Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The expansion of the global fuels industry has caused an increase in the quantity
of hydrocarbons produced as a by-product of refinery gas-to-liquid processes.
Conversion of hydrocarbons to higher value products is possible using bioprocesses,
which are sustainable and environmentally benign. Due to the deficiency
of oxygen in the alkane molecule, the supply of sufficient oxygen through aeration
is a major obstacle for the optimization of hydrocarbon bioprocesses. While the
oxygen solubility is increased in the presence of hydrocarbons, under certain process
conditions, the enhanced solubility is outweighed by an increase in viscosity,
causing a depression in overall volumetric oxygen transfer coefficient (KLa).
The rate at which oxygen is transferred is defined in terms of a concentration
driving force (oxygen solubility) and the overall volumetric oxygen transfer coefficient (KLa). The KLa term comprises an oxygen transfer coefficient (KL)
and the gas-liquid interfacial area (a), which are dependent on the
uid properties
and system hydrodynamics. This behaviour is not well understood for
hydrocarbon bioprocesses and in a bubble column reactor (BCR). To provide
an understanding of oxygen transfer behaviour, a model hydrocarbon bioprocess
was developed using a BCR with a porous sparger.
To evaluate the interfacial area, the Sauter mean bubble diameter (D32) was
measured using an image analysis algorithm and gas holdup (ϵG) was measured
by the change in liquid height in the column. Together the D32 and ϵG were
used in the calculation of interfacial area in the column.
The KLa was evaluated with incorporation of the probe response lag, allowing
more accurate representation of the KLa behaviour. The probe response lag
was measured at all experimental conditions to ensure accuracy and reliability
of data.
The model hydrocarbon bioprocess employed C14-20 alkane-aqueous dispersions
(2.5 - 20 vol% hydrocarbon) with suspended solids (0.5 - 6 g/l) at discrete super
ficial gas velocity (uG) (1 - 3 cm/s). For systems with inert solids (corn
our,
dp = 13.36 m), the interfacial area and KLa were measured and the behaviour
of KLa was described by separation of the in
uences of interfacial area and oxygen
transfer coefficient (KL). To further the understanding of oxygen transfer
behaviour, non-viable yeast cells (dp = 5.059 m) were used as the dispersed
solid phase and interfacial area behaviour was determined. This interfacial area
behaviour was compared with the behaviour of systems with inert solids to understand
the differences with change in solids type.
In systems using inert solids, a linear relationship was found between G and uG.
An empirical correlation fo rthe prediction of this behaviour showed an accuracy
of 83.34% across the experimental range. The interfacial area showed a similar relationship with uG and the empirical correlation provided an accuracy of 78.8%
for prediction across the experimental range.
In inert solids dispersions, the KLa increased with uG as the result of an increase
in interfacial area as well as increases in KL. An increase in solids loading indicated
an initial increase in KLa, due to the in
uence of liquid-film penetration
on KL, followed by a decrease in KL at solids loading greater than 2.5 g/l, due
to diffusion blocking effects.
In systems with yeast dispersions, the presence of surfactant molecules in the
media inhibited coalescence up to a yeast loading of about 3.5 g/l, and resulted
in a decrease in D32. Above this yeast loading, the fine yeast particles increased
the apparent viscosity of the dispersion sufficiently to overcome the in
uence of
surfactant and increase the D32.
The behaviour of G in yeast dispersions was similar to that found with inert
solids and demonstrated a linear increase with uG. However, in yeast dispersions,
the interaction between alkane concentration and yeast loading caused a
slight increase in dispersion viscosity and therefore G. An empirical correlation
to predict G behaviour with increased uG was developed with an accuracy of
72.55% for the experimental range considered. Comparison of yeast and inert
solids dispersions indicated a 37.5% lower G in yeast dispersions compared to
inert solids as a result of the apparent viscosity introduced by finer solid particles.
This G and D32 data resulted in a linear increase in interfacial area
with uG with no significant in
uence of alkane concentration and yeast loading.
This interfacial area was on average 6.7% lower than interfacial area found in
inert solid dispersions as a likely consequence of the apparent viscosity with finer
particles.
This study provides a fundamental understanding of the parameters which underpin
oxygen transfer in a model hydrocarbon bioprocess BCR under discrete
hydrodynamic conditions. This fundamental understanding provides a basis for
further investigation of hydrocarbon bioprocesses and the prediction of KLa behaviour
in these systems. / AFRIKAANSE OPSOMMING: Die uitbreiding van die internasionale brandstofbedryf het 'n toename veroorsaak
in die hoeveelheid koolwaterstowwe geproduseer as 'n deur-produk van raffinadery gas-tot-vloeistof prosesse. Omskakeling van koolwaterstowwe na hoër
waarde produkte is moontlik met behulp van bioprosesse, wat volhoubaar en
omgewingsvriendelik is. As gevolg van die tekort aan suurstof in die alkaan
molekule, is die verskaffing van voldoende suurstof deur deurlugting 'n groot
uitdaging vir die optimalisering van koolwaterstof bioprosesse. Terwyl die suurstof
oplosbaarheid verhoog in die teenwoordigheid van koolwaterstowwe, onder
sekere proses voorwaardes is die verhoogde oplosbaarheid oortref deur 'n
toename in viskositeit, wat 'n depressive veroorsaak in die algehele volumetriese
suurstofoordragkoëffisiënt (KLa).
