solvent development for aqueous absorption/stripping of...
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Solvent Development for Aqueous Absorption/Stripping of CO2
The University of Texas at AustinJ. Tim Cullinane and Gary T. Rochelle
April 27, 2004
Outline• Overview• Process Considerations• Solvent Development
– Experimental Methods– Development of Aqueous K+/PZ
• Other UT Research Activities– Degradation– Process Modeling– Pilot Plant/Packing Selection
• Conclusions
U.S. CO2 Emissions from Fossil Fuel Combustion by Sector
Commercial4.8%
Residential9.7% Power Plant - Petroleum
2.0%
Industrial31.8%
Power Plant - Coal47.1%
Power Plant - Natural Gas4.6%
Total U.S. Emissions = 3635.7 Tg CO2 Eq.Excludes Transportation, EPA (1999)
Advantages of Aqueous Absorption/Stripping
• Near Commercial Technology– Process used for treating H2 & natural gas– MEA demonstrated on small coal plants– Promoted K2CO3 used for H2 treating
• Post-process Technology Development– Lower cost and less risk to process– Resolve problems in small pilot plants– Demo Full-scale absorbers with 100 MW gas
• Problems– 20 - 40% energy use– High capital cost
Enhancing CO2 Capture by Amines
1. Contactor Development• Packing
2. Process Flowsheet Innovations• Multi-pressure stripper• Inter-cooling
3. Energy Integration• Power plant specific
4. Engineering Development• Large-scale equipment
5. Solvent Development
CO2 Capture by Amines
Sour Gas10% CO2
2-4 mol H2O/mol CO2Sweet Gas1% CO2
Rich Amine Lean Amine Reboiler
AbsorberT = 40–60oC
StripperT = 100–120oC
Cooler
PCO2* ~ 300 Pa
PCO2* ~ 3000 Pa
∆H = 20-25 kcal/mol CO2
Solvent Development K+/PZ
1. Thermodynamics2. Rates of Absorption3. Degradation
4. System Modeling
5. Pilot Plant
Bench-Scale Work
Fundamental
Process Flowsheet
Large-Scale Work
N NH H CO
ON N
O
OH
N+
NO
OHH
N NO
O O
O
O
O OCO
OO
H
H OO
OH
N NO
OH C
O
O
N NH H N+
NH
HH
N NO
OH
CO2 Absorption by K+/Piperazine
+
+
+
Carbonate Species
Piperazine Species
2+
PZ Speciation by 1H NMR
NHCH2
CH2
NHCH2
CH2
NCH2
CH2
NCH2
CH2
CO
CO
O
NHCH2
CH2
NCH2
CH2
CO
NHCH2
CH2
NHCH2
CH2
NCH2
CH2
NCH2
CH2
CO
OC
O
O
NHCH2
CH2
NCH2
CH2
CO
ONH
CH2
CH2
NCH2
CH2
CO
ONH
CH2
CH2
NCH2
CH2
CO
O
Wetted-Wall Column
WWC(38 cm2)
N2
CO2
Flow Controllers(4 – 6 L/min)
Saturator(25 – 110oC)
Heater(25 – 110oC) Solution Reservoir
(1000 cm3)
Condenser
IR CO2Analyzer
Sample Port
Pressure Control(35 – 60 psig)
Pump(2 – 4 cm3/s)
Fundamental Equilibrium Modeling
• Uses Electrolyte NRTL Model– Rigorous activity coefficient model
• Benefits– Versatile – can be used for broad range of conditions, systems– Develops/supported by theory – more accurate extrapolations– Predicts complicated behavior
• Challenges– Accurate representation of entire system– Meaningful results – can require a lot of data– Thermodynamic consistency
Model Parameter Summary
2037NMR, PCO2*PZ, K+, CO2, H2O
4063aNMR, PCO2*PZ, CO2, H2O
214UNIFACPZ, H2O
1204PCO2*KHCO3, K2CO3, H2O
6814Boiling pt. elev., PH2O*K2CO3, H2O
Data PointsParametersData TypesSystem
a. 6 parameters for equilibrium constants also regressed
Loading (mol CO2/mol PZ)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Frac
tion
of T
otal
PZ
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
PZ
H+PZCOO-
PZH+
PZ(COO-)2PZCOO-
Total ReactiveSpecies
Speciation in 1.8 m PZ at 60oC~300 Pa ~10000 Pa
Loading (mol CO2/(mol K+ + mol PZ)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Frac
tion
of T
otal
PZ
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
PZ
H+PZCOO-
PZH+
PZ(COO-)2
PZCOO-
Total ReactiveSpecies
Speciation in 5.0 m K+/2.5 m PZ at 60oC~300 Pa ~10000 Pa
Equilibrium in K+/PZ at 60oC
[CO2(aq)] Absorbed (m)0 1 2 3 4
P CO
2* (Pa
)
1
10
100
1000
10000
1.8 m
PZ
3.6 m
K+ /0.
