CAP Chemistry and phase equilibria
Kaj Thomsen, Associate Professor, [email protected]
CERE (Center for Energy Resources Engineering)
DTU Chemical Engineering
Technical University of Denmark
Background
•Currently supervising following projects in CCS:
–Industrial PhD: Chilled Ammonia Process
–PhD: amino acid salt solutions
–PhD: alkanolamine system
–Postdoc: Process simulation of amine process
–Postdoc: Ionic liquids for CO2 capture
2
DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Thermodynamic model
•Extended UNIQUAC model for the liquid phase
activity coefficients
–UNIQUAC local composition activity coefficient
model + Debye-Hückel term for electrostatic
interactions
•The Soave-Redlich-Kwong equation of state
(SRK) for the gas phase fugacities
•UNIQUAC volume, surface area, and interaction
parameters
•No model parameters are required for SRK
equation of state
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Experimental Data
•About 3700 experimental data points on this
system in our electrolyte data bank
–Vapor-liquid equilibrium data
–Solid-liquid equilibrium data
–Heat of mixing, heat capacity data
•Thermal properties and solid-liquid equilibrium
data: mostly from the first part of the previous
century, one paper from 1999
•Contradictions between solid-liquid equilibrium
data from different sources
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
ASPEN electrolyte model (ElecNRTL)
40
40C
35
5%
65C
87C
30
calc P (bar)
100C
25
120C
20
15
10
5
0
0
10
20
30
40
exp P (bar)
5
DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Speciation equilibria
•The following reactions are considered:
NH3 (aq)+ H2O (l) ⇔ NH4+ + OHCO2 (aq) + H2O (l) ⇔ HCO3- + H+
HCO3- ⇔ CO32- + H+
H2O (l) ⇔ H+ + OHNH3 (aq) + HCO3- ⇔ NH2COO- + H2O (l)
•The chemical potentials of the species are
calculated with the Extended UNIQUAC model
•This allows us to determine the amounts of the
species (speciation)
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Carbamate, NH2COO•Standard state properties for the carbamate ion
are not found in the usual tables
•The equilibrium constant of the carbamate
reaction was determined by a colorimetric
method at 0 and 18°C in 1921 at the Royal
Veterinary and Agricultural University of
Denmark by Carl Faurholt
•We used those values of the equilibrium
constant in our modelling of the system
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Comparison of correlations for carbamate equilibrium
constants from dissertation by Ute Lichtfers,
Kaiserslautern, 2000
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
12
NH3(aq), Extended UNIQUAC
Speciation at 40 °C in 12
molal NH3 measured by IR
spectrometry (Lichtfers, 2000)
NH4+, Extended UNIQUAC
NH2COO-, Extended UNIQUAC
NH3, Lichtfers, 2000
10
NH4+, Lichtfers, 2000
NH2COO-, Lichtfers, 2000
12
8
HCO3-, Extended UNIQUAC
CO3--, Extended UNIQUAC
CO2(aq), Extended UNIQUAC
CO2, Lichtfers, 2000
CO3--, Lichtfers, 2000
HCO3-, Lichtfers, 2000
10
6
8
4
6
2
4
0
0
2
4
6 2
m CO2 mol/kg
Same scale on the ordinate axis on
the two figures (mol/kg water)
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DTU Chemical Engineering, Technical University of Denmark
0
0
2
4
m CO2 mol/kg
CAP Chemistry and phase equilibria
6
02-12-2010
3
NH3(aq), Extended UNIQUAC
NH4+, Extended UNIQUAC
Speciation at 100 °C in 3
molal NH3 measured by IR
spectrometry (Lichtfers, 2000)
NH2COO-, Extended UNIQUAC
2.5
NH3, Lichtfers, 2000
NH4+, Lichtfers, 2000
NH2COO-, Lichtfers, 2000
2
3
1.5
2.5
1
2
1.5
0.5
0
HCO3-, Extended UNIQUAC
CO3--, Extended UNIQUAC
CO2(aq), Extended UNIQUAC
CO2, Lichtfers, 2000
CO3--, Lichtfers, 2000
HCO3-, Lichtfers, 2000
1
0
0.5
1
1.5
m CO2 mol/kg
Same scale on the ordinate axis on
the two figures (mol/kg water)
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DTU Chemical Engineering, Technical University of Denmark
2
2.5
0.5
0
0
0.5
1
1.5
m CAP
CO
2 mol/kg
Chemistry
and phase equilibria
2
02-12-2010
2.5
Vapor-liquid equilibrium
•Gas phase components:
–CO2 (g) ⇔ CO2 (aq)
–NH3 (g) ⇔ NH3 (aq)
–H2O (g) ⇔ H2O (l)
•Equilibrium requires that the chemical potentials
are the same in the two phases
•Chemical potentials of the volatile components
are calculated using:
–SRK in the gas phase
–Extended UNIQUAC in the liquid phase
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
0.025
Extended UNIQUAC
Van Krevelen et al. (1949)
Pexton & Badger (1938)
Otsuka et al. (1960)
