PHYSICAL CHEMISTRY PREDICTION OF VLE AND VLLE IN TERNARY SYSTEMS WATER+ALCOHOL+CO 2 * DAN GEANĂ a and RUDOLF STEINER b a Dept. Applied Physical Chemistry, University "Politehnica" Bucharest, Spl. Independentei 313, Bucharest, Romania, e-mail: d_geana@chim.upb.ro ; b Dept. Technical Chemistry, University Erlangen-Nürnberg, Germany, Egerlandstr. 3 , 91058 Erlangen, Germany, e-mail: Rudolf.Steiner@rzmail.uni-erlangen.de Received, June 27, 2001 The mixing rule models MHV1, MHV2 and WS were used to predict ternary VLE and VLLE at sub- and supercritical conditions of the systems water+methanol+carbon dioxide and water+ethanol+carbon dioxide. We used a cubic equation of state (SRK, PR) coupled with the G E -EOS mixing rules. For water+ethanol+carbon dioxide, it was possible to calculate and to locate the three-phase boundaries of VLLE. This three-phase region is very sensitive to the pressure. Key words: Vapor-Liquid-Liquid Equilibria; Equation of state; Ternary Phase Equilibria; High Pressure Systems. * Paper presented by Mircea D. Banciu INTRODUCTION A knowledge of the phase equilibria of water+alcohol+CO 2 systems is essential for applications in supercritical fluid extraction. Since abundant experimental data are available on the vapor-liquid equilibrium of the binary systems water+alcohol, and on the gas-liquid equilibrium of the binary systems water+carbon dioxide and alcohol+carbon dioxide, the most general procedure for calculating the ternary phase behavior is to use this information along with a predictive equation of state (EOS) model. We used a cubic equation of state (SRK, Soave 1 ; PR, Peng and Robinson 2 ) coupled with the G E -EOS mixing rules: SRK-MHV1, SRK-MHV2 (Michelsen 3 , Dahl and Michelsen 4 ) and PR-WS (Wong and Sandler 5 ). The purpose of this study was to evaluate the predictive capability of these models for vapor-liquid equilibria (VLE) and vapor-liquid-liquid equilibria (VLLE) in two ternary systems: water+methanol+carbon dioxide and water+ ethanol+carbon dioxide. For the system water+ethanol+carbon dioxide, it was possible to calculate and to locate the three-phase boundaries of VLLE. This three- phase region is very sensitive to the pressure. EQUATION OF STATE MODEL The general cubic equation of state (GEOS, Geană 6 ) has the form: c + d) - (V a(T) - b - V RT = P 2 (1) The equation (1) is reduced to SRK EOS by setting: 2 b - = d ; 4 b - = c 2 (2) The PR equation of state is obtained from (1) by setting: -b d ; -2b c 2 = = (3)