Improvement of the Thermodynamic description of polar molecules and their mixtures in the SAFT framework

i | P a g e ABSTRACT Chemical processes are designed around the manipulation of thermodynamic properties of chemical species and their mixtures. The ability to predict these properties accurately has driven the development of thermodynamic models from the humble beginnings of the van der Waals equation to the fundamental statistical mechanical theories we have today. Despite the successes of recent years resulting from improved fundamental understanding and availability of increased computational power, the accurate representation of certain chemical species and some thermodynamic properties remain elusive. This is particularly true of the holistic properties of polar components and their mixtures, which is the focus of the work presented here. Arguably the most successful product of the more fundamental approach to thermodynamic modelling is the Statistical Associating Fluid Theory (SAFT). This equation of state has allowed for the accurate representation of molecular geometries, molecular association and the accurate representation of polar interactions. The resulting model framework produces highly accurate predictions of mixture phase equilibria of many systems, which has been the focus of the model’s development. Exceptions still remain however, with one such systematic fault providing specific context for model improvement in this work. A central aim of this work was to establish whether there is deterioration in the predictive capacity of polar sPC-SAFT when applied to the phase equilibrium of different polar isomers. This follows from previous work, where such systematic deterioration was found for ketones, and is extended to ethers and esters here. A lack of previously measured data for isomers of these polar functional groups necessitated generation of such experimental data. Thus, new isobaric vapour-liquid equilibrium data were generated for two C6 ether isomers with n-heptane, and five C6 ester isomers with n-octane at 60 kPa. The experimental data showed that significant differences in mixture phase behaviour and resulting phase envelopes are present for each isomer considered, consistent with the experimental trend seen for ketones. Modelling of the experimental data using sPC-SAFT with the Jog & Chapman (JC) and Gross & Vrabec (GV) polar terms showed that similar trends to that witnessed for ketones previously are apparent for ethers and esters. Using pure component data alone, sPC-SAFTGV parameters were determined for all but one considered isomer, displaying a consistent level of accuracy in the prediction of mixture VLE. Significant parameter degeneracy was apparent for sPC-SAFTJC however, resulting in an inability to regress unique parameter sets for less polar isomers. The results constitute the first case of systematic superiority of the Gross & Vrabec polar term over that of Jog & Chapman. The modelling results served to clarify the relationship between isomer identity and predictability of the component properties. The predictive capacity of polar sPC-SAFT decreases as the behaviour of the isomer approaches that of an equivalent size/mass nonpolar component. In the case of ketones previously and esters here, this behaviour is linked to the shifting of the polar group centrally, while a Stellenbosch University https://scholar.sun.ac.za