0.02
Partial pressures at
20 °C
20°
1 molal NH3
0.015
0.01
0.12
0.5 m
0
0.5
1
CO2 mol
1.5
kg-1
K. Thomsen and P.
Rasmussen “Modeling of
Vapor-liquid-solid
equilibrium in gas-aqueous
electrolyte systems”,
Chemical Engineering
Science 54(1999)1787-1802
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DTU Chemical Engineering, Technical University of Denmark
0.08
2
0.06
20°
1 molal NH3
0
0.1
0.5 molal NH3
0.13
Extended UNIQUAC
Van Krevelen et al. (1949)
Pexton & Badger (1938)
Otsuka et al. (1960)
0.13 molal NH3
0.005
CO2 Partial pressure, bar
NH3 partial pressure, bar
2 molal NH3
2 molal NH3
0.04
0.02
0
0
0.5
1
1.5
CO2 mol kg-1
CAP Chemistry and phase equilibria
02-12-2010
2
2
1.6
9.0 m
80°C
1.4
1.2
Partial pressures at
80 °C
6.8 m
1
0.8
4.1 m
0.6
0.4
2m
0.2
90
0
0
2
4
6
CO2 mol kg-1
8
CO2 partial pressure, bar
NH3 partial pressure, bar
Extended UNIQUAC
Göppert and Maurer (1988)
Kurz et al. (1995)
12 molal NH3
1.8
Extended UNIQUAC
80°C
Göppert and Maurer (1988)5.9 molal NH3
10 Kurz et al. (1995)
6.8 molal NH3
9 molal NH3
80
70
60
50
0.6 m 1 m
40
2 molal
NH3
4.1 m
12 molal NH3
30
20
10
0
0
13
DTU Chemical Engineering, Technical University of Denmark
2
4
6
CO2 mol kg-1
CAP Chemistry and phase equilibria
8
02-12-2010
10
Solid-liquid equilibrium
•The following solids are considered
–NH2COONH4
Ammonium carbamate
–(NH4)2CO3·H2O
Ammonium carbonate
–(NH4)2CO3·2NH4HCO3
Sesquicarbonate
–NH4HCO3
Ammonium bicarbonate
–Ice
Solid water
•Solid-Liquid Equilibrium calculation by comparing
–Chemical potentials of species in the liquid
phase using the Extended UNIQUAC model
–Chemical potentials of the pure solids formed
in the system
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
The solid phases
•CO2 + 2NH3
•CO2 + 2NH3 + 2H2O
•3CO2 + 4NH3 + 3H2O
•CO2 + NH3 + H2O
•H2O
= NH2COONH4
=(NH4)2CO3·H2O
= (NH4)2CO3·2NH4HCO3
= NH4HCO3
= Ice
•Experimentally it is difficult to distinguish
between these salts as they are volatile
•Sesquicarbonate and ammonium bicarbonate
have almost identical compositions
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
0.7
NH4HCO3
0.6
CO2/NH3 mol ratio
(NH4)2CO3•2NH4HCO3
0.5
0.4
(NH4)2CO3•H2O
0.3
0.2
NH2COONH4
0.1
0
0
20
40
Extended UNIQUAC
Jänecke (1929)
Terres & Weiser (1921)
Terres & Behrens (1928)
Guyer & Piechowicz (1944)
60
80
100
Temperature, °C
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
17
DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Solid phases at 5°C
0.050
5°C, 1 bar total pressure, 27 wt % NH3
Partial Pressure, bar
0.040
0.030
CO2
NH3
(NH4)2CO3·H2O
0.020
NH4HCO3
0.010
0.000
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Loading (mol CO2/ mol NH3)
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Gibbs phase rule
•F = C – P + 2
F = Degrees of freedom
C = Number of independent components= 3
P = Number of phases
•P = 1 gas phase, 1 liquid phase, 2 solid phases
•F = 3 – 4 + 2 = 1
•The only degree of freedom is the composition of
the solid phase
•The compositions of liquid and gas phases have
to remain constant as long as there are 4 phases
and T and P are fixed!
