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CO 2 Chemical Solvent Screening

Author:L. Ji S.J. MikscheL.M. Rimpf G.A. Farthing

Babcock & Wilcox Power Genera on Group, Inc.Barberton, Ohio, U.S.A.

Presented to:

8th Annual Conference onCarbon Capture and Sequestra on – DOE/NETL

Date:May 4-7, 2009

Loca on:Pi sburgh, Pennsylvania, U.S.A.

Technical PaperBR-1823

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Babcock & Wilcox Power Generation Group 1

CO 2 Chemical Solvent Screening

L. Ji, S.J. Miksche, L.M. Rimpf and G.A. FarthingBabcock & Wilcox Power Generation Group, Inc.

Barberton, Ohio, U.S.A.

Presented at:8th Annual Conference on Carbon Capture and Sequestration – DOE/NETL

Pittsburgh, Pennsylvania, U.S.A.May 4-7, 2009

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AbstractCharacterization of the recently installed wetted-wall

column (WWC) apparatus in the CO 2 Control Laboratory atBabcock & Wilcox Power Generation Group, Inc. (B&W)was conducted at B&W’s Research Center in Barberton,Ohio. A correlation for the key parameter for deriving ratedata from the WWC, the gas- lm mass transfer coef cient,k g , was obtained with high accuracy. The average absoluterelative deviation between the experimental and modelresults for k g is only 4.8%. The criteria for determining theoperational limit of CO 2 partial pressure was establishedthrough theoretical analysis. This will allow for precise con-trol of experimental conditions to ensure the accuracy of ratedata obtained from the WWC. Reaction kinetics expressedin terms of liquid lm mass transfer coef cient, k g', andsolubility of CO 2 expressed in terms of equilibrium partial

pressure, P CO2 *, obtained in the WWC under differentexperimental conditions were comparatively benchmarkedagainst literature data with good agreement. The operating

protocol of the WWC was established.Screening of new solvents with the characterized WWC

was also performed. The preliminary results indicate promis-ing thermodynamic and kinetic properties for a new solventfor post-combustion CO 2 capture. Experimental work isongoing to gather fundamental data which are critical for later process performance simulation work.

1. IntroductionCarbon dioxide (CO 2) chemical absorption with amine-

based solvents is currently the state-of-the-art technologyfor post-combustion carbon capture from coal- red power

plants. Alkanoamines such as monoethanolamine (MEA),diethanolamine (DEA), and N-methyldiethanolamine(MDEA) are often the choice in other industries. The criteriafor selection of a certain amine are primarily based on theabsorption capacity, reaction kinetics, and the potential for regeneration. The wetted-wall column (WWC) apparatusis a differential reactor capable of determining kinetic andthermodynamic information of a speci c solvent, which arekey parameters in screening the potential solvent for CO 2

capture. Several research groups have already proven theeffectiveness of collecting kinetic and CO 2 solubility datawith a WWC [1-4].

B&W recently completed the installation of a WWCapparatus in its CO 2 Control Laboratory. The WWC wascharacterized with aqueous piperazine (PZ) solution todetermine the gas- lm mass transfer coef cient correlation,which is speci c to the apparatus. This correlation, alongwith the overall mass transfer coef cient obtained fromthe WWC, allows liquid-phase kinetic information to bedetermined from the overall rate of CO 2 removal. Further

benchmarking with MEA solution as the solvent was also performed to allow comparison with literature data andestablish con dence in the experimental methods used. The

purpose of these characterizations is to establish an experi-mental protocol which will allow for direct comparison of multiple solvent formulations. The WWC is used to collectreaction rate and CO 2 solubility data on new solvents withthe established experimental protocol. Rate and equilibrium

partial pressure of CO 2 data were measured at 40ºC and 60ºCunder various CO 2 loading conditions. Quantitative analysiswas conducted based on the collected rate and solubility datato justify the potential of the screened new solvents.

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2 Babcock & Wilcox Power Generation Group

2. Experimental Section

2.1. Chemical reagentsMEA (also known as ethanolamine) is liquid at standard

conditions and was supplied in 2.5 liter amber glass bottlesat 99% purity. PZ (also referred to as diethylenediamine)

anhydrous 99% is a crystalline solid and supplied in a 500gram opaque plastic bottle. Solvent A is a sterically hinderedamine commercially available from a chemical supplier.

