Page 348 - Chemical engineering design

DESIGN INFORMATION AND DATA
Example 8.6
Estimate the specific heat capacity of urea, CH 4 N 2 O. 323
Solution

Element mol. mass Heat capacity
C 12 7.5 D 7.5
H 4 4 ð 9.6 D 38.4
N 28 2 ð 26.0 D 52.0
O 16 16.7 D 16.7
Ž
60 114.6 J/mol C
114.6 Ž Ž
Specific heat capacity D D 1.91 J/g C kJ/kg C
60
Ž
Experimental value 1.34 kJ/kg C.
Kopp’s rule does not take into account the arrangement of the atoms in the molecule,
and, at best, gives only very approximate, “ball-park” values.
For organic liquids, the group contribution method proposed by Chueh and Swanson
(1973a,b) will give accurate predictions. The contributions to be assigned to each
molecular group are given in Table 8.3 and the method illustrated in Examples 8.7 and 8.8.
Liquid specific heats do not vary much with temperature, at temperatures well below
the critical temperature (reduced temperature <0.7).
The specific heats of liquid mixtures can be estimated, with sufficient accuracy for most
technical calculations, by taking heat capacities of the components as additive.
For dilute aqueous solutions it is usually sufficient to take the specific heat of the
solution as that of water.

Example 8.7
Using Chueh and Swanson’s method, estimate the specific heat capacity of ethyl bromide
Ž
at 20 C.

Solution
Ethyl bromide CH 3 CH 2 Br

Group Contribution No. of
CH 3 36.84 1 D 36.84
CH 2 30.40 1 D 30.40
Br 37.68 1 D 37.68
Ž
Total 104.92 kJ/kmol C