Evaluation and improvement of the sPC-SAFT equation of state for complex mixtures.

[摘要] ENGLISH ABSTRACT: Efficient process design commonly relies on equation-of-state (EOS) models to provide reliableestimates of thermodynamic properties. The accuracy of EOS models, in turn, depends on theextent to which they account for intermolecular forces. The aim of this project was to improve thesimplified Perturbed Chain - Statistical Associating Fluid Theory (sPC-SAFT), enabling it to accountmore accurately for complex molecular interactions. The more simple SAFT-basedCubic-Plus-Association (CPA) model was evaluated along similar lines for comparative purposes.A literature review showed that both sPC-SAFT and CPA have been widely applied in phaseequilibria problems, but not extensively for the prediction of other thermodynamic properties.Consequently, an initial evaluation was performed on the ability of sPC-SAFT and CPA to predictfirst- and second-order thermodynamic properties. The properties of non-polar, polar andhydrogen bonding fluids were considered, showing that:a) sPC-SAFT and CPA generally predict first-order properties with the same accuracy, butsPC-SAFT provides improved predictions of second-order properties. Significant errors are,however, still observed with sPC-SAFT.b) A parameter regression study with sPC-SAFT, using model parameters obtained byincluding second-order properties in the regression function, results in poor predictions ofthe saturated vapour pressure and liquid density.c) Treating strong polar and dispersive forces together as Van der Waals forces results inmany properties being poorly predicted by both sPC-SAFT and CPA.d) The major limitation of the association term in both CPA and sPC-SAFT is its inability toaccount for the influence of bond co-operativity, especially in alcohol/water mixtures.Based on these findings, the following improvements could be made:a) The development of a new association scheme for 1-alcohols, denoted the 2C associationscheme.b) The extension of sPC-SAFT with the polar theories of Jog & Chapman (JC) and Gross &Vrabec (GV) to obtain sPC-SAFT-JC and sPC-SAFT-GV.c) The extension of CPA with modified versions of the aforementioned polar theories toobtain CPA-JC and CPA-GV.d) The development of a new 'universal' cross-association approach.The new 2C association scheme consists of one bipolar association site and one negative electrondonor site and is a combination of the 1A and 2B/3B association schemes. Modelling 1-alcoholswith the 2C scheme in sPC-SAFT results in improved VLE predictions of alcohol/water andalcohol/alcohol mixtures, but alcohol/alkane VLE is predicted less accurately compared to the 2Band 3B association schemes. sPC-SAFT-JC and sPC-SAFT-GV provide improved VLE predictions of mixtures with non-associatingpolar components compared to sPC-SAFT. VLE of polar/alkane and polar/polar systems can berepresented accurately with no or only very small binary interaction parameters (BIPs). CPA-JC andCPA-GV also enable improved VLE predictions of the polar/alkane and polar/polar mixturescompared to CPA. sPC-SAFT-GV and sPC-SAFT-JC were also applied to several mixtures ofassociating components including alcohol/alkane, alcohol/alcohol and alcohol/water systems. Newalcohol model parameters for both sPC-SAFT-GV and sPC-SAFT-JC based on the 2C, 2B and 3Bassociation schemes were determined. The predictions of both sPC-SAFT-GV and sPC-SAFT-JC,based on any of the three association schemes, provide similar alcohol/alkane and alcohol/alcoholVLE representations, but the best phase equilibria predictions of water/alcohol systems areobtained when alcohols are modelled with the newly proposed 2C association scheme.The usefulness of a new 'universal' cross-association approach was demonstrated with bothsPC-SAFT-GV and sPC-SAFT-JC. The philosophy behind the new approach is to set the associationvolume value of the solvating component equal to the cross-associating volume value of the1-alcohol of the same molecular size and to determine an association energy value from binaryVLE data. This approach aims to characterize the solvating behaviour of the cross-associatingcomponent. Preliminary results are demonstrated with systems containing acetone, propylformate and ethyl acetate.Other thermodynamic properties, such as excess enthalpy and excess volume can be describedwith the new polar sPC-SAFT and CPA models. In the majority of cases, improvements areobserved compared to the normal sPC-SAFT and CPA models, but BIPs are still required to obtainaccurate correlations. However, these BIPs cannot be used in phase equilibria calculations and aregenerally property-specific.To summarise: Through the development of the 2C scheme, and the incorporation of polar termsinto the sPC-SAFT model structure, notable improvement in the VLE predictions of polar (nonhydrogenbonding)/alkane, alcohol/alkane, alcohol/water and polar/alcohol systems could beobtained if compared to the original sPC-SAFT EOS. As such, the research pesented in this thesisencapsulates some significant novel contributions, viz.:a) A systematic evaluation of sPC-SAFT and CPA, providing better insight into their ability topredict thermodynamic properties.b) The development of the new 2C association scheme for 1-alcohols, as published inInd. Eng. Chem. Res. 2011, 50, 8711–8725.c) The extension of sPC-SAFT with the polar theories of JC and GV, with application tonon-associating components, as published in Fluid Phase Equilib. 2011, 305, 174–184.d) The extension of CPA with the JC and GV polar theories, as published in Fluid PhaseEquilib. 2011, 312, 66–78.e) The application of sPC-SAFT-GV and sPC-SAFT-JC to associating components, includingresults with the new 2C association scheme.f) The development of the new 'universal' cross-association approach.

[发布机构] Stellenbosch University