Optimization of Blended Amines for CO2 Absorption in a Hollow-Fiber Membrane ContactorZhen Wang,†,§ Mengxiang Fang,*,† Shuiping Yan,‡ Hai Yu,§ Chiao-Chien Wei,§ and Zhongyang Luo†

†State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, China‡College of Engineering, Huazhong Agricultural University, Wuhan, 430070, China§CSIRO Energy Centre, Mayfield West, 2304, Australia

ABSTRACT: To understand and optimize CO2 absorption in binary amine systems, we experimentally and theoreticallyinvestigated CO2 absorption using typical amines and blended amines in a polypropylene hollow-fiber membrane contactor. Theamines studied were monoethanolamine (MEA), diethanolamine (DEA), and N-methyldiethanolamine (MDEA), and theiraqueous blends of MEA/MDEA, DEA/2-amino-2-methyl-1-propanol (AMP), and MDEA/piperazine (PZ). The predictedresults, including overall mass transfer coefficients and CO2 removal ratio, agreed very well with those determined experimentally.For single amines, the optimal concentration was around 30 wt % for MEA and 20 wt % for DEA. MDEA concentration had littleeffect on the overall mass transfer coefficient. We optimized the formulation of blended amines using theoretical analysis. Theoptimal compositions in MEA/MDEA, DEA/AMP, and MDEA/PZ systems were respectively 30 wt % MEA, with MDEA inproportions from 0.1 to 0.3; 15 wt % DEA, with AMP in proportions from 0.5 to 0.8; and 20 wt % MDEA, with PZ in aproportion of 0.3. To further understand the CO2 membrane absorption process, we also analyzed individual mass transferresistances as a function of additive concentration in blended amines and the effects of liquid velocity on the overall mass transfercoefficient. This shows that CO2 absorption is controlled by the liquid side for DEA/AMP blends and by combined liquid−gasphases for MEA/MDEA blends. For MDEA/PZ blends, control of CO2 absorption is characterized by a gradual transition fromliquid side controlled to liquid−gas combined controlled as the concentration of PZ increases.

1. INTRODUCTION

Carbon dioxide (CO2) is the major contributor to globalwarming, with 60% of CO2 emissions being produced by thepower and industrial sectors.1 Consequently, controlling CO2emissions from coal-fired power stations is a serious andpressing issue. Among various CO2 capture technologies,amine-based postcombustion capture (PCC) technologies,which typically use packed columns to achieve separation, arebelieved to be the most mature technologies for removing CO2from flue gas. This is due to their successful commercialapplications in many industries, their effectiveness in gasstreams of low CO2 partial pressure, and their tail-endtechnology, which can be retrofitted to existing power plants.2

However, the liquid and gas streams cannot be controlledindependently in amine-based PCC processes. This oftenresults in operating problems such as foaming, flooding, andentrainment, thus limiting amine-based PCC applications.3

To overcome these drawbacks, an alternative technologyCO2 membrane absorptionhas been proposed. Figure 1shows a schematic diagram of CO2 membrane absorption in ahydrophobic membrane. In this process, a suitable membraneconfiguration, such as a hollow-fiber membrane contactor, isused to control gas and liquid fluids independently. CO2molecules diffuse through the membrane pores and areabsorbed by the liquid absorbent. Membrane absorptiontherefore presents more operational flexibility, and avoids theoperating problems encountered in conventional contactingdevices. A membrane contactor also potentially offers a muchlarger gas−liquid interfacial area than traditional reactors, with areduction of 63−65% in absorber size;4 it can also be scaled up

linearly. Therefore, membrane contactors are a strongcandidate for CO2 absorption compared with a conventionalpacked tower, and have great potential for PCC.

Received: May 27, 2013Revised: July 19, 2013Accepted: July 30, 2013Published: July 30, 2013

Figure 1. Schematic diagram of CO2 membrane absorption inhydrophobic membrane.

