On the thermodynamics of ethanol + hexane at elevated temperatures and pressures

A recent experimental investigation of the system ethanol + hexane for elevated temperatures and pressures is thermodynamically evaluated and then compared to an activity coefficient model as well as to equation of state models. The thermodynamic evaluation starts from total vapor pressures and proceeds via the Gibbs-Duhem differential equation to calculate vapor-liquid-equilibria and Gibbs energies. The way via the Gibbs-Duhem equation is preferred because it does not involve possible artefacts of a peculiar parametric model. For evaluation of the vapor-liquid-equilibria at temperatures not far from the critical curve we used reasonable estimates of the non-idealities of the vapor phase up to order (pRT)2. The resulting excess Gibbs energy is compared to a 1-alkanol + alkane model developed previously for lower temperatures, with very statisfying agreement. It proved to be very difficult to assign equation of state models to this system which work satisfactorily for the phase equilibria from low temperatures up to the critical curve and which give good values for the excess volumes as well. Two compromises were discussed: The first on the basis of the Trebble-Bishnoi equation of state, with good volumetric properties, but relatively poor values for the phase equilibria; the second on the basis of the Peng-Robinson equation of state (using the modification by Stryjek and Vera) plus a Wong-Sandler mixing rule. Here the values of the phase equilibria are quite reasonable, but the volumetric properties are not. Other attempts tried a density correction to the Peng-Robinson equation of state according to Mathias et al. but the results were inferior whether using conventional mixing rules or a Wong-Sandler prescription.