Aspen Physical Property System - Physical Property Models

Where:
The parameters �1and �2depend on the equation-of-state used. In general a
cubic equation-of-state can be written as:
Values for �1and �2 for the Peng-Robinson and the Soave-Redlich-Kwong
equations of state are:
Equation-of-state � 1 � 2
Peng-Robinson
Redlich-Kwong-Soave 1 0
This expression can be used at any pressure as a mixing rule for the
parameter. The mixing rule for b is fixed by equation 3. Even when used at
other pressures, this expression contains the excess Gibbs energy at infinite
pressure. You can use any activity coeffecient model to evaluate the excess
Gibbs energy at infinite pressure. Binary interaction coefficients must be
regressed. The mixing rule used contains as many binary parameters as the
activity coefficient model chosen.
This mixing rule has been used successfully for polar mixtures at high
pressures, such as systems containing light gases. In theory, any activity
coefficient model can be used. But the NRTL equation (as modified by Huron
and Vidal) has demonstrated better performance.
The Huron-Vidal mixing rules combine extreme flexibility with thermodynamic
consistency, unlike many other mole-fraction-dependent equation-of-state
mixing rules. The Huron-Vidal mixing rules do not allow flexibility in the
description of the excess molar volume, but always predict reasonable excess
volumes.
The Huron-Vidal mixing rules are theoretically incorrect for low pressure,
because quadratic mole fraction dependence of the second virial coefficient (if
derived from the equation-of-state) is not preserved. Since equations of state
are primarily used at high pressure, the practical consequences of this
drawback are minimal.
The Gibbs energy at infinite pressure and the Gibbs energy at an arbitrary
high pressure are similar. But the correspondence is not close enough to
make the mixing rule predictive.
There are several methods for modifying the Huron-Vidal mixing rule to make
it more predictive. The following three methods are used in Aspen Physical
Property System equation-of-state models:
� The modified Huron-Vidal mixing rule, second order approximation
(MHV2)
� The Predictive SRK Method (PSRK)
� The Wong-Sandler modified Huron-Vidal mixing rule (WS)
80 2 Thermodynamic Property Models
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