ENGLISH ABSTRACT: Carbon dioxide (CO2) is classified as the main greenhouse gas (GHG) contributing to global warming.
Estimates by the Intergovernmental Panel on Climate Change (IPCC) suggest that CO2 emissions must be
reduced by between 50 to 85% by 2050 to avoid irreversible impacts. Carbon capture and storage (CCS)
strategies can be applied to de-carbonize the emissions from fossil-fueled power plants. Compared to
other CCS techniques, post-combustion capture (PCC) is most likely to be implemented effectively as a
retrofit option to existing power plants. At present however CCS is not yet commercially viable. The
main challenge with CCS is to reduce the inherent energy penalty of the CO2 separation stage on the
host plant.
Seventy-five to eighty percent of the total cost of CCS is associated with the separation stage. There are
several technologies available for separating CO2 from power plant flue gas streams. Reactive absorption
with aqueous amine solutions has the ability to treat low concentration, low pressure and large flux flue
gas streams in industrial-scale applications. It is most likely to be the first technology employed
commercially in the implementation of CCS. The energy required for solvent regeneration however, is
high for the standard solvent used in reactive absorption processes, i.e. MEA. This leads to a reduction in
thermal efficiency of the host plant of up to 15%. Alternative solvent formulations are being evaluated in
an attempt to reduce the energy intensity of the regeneration process.
The main objective of this study was to establish a novel, simplified thermodynamic method for solvent
screening. Partial solubility parameters (PSPs) were identified as the potential basis for such a method.
The major limitation of this approach is that the model doesn’t account for effects from chemical
reaction(s) between materials, e.g. CO2 reacting with aqueous alkanolamine solutions; considering only
the effects from dissolution. The EquiSolv software system was developed based on PSP theory. The
Hansen 3-set PSP approach was used to describe the equilibrium behaviour of CO2 absorbing in task
specific solvents. The Hansen theory was expanded to a 4-set approach to account for contributions
from electrostatic interactions between materials. The EquiSolv program was used successfully to screen
large sets of solvent data (up to 400 million formulations) in the search for suitable alternative solvent
formulations for CO2 absorption.
The secondary objective of this study was to evaluate the ability of the proposed PSP model to
accurately predict suitable alternative solvents for CO2 absorption through preliminary experimental
work. A series of CO2 absorption experiments were conducted to evaluate the absorption performance
of predicted alternative solvent formulations. The predicted alternative solvent formulations exhibited a
significant improvement in absorption performance (up to a 97% increase in the measured absorption
capacity) compared to conventional solvent formulations. Statistical analysis of the experimental results
has shown that there is a statistically significant concordant relationship between the predicted and
measured rankings for the absorption performance of the predicted solvent formulations. Based on this
it was concluded that PSP theory can be used to accurately predict the equilibrium behaviour of CO2
absorbing in task specific solvents.
Recently ionic liquids (ILs) have been identified as potential alternatives to alkanolamine solutions
conventionally used for CO2 absorption. Absorption experiments were conducted as a preliminary
assessment of the absorption performance of ILs. Results have shown ILs to have significantly improved
performance compared to conventional alkanolamine solvents; up to a 96% increase in the measured
absorption capacity compared to conventional solvents. Future work should focus on developing task
specific ionic liquids (TSILs) in an attempt to reduce the energy intensity of solvent regeneration in CO2
absorption processes.
AFRIKAANSE OPSOMMING: Koolsuurgas (CO2) word geklassifiseer as die vernaamste kweekhuis gas (GHG) wat bydra to globale
verwarming. Beramings deur die Interregeringspaneel oor Klimaatsverandering (IPKV) toon aan dat CO2
emissies teen 2050 verminder moet word met tussen 50 en 85% om onomkeerbare invloede te vermy.
Verskeie koolstof opvangs en bergings (KOB) strategieë kan toegepas word ten einde die koolstof
dioksied konsentrasie in die emissies van kragstasies wat fossielbrandstowwe gebruik, te verminder. Naverbranding
opvangs (NVO) is die mees aangewese KOB tegniek wat effektief toegepas kan word op
bestaande kragstasies. Tans is KOB egter nog nie kommersieël lewensvatbaarvatbaar nie. Die hoof
uitdaging wat KOB in die gesig staar is om die energie boete inherent aan die CO2 skeidingstap te
verminder.
