Rate-Based DEA Model

Aspen Plus

Rate-Based Model of the CO2 Capture Process by DEA using Aspen Plus

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Revision History 3

Revision History

Version Description

V7.0 First version

4 Contents

Contents

Introduction............................................................................................................5

1 Components .........................................................................................................6

2 Process Description..............................................................................................7

3 Physical Properties...............................................................................................8

4 Reactions ...........................................................................................................13

5 Simulation Approaches.......................................................................................16

6 Simulation Results .............................................................................................20

7 Conclusions ........................................................................................................22

References ............................................................................................................23

Introduction 5

Introduction

This document describes an Aspen Plus rate-based model of the CO2 capture process by DEA (Diethanolamine) from a gas mixture of CH4, C2H6, C3H8, N2, CO2 and H2S. The model consists of an absorber and a stripper. The operation data from a natural gas treating unit at Pyote, Texas(1975)[1] were used to specify feed conditions and unit operation block specifications. Thermophysical property models and reaction kinetic models are based on the works of Aspen Technology(2007)[2] and Rinker(1996)[3]. Transport property models and model parameters have been validated against experimental data from open literature.

The model presented here includes the following key features:

• True species including ions

• Electrolyte NRTL method for liquid and RK equation of state for vapor

• Concentration-based reaction kinetics

• Electrolyte transport property models

• Rate-based models for absorber and stripper with valve trays

6 1 Components

1 Components

The following components represent the chemical species present in the process:

Table 1. Components Used in the Model

ID Type Name Formula

H2O Conventional WATER H2O

DEA Conventional DIETHANOLAMINE C4H11NO2-1

CO2 Conventional CARBON-DIOXIDE CO2

H3O+ Conventional H3O+ H3O+

OH- Conventional OH- OH-

HCO3- Conventional HCO3- HCO3-

CO3-2 Conventional CO3-- CO3-2

DEAH+ Conventional DEA+ C4H12NO2+

DEACOO- Conventional DEACOO- C5H10NO4-

H2S Conventional HYDROGEN-SULFIDE H2S

HS- Conventional HS- HS-

S-2 Conventional S-- S-2

CH4 Conventional METHANE CH4

C2H6 Conventional ETHANE C2H6

C3H8 Conventional PROPANE C3H8

N2 Conventional NITROGEN N2

2 Process Description 7

2 Process Description

The Pyote Unit[1] flowsheet for CO2 capture by DEA includes three absorbers and two strippers. However, only one absorber and one stripper data are reported. Table 2 represents the typical operation data:

Table 2. Data of the Pyote Unit

Absorber

Diameter 66 inch

Tray 20

Stripper

Diameter 72 inch

Tray 31

Feeds and Products

Sour Gas to Absorber 2.92 MM scfh

Lean Amine to Absorber 188 gpm

Rich Amine to Stripper 380 gpm

CO2 in Sour Gas 0.013(mole fraction)

H2S in Sour Gas 0.00054(mole fraction)

CO2 in Sweet Gas 150 ppm (mole fraction)

H2S in Sweet Gas 0.15 ppm (mole fraction)

DEA solution 28.69%(wt)

8 3 Physical Properties

3 Physical Properties

The Electrolyte NRTL method and RK equation of state are used to compute liquid and vapor properties respectively in this rate-based DEA model. The NRTL parameters were regressed against VLE data from Maddox(1987, 1989)[4,5].

CH4, C2H6, C3H8, N2, CO2 and H2S are treated as Henry-components (solutes) to which Henry’s law is applied. Henry’s constants are retrieved from Aspen Plus databanks for these components with water and are regressed against CO2 solubility data from Maddox(1987, 1989)[4, 5] for CO2 with DEA. In the reactions calculations, the activity coefficient basis for the Henry’s components is chosen to be Aqueous. Therefore, in calculating the unsymmetric activity coefficients (GAMUS) of the solutes, the infinite dilution activity coefficients will be calculated based on infinite-dilution condition in pure water, instead of in mixed solvents.