Carbon dioxide gas blends in balance nitrogen (0.1%,1%, 10%, 30%) were supplied in 1A size (213 ft 3) gas cyl-inders to allow for variation of CO 2 concentrations duringexperimentation. High-purity (99.999%) house nitrogen gasis supplied from a lique ed storage tank and then vaporizedfor distribution.

High-purity water is available on site by treating citywater through an intense multi-step system: softening,activated charcoal lter, reverse osmosis, mixed-bed ionexchange columns, Liqui-Cels to remove dissolved oxygen

to 20 ppb, and ultraviolet light to eliminate any bacterialgrowth. The water is de-ionized to distilled water qualityand has conductivity of 18.4 meg-ohms.

2.2. Amine solution analysis by titrationThe Chittick carbon dioxide apparatus provides an ana-

lytical measurement of the amine solution concentration andamount of captured (loaded) CO 2 that has been absorbed bythe amine solution using a method of titration. A schematicof the device is shown in Figure 1. A liquid amine solution

sample of known volume is placed in the reaction ask. Theask is connected to a graduated gas measuring tube and

adjustable leveling bulb reservoir which contains a pink-colored non-reactive displacement solution. Acid titrant isintroduced to the reaction ask using a graduated titration bu -rette; methyl orange is used as an endpoint indicator. As CO 2 vapor evolves from the reaction, the uid in the reservoir isdisplaced, allowing for an evolved gas measurement.

Amine solution concentration can be determined withthe following relationship from the titration:

(1)

Where: C 1 = amine solution concentration(M=mole/Liter)

V 1 = amine solution sample volume (mL) C 2 = Acid concentration (M=mole/Liter) V 2 = Acid volume, from titration (mL) The amount of CO 2 absorbed by the amine solution can

be obtained in conjunction with the concentration deriva-tion. The captured CO 2 evolved through the titration can beanalyzed with the resulting equation, where CO 2 loading,α (alpha), is de ned as moles of CO 2 per mole of aminegroup:

(2)

Where: α = solution CO 2 loading(mole CO 2/mole amine group)

A = conversion constant (22.41 Liter/mole)

B = conversion constant (1000 mL/L) C 1= Amine solution concentration

(M = mole/Liter) P = barometric pressure (mmHg) T = room temperature (K) V 1 = amine solution sample volume (mL) V CO2 = volume of CO 2 collected (mL) at

STP conditions V gas = volume of displaced solution in the gas

measuring tube V acid = volume of acid titrant

2.3. WWC apparatusA WWC similar to that described by Cullinane [5] was

built and used in this study to measure the kinetics as wellas equilibrium partial pressure of CO 2 for the absorption

process by the solvent. A number of other research groupshave been using comparable WWC apparatuses to measuresimilar parameters [2-4]. The column in this study is madeof stainless steel and has a diameter of 1.27 cm (0.5 in.).The hydraulic diameter and height of gas-liquid reactionzone (annulus) are 0.43 cm and 9.13 cm, respectively. Theoverall ow diagram of the WWC apparatus is shown inFig. 1 Chittick carbon dioxide apparatus schematic.

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Figure 2. The column is kept in a circulating silicon oil bath,the temperature of which is maintained by a temperaturecontroller with an accuracy of ±0.1ºC.

Nitrogen and carbon dioxide are introduced to the systemthrough two separate mass ow controllers. The desired con -centration of CO 2 is achieved by varying ow rate of thesetwo gas species, while keeping the total gas ow rate thesame. The gas mixture is heated through the silicon oil bath,saturated with water, and then sent to the column. CO 2 in thegas mixture ows counter-currently past the thin solvent lmformed on the outside of the stainless steel column. Water vapor in the outlet gas is removed in a condenser and the CO 2 concentration is measured by a CO 2 analyzer. Changing theinlet CO 2 concentration allows tests under both absorptionor desorption conditions. Aqueous alkanolamine solutionsare circulated through the column at a constant ow rate of 3x10 -6 m3/s (180 mL/min) using a digital pump. The WWCwas operated under near atmosphere pressure for all the ex-

periments in this study. Gas ow rate was kept at 4 standardliters per minute unless otherwise speci ed.

