Drawing PT-phase envelopes and calculating critical points for multicomponent systems using flash calculations

Luciano Angel Toro

doi:10.32362/2410-6593-2020-15-1-46-54

Abstract

Objectives. This study aims to draw PT-phase envelopes and calculate the critical points for multicomponent systems using flash calculations.Methods. Flash calculations with an equation of state and a mixing rule were used to construct phase envelopes for multicomponent systems. In general, the methodology uses the Soave–RedlichKwong equation of state and Van der Waals mixing rules; and the Peng–Robinson equation of state with Wong–Sandler mixing rules and the non-random two-liquid activity coefficient model.Results. The method was applied to the following mixtures: ethane (1)–butane (2) (four different compositions); ethane (1)–propane (2) (four different compositions); butane (1)–carbon dioxide (2) (three different compositions); C2C3C4C5C6 (one composition); isobutane–methanol–methyl tertbutyl ether–1-butene (one composition); and propylene–water–isopropyl alcohol–diisopropyl ether (one composition).Conclusions. Our results agreed to a large extent with the experimental data available in the literature. For mixtures that contained CO2 , the best results were obtained using the PengRobinson equation of state and the Wong–Sandler mixing rules. Our methodology, based on flash calculations, equations of state, and mixing rules, may be viewed as a shortcut procedure for drawing phase envelopes and estimating critical points of multicomponent systems.

Highlights

The knowledge of phase envelopes and critical points is valuable for the calculation of phase equilibria and solving various problems in chemical engineering
We present a methodology of constructing phase envelopes of multicomponent mixtures, from which the critical points follow
The methodology was capable of reproducing phase envelopes for multicomponent mixtures reported in [11], where a method which included the solution of a set ofordinary differential equations was used for drawing the phase envelopes

Summary

Introduction

The knowledge of phase envelopes and critical points is valuable for the calculation of phase equilibria and solving various problems in chemical engineering. From a modeling point of view, knowledge of critical data is of paramount importance, as it provides information about real fluids, as well as characterizes phase change boundaries in mixtures with the help of phase diagrams. We present a methodology of constructing phase envelopes of multicomponent mixtures, from which the critical points follow. A methodology has been proposed [1] to calculate critical points of multicomponent mixtures using a modification of the Gibbs plane tangent. A group of researchers [2] solved the Heidemann and Khalil formulation [3] using a Newton method with defined intervals to calculate critical points of binary and ternary mixtures

Objectives

Results

Conclusion