Modelling of liquid-vapor-solid equilibria
in the NH3-CO2-H2O system
Maria G. Lioliou, Statoil ASA
Introduction
• CO2 capture by amines is state-of-the-art technology
• Characteristics of a desired technology
− Temperature stability over a temperature range
− High selectivity for CO2
− Non-corrosive
− High cycle life
− Low energy regeneration (stripping)
− Rapid kinetics for scrubbing and stripping
• Can ammonia processes challenge amines with respect to
− energy requirements
− reduced environmental impact
2-
Photo: Marit Hommedal, Statoil
Ammonia for CO2 capture
• Used in De-NOx processes & to remove SO2 and HCl
• Low environmental impact
• Several approaches on the thermodynamics
− Edwards (1978), Pawlikowski (1982), Bieling (1989), Kurz (1995), Krop (1999)
• Tool to predict phase behavior, mass & energy balances
3-
Components & equilibrium reactions
Vapour
Aqueous
CH 4 , N 2 , O2 , Ar
CO2
NH 3
H 2O
CO2
NH 3
H 2O
CO2 + H 2O ↔ H + + HCO3−
Soave-Redlich-Kwong
EOS for gas fugacities
Henry’s law
HCO3− ↔ H + + CO32−
NH 4+ ↔ H + + NH 3
NH 3 + HCO3− ↔ NH 2COO − + H 2O
H 2O ↔ H + + OH −
NH 4+ + HCO3− ↔ NH 4 HCO3 ( s )
Solids
4-
NH 4 HCO3 ( s )
Pitzer model & equilibrium
constants
Solubility
product
Pure solids
Thermal model
• Heat capacities, standard state enthalpies & entropies were used
T
H Tf • H 0f + ∫ C p (i ) (T )dt
•=
0
− For water:
C p ( H 2O=
C p, A + C p,B ⋅ T +
)
C p ,C ⋅10−5
T
2
− Aqueous & dissolved species: C p (i=)
+ C p , D ⋅10−6 T 2
C p, A + C p,B ⋅ T +
C p ,C
T − 200
− Solid(s): treated as particles/species in a water stream
− Gas components:
C idp=
(m)
C=
C idp (i ) + C pres (i )
p (i )
∑ x ⋅C
i
id
p (i )
i
C idp (i=
C p , A + C p , B ⋅ T + C p ,C ⋅ T 2 + C p , D ⋅ T 3
)
5-
residual term:
calculated through the EOS
Data
• Model is based entirely on open literature data
• Approximately 3500 experimental data on:
− VLE
− SLE
− heat capacities, enthalpies, entropies
• Scattered SLE data
• NH4HCO3(s) of primary importance
• Other possible salts: (NH4)2CO3
NH4CO2NH4
NH4HCO3-NH4CO2NH2
(NH4)2CO3-NH4HCO3-H2O
6-
Model testing – NH3-CO2-H2O system
0.20
80
Temp=20oC
)
(
o
T=60
Temp=60 C C
NH3=0.49752
NH3=0.5078
NH3=1.02846
NH3=0.1288
NH3=1.56242
NH3=2.11016
T=20oC
0.16
0.12
o
NH3=0.721
NH3=0.728
NH3=0.966
NH3=1.0285
NH3=1.129
NH3=2.1102
NH3=2.186
NH3=2.658
NH3=3.837
NH3=3.977
NH3=8.14
NH3=11.69
NH3=11.79
60
40
p
20
0.04
0.00
0
0.0
0.3
0.6
0.9
1.2
1.5
1.8
0.0
100
2.5
5.0
7.5
12.5
15.0
80
o
Temp=80
C oC
T=80
T=100oC
Temp=100oC
75
Reliable results at least
up to 100oC
)
60
NH3=1.079
NH3=3.12
NH3=9.57
NH3=11.2
NH3=14.19
(
50
NH3=0.591
NH3=1.087
NH3=2.006
NH3=4.14
NH3=5.93
NH3=9.03
NH3=12.17
25
0
40
20
0
0
2
4
6
8
10
12
0
2
4
6
CO2 concentration (mole/kgH2O)
7-
10.0
p
Vapor pressure (bar)
0.08
NH3: 0.5-13 mole/kg H2O
CO2: up to 15 mole/kg H2O
8
10
12
Data from: Pexton and Badger
(1938), Van Krevelen et al. (1949),
Otsuka et al. (1960), Kurz et al.
(1995), Verbrugge (1979), Göppert
and Maurer (1988), Müller et al.
