Chemical Engineering and Processing 43 (2004) 849–856

Modeling of CO2 capture by three typical amine solutionsin hollow fiber membrane contactors

R. Wang∗, D.F. Li, D.T. Liang

Institute of Environmental Science and Engineering, Nanyang Technological University, Innovation Center (NTU), Block 2, Unit 237, 18 Nanyang Drive,Singapore 637723, Singapore

Received 7 April 2003; received in revised form 26 May 2003; accepted 27 May 2003

Abstract

A theoretical simulation was performed to study CO2 capture by absorption in a hollow fiber membrane contactor. Three typical alkanolaminesolutions of 2-amino-2-methyl-1-propanol (AMP), diethanolamine (DEA) and methyldiethanoamine (MDEA) were employed as absorbentsin the analysis. The effects of different sorption systems, operating conditions and membrane characteristics on the removal behavior of CO2

were investigated. Simulation results indicate that AMP and DEA solutions have much higher CO2 absorption fluxes than MDEA solution, butthe concentrations of both AMP and DEA drop dramatically due to depletion. It implies that the separation efficiency and the consumption ofabsorbents should be taken into consideration simultaneously in terms of absorbent selection in practice applications. The liquid flow velocity,initial liquid concentration and the fiber length as well as fiber radius have significant impacts on the CO2 absorption by AMP and DEAbecause of their instantaneous reactions with CO2. The reaction kinetics of MDEA with CO2 has been found to be the controlling factor inthe process of CO2 capture in the membrane contactor. Theoretical solution also confirms that the non-wetted mode of operation is favored,by taking the advantage of higher gas diffusivity in order to optimize CO2 capture performance.© 2003 Elsevier B.V. All rights reserved.

Keywords: Carbon dioxide capture; Amine solutions; Absorption; Membrane contactors; Numerical simulation

1. Introduction

Today, fossil fuels supply about 80% of the world’s to-tal primary energy sources and produce over 60% of theworld’s electricity. However, the combustion of fossil fuelsinevitably results in the emissions of air pollutants and hugerelease of CO2—a greenhouse gas that has been associatedwith global climate change[1,2]. From the global environ-mental perspective, it is important to capture CO2 to avertthe threat of global warming, thereby attaining the carbonemission reduction targets set out by the Kyoto Agreement.Additionally, the CO2 concentrations are typically 3–5% ingas-fired power plants and 13–15% in coal plants. The needof concentrated CO2 by commercial factors such as the CO2merchant market and enhanced oil recovery makes CO2 cap-ture more attractive[3].

The most well-established method for CO2 capture is toremove CO2 by absorption into amine solutions in conven-

∗ Corresponding author. Tel.:+65-6794-3764; fax:+65-6792-1291.E-mail address: rwang@ntu.edu.sg (R. Wang).

tional equipment[4]. Amines are weak basic compoundsthat react with CO2 to form weak chemical bonds. Thesechemical bonds are easily broken upon mild heating, leadingto regeneration. Although chemical absorption technologyhas important commercial significance, judicious selectionof a competitive absorbent that is capable of high CO2loading, rapid absorption rate and low cost for regenerationremains a challenge. In terms of equipment, packed towers,bubble columns and spray towers are commonly applied inthe absorption processes. The substantial capital equipmentrequired by the use of such systems is an important aspectof CO2 capture. The associated initial fixed investment is amajor barrier for widespread utilization of this technology.Besides, phase dispersion and limited mass transfer areasare the drawbacks of these conventional equipment.

Microporous hollow fiber membrane contactors are ex-pected to overcome the disadvantages of the conventionalequipment when incorporated into the acid gas treatingprocesses[5]. The characteristic of microporous membranecontactors is that the gas stream flows on one side andthe absorbent liquid flows on the other side of the mem-

0255-2701/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0255-2701(03)00105-3

850 R. Wang et al. / Chemical Engineering and Processing 43 (2004) 849–856

brane without phase dispersion, thus avoiding the problemsoften encountered in the conventional equipment such asflooding, foaming and entrainment. The compact modularstructure of membrane contactors also provides much largergas–liquid interfaces with known area at the pore mouth ofthe membrane, as well as the flexibility to scale up or down.

