2.3. Relationship of activity and fugacity with
concentration
The framework of the thermodynamic equilibrium model
was built up from equilibrium constants, activity and fugac-
Table 5
Redox equilibrium constants in anaerobic digesters
Redox equilibrium reactions Constantsa log (Kredox)
2H2O (l)=O2 (g) + 2H2 −83.1221
CH4 (g) + 2H2O (l) =CO2 (g) + 4H2 −22.9174
C2H4O2 (aq.) + 2H2O (l) = 2CO2 (g) + 4H2 −13.8736
Note: aThese constants were calculated by Pourbaix’s method [24] at 25 ◦C.
ity. Table 6 shows the framework of equilibrium relations for
methanogenesis.
These properties (activity and fugacity) are related to the
observable concentration and partial pressures. The activity (ai)
is defined by actual mole fraction (xi) and activity coefficient
(γi) [15] as below
ai = γixi (13)
where the activity coefficient (γi) tends to one as (xi→1) except
in the critical region. The activity coefficient (γi) can be calculated
by ion interaction theories (Debye-H¨uckel and Pitzer
interactions) in solution phase.
The fugacity (fi) is defined by actual partial pressure (yiP)
and fugacity coefficient (Φi).
fi = ΦiyiP (14)
where the fugacity coefficient (Φi) is a function of PVT, and can
be calculated using the Ideal equation of State and Prausnitz’s
empirical equation of state [16].
2.3.1. Ideal model
The ideal model is useful to estimate the fundamental
behaviour of anaerobic degradation process without the introduction
of empirical parameters. The ideal equilibrium model
assumed no interactions among solvent and/or solute components
in solution phase or among molecules in gas phase. This
assumption holds in the infinitely dilute system at constant P
and T. The ideal model was developed by using the ideal gas
equation of state and ideal liquid (water) in which both fugacity
and activity coefficients are unity.
2.3.1.1. Fugacity coefficient. The ideal equilibrium model was
built up from ideal gas equation of state.
Z ≡ PV
NRT
= f (T, P) = 1 (15)