The liquid molar volume and transport property models have been updated and their model parameters regressed from literature experimental data. Specifications of these models include:

• For liquid molar volume, the Clarke model, called VAQCLK in Aspen Plus, is used with option code of 1 to use the quadratic mixing rule for solvents. The interaction parameter VLQKIJ for the quadratic mixing rule between DEA and H2O is regressed against experimental density data of the DEA-H2O system from Maham(1994)[15]. The Clarke model parameter VLCLK/1

is also regressed for main electrolytes (DEAH+, HCO −3 ), (DEAH+,

DEACOO − ) and (DEAH+, CO 23− ) against experimental density data of the

DEA-H2O-CO2 system from Weiland(1998)[6].

• For liquid viscosity, the Jones-Dole electrolyte correction model, called MUL2JONS in Aspen Plus, is used with the mass fraction based ASPEN liquid mixture viscosity model for the solvents. The three option codes for MUL2JONS are set to: 1 (mixture viscosity weighted by mass fraction), 1 (always use Jones and Dole equation when the parameters are available), and 2 (ASPEN liquid mixture viscosity model), respectively. The interaction parameters between DEA and H2O in the ASPEN liquid mixture viscosity model, MUKIJ and MULIJ, are regressed against experimental DEA-H2O viscosity data from Oyevaar(1989)[16], Rinker(1994)[17], Hsu(1997)[18], Weiland(1998)[19], and Mandal(2003)[20]. The Jones-Dole model parameters, IONMUB, for DEAH+ and DEACOO- are regressed

3 Physical Properties 9

against DEA-H2O-CO2 viscosity data from Weiland(1998)[6]; that of CO 23− is

regressed against K2CO3-H2O viscosity data from Pac et al.(1984)[21]; and that of HCO3

- is regressed against KHCO3-H2O viscosity data from Palaty(1992)[22].

• For liquid surface tension, the Onsager-Samaras model, called SIG2ONSG in Aspen Plus, is used with its option codes being -9 (exponent in mixing rule) and 1 (electrolyte system), respectively. No additional adjusted parameters are used in the surface tension model and the estimation results are somewhat higher than the experimental data from Weiland(1996)[7] (Figure 3).

• For thermal conductivity, the Riedel electrolyte correction model, called KL2RDL in Aspen Plus, is used.

• For binary diffusivity, the Nernst-Hartley model, called DL0NST in Aspen Plus, is used with option code of 1 (mixture viscosity weighted by mass fraction).

In addition to the updates to the above properties, the aqueous phase heat of formation at infinite dilution and 25°C (DHAQFM) for DEAH+ and DEACOO- and the heat capacity at infinite dilution (CPAQ0) for DEAH+ and DEACOO- are adjusted to fit to the literature data on heat of solution from Carson(2000)[9] and heat capacity from Weiland(1997)[8].

The estimation results of various transport and thermophysical properties are summarized in Figures 1-7:

900

950

1000

1050

1100

1150

1200

0 0.1 0.2 0.3 0.4 0.5

CO2 Loading, mol/mol

Den

sity

, kg/

m3

EXP DEA 10w t%EXP DEA 20w t%EXP DEA 30w t%EXP DEA 40w t%EST DEA 10w t%EST DEA 20w t%EST DEA 30w t%EST DEA 40w t%

Figure 1. Liquid Density of DEA-CO2-H2O at 298.15K, experimental data from Weiland (1998)[6]


0.1

1

10

0 0.1 0.2 0.3 0.4 0.5


Visc

osity

, mPa

S

EXP DEA 20w t%EXP DEA 30w t%EXP DEA 40w t%EST DEA 20w t%EST DEA 30w t%EST DEA 40w t%

Figure 2. Liquid Viscosity of DEA-CO2-H2O at 298.15K, experimental data from Weiland (1998)[6]

0

10

20

30

40

50

60

70

80

90

100

0.00 0.10 0.20 0.30 0.40 0.50


Surf

ace

Tens

ion,

mN

/m

EXP DEA 10w t%EST DEA 10w t%EXP DEA 20w t%EST DEA 20w t%EXP DEA 30w t%EST DEA 30w t%EXP DEA 40w t%EST DEA 40w t%

Figure 3. Surface tension of DEA-CO2-H2O at 298.15K, experimental data from Weiland (1996)[7]