Tussen vyf-en-sewentig en tagtig persent van die totale koste van KOB is gekoppel aan die skeidingstap.
Daar is verskeie metodes beskikbaar vir die skeiding van CO2 uit die uitlaatgasse van kragstasies.
Reaktiewe absorpsie met waterige oplossings van amiene kan gebruik word om lae konsentrasie, lae
druk en hoë vloei uitlaatgasstrome in industriële toepassings te behandel. Dit is hoogs waarskynlik die
eerste tegnologie wat kommersieël aangewend sal word in die toepassing van KOB. Die oplosmiddel wat
normalweg vir reaktiewe absorpsie gebruik word (d.w.s. MEA) benodig egter ‘n groot hoeveelheid
energie vir regenerasie. Dit lei tot ‘n afname in die termiese doeltreffendheid van die voeder aanleg van
tot 15%. Alternatiewe oplosmiddelstelsels word tans ondersoek in ‘n poging om the energie intensiteit
van die regenerasieproses te verminder.
Die hoof doelwit van hierdie studie was om ‘n nuwe, ongekompliseerde termodinamiese metode te
vestig vir die keuring van alternatiewe oplosmiddels. Parsiële oplosbaarheidsparameters (POPs) is
geïdentifiseer as ‘n moontlike grondslag vir so ‘n metode. Die model beskryf egter slegs die ontbindings
gedrag van materiale. Die effekte van chemise reaksie(s) tussen materiale, bv. die tussen CO2 en
waterige oplossings van alkanolamiene, word nie in ag geneem nie. Die POP teorie het gedien as
grondslag vir die ontwerp van die EquiSolv sagteware stelsel. Die Hansen stel van drie POPs is gebruik
om die ewewigsgedrag te beskryf van CO2 wat absorbeer in doelgerig-ontwerpte oplosmiddels. Die
Hansen teorie is verder uitgebrei na ‘n stel van vier POPs om die bydrae van elektrostatiese wisselwerking tussen materiale in ag te neem. Die EquiSolv program is verskeie kere met groot sukses
gebruik vir die sifting van groot stelle data (soveel as 400 miljoen formulasies) in die soektog na
alternatiewe oplosmiddels vir CO2 absorpsie.
Die sekondêre doelwit van die studie was om die vermoë van die voorgestelde POP model om geskikte
alternatiewe oplosmiddels vir CO2 absorpsie akkuraat te voorspel, te ondersoek deur voorlopige
eksperimentele werk. ‘n Reeks CO2 absorpsie eksperimente is gedoen ten einde die absorpsie
werkverrigting van die voorspelde alternatiewe oplosmidels te ondersoek. ‘n Verbetering in absorpsie
werkverrigting van tot 97% is gevind vir die voorspelde oplosmiddels vergeleke met die van
oplosmiddels wat tipies in die industrie gebruik word. Statistiese ontleding van die eksperimentele
resultate het getoon dat daar ‘n beduidende ooreenstemming tussen die voorspelde en gemete
rangskikking van die voorspelde oplosmiddels se werkverrigting bestaan. Dus kan POP teorie gebruik
word om die absorpsie van CO2 in doelgerig-ontwerpte oplosmiddels akkuraat te beskryf.
Ioniese vloeistowwe (IVs) is onlangs geïdentifiseer as moontlike alternatiewe oplosmidels vir die
alkanolamien oplossings wat normaalweg gebruik word vir CO2 absorpsie. Absorpsie eksperimente is
gedoen ten einde ‘n voorlopige raming van die absorpsie werkverrigting van IVs te bekom. Daar is
bevind dat IVs ‘n beduidende verbetering in werkverrigting toon in vergelyking met die alkanolamien
oplosmiddels wat normaalweg gebruik word. ‘n Verbetering in absorpsie werkverrigting van tot 96% is
gevind vir die voorspelde IV-bevattende oplosmiddels vergeleke met die van oplosmiddels wat tipies in
die industrie gebruik word. Die fokus van toekomstige navorsing moet val op die ontwikkeling van
doelgemaakte ioniese vloeistowwe (DGIVs) in ‘n poging om die energie intensiteit van oplosmiddel
regenerasie in CO2 absorpsie prosesse te verminder.
