Aspen Physical Property System - Physical Property Models

Where the L, M, and N are parameters that vary depending on the equation of
state and whether the temperature is above or below the critical temperature
of the component.
For Peng-Robinson equation of state:
Subcritical T Supercritical T
L (0)
M (0)
N (0)
L (1)
M (1)
N (1)
Twu Alpha Function
0.272838 0.373949
0.924779 4.73020
1.19764 -0.200000
0.625701 0.0239035
0.792014 1.24615
2.46022 -8.000000
The Twu alpha function is a theoretically-based function that is currently
recognized as the best available alpha function. It behaves better than other
functions at supercritical conditions (T > Tc).
Where the L, M, and N are substance-dependent parameters that must be
determined from regression of pure-component vapor pressure data or other
data such as liquid heat capacity.
Parameter
Name/Element
2 Thermodynamic Property Models 71
Symbol Default MDS Lower
Limit
Upper
Limit
PRTWUP/1 L — X — — —
PRTWUP/2 M 0 X — — —
PRTWUP/3 N 0 X — — —
Mathias-Copeman Alpha Function
Units
This is an extension of the Peng-Robinson alpha function which provides a
more accurate fit of vapor pressure for polar compounds.
For c2,i = 0 and c3,i = 0, this expression reduces to the standard Peng-
Robinson formulation if c2,i = mi. You can use vapor pressure data if the
temperature is subcritical to regress the constants. If the temperature is
supercritical, c2,i and c3,i are set to 0.
Parameter
Name/Element
Symbol Default MDS Lower
Limit
(7)
Upper
Limit
Units
TCPR T ci TC X 5.0 2000.0 TEMPERATURE
PCPR p ci PC X 10 5
10 8
PRESSURE