ON THE THERMODYNAMICS OF ETHANOL PLUS HEXANE AT ELEVATED-TEMPERATURESAND PRESSURES

A recent experimental investigation of the system ethanol + hexane for elevated temperatures and pressures is thermodynamically evaluated an d then compared to an activity coefficient model as well as to equatio n of state models. The thermodynamic evaluation starts from total vapo r 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( p/RT)(2), The resulting excess Gibbs energy is compared to a I-alkanol + alkane model developed previously for lower temperatures, with very satisfying agreement. It proved to be very difficult to ass ign equation of state models to this system which work satisfactorily for the phase equilibria from low temperatures up to the critical curv e and which give good values for the excess volumes as well. Two compr omises were discussed: The first on the basis of the Trebble-Bishnoi e quation 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 equili bria are quite reasonable, but the volumetric properties are not. Othe r attempts tried a density correction to the Peng-Robinson equation of state according to Mathias et al. but the results were inferior wheth er using conventional mixing rules or a Wong-Sandler prescription. (C) 1998 Published by Elsevier Science B.V, All rights reserved.