Property and Process Modeling of Aqueous Ammonia … · 1 1 Property and Process Modeling of...
Transcript of Property and Process Modeling of Aqueous Ammonia … · 1 1 Property and Process Modeling of...
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Property and Process Modeling of Aqueous Ammonia Processes for Post-Combustion Carbon Dioxide Capture
Paul M. Mathias
Satish Reddy
John P. O’Connell
AIChE 2008 100th Anniversary Meeting
November 16-21, Philadelphia, PA
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Claimed Advantages of Aqueous-Ammonia Processes
Low solvent cost
High stability
High capacity for CO2
Ability to capture other acid gases: SO2, NO2, etc.
Low heat of regeneration (claimed)– 800 Btu/lb CO2 for MEA Versus 260 Btu/lb CO2 for NH3
Regeneration flexibility: Low temperature or high pressure
Low slip of NH3 in the absorber
Process simulation using a rigorous thermodynamic model provides a powerful tool for objective analysis and process optimization.
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Thermodynamic Analysis of Chilled-Ammonia Process
Thermodynamic analysis provides a powerful tool to analyze chemical processes
Process simulation established for systems with electrolytes and solids
This presentation:– Describes the basis of the Fluor thermodynamic model for the
NH2-CO2-H2O system– Uses the thermodynamic model to analyze, evaluate and
optimize the chilled-ammonia process– Compares chilled-ammonia process to alkanolamine processes
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Fundamental Data – NH3-CO2-H2O System
Vapor-liquid equilibrium
Solid solubility – NH4HCO3(s) (or solid ammonium bicarbonate) is of primary interest
Solution speciation
Heat of solution – calorimetric data and derived values from VLE data
Fundamental data fit simultaneously using:– ElecNRTL model in Aspen Plus– Speciation Chemistry for NH3-CO2-H2O system
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Speciation Chemistry for NH3-CO2-H2O with NH4HCO3(s) Precipitation
)(
2
2
3434
2233
423
3323
3322
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sHCONHHCONH
OHCOONHHCONH
OHNHOHNH
COOHOHHCO
HCOOHOHCO
OHOHOH
↔+
+↔+
+↔+
+↔+
+↔+
+↔
−+
−−
−+
=−
−+
−+
+
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Partial Pressure of CO2
CO2 Partial Pressure, ~6 m NH3
0.1
1
10
100
1000
0 0.2 0.4 0.6 0.8 1
CO2 Loading (mol/mol)
PCO
2 (kP
a)
40ºC, 6.3 m60ºC, 6.0 m80ºC, 6.8 m
Enlarged data points identify NH4HCO3(s) precipitation
F Kurz, B. Rumpf and G. Maurer, Fluid Phase Equilibria, 104, 261-275 (1995)
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Partial Pressure of NH3
NH3 Partial Pressure, ~6 m NH3
0.1
1
10
100
0 0.2 0.4 0.6 0.8 1
CO2 Loading (mol/mol)
P NH
3 (kP
a)
40ºC, 6.3 m60ºC, 6.0 m80ºC, 6.8 m
Enlarged data point identif ies NH4HCO3(s) precipitation
F Kurz, B. Rumpf and G. Maurer, Fluid Phase Equilibria, 104, 261-275 (1995)
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Solubility of Solid Ammonium Bicarbonate
NH4HCO3(s) Solubility Limit
0
5
10
15
20
25
30
0 4 8 12 16 20
Wt% NH3
Wt%
CO
2
0ºC10ºC20ºC30ºC40ºC50ºC
NH4HCO3(s) Precipitates ↑
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Speciation in Ammonium Carbonate Solutions I. 295 K
0
1
2
3
4
0 0.5 1 1.5 2 2.5 3 3.5 4
Molality - (NH4)2CO3
Spec
ies
Mol
ality
NH3
CO3=
Speciation in (NH4)2CO3 Solutions @ 295 K
0
1
2
3
4
0 0.5 1 1.5 2 2.5 3 3.5 4
Spec
ies
Mol
ality
HCO3-
NH2COO-
NH4+
Wen and BrookerJ. Phys. Chem., 99, 359-368 (1995)
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Speciation in Ammonium Carbonate Solutions II. m=2.9
Speciation in (NH4)2CO3 Solutions @ m=2.9
0
1
2
3
4
280 300 320 340 360 380
Spec
ies
Mol
ality
HCO3-
NH2COO-
NH4+
0
1
2
3
280 300 320 340 360 380
Temperature (K)
Spec
ies
Mol
ality
NH3
CO3=
Wen and BrookerJ. Phys. Chem., 99, 359-368 (1995)
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Heat of Solution from VLE Data
CO2 Heat of Solution in 8 Wt% NH3 @ 100°F
550
575
600
625
650
675
700
725
0 0.2 0.4 0.6 0.8
CO2 Loading
Hea
t of S
olut
ion
(Btu
/lb) Thermodynamic Model
From VLE data of Kurtz (1995)
( )( ) Loading
COSolutionCO T
PRH
⎭⎬⎫
⎩⎨⎧
∂∂
−=/1
ln 22
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Vaporization of NH3-CO2-H2O Solutions
