Aspen Physical Property System - Physical Property Models

It should be understood that equations 6a-8a should be used only in
equations 3, 4, and 7. Ms, ds, and �s were already assumed as constants when
deriving equations 6 and 8 for mixed-solvent systems.
Local Interaction Contribution
The local interaction contribution is accounted for by the Non-Random Two
Liquid theory. The basic assumption of the NRTL model is that the nonideal
entropy of mixing is negligible compared to the heat of mixing: this is indeed
the case for electrolyte systems. This model was adopted because of its
algebraic simplicity and its applicability to mixtures that exhibit liquid phase
splitting. The model does not require specific volume or area data.
The effective local mole fractions Xji and Xii of species j and i, respectively, in
the neighborhood of i are related by:
Where:
Xj = xjCj
Gji
�ji
2 Thermodynamic Property Models 101
=
=
(9)
(Cj = zj for ions and Cj = unity for molecules)
�ji = Nonrandomness factor
gji and gii are energies of interaction between species j and i, and i and i,
respectively. Both gij and �ij are inherently symmetric (gij = gji and �ij = �ji).
Similarly,
Where:
Gji,ki
�ji,ki
=
=
�ji,ki = Nonrandomness factor
Apparent Binary Systems
(10)
The derivations that follow are based on a simple system of one completely
dissociated liquid electrolyte ca and one solvent B. They will be later extended