Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Hydrocarbons provide excellent feed stocks for bioconversion processes to produce
value added products using various micro-organisms. However, hydrocarbon-based
aerobic bioprocesses may exhibit transport problems where the bioconversion is
limited by oxygen supply rather than reaction kinetics. Consequently, the overall
volumetric oxygen transfer coefficient (KLa) becomes critical in designing, operating
and scaling up of these processes. In view of KLa importance in hydrocarbon-based
processes, this work evaluated KLa measurement methodologies as well as
quantification of KLa behavior in aerated agitated alkane-solid-aqueous dispersions.
A widely used KLa measurement methodology, the gassing out procedure (GOP) was
improved. This improvement was done to account for the dissolved oxygen (DO)
transfer resistances associated with probe. These resistances result in a lag in DO
response during KLa measurement. The DO probe response lag time, was
incorporated into the GOP resulting in the GOP (lag) methodology. The GOP (lag)
compared well with the pressure step procedure (PSP), as documented in literature,
which also incorporated the probe response lag time.
Using the GOP (lag), KLa was quantified in alkane-solid-aqueous dispersions, using
either inert compounds (corn flour and CaCO3) or inactive yeast cells as solids to
represent the micro-organisms in a hydrocarbon bioprocess. Influences of agitation,
alkane concentration, solids loading and solids particle sizes and their interactions on
KLa behavior in these systems were quantified.
In the application of an accurate KLa measurement methodology, the DO probe
response lag time was investigated. Factors affecting the lag, which included process
conditions such as agitation (600-1200rpm), alkane concentration (2.5-20% (v/v),
alkane chain length (n-C10-13 and n-C14-20), inert solids loading (1-10g/L) and solids
particle sizes (3-14μm) as well as probe characteristics such as membrane age and
electrolyte age (5 day usage), were investigated. Kp, the oxygen transfer coefficient of
the probe, was determined experimentally as the inverse of the time taken for the DO
to reach 63.2% of saturation after a step change in DO concentration. Kp dependence
on these factors was defined using 22 factorial design experiments. Kp decreased on
increased membrane age with an effect double that of Kp decrease due to electrolyte
age. Additionally, increased alkane concentration decreased Kp with an effect 7 times higher compared to that of Kp decrease due to increased alkane chain length. This
was in accordance to Pareto charts quantification.
KLa was then calculated, using the GOP (lag), according to equation [1] which
incorporates the influence of Kp. Equation 1 is derived from the simultaneous solution
of the models which describe the response of the system and of the probe to a step
change in DO.
1
1
*
L p
p
p K at K t
L
p p La
C
K e K ae
C K K
= - - - - -
[1]
The KLa values documented in literature from the PSP and KLa calculated by the
GOP (lag) showed only a 1.6% difference. However KLa values calculated by the
GOP (lag) were more accurate than KLa calculated by the GOP, with up to >40% error
observed in the latter according to t-tests analyses. These results demonstrated that
incorporating Kp markedly improved KLa accuracy. Consequently, the GOP (lag) was
chosen as the preferred KLa measurement methodology.
KLa was determined in n-C14-20-inert solid-aqueous dispersions. Experiments were
conducted in a stirred tank reactor with a 5L working volume at constant aeration of
0.8vvm, 22ºC and 101.3kPa. KLa behavior across a range of agitations (600-
1200rpm), alkane concentrations (2.5-20% (v/v)), inert solids loadings (1-10g/L) and
solids particle sizes (3-14μm) was defined using a 24 factorial design experiment. In
these dispersions, KLa increased significantly on increased agitation with an effect 5
times higher compared to that of KLa increase due to interaction of increased alkane
concentration and inert solids loading. Additionally, KLa decreased significantly on
increased alkane concentration with an effect 4 times higher compared to both that of
increased solids particle sizes and the interaction of increased agitation and solids
particle size.
