|dc.description.abstract||The measurement of vapour-liquid equilibrium data is extremely important as such data are crucial
for the accurate design, simulation and optimization of the majority of separation processes,
including distillation, extraction and absorption.
This study involved the measurement of vapour-liquid equilibrium data, using a modified version
of the static total pressure apparatus designed within the Thermodynamics Research Unit by J.D.
Raal and commissioned by Motchelaho, (Motchelaho, 2006 and Raal et al., 2011). This apparatus
provides a very simple and accurate means of obtaining P-x data using only isothermal total
pressure and overall composition (z) measurements. Phase sampling is not required.
Phase equilibrium measurement procedures using this type of apparatus are often tedious,
protracted and repetitive. It is therefore useful and realizable in the rapidly advancing digital age, to
incorporate computer-aided operation, to decrease the man hours required to perform such
The central objective of this work was to develop and implement a control scheme, to fully
automate the original static total pressure apparatus of Raal et al. (2011). The scheme incorporates
several pressure feedback closed loops, to execute process step re-initialization, valve positioning
and motion control in a stepwise fashion. High resolution stepper motors were used to engage the
dispensers, as they provided a very accurate method of regulating the introduction of precise
desired volumes of components into the cell. Once executed, the control scheme requires
approximately two days to produce a single forty data points (P-x) isotherm, and minimizes human
intervention to two to three hours. In addition to automation, the apparatus was modified to
perform moderate pressure measurements up to 1.5 MPa.
Vapour-liquid equilibrium test measurements were performed using both the manual and automated
operating modes to validate the operability and reproducibility of the apparatus. The test systems
measured include the water (1) + propan-1-ol (2) system at 313.15 K and the n-hexane (1) + butan-
2-ol system at 329.15 K.
Phase equilibrium data of binary systems, containing the solvent morpholine-4-carbaldehyde
(NFM) was then measured. The availability of vapour-liquid equilibrium data for binary systems
containing NFM is limited in the literature. The new systems measured include: n-hexane (1) +
NFM (2) at 343.15, 363.15 and 393.15 K, as well as n-heptane (1) + NFM (2) at 343.15, 363.15 and
The modified apparatus is quite efficient as combinations of the slightly volatile NFM with highly
volatile alkane constituents were easily and accurately measured. The apparatus also allows for
accurate vapour-liquid equilibrium measurements in the dilute composition regions.
A standard uncertainty in the equilibrium pressure reading, within the 0 to 100 kPa range was
calculated to be 0.106 kPa, and 1.06 kPa for the 100 to 1000 kPa pressure range. A standard
uncertainty in the equilibrium temperature of 0.05 K was calculated.
The isothermal data obtained were modelled using the combined (-) method described by Barker
(1953). This involved the calculation of binary interaction parameters, by fitting the data to various
thermodynamic models. The virial equation of state with the Hayden-O’Connell (1975) and
modified Tsonopoulos (Long et al., 2004) second virial coefficient correlations were used in this
work to account for vapour phase non-ideality. The Wilson (1964), NRTL (Renon and Prausnitz,
1968), Tsuboka-Katayama-Wilson (1975) and modified Universal Quasi-Chemical (Anderson and
Prausnitz, 1978) activity coefficient models were used to account for the liquid phase non-ideality.
A stability analysis was carried out on all the new systems measured to ensure that two-liquid phase
formation did not occur in the measured temperature range.
A model-free method based on the numerical integration of the coexistence equation was also used
to determine the vapour phase compositions and activity coefficients from the measured P-z data.
These results compare well with the results obtained by the model-dependent method.
The infinite dilution activity coefficients for the systems under consideration were determined by
the method of Maher and Smith (1979b), and by suitable extrapolation methods. Excess enthalpy
and excess entropy data were calculated for the systems measured, using the Gibbs-Helmholtz
equation in conjunction with the fundamental excess property relation.||en