| Summary: | Thesis (MScEng)-- Stellenbosch University, 2013. === ENGLISH ABSTRACT: The simplified Perturbed-Chain Statistical Associating Fluid Theory (sPC-SAFT) is characterised by the
dual advantages of decreased computational intensity, while remaining accurate for a variety of
systems. Vapour-liquid equilibrium data are used to generate equation of state parameters.
However, incorporating monomer fraction data into the parameter regression has long been
advocated as a good, or even preferred, practice. Therefore, the monomer fraction data of dilute
alcohol-acetone systems were analysed in this study. A small stainless steel sample vessel was
constructed with temperature control, manual pressure control and a mechanism for liquid phase
analysis via infrared spectroscopy.
The performance of the spectrometer was verified by comparison with the ethanol – n-hexane data
of von Solms et al. (2007), after which new monomer fraction data were obtained for dilute
solutions (between 0.01 and 1.5 mol%) of methanol, ethanol, 1-propanol and 2-propanol in acetone
near 23 °C.
For dilute alcohol-acetone systems it was found that the propanols had the highest monomer
fractions, and methanol the lowest. With increasing alcohol concentration, the monomer fractions
decreased exponentially to values of 0.4 and 0.1 for methanol and the other alcohols respectively.
The excess availability of hydrogen bond acceptors in the mixtures explains the equivalency
observed for ethanol, 1-propanol and 2-propanol.
For dilute acetone-alcohol systems it was found that, especially for methanol and ethanol, there was
a pronounced trend towards acetone monomer fractions of 1 at infinite dilution. For the acetone –
2-propanol system, a previously unrecorded monomer peak was observed and quantified. Acetone
monomer fractions tended to decrease as alcohol chain-length increased, showing that acetone
could more easily penetrate the hydrogen bond network of the solvent when the solvent-solvent
bonds were weaker. Monomer fraction data were compared to predictions for the sPC-SAFT scheme and parameters
combinations published in the literature. The experimental data were accurately modelled using
modified association parameters such that the solute associates strongly (εAB≈103 κ≈1), while the
solvent parameters were decreased (εAB≈102 κ≈10˗3) to give a weakened solvent association effect.
The difficulty for the dilute solute in penetrating the solvent bonding network appeared to be similar
to the hydrophobic effect. Two new association schemes were proposed for acetone, assigning a single (N) or two (2N)
negative association sites to represent the oxygen valence electron pairs. These schemes showed
relative success in modelling acetone as the solvent in the mixture, while not being able to predict
acetone monomer fractions when acetone was the solute. For dilute acetone-alcohol systems, the
data were best described using the 2B model for acetone, while the best choice of scheme for the
alcohol varied from system to system.
For dilute alcohol-acetone mixtures it was generally found that a 2B-N model (with modified
association parameters) provided the best fit to those experimental data. Accurate modelling below
0.1 mol% was difficult to attain with average errors decreasing to the order of 10% when this area
was excluded. In this highly dilute region, not one of the models could describe the rapid change in
(monomer fraction) gradient sufficiently while simultaneously offering accurate predictions over the
entire experimental range. === AFRIKAANSE OPSOMMING: Die sPC-SAFT of simplified Perturbed-Chain Statistical Associating Fluid Theory toestandsvergelykings
word wyd gebruik as gevolg van sy goeie akkuraatheid vir ‘n wye reeks sisteme, ten spyte van
verminderde berekeningsintensiteit. Die parameters vir dié toestandsvergelyk word afgelei van faseewewig
data, maar monomeer fraksie data word voorgestel vir die verbetering van (veral) die
assosiasie parameters. Ten opsigte hiervan, was alkohol-asetoon sisteme bestudeer en hul
monomeer fraksies gemeet. ‘n Staal reaktor was ontwerp (met ‘n temperatuurbeheerstelsel sowel
as drukbeheer) om vloeistof monsters voor te berei vir analise d.m.v. infrarooi-spektroskopie.
Die akkuraatheid van die eksperimentele apparaat is bewys deur nabootsing van etanol – n-heksaan
data van von Solms et al. (2007), waarna nuwe monomeer fraksie data gegenereer is vir verdunde
mengsels (0.01 tot 1.5 mol%) van metanol, etanol, 1- en 2-propanol met asetoon by 23 °C.
Metanol monomeer fraksies het eksponensieël afgeneem na 0.4, terwyl etanol en propanol fraksies
afgeneem het na ‘n gemene waarde van ongeveer 0.1. Hierdie tendens word toegeskryf aan ‘n
oormaat van toeganklike waterstofbindingontvangers in hierdie mengsels.
Vir verdunde asetoon-alkohol sisteme is daar ‘n tendens, (veral vir verdunnings met metanol en
etanol) vir die monomer fraksies om te neig na 1 by oneindige verdunning. ‘n Monomeer piek is ook
waargeneem vir die asetoon – 2-propanol sisteem. Hierdie piek is nie voorheen gesien in ander
studies nie en dit is ook die eerste keer wat sulke data gekwantifiseer is. Daar is bevind dat asetoon
monomeer fraksies afneem soos alkohol kettinglengte toeneem.
Die gegenereerde monomeer fraksie data word vergelyk met verskeie sPC-SAFT parameterstelle
vanuit die literatuur. Oor die algemeen, is die beraamde fraksie veel hoër as die eksperimentele data
wanneer die 2B/3B/2C skemas met ‘n nie-assosiërende asetoon molekuul gemodelleer word.
Wanneer die 2B parameters van von Solms et al. (2004) gebruik word, toon die beraming ‘n
drastiese onderskatting van die data. Om ‘n akkurate beraming van die monomeer fraksie data te kry, moet die assosiasie parameters van
die opgeloste stof vermeerder word (met εAB≈103 κ≈1) terwyl die oplosmiddel s’n drasties verswak
moet word (met εAB≈102 κ≈10-3). Hierdie patroon kan vergelyk word met die hidrofobiese effek waar
die kragte binne die oplosmiddel ‘n netwerk vorm wat die opgeloste stof uitstoot. Twee nuwe assosiasie skemas word ook voorgestel vir asetoon waar onderskeidelik een (N) en twee
(2N) negatiewe sones, wat die valenselektroonpare op die suurstofatoom voorstel, aan asetoon
geheg word. Hierdie twee skemas het relatiewe sukses getoon in die modellering van verdunde
alkohol-asetoon sisteme terwyl dit ‘n swak beskrywing van die verdunde asetoon-alkohol mengsels
voorspel het. ‘n Gewysigde 2B asetoon skema gee ‘n goeie beskrywing van die eksperimentele data.
In hierdie geval, is die keuse van alkohol skema minder belangrik, terwyl die waardes van die
assosiasie parameters verminder moet word.
Vir verdunde alkohol-asetoon mengels word daar bevind dat ‘n 2B-N model met nuwe assosiasie
parameters die beste passing van die eksperimentele data gee. Daar was ook bevind dat die modelle
se akkuraatheid drasties afneem (met fout vermeerdering in die orde van 10%) wanneer die
konsentrasie van die opgeloste stof minder as 0.1 mol% is.
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