3 Physical Properties 11

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6


Ther

mal

Con

duct

ivity

, Wat

t/m-K

DEA 10w t%DEA 20w t%DEA 30w t%DEA 40w t%

Figure 4. Liquid Thermal Conductivity of DEA-CO2-H2O at 298.15K

Figure 5. Liquid Heat Capacity of DEA-CO2-H2O at 298.15K, experimental data from Weiland (1997)[8]


Figure 6. Heat of Solution of DEA-CO2-H2O at 298.15K, experimental data from Carson (2000)[9]

Figure 7. CO2 partial pressure of DEA-CO2-H2O (DEA mass fraction = 0.20), experimental data from Maddox (1989)[5]

4 Reactions 13

4 Reactions

DEA is a secondary ethanolamine, as shown in Figure 8. It can associate with H3O+ to form DEAH+ ion and can also react with CO2 to form carbamate ion DEACOO-.

Figure 8. DEA Molecular Structure

The electrolyte solution chemistry has been represented with a CHEMISTRY model with ID of DEA, which is used as a global electrolyte calculation option in the simulation by specifying it on the Global sheet of the Properties | Specifications form. Chemical equilibrium is assumed with all the ionic reactions in the CHEMISTRY DEA. In addition, a REACTION model named DEA-REA has been created, which is used in calculations of the absorber and stripper by specifying it in the Reaction part of the absorber and stripper specifications. In DEA-REA, all reactions are assumed to be in chemical equilibrium except those of CO2 with OH- and CO2 with DEA.

A. Chemistry ID: DEA

1 Equilibrium ++ +↔+ OHDEAOHDEAH 32

2 Equilibrium −+ +↔+ 3322 HCOOHO2HCO

3 Equilibrium 2

3323 COOHOHHCO −+− +↔+

4 Equilibrium −− +↔+ 32 HCODEAOHDEACOO

5 Equilibrium −+ +↔ OHOHO2H 32

6 Equilibrium +− +↔+ OHHSSHOH 322

7 Equilibrium +−− +↔+ OHSHSOH 3

22

14 4 Reactions

B. Reaction ID: DEA-REA

1 Equilibrium ++ +↔+ OHDEAOHDEAH 32

2 Equilibrium −+ +↔ OHOHO2H 32

3 Equilibrium 2

3323 COOHOHHCO −+− +↔+

4 Kinetic −− →+ 32 HCOOHCO

5 Kinetic −− +→ OHCOHCO 23

6 Kinetic ++→++ OHDEACOOOHCODEA 3

-22

7 Kinetic 223- COOHDEAOHDEACOO ++→+ +

8 Equilibrium +− +↔+ OHHSSHOH 322

9 Equilibrium +−− +↔+ OHSHSOH 3

22

The equilibrium expressions for the reactions are taken from the work of Austgen et al.(1988)[10] and Jou et al.(1982, 1993)[11,12,13]. In addition, the power law expressions are used for the rate-controlled reactions (reactions 4-7 in DEA-REA) and the general power law expression is:

( ) ∏=

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟

⎠⎞

⎜⎝⎛ −=

N

i

ai

o

no

iCTTR

ETTkr1

11exp (1)

Where:

r = Rate of reaction;

k = Pre-exponential factor;

T = Absolute temperature;

T0 = Reference temperature;

n = Temperature exponent;

E = Activation energy;

R = Gas low constant;

N = Number of components in the reaction;

Ci = Concentration of component i;

ai = The stoichiometric number of component i in the reaction equation.

If To is not specified, the reduced power law expression is used:

∏=

−=N

i

ai

n iC)RTE(kTr

1

exp (2)

In this work, the reduced expressions are used. In equation (2), the concentration basis is Molarity and the factor n is zero. k and E are given in Table 3. The kinetic parameters for reactions 4-7 in Table 3 are derived from the work of Rinker(1996)[3]. The DEA concentration range of the data of

4 Reactions 15

Rinker is 0.25-2.8 M and the temperature range is 292-343 K, these conditions are the operating conditions of the absorber modeled in this work. The kinetic parameters for reaction 4 are taken from the work of Pinsent et al.(1956)[14] The kinetic parameters for reaction 5 are calculated by using the kinetic parameters of reaction 4 and the equilibrium constants of the reversible reactions 4 and 5.

