Rate-Based NaOH Model

Aspen Plus

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

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Revision History 1

Revision History

Version Description

V7.0 First version

2 Contents

Contents

Introduction............................................................................................................3

1 Components .........................................................................................................4

2 Process Description..............................................................................................5

3 Physical Properties...............................................................................................6

4 Reactions .............................................................................................................7

5 Simulation Approaches.......................................................................................13

6 Simulation Results .............................................................................................15

7 Conclusions ........................................................................................................17

References ............................................................................................................18

Introduction 3

Introduction

This document describes an Aspen Plus rate-based model of the CO2 capture process by NaOH (sodium hydroxide) from a gas mixture of N2, O2, H2O, and CO2. The model consists of an absorber. The operation data from a pilot plant[1] were used to specify feed conditions and unit operation block specifications in the model. The reaction kinetic models are based on the works of Pinsent(1956)[2]. The thermophysical property model and 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 phase properties and RK equation of state for vapor phase properties

• Concentration-based reaction kinetics

• Electrolyte transport property models

• Rate-based models for absorber with packing

4 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

CO2 Conventional CARBON-DIOXIDE CO2

NA2CO3 Conventional SODIUM-CARBONATE NA2CO3

H3O+ Conventional H3O+ H3O+

OH- Conventional OH- OH-

HCO3- Conventional HCO3- HCO3-

CO3-2 Conventional CO3-- CO3-2

NA+ Conventional NA+ NA+

NAOH Conventional SODIUM-HYDROXIDE NAOH

N2 Conventional NITROGEN N2

O2 Conventional OXYGEN O2

NAHCO3 Conventional SODIUM-BICARBONATE NAHCO3

2 Process Description 5

2 Process Description

The flowsheet for the pilot plant[1] for CO2 capture by NaOH includes an absorber. Table 2 represents typical operation data:

Table 2. Data of the pilot plant

Absorber

Diameter 0.1 m

Packing Type and Size 0.5 inch ceramic Berl Saddles

Packing Height 6.6 m

Feeds and Products

Sour Gas to Absorber 0.495 kmol/hr

Lean NaOH solution to Absorber 106.0 l/hr

CO2 in Feed (Gas) 15.4(Vol %)

CO2 in Outlet (Gas) 0.0(Vol %)

6 3 Physical Properties

3 Physical Properties

The electrolyte NRTL method is used for computing liquid phase properties while RK equation of state is used for computing vapor phase properties in this Rate-based NaOH model. The electrolyte pair parameters in the electrolyte NRTL model for (H2O, NaOH) were taken from the works of Aspen Technology(1985)[3]; those for (H2O, Na2CO3) were regressed in this work against vapor pressure data from Taylor (1955)[4] and osmotic coefficient data from Khvorostin et al.(1975)[5] and Goldberg et al.(1981)[6] of the Na2CO3-H2O system; and those for (H2O, NaHCO3) were also regressed in this work against CO2 solubility data in Na2CO3-H2O system from Knuutila et al.[7].

CO2, N2, and O2 are selected as Henry-components to which Henry’s law is applied and the Henry’s constants are retrieved from Aspen Plus databanks for these components with water. 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 model and transport property models have been updated and model parameters regressed from literature experimental data. Specifications of the transport property 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 Aspen Plus built-in databank values for the Clarke model parameter

VLCLK/1 of some main electrolytes (Na+, OH − ), (Na+, HCO −3 ) and (Na+,

CO 23− ) are used.

• 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 NaOH model Breslau and Miller equation instead of Jones and Dole equation when electrolyte concentration exceeds 0.1. The three option codes for MUL2JONS are set to 1 (mixture viscosity weighted by mass fraction), 0 (use Breslau and Miller equation instead of Jones and Dole equation when electrolyte concentration exceeds 0.1 M), and 2 (ASPEN liquid mixture viscosity model), respectively. The electrolyte correction model parameters, IONMUB, for HCO3

- is regressed against KHCO3-H2O viscosity data from Palaty(1992)[8];

and that of CO 23− is regressed against K2CO3-H2O viscosity data from Pac

3 Physical Properties 7

(1984)[9]. For Na+, IONMOB and IONMUB were regressed against viscosity data from Klochko et al. (1959)[10] and Vargaftik (1972)[11] of the NaOH-H2O system. For the other ions, Aspen Plus built-in databank values are used.

• 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.

