Separation processes are fundamental to all chemical engineering industries. Solvent
separation, either liquid-liquid extraction or extractive distillation, is a specialised
segment of separation processes. Solvents can be used either to optimise conventional
distillation processes or for azeotropic systems, which can not be separated by
conventional means. This work focuses on the performance of monoethanolamine
(MEA) as a solvent in extractive distillation. Furthermore, the methodology of solvent
evaluation is also studied.
The preliminary assessment of solvent selection requires the determination of selectivity
factors. The selectivity factor is defined as follows:
P• = y,." . y,
where y" is the activity coefficient at infinite dilution of the solute in the solvent.
Subscript 1 and 2 refer to solute 1 and 2. A large selectivity factor implies enhanced
separation of component 1 from 2 due to the solvent. Activity coefficients at infinite
dilution were determined experimentally (gas-liquid chromatography) and predicted
theoretically (UNIFAC group contribution method) for twenty-four solutes at three
temperatures. Solutes used were alkanes, alkenes, alkynes, cyclo-alkanes, aromatics,
ketones and alcohols. Most of this experimental work comprises data for systems which
have not been measured before.
Predicted and experimental values for y' were compared. For systems such as these
(with polar solvents and non-polar solutes), UNIFAC results are not accurate and
experimentation is vital. The experimental selectivity factors indicated tihat MEA could
be an excellent solvent for hydrocarbon separation. Three binary azeotropic systems
were chosen for further experimentation with MEA n-hexane (1) - benzene (2): fJ,~ = 31. Compared to other industrial solvents this
is one of the largest values and MEA could serve as an excellent solvent.
cyclohexane (1) - ethanol (2): fJ,~ = 148. This high value indicates an excellent
solvent for this system.
Acetone (1) - methanol (2): fJ,~ = 7.7.
Further work involved vapour-liquid equilibrium experimentation at sub-atmospheric
pressures in a dynamic recirculating stil l. The binary components with a certain amount
of MEA were added to the still. The vapour and liquid mole fractions for the binary
azeotropic components were measured and plotted on a solvent-free basis. The results
are summarised below:
n-hexane - benzene: Amount MEA added to still feed: 2%. MEA improved
separability slightly. Further addition of MEA resulted in two liquid phases forming.
cyclohexane - ethanol: Amount MEA added to still feed: 5% and 10%. Two liquid
phases were formed for cyclohexane rich mixtures. Addition of MEA improved
separability but did not remove the azeotrope.
acetone - methanol: Amount MEA added to still feed : 5%, 10% and 20%. The
ternary mixture remained homogenous and separability improved with addition of
MEA. The binary azeotrope was eliminated.
Due to the hetrogenous nature of the cyclohexane - ethanol system liquid-liquid
equilibrium experimentation was performed to complete the analysis. Viable separation
processes are possible for (a) cyclohexane - ethanol mixtures and for (b) acetone -
methanol mixtures using MEA as the solvent.
Comparison of various solvents used for the separation of acetone from methanol was
possible by constructing equivolatility curves for the ternary systems. Results showed
that MEA may possibly be the best solvent for this extractive distillation process. This study provides the following results and conclusions:
• New thermodynamic data, important for the understanding of MEA in the field of
solvent separations, was obtained.
• Results show that the UNIFAC contribution method cannot be used to accurately
predict polar solvent - non-polar solute y«> values. Experimentation is essential.
• Selectivity factors indicate that MEA could be an excellent solvent for hydrocarbon
separation.
• The separation of the azeotropic cyclohexane - ethanol mixture is possible with a
combination of extractive distillation and liquid-liquid extraction or simply liquid-liquid
extraction using MEA as the solvent.
• The separation of the azeotropic acetone methanol mixture is possible with
extractive distillation using MEA as the solvent. The solvent MEA is possibly the best
solvent for this separation. / Thesis (M.Sc.Eng.)-Univeristy of Natal, Durban, 2000.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/5454 |
| Date | January 2000 |
| Creators | Harris, Roger Allen. |
| Contributors | Letcher, Trevor M., Ramjugernath, Deresh D., Raal, Johan David. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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