Performance of Cubic Mixing Rules on the Description of High Pressure Fluid Phase Equilibria

M. CISMONDI; J.M. MOLLERUP; M.S. ZABALOY

Foz de Iguazú, Brasil

Conferencia; Primera Conferencia Iberoamericana de Fluidos Supercríticos - PROSCIBA 2007; 2007

PLAPIQUI, UNICAMP, Universidad de Valladolid, Universidade Nova de Lisboa

A high level of asymmetry among the system components is typical of the high pressure fluid systems found in technologies that make use of supercritical fluids.  Asymmetry is also found at moderate pressure in highly non-ideal systems. The models for representing the phase equilibria and the physico-chemical properties of such systems require the use of mixing rules more flexible than the classical quadratic van der Waals (vdW) mixing rules (QMR). Cubic mixing rules (CMR), implemented as a natural extension of the classical quadratic mixing rules, constitute the simplest alternative among different flexible approaches. They have the advantage of allowing correlation of multicomponent data by fitting ternary interaction parameters, while leaving invariant the description of the constituent binary systems. If preferred, or if multicomponent data are not available, ternary interaction parameters can be consistently estimated from binary parameters. In this work, we explore for the first time the possibilities that cubic mixing rules offer for modelling the high pressure phase behaviour of asymmetric binary systems as compared to conventional quadratic mixing rules. We study in the present work the effects of the interaction parameters k112, k122, l112 and l122 on critical lines, on liquid-liquid-vapor lines, and on solubility curves in the form of Txy / Pxy diagrams at high pressure. We focus in this work on the system carbon dioxide (1) + n-hexadecane (2), whose proper description by the currently available thermodynamic models is difficult. We present preliminary conclusions about the flexibility of cubic mixing rules for the correlation and prediction of the phase behavior of asymmetric systems over a wide range of conditions.