Rate Based MEA MDEA Model

Aspen Plus

Rate-Based Model of the CO2 Capture Process by MEA+MDEA Aqueous Solution using Aspen Plus

Copyright (c) 2008-2010 by Aspen Technology, Inc. All rights reserved. Aspen Plus, the aspen leaf logo and Plantelligence and Enterprise Optimization are trademarks or registered trademarks of Aspen Technology, Inc., Burlington, MA. All other brand and product names are trademarks or registered trademarks of their respective companies. This document is intended as a guide to using AspenTech's software. This documentation contains AspenTech proprietary and confidential information and may not be disclosed, used, or copied without the prior consent of AspenTech or as set forth in the applicable license agreement. Users are solely responsible for the proper use of the software and the application of the results obtained. Although AspenTech has tested the software and reviewed the documentation, the sole warranty for the software may be found in the applicable license agreement between AspenTech and the user. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH RESPECT TO THIS DOCUMENTATION, ITS QUALITY, PERFORMANCE, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE. Aspen Technology, Inc. 200 Wheeler Road Burlington, MA 01803-5501 USA Phone: (1) (781) 221-6400 Toll Free: (1) (888) 996-7100 URL: http://www.aspentech.com http://www.aspentech.com

Copyright (c) 2008-2010 by Aspen Technology, Inc. All rights reserved. Aspen Plus, the aspen leaf logo and Plantelligence and Enterprise Optimization are trademarks or registered trademarks of Aspen Technology, Inc., Burlington, MA. All other brand and product names are trademarks or registered trademarks of their respective companies. This document is intended as a guide to using AspenTech's software. This documentation contains AspenTech proprietary and confidential information and may not be disclosed, used, or copied without the prior consent of AspenTech or as set forth in the applicable license agreement. Users are solely responsible for the proper use of the software and the application of the results obtained. Although AspenTech has tested the software and reviewed the documentation, the sole warranty for the software may be found in the applicable license agreement between AspenTech and the user. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH RESPECT TO THIS DOCUMENTATION, ITS QUALITY, PERFORMANCE, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE. Aspen Technology, Inc. 200 Wheeler Road Burlington, MA 01803-5501 USA Phone: (1) (781) 221-6400 Toll Free: (1) (888) 996-7100 URL: http://www.aspentech.com http://www.aspentech.com

Revision History Vers Versio ion n

Des Descrip cripti tion on

V7.1 V7.1 CP1 V7.2

First version Add O2, CO and H2 to the model as Henry components Update results for V7.2

Revision History

1

Contents Introduction ............................................................................................................3 1 Components .........................................................................................................4 2 Process Description ..............................................................................................5 3 Physical Properties...............................................................................................6 4 Reactions ...........................................................................................................20 5 Simulation Approaches.......................................................................................25 6 Simulation Results .............................................................................................28 7 Conclusions ........................................................................................................29 References ............................................................................................................30

2

Contents

Introduction

This file describes an Aspen Plus rate-based model of the CO 2 capture process by mixed MEA and MDEA aqueous solution from a gas mixture of CO 2 and N2. The model consists of an absorber. The operating data for a bench-scale packed column from Aroonwilas (2004) [1] were used to specify feed conditions and unit operation block specifications in the model. Thermophysical property models and reaction kinetic models are based on the recent works of U.T. Austin (1988)[2], the work of Aspen Technology (2007) [3], Hikita (1977)[4] and Ramachandran (2006)[5] and Pinsent (1956)[6]. Transport property models and model parameters have been validated against experimental data from open literature[17-24]. The model 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 with packing

Introduction

3

1 Components

The following components represent the chemical species present in the process: Table 1. Components Used in the Model

4

ID

Type

Name

Formula

MEA H2O CO2 H3O+ OHHCO3CO3-2 MEAH+ MEACOOMDEA MDEAH+ N2 H2S HSS-2 O2 CO H2

Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional

MONOETHANOLAMINE WATER CARBON-DIOXIDE H3O+ OHHCO3CO3-MEA+ MEACOOMETHYL-DIETHANOLAMINE MDEA+ NITROGEN HYDROGEN SULFIDE HSS-OXYGEN CARBON-MONOXIDE HYDROGEN

C2H7NO H2O CO2 H3O+ OHHCO3CO3-2 C2H8NO+ C3H6NO3C5H13NO2 C5H14NO2+ N2 H2S HSS-2 O2 CO H2

1 Components

2 Process Description

The flowsheet for the bench-scale absorption unit [1] for CO2 capture by MEA+MDEA aqueous solution includes an absorber. Table 2 represents the absorber’s typical operation data: Table 2. Data of the Absorber Used in the CO2 Capture Process by MEA+MDEA Aqueous Solution Absorber