6
8
10
12
14
16
18
6 8 10 12 14 16 18
∆Hexp (kJ)
∆H
calc
(kJ) Fluor Model
Rumpf et al. (1998)
• Rumpf, Weyrich, Maurer, I&EC Res., 37, 2983-2995, 1998.• Calorimetric measurement of vaporization of NH3-CO2-H2O solutions.• According to Rumpf et al. analysis, about one-third of the vaporization
energy was due to species redistribution
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Heat of Solution – Fixed Speciation
-265.82NH4+ + CO3
= + H2O(l) + CO2(g) ↔ 2NH4+ + 2HCO3
-
-995.92NH3(l) + H2O(l) + CO2(g) ↔ 2NH4+ + CO3
=
-630.8NH3(l) + H2O(l) + CO2(g) ↔ NH4+ + HCO3
-
∆Hrxn
(Btu/lb CO2)Reaction
Fixed speciation is not a correct representation of the NH3-CO2-H2O system
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NH3 and CO2 Partial PressuresCO2 and NH3 Partial Pressures vs. NH3:CO2 Ratio
4 mol/kg NH3 (6.4 Wt% NH3) @ 5°C
NH3
CO2
10
100
1,000
10,000
1.0 1.5 2.0 2.5 3.0
NH3:CO2 Mole Ratio
CO
2 Par
tial P
ress
ure
(Pa)
NH3 Slip (ppmv)≈ PNH3*10
Mol% CO2 in clean FG≈ PCO2 / 1,000
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Process Modeling of Chilled-NH3 Process
Feed: 11 mol% H2O and 13.7 mol% CO2, 150,000 lbmol/hrModel specifications:– 90% capture of CO2– 10 ppmv slip NH3– Integrated plant in NH3 and H2O balance– Minimum approach temperature in cross exchangers = 10ºF– Condenser temperature = 100ºF– Absorber temperature: 30-50ºF– NH3 concentration in solvent: 15-30 wt% (CO2-free basis)
Key calculated results:– Solvent circulation rate– Stripper duties (solvent regeneration and NH3-abatement
regeneration)– Refrigeration loads (flue-gas, recycle solvent, and absorber)
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Process Modeling – Assumptions and Limitations
Reaction kinetics not considered – equilibrium assumed
Mass-transfer limitations in absorber ignored
Dissolution of solids in cross exchanger assumed to be at equilibrium
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Absorber at 50ºF
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
15 20 25 30
Wt % NH3 in Solvent (CO2-Free)
CO2
Load
ing
20
25
30
35
40
45
50
55
60
65
70
Wt%
Sol
ids
in R
ich
Sol
vent
Wt% Solids in Rich Solvent
0
1
2
3
4
5
6
7
8
9
10
15 20 25 30
Wt % NH3 in Solvent (CO2-Free0
Solv
ent F
low
(lb/
lb C
O 2)
980
990
1000
1010
1020
1030
1040
Strip
per D
uty
(Btu
/lb)
• NH3 slip constant at 2,230 ppmv• NH3 abatement regenerator duty = 1,022 Btu/lb CO2• Total LP steam requirement ≈ 2,000 Btu/lb• Total chilling load ≈ -1,160 Btu/lb
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26% Wt% NH3 (CO2-Free) – Vary Absorber Temperature
0
500
1,000
1,500
2,000
2,500
30 35 40 45 50
Absorber Temperature (°F)
NH3
Slip
(ppm
v)Lower absorber temperature is effective in reducing NH3 slip
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26% NH3 (CO2-Free) – Vary Absorber Temperature
0
250
500
750
1,000
1,250
1,500
1,750
2,000
30 35 40 45 50
Absorber Temperature (°F)
Dut
y (B
tu/lb
CO 2)
Total
Stripper
NH3 Abatement
LP Steam Refrigeration and Solids
-1,240
-1,230
-1,220
-1,210
-1,200
-1,190
-1,180
-1,170
-1,160
-1,150
30 35 40 45 50
Absorber Temperature (°F)
Chi
lling
Dut
y (B
tu/lb
CO 2)
0
1
2
3
4
5
6
7
Wt%
Sol
ids
Ric
h So
lven
t CXn
gr
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Thermodynamic Analysis of Chilled-Ammonia Process -Conclusions
Process simulation with rigorous thermodynamic model provides a powerful, reliable tool to analyze process:– Accurate model for vapor-liquid-solid equilibrium and heat of solution.– Integrated process model ensures mass and energy balance for entire
plant, and provides reliable estimates for utility loads.
Comparison between chilled-ammonia process and alkanolamine processes:– LP steam requirements of chilled-ammonia process are similar if the
absorber temperature is lowered, but rich solvent leaving cross exchanger will have solids for temperatures below 50ºF.
– Chilled-ammonia process requires refrigeration, which exceeds the benefits of relatively high-pressure CO2 product.
– Chilled-ammonia process has additional equipment (refrigeration system, NH3 abatement) and process complexity (solids handling).
– Kinetics and mass-transfer limitations will increase the utility loads of the chilled-ammonia process.