Aspen Physical Property System - Physical Property Models

Where both Ai* and Am are calculated by using equation 1. Ai* and Am are
written in terms of equation-of-state parameters.
The simplification of constant packing fraction (Vm / b) is used:
With:
Therefore:
The mixing rule is:
Where �' is slightly different from � for the Huron-Vidal mixing rule:
Where �1 and �2, depend on the equation-of-state (see Huron-Vidal Mixing
Rules). If equation 6 is applied at infinite pressure, the packing fraction goes
to 1. The excess Helmholtz energy is equal to the excess Gibbs energy. The
Huron-Vidal mixing rules are recovered.
The goal of these mixing rules is to be able to use binary interaction
parameters for activity coefficient models at any pressure. These parameters
have been optimized at low pressures. UNIFAC is chosen for its predictive
character. Two issues exist: the packing fraction is not equal to one, and the
excess Gibbs and Helmholtz energy are not equal at the low pressure where
the UNIFAC parameters have been derived.
Fischer (1993) determined that boiling point, the average packing fraction for
about 80 different liquids with different chemical natures was 1.1. Adopting
this value, the difference between liquid excess Gibbs energy and liquid
excess Helmholtz energy can be computed as:
The result is a predictive mixing rule for cubic equations of state. But the
original UNIFAC formulation gives the best performance for any binary pair
with interactions available from UNIFAC. Gas-solvent interactions are
unavailable. However, it has poor accuracy for highly asymmetric such as CH4
2 Thermodynamic Property Models 83
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