Determination of activity coefficients at infinite dilution using the inert gas stripping technique.
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Date
2008
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Abstract
The determination of limiting activity coefficients in liquid mixtures has become an important tool
in chemical engineering. It has been investigated intensively during the past in order to find new
alternatives and improved methods for its accurate detennination. The limiting activity coefficient
is a fundamental thermodynamic quantity which measures the solution non-ideality and acts as a
correction factor to deviations from Raoult's Law. This dissertation involves the determination of
limiting activity coefficients using the inert gas stripping (IGS) technique only. It is considered to
be the best method as it is a direct method involving exact concentrations of components in the
mixtures encountered in industry.
A comprehensive study of activity coefficients at infinite dilution for various systems, using the
inert gas stripping (IGS) method has been undertaken. Various other methods and their
suitability have also been discussed but preference is given to the superior quality of
measurements obtained using the inert gas stripping technique. Extensive research has been
conducted into the background and origination of the technique. Various improvements of the
equilibrium cell designed by various authors for different types of systems have been outlined
along with the various equations derived by the authors.
The equipment was designed for use with the double-cell technique as well as the single-cell
technique and in some cases both techniques were used. The techniques involve the use of a
dilutor cell in which the highly diluted, volatile solute is stripped from a liquid solution using the
inert gas nitrogen, introduced into the cell through capillaries and dispersed through the solution
as small bubbles, at a constant flow rate. Analysis of the stripped solution is accomplished
through the use of a gas chromatograph; the peak areas obtained from these analyses as well
as the residence times and other system data such as temperature, pressure, mass and flow
rate were used to compute the infinite dilution activity coefficient through the use of the various
equations available in literature.
The original equipment was designed for the use of the single cell technique by Soni (2004).
Various modifications have been made to the equipment in order to measure limiting activity
coefficients of more diverse systems with high accuracy. A major change to the equipment was
the introduction of a second saturation cell of similar design to the dilutor cell. This enabled the
determination of activity coefficients at infinite dilution of difficult systems i.e. systems where the
solvent volatility is high and for higher order systems. The equipment was redesigned and built using ideas and improvements by previous researchers in the field and commissioned using test
systems that have been classed as easy systems for this technique. The new equipment is now
applicable to almost all systems, however good separation in the GC column could be a problem
for complex systems.
The determination of infinite dilution activity coefficients for one-component solute + onecomponent
solvent systems and multi-component solvent systems were accomplished. The
systems that were investigated consisted of a mixture of components of alkanes, alkenes,
phenols and ketones, mostly in binary mixtures. Multi-component mixtures have also been
investigated in the form of ternary systems involving a binary solvent mixture at varying
concentrations, and a solute in order to show the diversity, uniqueness and efficiency of the IGS
technique. Major variables affecting the system (the dilutor cell), namely the stripping gas flow
rate and the dilutor cell temperature, were also investigated for all systems.
Two test systems, cyclohexane in 1-methyl-2-pyrrolidone (NMP) and n-heptane in NMP were
used to determine if the equipment is operating properly by comparing values obtained, to
literature values where the inert gas stripping technique was used to determine the activity
coefficients at infinite dilution. Another test system n-hexane in NMP was used to compare the
two techniques, Le. the results of the single cell technique with the results of the double cell
technique. The experimental results were thereafter compared to published literature values.
Systems where the inert gas stripping technique has not been used to determine activity
coefficients at infinite dilution were also investigated. These systems include 1-hexene in 0-
cresol as well as the ternary systems '-hexene in various concentrations of NMP + o-cresol.
A thorough literature survey has been completed and the relevant theory has been summarized.
The validity of the equations proposed by Bao and Han (1995), Duhem and Vidal (1978), Leroi
et a!. (1977), Hovorka and Dohnal (1997) and Krummen et al. (2000) for the determination of
activity coefficients at infinite dilution were investigated. The experimental values obtained were
consistent with literature values, with percentage errors of less than 1 % where the same
equation was used to determine the limiting activity coefficient. Comparing limiting activity
coefficients with the values obtained from other equations proposed by other authors mentioned
above resulted in deviations no greater than 2.5 %, and where possible limiting activity
coefficients were compared to values obtained from the single-cell technique.
The theory section of this thesis covers all the various formulae (and where possible a summary
of their derivation) used in the analysis of results. Some limiting activity coefficients for the
systems involving n-heptane, n-hexane, n-hexene, cyclohexane, o-cresol and n-methyl-2-pyrrolidone under various experimental conditions have been reported making it readily available
for use in other works. The effect of two major variables temperature and inert gas flow rate on
the limiting activity coefficients with regard to all the systems studied have also been
investigated and reported. This was also done in order to check that the data was reproducible.
A sensitivity analysis was also performed in order to check the effect that certain measured
variables would have on the limiting activity coefficient. These errors are estimated possible
errors and may not exist at all, so not much consideration was given to this when reporting
limiting activity coefficients for the various systems. The maximum error range for any given
limiting activity coefficient as determined by the sensitivity analysis is ±11 %. The inert gas
stripping technique is also extended to the determination of Hendry's constants. The actual
values for the Hendry's constants were not determined but a comprehensive study of its
determination was undertaken by Miyano et al. (2003) and summarized here.
In addition the suitability and diversity of the inert gas stripping technique has been outlined,
along with the advantages and disadvantages of the technique. The various designs of
equilibrium cells have been outlined taking into account mass transfer considerations as
proposed by Richon et al. (1980). The assumptions and limits of the method have also been
outlined and must be taken into consideration when using the technique. A detailed description
of the equipment setup and experimental procedure has been provided. The purpose, suitability,
operation and applicability, of the various pieces of equipment used to make up the final
equipment have been discussed in detail. Details for consideration when designing the
equilibrium cells have also been provided.
Description
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2008.
Keywords
Dilution., Activity coefficients., Theses--Chemical engineering.