Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Long-chain hydrocarbons are of value to numerous lucrative industries. Due to the low
volatility and close melting and boiling points of these solutes, traditional fractionation
methods lack the required selectivity for separation and cause thermal degradation of the
product.
This project investigates the feasibility of Supercritical Fluid Extraction (SFE) for processing
these systems, with the primary objective of modelling the high-pressure vapour-liquid
equilibrium (VLE) properties of hydrocarbon solutes with a light-weight solvent using a semiempirical
equation of state (EOS). Pure component vapour pressures and saturated liquid
volumes are also investigated.
A thorough investigation into the phase behaviour of the n-alkanes, 1-alcohols, carboxylic
acids and esters in light weight supercritical solvents CO2, ethane and propane revealed that
the solute structure and temperature largely influence the solute solubility and process
feasibility. Good selectivity amongst the various solutes was observed for all three solvents,
but very high pressures were required for complete miscibility using CO2 (exceeding 30
MPa). The quadrapole moment of CO2 further leads to complexities in phase behaviour such
as temperature and density inversions (CO2/alkanes and CO2/alcohols) and 3-phase regions
within the operating range. Simple linear trends in pressure vs. carbon number and
temperature were observed for all the considered series using ethane and propane and these
solvents were thus selected for conducting the modelling for this study.
A thorough review of semi-emperical EOS models from literature revealed that the simple
cubic equations of state (CEOSs) provide a promising modelling approach for SFE
applications due to their simplicity, flexibility and reliability.
The simple Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) EOSs provide good
correlation of vapour pressure (%AAD below 5 %) for all the series over a large carbon
number range (up to nC20), provided a two parameter alpha function is used. A 3rd parameter
in the volume dependence for Patel-Teja (PT) EOS provides considerable improvement over
the PR and SRK EOSs for satureate liquid volume correlations of the non-polar solutes
(alkanes and esters), but offers virtually no advantage for the more polar alcohols and acids.
The CEOSs therefore suffer clear limitations in simultaneous representation of these
saturation properties (vapour pressure and liquid molar volume) for the systems of interest.
Good correlations of high pressure binary VLE data were obtained using CEOSs available in
the Aspen Plus ® simulator (% AAD in P, T and X2 generally below 1 % and ranging from 4 to 12 % for Y2 for all series) provided that two binary interaction parameters (BIPs) are used
in the model mixing rules, irrespective of the model used. Aspen Plus ® was further validated
as a reliable thermodynamic tool by comparing model fits using the RK-ASPEN model with
parameters obtained from the Aspen Plus ® data regression routine and computational
methods used in self-developed MATLAB software. Very similar results were obtained for
both computational methods, which encourages the use of Aspen Plus ® for process
modelling in SFE applications.
A statistical sensitivity analysis into the relative effect and interactions between 6 modelling
factors in applying the CEOSs revealed that the mixing rules, temperature and solute structure
had the largest effect on the correlation of the high pressure VLE, with the pure component
limit having negligible effect once BIPs are fitted to data. A significant interaction was,
however, observed between the pure component model and the solute structure and
temperature, which suggest that accurate correlation of mixture VLE does not solely rely on
appropriate mixing rule selection, but also the pure model.
Binary interaction parameters (BIPs) in model mixing rules were found to become
intercorrelated when more than one are used, greatly impeding the development of
generalized correlations. BIPs were also found to be sensitive to the pure component limit
(alpha function and pure constants used), the temperature, the combining rules used and
possibly the fluid density. These factors should all be taken into account systematically for
developing generalized correlations which therefore fell outside the scope of this study.
Recommendations were, however, made on how the MATLAB software developed in this
study can be used to both expand the size of the statistical analysis already conducted into
relevant modelling factors and to develop new generalized correlations for BIPs and new
mixing rules. / AFRIKAANSE OPSOMMING: Lang-ketting koolwaterstowwe is van waarde in talle winsgewende industriële toepassings.
Vanweë die lae vlugbaarheiden ooreenstemmende kook- en smeltpunte van hierdie
molekules, toon tradisionele fraktioneringsmetodes nie die nodige selektiwiteit vir ekstraksie
nie en veroorsaak bonop termiese degradering van die produk.
Hierdie projek ondersoek dus die lewensvatbaarheid van superkritiese ekstraksie vir die
prosesering van hierdie sisteme, met primêre fokus op die modellering van die hoë-druk
damp-vloeistof ewewig eienskappe van koolwaterstowwe opgelos in ‘n lae-massa
oplosmiddel met gebruik van ‘n semi-empiriese toestandsvergelyking. Suiwer-komponent
dampdrukke en versadigde vloeistof volumes word ook ondersoek.
