Aspen Physical Property System - Physical Property Models

References
L. Haar, J.S. Gallagher, and J.H. Kell, "NBS/NRC Steam Tables," (Washington:
Hemisphere Publishing Corporation, 1984).
Nothnagel
The Nothnagel equation-of-state calculates thermodynamic properties for the
vapor phase. It is used in property methods NRTL-NTH, UNIQ-NTH, VANL-
NTH, and WILS-NTH. It is recommended for systems that exhibit strong vapor
phase association. The model incorporates the chemical theory of
dimerization to account for strong association and solvation effects, such as
those found in organic acids, like acetic acid. The equation-of-state is:
Where:
b =
bij
=
nc = Number of components in the mixture
The chemical reaction for the general case of a mixture of dimerizing
components i and j is:
The chemical equilibrium constant for the dimerization reaction on pressure
basis Kp is related to the true mole fractions and fugacity coefficients:
Where:
yi and yj = True mole fractions of monomers
yij = True mole fraction of dimer
�i = True fugacity coefficient of component i
Kij = Equilibrium constant for the dimerization of i and j, on a
pressure basis
When accounting for chemical reactions, the number of true species n t in the
mixture changes. The true molar volume V/n t
is calculated from the
equation-of-state. Since both V and n t change in about the same proportion,
this number does not change much. However, the reported molar volume is
the total volume over the apparent number of species: V/n a . Since the
apparent number of species is constant and the total volume decreases with
2 Thermodynamic Property Models 33