Page 356 - Chemical engineering design

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DESIGN INFORMATION AND DATA
Several equations have been developed to express vapour pressure as a function of
temperature. One of the most commonly used is the three-term Antoine equation, Antoine
(1888):
B
ln P D A 8.20
T C C
where P D vapour pressure, mmHg,
A, B, C D the Antoine coefficients,
T D temperature, K.
Vapour pressure data, in the form of the constants in the Antoine equation, are given in
several references; the compilations by Ohe (1976), Dreisbach (1952), Hala et al. (1968) and
Hirata et al. (1975) give values for several thousand compounds. Antoine vapour pressure
coefficients for the elements are given by Nesmeyanov (1963). Care must be taken when
using Antoine coefficients taken from the literature in equation 8.20, as the equation is often
written in different and ambiguous forms; the logarithm of the pressure may be to the base 10,
instead of the natural logarithm, and the temperature may be degrees Celsius, not absolute
temperature. Also, occasionally, the minus sign shown in equation 8.20 is included in the
constant B and the equation written with a plus sign. The pressure may also be in units other
than mm Hg. Always check the actual form of the equation used in the particular reference.
Antoine constants for use in equation 8.20 are given in Appendix C. Vapour pressure data
for hydrocarbons can be found in the four-volume handbook by Yaws (1994 1995).

8.12. DIFFUSION COEFFICIENTS (DIFFUSIVITIES)

Diffusion coefficients are needed in the design of mass transfer processes; such as gas
absorption, distillation and liquid-liquid extraction.
Experimental values for the more common systems can be often found in the literature,
but for most design work the values will have to be estimated. Methods for the prediction
of gas and liquid diffusivities are given in Volume 1, Chapter 10; some experimental
values are also given.

8.12.1. Gases
The equation developed by Fuller et al. (1966) is easy to apply and gives reliable
estimates:
1 1
1/2
7
1.013 ð 10 T 1.75 C
M a M b
D v D 8.21
2
1/3 1/3

P v i C v i
a b
2
where D v D diffusivity, m /s,
T D temperature, K,
M a ,M b D molecular masses of components a and b,
P D total pressure, bar,