Diameter Packing Type Packing Height

0.02m Sulzer DX 2m Feed Gas

Flow rate CO2 in Sour Gas Temperature Pressure

2.3648kmol/hr 10%(mole fraction) 298K 1atm Lean Amine

Flow rate MEA concentration MDEA concentration CO2 concentration Temperature Pressure

2 Process Description

0.4906m3 /hr 1.5M 1.5M 0.75M 298K 1atm

5

3 Physical Properties

The electrolyte NRTL method is used for liquid and RK equation of state for vapor in this Rate-based MEA+MDEA model. The phase equilibrium model parameters for the sub-systems CO2-MEA-H2O, CO2-MDEA-H2O and H2SMDEA-H2O, together with the Henry’s constant parameters and/or chemical equilibrium constant parameters, were regressed against:  CO2 solubility data in aqueous MEA solutions from Jou et al. (1995) [7], Maddox et al. (1987) [8], Isaacs et al. (1980)[9], Lawson and Garst (1976)[10], Muhlbauer and Monaghan (1957) [11]  CO2 solubility data in aqueous MDEA solutions from Jou et al. (1982, 1993)[ 12-14] , Kuranov et al. (1996) [15] and Kamps et al. (2001) [16]  H2S solubility data in aqueous MDEA solutions from Kuranov et al. (1996)[15] and Kamps et al. (2001) [16] Predictions for CO2 solubility in mixed MEA and MDEA aqueous solutions based on the phase equilibrium model parameters from the sub-systems were compared with literature data from Shen and Li (1992) [17]. CO2, H2S, N2, O2, CO and H2 are selected as Henry-components to which Henry’s law is applied. Henry’s constants for these components with water are retrieved from the Aspen Plus databanks. For solvents MEA and MDEA, the Henry’s constants are obtained as follows:  For CO2 with MEA, regressed from CO 2 solubility data[7-11] in aqueous MEA solutions  For CO2 with MDEA, regressed from CO2 solubility data[12-16] in aqueous MDEA solutions  For H2S with MDEA, regressed from H 2S solubility data[15-16] in aqueous MDEA solutions In the reactions calculations, the activity coefficient basis for the Henry’s components (solutes) is chosen to be Aqueous. Therefore, in calculating the unsymmetric activity coefficients (GAMUS) of the solutes, the infinite dilution activity coefficients are calculated based on infinite-dilution condition in pure water, instead of in mixed solvents. The liquid molar volume model and transport property model parameters are regressed from literature experimental data [18-25] for the sub-systems CO2MEA-H2O and CO2-MDEA-H2O. Predictions for the mixed MEA and MDEA aqueous solutions loaded with CO 2 are compared against literature data when possible. However, we did not evaluate these properties of the MEA and/or

6


MDEA systems loaded with H 2S. Specifications of the transport property models include:  For liquid molar volume, the Clarke model, called VAQCLK in Aspen Plus, is used with option code 1 to use the quadratic mixing rule for solvents. The interaction parameter VLQKIJ for the quadratic mixing rule between MEA and H2O is regressed against experimental MEA-H 2O density data from Kapadi et al. (2002) [18] and VLQKIJ between MDEA and H 2O is regressed against experimental MDEA-H2O density data from BernalGarcia (2003)[19]. The Clarke model parameter VLCLK/1 is also regressed for main electrolytes (MEAH+, HCO 3 ), (MEAH+, MEACOO  ) and (MEAH+, CO 32 ) against experimental CO2-MEA-H2O density data from Weiland (1996) [20] and for (MDEAH+, HCO 3 ) and (MDEAH +, CO 32 ) against experimental density data of the CO 2-MDEA-H2O system from Weiland (1998) [21].  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 solvent. There are three models for electrolyte correction and the MEA+MDEA model always uses the JonesDole correction model. 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 MEA and H 2O in the ASPEN liquid mixture viscosity model, MUKIJ and MULIJ, are regressed against experimental MEA-H2O viscosity data from Kapadi et al. (2002) [18] and Wadi et al. (1995) [22] . MUKIJ and MULIJ between MDEA and H 2O are regressed against experimental viscosity data of the MDEA-H2O system from Teng et al. (1994) [23]. The Jones-Dole model parameters, IONMUB, for MEAH +, and MEACOO- are regressed against CO2-MEA-H2O viscosity data from Weiland (1996) [20]; for MDEAH+, is regressed against CO2-MDEA-H2O viscosity data from Weiland (1998)[21]; for HCO3-, is regressed against KHCO3-H2O viscosity data from Palaty (1992) [24] and for CO32-, is regressed against K 2CO3-H2O viscosity data from Pac et al. (1984) [25].  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. Predictions for the subsystems CO2-MEA-H2O and CO2-MDEA-H2O are within the range of the experimental data from Weiland (1996) [20].  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 with the above transport properties, heat capacity at infinite dilution (CPAQ0) for MDEAH +, MEAH+ and MEACOO- are adjusted to fit to heat capacity data from Weiland (1996) [20].


