Aspen Physical Property System - Physical Property Models

The denominator of equation 8 is given by equation 6. The numerator (the
apparent partial pressure of HF) is the hypothetical partial pressure only if all
of the hexamer was dissociated. If you substitute equation 4, then equation 8
becomes:
K is known from Long et al., or can be regressed from (p,V,T) data. The
apparent mole fraction of HF, y a , is known to the user and the simulator, but
p1, or y = p1/p must also be known in order to calculate the thermodynamic
properties of the mixture. Equation 9 must be solved for p1.
2 Thermodynamic Property Models 27
(9)
Equation 9 can be written as a polynomial in p1 of degree 6:
K(6 - 5y a )(p1) 6 + p1 - py a = 0 (9a)
A second order Newton-Raphson technique is used to determine p1. Then p6
can be calculated by equation 5, and f is known (equation 7).
Gibbs Energy and Fugacity
The apparent fugacity coefficient is related to the true fugacity coefficient and
mole fractions:
(10)
Equation 10 represents a correction to the ideal mixing term of the fugacity.
The ratio of the true number of species to the apparent number of species is
similar to the correction applied in equation 2. Since the ideal gas law is
assumed, the apparent fugacity coefficient is given by the equation. All
variables on the right side are known.
For pure HF, y a = 1:
(11)
From the fugacity coefficient, the Gibbs energy departure of the mixture or
pure apparent components can be calculated:
Enthalpy and Entropy
(12)
(12a)
For the enthalpy departure, the heat of reaction is considered. For an
arbitrary gas phase reaction: