Thesis (PhD)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: In this study the deoiling and fractionation of paraffin wax using supercritical
fluid extraction (SCFE) has been investigated. SCFE was compared with
state-of-the-art processes such as wax crystallisation, static crystallisation and
short path distillation.
Ethane and carbon dioxide were investigated as supercritical solvents for the
supercritical fluid extraction of paraffin wax. Supercritical phase equilibrium
data for ethane - n-alkane and CO2 - n-alkane systems close to the mixture
critical region were obtained from the literature, and were correlated with
several equations of state. Statistical mechanical equations of state failed to
correlate the data close to the mixture critical region due to the neglect of
density fluctuations which influences phase behaviour close to critical points,
or due to inadequate mixing rules. It was found that simple cubic equations of
state such as Soave-Redlich-Kwong, Peng-Robinson and Patel-Teja could
correlate the data using two interaction parameters. This can be attributed
more to their flexibility as correlating tools than to their fundamental accuracy.
The Patel-Teja EOS was modified by fitting it to predict low vapour pressure
data for long-chain n-alkanes. This modified Patel-Teja EOS was then fitted
to the phase equilibria by adjusting two interaction parameters per binary
system. The interaction parameters for each solvent (ethane or CO2) system
were then fitted to generalised correlations to enable extrapolation to solvent
- n-alkane systems for which no equilibrium data were available. The
Simplified Perturbed Hardchain theory (SPHC) equation of state was used to
correlate lower-pressure solubility data used to model the extract separator.
A pilot plant SCFE unit was constructed and used to obtain experimental
fractionation data of a low-molecular weight Fischer-Tropsch wax. The
experimental results indicate that fractionation of the wax is possible and that
the separation efficiency is enhanced by returning some of the extract to the
column as reflux. An equilibrium stage model was constructed and used to
simulate the extraction experiments. It was possible to obtain good
agreement between the experimental results and model predictions.
Deoiling of petroleum waxes with a low n-paraffin content (which are not
currently deoiled commercially) was investigated. Experimental SCFE and
SPD results indicated that selective deoiling is not possible, since the
separation is based on differences in molecular weight (or vapour pressure).
Simulations of wax crystallisation or solvent extraction and practical tests
indicate that deoiling is possible, based on differences in structure and therefore melting point of the components in the wax. Practical problems
associated with crystallisation or solvent extraction such as filtration, the use
of chlorinated solvents and low yields currently prevent the commercial
deoiling of these waxes. During crystallisation of these waxes a soft wax cake
is formed which impedes the operation of static crystallisation.
A detailed study of the economics of n-paraffin wax deoiling using SCFE was
conducted. Flow sheets were proposed to minimise the energy consumption
of the SCFE process. Comparison of SPD, static crystallisation and SCFE
indicates that a SPD plant will be the cheapest option for deoiling the wax
feed investigated. Fractionation of heavier waxes using SPD might not be
economically feasible, since the distillation temperature increases dramatically
with increasing molecular weight, which leads to higher energy cost. For
medium to long chain n-paraffin waxes SCFE should be very competitive,
since the capacity of the supercritical solvent can be manipulated to extract
longer chain waxes without increasing the extraction temperature. Static
crystallisation appears to be the more expensive deoiling option, due primarily
to the large initial capital investment cost. / AFRIKAANSE OPSOMMING: Die olieverwydering en fraksionering van paraffienwasse met behulp van
superkritiese ekstraksie is in hierdie studie ondersoek. Die modellering van
moderne olieverwyderings- en fraksioneringsmetodes soos waskristallisasie,
statiese kristallisasie en kortpad distillasie is ook ondersoek.