3. TheoryFor CO 2 absorption with amine, the mass transfer pro-

cess consists of three series resistances: gas lm resistanceR g, reaction lm resistance R rxn , and liquid lm resistanceR Leq .

R T = R g + R rxn + R Leq , [6] or :

(3)

Where K G is the overall mass transfer coef cient, k g isthe gas lm mass transfer coef cient, H CO2 is the Henry’sconstant for CO 2 in amine solution, E is the enhancementfactor, k l 0 is the liquid phase mass transfer coef cient of CO 2, and k l,prod is the liquid phase mass transfer coef cientof reaction product. The k l 0 used in this study was calculatedfrom the correlation developed by Pacheco [7].

Fig. 2 Schematic of the WWC apparatus.

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When the purpose of an experiment is to derive reactionkinetics, it is critical to ensure the experiment is carried out inthe pseudo rst order (PFO) regime. In this regime, the freeamine concentration is much higher than the CO 2 concentra-tion, so the free amine concentration remains mostly constantat the gas-liquid interface and throughout the reaction lm.The reaction starts at the gas-liquid interface and completeswithin the reaction lm, as shown in Schematic A of Figure

3. This eliminates the mass transfer resistance caused by thereactant diffusion through the liquid lm, which would bemore pronounced if the free amine concentration were lower (shown in Schematic B of Figure 3).

Since the concentration or partial pressure of CO 2 isconsiderably low compared to that of free amine, the amountof reaction product produced in the reaction lm providesnegligible mass transfer resistance. So the third term inEquation 3, which accounts for the diffusion resistance of reactants and products, can be neglected. Mass transfer resis-tance due to kinetics, shown in the second term of Equation3, can be calculated directly from experimental data, sincethe rst term is addressed through a correlation speci c tothe WWC device and the third term can be neglected under the PFO regime. This kinetic resistance is also consideredrepresentative of the total liquid lm resistance.

When the PFO assumptions apply, the enhancement fac-tor, E , is considered to be equal to the Hatta number, Ha ,which by de nition is [8]:

(4)

Where M is a dimensionless number, DCO2 is the diffu-sion coef cient of CO 2 in the amine solution, [Am] is the

free amine concentration, and k 2 is the second order reactionrate constant. By substituting Equation 4 into Equation 3 andconsidering the PFO assumption:

(5)

To simplify Equation 5, the liquid lm resistance is oftenshown as 1/kg' [1]:

(6)

To determine whether the experimental conditions arewithin the PFO regime, the relative magnitude of the Hattanumber is compared to the in nite enhancement factor E i,which is de ned by penetration theory as [8]:

(7)

Where Dam is the diffusion coef cient of amine in solu -tion, γam is the reaction stoichiometric number of amine,mCO2 is the distribution coef cient of CO 2, and P CO2 b is the

bulk phase CO 2 partial pressure. The general rule used in thisstudy is that E i /Ha ≥ 10 to ensure the PFO condition.

4. Results and Discussion

4.1. Gas lm mass transfer coef cient cor -relation measurement

The overall mass transfer coef cient K G can be obtainedfrom CO 2 absorption and desorption experiments in theWWC. To determine the liquid lm mass transfer coef -cient, k g ' , the gas lm mass transfer coef cient, k g , must beaccounted for; these terms are shown in Equation 6.

The gas lm mass transfer coef cient, k g , was deter-mined by the analytical correlation proposed by Hobler for short columns [9].

(8)

The Sherwood number, Reynolds number, and Schmidtnumber are de ned as:

(9)

(10)

(11)

Where R is the gas constant, T is the temperature, d is thehydraulic diameter of the annulus (0.43 cm), h is the heightof the WWC (9.13 cm), DCO2 is the diffusion coef cient of CO 2 in the gas phase, u is the gas velocity, ρ is the gas den-sity, and μ is the gas viscosity. These dimensionless numberscan be calculated under speci c experiment conditions and

Fig. 3 Interface behavior for the liquid-phase reaction(Two-Film Theory).

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the constants A and B can then be regressed according toEquation 8. Once A and B are obtained, by solving Equa-tions 8-11, we can have the correlation for predicting k g , asshown in Equation 12.

