Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Distillation is the most widely used separation technique in the chemical process industry and typically accounts
for approximately one-third of the total capital cost and more than half of the total energy consumption of a
typical petrochemical-chemical plant. Therefore, the design and optimization of the distillation sequence are of
critical importance to the economics of the entire process. Azeotropic mixtures cannot be separated into their
pure components via normal distillation. Enhanced distillation techniques such as heterogeneous azeotropic
distillation should be considered for these mixtures. Isobaric vapour-liquid-liquid equilibrium (VLLE) data are
highly important for the design and analysis of heterogeneous distillation columns. However, limited VLLE
data are available in literature due to the difficulties involved with measuring such data.
The objective of this work was to systematically evaluate and compare the performance of selected entrainers
(including benzene, DIPE and cyclohexane) for the dehydration of C2 and C3 alcohols. To meet this objective,
phase equilibrium data had to be measured. Isobaric VLLE at standard atmospheric conditions were measured
with a dynamic Guillespie unit equipped with an ultrasonic homogenizer, which prevented liquid-liquid
separation.
Vapour-liquid equilibrium (VLE) and VLLE data were measured for ethanol/water/di-isopropyl ether (DIPE),
n-propanol/water/DIPE and n-propanol/water/isooctane. The VLE data were found to be thermodynamically
consistent according to the L-W (Wisniak 1993) and McDermott-Ellis consistency tests. No thermodynamic
consistency test, specifically for VLLE data, could be found in literature, but the LLE part of the data followed a
regular profile according to the Othmer-Tobias correlation. The binary DIPE/water and isooctane/water
azeotropes, as well as ternary ethanol/DIPE/water and n-propanol/isooctane/water azeotropes, as measured in
this work, agree well with those found in literature.
Regressed parameters for the NRTL, UNIQUAC, and UNIFAC models, generally improved the model
predictions compared with built-in Aspen parameters. This confirmed the importance of having actual measured
VLLE data available for evaluation and improvement of estimations by thermodynamic models. NRTL
predicted the ethanol/DIPE/water and n-propanol/DIPE/water VLLE most accurately. Despite the improved
regressed parameters for n-propanol/isooctane/water predictions, the models are still considered unsuitable for
accurate prediction of the VLLE behaviour of this system. Separation sequences were simulated in Aspen with
built-in and regressed parameters, respectively, to illustrate the significant effect such inaccurate parameters
have on these simulations.
Phase diagram (VLLE data) evaluation of ethanol and isopropanol (IPA) with various entrainers, as found in
literature, indicated that DIPE might be a good entrainer for the dehydration of these alcohols. Benzene and
cyclohexane are generally used as entrainers in industry for these processes. Benzene is however carcinogenic
and therefore an alternative has to be found (United States Department of Labour - Occupational Safety &
Health Administration 2011). Separation sequences were simulated for ethanol dehydration with benzene and
DIPE as entrainers, respectively. Taking cost and safety into account, DIPE can be considered an acceptable
replacement for benzene as entrainer for ethanol dehydration. A separation sequence was also simulated for the dehydration of IPA with DIPE as entrainer and compared to a
simulation with cyclohexane (Arifin, Chien 2007) as entrainer. DIPE was found to be a reasonable alternative
to cyclohexane as entrainer for IPA dehydration. Two other separation sequences were simulated as practical examples where DIPE could be used as entrainer for the recovery of ethanol or n-propanol from aqueous Fischer
Tropsch waste streams.
DIPE is therefore found to be a feasible alternative entrainer to benzene and cyclohexane for the
dehydration of ethanol and IPA via heterogeneous azeotropic distillation, based on pre-liminary cost
considerations, separation ability and safety. Better entrainers than DIPE may exist, but from the data
available in literature and the measurements made in this work DIPE appears to be superior to benzene,
cyclohexane and isooctane. / AFRIKAANSE OPSOMMING: Distillasie is die mees algemeen-gebruikte skeidingstegniek in die chemiese proses-industrie. Dit is tipies
verantwoordelik vir ʼn derde van die totale kapitaalkoste en meer as die helfte van die totale energie verbruik op
ʼn tipiese petrochemiese chemiese aanleg. Daarom is die ontwerp en optimering van ʼn distillasie trein van
kardinale belang vir die winsgewendheid van die proses. Azeotropiese mengsels kan nie slegs deur normale
distillasie in suiwer komponente geskei word nie. Gevorderde distillasie tegnieke soos heterogene azeotropiese
distillasie moet dus oorweeg word vir sulke mengsels. Isobariese damp-vloeistof-vloeistof ewewigsdata is een
van die belangrikste fisiese eienskappe vir die ontwerp van heterogene distillasie kolomme. Die hoeveelheid
damp-vloeistof-vloeistof ewewigsdata wat beskikbaar is in die literatuur is egter baie beperk omdat dit moeilik
is om die data te meet.
