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1 1 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

Property and Process Modeling of Aqueous Ammonia … and Process Modeling of Aqueous Ammonia Processes for Post-Combustion Carbon Dioxide ... – ElecNRTL model in Aspen Plus ... Reaction

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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

2

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.

3

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

4

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

5

Speciation Chemistry for NH3-CO2-H2O with NH4HCO3(s) Precipitation

)(

2

2

3434

2233

423

3323

3322

32

sHCONHHCONH

OHCOONHHCONH

OHNHOHNH

COOHOHHCO

HCOOHOHCO

OHOHOH

↔+

+↔+

+↔+

+↔+

+↔+

+↔

−+

−−

−+

=−

−+

−+

+

6

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)

7

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)

8

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 ↑

9

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)

10

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)

11

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

12

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

13

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

14

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

15

Chilled-NH3 Process – Absorption/Stripping

16

Chilled-NH3 Process – NH3 Abatement

17

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)

18

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

19

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

21

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.

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Questions?