Page 379 - Chemical engineering design

8.16.12. Use of equations of state to estimate specific enthalpy
and density DESIGN INFORMATION AND DATA 353
Computer aided packages for the design and simulation of separation processes will
contain sub-routines for the estimation of excess enthalpy and liquid and vapour density
from the appropriate equation of state.

Specific enthalpy
For the vapour phase, the deviation of the specific enthalpy from the ideal state can be
illustrated using the Redlich-Kwong equation, written in the form:

2
3
2
z C z C z B C B A D 0
where z D the compressibility factor
a ð P
A D
2
R ð T 2.5
b ð P
B D
R ð T
The fugacity coefficient is given by:
A B

ln D z 1 ln z b ln 1
B z

Ž dP
and the excess enthalpy H H D RT C T P d
0 dT v
Ž
where H is enthalpy at the system temperature and pressure and H enthalpy at the ideal
state.
Unless liquid phase activity coefficients have been used, it is best to use the same
equation of state for excess enthalpy that was selected for the vapour-liquid equilibria. If
liquid-phase activity coefficients have been specified, then a correlation appropriate for
the activity coefficient method should be used.

Density

For vapours, use the equation of state selected for predicting the vapour-liquid equilibria.
For liquids, use the same equation if it is suitable for estimating liquid density.

8.17. REFERENCES

AIChE (1983) Design Institute for Physical Property Data, Manual for Predicting Chemical Process Design
Data (AIChemE).
AIChE (1985) Design Institute for Physical Property Data, Data Compilation, Part II (AIChemE).
ANTOINE, C. (1888) Compte rend. 107, 681 and 836. Tensions des vapeurs: nouvelle relation entre les tensions
et les temp´ eratures.
AUGER, C. P. (ed.) (1992) Information Sources in Patents (Bower-Saur).