Aspen Physical Property System - Physical Property Models

However, q(�) can only be written explicitly for � = 5.8. Only an
approximation is possible below that threshold. Dahl and Michelsen use a
second order polynomial fitted to the analytical solution for 10 < � < 13 that
can be extrapolated to low alpha:
Since q(�)is a universal function (for each equation-of-state), the
combination of equations 6 and 7 form the MHV2 mixing rule. Excess Gibbs
energies, from any activity coefficient model with parameters optimized at
low pressures, can be used to determine �, if �i, bi, and b are known. To
compute b, a linear mixing rule is assumed as in the original Huron-Vidal
mixing rules:
This equation is equivalent to the assumption of zero excess molar volume.
The MHV2 mixing rule was the first successful predictive mixing rule for
equations of state. This mixing rule uses previously determined activity
coefficient parameters for predictions at high pressures. UNIFAC was chosen
as a default for its predictive character. The Lyngby modified UNIFAC
formulation was chosen for optimum performance (see UNIFAC (Lyngby
Modified)). However, any activity coefficient model can be used when its
binary interaction parameters are known.
Like the Huron-Vidal mixing rules, the MHV2 mixing rules are not flexible in
the description of the excess molar volume. The MHV2 mixing rules are
theoretically incorrect at the low pressure limit. But the practical
consequences of this drawback are minimal (see Huron-Vidal Mixing Rules,
this chapter).
Reference: S. Dahl and M.L. Michelsen, "High-Pressure Vapor-Liquid
Equilibrium with a UNIFAC-based Equation-of-state," AIChE J., Vol. 36,
(1990), pp. 1829-1836.
Predictive Soave-Redlich-Kwong-Gmehling
Mixing Rules
These mixing rules by Holderbaum and Gmehling (1991) use a relationship
between the excess Helmholtz energy and equation-of-state. They do not use
a relationship between equation-of-state properties and excess Gibbs energy,
as in the Huron-Vidal mixing rules. The pressure-explicit expression for the
equation-of-state is substituted in the thermodynamic equation:
The Helmholtz energy is calculated by integration. A E is obtained by:
82 2 Thermodynamic Property Models
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