For many years the problem of separating aliphatic and aromatic compounds has been at the
forefront of the petroleum and oil refining industries. This separation is often effected using
liquid-liquid extraction or extractive distillation. Both of these processes require the addition of a
solvent to bring about separation.
The aims of this work were to investigate the use of "mixed" solvents, such as those used in the
Arosolvan process, for their application in liquid-liquid extraction and extractive distillation as
well as to provide related thelmodynamic data for systems containing mixed solvents. In the last
part of this work, a computer program was developed to theoretically predict the effectiveness of
a number of solvents on a user-defined separation.
The solvents used for liquid-liquid extraction were chosen based on their similarities to those in
the Arosolvan process and were of the form, {N-methyl-2-pyrollidone (NMP) + glycerol, a glycol
or water} where the glycol was either monoethylene glycol (MEG), diethylene glycol (DEG) or
triethylene glycol (TEG). The additives were combined in various mixing ratios to NMP to
determine a mixing ratio for which the effect of the solvent is possibly optimized (a list of all
solvents and mixing ratios used are presented in this work).
Solvent selectivity and the range of compositions over which separation could occur determined
the effectiveness of the solvents. This work dealt with the separation of n-hexane and toluene. In
order to determine the selectivity and range of compositions, the liquid-liquid equilibria (LLE) of
systems containing n-hexane + toluene + solvent had to be determined. LLE was measured using
a simple equilibrium cell at 298 K and 1 atm. The phase separation boundaries (binodal curves)
were determined using a titration method.
The results obtained in this work showed an increase in the range of compositions over which the
mixture of n-hexane and toluene could be separated (i.e a larger range of mixing ratios over which
these components could be separated from each other) from the pure NMP solvent to the mixed
solvent cases. This implies that there is a The range of compositions over which separation could
be affected is given (for the solvents) in descending order:
NMP + 50% glycerol> NMP + 10% water > NMP + 30% MEG > NMP + 5% water > NMP +
30% glycerol> NMP + 10% glycerol > NMP + 10% MEG > NMP + 10% DEG > NMP + 10%
TEG > NMP + 5% DEG > 100% NMP.
The selectivities of the solvents showed a remarkable increase from the pure NMP case to the
mixed solvent cases. The maximum selectivity obtained for the NMP + 10% DEG system was
over 1200 compared to a maximum selectivity of just 6 for the pure NMP system. The maximum
selectivities obtained in descending order were as follows:
NMP + 10% DEG > NMP + 10% TEG > NMP + 10% glycerol > NMP + 10% MEG > NMP +
30% MEG > NMP + 50% glycerol > NMP + 10% water > NMP + 5% water > NMP + 30%
glycerol > NMP + 5% DEG > 100% NMP.
The binodal curves were modelled using the Hlavaty, ,8-density and log-y functions. The
maximum standard deviations obtained were 0.075, 0.078 and 0.05 for each of the functions
respectively. The equilibrium data was modelled using the UNIQUAC and NRTL
thermodynamic models and showed excellent agreement. This work showed better agreement to
the NRTL functions due to the fact that the non-randomness parameter, a ij , may be chosen
arbitrarily.
The results obtained in this work indicate that the use of mixed solvents greatly increases the
effectiveness ofNMP used for the separation of n-hexane and toluene. It is suggested that further
studies be performed on a wider range of aliphatic and aromatic compounds in order to determine
whether this is a generic behaviour or just true for n-hexane and toluene.
The effectiveness of each solvent for extractive distillation was determined by its separation
factor. In order to determine separation factors, the activity coefficients at infinite dilution
(IDACs) had to be measured. This was done using a gas-liquid chromatography technique. The
solvents employed in this study were NMP, Glycerol, MEG, TEG, NMP + 10% glycerol, NMP +
10% MEG, NMP + 10% DEG, NMP + 10% TEG. The solutes used were: pentane, heptane,
hexane, toluene and benzene.
The separation factors were determined for each alkane/aromatic pair per solvent. The pure
solvent cases were then compared to the mixed solvent cases. The mixed solvents did not show
results as promising for extractive distillation applications as they did for liquid-liquid extraction.
TEG displayed the best selectivities for each of the alkane/aromatic separations except for the
heptane/benzene pair, for which NMP + 10% glycerol proved to be the most effective solvent.
When compared to the results obtained from the original UNIF AC model, the IDACs obtained in
this work showed up to a 99% deviation. This is due to the fact that the model does not work well
for all types of molecules and does not predict the equilibrium of "unlike" molecules adequately.
It is suggested that other mixing ratios and different solvents be used to further investigate the
effectiveness of mixed solvents for extractive distillation applications. It is further recommended
that a computer aided data logging system be developed to determine residence times. This
would not only provide more accurate results, but also provide a database for future reference.
The computer program that was developed using the original UNIF AC method contains a
database of 28 commonly used industrial solvents. This program enables the user to compare
graphically the effectiveness of each of the solvents on the desired separation. Due to the
limitations of the original UNIF AC method, the program does not work well for all types of
molecules. However, the model can be changed without altering the prografnming structure to
include a modified version of the UNIFAC model depending on the users needs. The program
although written from an extractive distillation standpoint can be extended to include liquid-liquid
equilibrium predictions. The main benefit of such a program is to eliminate time-consuming
experimental work required to narrow down a long list of solvents required for a particular
separation by theoretically predicting the best solvents for the job. The solvent database can also
be expanded when new solvents become available or the user needs change / Thesis (M.Sc. Eng)-University of Natal, Durban, 2003.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/5083 |
| Date | January 2003 |
| Creators | Naidoo, Rolandra D. |
| Contributors | Letcher, Trevor M., Ramjugernath, Deresh D. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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