VAPOUR LIQUID EQUILIBRIUM PREDICTION FOR n-ALKANES USING THE GENERAL EQUATION OF STATE Anca Duţă ｼ and D. Geană A general cubic equation of state was developed and presented in previous papers. The form of the equation we use in this paper has four parameters; among these, three are depending on the critical constants of the components and the fourth supplementary depends on three constants (C1, C2, C3). Based on a database of PVT data for n-alkanes up to hexacontane there are proposed and comparatively discussed prediction functions for these three parameters. Introduction Describing the vapour – liquid equilibrium at high pressures is traditionally done using equations of state. A large group of them derives from the van der Waals equation of state and yield the dependence of pressure on volume as a third degree polynomial being therefore described as cubic equations of state. The initial equation has two parameters, with a molecular significance and is valid at low pressures. Most of the successors have two, three or four empiric parameters but maintain the original advantage of a simple, easy to handle, form. A general cubic equation of state, GEOS, was developed and extended by Geană [1,2,3] and used by the authors [4,5] in describing vapour-liquid equilibrium in systems involving n-alkanes. The four parameters of GEOS (a, b, c, d) can be calculated based on the critical constants of the pure components (T c , P c , V c ) and, in the original form, on two additional constants (m and α c ). Further papers, [6,7] introduced a new parameterisation, called GEOS3C which bases the calculation of the temperature dependent parameter “a” on three constants (C 1 , C 2 , C 3 ) instead of the parameter “m”. The C 1 , C 2 , C 3 parameters can be calculated, by matching points on the saturation curve together with the corresponding liquid volumes and details of the algorithm are presented by Geană and Feroiu [8,9]. Using this type of calculations, the parameters were calculated for n-alkanes up to eicosane and based on these results the paper presents some new correlation that are developed in order to predict their values for a broader group of n-alkanes. The results show that the polynomial dependencies on the accentric factor are suitable for VLE prediction for n-alkanes up to hexacontane. ｼ Dept. of Chemistry, “Transilvania” University of Brasov, I. Maniu 50, 2200 Brasov, Romania, e-mail: a.duta@info.unitbv.ro