Organic molecular solvents are used in different industrial
chemical processes with satisfying technical results. However, most of these
chemicals represent a risk to human health and are ecologically unfriendly due
to their volatility. A good illustration of how destructive modern chemical
products can be is given by this non-exhaustive list of disasters (Moutiers et
al., 2003):
· Toxic cloud over Seveso in Italy, 1976;
· 3500 lives claimed when in 1984, the Bhopal based
pesticide factory released toxic emissions in India;
· The devastating explosion of the AZF chemical plant in
the French city of Toulouse in 2001.
Such accidents contribute to the increased awareness of
environmental and health threats resulting from the large number of industrial
processes. The development of less-polluting solvents and safe processes is
nowadays a high priority on the agenda for green chemistry?. There is a
pressing need to rethink the design of unsafe chemical processes so that
younger generations inherit a healthy and safer environment.
In recent years, ionic liquids, salts that are liquids at low
temperatures (Plechkova and Seddon, 2008) received the most attention as
alternatives to volatile organic solvents in reactive chemistry, as well as,
separation processes due to their negligible vapour pressure and wide liquid
range.
Researchers at the Thermodynamics Research Unit, in the School
of Chemical Engineering,
University of KwaZulu-Natal in South Africa along
with other academic institutions carried out
systematic measurements of infinite dilution activity
coefficients ( ) for a wide range of
organic solutes in various ionic liquids in order to assess
their effectiveness as extracting solvents.
Though encouraging results were achieved, there is still an
obvious appeal for more investigations in order to:
· Assess the solvent potential of the large number of ionic
liquids that have not yet been investigated;
· Correlate the ionic liquid`s structure and its ability to
separate particular mixtures.
The present work has been conducted to increase the
understanding of these two phenomena. Moreover, South Africa has nearly a third
of the world`s fluorspar (CaF2) reserves (Meshri, 2000) and in the context of
the Fluorochemical Expansion Initiative recently launched by its government,
there are no better candidates for investigation than fluorinated ionic
liquids. Thus this work is a focus on the measurement of infinite dilution
activity coefficients in Fluorinated Ionic Liquids (FILs), bearing in mind
their performance in separation processes.
An inert gas stripping apparatus for measurement of infinite
dilution activity coefficients, (IDACs), for systems involving ionic liquids
was constructed. The objective of this work was to answer the following
questions:
· What performance can be expected from FILs as extracting
solvents in extractive distillation and liquid-liquid extraction?
· How is the selectivity towards various systems affected
by the anion-cation combination found in the structure of FILs?
· Can a reliable dilutor technique apparatus be locally
designed, constructed and commissioned for use with small amounts of ionic
liquids as solvent?
To address the above questions the methodological approach in
this work consisted of:
· Generating new experimental activity coefficient data for
organic solutes in selected fluorinated ionic liquids;
· Using experimental IDACs to determine limiting
selectivity and capacity values for all FILs newly investigated, as well as,
data reported in the literature;
· Analyzing the trends of variation of IDACs to gain
insight into the influence of structure on limiting selectivity and
capacity;
· Reviewing advances recorded in the use of the inert gas
stripping technique;
· Constructing an apparatus similar to one developed
previously in the Thermodynamics Research Unit (George, 2008) and reported in
the open literature;
· Assessing its reliability in measuring infinite
dilution activity coefficients by comparison of IDAC results obtained for the
same systems using both Gas-Liquid Chromatography (GLC) and the Inert Gas
Stripping Technique (IGST).
All experimental procedures have been conducted in the
Thermodynamics Research Unit laboratories, in the School of Chemical
Engineering, University of KwaZulu-Natal. All appropriate experimental
equipment and necessary facilities were available to undertake the research
work. Experimentally investigated ionic liquids are:
· Trihexyltetradecylphosphonium tetrafluoroborate,
[3C6C14P] [BF4]
· Trihexyltetradecylphosphonium
bis(trifluoromethylsulfonyl) imide, [3C6C14P] [Tf2N]
· Trihexyltetradecylphosphonium hexafluorophosphate,
[3C6C14P] [PF6]
· Methyltrioctylammonium bis (trifluoromethylsulfonyl)
imide, [C13C8N] [Tf2N]
· 1-ethyl-3-methylimidazolium trifluoromethanesulfonate,
[EMIM] [TfO]
· 1-butyl-3-methylimidazolium hexafluoroantimonate, [BMIM]
[SbF6]
· 1-methyl-3-octylimidazolium hexafluorophosphate.
[MOIM][PF6]
Data have been generated at temperatures ranging from (313.15
to 373.15) K whereas solutes were selected among n-alkanes, alk-1-enes,
alk-1-ynes, cycloalkanes, alkan-1-ols, alkylbenzenes and ket-2-ones. Separation
problems are discussed through the n-hexane /benzene, methanol/benzene,
methanol/acetone, n-hexane/hex-1-ene, ethanol/butan-2-one and
benzene/butan-2-one systems.
During this study, contributions were made to publications by
Olivier et al. (2010a, b and c), as well as, Gwala et al. (2010). This
consisted of producing part of the published data i.e. infinite dilution
activity coefficients in [EMIM] [TfO], [MOIM][PF6], [BMIM] [SbF6] and [C13C8N]
[Tf2N]. Full results obtained by these authors are used in this dissertation
with their authorization.
This thesis is organized as follows. After this brief
introduction (as chapter one), previous scientific work related to ionic
liquids, generalities on activity coefficient at infinite dilution and recent
advances in the design of inert gas stripping equipment are reviewed in chapter
two. The third chapter provides details about the theory behind GLC and the
IGST, as well as, equations used to compute infinite dilution activity
coefficients. Chapter four is a description of the materials, the experimental
set up and the experimental procedure used in this work. Infinite dilution
activity coefficient experimental data are presented in chapter five. Results
are discussed in the sixth chapter. Lastly, chapter seven is devoted to the
conclusion and the recommendations for future investigations.