Because of the advantages offered by membrane con-tactors, considerable academic and industrial research hasbeen carried out. Qi and Clussler[6] first explored thepossibility to use microporous polypropylene membranesfor CO2 absorption in a NaOH solution. Karoor and Sirkar[7] conducted comprehensive experiments and simulatedthe gas–liquid absorption in a microporous hydropho-bic hollow fiber device using CO2, SO2, CO2–N2 andSO2–air mixtures as feed gases and water as an absorbent.Kreulen et al.[8] studied the influence of a chemical re-action on the mass transfer by means of simulating andtesting the absorption of CO2 in a KOH solution. Rang-wala [9] presented correlations for predicting liquid- andgas-phase mass transfer coefficients in the systems of CO2absorption into water, aqueous NaOH and aqueous di-ethanolamine (DEA) solutions in membrane modules. Kimand Yang[10] used aqueous monoethanolamine (MEA),2-amino-2-methyl-1-propanol (AMP) and DEA solutionsas absorbents to separate CO2–N2 mixture in PTFE hol-low fiber membrane contactors. Lee et al.[11] numericallyanalyzed CO2 removal by an aqueous KOH solution. Re-cently, Kumar et al.[12] theoretically and experimentallystudied a new absorbent named as CORAL for CO2 capturefrom dilute gas streams. Feron and Jansen[13] also appliedCORAL absorbents in polyolefin membrane contactorsfor CO2 separation. In industry, the Kvaerner membranecontactors were used for the removal of acid gases fromnatural gas and flue gases from conventional gas turbines[14].

However, current work is still in the proof of conceptstage. It is observed that most of the works have em-ployed water, NaOH or KOH solutions as absorbents in themembrane-based CO2 capture study possibly to avoid com-plicated chemical reactions involved in the absorption tosimplify the analysis. In fact, the utilization of amine solu-tions instead of water, NaOH or KOH solutions in membranecontactors is more realistic for fully understanding this inte-grated system, as they are absorbents often used in the acidgas treatment. The CO2 Capture Project launched by Inter-national Energy Agency (IEA) presently has also consideredthe absorption of CO2 by amine solutions in membrane

Fig. 1. Chemical structures of three amines.

contactors is a promising approach as one of CO2 capturetechnologies (seehttp://www.CO2captureproject.orgformore information).

The objective of this paper is to numerically simulate CO2capture by the absorption of three typical amine solutionsin hollow fiber membrane contactors to provide a better in-sight on the characteristics of CO2 absorption through mem-branes. The effects of different sorption systems, operatingconditions and membrane features on the removal behaviorof CO2 will be investigated as a basis for further experimen-tal study.

2. Theory

2.1. Reaction mechanisms of amines with CO2

Three amines named AMP, DEA and methyldiethano-amine (MDEA) were chosen in this study. Their chemicalstructures are shown inFig. 1 and their physical propertiesare tabulated inTable 1. Normally, amines are classified asprimary, secondary and tertiary types. AMP, a primary ster-ically hindered amine in which the amino group is attachedto a tertiary carbon atom, was identified as a promisingabsorbent to offer capacity and rate advantages over con-ventional amines[15,16]. While for the secondary amines,DEA is a popular commercially used absorbent with twoethanol groups attached to the nitrogen atom[17]. Sec-ondary amines are less corrosive and require less heat toregenerate, as the additional ethanol group draws most ofthe free electron character away from the nitrogen atomcompared with primary ones. However, they are not ef-fective for extended CO2 removal. The tertiary amine ofMDEA has two ethanol groups attached to the nitrogen atomalong with a methyl group. Aqueous MDEA solutions arewidely used for H2S removal[18]. They are also normallyemployed as an alternative to the primary and secondaryamines in bulk CO2 removal due to its lowest regenerationheat and lowest corrosivity among three types of amines[17,19,20].

The reaction of primary or secondary amines (R1R2NH)with dissolved CO2 is generally described by the zwitterionmechanism in a two-step sequence, i.e. the fist step is theformation of an intermediate zwitterion:

CO2 + R1R2NHk2�k−1

R1R2NH+CO−2 (1)

R. Wang et al. / Chemical Engineering and Processing 43 (2004) 849–856 851

Table 1Physical properties of three amines at 298 K

Aqueous amine solution m DA (109 m2 s−1) DB (109 m2 s−1) Amine concentration (kmol m−3) Literature

AMP 0.79 1.33 9.7a 1.56 [15]a, [21]DEA 0.79 1.25b 5.95 1.2 [23]b, estimatedMDEA 0.78 1.25 9.74 1.2 [23]

a CO2.b Amines.

