Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Bioconversion of hydrocarbons to value-added intermediates and products has significant industrial
potential using both prokaryotic and eukaryotic organisms. In particular, alkanes can
be converted to an expansive range of commercially important products using aerobic bioprocesses
under mild process conditions. Coupled with the relative abundance of alkanes derived
from gas to liquid (GTL) technologies, such as those employed by SASOL, South Africa, the
commercial potential for bioconverison of alkanes is large. However, unlike carbohydrate substrates,
alkane feedstocks are devoid of oxygen in their molecular structure. This means that
the entire oxygen demand needs to be met by oxygen transfer. Furthermore, a decline in oxygen
transfer in aqueous-hydrocarbon dispersions with increasing alkane concentration has been
observed to result from depression of the overall volumetric oxygen transfer coefficient (KLa).
Therefore, understanding KLa and the fundamental parameters underpinning its behaviour is
critical to ensuring the bioprocess is kinetically, rather than transport, limited in terms of both
operation and scale-up.
Previous studies have examined KLa in aerated-alkane-aqueous systems. In light of the importance
of oxygen transfer in bioprocesses, this study expands on the KLa understanding in
3-phase studies by including a fourth solid phase, thus more closely representing a hydrocarbonbased
bioprocess. The project aimed to determine the impact of agitation, alkane concentration
and solid loading on the Sauter mean bubble diameter (DSM), gas hold-up and specific interfacial
area (a) and correlate these parameters to KLa. This ultimately determined which parameter
was dominant over a range of process conditions. Furthermore, concurrent measurement of the
KLa and interfacial area meant the behaviour of the liquid side oxygen transfer coefficient (KL)
could be defined, providing further insight into how changes in the process conditions impact
on KLa.
Experiments were conducted in a 5 litre stirred tank bioreactor containing n-C14-20 straight chain
alkane, sparged with air at 0.8 vvm. In line with process conditions typical of a hydrocarbonbased
bioprocess, KLa and a were measured for agitation rates from 450 to 1000 RPM, alkane
concentrations from 2 to 20% v/v and yeast solids from 1 to 10 g/l. KLa was measured using
the gassing out procedure using a dissolved oxygen (DO) probe which measured the response
of the system to a step change in the sparge gas oxygen pressure. The probe response lag ( P), equal to the time taken for the probe to reach 63.2% of the saturation DO concentration, was
determined for every set of process conditions. The inverse of P, KP was taken into account
when calculating KLa from the DO probe response. The area was calculated from DSM and gas
hold-up. DSM was quantified using high speed photography and image analysis was performed
in Matlab® using bespoke routines. Elimination of optical distortion and the development of
an adequate light source was key to acquiring clear images.
Both KLa and interfacial area were found to be affected by changes in agitation, alkane concentration
and yeast loading. An increase in agitation increased the KLa over the entire range
of alkane concentration and yeast loading. Similarly, an increase in agitation resulted in an
increase in interfacial area, underpinned by a decrease in the DSM. It is therefore likely that the
interfacial area plays a dominant role in defining KLa when considering an increase in agitation.
Increases in alkane concentration resulted in a peak in KLa between 2.5 and 5% alkane
concentration while further increases in alkane concentration depressed KLa. This peak was
not observed in interfacial area, where an increase in alkane concentration resulted only in a
decrease in interfacial area, thus indicating a positive influence of KL on KLa at low alkane
concentrations. Further increases in alkane concentration beyond those creating the peak KLa
resulted in KLa depression, suggesting that the increasing viscosity imparted by the alkane decreases
both KL and interfacial area.
Increased yeast loading had opposing effects at low and high agitation rates. At low agitation
rates, increased loadings were observed to increase KLa, while increased loadings at high
agitation rates caused a decrease in KLa. This behaviour was also evident in interfacial area,
suggesting that in this regime KLa was defined by interfacial area behaviour.