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The aqueous phase heat of formation at infinite dilution and 25°C (DHAQFM) for MEAH+ and MEACOO- are adjusted to fit to the literature heat of solution data from Carson et al. (2000) [26] of the sub-system CO 2-MEA-H2O. For MDEAH+, the databank value of DHAQFM is -5.0471x10 8 J/kmol, which results in heat of solution predictions for the sub-system CO 2-MDEA-H2O as shown in Figure 6b-1 together with the data from Carson et al. (2000) [26]. However, to match the temperature profile data of an plant absorber for CO 2 capture by aqueous MDEA solutions [27], it was found that a value of -5.0 x10 8J/kmol for DHAQFM of MDEAH+ is better. This value is used in the current simulation and results in heat of solution predictions for sub-system CO 2-MDEA-H2O as shown in Figure 6b-2. The estimation results of various transport and thermal properties are summarized in Figures 1-8:

1200

EXP MEA EXP MEA EXP MEA EXP MEA EST MEA EST MEA EST MEA EST MEA

1150

1100

3 m / g k 1050 , y t i s n e D 1000

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

950

900 0

0.1

0.2

0.3

0.4

0.5

CO2 Loading

Figure 1a. Liquid Density of MEA-CO2-H 2O at 298.15K, experimental data from Weiland (1996)[20]

8


1200

1150

3 1100 m / g k , y 1050 t i s n e D 1000

EXP MDEA EXP MDEA EXP MDEA EXP MDEA EST MDEA EST MDEA EST MDEA EST MDEA

950


900 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

CO2 Loading, m ol/mol

Figure 1b. Liquid Density of MDEA-CO2-H 2O at 298.15K, experimental data from Weiland (1998)[21]

1200

1150

3 1100 m / g k , y 1050 t i s n e D 1000

EXP MEA5w t%+MDEA45w t% EST MEA5w t%+MDEA45w t% EXP MEA10w t%+MDEA40w t% EST MEA10w t%+MDEA40w t%

950

EXP MEA20w t%+MDEA30w t% EST MEA20w t%+MDEA30w t%

900 0

0.1

0.2

0.3

0.4

0.5

CO2 Loading

Figure 1c. Liquid Density of MEA+MDEA-CO2-H 2O at 298.15K, experimental data from Weiland (1998)[21]


9

100

S a P m , y t i s o c s i V

EXP MEA EXP MEA EXP MEA EST MEA EST MEA EST MEA

20w t% 30w t% 40w t% 20w t% 30w t% 40w t%

10

1

0.1 0

0.1

0.2

0.3

0.4

0.5

CO2 Loading, mol/mol

Figure 2a. Liquid Viscosity of MEA-CO2-H 2O at 298.15K, experimental data from Weiland (1996)[20]

1000.00

) S a P m , y t i s o c s i V ( g o L

EXP MDEA EST MDEA EXP MDEA EST MDEA EXP MDEA EST MDEA EXP MDEA EST MDEA

100.00


10.00

1.00 0

0.1

0.2

0.3

0.4

0.5

CO2 Loading

Figure 2b. Liquid Viscosity of MDEA-CO2-H 2O at 298.15K, experimental data from Weiland (1998)[21]

10


0.1

EXP MEA5w t%+MDEA45w t% EST MEA5w t%+MDEA45w t% EXP MEA10w t%+MDEA40w t% EST MEA10w t%+MDEA40w t% EXP MEA20w t%+MDEA30w t%

S a P , y t i s o c s i V

EST MEA20w t%+MDEA30w t% 0.01

0.001 0

0.1

0.2

0.3

0.4

0.5

CO2 Loading

Figure 2c. Liquid Viscosity of MEA+MDEA-CO2-H 2O at 298.15K, experimental data from Weiland (1998)[21]

0.1

0.09

0.08

m0.07 / N , n 0.06 o i s n 0.05 e T e c 0.04 a f r u S 0.03

EXP MEA EXP MEA EXP MEA EXP MEA EST MEA EST MEA EST MEA EST MEA

0.02

0.01

10wt% 20wt% 30wt% 40wt% 10wt% 20wt% 30wt% 40wt%

0 0

0.1

0.2

0.3

0.4

0.5

0.6


Figure 3a. Surface tension of MEA-CO2-H 2O at 298.15K, experimental data from Weiland (1996)[20]