Then, the zwitterion is deprotonated by the bases presentin the solution, forming a carbamate ion and a protonatedbase:

R1R2NH+CO−2 + b

kb�R1R2NCO−2 + bH+ (2)

where b denotes H2O, OH− and R1R2NH in an aqueousamine solution, respectively. Based on the assumption ofquasi-steady-state condition for the zwitterions concentra-tion, the rate of CO2 reaction with primary or secondaryamines is expressed by[15,16,21]

RCO2 = k2[CO2][R1R2NH]

1 + (k−1/kH2O[H2O]) + (k−1/kOH− [OH−]) + (k−1/kR1R2NH[R1R2NH])(3)

where the contribution of OH− was assumed to be negligiblenormally, as its concentration was low compared with thoseof H2O and R1R2NH [16]. The kinetic constants for AMPand DEA amine solutions are listed inTable 2.

However, a tertiary amine cannot undergoreactions (1)and (2)since no hydrogen binding with nitrogen atom isavailable. The reaction of a tertiary amine with CO2 proceedsby the formation of a protonated amine and a bicarbonateanion:

R1R2R3N + H2O + CO2k2−→R1R2R3NH+ + HCO−

3 (4)

The rate of CO2 in MDEA solutions is second order, firstorder with respect to MDEA concentration and first orderwith respect to the CO2 concentration[22]:

RCO2 = k2[MDEA][CO2] (5)

k2 = 8.741× 1012 exp

(−8625

T

)(6)

2.2. Formulation of mass transfer and absorption

In terms of mass transfer and chemical reactions occurredin a hollow fiber membrane contactor, the analysis givenbelow is restricted to the case of pure CO2 absorption in

Table 2Kinetic parameters of three amines at 298 K

Aqueous amine solution k2 (m3 kmol−1 s−1) k2kR1R2NH/k−1 (m6 kmol−2 s−1) k2kH2O/k−1 (m6 kmol−2 s−1) Reference

AMP 810 2335 2.64 [16]DEA 2375 437 2.2 [16]MDEA 2.47 [22]

non-wetted hollow fibers, i.e. the liquid flows under lami-nar conditions through the fiber lumen and the pores of themembrane are filled with pure gas flowing in the sell side, asshown schematically inFig. 2. In the mathematical model,following assumptions have been adopted:

1) A laminar parabolic velocity profile is applied in thelumen of fibers.

2) The contactor is under steady-state isothermal operation.3) The axial diffusion is negligible.

4) Ideal gas behavior is imposed.5) The Henry’s law is applicable.

Subject to these assumptions, governing equations for ab-sorption in a hollow fiber membrane contactor can be de-rived as follows.

The differential mass balances to describe the concentra-tion profiles of components in the liquid may be written as

vr∂CA

∂z= DA

(∂2CA

∂r2+ 1

r

∂CA

∂r

)− RA (7)

vr∂CB

∂z= DB

(∂2CB

∂r2+ 1

r

∂CB

∂r

)− RB (8)

where the subscripts A and B denote CO2 and amines, re-spectively. The radial velocity profile is given as

vr = 2vav

[1 −

( r

R

)2]

(9)

The initial and boundary conditions are

z = 0, CA = 0, CB = CB0 (10)

r = 0,∂CA

∂r= 0,

∂CB

∂r= 0 (11)

852 R. Wang et al. / Chemical Engineering and Processing 43 (2004) 849–856

Fig. 2. Non-wetted mode of a hollow fiber membrane contactor.

r = R, DA

(∂CA

∂r

)= kex(CAg − CAg,i),

∂CB

∂r= 0 (12)

where a symmetry in the radial direction of hollow fibers andnon-volatile amines are assumed. The CO2 flux in the liquidis equal to that in the gas phase based on mass conservationat the gas–liquid interface. The Henry’s law is applied toconnect CO2 interfacial concentrations in the liquid and gasphase:

CA,i = mCAg,i (13)

The external mass transfer coefficientkex in Eq. (12)is acombination of mass transfer coefficients in the gas and inthe membrane:

1

kex= 1

kg+ 1

km(14)

2.3. Numerical scheme

The set of partial differentialequations (7) and (8)couldbe solved numerically along with the initial and boundaryconditions as well as the reaction rates of CO2 with aminesolutions (Eq. (3) or Eq. (5)). All the equations were rear-ranged in a dimensionless form. Then they were written ina form suitable for their solution by the orthogonal collo-cation method, thus reducing the set of partial differentialequations to a set of ordinary differential equations. The re-sulting ordinary differential equations were integrated in thedomain of fiber length using the Gear method.

