Phase equilibrium measurements at low-to-moderate pressures for systems containing n-Hexane, 1-Hexene and n-Methyl-2-pyrrolidone.
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2012
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Abstract
The primary focus of this study is the measurement and modeling of binary and ternary VLE
data. The measurements of binary and ternary systems were undertaken on a fully automated
dynamic VLE apparatus. The glass dynamic VLE still was modified to handle pressures
ranging from 0 to 500 kPa, however, the safest maximum pressure to which tests had been
conducted was 350 kPa. Thus, this limit was not to be exceeded during the measurement of
experimental data.
The systems under investigation included the binary and ternary combinations of the
following chemicals: n-hexane, 1-hexene and n-methyl-2-pyrrolidone (NMP) at isothermal
conditions. A test system consisting of ethanol + cyclohexane was measured at 40 kPa, as
well as the system of 1-hexene + NMP at 363.15 K and n-hexane + NMP at 363.15 K.
Published literature data for these test systems were employed to verify the measured data
for the test systems complied with thermodynamic consistency. All other data constitutes new
data, currently unavailable in literature.
The following isotherms were measured:
1) 1-hexene (1) + NMP (2) at 323.15, 343.15, 353.15 and 363.15 K
2) n-hexane (1) + NMP (2) at 353.15, 363.15, 378.15 and 383.15 K
3) 1-hexene (1) + n-hexane (2) at 343.15, 363.15 and 373.15 K, and
4) 1-hexene (1) +n-hexane (2) + NMP (2) at 363.15 K
All system measurements were carried out on the glass low-to-medium pressure VLE still of
Lilwanth (2011), with the exception of the test system ethanol + cyclohexane, which was
carried out on the low pressure VLE glass still of Hirawan (2007).
The two VLE stills, utilized to carry out measurements in this work, can operate isobarically
and isothermally. The temperature on the stills of Hirawan (2007) and Lilwanth (2011) were
controlled to within ±0.425 and ±0.089 K respectively and the accuracy of pressure control is
to within ±0.320 and ±0.440 kPa respectively. In addition, for the calibration of the various
systems: ethanol + cyclohexane, 1-hexene + NMP, n-hexane + NMP, 1-hexene + n-hexane
and 1-hexene + n-hexane + NMP, the accuracies are: ±0.002, ±0.0034, ±0.0033, ±0.0066 and
±0.0083 of a mole fraction respectively.
The binary interaction parameters obtained from modeling the three binary systems were
used to predict the ternary system data. Thereafter, the experimentally measured data for the
ternary system was then compared to the model prediction, which was completed on
Dortmund Data Bank (DDB, 2011).
The measured binary data was regressed utilizing the combined and the direct methods. For
the direct method, the cubic equations of state (CEoS) used to describe the vapour phase
included the Peng-Robinson (1976) and Soave-Redlich-Kwong (1972) equations combined
with the mixing rule of Wong and Sandler (1992) in conjunction with the Gibbs excess
energy models, namely the NRTL (1968) and UNIQUAC (1975) models, to describe the
liquid phase non-idealities.
For the combined method, the Gibbs excess energy activity coefficient models mentioned
above were employed to represent the liquid phase imperfections and the vapour phase nonidealities
were represented by cubic equations of state, as mentioned above, as well as the
Hayden and O‟Connell (1975) virial equation of state for the calculation of the virial
coefficients.
To verify whether the measured data is thermodynamically consistent the point and direct
tests were applied. Even though the direct test is a more stringent approach to testing
thermodynamic consistency, for the systems 1-hexene + NMP and n-hexane + NMP, the
point test was utilized as the primary means by which to quantify the data, as the associative
effects of the NMP molecule effect the results obtained. For the system 1-hexene + n-hexane
the direct test was used as the primary means to test the consistency of data, as no cross- or
self-association is present.
After extensive modeling was carried out, it was found that for the systems 1-hexene + NMP
and n-hexane + NMP the model which enabled the best fit of the experimental data are the
NRTL activity coefficient model in conjunction with the Hayden and O‟Connell virial
equation of state (EoS). For the system 1-hexene + n-hexane the overall best fit model is
the Peng-Robinson EoS in conjunction with the Wong-Sandler mixing rule and the NRTL
activity coefficient model.
A single set of binary interaction parameters for each of the three binary systems was
obtained (via regression on Aspen Plus®) using the NRTL-HOC models. However, since
Aspen Plus® cannot predict ternary system behaviour using the binary interaction parameters
of the constituent systems, DDB was utilized. Further, DDB did not have available the HOC
virial EoS (for enabling predictions), thus, it was decided to use the ideal gas model for
representation of the vapour phase in conjunction with the NRTL activity coefficient model.
The use of the ideal gas model does not compromise the integrity of the prediction in any
way since the ternary system measurements were carried out in the dilute NMP region. Thus,
since the main components in the ternary mixture at any one instant were 1-hexene and nhexane,
and these components behave ideally, the ideal gas model is applicable.
After the predicted behaviour for the ternary system was compared to the experimental data
for the same system, the maximum percentage error encountered between the two data sets is
5%.
Description
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2012
Keywords
Phase rule and equilibrium., Chemical equilibrium., Theses--Chemical engineering.