11

0.08 EXP MDEA EST MDEA EXP MDEA EST MDEA EXP MDEA EST MDEA EXP MDEA EST MDEA

0.07

m / N , n 0.06 o i s n e T e 0.05 c a f r u S


0.04

0.03 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

CO2 Loading

Figure 3b. Surface tension of MDEA-CO2-H 2O at 298.15K, experimental data from Weiland (1996)[20]

1 EST MEA 10w t%

K m / t t a W , y t i v i t c u d n o C l a m r e h T

0.9

EST MEA 20w t% EST MEA 30w t%

0.8

EST MEA 40w t%

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.1

0.2

0.3

0.4

0.5

CO2 Loading

Figure 4a. Liquid Thermal Conductivity of MEA-CO2-H 2O at 298.15K

12


0.4

K 0.35 m / t t 0.3 a W , y 0.25 t i v i t c 0.2 u d n o 0.15 C l a 0.1 m r e h T 0.05

EST MDEA 30w t% EST MDEA 40w t% EST MDEA 40w t% EST MDEA 60w t%

0 0

0.1

0.2

0.3

0.4

0.5


Figure 4b. Liquid Thermal Conductivity of MDEA-CO2-H 2O at 298.15K

4500

4000

) g k / 3500 J ( y t i c a p3000 a C t a e H2500

EXP MEA 10w t% EXP MEA 20w t% EXP MEA 30w t% EXP MEA 40w t% EST MEA10w t% EST MEA 20w t% EST MEA 30w t% EST MEA 40w t%

2000 0

0.1

0.2

0.3

0.4

0.5

CO2 Loading

Figure 5a. Liquid Heat Capacity of MEA-CO2-H 2O at 298.15K, experimental data from Weiland (1996)[20]


13

160

140

120

K l o m100 / J , y t i 80 c a p a C 60 t a e H 40

EXP MDEA EXP MDEA EXP MDEA EXP MDEA EST MDEA EST MDEA EST MDEA EST MDEA

20


0 0

0.1

0.2

0.3

0.4

0.5

CO2 Loading

Figure 5b. Liquid Heat Capacity of MDEA-CO2-H 2O at 298.15K, experimental data from Weiland (1996)[20]

4000 3500

K - 3000 g k / J 2500 , y t i c 2000 a p a C 1500 t a e H 1000

EXP MEA5w t%+MDEA45w t%5 EST MEA5w t%+MDEA45w t% EXP MEA10w t%+MDEA40w t% EST MEA10w t%+MDEA40w t% EXP MEA10w t%+MDEA40w t%

500

EST MEA10w t%+MDEA40w t% 0 0

0.1

0.2

0.3

0.4

0.5

CO2 Loading

Figure 5c. Liquid Heat Capacity of MEA+MDEA-CO2-H 2O at 298.15K, experimental data from Weiland (1997)[28]

14


-70000

EXP MEA 10wt% EXP MEA 20wt%

-72000

EXP MEA 30wt% 1 EXP MEA 30wt% 2

-74000

EST MEA 10wt%

-76000

EST MEA 20wt% EST MEA 30wt%

-78000 -80000 -82000 -84000 -86000 -88000 -90000 0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Figure 6a. Heat of Solution of CO2 in MEA-H 2O at 298.15K, experimental data from Carson et al. (2000)[26]

Figure 6b-1. Heat of Solution of CO2 in MDEA-H 2O using ASPEN databank DHAQFM(MDEAH + ) value (-5.0471E8J/kmol), experimental data from Carson et al. (2000)[26]


15

Figure 6b-2. Heat of Solution of CO2 in MDEA-H 2O using DHAQFM(MDEAH + )=5.0E8J/kmol, experimental data from Carson et al. (2000) [26]

0

EXP1 MEA7w t%+MDEA27w t% EXP2 MEA7w t%+MDEA27w t% EST MEA7w t%+MDEA27w t%

l o m / J , n o i t u l o S f o t a e H

-20000

EST MEA34w t% EST MDEA34w t%

-40000

-60000

-80000

-100000 0

0.02

0.04

0.06

0.08

0.1

0.12

CO2 Loading

Figure 6c. Heat of Solution of CO2 in MEA+MDEA-H 2O at 298.15K, experimental data from Carson et al. (2000) [26]

