Aspen Physical Property System - Physical Property Models

156 2 Thermodynamic Property Models
(14)
(15)
The symmetric reference state defined by Eq. 6 is restricted to systems
containing a single electrolyte component. For multi-electrolyte systems, the
symmetric reference state can be generalized from Eq. 6 as follows:
(16)
where m applies to all molecular components in the system. The symmetric
reference state is a molecular-component-free medium.
Infinite-dilution aqueous solution
The condition of infinite-dilution aqueous solution for ionic components can be
written as follows:
xc=xa=0 (17)
This condition applies to all ionic components in the solution, and water must
be present in the solution for this reference state. In terms of the activity
coefficients for ionic components, the condition for the aqueous solution as
the unsymmetric reference state can be written as follows (where w=water):
(18)
This equation applies to all ionic components in the solution.
Local Interaction Term
In an electrolyte system, all component species can be categorized as one of
three types: molecular species (solvents), m; cationic species (cations), c;
and anionic species (anions), a. The model assumes that there are three
types of local composition interactions. The first type consists of a central
molecular species with other molecular species, cationic species, and anionic
species in the immediate neighbourhood. Here, local electroneutrality is
maintained. The other two types are based on the like-ion repulsion
assumption and have either a cationic or anionic species as the central
species. They also have an immediate neighbourhood consisting of molecular
species and oppositely charged ionic species. Accordingly, the excess Gibbs
energy from local interactions for an electrolyte system can be written as
follows:
or
(19)