Accounting for quadrupolar interactions in the CPA equation of state: A modelling approach for binary mixtures of CO2 or benzene with n-alkanes and 1-alkanols

Abstract

The Cubic Plus Association (CPA) equation of state (EoS) is extended with the theory for quadrupolar molecules of Gross, yielding the CPA-G model. CPA-G is compared to the recently developed qCPA, which uses the polar theory of Larsen and coworkers. Each model requires an additional adjustable parameter for quadrupolar molecules.The models are evaluated by means of their ability to model the phase equilibria of binary mixtures of CO2 or benzene with n-alkanes and 1-alkanols, as well as binary mixtures of quadrupolar molecules. A refined technique of accounting for cross-association in the CO2 + 1-alkanol systems is proposed, allowing an improved description of these systems without the use of a binary interaction parameter. Accounting for cross-association in the benzene + 1-alkanol systems investigated in this work did not lead to improved results. Both CPA-G and qCPA provide significant improvements over nonpolar CPA in both n-alkane and 1-alkanol mixtures, with excellent results obtained in the subcritical region without the use of binary interaction parameters. qCPA continues to provide excellent predictions in the supercritical region, while CPA-G overestimates the solubility pressures in the supercritical region. Both quadrupolar models can qualitatively predict the existence of three-phase equilibrium in the CO2 + n-alkane and CO2 + 1-alkanol systems. The simple combining rules used for cross-quadrupolar interactions in both CPA-G and qCPA do not take molecular orientation into account and are unable to truly capture the phase behaviour in the CO2 + benzene system.