Die suurstof oordrag tempo word gedefinieer in terme van 'n konsentrasie dryfkrag
(suurstof oplosbaarheid) en KLa. Die KLa term behels 'n suurstofoordragkoëffisiënt
(KL) en die gas-vloeistof oppervlakarea (a), wat afhanklik is van die vloeistof
eienskappe en stelsel hidrodinamika. Hierdie gedrag is nie goed verstaan vir
koolwaterstof bioprosesse nie, asook in kolom reaktors (BCR). Om 'n begrip
van suurstof oordrag gedrag te voorsien, is 'n model koolwaterstof bioproses
ontwikkel met 'n BCR met 'n poreuse besproeier.
Om die oppervlakarea te evalueer, is die gemiddelde Sauter deursnit (D32)
gemeet deur 'n foto-analise algoritme en gas vasvanging ( G) is gemeet deur
die verandering in vloeibare hoogte in die kolom. Saam is die D32 en G gebruik
in die berekening van die oppervlakarea in die kolom.
Die KLa is geëvalueer met insluiting van die meter se reaksie sloering, om n
meer akkurate voorstelling van die KLa gedrag te bereken. Die meter reaksie
sloering was gemeet op alle eksperimentele toestande om die akkuraatheid en
betroubaarheid van data te verseker.
Die model koolwaterstof bioproses gebruik n-C14-20 alkaan-water dispersies (2.5 -
20 vol% koolwaterstof) solide partikels (0.5 - 6 g/l) op diskrete oppervlakkige gas
snelhede (1 - 3 cm/s). Vir stelsels met inerte solides (koring meel, dp = 13.36 m),
is die oppervlakarea en KLa gemeet en die gedrag van KLa beskryf deur skeiding
van die invloede van oppervlakarea en KL. Om die begrip van suurstof oordrag
se gedrag te bevorder, is nie-lewensvatbare gisselle (dp = 5.059 m) gebruik as die
verspreide solide fase en oppervlakarea is bepaal. Hierdie oppervlakarea gedrag is
vergelyk met die van stelsels met inerte solides om die verskille met verandering
in solide tipes te verstaan.
In stelsels met inerte solides, is 'n line^ere verwantskap gevind tussen G en uG.
'n Empiriese korrelasie vir die voorspelling van hierdie gedrag is opgestel met
'n akkuraatheid van 83.34% in die eksperimentele reeks. Die oppervlakarea het 'n soortgelyke verhouding met uG en die empiriese korrelasie verskaf 'n akkuraatheid
van 78,8% vir die voorspelling van oppervlakarea oor die eksperimentele
reeks.
In inerte solide dispersies, het die KLa toegeneem met uG as die gevolg van 'n
toename in grens oppervlak asook stygings in KL. 'n Toename in solides belading
het n aanvanklike styging in KLa aangedui, as gevolg van die invloed van die
vloeistof-film penetrasie op KL, gevolg deur 'n afname in KL op vastestowwe
ladings groter as 2.5 g/l, te danke aan diffusie blokkeer effekte.
In stelsels met gis dispersies, het die teenwoordigheid van benattings molekules
in die media samesmelting geïnhibeer tot 'n gis lading van ongeveer 3.5 g/l, en
het gelei tot 'n afname in D32. Bo hierdie gis lading, het die fyn gis partikels
die skynbare viskositeit van die verspreiding verhoog genoegsaam om die invloed
van benattings molekules te oorkom en die D32 te verhoog.
Die gedrag van G in gis dispersies was soortgelyk aan die van inerte solides en
dui op 'n lineêre toename met uG. Maar in gis dispersies, het die interaksie tussen
alkaan konsentrasie en gis lading 'n effense toename veroorsaak in die verstrooiing
viskositeit en dus in G. 'n Empiriese korrelasie is ontwikkel om G gedrag te
voorspel en het 'n akkuraatheid van 72,55% vir die eksperimentele verskeidenheid
beskou. Vergelyking van gis en inerte patrikel dispersies wys 'n 37.5% laer G
in gis dispersies in vergelyking met inerte vaste stowwe as 'n gevolg van die
skynbare viskositeit bekendgestel deur fyner vastestowwe partikels. Hierdie G
en D32 data het gelei tot 'n linere toename in grens oppervlak met uG met geen
beduidende invloed van alkaan konsentrasie en gis lading nie. Die oppervlakarea
was gemiddeld 6.7% laer as oppervlakarea gevind in inerte partikel dispersies as
'n waarskynlike gevolg van die skynbare viskositeit met fyner partikels.
Hierdie studie bied 'n fundamentele begrip van die veranderlikes wat die suurstof
oordrag definieer in 'n model koolwaterstof bioproses BCR onder diskrete hidrodinamiese
voorwaardes. Hierdie fundamentele begrip bied n basis vir verdere
ondersoek van koolwaterstof bioprosesse en en die voorspelling van KLa gedrag
in hierdie stelsels.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96981 |
| Date | 03 1900 |
| Creators | Cloete, Jannean Christelle |
| Contributors | Clarke, Kim Gail, 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 | English |
| Type | Thesis |
| Format | 175 pages : illustrations |
| Rights | Stellenbosch University |
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