Stability analysis for solid-fluid isopleths in binary asymmetric mixtures

S.B. RODRIGUEZ REARTES; M. CISMONDI; M.S. ZABALOY

Praia da Rocha Portugal

Congreso; EQUIFASE ´09 (VIII Iberoamerican Conference on Phase Equilibria and Fluid Properties for Process Desing) - 17 al 21 de Octubre de 2009; 2009

LSRE |Chemical Engineering Department - Faculdade de Engenharia | Universidade do Porto, Portugal

Models for the description of the solid-fluid equilibrium of multicomponent asymmetric mixtures of technological interest should behave properly for the corresponding binary subsystems. The study of binary asymmetric systems, such as methane + n-triacontane, in wide ranges of conditions, requires, on one hand, robust calculation procedures that achieve convergence and, on the other hand, strategies for assessing the thermodynamic stability of the computed equilibria. When the set fluid phase composition, of a binary solid-fluid equilibrium isopleth, falls within the composition range of existence of the solid-fluid-fluid equilibrium line, then, the isopleth meets a solid-fluid-fluid equilibrium point. Such intersection point splits the isopleth into two segments. Only one of them is thermodynamically stable. It is not a trivial matter to properly identify such stable segment. In this work, we apply a novel strategy that makes it possible to identify stable binary solid-fluid isopleth segments. The test is differential and does not require iterative calculations, provided that the solid-fluid-fluid equilibrium line has already been calculated. The test is based on the computation of some derivative of certain tangent plane distance, under the constraint of solid-fluid equilibrium at the specified isopleth composition. Such derivative is evaluated at a special composition value.  We illustrate the application of the test for the methane + n-triacontane system.