Aspen Physical Property System - Physical Property Models

76 2 Thermodynamic Property Models
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This alpha function is used in the Redlich-Kwong-UNIFAC model which is the
basis for the SR-POLAR property method.
Parameter Symbol Default MDS Lower Upper Units
Name/Element
Limit Limit
TCRKU T ci TC X 5.0 2000.0 TEMPERATURE
PCRKU p ci PC X 10 5
OMGRKU � i OMEGA X -0.5 2.0 —
RKUPP0 p 1,i — X — — —
RKUPP1 p 2,i 0 X — — —
RKUPP2 p 3,i 0 X — — —
Mathias-Copeman Alpha Function
10 8
PRESSURE
The Mathias-Copeman alpha function is suitable for use with both polar and
nonpolar components. It has the flexibility to fit the vapor pressure of most
substances from the triple point to the critical point.
(14)
For c2,i=0 and c3,i=0 this expression reduces to the standard Redlich-Kwong-
Soave formulation if c1,i=mi. If the temperature is subcritical, use vapor
pressure data to regress the constants. If the temperature is supercritical, set
c2,i and c3,i to 0.
Parameter
Name/Element
Symbol Default MDS Lower
Limit
Upper
Limit
Units
TCRKS T ci TC X 5.0 2000.0 TEMPERATURE
PCRKS p ci PC X 10 5
RKSMCP/1 c 1,i — X — — —
RKSMCP/2 c 2,i 0 X — — —
RKSMCP/3 c 3,i 0 X — — —
10 8
PRESSURE
Schwartzentruber-Renon-Watanasiri Alpha Function
The Schwartzentruber-Renon-Watanasiri alpha function is:
(15)
Where mi is computed by equation 6 and the polar parameters p1,i, p2,i and
p3,i are comparable with the c parameters of the Mathias-Copeman
expression. Equation 15 reduces to the standard Redlich-Kwong-Soave
formulation if the polar parameters are zero. Equation 15 is very similar to
the extended Mathias equation, but it is easier to use in data regression. It is
used only for temperatures below critical. The Boston-Mathias extrapolation is
used for temperatures above critical, that is, use equation 7 with:
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