The Separation of Detergent Range Alkanes and Alcohol Isomers with Supercritical Carbon Dioxide

513 Pages·2014·6.21 MB·English

The Separation of Detergent Range
Alkanes and Alcohol Isomers with
Supercritical Carbon Dioxide

by

Michelle Zamudio

Dissertation presented for the Degree

of

DOCTOR OF PHILOSOPHY
(Chemical Engineering)

in the Faculty of Engineering
at Stellenbosch University

Prof. J.H. Knoetze

Dr. C.E. Schwarz

April 2014
Stellenbosch University http://scholar.sun.ac.za
Stellenbosch University http://scholar.sun.ac.za
D
ECLARATION

By submitting this dissertation electronically, I declare that the entirety of the work contained
therein is my own, original work, that I am the sole author thereof (save to the extent explicitly
otherwise stated), that reproduction and publication thereof by Stellenbosch University will not
infringe any third party rights and that I have not previously in its entirety or in part submitted it for
obtaining any qualification.

Michelle Zamudio February 2014
…………………………… …………………….
Signature Date

Copyright © 2014 Stellenbosch University
All rights reserved
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BSTRACT

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, CO , is used as the separating agent;
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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: CO + (20 % n-dodecane + 70 % 1-decanol + 10 %
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3,7-dimethyl-1-octanol) and CO + (25 % n-decane + 25 % 1-decanol + 25 % 3,7-dimethyl-1-octanol
2
+ 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 CO and a C -alkane or C -alcohol isomer at temperatures
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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 CO or ethane, and these compounds will in all
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likelihood not be separated in a supercritical fluid fractionation process. The phase transition
pressures measured for the C -alcohol isomers decreased in both CO and ethane in the following
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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, CO , in further
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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 CO +
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(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 CO +
2
(25 % n-decane + 25 % 1-decanol + 25 % 3,7-dimethyl-1-octanol + 25 % 2,6-dimethyl-2-octanol) and
CO + (20 % n-dodecane + 70 % 1-decanol + 10 % 3,7-dimethyl-1-octanol) mixtures showed that the
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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
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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 C -alkane and C -alcohol isomers in
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supercritical ethane, as published in The Journal of Supercritical Fluids 58 (2011) 330 –
342.
• New phase equilibrium data were measured for C -alkane and C -alcohol isomers in
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supercritical CO , as published in The Journal of Supercritical Fluids 59 (2011) 14 – 26.
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• 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 CO , as published in The Journal of
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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: CO + (25 % n-decane + 25 % 1-decanol + 25 %
2
3,7-dimethyl-1-octanol + 25 % 2,6-dimethyl-2-octanol) and CO + (20 % n-dodecane +
2
70 % 1-decanol + 10 % 3,7-dimethyl-1-octanol).
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PSOMMING

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 C – C alkane en alkohol isomere te skei, in Aspen Plus® tot stand te bring
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om die omvang van die skeiding van ’n superkritiese fraksioneringsproses te meet.
Tans word azeotropiese distillasie gebruik vir die skeiding van C – C alkane en alkohol-
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isomere. 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, CO , word as die skeidingsagent gebruik;
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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 C – C alkane en alkohole die beste voorspel binne die
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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: CO + (20 % n-dodekaan + 70 % 1-dekanol +
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Ops om m i ng
10 % 3,7-dimetiel-1-oktanol) and CO + (25 % n-dekaan + 25 % 1-dekanol + 25 %
2
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 CO en ’n C -alkaan- of C -alkohol-isomeer bestaan, gemeet
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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 CO nie en dus sal hierdie stowwe in
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alle waarkynlikheid nie met ’n superkritiese fraksioneringsproses geskei kan word nie. Die
faseoorgangsdrukke wat vir die C -alkohol gemeet is, het in beide etaan en CO afgeneem in die
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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, CO , vir verdere ondersoeke
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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 CO + (25 % n-dekaan + 25 % 1-dekanol + 25 %
2
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 CO + (25 %
2
n-dekaan + 25 % 1-dekanol + 25 % 3,7-dimetiel-1-oktanol + 25 % 2,6-dimetiel-2-oktanol) en CO +
2
(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
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