Increased yeast loading was observed to depress the KLa for all alkane concentrations when
examined at a constant midpoint agitation rate. This trend was not evident in interfacial area,
which increased with increasing yeast loading at the same agitation rate. The positive influence
of yeast on interfacial area was likely caused by adhesion of the yeast particles to the bubble
surface, lowering the DSM by preventing coalescence. The disagreement between the KLa and
interfacial area results suggested that yeast loading impacted negatively on KL, which had an
over-riding negative impact on KLa.
The use of reliable methods for the determination of both interfacial area and KLa were demonstrated
for application in model hydrocarbon-based bioprocesses. The combined results offer
a unique insight into how changes in the process conditions impact independently on KL and
interfacial area, which when combined ultimately defined the KLa behaviour. Quantification of
the relative magnitude of the impact each parameter had on KLa contributed toward a fundamental
understanding of oxygen transfer in model hydrocarbon-based bioprocesses. / AFRIKAANSE OPSOMMING: Biologiese omsetting van koolwaterstowwe na produkte met finansiële waarde het beduidende
industriële potensiaal met behulp van beide prokariotiese en eukariotiese organismes. In die
besonder, kan alkane omgeskakel word na ’n uitgebreide reeks van kommersieel belangrike
produkte met behulp van aerobiese bioprosesse onder ligte proses voorwaardes. Tesame met die
relatiewe oorvloed van alkane afgelei van GTL tegnologie, soos dié van Sasol, Suid-Afrika, die
kommersiële potensiaal vir bioconverison van alkane is groot. Maar, in teenstelling koolhidrate
substrate, alkaan voerstowwe is beroof van suurstof in hul molekulêre struktuur. Dit beteken
dat die hele suurstof vereiste moet nagekom word deur suurstof oordrag. Verder het ’n afname
in suurstof oordrag in waterige-koolwaterstof dispersies met toenemende alkaan konsentrasie
waargeneem te lei van depressie van die algehele volumetriese suurstofoordragkoëffisiënt (KLa).
Daarom verstaan KLa en die fundamentele parameters onderliggend sy gedrag is van kritieke
belang om te verseker dat die bioprocess is kineties, eerder as vervoer, beperk in terme van
beide werking en skaal-up van bioprosesse.
Vorige studies het KLa in deurlug-alkaan-waterige stelsels ondersoek. In die lig van die belangrikheid
van suurstof oordrag in bioprosesse hierdie studie brei uit op die KLa begrip in driefase
studies deur die insluiting van ’n vierde soliede fase, dus meer nou wat ’n koolwaterstofgebaseerde
bioprocess. Die doel van die projek is om die impak van vermengingstempo, alkaan
konsentrasie en soliede inhout op die Sauter gemiddelde borrel deursnee (DSM), gas-vasvanging
en spesifieke gas-vloistof oppervlakarea (a) te kwantifiseer en korreleer met KLa gedrag. Dit
sou defineer die dominante parameter oor ’n verskeidenheid van proses voorwaardes. Verder,
gelyktydige meting van die KLa en oppervlakarea kan die gedrag van die vloeistof-kant suurstofoordragkoëffisiënt (KL) gedefinieer. Dit sal verskaf verdere insig in hoe die veranderinge in die
proses voorwaardes impak op KLa.
Eksperimente was uitgevoer in ’n 5 liter belugte geroerde tenk bioreaktor bevat n - C14-20 reguitketting
alkane, met lug met lug deurgeborrel by 0.8 VVM. In lyn met die proses voorwaardes
tipies van ’n koolwaterstof-gebaseerde bioprocess, KLa en a was gemeet vir vermengignstempos
van 450-1000 RPM, alkaan konsentrasies van 2-20 % v/v en gis vastestowwe van 1 tot 10
g / l. KLa is gemeet deur die vergassinguit prosedure met behulp van ’n suurstofmeter wat die
reaksie van die stelsel na ’n stap verandering in die voer gas suurstof druk gemeet het.
