Browsing by Author "Ferreira, Machelle"
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- ItemPhase Equilibria & thermodynamic modelling of the ternary system CO2 + 1-decanol + N-Tetradecane(Stellenbosch : Stellenbosch University, 2018-12) Ferreira, Machelle; Schwarz, C. E.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: Experimental data and predictive process models, tested at various operating conditions, have shown that supercritical fluid fractionation is a feasible process when aimed at the separation of detergent range 1-alcohols and n-alkanes with similar boiling points. Although this process shows good separation performance, it was previously found that distinct solute + solute interactions occur that influence the predictive capability of thermodynamic models. The aim of this study was to obtain a fundamental understanding of the solute + solute interactions in the CO2 + 1-decanol + n-tetradecane ternary system; firstly, through the generation of phase equilibria data and secondly, through the evaluation of thermodynamic models, with solute + solute binary interaction parameters (BIPs) incorporated into their algorithm, to correlate the new VLE data. The aim was achieved through the following objectives: (1) Studying the high-pressure phase equilibria of the CO2 + 1-decanol + n-tetradecane ternary system; (2) Studying the low-pressure phase equilibria of the 1-decanol + n-tetradecane binary system; (3) Selecting 4 suitable thermodynamic models available within a commercial process simulator and studying the modelling of the ternary and binary phase equilibria data with new solute + solute BIPs obtained from the experimental data. The first objective was met in two parts namely, the measurement of new high-pressure bubble- and dew-point data (HPBDP) and the measurement of new high-pressure vapour-liquid equilibria data (HPVLE). The HPBDP experiments were conducted between T = 308 K and T = 358 K using a visual static synthetic method. CO2 free n-tetradecane mass fractions (wcred) of 0.2405, 0.5000, 0.6399, 0.7698, 0.8162 and 0.9200 g/g were investigated, and the total solute mass fractions were varied between 0.015 g/g and 0.65 g/g. An increase in the solute + solute interactions were observed when increasing the n-tetradecane composition and decreasing the temperature. The distinct solute + solute interactions lead to the formation of a liquid-gas hole in the three-phase surface, cosolvency effects and miscibility windows. For the HPVLE data, a state of the art high-pressure analytical view cell was used to study four ternary mixtures at T = 308 K, 328 K and 348 K and pressures between P = 8.0 and 16.4 MPa. The equipment allowed for equilibrium to be achieved after which samples of the co-existing phases were taken simultaneously. Phase composition data for four tie lines were obtained and ternary phase diagrams constructed. A similar outcome to the HPBDP experimental results were observed. In general, for wcred ≥ 0.9004 g/g, 1-decanol will be the more soluble compound and for wcred ≤ 0.2403 g/g, n-tetradecane will be the more soluble compound. Furthermore, within the complex phase behaviour region (wcred = ± 0.6245 g/g), separation of residual n-tetradecane from 1-decanol in the mixtures are postulated to be impossible. However, separation experiments are required on a pilot plant setup to verify this assumption. To achieve the second objective, the phase behaviour complexities brought on by the 1-decanol + n-tetradecane interactions were further evaluated through the measurement of new low-pressure vapour-liquid equilibria data (LPVLE). The experiments were conducted at sub-atmospheric pressure (P = 40 kPa) using an all glass dynamic recirculating still. The binary system displayed positive azeotropy, inferring Type I-A fluid phase behaviour. The presence of the azeotrope and the non-unity activity coefficients confirmed that the binary system exhibits non-ideal phase behaviour. Four thermodynamic models, available within Aspen Plus®, were evaluated for their ability to correlate (RK-Aspen, SR-Polar and PC-SAFT) and to predict (PSRK) all three sets of experimental data. PSRK made use of previously determined low-pressure activity coefficient group-group parameters and thus served as a purely predictive model. For the remaining three models, the HPVLE and LPVLE data were used with the built-in data regression function in Aspen Plus® to regress solute + solute BIPs. For the HPBDP BIPs a plug-and-play method was applied to manually regress representative values instead of exact values. Objective 3 was achieved by evaluating the performance of the models with varying solute + solute BIPs in their specific model algorithm, i.e. BIPs regressed using low-pressure data were used to represent high-pressure data and vice versa as summarised in Table i. RK-Aspen was the only model to produce an accurate representation of each set of experimental data, which included the complex phase behaviour regions. SR-Polar was a close second, lagging in the representation of the LPVLE data. On a purely predictive front, PSRK can be used to represent accurate HPBDP and LPVLE data but should not be used to predict HPVLE data. Lastly, PC-SAFT can produce reasonable LPVLE and HPVLE data but failed to correlate the HPBDP data accurately. The models, presented in order of decreasing performance, were RK-Aspen > SR-Polar > PSRK > PC-SAFT. Lastly, the RK-Aspen model with HPBDP solute + solute BIPs provided the most accurate model fit (quantitatively and qualitatively) within the complex phase behaviour region of the HPBDP, HPVLE and LPVLE data. Overall, this thesis provided valuable insight into the role that solute + solute interactions play in generating complex phase behaviour and fractionation difficulties. In future studies, solute + solute interactions should not be ignored as they will help improve not only the design of pilot plant experiments, but also process models.