Article

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© 2013 American Chemical Society 12170 dx.doi.org/10.1021/ie401676t | Ind. Eng. Chem. Res. 2013, 52, 12170−12182

Unlike a traditional gas separation membrane, CO2selectivity during membrane absorption is provided by solvents,rather than by the membrane itself. The hydrophobicmembrane, which prevents flooding problems in the membranecontactor, acts as a barrier between the liquid and gas phases toincrease the gas−liquid contact area without dispersing onephase into another. Screening CO2 absorbents for membraneabsorption is therefore of vital importance to improve CO2absorption performance and reduce liquid-side mass transferresistance. Amine solvents such as monoethanolamine (MEA),diethanolamine (DEA), and N-methyldiethanolamine(MDEA), which are preferred in conventional packed towers,are also extensively reported as absorbents in membraneabsorption processes.5−7 Because these absorbents requireregeneration after CO2 absorption, regeneration energyconsumption is an important consideration. Until now, nosingle amine has been reported to achieve satisfactoryperformance in both absorption and desorption.8 Generallyspeaking, primary (e.g., MEA) or secondary (e.g., DEA) aminespresent fast reaction kinetics with CO2, but are difficult toregenerate. MDEA, which is a tertiary amine, has a very lowCO2 reaction rate, but is easily regenerated. Hence, there hasbeen rising interest in blending different amines to improveperformance.Most studies of blended amine absorbents have focused on

conventional gas−liquid reactors,9−15 with limited research intothe use of amine blends in a hollow-fiber membrane contactor.Gong et al.16 found that CO2 absorption flux increases withMEA content in a blended MEA/MDEA solution. Lu et al.17

activated MDEA with piperazine (PZ) in a polypropylene (PP)hollow-fiber membrane and found that the average value of theoverall mass transfer coefficient in MDEA/PZ is 2.25 timeshigher than in MEA. Paul et al.18 simulated the performances ofdifferent single and blended alkanolamine solvents in a hollow-fiber membrane contactor, at a total amine concentration of 10wt % in all cases. However, the combined experimental andtheoretical analyses of CO2 absorption in different blends ofamine solvents in the hollow-fiber membrane contactor werevery limited. CO2 absorption can be improved by increasing theamine concentration, but it is impossible to improve the CO2absorption performance to a very high level with increasingbase or additive amine concentrations. This is because theviscosity changes with amine concentration and further reducesthe mass transfer between the gas and liquid phases. Moreover,a higher concentration of absorbent can accelerate corrosion offacilities and amine volatile loss, and increase the operation andmaintenance costs of PCC. Therefore, optimizing theformulation of blended amines is of practical significance forefficient, effective CO2 membrane absorption. To the best ofour knowledge, the optimization of binary amine systems forCO2 removal in hollow-fiber membrane contactors has not yetbeen reported.In the present study, we investigated the performances of

CO2 absorption from CO2/N2 gas mixtures in a hollow-fibermembrane contactor using three different single amines (MEA,DEA, and MDEA) and three blended amines (MEA/MDEA,DEA/2-amino-2-methyl-1-propanol (AMP), and MDEA/PZ).Polytetrafluoroethylene is recommended as the best materialfor membrane absorption due to its high hydrophobicity;19

however, in favor of commercial applications, we used acommercially available, low-cost PP membrane. The effects ofamine concentration and liquid velocity were investigated andcompared with our theoretical analysis. We also analyzed the

overall mass transfer coefficients and individual mass transferresistances for different blended amines in the CO2 membraneabsorption process. We used theoretical analysis to optimizethe formulation of blended amine systems, to obtain the highestpossible overall mass transfer coefficient between gas and liquidphases while minimizing the use of amines.

2. THEORY2.1. CO2−Amine Reaction Mechanism. The reaction of

CO2 with primary or secondary amines can be described by atwo-step zwitterionic mechanism, which was first proposed byCaplow20 and reintroduced by Danckwerts.21 The first step ofthe reaction is to form a zwitterion as an intermediate, whichcan be expressed as

+ + −

−

X YooooooCO R R NH R R NH COOk

k

2 1 2 1 21

2,amine

(1)

The zwitterion then undergoes deprotonation by a base B, asshown by

+ → ++ − − +R R NH COO B R R NCOO BHk

1 2 1 2b

(2)

where B could be an amine, OH−, or H2O. The contribution ofOH− can usually be ignored, due to its very low concentrationin solution.With the assumption of quasi-steady-state condition for the

zwitterion concentration, the change of zwitterion concen-tration is negligible. Then the CO2 reaction rate at quasi-steadystate can be expressed by

=+

−

+−

rk [CO ][R R NH]

1 kk k

CO R R NH2,R R NH 2 1 2

[H O] [R R NH]2 1 2

1 2

1

H2O 2 R1R2NH 1 2 (3)

For the reaction of CO2 with tertiary alkanolamines, such asMDEA, the reaction with CO2 is a base-catalyzed hydration ofCO2,

