyde (FA), and water can form an azeotropic system at atmospheric pressure, which causes the TOX produced by reactive distillation to consume a lot of energy to obtain a high-purity TOX product through the subsequent separation and purification process. Therefore, four inorganic salts (MgSO4, MgCl2, CaCl2, and the mixture of MgCl2 and CaCl2) were selected as extractive distillation agents to investigate the effects of inorganic salts on the azeotropic system of TOX + FA + water. Isobaric vapor–liquid equilibrium data were obtained by the experiments on TOX + FA + water and TOX + FA + water + salt at atmospheric pressure. The experimental results show that the addition of inorganic salts can effectively change the azeotropic composition, so that the mass fraction of TOX in the azeotropic system increases from 70.22% to a maximum of 80.66%, which can significantly reduce the energy consumption required for subsequent the separation and purification process. Finally, the data were fitted and correlated with the extended UNIFAC model. The correlation was very consistent with the experimental results, indicating that the extended UNIFAC model has good applicability to the system.