| Summary: | Thesis (PhD)--Stellenbosch University, 2012. === ENGLISH ABSTRACT: Second-generation bioethanol is an alternative transportation fuel currently being
investigated whereby cellulose, specifically lignocellulosic (woody) portions, of
any plant mass can be converted to ethanol. To date, the technology had only been
successfully implemented with demonstration scale facilities. Despite intensive
research efforts at laboratory scale, no-one is certain what the secondary effects of
scale-up to large systems are. The objective of this project was to develop threedimensional
numerical models of a laboratory scale fermenter which could predict
the effects of particulate mixing and reaction kinetics for future scale-up
investigations.
A numerical model of the reaction kinetics for simultaneous saccharification and
fermentation of Avicel (microcrystalline cellulose) particles to ethanol is
presented. The novelty of this model is the separation of the two primary
cellulase enzyme-kinetics, which generated the capability to predict the
heterogeneous behaviour of the enzyme-substrate interactions. This model
improves the understanding of these systems while maintaining sufficient
simplicity for implementation alongside a commercial computational fluid
dynamics environment.
Effects of the various fermentation medium constituents and the influence of each
on the dynamic viscosity of the medium were also investigated. Results indicated
that particle volume fraction had the dominant effect on the apparent dynamic
viscosity resulting in further research of the particle properties. Due to the
irregular shapes of Avicel particles, tests were conducted to determine drag and
settling behaviour, which led to the development and modification of models to
account for these phenomena. This investigation is unique as it allows a more
accurate calculation of particle transportation through a three-dimensional
environment including the effects of natural packing density. At lower particle
volume fraction the concentration of ethanol and glycerol had the greatest effect
on the apparent dynamic viscosity and was calculated from models obtained from
literature.
Validation of the physics and the incorporation thereof in the simulations resulted
in the modification of various generic models which either improved numerical
stability or accuracy, or both. Contributions included a modified form of the
pressure force model, which proved significantly more stable and accurate than
previous models proposed in literature. The models developed for capturing the
effects of particles on the apparent dynamic viscosity proved effective for this
specific substrate.
Results from cross-coupling the reaction models with computational fluid
dynamic simulations provide a novel approach to capturing the secondary effect
of substrate conversion and particle distribution on the performance of the
fermentation vessels. This is the first time where that biological reactions were successfully combined with particle dynamics and fluid flow fields to investigate
the secondary effects which occur in fermenters.
This work served as a foundation for future research and development within the
bioethanol field with significant potential for expansion into other biochemical
disciplines. === AFRIKAANSE OPSOMMING: Tweede-generasie bioetanol is ’n alternatiewe vervoerbrandstof wat tans
ondersoek word waar sellulose, spesifiek lignosellulosiese (houtagtige) gedeeltes,
van enige plantmassa na etanol omgesit kan word. Tot op hede was die
tegnologie slegs suksesvol geïmplimenteer in demonstrasieskaal fasiliteite. Ten
spyte van intensiewe navorsingpogings op laboratoriumskaal, is niemand seker
wat die sekondêre effekte van die opskaal tot groot stelsels sal wees nie. Die
doelwit van die projek was om drie-dimensionele modelle te ontwikkel van ’n
laboratoriumskaal fermentor wat die effekte van partikulêre vermenging en
reaksiekinetika kan voorspel vir toekomstige opskaal navorsing.
’n Numeriese model van die reaksiekinetika vir gelyktydige versuikering en
fermentasie van Avicel (mikrokristallyne sellulose) partikels tot etanol word
aangebied. Die oorspronklikheid van die model is geleë in die skeiding van die
twee primêre sellulase ensiemkinetika, wat lei tot die vermoë om die heterogene
gedrag van die ensiem-substraat interaksies te voorspel. Hierdie model verbeter
die kennis van die stelsels, terwyl voldoende eenvoud behoue bly vir
implementering parallel aan kommersiële berekeningsvloeidinamika sagteware.
Effekte van die verskillende bestanddele van die fermentasiemedium en die
invloed van elk op die dinamiese viskositeit van die medium is ook ondersoek.
Resultate dui aan dat partikel volume fraksie die dominante invloed op die
skynbare dinamiese viskositeit het, wat gelei het tot verdere ondersoek van die
partikel eienskappe. As gevolg van die onreëlmatige vorms van Avicel partikels,
is toetse gedoen om die sleur-en uitsakkingsgedrag te bepaal, wat gelei het tot die
ontwikkeling en aanpassing van modelle om hierdie verskynsels in ag te neem.
Hierdie ondersoek is uniek, want dit laat meer akkurate berekening van
partikelvervoer deur ’n drie-dimensionele omgewing toe, insluitend die effekte
van natuurlike verpakkingsdigtheid. By laer partikel volume fraksie het die
konsentrasie van etanol en gliserol die grootste effek op die skynbare dinamiese
viskositeit gehad en was bereken vanaf modelle in die literatuur.
Bevestiging van die fisika en die insluiting daarvan in die simulasies het gelei tot
die aanpasing van verskillende generiese modelle wat óf numeriese stabiliteit óf
akkuraatheid óf beide verbeter. Bydraes gemaak sluit ’n aangepaste vorm van die
drukkragmodel in, wat heelwat meer stabiel en akkuraat was as die vorige modelle
voorgestel in die literatuur. Die modelle wat ontwikkel is om die effek van
partikels op die skynbare viskositeit vas te vang, was effektief bewys vir hierdie
spesifieke substraat.
Resultate van die kruiskoppeling van inligting vanaf die reaksiemodelle met
berekeningsvloeidinamika simulasies lewer ’n nuwe benadering tot die bepaling
van die sekondêre effek van substraatomskakeling en partikeldistribusie op die
uitvoering van die fermentasie toestel. Hierdie is die eerste poging om biologiese reaksies met partikel dinamika en vloeivelde te kombineer om die sekondêre
effekte wat in fermenter plaasvind, te ondersoek.
Hierdie werk dien as ’n grondslag vir toekomstige navorsing en ontwikkeling
binne die bioetanolveld, met beduidende potensiaal vir uitbreiding na ander
biochemiese dissiplines.
|
|---|