• For thermal conductivity, the Riedel electrolyte correction model, called KL2RDL in Aspen Plus, is used. The model parameter IONRDL for Na+, CO3

-2, and HCO3- was regressed in this work against data from

Riedel(1951)[12], Vargaftik et al.(1956)[13], Vargaftik(1972)[11] and Cederberg(1965)[14], Chernen'kaya et al.(1973)[15], Chernen'kaya et al.(1972)[16].

• 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, the heat

capacity at infinite dilution (CPAQ0) for CO 23− and HCO3

- are adjusted to keep

constant with the rate-based amines models, and that of Na+ was regressed in this work against the heat capacity data from Puchkov et al.(1978)[17] and Chernen'kaya(1971)[18] together with the vapor pressure data from Taylor(1955)[4] and osmotic coefficient data from Khvorostin et al.(1975)[5] and Goldberg et al.(1981)[6] of the Na2CO3-H2O system.

The estimation results of the transport and thermal properties are summarized in Figures 1-6:

980

1000

1020

1040

1060

1080

1100

1120

1140

0 0.02 0.04 0.06 0.08 0.1 0.12

NaOH Mass Fraction

Den

sity

, kg/

m3

EXP 298.15K

EST 298.15K

EXP 348.15K

EST 348.15K

Figure 1. Liquid Density of NaOH-H2O, experimental data from Herrington et al. (1986)[19]


0.0001

0.001

0.01

0.1

1

10

0 0.2 0.4 0.6 0.8

NaOH Mass Fraction

Visc

osity

, PaS

EXP Klochko59 298.15KEST 298.15KEXP Klochko59 348.15KEST 348.15KEXP Klochko59 373.15KEST 373.15KEXP Vargaftik72 293.15KEXP Vargaftik72 303.15KEXP Vargaftik72 313.15K

Figure 2. Liquid Viscosity of NaOH-H2O, experimental data from Klochko et al.(1959)[10] and Vargaftik(1972)[11]

0.05

0.06

0.07

0.08

0.09

0.1

290 300 310 320 330 340 350T, K

Surf

ace

Tens

ion,

N/m

EXP NAOH 5WT%EST NAOH 5WT%EXP NAOH 10WT%EST NAOH 10WT%EXP NAOH 15WT%EST NAOH 15WT%

Figure 3. Surface Tension of NaOH-H2O, experimental data from Gel'perin et al.(1969)[20]

3 Physical Properties 9

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6NaOH Mass Fraction

Ther

mal

CO

nduc

tivity

, Wat

t/m-K

EXP Riedel51 293.15KEXP Vargaftik56 293.15KEXP Vargaftik72 293.15KEXP Cederberg65 293.15KEST 293.15K

Figure 4. Liquid Thermal Conductivity of NaOH-H2O, experimental data Riedel(1951)[12], Vargaftik et al.(1956)[13], Vargaftik(1972)[11] and Cederberg(1965)[14]

0

1000

2000

3000

4000

5000

6000

0 0.01 0.02 0.03 0.04 0.05 0.06

NaOH Mas s Frac tion

Hea

t C

apac

ity,

J/kg

-K

EXP Conti88 325K EST 325KEXP Conti88 373K EST 373KEXP Conti88 422K EST 422KEXP Conti88 470K EST 470KEXP Conti88 521K EST 521K

Figure 5. Liquid Heat Capacity of NaOH-H2O at 1atm, experimental data from Conti et al.(1988)[21]


0.01

0.1

1

10

100

0.1 1

CO2 loading

P C

O2,

kPa

EXP 80C EXP 60CEXP 40C EST 80CEST 60C EST 40C

Figure 6. CO2 partial pressure of Na2CO3-CO2-H2O (Na2CO3 mass fraction = 0.08), experimental data from Knuutila et al.[7]

4 Reactions 11

4 Reactions

The electrolyte solution chemistry has been modeled with a CHEMISTRY model with CHEMISTRY ID of CAUSTIC, 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 CHEMISTRY CAUSTIC. In addition, a kinetic REACTION model named CAUST-R has been created, which is used in calculations of the absorber by specifying it in the Reaction part of the absorber specifications. In CAUST-R, all reactions are assumed to be in chemical equilibrium except the reactions of CO2 with OH-.

