Page 354 - Chemical engineering design

DESIGN INFORMATION AND DATA
where L v D latent heat at the required temperature, kJ/kmol,
T D temperature, K, 329
B, C D coefficients in the Antoine equation (equation 8.20),
z D z gas z liquid (where z is the compressibility constant), calculated
from the equation:
0.5
P r
z D 1 (8.17)
T 3
r
P r D reduced pressure,
T r D reduced temperature.
If an experimental value of the latent heat at the boiling point is known, the Watson
equation (Watson, 1943), can be used to estimate the latent heat at other temperatures.
0.38
T c T

L v D L v,b 8.18
T c T b
where L v D latent heat at temperature T, kJ/kmol,
L v,b D latent heat at the normal boiling point, kJ/kmol,
T b D boiling point, K,
T c D critical temperature, K,
T D temperature, K.
Ž
Over a limited range of temperature, up to 100 C, the variation of latent heat with temper-
ature can usually be taken as linear.

8.10.1. Mixtures
For design purposes it is usually sufficiently accurate to take the latent heats of the
components of a mixture as additive:
L v mixture D L v1 x 1 C L v2 x 2 CÐ Ð Ð 8.19
where L v1 ,L v2 D latent heats of the components kJ/kmol,
x 1 ,x 2 D mol fractions of components.

Example 8.10
Estimate the latent heat of vaporisation of acetic anhydride, C 4 H 6 O 3 , at its boiling point,
Ž
Ž
139.6 C (412.7 K), and at 200 C (473 K).
Solution
For acetic anhydride T c D 569.1K, P c D 46 bar,
Antoine constants A D 16.3982
B D 3287.56
C D 75.11
Experimental value at the boiling point 41,242 kJ/kmol.