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CHEMICAL ENGINEERING
Example 8.13
3
Ž
Estimate the surface tension of pure methanol at 20 C, density 791.7 kg/m , molecular
weight 32.04.
Solution
Calculation of parachor, CH 3 OH, Table 8.7.
Group Contribution No.

C 4.8 ð 1 D 4.8
H O 11.3 ð 1 D 11.3
H C 17.1 ð 3 D 51.3
O 20.0 ð 1 D 20.0
87.4

4

87.4 ð 791.7 12 2
D ð 10 D 21.8mJ/m (8.23)
32.04
2
Experimental value 22.5 mJ/m .
8.14. CRITICAL CONSTANTS
Values of the critical temperature and pressure will be needed for prediction methods that
correlate physical properties with the reduced conditions. Experimental values for many
substances can be found in various handbooks; and in Appendix C. Critical reviews of the
literature on critical constants, and summaries of selected values, have been published by
Kudchadker et al. (1968), for organic compounds, and by Mathews (1972), for inorganic
compounds. An earlier review was published by Kobe and Lynn (1953).
If reliable experimental values cannot be found, techniques are available for estimating
the critical constants with sufficient accuracy for most design purposes. For organic
compounds Lydersen’s method is normally used, Lydersen (1955):

T b
T c D 8.25
2
[0.567 C T T ]
M
P c D 8.26
0.34 C P 2
V c D 0.04 C V 8.27
where T c D critical temperature, K,
P c D critical pressure, atm (1.0133 bar),
3
V c D molar volume at the critical conditions, m /kmol,
T b D normal boiling point, K,
M D relative molecular mass,
T D critical temperature increments, Table 8.8,
P D critical pressure increments, Table 8.8,
V D molar volume increments, Table 8.8.