There were in total 2M numbers of coupled equations forintegration, whereM = 16 is the collocation point along the

radial direction of a fiber. From this scheme, the concentra-tion profiles of CO2 and amines in the liquid phase wereobtained. Subsequently, the local CO2 absorption flux couldbe calculated. The integration of the local CO2 flux overthe fiber length led to the average CO2 flux of absorption indifferent amine solutions.

3. Results and discussion

Prior to the analysis of CO2 absorption in three aminesolutions was carried out theoretically, a comparison withliterature was conducted in order to verify the correctnessof the model developed under the current study for CO2absorption in a membrane contactor. As shown inFig. 3,the present simulations are in an excellent accordance withthe literature work. The parameters used in the calculationare the same as those in Ref.[8]. It demonstrates that themodeling and numerical scheme for solution developed hereare reliable.

Fig. 4 shows the average CO2 absorption flux over thefiber length as a function of the external mass transfer coef-ficient. Whenkex is small, the mass transfer has a significantinfluence on the flux. For non-wetted mode of pure CO2absorption, the pores of fibers are filled with CO2. Thereis no transport resistance in the gas phase. Thus, the exter-nal mass transfer resistance is determined by the membranemass transfer resistance, as indicated inEq. (14). The masstransfer inside the pores of membrane is governed by gasdiffusion and membrane structure such as fiber pore radius,fiber porosity and tortuosity as well as fiber wall thickness.Thekm was estimated to be between 0.012 and 0.077 m s−1

[24]. Within this range, the CO2 absorption flux tends toreach its highest level. That is why the non-wetted mode ofoperation is usually preferred in order to take the advantageof the higher gas diffusivity[5].

Fig. 3. Comparison study of simulated local flux along the fiber lengthat two reaction rate constants (DA = 1.0× 10−9 m2 s−1, DB = 2.0× 10−9

m2 s−1, m = 0.5, n = 2, L = 0.4 m, R = 1.0× 10−3 m, vl = 0.1 m s−1,CAg = 41.6 mol m−3, CB0 = 1200 mol m−3).

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Fig. 4. Average CO2 absorption flux over the fiber length versus externalmass transfer coefficient (L = 0.2 m, R = 2.0× 10−4 m, vl = 0.1 m s−1,CAg = 40.9 mol m−3, CB0 = 1200 mol m−3).

In addition, it can be seen that AMP solution hasthe best CO2 absorption capacity followed by DEA andMDEA solutions in sequence. The CO2 absorption fluxesfor AMP and DEA are about 3.5 and three times higherthan that of MDEA, respectively, whenkex is larger than0.001 m s−1. This is not surprising when consideringtheir reaction kinetics with CO2. For the same initialamine concentration ofCB0 = 1200 mol m−3, the reactionrates of CO2 with three amines could be written roughlyas RCO2=0.1083[AMP][CO2], 0.0856[DEA][CO2] and0.00247[MDEA][CO2] for AMP, DEA and MDEA, respec-tively, based onEqs. (3) and (5)as well asTable 2. MDEAis a much weaker amine compared with other primaryand the secondary amines. In addition, the solubility anddiffusivity of CO2 in three amines have no significant dif-ferences. Therefore, the different CO2 absorption fluxes bythree amine solutions in the contactor are mainly attributedto their different reaction kinetics.

Fig. 5 plots CO2 concentration in the liquid phase at theliquid–gas interface versus the external mass transfer coef-ficient for three amines. Whenkex is small, the interfacialCO2 concentration in MDEA solution is obviously higherthan the other two cases, because the reaction rate of CO2with MDEA is much slower than that of AMP and DEA.Whenkex is high enough to provide sufficient CO2 for ab-sorption, the interfacial CO2 concentration in each aminesolution tends to reach equilibrium with the bulk CO2 con-centration in the gas phase. In such circumstances, the ab-sorption is principally controlled by the chemical reactionand diffusion in the liquid phase.

