Solvent Development for Aqueous Absorption/Stripping of CO2 · Condenser IR CO2 Analyzer Sample Port Pressure Control (35 – 60 psig) Pump (2 – 4 cm3/s) Fundamental Equilibrium

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?