‘n Deeglike ondersoek na die fasegedrag van die n-alkane, 1-alkohole, korboksiel-sure asook
esters in lae-massa superkritiese oplosmidds CO2, etaan en propaan toon dat die struktuur van
die opgeloste stof en die temperatuur ‘n groot invloed het op die oplosbaarheid en proses
lewensvatbaarheid. Goeie selektiwiteit tussen die verskillende koolwaterstowwe was
waargeneem vir al drie oplosmiddels, alhoewel baie hoë drukke nodig was vir totale
vermenging van die fases in CO2 (hoër as 30 MPa). Die quadrupool moment van CO2
veroorsaak verder ongewenste kompleksiteite in fase gedrag soos temperatuuren digtheid
inversies (CO2/alkane en CO2/alkohole) en 3-fase-gebiede in die bedryfs-kondisies.
Eenvoudige lineêre tendense in druk tenoor die koolstofnommer van die opgeloste stof asook
die temperatuur was waargeneem vir al die ondersoekte koolwaterstof reekse in etaan en
propaan en hierdie oplosmiddels was dus gekies vir die modellering vir hierdie studie.
'n Deeglike oorsig van semi-empiriese toestandsvergelykings uit die literatuur het getoon dat
die eenvoudige kubiese toestandsvergelykings ‘n belowende modelleringsbenadering bied vir
superkritiese ekstraksie toepassings vanweë hul eenvoudigeid, buigsaamheid
enbetroubaarheid.
Die eenvoudige Peng-Robinson (PR) en Soave-Redlich-Kwong (SRK) toestandsvergelykings
bied goeie korrelasie van suiwer dampdruk (foute laer as 5 %) vir alle koolwaterstowwe oor
‘n groot koolstofnommer gebied (tot by nC20), met die voorwaarde dat ‘n 2 parameter alpha
funksie gebruik word. ‘n 3rde parameter in die volume afhanklikheid van die Patel-Teja (PT)
toestandsvergelyking bied ‘n beduidende verbetering in die passing van die versadigde
vloeistof volume vir die nie-polêre koolwaterstowwe (n-alkane en die esters), maar bied geen
voordeel vir die meer polêre alkohole en karkoksiel sure nie. Die kubiese modelle toon dus
duidelike beperkings vir die gelyktydige voorstelling van hierdie versadigingde eienskappe
(dampdruk en vloeistof volume) vir die sisteme van belang.
Goeie korrelasie van hoë druk binêre damp-vloeistof ewewig data was verkry deur gebruik
van die kubiese toestandsvergelykings beskikbaar inAspen Plus ® (fout in P, T en X2 tipies
laer as 1 % en van 4 tot 12 % vir Y2 vir alle sisteme), met die voorwaarde dat 2 binêre
interaksie parameters gebuik word in die model mengreëls, onafhanklik van die model. Aspen
Plus ® was verder bekraktig as ‘n betroubare termodinamiese hulpmiddel deur model
passings te vergelyk met die RK-ASPEN model tussen gevalle waar parameters verkry is deur
die beskikbare regressie metode in Aspen Plus ® en metodes gebruik in self-ontwikkelde
MATLAB sagteware. Eenderse resultate was verkry vir beide berekeningsmetodes, wat die
gebruik van Aspen Plus ® vir prosesmodellering in superkritiese ekstrasie toepassings
aanmoedig.
‘n Satistiese sensitiwiteits analise op die relatiewe effek en interaksies tussen 6
modelleringsfaktore in die toepassing van die kubiese toestandsvergelykings het gevind dat
die mengreëls, temperatuur en die stuktuur van die opgeloste stof die grootste effek op die
korrelasie van hoë druk binêre damp-vloeistof ewewig het, met ‘n weglaatbare effek vandie
suiwerkomponent limiet waargeneem sodra binêre interaksie parameters gepas is aan data. ‘n
Beduidende interaksie was wel waargeneem tussen die suiwerkomponent model en die
struktuur van die opgeloste stof asook die temperatuur, wat daarop dui dat akurate korrelasie
van mengsel damp-vloeistof ewewig nie slegs afhanklink is van ‘n gepaste keuse van
mengreëls nie, maar ook die suiwer-komponent model.
Binêre interaksie parameters in die model mengreëls ondergaan inter-korrelasie wanneer
meer as een interaksie parameter gebruik word, wat die ontwikkeling van algemeen toepaslike
korrelasies grotendeels belemmer. Binêre interaksie parameters was ook bevind om sensitief
te wees tot die suiwer component limiet (alpha funksie en suiwer konstantes wat gebruik is),
die temperatuur, die kombineringsreëls en moontlik die vloeistof digtheid. Hierdie faktore
moet dus almal sistematies in ag geneem word wanneer algemeen toepaslike korrelasies
ontwikkel word, wat dus buite die omvang van die huidge studie val. Aanbevelings was wel
gemaak vir hoe die MATLAB sagteware ontwikkel vir hierdie studie gebruik kan word om
beide die betaande statistiese analise uit te brei, asook nuwe korrelasies vir binêre interaksies
parameters en nuwe mengreëls te ontwikkel.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96844 |
| Date | 03 1900 |
| Creators | Lombard, James Edward |
| Contributors | Knoetze, J. H., Schwarz, Cara E., 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 | 360 pages : illustrations |
| Rights | Stellenbosch University |
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