(12)

Pacheco [7] used CO 2 absorption into 2 M MEA to de-termine the correlation of k g , while Bishnoi [10] used 0.1 Msodium hydroxide (NaOH) to absorb SO 2. In this study, a1.7 M PZ aqueous solution was used to absorb CO 2 due toits fast reaction rate with CO 2. The experimental conditionsincluded three gas ow rates (2, 3, and 4 standard liters per minute), two solvent temperatures (40ºC and 60ºC), and CO 2

partial pressures ranging from 25-600 Pa. A total of 22 data points were regressed and constants A and B were deter-mined to be 1.05 and 0.724, respectively. All 22 data pointsused in the regression were collected under conditions wherethe mass transfer was at least 50% gas lm controlled; thisensured accuracy of the regressed parameters. The averageabsolute relative deviation between the experimental andmodel results for k g is approximately 4.8%.

4.2. MEA baseline measurementsThe WWC was further characterized by comparing

the measured k g ' and equilibrium partial pressure of CO 2 ( P CO2 *) to literature data under similar experimental condi-tions. MEA solution was chosen due to its readily availablekinetic and thermodynamic data from the literature. TwoMEA concentrations [7 molal (m) and 3.5 m], two solventtemperatures (40ºC and 60ºC), and one CO 2 loading (0.4

mol/mol alk ), were selected as the experimental conditions.CO2 absorption and desorption experiments in the WWCare performed following the protocol described by Dugas[1]. Six inlet CO 2 partial pressures are used for each solventtesting condition: three of these are absorption points andthe other three are desorption points. The CO 2 ux is relatedto the log mean CO 2 partial pressure driving force, and theoverall mass transfer coef cient, K G, is equal to the slope of the tted line. The CO 2 equilibrium partial pressure is ob-tained by iterating to the zero ux partial pressure. A typical

ux versus driving force plot is shown in Figure 4.The measured CO 2 equilibrium partial pressure and rate

data for MEA are listed in Table 1.

Gabrielsen [11] developed a model for estimating CO 2 equilibrium partial pressure in aqueous alkanolamines withthe parameters in the model obtained by regressing differentsources of literature CO 2 solubility data. P CO2 * calculatedunder the exact experiment conditions with Gabrielsen’smodel are also listed in Table 1 for comparison. The ex-

perimental results shown here match very well with the data predicted by the model. The difference could be due to themodel parameters’ relative accuracy, and the possible error inestimating the CO 2 loading with the titration method. MEAconcentration seemed to have a limited impact on CO 2 solu-

bility when other conditions are identical; this is consistentwith the discovery from another research group [1].

The liquid side mass transfer coef cient, k g ' , obtainedfrom this study was also compared to those available fromliterature. Dugas [1] reported k g ' values of 1.4x10 -6 mol/s.m2Pa and 1.7x10 -6 mol/s .m2Pa at 40ºC and 60ºC, respec-tively, for 7 m MEA with CO 2 loading of 0.351. The cor-responding values of k g ' from this study under the similar conditions are 9.01x10 -7 mol/s .m2Pa and 1.16x10 -6 mol/s.m2Pa. Since the CO 2 loading in this study is 0.397, which isexpected to lower the k g ' value due to the increased diffusionresistance, the rate data generated from the WWC seems to

be consistent with the literature values. The values of k g ' determined from the WWC under selected experimentalconditions are very similar. It seems that temperature andamine concentration do not have a signi cant effect on k g ' ;this is consistent with the conclusion of the other researchgroup [1]. The gas lm resistances (shown in Table 1) for allexperiments are between 10 to 15% of the total resistance,which indicates that the liquid lm resistance dominates the

absorption process.

Fig. 4 Representative plot of flux-driving force dependence(3.5 m MEA, 0.387 mol/mol, 60 o C).

Table1CO 2 Equilibrium Partial Pressure and Rate Data for 3.5 and 7 m MEA Solutions at 40 oC and 60 oC

MEA T CO 2 Loading P CO2 * PCO2 *[11] K G kg kg ' Gas film control

m o C mol/mol alk

Pa Pa mol/s m 2 Pa mol/s m 2 Pa mol/s m 2 Pa %

3.540 0.387 165 143 1.00E-06 7.14E-06 1.16E-06 14.0

60 0.387 1000 1091 1.00E-06 7.23E-06 1.16E-06 13.8

7 40 0.397 180 155 8.00E-07 7.14E-06 9.01E-07 11.2

60 0.397 1800 1184 1.00E-06 7.23E-06 1.16E-06 13.8

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In summary, the MEA baseline experimental resultsillustrate that the B&W WWC can generate high qualitydata, and that those data are very consistent with literaturevalues. Screening of new solvents by evaluating the reactionrate and CO 2 solubility data from this WWC will deliver very accurate and reliable results.

