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CHEMICAL ENGINEERING
From Trouton’s rule:
L v,b D 100 ð 412.7 D 41,270 kJ/kmol
Note: the close approximation to the experimental value is fortuitous, the rule normally
gives only a very approximate estimate.
From Haggenmacher’s equation:
1
at the b.p. P r D D 0.02124
46
412.7
T r D D 0.7252
569.1
0.02124
0.5
z D 1 D 0.972
0.7252 3
2
8.32 ð 3287.6 ð 412.7 ð 0.972
L v,b D D 39,733 kJ/mol
412.7 75.11 2
Ž
At 200 C, the vapour pressure must first be estimated, from the Antoine equation:
B
ln P D A
T C C
3287.56
ln P D 16.3982 D 8.14
473 75.11
P D 3421.35 mmHg D 4.5bar
4.5
P c D D 0.098
46
473
T c D D 0.831
569.1
0.098
0.5
z D 1 D 0.911
0.831 3
2
8.32 ð 3287.6 ð 473 ð 0.911
L v D D 35,211 kJ/kmol
473 75.11 2
Using Watson’s equation and the experimental value at the b.p.
0.38
569.1 473

L v D 41,242 D 34,260 kJ/kmol
569.1 412.7
8.11. VAPOUR PRESSURE

If the normal boiling point (vapour pressure D 1 atm) and the critical temperature and
pressure are known, then a straight line drawn through these two points on a plot of log-
pressure versus reciprocal absolute temperature can be used to make a rough estimation
of the vapour pressure at intermediate temperatures.