Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Data on the process performance at different operating conditions are required to determine the
feasibility of a separation process. Such data can be experimentally measured, but due to the time and
costs associated with pilot plant scale experiments, the use of predictive process models are often
preferred. The main aim of this project is to establish a working process model in Aspen Plus® that
can be used to predict the separation performance of a supercritical fluid fractionation process aimed
at the separation of mixtures of detergent range alkanes and alcohol isomers where similar boiling
points or low relative volatilities can occur.
Currently, an azeotropic distillation process is employed for the separation of detergent range
alkanes and alcohols. Although this process shows good separation performance, some concerns
regarding the operating conditions are raised: the preferred entrainer, diethylene glycol, is toxic to
humans; very low operating pressures of 0.016 – 0.031 MPa and high temperatures of 473 K are
required; additional processing units and materials are required to remove the entrainer from the
product streams. An alternative process, supercritical fluid fractionation, is proposed in this work
after previous studies have reported that this process have potential for the separation of alkanes and
alcohols. The supercritical fluid fractionation process addresses the concerns of the azeotropic
distillation process in the following ways: a non-toxic solvent, CO2, is used as the separating agent;
mild temperatures of 344 K is proposed, but at the cost of the low operating pressures of the
azeotropic process; and a single process unit and no additional material is required to separate the
solvent from the product streams. A process model was developed in Aspen Plus® to evaluate the separation performance of the
newly proposed supercritical fluid fractionation process and compare it to the current azeotropic
distillation process. The development of the process model included the development of an accurate
thermodynamic model in Aspen Plus®. After thorough evaluation of a number of cubic equations of
state, the RK-ASPEN model was found to be superior in its representation and prediction of phase
transition pressures for multi-component mixtures of detergent range alkanes and alcohols in the
temperature range 318 – 348 K. Phase transition pressures could be predicted with an error of less
than 6 % with the inclusion of regressed polar parameters and binary solute-solvent interaction
parameters for two multi-component mixtures: CO2 + (20 % n-dodecane + 70 % 1-decanol + 10 % 3,7-dimethyl-1-octanol) and CO2 + (25 % n-decane + 25 % 1-decanol + 25 % 3,7-dimethyl-1-octanol
+ 25 % 2,6-dimethyl-2-octanol).
Polar parameters were regressed from pure component vapour pressure data predicted with
correlations available in Aspen Plus®. Binary interaction parameters were regressed from
experimental bubble and dew point data. Binary bubble and dew point data were measured for a
number of systems containing ethane or CO2 and a C10-alkane or C10-alcohol isomer at temperatures
between 308 K and 353 K, and compositions ranging between 0.01 and 0.7 mass fraction solute. A
comparison between the phase equilibrium data measured for these systems revealed that the structure
of the molecule, and not only the molecular weight, influences its solubility in the supercritical
solvent. The phase transition pressures of n-decane, 2-methylnonane, 3-methylnonane and
4-methylnonane did not differ significantly in CO2 or ethane, and these compounds will in all
likelihood not be separated in a supercritical fluid fractionation process. The phase transition
pressures measured for the C10-alcohol isomers decreased in both CO2 and ethane in the following
order: 1-decanol, 3,7-dimethyl-1-octanol, 2-decanol, 2,6-dimethyl-2-octanol and
3,7-dimethyl-3-octanol. The position of the hydroxyl group and the number, length and position of
the side branches, all influence the solubility behaviour and phase transition pressures of the isomeric
alcohols in the supercritical solvent. Since the use of ethane did not show any significant benefits
with regard to selectivity, the use of the less harmful and less expensive solvent, CO2, in further
investigations was justified. The RK-ASPEN thermodynamic model, with the inclusion of the regressed polar and binary
solute-solvent interaction parameters, was implemented in the process model and the separation
performance of the process was simulated at different operating conditions for the CO2 +
(25 % n-decane + 25 % 1-decanol + 25 % 3,7-dimethyl-1-octanol + 25 % 2,6-dimethyl-2-octanol)
mixture. A comparison to experimental pilot plant data revealed that the model cannot be used to
predict the separation performance at low fractionation temperatures (316 K) due to shortcomings in
the thermodynamic model. However, the performance of the process at high fractionation
temperatures (344 K) could be predicted well, with an error of 10 – 36 %. Simulations for the CO2 +
(25 % n-decane + 25 % 1-decanol + 25 % 3,7-dimethyl-1-octanol + 25 % 2,6-dimethyl-2-octanol) and
CO2 + (20 % n-dodecane + 70 % 1-decanol + 10 % 3,7-dimethyl-1-octanol) mixtures showed that the
composition of the feed mixture have a significant effect on the location and size of the operating
window and optimum operating conditions. The optimum operating conditions were defined as the
conditions where an acceptable selectivity ratio and alcohol recovery occurred simultaneously. Since the selectivity ratio and alcohol recovery have opposing optimization approaches, a number of
possible optimum operating conditions exist, based on the product specifications. When an alcohol
and an alkane with similar phase behaviour exist in a mixture, a distinct minimum selectivity ratio
will occur at a point within the extract-to-feed ratio limits of the process. When the alkanes and
alcohols present in a mixture do not have similar or overlapping phase transition pressures, the
minimum selectivity ratio will typically cover a small range of extract-to-feed ratios at the high end
limit of the extract-to-feed ratio range.