A. Chemistry ID: CAUSTIC

1 Equilibrium −+ +↔+ 3322 HCOOHO2HCO

2 Equilibrium 2

3323 COOHOHHCO −+− +↔+

3 Equilibrium −+ +↔ OHOHO2H 32

4 Dissociation −+ +↔ OHNaNaOH

5 Dissociation 2332 CO2NaCONa −+ +↔

6 Dissociation 233 CONaNaHCO −+ +↔ H

B. Reaction ID: CAUST-R

1 Equilibrium −+ +↔ OHOHO2H 32

2 Equilibrium 23323 COOHOHHCO −+− +↔+

3 Kinetic −− →+ 32 HCOOHCO

4 Kinetic −− +→ OHCOHCO 23

The equilibrium expressions for the reactions are taken from the work of Aspen Technology(1985)[3]. In addition, the power law expressions are used for the rate-controlled reactions (reactions 3-4 in CAUST-R) and the general power law expression is:

12 4 Reactions

( ) ∏=

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟

⎠⎞

⎜⎝⎛ −=

N

i

ai

n iCTTR

ETTkr10

011exp (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 T0 is not specified, the reduced power law expression is used:

∏=

−=N

i

ai

n iC)RTE(kTr

1

exp (2)

In this file, the reduced expressions are used. In equation (2), the concentration basis is Molarity, the factor n is zero, k and E are given in Table 3. The kinetic parameters for reaction 3 are taken from the work of Pinsent et al.(1956)[3], and the kinetic parameters for reaction 4 are calculated by using the kinetic parameters of reaction 3 and the equilibrium constants of the reversible reactions 3 and 4.

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

Reaction No. k E (cal/mol)

2 4.32e+13 13249

3 2.83e+17 29451

5 Simulation Approaches 13

5 Simulation Approaches

Case 12 for the absorber of the pilot plant[1] for CO2 capture by NaOH are used in this study.

Simulation Flowsheet – The pilot plant has been modeled with the following simulation flowsheet in Aspen Plus, shown in Figure 7.

LEANIN

GASIN

GASOUT

RICHOUT

ABSORBER

Figure 7. Rate-Based NaOH Simulation Flowsheet in Aspen Plus

14 5 Simulation Approaches

Unit Operations - Major unit operations in this model have been represented by Aspen Plus Blocks as outlined in Table 4.

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

Unit Operation Aspen Plus Block Comments / Specifications

Absorber RadFrac 1. Calculation type: Rate-Based

2. 18 Stages

3. Top Pressure: 101.15 kPa

4. Reaction condition factor: 0.9

5. Film discretization ratio: 5

6. Reaction: Reaction ID is CAUST-R for all stages; when calculation type is equilibrium stages, Liquid Holdup is used, and in this file, the Liquid Holdup = 0.0003 cum

7. Packing Type: GENERIC, 0.5-IN, CERAMIC, BERL

8. Packing height: 6.6 m and Section diameter: 0.1m

9. Mass transfer coefficient method: Onda et al (1968)

10. Interfacial area method: Onda et al (1968)

11. Interfacial area factor: 2

12. Heat transfer coefficient method: Chilton and Colburn

13. Holdup correlation: Stichlmair et al. (1989)

14. Film resistance options: Discrxn for liquid film, and Film for vapor film

15. Additional discretization points for liquid film: 5

16. Flow model: Mixed

Streams - Feeds to the absorber are gas stream GASIN containing N2, O2, H2O, and CO2 and liquid solvent stream LEANIN containing aqueous NaOH solution. Feed conditions are summarized in Table 5.

Table 5. Feed specifications Stream ID GASIN LEANIN

Substream: MIXED

Temperature: C 20 21

Pressure: kPa 103.15 103.15

Total flow 0.138 mol/sec 0.106 cum/hr

Mole-Frac Mole-Conc

H2O 0.03

CO2 0.155

NAOH 0 1.9 kmol/cum

N2 0.65

O2 0.165

6 Simulation Results 15

6 Simulation Results

The simulation was performed using Aspen Plus V7.0. The measured versus calculated absorber liquid temperature and concentration profiles are presented in Figures 8-10.

15

20

25

30

35

40

45

0 2 4 6 8

Distance from top, m

Tem

pera

ture

, C

Experimental

Rate-Based NaOH Model

Figure 8. The Absorber Temperature Profile

16 6 Simulation Results

0

0.04

0.08

0.12

0.16

0.2

0 2 4 6 8


CO

2 co

ncen

trat

ion,

mol

e fr

actio

n Experimental


Figure 9. The Absorber CO2 Concentration Profiles in Vapor Phase

0

0.4

0.8

1.2

1.6

2

0 2 4 6 8


OH

- con

ncen

trat

ion,

mol

/l

Experimental


Figure 10. The Absorber OH- Concentration Profiles in Liquid Phase

7 Conclusions 17

7 Conclusions

The Rate-Based NaOH 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 NaOH. You may use it as a starting point for more sophisticated models for process development, debottlenecking, plant and equipment design, among others.