(1988), Pawlikowski et al. (1982),
Mezger and Payer (1925)
Model testing – NH4HCO3 solubility
 NH 4+   HCO3− 
SR =
K sp ( NH 4 HCO3 )
Supersaturation, SR
3.0
2.5
SR=1 perfect match
SR>1 model predicts too low
solubility
2.0
SR<1 model predicts too high
solubility
1.5
1.0
0.5
0
20
40
60
80
Temperature (oC)
8-
100
120
Data from:
Jänecke (1927), Jänecke (1929)
Simulation tool
• Excel based process simulator, thermodynamic calculations in dll
9-
Application in a generic ammonia process
Pure CO2
• Chilled Ammonia Process patented
by Eli Gal (2006)
Cleaned gas
Air cooler
• Powerspan’s ECO2® technology
• CSIRO’s ammonia PCC
• CAER pilot studies (Univ. of
Kentucky)
Desorber
Heat
exchanger
Absorber
Flue gas
CO2 rich
CO2 lean
10 -
Cleaned
gas
Simulation results
Solven
t
Absorber
Flue
gas
NH3 in cleaned gas
CO2
rich
3.5% CO2, G: 10 kg/s, L: 200 kg/s, (NH4)2CO3: 3,4,6 mol/L
30
3.5% CO2, G: 10 kg/s, L: 200 kg/s, (NH4)2CO3: 4 mol/L
20
2400
15
Flue gas temperature
80oC
90oC
120oC
2000
10
5
0
5
10
15
20
25
30
35
o
Temperature in absorber ( C)
40
45
Cooling duty of flue gas (kJ/s)
NH3 slip (mmole/s)
Cooling duty of flue gas
Solvent: (NH4)2CO3
3 mol/L
4 mol/L
6 mol/L
25
1600
1200
800
400
0
5
10
15
20
25
30
Temperature in absorber (oC)
11 -
35
40
45
Simulation results
Energy needed to heat CO2 rich stream
Pure CO2
T(regen): 60, 70, 80, 90oC, T(abs): 12, 20, 35oC
250
T in absorber
15oC
20oC
35oC
Desorber
CO2
lean
Heating duty (kJ/s)
200
150
100
50
0
60
65
70
75
80
85
o
Temperature in desorber ( C)
12 -
90
Thank you
Modelling of liquid- vapor- solid equilibria in the NH3- CO2- H2O system
Maria G. Lioliou
TNE RD NEH CCA
mlio@statoil.com, tel: +47 94424249
www.statoil.com
13 -
Backup slides
14 -
Calculations
• Calculate thermodynamic equilibrium and other P-T constants
• EOS solved for gas  fugacity coefficients
• Pitzer solved for aqueous components  activity coefficients
• Equilibria, mass balances for CO2, NH3, H2O & alkalinity equation
• Update EOS and SRK with new values
• Typically 3-6 iterations
15 -
Parameters & simplifications in Pitzer model
• Similar to Bieling et al. (1995)
• Theoretically: 36 binary and 120 ternary parameters
• Interactions neglected:
− H+/[OH-] << NH3/NH4+ and CO2/HCO3-/CO32− CO2 and NH3 cannot coexist
− ions of the same sign
− ion and a neutral
• Ternary parameters: important only at very high concentrations
• Same approach for triple interactions
16 -
NH3 solubility –
Comparison with HYSYS
x% NH3
20 kg/s
• oC
Gas out, ToC
3.0x107
HYSYS 40% NH3
Model 40% NH3
HYSYS 60% NH3
Model 60% NH3
HYSYS 80% NH3
Model 80% NH3
2.5x107
2.0x107
Heat Flow (J/s)
1.5x107
1.0x107
Water
10 kg/s
• oC
Water: 10 kg/s
Gas: 20 kg/s
Liquid out, ToC
Q
50
5.0x106
0.0
experimental points
developed model
Peng-Robinson
NRTL
40
6
-5.0x10
NH3 solubility
-1.0x107
-1.5x107
20
30
40
50
60
70
o
Temperature ( C)
30
20
-9.00x105
10
-1.05x106
Heat Flow (J/s)
-1.20x106
0
Peng Robinson EOS model
developed model
Redlich Kwong
6
-1.35x10
0
20
30
40
50
60
70
80
90
100
Temperature (oC)
-1.50x106
• Different fluid packages in HYSYS – non ideal behaviour
of NH3 in water
-1.65x106
-1.80x106
-1.95x106
-2.10x106
10
20
30
40
50
Temperature (oC)
17 -
10
60
70
80
• Solubility of NH3 is calculated and compared to various
models