Aspen Physical Property System - Physical Property Models

The equations for activity coefficients and the Gibbs free energy are the same
as equations 3 through 6.
Parameters
The Pitzer model in the Aspen Physical Property System involves usersupplied
parameters. These parameters are used in the calculation of binary
and ternary parameters for the electrolyte system. These parameters include
the cation-anion parameters � (0) , � (1) , � (2) , � (3) and C � , cation-cation
parameter �cc', anion-anion parameter �aa', cation1-cation2-common anion
parameter �cc'a, anion1-anion2-common cation parameter �caa', and the
molecule-ion and molecule-molecule parameters � (0) , � (1) , and, C � . The
parameter names in the Aspen Physical Property System and their
requirements are discussed in Pitzer Activity Coefficient Model.
Parameter Conversion
For n-m electrolytes, n and m>1 (2-2, 2-3, 3-4, and so on), the parameter
� (3) corresponds to Pitzer's � (1) . � (2) is the same in both the Aspen Physical
Property System and original Pitzer models. Pitzer refers to the n-m
electrolyte parameters as � (1) , � (2) , � (0) . � (0) and � (2) retain their meanings in
both models, but Pitzer's � (1) is � (3) in the Aspen Physical Property System. Be
careful to make this distinction when entering n-m electrolyte parameters.
Pitzer often gives values of � (0) , � (1) , � (2) , � (3) , and C � that are corrected by
some factors (see Pitzer and Mayorga (1973) for examples). These factors
originate from one of Pitzer's earlier expressions for the excess Gibbs energy:
Where:
144 2 Thermodynamic Property Models
=
na = Mole number of anions
nc = Mole number of cation
(18)
Here � (0) , � (1) , � (2) , and � (3) are multiplied by a factor of 2ncna. C is multiplied
by a factor of 2(ncna) 3/2 .
Aspen Physical Property System accounts for these correcting factors. Enter
the parameters without their correcting factors.
For example, Pitzer gives the values of parameters for MgCl2 as:
4/3� (0) = 0.4698
4/3� (1) = 2.242