18 References

References

[1] Thiele R., Faber R., Repke J.-U., etc. “Design of Industrial Reactive Absorption Processes in Sour Gas Treatment Using Rigorous Modelling and Accurate Experimentation”, Chemical Engineering Research and Design, Vol. 85, 7007)

[2] 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)

[3] Aspen Technology, (1985)

[4] Taylor, C.E., “Thermodynamics of Sodium Carbonate in Solution”, J. Phys. Chem., Vol. 59, 653-657(1955)

[5] Khvorostin, Yu.S., Filippov, V.K., Reshetova, L.I., “Isopiestic determination of the activity coefficients of sodium carbonate solutions at 25 C”, Russ. J. Phys. Chem., Vol. 49, Issue 5, 743-744(1975)

[6] Goldberg,R.N., “Evaluated Activity and Osmotic Coefficients for Aqueous Solutions: Thirty-Six Uni-Bivalent Electrolytes”, J. Phys. Chem. Ref. Data, 10, 3, 671-764(1981)

[7] Hanna Knuutila, Mikko Anttila, Eli Børresen, Olav Juliussen and Hallvard F. Svendsen, “CO2 capture with sodium carbonate”, paper presented at the 8th International Conference on Greenhouse Gas Control Technologies, Trondheim, Norway, 19-22 June 2006

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

[9] 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)

[10] Klochko,M.A., Godneva,M.M., “Electrical conductivity and viscosity of aqueous solutions of NaOH and KOH”, Russ. J. Inorg. Chem., Vol. 4, Issue 9, 964-968(1959)

[11] Vargaftik N.B., “Dictionary of Thermophysical Properties of Gases and Liquids”, Moskva, (1972)Puchkov L.V., Matveeva R.P., Dankova,I.S., “Heat capacity of aqueous solutions of lithium, sodium, and potassium sulfate and of sodium and potassium carbonate at temperatures up to 350 C”, NIITEKHIM N1716(1978)

References 19

[12] Riedel,L., "The Thermal Conductivity of Aqueous Solutions of Strong Electrolytes", Chem. Ing. Tech., 23, 3, 59-64(1951)

[13] Vargaftik,N.B.;Os'minin,Yu.P., "Thermal Conductivity of Aqueous Solutions of Salts, Acids and Alkalis", Teploenergetika, 3, 7, 11-15(1956)

[14] Cederberg,N.V.(Ed.), "Thermal Conductivity of Gases and Liquids. X. Thermal Conductivity of Aqueous Electrolyte Solutions", Edward Arnold; London Pub., 223-233(1965)

[15] Chernen'kaya,E.I.;Vernigora,G.A., "Experimental Determination and Calculation of Thermal Conductivity of Liquors of the Ammonia-Soda Process", J. Appl. Chem. USSR, 46, 6, 1306-1309(1973)

[16] Chernen'kaya,E.I., Vernigora,G.A., “Experimental Determination of Thermal Conductivities of Aqueous Solutions of Salts and Ammonia at 25 and 50 C”, J. Appl. Chem. USSR, Vol. 45, Issue 8, 1779-1782(1972)

[17] Puchkov,L.V., Matveeva,R.P., Dankova,I.S., “Heat capacity of aqueous solutions of lithium, sodium, and potassium sulfate and of sodium and potassium carbonate at temperatures up to 350 C”, Deposited Doc. 1978 NIITEKHIM N1716/78 dep. 14 pp. Avail.CHEMIE BERLIN, 14(1978)

[18] Chernen'kaya,E.I., “Experimental Determination of the Specific Heats of Aqueous Solutions of NH4HCO3, NaHCO3, Na2CO3, NH3, and of Liquors of the Soda Industry at 25 C”, J. Appl. Chem. USSR, Vol. 44, Issue 7, 1562-1566(1971)

[19]Herrington,N.M., Pethybridge,A.D., Roffey,M.G., “Densities of Aqueous Lithium, Sodium, and Potassium Hydroxides From 25 to 75 C at 1 Atm”, J. Chem. Eng. Data, Vol. 31, Issue 1, 31-34(1986)

[20] Gel'perin,N.I.;Gurovich,B.M.;Dubinchik,K.Kh., “The Relation between the Surface Tension of Aqueous Solutions of Inorganic Substances and Concentration and Temperature”, J. Appl. Chem. USSR, Vol. 42, Issue 1, 190-192(1969)

[21] Conti,G., Gianni,P., Papini,A., Matteoli,E., ” Apparent Molar Heat Capacity and Relative Enthalpy of Aqueous NaOH between 323 and 523 K”, J. Solution Chem., Vol. 17, Issue 5, 487-491(1988)

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