McGill University, Department of Chemical Engineering - CHEE351: Separation Processes
Distillation - 1
Notice: These notes have not been revised and may contain errors. Use your textbook as the
primary learning reference. The notes are almost entirely based on your textbook.
Vapor-Liquid Separation Processes
VAPOR-LIQUID EQUILIBRIUM RELATIONS - Phase Rule and Raoult’s Law
One Component
As in the gas-liquid systems, the equilibrium in vapor-liquid systems is restricted by the phase rule:
F=C-P+2
For an one-component system: C=1, P=2
→
F=1 degrees of freedom
Four working variables are (but the last two do not change,
i.e.
x=y=
1)
¾
temperature,
¾
pressure,
¾
composition of
y
A
in the vapor phase, and
¾
x
A
in the liquid phase.
So, if we set up the pressure, the temperature is automatically set. The relationship between temperature and
pressure for which two phases co-exist at equilibrium is called the
vapor pressure
curve.
This diagram
summarizes all the vapor-liquid phase behavior for a one-component system.
Two Components
Now, for a two-component system, the phase rule tells: F = C - P + 2
C=2, P=2
→
F=2 degrees of freedom
Four working variables are:
¾
temperature,
¾
pressure,
¾
composition of
y
A
in the vapor phase, and
¾
x
A
in the liquid phase.
The composition of water (
B
)
is fixed if
y
A
or
x
A
is specified, since:
y
A
+
y
B
= 1.0
and
x
A
+
x
B
= 1.0.
Now, considering the four variables and if we fix
p
, then only one more variable can be set.
If we set the liquid composition, the temperature and vapor composition are automatically set.
As we can see, the vapor-liquid phase behavior of two-component systems is more complicated then the one-
component system, because we have an additional variable
→
composition
, as well as
T
and
P.
Instead of a single temperature at which both liquid and vapor can co-exist, there is a range of temperatures.
©
S.Omanovic, 2005