Fig. 6 illustrates the effect of liquid flow velocity on theabsorption rate of CO2 for three amines. The CO2 absorp-tion flux in MDEA solution is virtually unaffected by theliquid flow velocity within the calculated range. However,for both AMP and DEA solutions, the CO2 absorptionflux increases with an increase in the liquid flow velocitybecause of instantaneous absorptions. The instantaneous

Fig. 5. CO2 concentration at the interface versus external mass transfercoefficient (L = 0.2 m,R = 2.0× 10−4 m, vl = 0.1 m s−1, CAg = 40.9 molm−3, CB0 = 1200 mol m−3).

absorptions of CO2 in AMP and DEA solutions are con-firmed by the results revealed inFig. 7. Fig. 7 depicts theradial concentration profiles of three amines and CO2 inthe liquid phase at the exit of the fiber. It can be seen thatthere are significant depletions of AMP and DEA at theliquid–gas interface, while MDEA concentration changesa little for the same initial amine concentrations.Fig. 6suggests that a larger liquid flow velocity may be suitablefor the operation of the membrane contactor in order tofully utilize its separation capacity when AMP or DEA isemployed.

It is also found fromFig. 7 that CO2 concentrations inAMP and DEA solutions are lower than that in MDEAsolution. This is because the reaction rate constant for CO2with MDEA is much smaller than with other two amines asmentioned previously. Additionally, each mole of CO2 has

Fig. 6. Average CO2 absorption flux over the fiber length as a functionof liquid velocity (L = 0.2 m, R = 2.0× 10−4 m, CAg = 40.9 mol m−3,CB0 = 1200 mol m−3, kex = 100 m s−1).

854 R. Wang et al. / Chemical Engineering and Processing 43 (2004) 849–856

Fig. 7. Radial concentrations of amines and CO2 in the liquid solutions atthe liquid exit of the fiber (A: CO2, B: amines) (L = 0.2 m,R = 2.0× 10−4

m, vl = 0.1 m s−1, CAg = 40.9 mol m−3, CB0 = 1200 mol m−3, kex = 100m s−1).

to consume 2 mol of AMP and DEA, respectively, duringthe chemical reactions, while MDEA reacts with CO2 onan equal mole base. Therefore, the consumption of MDEAis quite little and MDEA concentration can still maintain ahigh value. Except the absorption by MDEA, there is a rel-atively larger amount of CO2 accumulated at the interface,which can diffuse into the inner place of the hollow fiberstill r/R = 0.5 position.

The dependence of CO2 absorption flux on initial amineconcentration is plotted inFig. 8. Initial MDEA concentra-tion seems to have a little influence on the CO2 absorptionflux. The CO2 absorption is essentially limited by the re-action kinetics. In contrast, the increase of initial AMP orDEA concentration enhances the CO2 absorption signif-icantly. However, this result does not mean that a higher

Fig. 8. Average CO2 absorption fluxes over the fiber length as a functionof initial amine concentration (L = 0.2 m, R = 2.0× 10−4 m, vl = 0.1 ms−1, CAg = 40.9 mol m−3, kex = 100 m s−1).

Fig. 9. Average CO2 absorption flux over the fiber length as a functionof fiber length (R = 2.0× 10−4 m, vl = 0.1 m s−1, CAg = 40.9 mol m−3,CB0 = 1200 mol m−3, kex = 100 m s−1).

initial AMP or DEA concentration would be a better choicein real operations. AMP and DEA solutions with a highconcentration may cause a serious damage of the membranesurface and corrosion in the equipment used. A compromisehas to be taken among these factors.

Fig. 9 shows the influence of the fiber length on the CO2absorption flux. The variation of the fiber length has no im-pact on the CO2 absorption in MDEA solution under sim-ulated conditions due to its smaller reaction rate constant.But the CO2 absorption fluxes for AMP and DEA decreasewhen the fiber length is increased. For the membrane con-tactor with longer fibers, the residence time of CO2 is longerso that it consumes more AMP or DEA to react with. Sincethe reaction rate is a function of the amine concentrationand it keeps dropped along the fiber length as shown inFig. 9, the average CO2 absorption flux would decreaseinevitably.