To summarize: A process model was established in Aspen Plus® that can be used to determine
the feasibility and separation performance of a supercritical fractionation process for a feed mixture of
detergent range alkane and alcohol isomers. The model was used to prove that an SFF process is a
feasible alternative process to consider for the removal of alkanes from mixtures of detergent range
alcohol isomers, even where overlapping boiling points or low relative volatilities occur. During the
development of the process model, the following significant novel contributions were made:
· New phase equilibrium data were measured for C10-alkane and C10-alcohol isomers in
supercritical ethane, as published in The Journal of Supercritical Fluids 58 (2011) 330 –
342.
· New phase equilibrium data were measured for C10-alkane and C10-alcohol isomers in
supercritical CO2, as published in The Journal of Supercritical Fluids 59 (2011) 14 – 26.
· A thermodynamic model was developed in Aspen Plus® that can accurately predict the
phase transition pressures of binary, ternary and multi-component mixtures of detergent
range alkanes and alcohols in supercritical CO2, as published in The Journal of
Supercritical Fluids 84 (2013) 132 – 145.
· A process model was developed in Aspen Plus® that can be used to predict the separation
performance of a supercritical fluid fractionation process for the separation of mixtures of
detergent range alkanes and alcohols.
· Experimental and simulated results indicated that a supercritical fluid fractionation process
can be implemented successfully to separate an alkane from a mixture of alcohol isomers,
as was shown for two mixtures: CO2 + (25 % n-decane + 25 % 1-decanol + 25 %
3,7-dimethyl-1-octanol + 25 % 2,6-dimethyl-2-octanol) and CO2 + (20 % n-dodecane +
70 % 1-decanol + 10 % 3,7-dimethyl-1-octanol). / AFRIKAANSE OPSOMMING: Data oor die omvang van skeiding by verskillende bedryfstoestande word benodig om die
lewensvatbaarheid van ’n skeidingsproses te bepaal. Sulke data kan eksperimenteel gemeet word,
maar as gevolg van die tyd en kostes geassosieer met eksperimente op loodsaanlegskaal, word die
gebruik van prosesmodelle verkies. Die hoofdoel van hierdie projek is om ’n werkende prosesmodel,
wat daarop gemik is om C8 – C20 alkane en alkohol isomere te skei, in Aspen Plus® tot stand te bring
om die omvang van die skeiding van ’n superkritiese fraksioneringsproses te meet.
Tans word azeotropiese distillasie gebruik vir die skeiding van C8 – C20 alkane en alkoholisomere.
Alhoewel goeie skeiding met hierdie proses bewerkstellig word, is daar sekere eienskappe
van die proses wat aandag vereis: die voorgestelde skeidingsagent, dietileen glikol, is giftig vir mense;
baie lae bedryfsdrukke van 0.016 – 0.031 MPa en hoë temperature van 473 K word benodig;
addisionele proseseenhede en materiaal is nodig om die skeidingsagent van die produkte te verwyder.
Die gebruik van ’n alternatiewe proses - superkritiese fraksionering - word in hierdie werk voorgestel
nadat vorige studies getoon het dat hierdie proses die potensiaal het om alkane en alkohole te skei.