4.3 Solvent screeningWWC experiments were performed on Solvent A, a steri-

cally hindered amine, to establish its potential as a viableCO2 solvent. Rate and CO 2 equilibrium partial pressure datawere obtained under different experimental conditions andthe results are summarized in Table 2.

Literature CO 2 equilibrium partial pressure data for MEA and 2-amino-2-methyl-1-propanol (AMP) under ex-

perimental conditions of the current work were estimatedusing the model developed by Gabrielsen [11, 12], and arelisted in Table 2 for comparison. The equilibrium partial

pressure of CO 2, P CO2 *, for Solvent A is higher than thatfor MEA at the loading of approximately 0.2; both solventshave similar P CO2 * at the loading of about 0.4, as can beseen from Table 2. A plot of P

CO2* versus CO

2loading is

shown in Figure 5 to compare the CO 2 solubility of SolventA to that of AMP solution.

The P CO2 * points for Solvent A are substantially lower than those for AMP, under all experimental conditions.Vapor-liquid equilibrium (VLE) data collection for SolventA is still in progress and conclusions can only be made oncethe collection of VLE data for Solvent A is complete.

Fig. 6 CO 2 absorption rate data for 30 wt% A and MEA solu-tions at 40 o C and 60 o C.

The comparison of k g ' for MEA and Solvent A can beseen from Figure 6. The k g ' for Solvent A is smaller thanthat for MEA under the CO 2 loading of approximately 0.2,for both 40ºC and 60ºC conditions. However, for the loadingof about 0.4, Solvent A and MEA share almost identical k g ' ,even though the intrinsic second order reaction rate constant,k 2, of Solvent A is much smaller than that of MEA.

5. ConclusionsA newly installed WWC apparatus at the B&W Re-

search Center was characterized and a good correlation for the gas- lm mass transfer coef cient was obtained withhigh accuracy. The operating protocol of this WWC wasestablished through theoretical analysis. Reaction rate andVLE data were collected for MEA solution under variousexperimental conditions and the results were benchmarkedagainst literature values with good agreement.

Preliminary rate as well as VLE data for Solvent A, asterically hindered amine, were collected from the character-ized WWC. Initial comparison of Solvent A with other sol-vents, MEA and AMP, was performed. The results indicate

that Solvent A looks promising due to its reasonable reactionrate and CO 2 solubility, and its potential for yielding a lower regeneration energy. Complete understanding of the physico-chemical properties, CO 2 working capacity, and regeneration

potential will be needed before a nal decision can be madeon the potential of Solvent A as an ef cient CO 2 solvent.Experimental work toward this target continues.

Table 2CO 2 Equilibrium Partial Pressure and Rate Data for 30 wt% A, MEA, and AMP Solutions at 40 oC and 60 oC

Solvent TCO 2 Loading

[1]P CO2 * [11] k g ' [1] Solvent CO 2 Loading P CO2 * kg ' Solvent P CO2 * [12]

30 wt% o C mol/mol alk

Pa mol/s m 2 Pa 30 wt% mol/mol alk

Pa mol/s m 2 Pa 30 wt% Pa

MEA

40 0.252 5.5 3.34E-6

A

0.201 56 2.78E-6

AMP

317

60 0.252 42.2 2.92E-6 0.201 230 1.16E-6 1510

40 0.351 155 1.40E-6 0.397 196 1.16E-61719

60 0.351 1184 1.70E-6 0.397 1090 1.03E-6 8138

Fig. 5 CO 2 equilibrium partial pressure data for 30 wt% A and AMP solutions at 40 o C and 60 o C.

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Copyright © 2009 by Babcock & Wilcox Power Genera on Group, Inc.a Babcock & Wilcox company

All rights reserved.