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Solid phases at 10°C
0.050
10°C, 1 bar total pressure, 27 wt % NH3
Partial Pressure, bar
0.040
(NH4)2CO3·H2O
0.030
CO2
NH3
0.020
NH4HCO3
0.010
0.000
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Loading (mol CO2/ mol NH3)
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
The amounts of salts formed
20
10°C, 1 bar total pressure, 27 wt % NH3
18
16
mol salt
14
12
NH4HCO3
10
8
(NH4)2CO3·H2O
6
4
2
0
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Loading (mol CO2/ mol NH3)
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Solid phases at 20°C
0.100
20°C, 1 bar total pressure, 27 wt %
0.090
Partial Pressure, bar
0.080
0.070
(NH4)2CO3·H2O
0.060
CO2
NH3
0.050
0.040
(NH4)2CO3∙2NH4HCO3
NH4HCO3
0.030
0.020
0.010
0.000
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Loading (mol CO2/ mol NH3)
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
No solid phases at 50°C, 1 bar !!!
0.500
50°C, 1 bar total pressure, 27 wt % NH3
0.450
0.400
CO2
Partial Pressure, bar
0.350
NH3
0.300
0.250
0.200
0.150
0.100
0.050
0.000
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Loading (mol CO2/ mol NH3)
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Solid phases 15 wt % NH3, 5°C
0.050
5°C, 1 bar total pressure, 15 wt% NH3
Partial Pressure. bar
0.040
CO2
NH3
0.030
0.020
NH4HCO3
0.010
0.000
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Loading (mol CO2/ mol NH3)
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Solid phases, 5 wt % NH3, 5°C
0.050
5°C, 1 bar total pressure, 5 wt% NH3
NH4HCO3
Partial Pressure. bar
0.040
CO2
NH3
0.030
0.020
0.010
0.000
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Loading (mol CO2/ mol NH3)
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Enthalpy change
•A large amount of heat is developed when CO2 is
dissolved in aqueous ammonia
–Heat of reaction from speciation reactions
–Excess enthalpy of the ionic solution
–Heat of crystallization
•These terms are calculated with the Extended
UNIQUAC model
•Excess enthalpy is small compared with heat of
reaction and crystallization in CO2-NH3-H2O
•A similar amount of heat is required to
evaporate CO2 from the slurry in the desorber
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
T-P Hou, 1942
•Solvay process:
• Shell and tube heat
exchangers are used
under each mushroom
shaped stage
• Enthalpy from
crystallization: 25%
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Heat of dilution to infinite dilution
8
7
kJ mol-1
6
5
4
3
2
Extended UNIQUAC, 14°C
Baud & Gay (1909), 12-13°C
Berthelot (1875), 14°C
Wrewsky & Sawaritzky (1924), 14°C
1
0
0
10
20
30
40
50
60
70
80
NH3 mol kg-1
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Experimental enthalpy data
•One recent investigation
–Rumpf B., Weyrich F., Maurer G., „Enthalpy
changes upon Partial Evaporation of Aqueous
Solutions containing Ammonia and Carbon
Dioxide”, Ind. Eng. Chem. Res.
37(1998)2983-2995
•These data were published shortly before this
model and were therefore not used for
parameter estimation in my 1999 paper.
•The data were included in the new version of the
model published by Darde et al. 2011.
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Experimental and calculated
enthalpy change by partial
evaporation
Experimental data from Rumpf et al., 1998
Calculations are within the experimental accuracy
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Application of the model (V.
Darde)
•Energy requirement desorber MEA: 3700kJ/kg
CO2 captured (CASTOR project)
•Energy requirement desorber chilled ammonia :
less than 2000 kJ/kg CO2 captured
Significant reduction of the energy consumption
in the desorber
•Additional energy savings during compression
but energy requirement for the chilling of the
flue gas and solvent
•Optimization of the configuration of the process
to minimize the energy consumption
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010
Conclusion
•Phase equilibria and thermal properties of CO2NH3-H2O mixtures can be accurately described
by the Extended UNIQUAC model combined with
the Soave-Redlich-Kwong cubic equation of state
in the temperature range from the freezing point
of the solutions to 110 °C. (150°C)
•Data on heat of absorption and heat capacity for
this system are scarce. Such data are essential
for improving the thermodynamic models so
they can give better estimates of the energy
requirement.
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DTU Chemical Engineering, Technical University of Denmark
CAP Chemistry and phase equilibria
02-12-2010