A STRATEGY TO COMPUTE CRITICAL SURFACES FOR HIGHLY ASYMMETRIC TERNARY SYSTEMS

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

Nova Friburgo

Congreso; CBTERMO 2019 (X Congresso Brasileiro de Termodinamica Aplicada); 2019

The phase behavior ofternary systems may be quite complex, including critical phenomena. A ternary CriticalPoint (T-CP) has two degrees of freedom. This gives rise to ternary criticalsurfaces (T-CSs). The fast and robust computation of such surfaces, usingmodels of the equation of state type, is one of the purposes of this work. Forrelatively simple ternary systems, when the computed phase behavior of thebinary sub-systems (binary critical lines) is available, it is possible toguess in advance what the range of existence is, and what the boundaries are,for the T-CSs to be calculated. Such guess is not possible when the ternarysystem 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 ofequations, which makes the calculation more difficult. In particular, we deal inthis work with a ternary system (carbon dioxide + water + alcohol) that hascomplexities not considered previously in the literature related to thecomputation of T-CSs, e.g., the presence of a tricritical point in the boundaryof a T-CS. In this work, we further develop a method that makes possible to efficientlycalculate a set of T-CLs, i.e., a T-CS, with minimal user intervention, exploitinginformation available in the previously computed boundaries of the T-CS. Thecalculation of a T-CL starts at a point of a boundary of the T-CS, e.g., at aternary critical end point (T-CEP). T-CEPs were used as starting points forcomputing 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 separatedbranches that never meet, or at least do not meet within the wide (although limited)pressure range within which the computations were carried out. The calculationof a given T-CL is completed with the help of a numerical continuation method.