Table 3. Parameters k and E in Equation (2)

Reaction No. k E (cal/mol)

4 4.32e+13 13249

5 2.38e+17 29451

6 6480000 5072

7 1.34e+17 11497

16 5 Simulation Approaches

5 Simulation Approaches

The natural gas treating unit at Pyote[1] is simulated in this work and is described in this section.

Simulation Flowsheet – The Pyote Unit has been modeled with the following simulation flowsheet in Aspen Plus as shown in Figure 9.

GASIN

LEANIN

GASOUT

RICHOUT

RICHIN

CO2OUT

LEANOUT

ABSORBER STRIPPER

Figure 9. Rate-Based DEA Simulation Flowsheet in Aspen Plus

5 Simulation Approaches 17

Unit Operations – The major unit operation models have been represented by Aspen Plus Blocks as outlined in Table 4.

Table 4. Aspen Plus Unit Operation Blocks Used in the Rate-Based MEA Model

Unit Operation Aspen Plus Block Comments / Specifications

Absorber RadFrac 1. Calculation type: Rate-Based

2. 20 Stages

3. Top Pressure: 900 psig

4. Reaction: Reaction ID is DEA-REA for all stages; when calculation type is equilibrium, Holdup is used, and in this file, Holdup=0.001 m3, which is intended to aid convergence

5. Tray Type: Glitsch Ballast

6. Tray Diameter: 66 inch

7. Weir height: 0.06 m

8. Mass transfer coefficient method: Scheffe and Weiland (1987)

9. Interfacial area method: Scheffe and Weiland (1987)

10. Interfacial area factor: 0.72

11. Heat transfer coefficient method: Chilton and Colburn

12. Holdup correlation: Bennett et al. (1983)

13. Holdup scale factor: 1

14. Film resistance: Discrxn for liquid film; Film for vapor film

15. Additional discretization points for liquid film: 0.0001, 0.001, 0.01, 0.1 and 0.5

16. Flow model: Mixed

17. Estimates: provide temperature estimates for all stages. These estimates are intended to aid convergence

18 5 Simulation Approaches

Unit Operation Aspen Plus Block Comments / Specifications

Stripper RadFrac 1. Calculation type: Rate-Based

2. 33 Stages

3. Partial vapor condenser

4. Kettle reboiler

5. Mole distillate rate: initialized at 0.03kmol/s with final value of 0.02804509kmol/s obtained by Design-Spec 1

6. Mole reflux rates: 0.04kmol/sec

7. Top Pressure: 9.8 psig

8. Reaction: Reaction ID is DEA-REA for all stages; when calculation type is equilibrium, Holdup is used, and in this file, Holdup=0.001 m3, which is intended to aid convergence.

9. Design specs: Stage 1 temperature = 291.5K

10. Vary: Distillate rate

11. Tray Type: Glitsch Ballast

12. Tray Diameter: 72 inch

13. Mass transfer coefficient method: Scheffe and Weiland (1987)

14. Interfacial area method: Scheffe and Weiland (1987)

15. Interfacial area factor: 1

16. Heat transfer coefficient method: Chilton and Colburn

17. Holdup correlation: Bennett et al. (1983)

18. Film resistance: Discxrn for liquid film; Film for vapor film

19. Additional discretization points for liquid film: 0.0001, 0.001, 0.01, 0.1 and 0.5

20. Flow model: Mixed

21. Estimates: provide temperature estimates for all stages; these estimates are intended to aid convergence

5 Simulation Approaches 19

Streams - Feeds to the absorber are gas stream GASIN containing CH4, C2H6, C3H8, N2, CO2 and H2S and liquid solvent stream LEANIN containing aqueous DEA solution with few CO2 loaded in. Feed to the stripper is rich solvent stream RICHIN containing aqueous DEA solution with absorbed CO2. Feed streams conditions are summarized in Table 5.

