Simulation of double retrograde vaporization using the Peng–Robinson equation of state

Abstract

Double retrograde vaporization is a phenomenon characterized by an unexpected retrograde dew point curve at compositions approaching nearly the pure volatile component (component A) and at temperatures very close to the critical temperature of the more volatile component ( Tc A). On the p– x– y diagram, instead of the single-domed dew point curve in the familiar “single” retrograde vaporization, double retrograde vaporization shows two “domes” at temperatures above but close to Tc A. At temperatures below but close to Tc A, the dew point curve has an “S”-shape. This results respectively in quadruple- or triple-valued dew points at a specific composition. In this work, the phenomenon of double retrograde vaporization has been simulated using a cubic equation of state. Both the “double-dome” and the “S”-shape curves for the binary systems (ethane+linalool) and (ethane+ d-limonene) were successfully modelled, even without the use of binary interaction parameters. Results are also obtained by optimizing interaction parameters using experimental bubble point data. Even though double retrograde vaporization has rarely been observed in literature, we believe that it is the normal behaviour that always occurs in binary mixtures in which the two components differ largely enough in molecular symmetry to produce a very steep dew point curve. To further verify this generality, simulations were performed on a number of binary mixtures of different families. Double retrograde vaporization was estimated in every system with a steep dew point curve.