Fig. 10 shows typical three-dimensional concentrationprofiles of AMP and CO2 in the membrane contactor. Start-ing from the inlet, AMP concentration decreases from thecenterline to the liquid–gas interface (r/R = 1) along thewhole fiber length. For the same radial position, AMP con-centration also drops from the inlet to the outlet due to moreand more serious depletion. Nevertheless, the variation ofCO2 concentration in AMP solution follows an inverse trend.When moving in the axial direction, CO2 may diffuse intoradial spaces away from the interface.

The effect of fiber inner radius on CO2 absorption in threeamines is shown inFig. 11. The liquid flow velocity was keptthe same in the simulation. Increase in the fiber inner radiuscauses a reduction of CO2 absorption flux in AMP or DEAsolution, but it has no influence for the case of MDEA–CO2system. Clearly, the hollow fibers with a smaller inner diam-eter have better performances for instantaneous absorptionprocesses.

R. Wang et al. / Chemical Engineering and Processing 43 (2004) 849–856 855

Fig. 10. Concentration profiles of AMP and CO2 in the aqueous AMP solution (L = 0.2 m, R = 2.0× 10−4 m, vl = 0.1 m s−1, CAg = 40.9 mol m−3,CB0 = 1200 mol m−3, kex = 100 m s−1).

Fig. 11. Average CO2 absorption flux over the fiber length as a functionof fiber inner radius (L = 0.2 m, vl = 0.1 m s−1, CAg = 40.9 mol m−3,CB0 = 1200 mol m−3, kex = 100 m s−1).

4. Conclusions

A theoretical simulation was performed to study CO2capture by absorption in a hollow fiber membrane contac-tor. Three typical alkanolamine solutions of AMP, DEA andMDEA were employed as absorbents in the analysis. Theeffects of different sorption systems, operating conditionsand membrane characteristics on the removal behavior ofCO2 were investigated.

Simulation results indicate that AMP and DEA solutionshave much higher CO2 absorption fluxes than MDEA solu-tion, but the concentrations of both AMP and DEA drop dra-

matically due to depletion. The liquid flow velocity, initialliquid concentration and the fiber length as well as fiber ra-dius have significant impacts on the CO2 absorption by AMPand DEA because of their instantaneous reactions with CO2.Reaction kinetics of MDEA with CO2 has been found to bethe controlling factor in the process of CO2 capture in themembrane contactor. Theoretical solution also confirms thatthe non-wetted mode of operation is favored, by taking theadvantage of higher gas diffusivity in order to optimize CO2capture performance. In practice, the separation efficiencyand the consumption of absorbents should be taken into con-sideration simultaneously in terms of absorbent selection.

Acknowledgements

The authors gratefully acknowledge the support ofAgency of Science, Technology and Research of Singapore(A*STAR) for funding the IESE programme. Special thanksare due to Dr. Rong Yan for her kind help.

Appendix A. Nomenclature

C liquid concentration (mol m−3)CAg CO2 bulk concentration in gas phase (mol m−3)CB0 initial amine concentration (mol m−3)D diffusivity (m2 s−1)JA CO2 absorption flux (mol m−2 s−1)Jloc local CO2 absorption flux (mol m−2 s−1)k−1 reverse first-order reaction rate constant (s−1)k2 second-order reaction rate constant (m3 mol−1 s−1)

856 R. Wang et al. / Chemical Engineering and Processing 43 (2004) 849–856

kb second-order reaction rate constant for base b(m3 mol−1 s−1)

kex external mass transfer coefficient (m s−1)kg gas-phase mass transfer coefficient (m s−1)km membrane mass transfer coefficient (m s−1)L length of a hollow fiber (m)m distribution coefficient (m = [CO2]l /[CO2]g at

equilibrium)M radial number of collocation pointsn stoichiometric coefficientr radial coordinate (m)R fiber inner radius (m)RCO2 rate of CO2 reaction (mol m−2 s−1)vav average liquid flow velocity (m s−1)vr liquid flow velocity (m s−1)Z axial coordinate (m)

SubscriptsA CO2B aminei at liquid–gas interface

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