In n-C14-20-yeast-aqueous dispersions, KLa was determined under narrowed process
conditions better representing typical bioprocess conditions. KLa behavior across a
range of agitations (600-900rpm), alkane concentrations (2.5-11.25% (v/v)) and yeast
loadings (1-5.5g/L) using a 5μm-yeast cell was defined using a 23 factorial design
experiment. In these dispersions, KLa increased significantly on increased agitation.
Additionally, KLa decreased significantly on increased yeast loading with an effect 1.2
times higher compared to that of KLa decrease due to interaction of increased alkane
concentration and yeast loading. In this study, the importance of Kp for accurate KLa measurement in alkane based
systems has been quantified and an accurate and less complex methodology for its
measurement applied. Further, KLa behavior in aerated alkane-solid-aqueous
dispersions was quantified, demonstrating KLa enhancement on increased agitation
and KLa depression on increased alkane concentration, solids loading and solids
particle sizes. / AFRIKAANSE OPSOMMING: Koolwaterstowwe dien as uitstekende voervoorraad vir ´n verskeidenheid van mikroorganismes
wat aangewend word in biologiese omsettingsprosesse ter vervaardiging
van waardetoevoegende produkte. Hierdie biologiese omsettingsprosesse word egter
vertraag weens die gebrek aan suurstoftoevoer onder aerobiese toestande. Die
tempo van omsetting word dus beheer deur die volumetriese
suurstofoordragkoeffisiënt (KLa) eerder as die toepaslike reaksiekinetika. Die bepaling
van ´n akkurate KLa word dus krities tydens die ontwerp en opskalering van hierdie
prosesse. Met dit in gedagte het hierdie studie die huidige metodes om KLa te bepaal
geëvalueer en die gedrag van KLa in goed vermengde en belugde waterige
alkaanmengsels met inerte vastestowwe, soos gisselle, in suspensie ondersoek.
´n Deesdae populêre metode om KLa te bepaal, die sogenaamde
gasvrylatingsprosedure (GOP) is in hierdie studie verbeter. Die verbetering berus op
die ontwikkeling van ´n prosedure om die suurstofoordragsweerstand van die pobe
wat die hoeveelheid opgeloste suurstof (DO) meet, in berekening te bring. Hierdie
weerstand veroorsaak ´n vertragin in the responstyd van die probe. Die verbeterde
metode, GOP (lag), vergelyk goed met die gepubliseerde resultate van die
drukstaptegniek (PSP) wat ook die responstyd in ag neem.
GOP (lag) is ingespan om KLa te gekwantifiseer vir waterige alkaan-vastestof
suspensies. Inerte componente soos mieliemeel, kalsiumkarbonaat en onaktiewe
gisselle het gedien as die vastestof in suspensie verteenwoordigend van die
mikroörganismes in ´n koolwaterstof bio-proses. Die invloed van vermengingstempo,
alkaan konsentrasie, vastestof konsentrasie en partikelgrootte asook die interaksie
van al die bogenoemde op KLa is kwatitatief bepaal in hierdie studie.
Faktore wat die responstyd van die DO probe beïnvloed is ondersoek. Hierdie faktore
is onder meer vermengingstempo (600-1200opm), alkaankonsentrasie (2.5-20%
(v/v)), alkaankettinglengte (n-C10-13 en n-C14-20), vastestofkonsentrasie (1-10g/L) en
partikelgrootte (3-14 μm). Faktore wat die eienskappe van die probe beïnvoed,
naamlik membraan-en elektrolietouderdom (5 dae verbruik), is ook ondersoek. Kp,
die suurstofoordragskoeffisiënt, is bepaal deur ´n inkrementele verandering in die
suurstofkonsentrasie van die mengsel te maak en die tyd vir 63.2% versadiging van
die probelesing te noteer. Die genoteerde tyd is die response tyd van die probe en
Kp, die inverse van hierdie tyd. Die afhanklikheid van Kp op die bogenoemde faktore is ondersoek in ´n 22 faktorieël ontwerpte reeks eksperimente. Kp toon ´n afname met
´n toename in membraanouderdom. Hierdie afname is dubbel in grootte as dit
vergelyk word met die afname relatief tot die toename in elektrolietouderdom. Verder
toon Kp ´n afname met ´n toename in alkaankonsentrasie. Hierdie afname is 7 keer
groter relatief tot die afname gesien met die toename in alkaan kettinglengte. Hierdie
is in goeie ooreenstemming met Pareto kaarte as kwantifiseringsmetode.
