Aspen Physical Property System - Physical Property Models

Boston-Mathias Extrapolation
For light gases at high reduced temperatures (> 5), equation 4 gives
unrealistic results. The boundary conditions are that attraction between
molecules should vanish for extremely high temperatures, and � reduces
asymptotically to zero. Boston and Mathias derived an alternative function for
temperatures higher than critical:
With
=
=
Where mi is computed by equation 5, and equation 4 is used for subcritical
temperatures. Additional parameters are not needed.
Extended Gibbons-Laughton Alpha Function
The extended Gibbons-Laughton 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.
Where Tr is the reduced temperature; Xi, Yi and Zi are substance dependent
parameters.
This function is equivalent to the standard Peng-Robinson alpha function if
Parameter
Name/Element
70 2 Thermodynamic Property Models
Symbol Default MDS Lower
Limit
(6)
Upper
Limit
PRGLP/1 X — X — — —
PRGLP/2 Y 0 X — — —
PRGLP/3 Z 0 X — — —
PRGLP/4 n 2 X — — —
Units
PRGLP/5 T lower 0 X — — TEMPERATURE
PRGLP/6 T upper 1000 X — — TEMPERATURE
Twu Generalized Alpha Function
The Twu generalized 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) and when the acentric
factor is large. The improved behavior at high values of acentric factor is
important for high molecular weight pseudocomponents. There is no limit on
the minimum value of acentric factor that can be used with this function.