Peter FrailieGary T. Rochelle

The University of Texas at AustinLuminant Carbon Management Program

TCCS-6June 16, 2011

Modeling Energy Performance of Aqueous MDEA/PZ for CO2 Capture

Overview Why MDEA/PZ? MDEA/PZ Aspen Plus® Framework Thermodynamics Hydraulics Kinetics

Process Modeling Absorber Intercooling

Stripper Simple stripper vs. 2-Stage Flash

Conclusions

Why MDEA/PZ? High capacity7m MDEA/2m PZ0.83 mol CO2/kg solvent7m MEA (0.60) 8m PZ (0.76)

High CO2 Absorption Ratekg’ comparable to 8m PZ at 40oC

Does not exhibit solubility limitations of conc. PZ Commercially used for H2 and CH4 treating MDEA is less expensive than PZ

Amine Modeling

Aspen Plus® Modeling - Thermo Overall goal: construct 1 model that represents MDEA, PZ and

MDEA/PZ using Aspen Plus® eNRTL method Over wide temperature, loading, and amine concentration ranges

Sequential regression: amineamine/H2Oamine/H2O/CO2

Minimizes the number of regressed parameters Process models more likely to converge Improves confidence in parameter values

Thermodynamically consistent methodology Speciation and thermodynamic properties calculated using same set

of thermodynamic parameters

Aspen Plus® Modeling - Thermo Incorporated all available experimental dataCP, VLE, amine volatility, speciation, ∆HABS, pKa,γCO2

Improves thermodynamic consistency

Final model utilized 54 independently adjusted parameters MDEA (17), PZ (33), MDEA/PZ (4)

Focused on operationally significant conditions Loading 0.5 and 5 kPa CO2

Temperature 40 oC to 150 oCAmine concentration 35-50 wt%

Aspen Plus® Modeling - Hydraulics FORTRAN subroutines used to fit data Functions of amine concentration, loading, and

temperatureDensity Dugas (2009)Viscosity Weiland (1998)Diffusivity Dugas (2009)

Fit over same temperature, loading, and amine concentration ranges as thermodynamic data

Aspen Plus® Modeling - Kinetics Fit using WWC simulation in Aspen Plus® RateSepTM

Adjusted k0 and EA for select kinetic reactionsReactions selected based on predicted speciation

Final model uses 7 independently adjusted parameters 3 k0, 3 EA, and D0

−−=

KTREkk A

15.29811exp0

Amine System Temperature (oC)CO2 Loading

(mol/mol alk)

8m PZ 40-100 0.20-0.40

7m MDEA/2m PZ 40-100 0.10-0.26

5m MDEA/5m PZ 40-100 0.18-0.37

0,7

0,8

0,9

1,0

1,1

1,2

1,3

0,01 0,1 1 10 100

Flux

pred

/Flu

x exp

PCO2 (kPa)

40oC60oC

100oC80oC

RichLean

∆ = 8m PZ◊ = 7m MDEA/2m PZ

= 5m MDEA/5m PZ

Erroravg = 6.7%

Process Modeling - Absorber

Absorber

L/Lmin=1.1100 kPaMellapak

250X

Intercooling to 40oC

Lean40oC

Rich45-55oC

Column diameter set to 80% flood in bottom stage.

12 kPa CO240oC100 kPa

~1.2 kPa CO2(90% removal)

4

5

6

7

8

9

10

11

12

0 5 10 15 20 25

L/G

(m

ol b

asis

)

Absorber Height (m)

5m MDEA/5m PZ0.24 mol CO2/mol alkNot Intercooled

4

5

6

7

8

9

10

11

12

0 5 10 15 20 25

L/G

(m

ol b

asis

)

Absorber Height (m)

6.4

5m MDEA/5m PZ0.24 mol CO2/mol alkNot Intercooled

4

5

6

7

8

9

10

11

12

0 5 10 15 20 25

L/G

(m

ol b

asis

)

Absorber Height (m)

6.4

7.04

5m MDEA/5m PZ0.24 mol CO2/mol alkNot Intercooled

11.5 m

0,4

0,5

0,6

0,7

0,8

0,9

1

0,1 1

Cap

acit

y (m

ol C

O2/

kg H

2O +

Am

ine)

PCO2 at 40oC (kPa)

8m PZ

7m MDEA/2m PZ

5m MDEA/5m PZ

Isothermal

0.5

0,4

0,5

0,6

0,7

0,8

0,9

1

0,1 1

Cap

acit

y (m

ol C

O2/

kg H

2O +

Am

ine)

PCO2 at 40oC (kPa)

8m PZ

7m MDEA/2m PZ

5m MDEA/5m PZ

Not Intercooled

0.5

0,4

0,5

0,6

0,7

0,8

0,9

1

0,1 1

Cap

acit

y (m

ol C

O2/

kg H

2O +

Am

ine)

PCO2 at 40oC (kPa)

8m PZ

7m MDEA/2m PZ

5m MDEA/5m PZIntercooled

Not Intercooled

0.5

Process Modeling - Stripper

Simple Stripper

120-150oC4-14 barMellapak

250X

Rich Pump

Lean Pump

HeatXCold ∆T = 5oC

Trim Cooler

Rich conditions set by absorber results

40oC 150 bar99.9% CO2

2 Stage Flash

Rich Pump

Lean Pump

HeatXCold ∆T = 5oC

Trim Cooler

Rich conditions set by absorber results

40oC 150 bar99.9% CO2

HP Flash

LP Flash

•HP and LP flashes at same temperature•Equal vapor flow rates

Equivalent Work Analysis (0.5 kPa Lean Loading)

Amine Stripper T (oC)

IC? WEQ, SS(kJ/mol CO2)

WEQ, 2SF(kJ/mol CO2)

Abs Ht (m)

7m MDEA/2m PZ 120No 36 37.2 14

Yes 33.9 35.2 16

5m MDEA/5m PZ 120No 36 37.1 10

Yes 33 34.2 17

8m PZ 120No 36.6 38.4 11

Yes 33.7 35.3 16

8m PZ 150No 37.3 38.5 11

Yes 33.5 34.6 16

compspumps

n

i i

kiieq WW

KTTKTQW

reboilers

++

+−+

∗= ∑=1

sin

5575.0

Conclusions Thermodynamic, hydraulic, and kinetic data can be

simultaneously fit for MDEA, PZ, and MDEA/PZ using eNRTL model and RateSepTM in Aspen Plus®

Intercooling significantly improved the capacity of each solvent tested Also improved associated WEQ

Increased absorber height WEQ for 2SF systematically higher (~1.5 kJ/mol CO2) than

that of SS. Higher stripper temperature did not necessarily improve

energy performance Best WEQ observed for 5m MDEA/5m PZ with an

intercooled absorber

Questions?

Reaction Rate Constants

Nine possible amine/base combinations for MDEA/PZ Cut down to 6 reactions by analyzing predicted speciation 12 total parameters (6 k0 and 6 EA) further reduced to 8

+− +→++ BHAmCOOCOBAm 2

−+ +→++ 322 HCOPZHCOOHPZ−+ +→+ PZCOOPZHCOPZ 22

−+ +→++ PZCOOMDEAHCOMDEAPZ 2

( ) −+− +→++ 222 COOPZMDEAHCOMDEAPZCOO

( ) −−+− +→+ 2222 COOPZPZCOOHCOPZCOO

−+ +→++ 322 HCOMDEAHCOOHMDEA(kf1)

(kf2)

(kf3)

(kf4)

(kf5)

(kf6)