In hierdie werk is isobariese damp-vloeistof-vloeistof ewewigsdata met ʼn dinamiese Guillespie eenheid, by
standaard atmosferiese druk gemeet. Die eenheid is toegerus met ʼn ultrasoniese homogeniseerder om
vloeistof-vloeistof skeiding te voorkom. Temperatuur is gemeet met ʼn akkuraatheid van 0.03oC by 0oC. Die
sisteem se druk is konstant gehou op 101.3 kPa met ʼn akkuraatheid van 0.35 % VSU (Vol Skaal Uitset). Die
ewewigsamestellings is met ʼn relatiewe akkuraatheid van 2 % gemeet.
Daar is damp-vloeistof en damp-vloeistof-vloeistof ewewigsdata van etanol/water/di-isopropiel eter (DIPE), npropanol/
water/ DIPE en n-propanol/water/iso-oktaan gemeet. Die damp-vloeistof ewewigsdata is deur die LW
(Wisniak 1993) en McDermott-Ellis termodinamiese konsistensie toetse getoets en konsistent bevind. Geen
termodinamiese konsistensie toets spesifiek vir damp-vloeistof-vloeistof ewewigsdata kon gevind word nie. Die
Othmer-Tobias korrelasie dui egter aan dat die vloeistof-vloeistof ewewig gedeelte van die data ʼn reëlmatige
gang volg. Die binêre DIPE/water en iso-oktaan/water azeotrope en ternêre etanol/DIPE/water en
n-propanol/iso-oktaan/water fase-ewewigte wat in hierdie werk gemeet is, stem ooreen met die wat in die
literatuur te vind is.
Die parameters vir die modelle (NRTL, UNIQUAC en UNIFAC) wat in hierdie werk bestudeer is, is in die
algemeen verbeter deur regressie van die eksperimentele data. Dit dui daarop dat dit belangrik is om
eksperimentele damp-vloeistof-vloeistof ewewigsdata te hê om die voorspellings van termodinamiese modelle
mee te evalueer en te verbeter. Die etanol/water/ DIPE en n-propanol/water/DIPE damp-vloeistof-vloeistof
ewewigsdata is die akkuraatste deur NRTL voorspel. Ten spyte van die verbeteringe wat deur regressie behaal
is met die NRTL en UNIQUAC parameters vir n-propanol/water/isooktaan, word hierdie modelle steeds nie as
gepas vir die voorspelling van hierdie datastel beskou nie.
Skeidingsreekse is gesimuleer met die ingeboude Aspen parameters en regressie parameters, onderskeidelik, om
te illustreer dat onakkurate parameters ʼn beduidende effek op sulke simulasies het.
Die evaluasie van fase diagramme van etanol en IPA met verskeie skeidingsagente, wat in die literatuur te vind
is, dui aan dat DIPE ʼn goeie skeidingsagent kan wees vir die dehidrasie van hierdie alkohole. Skeidingsreekse
vir die dehidrasie van etanol met benseen en DIPE, onderskeidelik, is gesimuleer. Met koste en veiligheid in ag
geneem, is daar gevind dat DIPE ʼn aanvaarbare plaasvervanger vir benseen as skeidingsagent vir etanol
dehidrasie kan wees. Daar is ook ʼn skeidingsreeks vir die dehidrasie van IPA met DIPE as skeidingsagent gesimuleer en met ʼn
simulasie (Arifin, Chien 2007) wat sikloheksaan as skeidingsagent gebruik, vergelyk. Daar is bevind dat DIPE ʼn
redelike alternatief vir sikloheksaan kan wees as skeidingsagent vir IPA dehidrasie. Nog twee skeidingsreekse is gesimuleer om as praktiese voorbeelde te dien van die gebruik van DIPE as skeidingsagent om etanol of
n-propanol vanaf waterige Fischer-Tropsch afvalstrome te herwin.
Daarom is daar bevind dat DIPE ʼn geldige alternatiewe skeidingsagent vir benseen en sikloheksaan is,
gebaseer op koste, skeidingsvermoë en veiligheid. Daar kan beter skeidingsagente as DIPE bestaan, maar
vanuit die data beskikbaar in literatuur en die metings geneem in hierdie werk, is DIPE die beste.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/20357 |
| Date | 03 1900 |
| Creators | Pienaar, Cornelia |
| Contributors | Burger, A. J., Knoetze, J. H., 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 | Unknown |
| Type | Thesis |
| Format | 234 p. : ill. |
| Rights | Stellenbosch University |
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