Table 5. Feed specifications Stream ID GASIN LEANIN RICHIN

Substream: MIXED

Temperature: F 72 102 212

Pressure: psig 900 900 10

Total flow 3689 kmol/hr 188 gal/min 380 gal/min

Mole-Frac Mass-Frac Mole-Frac

H2O 0 0.7118 0.91

DEA 0 0.2869 0.063

CO2 0.013 0.0012 0.026

H2S 0.00054 0.0001 0.001

CH4 0.924 0 0

C2H6 0.036 0 0

C3H8 0.016 0 0

N2 0.01 0 0

20 6 Simulation Results

6 Simulation Results

The simulation was performed using Aspen Plus V7.0. Key simulation results are presented in Table 6. The measured versus calculated absorber and stripper liquid temperature profiles are presented in Figures 10 and 11, respectively.

Table 6. Key Simulation Results

Variable Measurement Rate-Based DEA Model

Absorber

CO2 mole fraction in GASOUT 150 ppm 178ppm

Loading of RICHOUT, MolAcidGas/MolDEA 0.50 0.42

Stripper

Loading of LEANOUT, MolAcidGas/MolDEA 0.01 0.007

Reboiler duty, lbs. steam/ gal. circ. solution 0.85 0.87

6 Simulation Results 21

0

50

100

150

200

0 5 10 15 20Stage number

Tem

pera

ture

, F

MeasureAspenPlus: LiquidAspenPlus: Vapor

Figure 10. The Absorber Liquid Temperature Profile

50

100

150

200

250

0 5 10 15 20 25 30 35

Stage number

Tem

pera

ture

, F

MeasurementAspenPlus: LiquidAspenPlus: Vapor

Figure 11. The Stripper Liquid Temperature Profile

22 7 Conclusions

7 Conclusions

The rate-based DEA model provides a rate-based rigorous simulation of the process. Key features of this rigorous simulation include electrolyte thermodynamics and solution chemistry, reaction kinetics for the liquid phase reactions, rigorous transport property modeling, rate-based multi-stage simulation with Aspen Rate-Based Distillation which incorporates heat and mass transfer correlations accounting for columns specifics and hydraulics.

The model is meant to be used as a guide for modeling the CO2 capture process with DEA. You may use it as a starting point for more sophisticated models for process development, debottlenecking, plant and equipment design, among others.

References 23

References

[1] Butwell, F. B., Perry C. R., Performance of Gas Purification Systems Utilizing DEA Solutions. Laurance Reid Gas Conditioning Conference, 1975

[2] Aspen Technology Inc., 2007

[3] Rinker E. B., Ashour S. S., Sandall O. C., “Kinetics and Modeling of Carbon Dioxide Absorption into Aqueous Solutions of Diethanolamine”, Ind. Eng. Chem. Res., Vol. 35, 1107 (1996)

[4] Maddox R. N., Bhairl A. H., Diers J. R., Thomas P. A., “Equilibrium Solubility of Carbon Dioxide or Hydrogen Sulfide in Aqueous Solutions of Monoethanolamine, Diglycolamine, Diethanolamine and Methldiethanolamine”, GPA Research Report RR-104, 1987

[5] Maddox R. N., Elizondo E. M., “Equilibrium Solubility of Carbon Dioxide or Hydrogen Sulfide in Aqueous Solutions of Diethanolamine at Low Partial Pressures”, GPA Research Report RR-124, 1989

[6] Weiland R.H., Dingman J.C., Cronin D.B., Browning G.J., “Density and viscosity of some partially carbonated aqueous alkanolamine solutions and their blends”, J. Chem. Eng. Data, Vol. 43, 378 (1998)

[7] Weiland R.H., “Physical Properties of MEA, DEA, MDEA and MDEA-Based Blends Loaded with CO2”, GPA Research Report No. 152, 1996

[8] Weiland R.H., Dingman J.C., Cronin D.B., “Heat capacity of aqueous monoethanolamine, diethanolamine, N- methyldiethanolamine, and N-methyldiethanolamine-based blends with carbon dioxide”, J. Chem. Eng. Data, Vol. 42, 1004 (1997)

