Aspen Physical Property System - Physical Property Models

mi = Computed by equation 6
Equation 5 = Used for subcritical temperatures
Additional parameters are not needed.
Mathias Alpha Function
This is an extension of the Soave alpha function which provides a more
accurate fit of vapor pressure for polar compounds.
For �i=0, equation 8 reduces to the standard Redlich-Kwong-Soave
formulation, equations 5 and 6. For temperatures above critical, the Boston-
Mathias extrapolation is used, that is, equation 7 with:
2 Thermodynamic Property Models 75
(8)
(9)
(10)
The Mathias alpha function is used in the Redlich-Kwong-Aspen model, which
is the basis for the RK-ASPEN property method. This alpha function is also
available as an option for SRK, SRKKD, SRK-ML, RK-SOAVE, and RKS-BM.
See Soave-Redlich-Kwong Option Codes for more information.
Parameter
Name/Element
Symbol Default MDS Lower
Limit
Upper
Limit
Units
TCRKA T ci TC X 5.0 2000.0 TEMPERATURE
PCRKA p ci PC X 10 5
OMGRKA � i OMEGA X -0.5 2.0 —
† � i — X -2.0 2.0 —
10 8
PRESSURE
† RKAPOL for Redlich-Kwong-Aspen, SRKPOL for SRK and SRKKD, SRKMLP for
SRK-ML, RKSPOL for RKS-BM, or RKSSPO for RK-SOAVE.
Extended Mathias Alpha Function
An extension of the Mathias approach is:
(11)
Where mi is computed by equation 6. If the polar parameters p1,i, p2,i and p3,i
are zero, equation 11 reduces to the standard Redlich-Kwong-Soave
formulation. You can use vapor pressure data to regress the constants if the
temperature is subcritical. Equation 11 is used only for temperatures below
critical.
The Boston-Mathias extrapolation is used for temperatures above critical, that
is, with:
(12)