Die suurstofmeter reaksie vertraging ( P), gelyk aan die tyd wat dit neem vir die suurstofmeter
63.2 % van die versadiging DO konsentrasie te bereik, is bepaal vir elke procesopset. Die
inverse van P, KP is in ag geneem by die berekening van KLa uit die suurstofmeter reaksie. Die
gas-vloistof oppervlak is bereken vanaf DSM en gas hold-up. DSM is gekwantifiseer met behulp
van hoë spoed fotografie en beeld analise is uitgevoer in Matlab ® roetines. Uitskakeling
van optiese vervorming en die ontwikkeling van ’n voldoende ligbron was die sleutel tot die
verkryging van helder beelde.
Beide KLa en grens oppervlakarea gevind geraak word deur veranderinge in vermengignstempo,
alkaan konsentrasie en gis laai. ’N toename in geroer het die KLa verbeter oor die hele reeks
van alkaan konsentrasie en gis laai. Net so, ’n toename in geroer het gelei tot ’n toename in
grens oppervlak, ondersteun deur ’n afname in die DSM. Dit is dus waarskynlik dat die grens
oppervlak speel ’n dominante rol in die definisie van KLa by die oorweging van ’n toename in
roering. Stygings in alkaan konsentrasie gelei tot ’n hoogtepunt in KLa tussen 2.5 en 5 % alkaan
konsentrasie terwyl verdere verhogings in alkaan konsentrasie druk die KLa af. Die piek was
nie in oppervlakarea duidelik, waar ’n toename in alkaan konsentrasie gelei net tot ’n afname
in oppervlakarea, dus dui op ’n positiewe invloed van KL op KLa teen lae alkaan konsentrasies
waargeneem. Verdere stygings in alkaan konsentrasie verder as die skep van die piek
KLa gelei tot KLa depressie, wat daarop dui dat die toenemende viskositeit meegedeel deur die
alkaan verminder beide KL en grens oppervlak.
Verhoogde gis laai het opponerende effekte teen ’n lae en hoë vermengingstempo. By lae vermengingstempo,
’n verhoging in gis laai waargeneem KLa te verhoog, terwyl ’n verhoging in
gis laai op ’n hoë vermengingstempo veroorsaak ’n afname in KLa . Hierdie gedrag was ook
duidelik in grens oppervlak, wat daarop dui dat daar in hierdie regime KLa gedefinieer deur
grens oppervlak gedrag.
Verhoogde gis laai waargeneem die KLa te onderdruk vir alle alkaan konsentrasies wanneer
ondersoek teen ’n konstante middelpunt vermengingstempo. Hierdie tendens was nie duidelik
in tussenvlak gebied, wat verhoog met toenemende gis laai op dieselfde geroer koers. Die
positiewe invloed van gis op grens oppervlak is waarskynlik veroorsaak deur adhesie van die
gis deeltjies aan die borrel oppervlak, die verlaging van die DSM deur die voorkoming van
die saamsmelting van gasborrels. Die meningsverskil tussen die KLa en grens oppervlakarea
resultate voorgestel dat gis laai negatiewe uitwerking op KL, met ’n dominante negatiewe impak
op KLa.
Die gebruik van ’n betroubare metodes vir die bepaling van beide oppervlakarea en KLa gedemonstreer
vir toepassing in model koolwaterstof-gebaseerde bioprosesse. Die gekombineerde resultate
bied ’n unieke insig in hoe die veranderinge in die proses voorwaardes impak onafhanklik
op KL en oppervlakarea, wat wanneer gekombineer gedefinieer die KLa gedrag. Kwantifisering
van die relatiewe grootte van die impak elke parameter het op KLa bygedra tot ’n fundamentele begrip van suurstof oordrag in model koolwaterstof-gebaseerde bioprosesse.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96868 |
| Date | 03 1900 |
| Creators | Hollis, Peter Graham |
| 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 | 209 pages : illustrations |
| Rights | Stellenbosch University |
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