[9] Carson J.K., Marsh K.N., Mather A.E., “Enthalpy of solution of carbon dioxide in (water + monoethanolamine, or diethanolamine, or N- methyldiethanolamine) and (water + monoethanolamine + N- methyldiethanolamine) at T = 298.15 K”, J. Chem. Thermodyn., Vol. 32, 1285 (2000)

[10] Austgen D.M., Rochelle G.T., Peng X., Chen C.C., “A Model of Vapor-Liquid Equilibria in the Aqueous Acid Gas-Alkanolamine System Using the Electrolyte-NRTL Equation”, paper presented at the New Orleans AIChE meeting, March 1988

[11] Jou F. Y., Mather A. E., Otto F. D., “Solubility of Hydrogen Sulfide and Carbon Dioxide in Aqueous Methyldiethanolamine Solutions”, Ind. Eng. Chem. Proc. Des. Dev., Vol. 21, 539 (1982)

24 References

[12] Jou F. Y., Carroll J. J., Mather A. E., Otto F. D., “Solubility of Mixtures of Hydrogen Sulfide And Carbon Dioxide in Aqueous N-Methyldiethanolamine Solutions”, J. Chem. Eng. Data, Vol. 38, 75 (1993)

[13] Jou F. Y., Carroll J. J., Mather A. E., Otto F. D., “The Solubility of Carbon Dioxide and Hydrogen Sulfide in a 35 wt% Aqueous Solution of Methyldiethanolamine”, Can. J. Chem. Eng., Vol. 71, 264 (1993)

[14] Pinsent B. R., Pearson L., Roughton F. J. W., “The Kinetics of Combination of Carbon Dioxide with Hydroxide Ions”, Trans. Faraday Soc., Vol. 52, 1512 (1956)

[15] Maham, Y., Teng, T. T., Hepler, L. G., Mather, A. E., “Densities, excess molar volumes, and partial molar volumes for binary mixtures ofwater with monoethanolamine, diethanolamine, and triethanolamine from 25 to 80°C”, J. Solution Chem., Vol. 23, Issue. 2, 195(1994)

[16] Oyevaar, M. H., Morsinkhof, R. W. J., Westerterp, K. R., “Density, Viscosity, Solubility, and Diffusivity of Diethanolamine in Aqueous Ethlyene Glycol at 298K“, J. Chem. Eng. Data., Vol. 34, Issue. 1, 77(1989)

[17] Rinker, E.B., Oleschager, D.W., Colussi, A. T., Henry, K. R., Sandall, O. C., “Viscosity, Density, and Surface Tension of Binary Mixtures of Water and N-Methyldiethanolamine and Water and Diethanolamine and Tertiary Mixtures of These Amines with Water over the Temperature Range 20-100°C” , J. Chem. Eng. Data., Vol. 39, Issue. 2, 392(1994)

[18] Hsu, C.H., Li, M. H., “Viscosities of aqueous blended amines”, J. Chem. Eng. Data., Vol. 42, Issue. 4, 714(1997)

[19] Weiland, R. H., Dingman, J. C., Cronin, D. B., Browning, G. J., “Density and viscosity of some partially carbonated aqueous alkanoamine solutions and their blends”, J. Chem. Eng. Data., Vol. 43, Issue. 3, 378(1998)

[20] Mandal, B. P., Kundu, M., Bandyopadhyay, S. S., “Density and viscosity of aqueous solutions of (N-methyldiethanolamine + monoethanolamine), (N-methyldiethanolamine + diethanolamine), (2-amino-2-methyl-1-propanol + monoethanolamine), and (2-amino-2-methyl-1-propanol + diethanolamine)”, J. Chem. Eng. Data., Vol. 48, Issue. 3, 703(2003)

[21] Pac, J. S., Maksimova, I. N., Glushenko, L. V., ”Viscosity of Alkali Salt Solutions and Comparative Calculation Method”, J. Appl. Chem. USSR, Vol. 57, 846 (1984)

[22] Palaty, Z., “Viscosity of diluted aqueous K2CO3/KHCO3 solutions”, Collect. Czech. Chem. Commun., Vol. 57, Issue. 9, 1879(1992)

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Rate-Based DEA Model