KLa is bereken met die inagname van Kp volgens vergelyking [1]:
1
1
*
L p
p
p K at K t
L
p p La
C
K e K ae
C K K
= - - - - -
[1]
Vergelyking [1] is afgelei vanaf die gelyktydige oplossing van die bestaande modelle
wat die responstyd van die pobe vir ´n stapverandering in DO bereken.
Die KLa waardes van die PSP metode uit literatuur verskil in die orde van 1.6% van
dié bereken deur vergelyking [2]. Hierdie verskil is weglaatbaar. Die KLa waardes
verkry uit die GOP metode wat nie Kp in berekening bring nie, verskil met meer as
40% van die huidige, verbeterde metode volgens die statistiese t-test analiese. Dit
bewys dat die inagname van Kp ´n merkwaardige verbetering in die akuraatheid van
KLa teweeg bring. GOP (lag) kry dus voorkeur vir die berekening van KLa verder aan
in hierdie studie.
KLa is bereken vir n-C14-20-water mengsels met inerte vastestofsuspensies. Die
eksperimente is uitgevoer in ´n 5L geroerde reaktor met ´n konstante belugting van
0.8vvm (volume lug per volume supensie per minuut), 22ºC en 101.3kPa. Die gedrag
van KLa met betrekking tot vermengingstempo (600-1200opm), alkaankonsentrasie
(2.5-20% (v/v)), vastestofkonsentrasie (1-10g/L) en partikelgrootte (3-14μm) is
ondersoek in ´n 24 faktorieël ontwerpte reeks eksperimente. Verder is die invloed van
vloeistofviskositeit en oppervlakspanning op KLa ondersoek in ´n 23 faktorieël
ontwerpte reeks eksperimente. KLa het ´n beduidende toename getoon met ´n
toename in vermengingstempo. Hierdie toename was 5 keer groter as die toename
relatief tot die interaksie van alkaan-en vastestofkonsentrasie. KLa het ook beduidend
afgeneem met ´n toename in alkaankonsentrasie. Die toename was 4 keer groter as
die toename relatief tot die toename in partikelgrootte en die interaksie van
vermengingstempo en partikelgrootte.
In n-C14-20-water mengsels met gisselsuspensies is KLa bepaal onder kondisies
verteenwoordigend van tipiesie biologiese omsettingsprosesse. Die gedrag van KLa met betrekking tot vermengingstempo (600-900opm), alkaankonsentrasie
(2.5-11.25% (v/v)) en giskonsentrasie (1-5.5g/L) met ´n partikelgroote van 5μm is
ondersoek in ´n 23 faktorieël ontwerpte reeks eksperimente. Hierdie eksperimente het
´n beduidende toename in KLa met ´n toename in vermengingstempo getoon sowel
as ´n beduidende afname met ´n toename in giskonsentrasie. Hierdie afname is in
die orde van 1.2 keer groter in vergelyking met die interaksie van alkeen- en
giskonsentrasie.
Hierdie studie bring die kritieke rol wat Kp speel in die akkurate bepaling van KLa in
waterige alkaansisteme met inerte vastestofsuspensies na vore. Dit stel verder ´n
metodiek voor vir die akurate meting van en kwantifisering van beide Kp en KLa onder
aerobiese toestande met betrekking tot vermengingstempo, alkaankonsentrasie,
vastestofkonsentrasie en partikelgrootte.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/5329 |
| Date | 12 1900 |
| Creators | Manyuchi, Musaida Mercy |
| Contributors | Clarke, K. G., University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 148 p. : ill. |
| Rights | University of Stellenbosch |
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