16


1000000 100000 10000

a P k , e r u s s e r P 2 O C

1000 100

EXP 40C EST 40C EXP 60C EST 60C EXP 80C EST 80C EXP 100C EST 100C EXP 120C EST 120C

10 1 0.1 0.01 0.001 0.001

0.01

0.1

1

10

CO2 Loading

Figure 7a-1. CO2 Partial Pressure of MEA-CO 2-H 2O (MEA mass fraction = 0.3), experimental data from Jou et al. (1995) [7]

10000

1000

a P K , e r u s s e r P 2 O C

100

10 EXP 298.15K EST 298.15K EXP 333.15K EST 333.15K EXP 353.15K EST 353.15K

1

0.1 0.1

1

10

CO2 Loading

Figure 7a-2. CO2 Partial Pressure of MEA-CO 2-H 2O (MEA mass fraction = 0.153), experimental data from Maddox et al. (1987)[8]


17

10000

1000


100

EXP 310.9K EST 310.9K EXP 338.7K EST 338.7K EXP 388.7K EST 388.7K

10

1 0.1

1

10

CO2 Loading

Figure 7b. CO2 Partial Pressure of MDEA-CO2-H 2O (MDEA mass fraction = 0.20), experimental data from Maddox et al. (1987)[8]

10000

EXP 40C EST 40C EXP 60C

1000


EST 60C EXP 80C EST 80C

100

EXP 100C EST 100C

10

1

0.1 0.1

1

CO2 Loading

Figure 7c-1. CO2 Partial Pressure of MEA+MDEA-CO2-H 2O (MEA mass fraction = 0.12 and MDEA mass fraction = 0.18), experimental data from Shen and Li (1992)[17]

18


10000 EXP 40C EST 40C EXP 60C EST 60C EXP 80C EST 80C EXP 100C EST 100C

1000


100

10

1

0.1 0.1

1

CO2 Loading

Figure 7c-2. CO2 Partial Pressure of MEA+MDEA-CO2-H 2O (MEA mass fraction = 0.24 and MDEA mass fraction = 0.06), experimental data from Shen and Li (1992)[17]

10

a P M , e r u s s e r P S 2 H

EXP 313.15K EST 313.15K EXP 333.15K EST 333.15K EXP 373.15K EST 373.15K EXP 393.15K EST 393.15K EXP 413.15K EST 413.15K

1

0.1 1

10

m_H2S, mol/kg

Figure 8. H 2S Partial Pressure of MDEA-H 2S-H 2O (MDEA molality = 4), experimental data from Kuranov et al. (1996) [15]


19

4 Reactions

MEA is a primary ethanolamine, as shown in Figure 9. It can associate with H + to form an ion MEAH +, and react with CO2 to form a carbamate ion MEACOO -.

Figure 9. MEA Molecular Structure

MDEA is a tertiary ethanolamine, as shown below in Figure 10. It can associate with H+ to form MDEAH+ but cannot react with CO 2 to produce carbamate as is the case for primary or secondary ethanolamines.

Figure 11. MDEA Molecular Structure

The electrolyte solution chemistry for the mixed amines, MEA+MDEA with CO 2 and H2S, has been modeled with a CHEMISTRY model with CHEMISTRY ID = MEAMDEA. This CHEMISTRY ID is used as the 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 MEAMDEA. In addition, a kinetic REACTION model called RMEAMDEA has been created and used in the calculations of the absorber by specifying it in the Reaction part of the absorber specifications. In RMEAMDEA, all reactions are assumed to be in chemical equilibrium except those of CO 2 with OH-, CO2 with MEA and CO 2 with MDEA.

