| Vapour Liquid Equilibria |
VAPOUR LIQUID EQUILIBRIA
Distillation columns
are designed based on the boiling point properties of the components in the
mixtures being separated. Thus the sizes, particularly the height, of
distillation columns are determined by the vapour liquid equilibrium (VLE) data
for the mixtures.
Vapour-Liquid-Equilibrium (VLE) Curves
Constant
pressure VLE data is obtained from
boiling point
diagrams. VLE data of binary mixtures is often presented as a plot, as shown
in the figure on the right. The VLE plot expresses the bubble-point and the
dew-point of a binary mixture at constant pressure. The curved line is called
the equilibrium line and describes the compositions
of the liquid and vapour in equilibrium at some fixed pressure.
This particular VLE plot shows a binary mixture
that has a uniform vapour-liquid equilibrium that is relatively easy to
separate. The next two VLE plots below on the other hand, shows non-ideal
systems which will present more difficult separations. We can tell from the
shapes of the curves and this will be explained further later on.
The most intriguing
VLE curves are generated by azeotropic systems. An
azeotrope is a liquid mixture which when vaporised, produces the same
composition as the liquid. The two VLE plots below, show two different
azeotropic systems, one with a minimum boiling point and one with a maximum
boiling point. In both plots, the equilibrium curves cross the diagonal lines,
and this are azeotropic points where the azeotropes
occur. In other words azeotropic systems give rise to VLE plots where the
equilibrium curves crosses the diagonals.
Note the shapes
of the respective equilibrium lines in relation to the diagonal lines that
bisect the VLE plots.
Both plots are
however, obtained from homogenous azeotropic systems. An azeotrope that contains
one liquid phase in contact with vapour is called a
homogenous azeotrope. A homogenous azeotrope cannot be separated by
conventional distillation. However, vacumn distillation
may be used as the lower pressures can shift the azeotropic point.Alternatively,
an additional substance may added to shift the azeotropic point to a more
�favourable� position.
When this additional
component appears in appreciable amounts at the top of the column, the operation
is called azeotropic distillation.
When the additional
component appears mostly at the bottom of the column, the operation is called
extractive distillation
 The
VLE curve on the left is also generated by an azeotropic system, in this case a
heterogenous azeotrope. Heterogenous azeotropes can
be identified by the �flat� portion on the equilibrium diagram.
They may be separated in 2 distillation columns since these
substances usually form two liquid phases with widely differing
compositions. The phases may be separated using settling tanks
under appropriate conditions.
Next, we will look
at how VLE plots/data are used to design distillation columns.
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