Etaan en koolstofdioksied is ondersoek as superkritiese oplosmiddels vir die
superkritiese ekstraksie van paraffienwasse. Literatuurdata van superkritiese
fase-ewewigte vir etaan - n-alkaan en CO2 - n-alkaan stelsels naby die
mengselkritiese punt is versamel en gekorreleer met verskeie
toestandsvergelykings. Statisities-meganiese toestandsvergelykings kon nie
data naby mengselkritiese punte korreleer nie, moontlik weens
digtheidsvariasies wat afwykings van klassieke gedrag teweegbring, of
onakkurate mengreëls. Eenvoudige kubiese toestandsvergelykings soos
Soave-Redlich-Kwong, Peng-Robinson en Patel-Teja kon op die ewewigsdata
gepas word deur gebruik van twee interaksieparameters. Dit kan eerder
toegeskryf word aan hulle buigsaamheid eerder as hulle fundamentele
akkuraatheid. Die Patel-Teja toestandsvergelyking is gemodifiseer deur dit te
pas op lae dampdruk data van langketting n-alkane. Hierdie gemodifiseerde
toestandsvergelyking is gepas op die fase-ewewig data deur twee
interaksieparameters te gebruik per binêre sisteem. Die interaksieparameters
vir die oplosmiddel stelsels (etaan of CO2) is gekorreleer met algemene
vergelykings sodat dit vir ekstrapolasie na oplosmiddel - n-alkaan stelsels
gebruik kan word waarvoor ewewigsdata nie beskikbaar is nie. Die
"Simplified Perturbed Hardchain" teorie (SPHC) toestandsvergelyking is
gebruik om laer druk oplosbaarheidsdata te korreleer vir gebruik in die
modellering van die ekstrak skeier.
In Superkritiese ekstraksie loodsaanleg is gebou en gebruik om
eksperimentele fraksioneringsdata van 'n lae molekulêre massa Fischer-
Tropsch was te genereer. Vanaf die eksperimentele resultate blyk
fraksionering van was moontlik te wees. Die doeltreffendheid van die
skeiding kan verhoog word deur terugvloei van ekstrak na die kolom. 'n
Ewewigsmodel is opgestel en gebruik om die ekstraksie eksperimente te
modelleer. Deur die ekstraksiedruk en aantal stadia te verander kon goeie
ooreenstemming met eksperimentele resultate verkry word.
Die verwydering van olie uit petroleumwasse met In lae n-paraffien inhoud
(wat nie tans kommersiëel ontolie word nie) is ondersoek. Eksperimentele
resultate vir superkritiese ekstraksie en kortpad distillasie dui daarop dat
selektiewe olieverwydering nie moontlik is nie, omdat die skeiding gebaseer is op verskille in molekulêre massas en dus (of dampdrukke). Simulasies van
waskristallisasie dui op die moontlikheid van olieverwydering gebaseer op
verskille in strukture van die komponente in die was. Praktiese probleme
geassosieer met kristallisasie of oplosmiddel ekstraksie soos filtrasie, lae
opbrengste en gebruik van gechlorineerde koolwaterstowwe as oplosmiddel
belemmer die kommersialisering van olieverwydering vir hierdie tipes wasse.
'n Gedetaileerde studie van die ekonomiese lewensvatbaarheid van
superkritiese olieverwydering is uitgevoer. Vloeidiagramme is voorgestelom
die energieverbruik van die superkritiese ekstraksieproses te minimeer.
Vergelyking van kortpad distillasie, statiese kristallisasie en superkritiese
ekstraksie dui daarop dat kortpad distillasie die goedkoper opsie vir die
olieverwydering van die spesifieke was is. Fraksionering van swaarder wasse
met kortpad distillasie sal moontlik nie haalbaar wees nie omdat die
distillasietemperatuur drasties toeneem met molekulêre massa. Die skeiding
van medium tot langketting wasse met superkritiese ekstraksie behoort meer
mededingend te wees, want die kapasiteit van die superkritiese oplosmiddel
kan maklik verstel word om langer kettinglengtes wasse te ekstraeer sonder
om die temperatuur te verhoog. Statiese kristallisasie blyk die duurder
olieverwyderingsopsie te wees hoofsaaklik weens die hoë kapitaalkoste van
so 'n aanleg.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52471 |
| Date | 12 1900 |
| Creators | Crause, J. C. (James Christoffel) |
| Contributors | Nieuwoudt, I., Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 277 p. |
| Rights | Stellenbosch University |
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