Die superkritiese fraksioneringsproses spreek al die kommerwekkende eienskappe van azeotropiese
distillasie aan soos volg: ’n veilige oplosmiddel, CO2, word as die skeidingsagent gebruik;
gemiddelde temperature van 344 K word voorgestel, maar ten koste van lae bedryfsdrukke; ’n enkele
proseseenheid en geen addisionele materiaal word benodig om die oplosmiddel van die produkte te
skei nie. ’n Prosesmodel is in Aspen Plus® ontwikkel om die omvang van die skeiding wat deur die
voorgestelde superkritiese fraksioneringsproses teweeggebring is, te evalueer en te vergelyk met die
azeotropiese distillasieproses wat tans in gebruik is. Die ontwikkeling van die prosesmodel sluit die
ontwikkeling van ’n akkurate termodinamiese model in Aspen Plus® in. Na deeglike evaluasie van ’n
aantal kubiese toestandsvergelykings is gevind dat die RK-ASPEN-model die faseoorgangsdrukke
van multi-komponentmengsels van C8 – C20 alkane en alkohole die beste voorspel binne die
temperatuurbereik van 318 – 348 K. Faseoorgangsdrukke kon voorspel word met ’n fout van minder
as 6 % met die insluiting van voorafbepaalde polêre parameters en binêre interaksie-parameters vir
twee multi-komponentmengsels: CO2 + (20 % n-dodekaan + 70 % 1-dekanol + 10 % 3,7-dimetiel-1-oktanol) and CO2 + (25 % n-dekaan + 25 % 1-dekanol + 25 %
3,7-dimetiel-1-oktanol + 25 % 2,6-dimetiel-2-oktanol).
Polêre parameters is bepaal met dampdruk data, wat voorspel is met korrelasies in Aspen Plus®.
Binêre interaksieparameters is van eksperimentele faseoorgangsdata bepaal. Binêre faseoorgangsdata
is vir ’n aantal sisteme wat uit etaan of CO2 en ’n C10-alkaan- of C10-alkohol-isomeer bestaan, gemeet
by temperature tussen 308 K en 353 K en samestellings van tussen 0.01 en 0.7 massafraksie van die
opgeloste stof. ’n Vergelyking tussen die gemete fase-ewewigsdata het onthul dat die struktuur van
die molekuul, en nie net die molekulêre massa nie, die oplosbaarheid van die stof in die superkritiese
oplosmiddel beïnvloed. Die faseoorgangsdrukke van n-dekaan, 2-metielnonaan, 3-metielnonaan en
4-metielnonaan het geen skynbare verskille getoon in etaan of CO2 nie en dus sal hierdie stowwe in
alle waarkynlikheid nie met ’n superkritiese fraksioneringsproses geskei kan word nie. Die
faseoorgangsdrukke wat vir die C10-alkohol gemeet is, het in beide etaan en CO2 afgeneem in die
volgende volgorde: 1-dekanol, 3,7-dimetiel-1-oktanol, 2-dekanol, 2,6-dimetiel-2-oktanol en
3,7-dimetiel-3-oktanol. Die posisie van die hidroksielgroep en die aantal, lengte en posisie van die
sytakke beïnvloed die oplosbaarheidsgedrag van die alkohol-isomere in die superkritiese oplosmiddel.
