Heterogeneous distillation of the system water-acetic acid- p-Xylene : study of its fluid phase equilibria, micro-pilot column experimental results and computer simulation

The separation of the binary mixture water-acetic acid by direct distillation is not suitable for industrial applications because of the presence of a tangent pinch on the pure water end. The problem is overcome via heterogeneous azeotropic distillation adding a third component, the entrainer, generally immiscible with water. With particular reference to terephthalic acid production process, we selected p-xylene being both a good entrainer and the principal raw material for the terephthalic acid production. The phase equilibria (vapor-liquid and vapor-liquid-liquid) of the ternary system water/acetic acid/p-xylene were analysed in order to choose a right thermodynamic model suitable for the simulation of this system. The simulations obtained by SIMSCI PRO II software adopting the Lyngby modified UNIFAC equation to calculate the activity coefficients and the Hayden-O’Connel correlation to take into account the acetic acid association gave very satisfactory results. As further check, some experimental data were produced by a continuous micro-pilot distillation column, 5 m height with 15 trays. These data, successfully simulated, showed the possibility to use p-xylene as entrainer. A new system based on two operative column for this separation is then proposed. The first column, without entrainer, is used to bring the composition of the distillate near the tangent pinch, while the second one is used to complete the separation