20

4 Reactions

A. Chemistry ID: MEAMDEA

1 Equilibrium

2H 2 O  H 3 O   OH 

2 Equilibrium

CO 2  2H 2 O  H 3 O   HCO 3

3 Equilibrium

HCO 3  H 2 O  H 3 O  CO 3

4 Equilibrium

MEAH   H 2 O  MEA  H 3 O 

5 Equilibrium

MEACOO   H 2 O  MEA  HCO 3

6 Equilibrium

MDEAH   H 2 O  MDEA  H 3 O 

7 Equilibrium

H 2 O  H 2 S  HS  H 3 O

8 Equilibrium

H 2 O  HS   S 2  H 3 O 





2





B. Reaction ID: RMEAMDEA

1 Equilibrium

MEAH   H 2 O  MEA  H 3 O 

2 Equilibrium

2H 2 O  H 3 O  OH

3 Equilibrium

HCO 3  H 2 O  CO 3   H 3 O 

4 Equilibrium

MDEAH   H 2 O  MDEA  H 3 O 

5 Equilibrium

H 2 O  H 2 S  HS   H 3 O 

6 Equilibrium

H 2 O  HS   S

7 Kinetic

CO 2  OH   HCO 3

8 Kinetic

HCO 3  CO 2  OH 

9 Kinetic

MEA  CO 2  H 2 O  MEACOO  H 3 O 

10 Kinetic

MEACOO  H 3 O   MEA  H 2 O  CO 2

11 Kinetic

MDEA  CO 2  H 2 O  MDEAH  HCO 3

12 Kinetic

MDEAH   HCO -3  MDEA  CO 2  H 2 O





2

2

 H 3O



-

-



-

The equilibrium expressions for the reactions are taken from the work of Austgen et al.[2] and Jou et al.[12-14], as well as the parameters for the equilibrium constants. However, for the fifth equilibrium reaction, its equilibrium constant parameters were regressed using the CO 2 solubility data[7-11] in aqueous MEA solutions together with the NRTL parameters and the Henry’s constants parameters. The power law expressions (T 0 not specified) are used for the rate-controlled reactions (reactions 7-12 in RMEAMDEA): n

r  kT exp ( 

E RT

N

) C i

ai

(1)

i 1

Where:

4 Reactions

21

r =

Rate of reaction; k = Pre-exponential factor; T =

Absolute temperature; n = Temperature exponent; E =

Activation energy; R = Universal gas constant; N =

Number of components in the reaction; C i = Concentration of component i ; ai =

The stoichiometric coefficient of component i in the reaction equation.

In equation (1), the concentration basis is Molarity, the factor n is zero, k and E are given in Table 3. Ramachandran et al.[5] reported that reaction kinetics of the mixed MEA and MDEA system cannot be interpreted by the mechanism of the single amines. We assume that free MEA can transfer CO 2 to MDEA and then regenerate by itself simultaneously: (A) MEA  CO 2  MEA  CO 2 (B) MEA  CO 2  MDEA  MDEA  CO 2  MEA (C) MDEA  CO 2  H 2 O  MDEAH   HCO 3We combine these three reactions and obtain the following reaction: (D) MDEA  CO 2  H 2 O  MDEAH   HCO 3Reaction (D) is used to represent the chemical equilibrium between MDEA and CO2, and the following rate expression is used to represent the catalytic effect of MEA on reaction (D): n

r  kT exp ( 

E RT

)C MDEAC MEA C CO2

(2)

To implement the catalytic effect of MEA on reaction (D), we set the stoichiometric coefficient of MEA to 0 and the concentration exponent of MEA to 1 when we edit reactions 11 and 12 of RMEAMDEA in Reactions (Figures 11a and 11b).

22

4 Reactions

Figure 11a. Specifications of Reaction 11

Figure 11b. Specifications of Reaction 12

The kinetic parameters for reactions 7, 9 and 11 in Table 3 are derived from the work of Pinsent [6], Hikita[3] and Ramachandran[5]. The kinetic parameters for the corresponding reversible reactions 8, 10 and 12 are calculated by using the kinetic parameters and the equilibrium constants of the forward reactions 7, 9 and 11.

4 Reactions

23

Table 3. Parameters k and E in Equation (1)

24

Reaction No.

k

E , cal/mol

7 8 9 10 11 12

4.32e+13 2.38e+17 9.77e+10 2.80e+20 2.21e+8 8.89e+11

13249 29451 9856 17230 7432 15334

4 Reactions

5 Simulation Approaches

The case from Aroonwilas [1] is used in this simulation. Simulation Flowsheet – The bench-scaled absorber has been modeled with

the following simulation flowsheet in Aspen Plus, shown in Figure 12.

GASOUT

LEANIN

ABSORBER GASIN RICHOUT

Figure 12. Rate-Based MEA+MDEA Simulation Flowsheet in Aspen Plus


25

Unit Operations - Major unit operations in this model have been represented

by Aspen Plus Blocks as outlined in Table 4. As there is no Sulzer DX in the packing type option of Aspen Plus, the Sulzer BX packing is used, the column diameter is scaled up to 0.25m and the packing height is set to 6m. Table 4. Aspen Plus Unit Operation Blocks Used in the Rate-Based MEA+MDEA Model Unit Operation