22 and can be described by

+ + ← →⎯⎯⎯ ++ −CO R R R NH H O R R R NH HCOk

2 1 2 3 2 1 2 3 32,amine

(4)

Therefore, the reaction of CO2 with tertiary alkanolaminescan be assumed to be of pseudofirst order with respect to bothCO2 and amine. The reaction rate of CO2 can be expressed as

=−r k [CO ][R R R N]CO R R R N 2,R R R N 2 1 2 32 1 2 3 1 2 3 (5)

For the reaction of CO2 with PZ, the reaction can be treatedas a second-order reaction, as shown by23

=−r k [CO ][PZ]CO PZ 2,PZ 22 (6)

For absorption of CO2 into blended amine aqueoussolutions, the overall CO2 reaction rate can be expressed asfollows:

= + + +− − − −−r r r r robs CO amine1 CO amine2 CO OH CO H O2 2 2 2 2

(7)

In eq 7, the reaction of CO2 with H2O and OH− can beignored, due to their small contributions.24 Therefore, theoverall CO2 reaction rate in blended amines can be simplified as

= +− −r r robs CO amine1 CO amine22 2 (8)

In this work, for the reaction of CO2 with blended aminesystems, the overall CO2 reaction rate can be expressed as thefollowing equations, respectively.

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for blended MEA/MDEA:

=+

+∑

−r

k C Ck C C

1 kk C

obs2,MEA CO MEA

2,MDEA CO MDEA2

1

b b

2

(9)

for blended DEA/AMP:

=+

++∑ ∑

− −r

k C C k C C

1 1kk C

kk C

obs2,DEA CO DEA 2,AMP CO AMP2

1

b b

2

1

b b (10)

for blended MDEA/PZ:

= +r k C C k C Cobs 2,MDEA CO MDEA 2,PZ CO PZ2 2 (11)

Since the change of amine concentration is very small in theliquid phase, the overall reaction rate of CO2 with aqueousamines can be expressed as a pseudo-first-order reaction withrespect to CO2 as follows:

=r k [CO ]obs obs 2 (12)

Table 1 presents the specific rate expressions for differentamine systems. The values of kinetic constants for all thereactions of CO2 with amine systems at the temperature of 313K are shown in Table 2.2.2. Overall Mass Transfer Coefficient. For a hydro-

phobic hollow-fiber membrane with gas-filled pores and liquidabsorbent in the lumen side, the overall gas phase mass transfercoefficient can be expressed by a resistance in series model:31

= + +K mEk k

dd k

dd

1 1 1 1

O L m

i

m g

i

o (13)

where KO is the overall mass transfer coefficient; kL, km, and kgare the individual mass transfer coefficients in the liquid,membrane, and gas phases, respectively; E is the enhancementfactor; m is the distribution coefficient between liquid and gas;di, dm, and do are the inner, log mean, and outer diameters ofthe hollow-fiber membrane; and dm = (do − di)/ln(do/di).In eq 13, the enhancement factor E is a function of pseudo-

first-order reaction rate constant kobs; the detailed calculation ofE is presented in section A.7.2.2.1. Tube-Side Mass Transfer Coefficient. Generally, fluid

flow in the tube side of a hollow-fiber membrane is consideredto be a fully developed laminar flow. The Graetz−Levequesolution can be used to predict the mass transfer coefficient inthe tube side. Kerulen et al.32 proposed a generalized solutionof the Graetz−Leveque equation that is applicable for thecomplete range of Graetz numbers in the laminar flow:

= = +Shk d

DGz(3.67 1.62 )L i

CO ,L

3 3 1/3

2 (14)

where Sh is the Sherwood number, DCO2,L is the diffusioncoefficient of CO2 in solvent, and Gz is the Graetz number,expressed by

=Gzu d

D LL i

2

CO ,L2 (15)

where uL is the liquid mean velocity and L is the effective lengthof the fiber membrane.2.2.2. Membrane-Side Mass Transfer Coefficient. On the

assumption that CO2 transport through the membrane is onlycaused by diffusion and the membrane is operated without a Table

1.Expressions

ofCO

2ReactionRates

forDifferentAmineSystem

s

amines

r CO

2AM1

r CO

2AM2

singleam

ines

MEA

(AM1),D

EA(AM1)

++

−−

k

kk

kk

[CO

][AM

1]

11/

((/

)[H

O]

(/

)[AM

1])

2,AM

12

HO

12

AM1

12

MDEA

(AM1)

k[C

O][

AM1]

