Spectroscopic measurement of associating and solvating binary mixtures for the determination of monomer fraction data towards thermodynamic model improvement
Date
2024-03
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Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Amidst an energy transition within the broader chemical engineering sector, there is a strong drive towards making processes more sustainable by enhancing energy efficiency by adopting cleaner alternatives like green solvents and biodiesels. However, the presence of hydrogen bonding phenomena in such chemicals poses a longstanding challenge for thermodynamic modelling and process design. The monomer fraction, indicative of the extent of hydrogen bonding, can be measured using Fourier Transform Infrared (FTIR) spectroscopy. Yet surprisingly, its practical application in testing thermodynamic model performance remains unexplored, partly due to limited data and discrepancies in measurement techniques. Thus this was the overall focus of this work, to propose a rigorous alternative for monomer fraction quantification in associating and solvating systems and thoroughly assess its use in testing thermodynamic model performance. A detailed examination of the uncertainty associated with the single-point calibration in an ethanol/nhexane system revealed that there was a significant dependence of the ethanolic monomer fraction on the choice of the calibration method. With this in mind, establishing a definitive calibration choice proved to be challenging, prompting an alternative methodology which was based on multivariate curve resolution with alternating least squares (MCR-ALS). The approach leveraged purely physical constraints and resulted in physically meaningful profiles. The rotational ambiguity uncertainty in each of the resolved profiles was completely suppressed and the resulting lack of fit (LOF) was determined to be 2.55%. The spectral profiles exhibited features that allowed the assignment of α, β, γ, and δ, bonds where the α-bond type was used to determine the true monomer fraction while the fraction of free hydroxyls (α+β) compared well to previously reported data. Probing for ethanolic monomer features in solution with C4 esters was performed using the hydroxyl band. In each of the three binary systems, no features could be attributed to free ethanol. Thus, it was concluded that ethanol was not present in a free monomeric form and the monomer fraction of ethanol in all C4 ester systems was zero. Conversely, C4 ester monomer fractions, in solution with ethanol, were determined by probing the alkoxyl ester band as this allowed spectra across the entire compositional space to be studied. It was found that the monomer fraction of methyl propionate and ethyl formate had similar gradual decreasing trends while the monomer fraction trend of propyl formate decreased significantly as a function of increasing ethanol composition. The monomer fractions of C4 esters were thermodynamically modelled using the SAFT-VR Mie + GV equation of state (EoS) under non-solvating and solvating conditions. Using the N scheme, improved monomer fraction predictions were seen but were still qualitatively poor. Although regression of new association parameters, against C4 ester monomer fractions, brought improved monomer fraction correlations, significant deterioration in VLE predictions was observed. Marginal improvements were determined when using parameters obtained from a combined monomer fraction and VLE discretised sensitivity analysis. The inclusion of monomer fraction data in thermodynamic model parameterisation highlighted the model’s inability to accurately describe monomer fraction data. This suggests a possible shortcoming of Wertheim’s association theory, indicating the need for further development of the theory.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (MEng)--Stellenbosch University, 2024.