298.15 K.
ln γi = −Az2i
√
I (25)
A =
√ e
DkT
3
×
%
2πρWNA
1000
(26)
I = 0.5
i
mi(zi)2 (27)
where zi is the charge of ion component (i). A is the Debye-
H¨uckel constant. Electronic charge e is 1.60206×10−19
Coulomb. k is Boltzmann’s constant. NA is Avagadro’s number.
Dielectric constant (D) of water is 78 (F/m) at 25◦C. T is
temperature. The valid range of ionic strength (I) is less than
0.005 (mol/kg) [15]. Pure water density (ρW) was considered
in Eq. (28).
ρW (g/l) = 322.0 ×
1 + 1.99274064τ(1/3),+1.09965342τ(2/3),−0.510839303τ(5/3),
−1.75493479τ(16/3),−45.5170352τ(43/3),−674694.450τ(110/3),
(28)
where τ = (1−T/647.14) [17].
The water activity of Debye-H¨uckel–Praunitz model is estimated
from the partial water pressure over saturated water
vapour pressure.
ln aW = ln
ΦWyWP
Φsat
WPsat
W exp
&' P
Psat
W
(υ
Liquid
W /RT) dp
( (29)
where Psat
i is the vapour pressure and superscript sat stands for
saturation.Φsat
i is the fugacity coefficient at the saturated vapour
pressure, and the Poynting factor was used to describe the non
ideal effect of pressure in the solution phase.
poynting factor = exp
P
Psat
i
υ
Liquid
i
RT
dp (30)
where the molar liquid volume (υ
Liquid
i ) is the ratio of the molecular
weight to the density. It represents the pressure effect at
constant T. The molar water liquid volume (υ
Liquid
W ) can be
calculated from Edward and Maurer’s empirical method [18].
υ
Liquid
W (l/mol)=7.1655 × 10−8T 2 − 3.9145 · 10−5T + 0.0234
(31)
For a mixture, the molar volume (υ
∞
i−W) of solute (i) can be
calculated from the equations of Brelvi and O’Connell [19].
υ
∞
i−W
= K
∞
WRT(1 + C
∞
i−W) (32)
where υ
∞
i−W is partial molar volume at infinite dilution (l/mol).
The isothermal compressibility K
∞
W (atm−1) at infinite dilution
was calculated as follows:
1
K
∞
W
= ρWRT
18.02
exp
)
−0.42704
ρWv
⊗
W
18.02
− 1
+ 2.089
ρWv
⊗
W
18.02
− 1
2
− 0.42367
ρWv
⊗
W
18.02
− 1
3*
− 1
(33)
where the characteristic volume (v
⊗
W) of water is 0.0464 (l/mol).
Experimental compressibility data may be used to determine v
⊗
i
for component (i) in anaerobic digesters, but the critical volume
(vc) may also be used. The reduced volume integral (C
∞
i−W) is
the reduced volume integral of the molecular direct correlation
function at infinite dilution. The reduced volume integral (C
∞
i−W)
was calculated using one of the following empirical equations
[19]:
when 2.0 ≤ ρWv
⊗
W
18.02
≤ 2.785
C
∞
i−W
= exp
−2.4467 + 2.12074
ρWv
⊗
W
18.02
v
⊗
i
v
⊗
W
0.62
(34)
when 2.785 ≤ ρWv
⊗
W
18.02
≤ 3.2
C
∞
i−W
= exp
)
3.02214 − 1.87085
ρWv
⊗
W
18.02
+0.71955
ρWv
⊗
W
18.02
2*
v
⊗
i
v
⊗
W
0.62
(35)
However, a liquid is nearly incompressible at a temperature
well belowthe critical. In that case the pressure effect on solution
phase is not large unless the pressure is very high or the temperature
is very low. The Poynting factor can be approximated to
unity.
2.3.3. Pitzer–Praunitz Model
In Pitzer–Prausnitz model both activity and fugacity are
described by empirical parameters. The empirical parameters
are defined as ion–ion, ion–molecule, and molecule–molecule
interactions. The Pitzer–Prausnitz model can be used out of
the range of Debye-H¨uckel–Prausnitz model (0.005≤I≤20)
[20]. The empirical Pitzer interactions were superimposed on
the Debye-H¨uckel–Praunitz model.
2.3.3.1. Activity coefficient. The Pitzer interaction uses an
empirical expression in addition to the Debye-H¨uckel interaction.
This model, characterised by three empirical parameters
(ion–ion, ion–molecule, and molecule–molecule interactions),
can be used outside the range of Debye-H¨uckel–Praunitz model