Computation and analysis of binary multiphase isochores

Abstract

In this work, a systematic and robust method for computing and studying binary fluid phase equilibrium loci of constant overall composition, and constant overall density (isochores, ICs), is proposed and evaluated. The method uses information available for previously computed binary phase envelopes (B-PEs) and binary three-phase lines (B-3PLs), and it is applicable to isochores having any number of two-phase and three-phase segments. Also, a direct way of computing the change in IC pressure versus temperature slope at the intersection point with the phase envelope is proposed. The proposed methods are reliable and facilitate the understanding of the behavior of ICs. Their application is illustrated using models of the equation of state type. We have also considered the actual situation found in the laboratory, in which the inner volume of the equilibrium cell depends on temperature, due to the thermal expansivity of the solid material of which the equilibrium cell is made, i.e., the computation of quasi-isochores (q-ICs) has also been accounted for in this work. It is shown that the temperature-dependency of the global molar volume of the fluid system may have a significant influence on the trace of, e.g., liquid-liquid q-IC segments.