Modelling of liquid-vapor-solid equilibria in the NH3-CO2-H2O system
Maria G. Lioliou, Baard Kaasa, Svein Berg
Statoil ASA, R&D, N-7005, Trondheim, Norway
[email protected]
Keywords MultiScale©; thermodynamic model; electrolyte; Pitzer model; chilled ammonia
Introduction
A thermodynamic model has been developed to describe CO2 capture processes from ammonia
solutions. MultiScale©1 was used as a basis for the development of both the model and a new
process simulation tool. The model consists of (a) components and equilibrium reactions,
including mass balances and electro neutrality, (b) a numerical method to solve all the equilibria,
(c) thermodynamic relations describing the pressure, temperature and composition dependencies
of the ideal system and (d) phase models that describe the non-ideal behavior of the components
and phases. The calculations are based on an extended database with salt solubility data, VLE
data and thermal properties of the chemical species.
Components and equilibria
The components and main reactions of the system are given in equations (1)-(9). The respective
thermodynamic equilibrium expressions were introduced in the model.
CO2 ( g ) = CO2 (aq)
NH 3 ( g ) = NH 3 ( aq)
H 2O( g ) = H 2O(aq)
H 2O = H + + OH −
K H0 (CO2 ) =
K H0 ( NH3 ) =
a CO2
f CO2
−
3
CO2 + H 2 O = H + HCO
a NH3
f NH3
f H 2O =a H2O PH*2O
K w0 =
(1)
+
(2)
−
3
+
2−
3
HCO = H + CO
+
4
+
(3)
NH = H + NH 3
(4)
NH 4+ + HCO3− = NH 4 HCO3 (s)
K10 ( CO2 ) =
K 20 ( CO2 ) =
a H + a HCO−
3
aCO2 aH 2O
3
aHCO−
(6)
3
K 0 ( NH4+ ) =
(5)
a H + a CO2−
a H + a NH
aNH +
4
3
(7)
a H + a OH −
NH 4+ + HCO3− = NH 2COO− + H 2O
aH 2O
0
K carbam
=
K 0 sp = aNH + aHCO−
4
3
(8)
a NH 2COO− a H 2O
aNH 4 aHCO−
3
(9)
Phase models
There are three possible phases present in the NH3-CO2-H2O system: gas, liquid and solid. Most
of the reactions are taking place at low pressure, conditions under which the gas phase may be
treated as ideal gas. The SRK equation of state, given by:
P=
RT
a
−
V − b V (V + b )
(10)
was included to describe possible non-ideal behavior of the gas phase in sections where the
pressure might be high, such as a regenerator operating under high pressure. The aqueous phase
is expected to be far from ideal due to high concentrations of dissolved CO2 and NH3. The Pitzer
semi-empirical equation for the excess Gibbs energy of aqueous, salt-containing systems was
used to describe the non-ideal behavior of the aqueous phase. Simplifications of the system were
adopted, thus the only parameters needed are the binary and ternary interaction parameters βij0
and τijk, which are expressed through temperature dependent equations.
Model testing
The vapor pressure as a function of CO2 concentration when NH3 concentration is kept constant,
at various temperatures is given in Figure 1. The total molality of ammonia is in the range 0.5-15
mole/kg H2O and of CO2 up to 15 mole/kg H2O. Simulations were compared to approximately
500 data from the open literature2,3,4,5,6. The thermodynamic model is capable of describing
reliably the vapor-liquid in the system in a temperature range from 20 to 100oC up to very
concentrated solutions.
0.20
80
NH3=0.49752
o
Temp=20 C
o
Temp=60 C
NH3=0.721
NH3=0.5078
0.16
NH3=1.02846
NH3=0.728
60
NH3=0.966
NH3=0.1288
NH3=1.0285
NH3=1.56242
0.12
NH3=1.129
NH3=2.11016
NH3=2.1102
40
NH3=2.186
NH3=2.658
0.08
NH3=3.837
NH3=8.14
0.04
NH3=11.69
NH3=11.79
0
0.00
0.0
0.3
0.6
0.9
1.2
1.5
1.8
0.0
100
2.5
5.0
7.5
10.0
12.5
15.0
80
o
o
Temp=100 C
Temp=80 C
75
60
50
NH3=0.591
NH3=1.087
NH3=2.006
NH3=4.14
25
NH3=5.93
Vapour Pressure (bar)
Vapour Pressure (bar)
Vapor pressure (bar)
NH3=3.977
20
NH3=1.079
NH3=3.12
NH3=9.57
NH3=11.2
NH3=14.19
40
20
NH3=9.03
NH3=12.17
0
0
0
2
4
6
8
CO concentration (mole/kgH O)
10
12
0
2
4
6
8
10
12
CO concentration (mmol/kgH O)
CO2 concentration (mole / kg H2O)
Figure 1: Vapour pressure for CO2-NH3-H2O solutions, as a function of CO2 concentration at different
temperatures. Symbols are experimental values of vapour pressure, while lines represent the simulations.
References
1
MultiScale 7.1, Petrotech, P.O.Box 575, 5501, Haugesund, Norway: [email protected]
2
Göppert U., Maurer G., Fluid Phase Equilibria, 41 (1988) 153-185
3
Jänecke, E., Zeitschrift für Electrochemie, 35 (1929) 716-728
4
Van Krevelen D. W., Hoftijzer P. J., Huntjens F., Recueil des travaux chimiques des Pays-Bas,
68 (1949) 191-216
5
Kurz, F., Rumpf, B., Maurer, G., Fluid Phase Equilibria, 104 (1995) 261-2758
6
Pexton S., Badger E. H. M., Journal of the Society of Chemical Industry, 57 (1938) 107-110