Browsing by Author "Smith, Sonja Almi Milé"

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    Extending SAFT-VR Mie to the global phase behaviour of CO2 and its mixtures
    (Stellenbosch : Stellenbosch University, 2022-04) Smith, Sonja Almi Milé; Schwarz, Cara Elsbeth; Cripwell, Jamie Theo; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Understanding the phase behaviour of CO2-containing mixtures is important for many industrial processes, amongst others supercritical fluid fractionation and enhanced oil recovery. These mixtures are complicated by the CO2 quadrupole moment, and, because these processes are often conducted near CO2’s critical point, critical phenomena. These characteristics make thermodynamic modelling of CO2-containing systems challenging. Many equations of state (EoSs) with firm theoretical foundations have been developed. The Statistical Associating Fluid Theory, or SAFT EoS, is rooted in statistical mechanics where macroscopic properties are calculated by considering the energy contributions of molecular interactions. The SAFT with Variable Range Mie-potential (SAFT-VR Mie) model was the focus of this project, because it is arguably the most advanced of the SAFT-variants and shows promise as a holistic predictive tool. The industrially relevant Cubic Plus Association (CPA) model was included for comparative purposes. The overarching aim of this project was to improve the predictive modelling of CO2- containing mixtures, thereby developing a single model that describes the global phase behaviour of these mixtures. To achieve this, the models’ descriptions of quadrupolar interactions and of the critical region needed improvement. To account for quadrupolar interactions, SAFT-VR Mie (VRM) and CPA were extended with the quadrupolar terms of Gross (G) and Larsen & coworkers (L), and three new models were proposed: VRM-G, VRM-L, and CPA-G. CPA extended with the Larsen quadrupolar term was developed in previous work (Bjørner & Kontogeorgis, Fluid Phase Equilibria 2016;408:151􀀀69), and is called qCPA. The quadrupolar models were evaluated by modelling the phase equilibria of binary mixtures containing CO2 or benzene + n-alkanes, 1-alkanols, water, or esters. The quadrupolar models’ improvements are most pronounced in the CO2 + n-alkane systems. The quadrupolar models predict these systems’ phase behaviour accurately at subcritical conditions, and offer improved qualitative descriptions at supercritical conditions. In the CO2 + 1-alkanol systems, good predictions are obtained when accounting for both quadrupolar and cross-association interactions. A single set of CO2 association parameters, determined from a sensitivity analysis, were used to predict the VLE behaviour of CO2 + 1-alkanol mixtures ranging from ethanol to 1-decanol. There is still room for improvement, specifically regarding the water- and ester mixtures. In the water mixtures, the additional quadrupolar terms do not improve the descriptions of the nonpolar models. To obtain good qualitative descriptions of the phase boundaries, the cross-association description is the most important. In the ester mixtures, the polar models do not capture the balance between dipolar, quadrupolar, and dipole-quadrupole interactions adequately. Based on the results for the CO2 + n-alkane and CO2 + 1-alkanol mixtures, VRM-G and qCPA were identified as the best quadrupolar model options in SAFT-VR Mie and CPA, respectively. These models are based in mean-field theory, and therefore cannot describe the critical region. To this end, VRM-G and qCPA were treated with renormalisation corrections, yielding VRM-G + RG and qCPA + RG. Both models improve the description of pure component properties in and around the critical region, without losing accuracy outside the critical region. The RG-models were extended to mixtures using the isomorphism approach and applied to binary n-alkane and CO2 + n-alkane systems. qCPA + RG only offers significant improvement for the more symmetric systems; this improvement, however, does not worsen prediction of binary VLE outside the critical region. In VRM-G + RG, remarkable predictions of the critical loci are obtained without binary interaction parameters. VRM-G + RG also describes the phase behaviour of these systems outside the critical region accurately, thus achieving the overarching aim of developing a global model for CO2-containing mixtures. The following contributions stem from this research: 1. The development of VRM-G and VRM-L, published in Journal of Chemical & Engineering Data 2020;65(12):5778 􀀀 5800; 2. The development of CPA-G, published in Fluid Phase Equilibria 2021;528:112848; 3. The development of VRM-G + RG.