Thesis (MScEng)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: Supercritical extraction is being investigated as a possible alternative to the processes
currently used in the fractionation of paraffinic waxes. By removing the lighter carbon
fractions from the wax, the wax hardness will be improved and its melting temperature
range reduced, hence improving the performance of the wax product in certain
applications. In order to evaluate and operate such an extraction process optimally, it is
necessary to have a thermodynamic model that accurately represents the process system.
There are, however, currently no predictive models available for these systems. In order to
fit present models to the systems, accurate phase equilibrium data of the supercritical
solvent - n-alkane systems are needed. Unfortunately, the amount of reliable published
data on these systems in the required operating range is very limited.
A view cell was designed and developed with which these high pressure equilibria could
be studied. The binary phase equilibria of supercritical CO2 with n-CI2, n-CI6, n-C20, n-C24,
n-C28 and n-C36 and of supercritical ethane with n-CI6, n-C24 and n-C28 were measured in
the temperature range 313 - 367 K. It was found that the systems with these two solvents
have very different types of phase behaviour. The n-alkane solubility is much higher in
ethane, but supercritical CO2 will provide a much better degree of control over the
selectivity achieved in an extraction process.
Of the various equations of state investigated, it was found that the Patel Teja equation of
state provided the best fit of the CO2 - n-alkane systems and that the Soave-Redlich-
Kwong equation fitted the ethane - n-alkane systems the best. The interaction parameters
of both these equations of state display a functional relationship with temperature and nalkane
acentric factor, making it possible to determine parameter values for application at
other operating temperatures and with other n-alkane systems.
It was found that the current equations of state were not able to represent the phase
equilibria accurately over the entire range of operating conditions. The poor performance
of the equations of state can be attributed to inherent flaws in the existing equations of
state. / AFRIKAANSE OPSOMMING: Superkritiese ekstraksie word tans ondersoek as 'n moontlike altematief vir die prosesse
wat huidiglik gebruik word om paraffiese wasse te fraksioneer. Die Iigter koolstofwasse
word verwyder om die washardheid te verhoog en die temperatuurgebied waaroor die was
smelt te verklein. Dit verbeter dan die was se kwaliteit en werkverrigting. Modelle wat die
superkritiese ekstraksie proses akkuraat kan voorstel word egter benodig om die ekstraksie
proses te kan evalueer en optimaal te bedryf. Daar is tans geen modelle beskikbaar wat die
proses direk kan voorstel nie. Akkurate fase-ewewigsdata word benodig om bestaande
modelle aan te pas vir gebruik in hierdie sisteme. Daar is egter baie min betroubare faseewewigsdata
vir die superkritiese oplosmiddel - n-alkaan sisteme beskikbaar in die
literatuur.
'n Sig-sel, waarrnee hierdie hoe druk data gemeet kan word, is ontwerp en ontwikkel. Die
volgende binere fase ewewigte is in die temperatuur gebied 313 - 367 K gemeet:
superkritiese CO2 met n-CI2, n-CI6, n-C20, n-C24, n-C28 en n-C36, en superkritiese Etaan met
n-CI6, n-C24 en n-C28. Daar is gevind dat hierdie twee superkritiese oplosmiddelsisteme
verskillende tipes fase-ewewigsgedragte openbaar. Die n-alkane het 'n baie boer
oplosbaarheid in Etaan, maar deur superkritiese C02 in 'n ekstraksie kolom te gebruik, sal
tot beheer oor die selektiwiteit van die ekstraksieproses lei.
Uit die verskillende toestandsvergelykings wat ondersoek is, is daar gevind dat die Patel-
Teja vergelyking die CO2 sisteme die beste kon beskryf en dat die Soave-Redlich-Kwong
vergelyking die beste vergelyking was om die Etaan sisteme mee te modelleer. Beide die
toestandsvergelykings se interaksie parameters het 'n funksionele verband met temperatuur
en die n-alkaan asentrise faktor getoon. Dit is dus moontlik om waardes vir die parameters
vir sisteme by ander temperature en met ander n-alkaan tipes te bepaal.
Daar was gevind dat die bestaande toestandsvergelykings nie die die fase-ewewigte oor die
hele eksperimenele gebied akkuraat kon voorstel nie. Dit kan toegeskryf word aan foute
wat inherent is aan die vergelykings.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/51789 |
| Date | 12 1900 |
| Creators | Du Rand, Marlie |
| 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 | 168 p. : ill. |
| Rights | Stellenbosch University |
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