2.4. Solution algorithm
The thermodynamic equilibrium model was evaluated by
using a Newton–Raphson method [21]. The convergence constant
in the iterative solution algorithm was 10−14. The
solution algorithm was evaluated in a sequence of stages
commencing with the Ideal activity model followed by
the Debye-H¨uckel–Prausnitz, and finally the Pitzer–Prausnitz
model. Initially the ideal model is converged then the ideal
solution is used to initialize the Debye-H¨uckel–Prausnitz
model. The converged the Debye-H¨uckel–Prausnitz solution
is subsequently used to initialise the Pitzer–Prausnitz
model.
3. Results and discussions
For the catabolic reactions of methanogenesis in the
anaerobic digestion process, the three thermodynamic equilibrium
models (the ideal, the Debye-H¨uckel–Praunitz, and the
Pitzer–Praunitz) were considered under isothermal and isobaric
conditions.
One kilogram of acetic acid solution injected into a closed
batch reactor was modelled at constant P (1 atm) and T
(298.15 K). As the initial moles of acetic acid (HAc) in
the system was increased, the initial moles of solvent water
(H2O) was decreased to conserve the system mass of 1 kg
(H2O·18.02 + HAc·60.04 = 1000(g)).
Fig. 1.