A Strategy to Compute Critical Surfaces for Highly Asymmetric Ternary Systems

PISONI, G. O.; CISMONDI, MARTÌN; ZABALOY, MARCELO S.

Nova Friburgo

Congreso; X Congresso Brasileiro de Termodinâmica Aplicada (CBTermo) e VI Escola de Termodinâmica; 2019

Universidad del Estado de Río de Janeiro

The phase behavior of ternary systems may be quite complex, including critical phenomena. A ternary Critical Point (T-CP) has two degrees of freedom. This gives rise to ternary critical surfaces (T-CSs). The fast and robust computation of such surfaces, using models of the equation of state type, is one of the purposes of this work. For relatively simple ternary systems, when the computed phase behavior of the binary sub-systems (binary critical lines) is available, it is possible to guess in advance what the range of existence is, and what theboundaries are, for the T-CSs to be calculated. Such guess is not possible when the ternary system is highly asymmetric, as shown by the results obtained in this work. Besides, a higher asymmetry implies a higher non linearity for the T-CP system of equations, which makes the calculation more difficult. In particular, we deal in this work with a ternary system (carbon dioxide + water + alcohol) that has complexities not considered previously in the literaturerelated to the computation of T-CSs, e.g., the presence of a tricritical point in the boundary of a T-CS. In this work, we further develop a method that makes possible to efficiently calculate a set of T-CLs, i.e., a T-CS, with minimal user intervention, exploiting information available in the previously computed boundaries of the T-CS. The calculation of a T-CL starts at a point of a boundary of the T-CS, e.g., at a ternary critical end point (T-CEP). T-CEPs wereused as starting points for computing T-CLs neither in our previous works nor (to our knowledge) in the literature. In particular we have calculated in this work T-CLs made of two separated branches that never meet, or at least do not meet within the wide (although limited) pressure range within which the computations were carried out. The calculation of a given TCL is completed with the help of a numerical continuation method.