Aangesien die gebruik van etaan nie enige voordele ten opsigte van selektiwiteit inhou nie, is die
gebruik van die minder skadelike en goedkoper oplosmiddel, CO2, vir verdere ondersoeke
geregverdig. Die ontwikkelde termodinamiese model, met die insluiting van die polêre parameters en binêre
interaksieparameters, is in die prosesmodel ingesluit en die omvang van die skeiding van die proses is
gesimuleer by verskillende bedryfstoestande vir die CO2 + (25 % n-dekaan + 25 % 1-dekanol + 25 %
3,7-dimetiel-1-oktanol + 25 % 2,6-dimetiel-2-oktanol) mengsel. ’n Vergelyking tussen die
gesimuleerde data en die eksperimentele loodsaanlegdata het onthul dat die model nie die omvang van
die skeiding kan voorspel by lae fraksioneringstemperature (316 K) nie as gevolg van die
tekortkominge in die termodinamiese model. Die omvang van die skeiding by hoë temperature
(344 K) kon egter goed voorspel word met ’n fout van 10 – 36 %. Simulasies van die CO2 + (25 %
n-dekaan + 25 % 1-dekanol + 25 % 3,7-dimetiel-1-oktanol + 25 % 2,6-dimetiel-2-oktanol) en CO2 +
(20 % n-dodekaan + 70 % 1-dekanol + 10 % 3,7-dimetiel-1-oktanol) mengsels het getoon dat die
samestelling van die voermengsel ’n beduidende effek op die grootte van die bedryfsvenster en
optimum bedryfstoestande het. Die optimum bedryfstoestande word gedefinieer as die toestande waar
’n aanvaarbare selektiwiteitsverhouding en alkoholherwinning terselfdertyd voorkom. Aangesien die
selektiwiteitsverhouding en alkoholherwinning teenstrydige optimeringsbenaderings het, bestaan daar ’n aantal optimum bedryfstoestande gebaseer op die produkspesifikasies. Wanneer ’n alkohol en ’n
alkaan met ooreenstemmende fasegedrag saam in ’n mengsel voorkom, bestaan daar ’n duidelike
minimum selektiwiteitsverhouding by ’n punt binne die ekstrak-tot-voer-verhoudingslimiete van die
proses. Wanneer die alkane en alkohole in ’n mengsel nie ooreenstemmende fasegedrag toon nie, sal
die minimum selektiwiteitsverhouding oor ’n reeks ekstrak-tot-voer-verhoudings voorkom, tipies by
die hoë limiet van die ekstrak-tot-voer-verhoudingsreeks.
Om op te som: ’n Prosesmodel is in Aspen Plus® tot stand gebring wat die lewensvatbaarheid
en omvang van die moontlike skeiding van ’n superkritiese fraksioneringsproses vir voermengsels van
C8 – C20 alkane en alkohol-isomere kan voorspel. Die model is gebruik om te bewys dat ’n
superkritiese proses ’n lewensvatbare alternatiewe proses is om te oorweeg vir die verwydering van
alkane uit mengsels van alkohol-isomere, self waar ooreenstemmende kookpunte of lae relatiewe
vlugtigheid tussen komponente voorkom. Tydens die ontwikkeling van die prosesmodel is die
volgende beduidende nuwe bydraes gemaak:
· Nuwe fase-ewewigsdata is gemeet vir C10-alkaan- en C10-alkohol-isomere in superkritiese
etaan, soos gepubliseer in The Journal of Supercritical Fluids 58 (2011) 330 – 342.
· Nuwe fase-ewewigsdata is gemeet vir C10-alkaan and C10-alkohol isomere in superkritiese
CO2, soos gepubliseer in The Journal of Supercritical Fluids 59 (2011) 14 – 26.
· ’n Termodinamiese model is ontwikkel in Aspen Plus® wat die faseoorgangsdrukke van
binêre, ternêre en multi-komponent mengsels van C8 – C20 alkane en alkohol-isomere in
superkritiese CO2 akkuraat kan voorspel, soos gepubliseer in The Journal of Supercritical
Fluids 84 (2013) 132 – 145.
· ’n Prosesmodel is ontwikkel in Aspen Plus® wat die omvang van die moontlike skeiding
van ’n superkritiese fraksioneringsproses, gemik op die skeiding van mengsels van C8 –
C20 alkane en alkohol-isomere, kan voorspel.
· Eksperimentele en gesimuleerde resultate toon aan dat ’n superkritiese
fraksioneringsproses suksesvol geïmplementeer kan word vir die skeiding van ’n alkaan
vanuit ’n mengsel van alkohol-isomere, soos bewys vir twee mengsels: CO2 + (25 %
n-dekaan + 25 % 1-dekanol + 25 % 3,7-dimetiel-1-oktanol + 25 % 2,6-dimetiel-2-oktanol)
en CO2 + (20 % n-dodekaan + 70 % 1-dekanol + 10 % 3,7-dimetiel-1-oktanol).
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/86434 |
| Date | 04 1900 |
| Creators | Zamudio, Michelle |
| Contributors | Knoetze, J. H., Schwarz, C. E., 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 | xvi, 322, clxxiii p. : ill. |
| Rights | Stellenbosch University |
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