6 m P
Z
3.6 m
K+ /1.
8 m P
Z5.0
m K
+ /2.5 m
PZ
7 m (30wt%) M
EA
6.2 m K
+ /1.2 m PZ
Heat of Absorption
2*[PZ] 2*[PZ] + [K+]
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
- ∆H
abs (
kcal
/mol
CO
2)
6
8
10
12
14
16
18
20
Other Model Predictions3.6 m K+ Experimental Points
3.6 m K+ Model Predictions
1.8 m PZ
5.0 m K+/2.5 m PZ
6.2 m K+/1.2 m PZ
3000 Pa at 60oC
Normalized Flux at 60oC
PCO2* (Pa)
100 1000 10000
Nor
mal
ized
Flu
x (m
ol/P
a-cm
2 -s)
1e-10
5.0 M MEA
3.6 m K+/0.6 m PZ
3.6 m K+/1.8 m PZ
6.2 m K+/1.8 m PZ
3.6 m K+/3.6 m PZ
2.5 m K+/2.5 m PZ
5.0 m K+/2.5 m PZ
Absorption Rate in 5.0 m K+/2.5 m PZ
PCO2* (Pa)
100 1000 10000
Nor
mal
ized
Flu
x (m
ol/c
m2 -P
a-s)
1e-10
1e-9
40oC80oC
60oC
100oC
110oC
Research Activities at UT• Bench-scale
– Wetted-wall Column – VLE, rates– NMR – speciation– Degradation– Other – solid solubility, transport properties
• Modeling– Thermodynamics– Rate– Process
• Pilot Plant– Contactor Testing– Solvent Testing
Oxidative Degradation of MEA
OH CH2 CH2 NH
H
?νO2
NH3
Formaldehyde
Formate, Acetate
•Rate is measured by NH3 evolution from a sparged reactor vessel•Gas analysis is quick/liquid analysis requires long experiments•Uncertainty in the stoichiometry of O2 in the reaction
Degradation Results
Conclusion: Mass Transfer Limited?
45.816.7Air w/ Agitation
27.825.0AirGoff and Rochelle
12.920.0AirChi and Rochelle5.02.9Pure O2Hofmeyer et al.2.61.050% O2Girdler0.81.0AirBlachly and Ravner0.40.006AirRooney et al.
Max. Rate (mM/hr)
Gas Flow/Liq. Vol (min-1)
SpargeGasStudy
Process Modeling• Explore Optimum Operating Conditions
30
40
50
60
3 3.2 3.4 3.6 3.8 4 4.2Hea
t req
uire
men
t (kc
al/g
mol
CO
2)
lean loading (m)
10
5
2.5
P*CO2
1.25 kPa
optimumlean
OptimalMEA
40C Absorber1.6 atm stripper
Process Configuration• Explore unique flowsheets
MultistageCompressor
W=7.4 kc/mol CO2
CO2130 atm
Q=20 kc/mol CO2
118 C
113 C
Multipressure Stripper
Leanldg=0.34
Richldg=0.46 115 C
4 atm
2.8 atm
2 atm
Pilot Plant at UT-SRP
• 18” PVC columns, 20 ft of packing– Accommodates commercial, structured packing– Operation as absorber or absorber/stripper
• Vacuum stripping• Air/CO2 Fed
– Wide Range of Concentrations Possible
Pilot Plant Operation• NaOH/Air – Screen packing areas
– Packing areas based on 0.75” H2O/ft, 5 gpm/ft2
• Solvent – Simulation of absorber/stripper– Quantify “real” solvent performance– Includes impurities (Fe2+, degradation, etc.)
Packing Wetted Area (ft2/ft3) CMR 2”, plastic 27
IMTP #40 44 CMR 2”, metal 48 Montz B1-250 64 Montz B1-350 91
Conclusions• E-NRTL model describes speciation and VLE• K+ increases the amount of reactive species in
solution– CO3
2-/HCO3- is an effective buffer
– Apparent carbamate stability is increased w/ K+
• Solvent capacity increases with concentration and is comparable to MEA
• ∆Habs can be lower than other amine-based systems and depends on the ratio of K+:PZ
• Absorption rate is 1.5 to 4 times faster than MEA or other amine-promoted K2CO3 solutions
Acknowledgements
• Texas Advanced Technology Program: contract 003658-0534-2001
• George Goff – Degradation• Tunde Oyenekan – Process Modeling• Dr. Ben Shoulders – The University of
Texas at Austin, Department of Chemistry
Questions?