Aspen Physical Property System - Physical Property Models

Parameter
Name/Element
2 Thermodynamic Property Models 77
Symbol Default MDS Lower
Limit
(17)
Upper
Limit
Units
TCRKS T ci TC X 5.0 2000.0 TEMPERATURE
PCRKS p ci PC X 10 5
OMGRKS � i OMEGA X -0.5 2.0 —
RKSSRP/1 p 1,i — X — — —
RKSSRP/2 p 2,i 0 X — — —
RKSSRP/3 p 3,i 0 X — — —
10 8
Extended Gibbons-Laughton Alpha Function
PRESSURE
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 Soave alpha function if
This function is not intended for use in supercritical conditions. To avoid
predicting negative alpha, when Tri>1 the Boston-Mathias alpha function is
used instead.
Parameter
Name/Element
Symbol Default MDS Lower
Limit
Upper
Limit
SRKGLP/1 X — X — — —
SRKGLP/2 Y 0 X — — —
SRKGLP/3 Z 0 X — — —
SRKGLP/4 n 2 X — — —
Units
SRKGLP/5 T lower 0 X — — TEMPERATURE
SRKGLP/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.