Analysis using Constant Pressure
Phase Diagram: Benzene-Toluene System (ideal solution) at 1 atm
below. Consider a container whereby a fixed amount of liquid benzene-toluene mixture
is gradually heated.
will study the changes in composition (mole fraction benzene)
and temperature as distillation progresses from point
(a) to point (e) using the constant pressure
phase diagram as shown in the Figure
below. The original concentration in the benzene-toluene mixture is 0.40 mole
fraction benzene (i.e. 0.60 mole fraction toluene).
Liquid-phase; no vapour Temperature 86 oC
Concentration of benzene Liquid: x = 0.40; Vapour: y = 0.00
(b): Liquid-phase; first bubble of vapour produced Temperature 95.2 oC
(Bubble Point - solution about to boil) Concentration of benzene Liquid: x = 0.40;
Vapour: y = 0.61
Point (c): Vapour-Liquid Mixture;
continued vaporization of liquid as heat is added Temperature 98.0 oC
Concentration of benzene Liquid: x = 0.31; Vapour: y = 0.52
(d): Vapour-phase; last droplet of liquid
remains Temperature 101.6 oC (dew point) Concentration
of benzene Liquid: x = 0.21; Vapour: y = 0.40
(e): Vapour-phase only, no liquid Temperature 108 oC
The process is unsteady-state in nature.
from the above Figure
heating continues, vaporization occurs over a range
of boiling points.
any time when 2 phases - vapour and liquid - are present, the concentration
of benzene (more volatile component)
is the vapour (y) is always
higher than that in the liquid (x).
The reverse is true for toluene (less volatile component).
a fixed original amount of benzene-toluene mixture; the temperature,
mole fractions in the vapour phase and liquid phase change continuously
over the entire distillation process: both x
and y decreases as temperature increases.
important observation here that has direct impact on the analysis of
distillation process is that for separation by distillation to be useful,
2 phases must be present - vapour and liquid. The more
volatile component will distribute itself more
in the vapour phase than the liquid phase. No separation is achieved
if all the liquid is vapourised.
of concentration changes using phase diagram does
not take into consideration the quantity
changes: as heating continues, the quantity of liquid remaining in the
container gradually decreases as more is turned into vapour. The quantity
of vapour produced increases.
important observation is that as the quantity
of vapour obtained increases,
the concentration of the more volatile component
in the vapour decreases: see for
examples, changes from point (b) to (d). This is because as heating
continues and temperature in the residual liquid increases, more of
the less volatile component is being vapourized as well.
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