No part of this work may be published, translated or reproduced in any form or by any means, or incorporatedinto any informa on retrieval system, without the wri en permission of the copyright holder. Permission re -quests should be addressed to: Marke ng Communica ons, Babcock & Wilcox Power Genera on Group, P.O.Box 351, Barberton, Ohio, U.S.A. 44203-0351. Or, contact us from our Web site at www.babcock.com.

Disclaimer Although the informa on presented in this work is believed to be reliable, this work is published with theunderstanding that Babcock & Wilcox Power Genera on Group and the authors are supplying general infor -ma on and are not a emp ng to render or provide engineering or professional services. Neither Babcock &Wilcox Power Genera on Group nor any of its employees make any warranty, guarantee, or representa on,whether expressed or implied, with respect to the accuracy, completeness or usefulness of any informa on,product, process or apparatus discussed in this work; and neither Babcock & Wilcox Power Genera on Groupnor any of its employees shall be liable for any losses or damages with respect to or resul ng from the use of,or the inability to use, any informa on, product, process or apparatus discussed in this work.

6. AcknowledgementsThe authors would like to express their deep appreciation

to Ruyu Zhang and Egilda Purusha Bonnin-Nartker, fellowmembers of the CO 2 Focus Team at the B&W ResearchCenter, for their valuable advice and support. We would alsolike to thank Dr. Shengteng Hu for his help in programmingthe data acquisition system for the WWC apparatus. We

would also like to express our special thanks to Professor Gary Rochelle and his research group at The Universityof Texas for their help in designing and characterizing theWWC device, and for their very thoughtful advice in inter-

preting the results.

7. References1. Dugas R., G.T. Rochelle., Absorption and desorption

rates of carbon dioxide with monoethanolamine and pipera-zine. Energy Procedia, 2009. 1(1): p. 1163-1169.

2. Bougie, F., M.C. Iliuta, Kinetics of absorption of carbon dioxide into aqueous solutions of 2-amino-2-

hydroxymethyl-1,3-propanediol. Chemical EngineeringScience, 2009. 64(1): p. 153-162.

3. Paul, S., A.K. Ghoshal, and B. Mandal, Absorption of Carbon Dioxide into Aqueous Solutions of 2-Piperidineetha-nol: Kinetics Analysis. Industrial & Engineering ChemistryResearch, 2009. 48(3): p. 1414-1419.

4. Samanta, A. and S.S. Bandyopadhyay, Absorption of carbon dioxide into aqueous solutions of piperazine activated2-amino-2-methyl-1-propanol. Chemical Engineering Sci-ence, 2009. 64(6): p. 1185-1194.

5. Cullinane, J.T. and G.T. Rochelle, Kinetics of CarbonDioxide Absorption into Aqueous Potassium Carbonate andPiperazine. Industrial & Engineering Chemistry Research,2006. 45(8): p. 2531-2545.

6. Oyenekan B.A., G.T. Rochelle., Rate modeling of CO 2 stripping from potassium carbonate promoted by piperazine.International Journal of Greenhouse Gas Control, 2009. 3:

p. 121-132.7. Pacheco, M.A., Mass Transfer, Kinetics, and Rate-

based Modeling of Reactive Absorption, in Deparment of Chemical Engineering. 1998, University of Texas at Austin:Austin, Texas. p. 318.

8. Derks, P.W.J., et al., Kinetics of absorption of carbondioxide in aqueous piperazine solutions. Chemical Engineer-ing Science, 2006. 61(20): p. Pages 6837-6854.

9. Hobler, T., Mass Transfer and Absorbers. 1966, Ox-ford: Permagon Press.

10. Bishnoi, S., Carbon dioxide absorption and solutionequilibrium in piperazine activated methyldiethanolamine,in Department of Chemical Engineering. 2000, Universityof Texas at Austin: Austin, Texas. p. 219.

11. Gabrielsen, J., et al., A Model for Estimating CO 2 Solubility in Aqueous Alkanolamines. Ind. Eng. Chem. Res.,2005. 44(9): p. 3348-3354.

12. Gabrielsen J., M.L. Michelsen, E.H. Stenby, G.M.Kontogeorgis, Modeling of CO 2 Absorber Using an AMPSolution. AIChE Journal, 2006. 52: p. 3443-3451.

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