Aspen Plus Block

Comments / Specifications

Absorber

RadFrac

1. Calculation type: Rate-Based 2. 20 Stages 3. Top Pressure: 1atm 4. Reaction condition factor: 0.5 5. Film discretization ratio: 2 6. Heater Cooler: Heat loss is ignored for the absorber 7. Reaction: Reaction ID is RMEAMDEA for all stages 8. Packing Type: Sulzer BX 9. Section Diameter: 0.25m 10. Packing Height: 6m 11. Mass transfer coefficient method: Bravo et al (1985) 12. Interfacial area method: Bravo et al (1985) 13. Interfacial area factor: 0.5 14. Heat transfer coefficient method: Chilton and Colburn 15. Holdup correlation: Bravo et al (1992) 16. Film resistance: Discrxn for liquid film; Film for vapor film 17. Additional discretization points for liquid film: 5 18. Flow model: CounterCurrent

26


Streams -

Feeds to the absorber are GASIN containing N 2 and CO2 and LEANIN containing aqueous MEA and MDEA solution loaded with some CO 2. Feed conditions are summarized in Table 5. The flow rates of feeds are scaled up in accordance with the scaled-up column diameter. Table 5. Feed specifications Stream ID

GASIN

Substream: MIXED Temperature: K 298 Pressure: atm 1 Total flow 2.3648kmol/hr Mole-Frac H2O 0 CO2 0.1 MEA 0 MDEA 0 N2

0.9

LEANIN 298 1

0.4906cum/hr Mole-Conc solvent 0.75kmol/cum 1.5kmol/cum 1.5kmol/cum 0

Prop-Sets -

A Prop-Set, XAPP, has been created to report apparent mole fraction of CO2, MEA and MDEA in liquid streams to facilitate calculations of CO2 loadings of the streams.


27

6 Simulation Results

The simulation was performed using Aspen Plus V7.2. The measured versus calculated absorber CO2 concentration profile in the gas phase is shown in Figure 13. 2

m1.5 , m o t t o b 1 m o r f t h g i e 0.5 H

0 0

0.02

0.04

0.06

0.08

0.1

CO2 concentration in gas(%)

Figure 13. The Absorber CO2 concentration Profile in Gas Phase

() Experimental data of 3M MEA, ( ) Experimental data of 1.5M MEA and 1.5M MDEA, () Experimental data of 3M MDEA, (—) Simulation results of 3M MEA, (–––) Simulation results of 1.5M MEA and 1.5M MDEA, (----) Simulation results of 3M MDEA ■

28

6 Simulation Results

7 Conclusions

The Rate-Based MEA+MDEA 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 CO 2 capture process with MEA+MDEA. Users may use it as a starting point for more sophisticated models for process development, debottlenecking, plant and equipment design, among others.

7 Conclusions

29

References

[1] A. Aroonwilas, A. Veawab, “Characterization and Comparison of the CO 2 Absorption Performance into Single and Blended Alkanolamines in a Packed Column”, Ind. Eng. Chem. Res., 43, 2228-2237 (2004) [2] D.M. Austgen, G.T. Rochelle, X. Peng, and C.-C. Chen, "A Model of VaporLiquid Equilibria in the Aqueous Acid Gas-Alkanolamine System Using the Electrolyte-NRTL Equation," Paper presented at the New Orleans AIChE Meeting, March 1988 [3] Aspen Technology Inc., 2007 [4] H. Hikita, S. Asai, H. Ishikawa, M. Honda, “The Kinetics of Reactions of Carbon Dioxide with Monoethanolamine, Diethanolamine, and Triethanolamine by a Rapid Mixing Method”, Chem. Eng. J., 13, 7-12 (1977). [5] N. Ramachandran, A. Aboudheir, R. Idem, P. Tontiwachwuthikul, “Kinetics of the Absorption of CO 2 into Mixed Aqueous Loaded Solutions of Monoethanolamine and Methyldiethanolamine”, Ind. Eng. Chem. Res., 45, 2608-2616 (2006) [6] B.R. Pinsent, L. Pearson, F.J.W. Roughton, “The Kinetics of Combination of Carbon Dioxide with Hydroxide Ions”, Trans. Faraday Soc., 52, 1512-1520 (1956) [7] F.-Y. Jou, A.E. Mather, F.D. Otto, "The Solubility of CO2 in a 30 Mass Percent Monoethanolamine Solution", Can. J. Chem. Eng., 3, 140-147 (1995) [8] R.N. Maddox, A.H. Bhairi, J.R. Diers, P.A. Thomas, “Equilibrium Solubility of Carbon Dioxide or Hydrogen Sulfide in Aqueous Solutions of Monoethanolamine, Diglycolamine, Diethanolamine and Methyldiethanolamine”, GPA Research Report No.104, 1987 [9] E.E. Isaacs, F.D. Otto, A.E. Mather, "Solubility of Mixtures of H 2S and CO2 in a Monoethanolamine Solution at Low Partial Pressures", J. Chem. Eng. Data, 25, 118-120 (1980) [10] J.D. Lawson, A.W. Garst, "Gas Sweetening Data: Equilibrium Solubility of Hydrogen Sulfide and Carbon Dioxide in Aqueous Monoethanolamine and Aqueous Diethanolamine Solutions", J. Chem. Eng. Data, 21, 20-30 (1976) [11] H.G. Muhlbauer, P.R. Monaghan, "Sweetening Natural Gas With Ethanolamine Solutions", Oil Gas J. 55(17), 139 (1957)

