University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
PVT relationship for liquids
A generalized correlation to estimate the molar volume of saturated liquid
proposed by Rackett
(1)
Critical constants VC, ZC, and TC (APP. B.) (J.M. Smith, Introduction to Chemical
Engineering Thermodynamics, 4thEd., 1987, McGraw-Hill).
Lydrson and coworkers developed a general method for estimation of liquid
volume based on the principles of corresponding state
(2)
ρr is reduced density , where ρC is density at critical point.
The generalized correlation is shown by below figure; this figure may be used
directly with Eq. (1) for determination of liquid volumes if the value of the critical
volume is known.
1
University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
A better procedure is to make use of a single known liquid volume (state 1) by the
identity,
V2  V1
 r1
r 2
V2 = required volume
V1 = known volume
r1,r2 = reduced densities read from figure
Example: a) Estimate the density of saturated liquid ammonia at 310K. b) Estimate
the density of liquid ammonia at 310 K and 100bar.
310
Tr 
 0.7643
405.6
VC  72.5 and Z C  0.242 ( from App.B)we get
V Sat  VC Z C(1Tr )
errer % 
0.2857
 (72.5)(0.242) ( 0.2357)
0.2857
 28.35 cm 3 mol 1
theoratica l 29.14  28.35
 2.71%
exp erimental
29.14
b) The reduced conditions
100
Tr  0.7643 , and Pr 
 0.887
112.8
From figure we have ρr = 2.38
V 
VC
r

72.5
 30.5 cm 3 mol 1
2.38
30.5  28.6
 6.64%
28.6
For saturated liquid at Tr = 0.764, we find from figure that ρr = 2.34. Substituation
of known value into below equation
errer % 
V2  V1
 r1
 2.34 
3
1
 (29.14)
  28.65 cm mol
r 2
 2.38 
2