Separation of water and acetic acid by distillation using p-xylene as entrainer : experimental data and computer simulation

The separation of the binary mixture acetic acid and water by simple distillation procedures is not suitable in industrial application due to its non-ideal behaviour; this system in fact presents a tangent pinch on the pure water end. The most widely used solution is the use of an entrainer via a heterogeneous azeotropic column. This separation is a crucial step in the terephtalic acid production process; in this work we studied the possibility to use p-xylene (reactant of terephtalic acid process) as suitable entrainer. The separation of this ternary mixture was experimentally performed using a micropilot distillation column with 15 trays (height of the column 5 m) [1] and a process simulation software provided by SIMSCI (PRO II v.8.3) to simulate the experimental data. The system W/AcAc was studied with and without the entrainer and an increase of the relative volatility was observed when XY was added (Fig. 1). A comparison between some activity coefficient models (UNIQUAC, UNIFAC and NRTL) using LLE data of the ternary mixture available in literature [2] was carried out, selecting Lingby modified UNIFAC model (i.e. a temperature dependent previsional model) as suitable for this system. In any case, the Hayden O’Connel correlation (HOC) must be considered to correctly calculate the fugacity of the components. Both experimental results and simulation calculations determined a tray efficiency equal to 0.4 for LLV equilibrium (i.e. two liquid phases in the tray) and 0.6 for LV equilibrium (i.e. one liquid phase in the tray). Low amount of AcAc in distillate and W in residue have been obtained and correctly simulated.