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[12] F.-Y. Jou, A.E. Mather, F.D. Otto, “Solubility of Hydrogen Sulfide and Carbon Dioxide in Aqueous Methyldiethanolamine Solutions”, Ind. Eng. Chem. Proc. Des. Dev., 21, 539-544 (1982) [13] F.-Y. Jou, J.J. Carroll, A.E. Mather, F.D. Otto, “Solubility of Mixtures of Hydrogen Sulfide And Carbon Dioxide in Aqueous N-Methyldiethanolamine Solutions”, J. Chem. Eng. Data, 38, 75-77 (1993) [14] F.-Y. Jou, J.J. Carroll, A.E. Mather, F.D. Otto, “The Solubility of Carbon Dioxide and Hydrogen Sulfide in a 35 wt% Aqueous Solution of Methyldiethanolamine”, Can. J. Chem. Eng., 71, 264-268 (1993) [15] G. Kuranov, B. Rumpf, N.A. Smirnova, G. Maurer, "Solubility of Single Gases Carbon Dioxide and Hydrogen Sulfide in Aqueous Solutions of NMethyldiethanolamine in the Temperature Range 313-413 K at Pressures up to 5 MPa", Ind. Eng. Chem. Res., 35, 1959-1966 (1996) [16] Á. P.-S. Kamps, A. Balaban, M. Jödecke, G. Kuranov, N.A. Smirnova, G. Maurer, "Solubility of Single Gases Carbon Dioxide and Hydrogen Sulfide in Aqueous Solutions of N-Methyldiethanolamine at Temperatures from 313 to 393 K and Pressures up to 7.6 MPa: New Experimental Data and Model Extension", Ind. Eng. Chem. Res., 40, 696-706 (2001) [17] K.-P. Shen and M.-H. Li, "Solubility of Carbon Dioxide in Aqueous Mixtures of Monoethanolamine with Methyldiethanolamine", J. Chem. Eng. Data 37, 96-100 (1992) [18] U.R. Kapadi, D.G. Hundiwale, N.B. Patil, M.K. Lande, "Viscosities, excess molar volume of binary mixtures of ethanolamine with water at 303.15, 308.15, 313.15 and 318.15 K", Fluid Phase Equilibria, 201, 335-341 (2002) [19] J.M. Bernal-Garcia, M. Romas-Estrada, G.A. Iglesias-Silva, “Densities and Excess Molar Volumes of Aqueous Solutions of NMethyldiethanolamine(MDEA) at Temperatures from (283.15 to 363.15)K”, J. Chem. Eng. Data, 48, 1442-1445 (2003) [20] R.H. Weiland, “Physical Properties of MEA, DEA, MDEA and MDEA-Based Blends Loaded with CO2” , GPA Research Report No. 152, August 1996 [21] R.H. Weiland, J.C. Dingman, D.B. Cronin, G.J. Browning, “Density and Viscosity of Some Partially Carbonated Aqueous Alkanolamine Solutions and Their Blends”, J. Chem. Eng. Data, 43, 378-382 (1998) [22] R.K. Wadi, P. Saxena, "Molar Conductivity of Alkali-Halides in Ethanolamine and Water Plus Ethanolamine at 298.15 K", Indian J. Chem. Sect. A, 34, 273 (1995) [23] T.T. Teng, Y. Maham, L.G. Hepler, A.E. Mather, “Viscosity of Aqueous Solution of N-Methyldiethanolamine and of Diethanolamine”, J. Chem. Eng. Data., 39, 290-293 (1994) [24] Z. Palaty, “Viscosity of diluted aqueous K2CO3/KHCO3 solutions”, Collect. Czech. Chem. Commun., 57, 1879 (1992) [25] J.S. Pac, I.N. Maksimova, L.V. Glushenko, ”Viscosity of Alkali Salt Solutions and Comparative Calculation Method”, J. Appl. Chem. USSR, 57, 846 (1984) [26] J.K Carson, K.N. Marsh, A.E. Mather, "Enthalpy of solution of carbon dioxide in (water + monoethanolamine, or diethanolamine, or N -

References

31

Rate Based MEA MDEA Model

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