Environmental Engineering Reference
In-Depth Information
Important additional point.
The value of the variables specified has to be reasonable for
the system. For example, if the feed stream for a two-phase (vapor and liquid) equilibrium-
limited process is water-based, and the equilibrium pressure is specified as 1 atm and the
temperature as 200
◦
C, there will not be a two-phase system at equilibrium.
3.4
Phase equilibrium
Phase equilibrium information characterizes partitioning between phases for a system
and is important for describing separation processes. For equilibrium-limited processes,
these values dictate the limits for separation in a single stage. For mass transfer-limited
processes, the partitioning between phases is an important parameter in the analysis.
The data can be presented in tabular form. But this approach is restricted in application,
since an analysis typically requires phase equilibrium values that are not explicitly listed
in the table. So, graphical representation and computational methods are usually more
useful.
3.4.1
Vapor-liquid equilibrium (VLE)
For binary systems, graphical representation is typically plotted (Figure 3.7(a)) as the
vapor-phase mole fraction of the more volatile component (
y
A
) vs liquid-phase mole
fraction (
x
A
). The
y
x
diagonal line is included for reference. The data are usually
plotted for a constant total pressure.
Figure 3.7(b) illustrates the presence of an azeotrope (a point at which
y
A
=
=
x
A
). The
presence of an azeotrope is important since it represents a limiting value in distillation.
An alternative graphical method, shown in Figure 3.8, is a
T
-
x
-
y
phase diagram. Again,
the total pressure is constant. To use the graph, select a temperature, draw a horizontal
(a)
(b)
1
1
azeotrope
y
A
y
A
y
=
x
line
0
0
x
A
1
x
A
1
Figure
3.7
(a)
Equilibrium
line
(solid)
on
a
typical
VLE
x
-
y
diagram;
(b) graphical representation of an azeotrope in a VLE system.
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