2,AM

12

blendedam

ines

MEA

(AM1)/M

DEA

(AM2)

++

+−

−−

k

kk

kk

kk

[CO

][AM

1]

11/

((/

)[H

O]

(/

)[AM

1](

/)[

AM2]

)2,

AM1

2

HO

12

AM1

1AM

21

2

k[C

O][

AM2]

2,AM

22

DEA

(AM1)/A

MP(AM2)

++

+−

−−

k

kk

kk

kk

[CO

][AM

1]

11/

((/

)[H

O]

(/

)[AM

1](

/)[

AM2]

)2,

AM1

2

HO

12

AM1

1AM

21

2+

++

−−

−

k

kk

kk

kk

[CO

][AM

2]

11/

((/

)[H

O]

(/

)[AM

2](

/)[

AM1]

)2,

AM2

2

HO

12

AM2

1AM

11

2

MDEA

(AM1)/PZ(AM2)

k[C

O][

AM1]

2,AM

12

k[C

O][

AM2]

2,AM

22

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pore-wetting problem, the mass transfer coefficient in themembrane can be expressed as33

τδς

=k D1

m ige

(16)

where ς is the membrane porosity, Dige is the effective

membrane diffusion coefficient in pure gas-filled pores, δ isthe membrane thickness, and τ is tortuosity; τ = 1/ς2.The calculation of Dig

e was determined by the mean porediameter of the membrane pore. In this work, the porediameter is in the range 1 × 10−7−1 × 10−5 m. Therefore,diffusion through the membrane is the combination ofmolecular diffusion and Knudsen diffusion, and Dig

e can bedetermined by

= +D D D1 1 1

ige

igm

igk

(17)

in which Digm and Dig

k are the molecular and Knudsen diffusioncoefficients of CO2, respectively.2.2.3. Shell-Side Mass Transfer Coefficient. Many studies

have been reported concerning shell-side mass transfer,34−36

but shell-side fluid flow is not yet fully understood. The fluidhydrodynamics of the shell side varies significantly and isaffected by a range of parameters, such as the packing density,the irregularity of fiber spacing, the polydispersity of the fiberdiameter, and the inlet and outlet effects. In this work, takinginto consideration the operating conditions of shell-side flow,the following equation is used to predict the mass transfercoefficient at the given conditions of 0 < Re < 500, 0.04 < φ <0.4:37

φ= −Sh d L Re Sc5.85(1 )( / )h0.6 0.33

(18)

where φ is the packing density, dh is the hydraulic diameter, andRe and Sc are the Reynolds and Schmidt numbers, respectively.Table 3 lists some of the main parameters of the single

amines at a concentration of 10 wt % concentration that wereused to calculate theoretical mass transfer coefficients. Thedetailed calculations of these physicochemical parameters, suchas the Henry’s constant, enhancement factor, viscosity, anddiffusion coefficient, can be found in the Appendix.

3. EXPERIMENTAL SECTION3.1. Materials. MEA (purity, >99%), DEA (purity, >98%),

MDEA (purity, >98%), and PZ (purity, 99.5%) were purchasedfrom Sinopharm Co. AMP (purity, >97%) was purchased fromFluka. All amines were used as received without furtherpurification. The solutions were prepared to the desiredconcentrations by dissolving the amines in deionized water.High-purity (>99.9 mol %) CO2 and N2 gases were purchasedfrom Hangzhou Jingong Gas Co. Ltd.The hydrophobic microporous PP hollow-fiber membrane

modules were provided by Hangzhou Joint-future MembraneCo. The specifications of the membrane modules are listed inTable 4.

3.2. Experimental Apparatus and Procedure. Aschematic diagram of the experimental apparatus for CO2membrane absorption is shown in Figure 2. Aqueous amineabsorbent in the lean solvent reservoir is first heated to thedesired temperature and then pumped into the tube side of thehollow-fiber membrane contactor. The liquid flow rate isregulated by the flow controller, and the liquid-side pressure isadjusted at around 135 kPa by a back-pressure valve placed on

Table 2. Kinetic Parameters of Single and Blended Amine Systems (313 K)

aqueous aminesystems

k2,AM1(m3 mol−1 s−1)

k2,AM1kH2O/k−1(m6 mol−2 s−1)

k2,AM1kAM1/k−1(m6 mol−2 s−1)

k2,AM1kAM2/k−1(m6 mol−2 s−1)

k2,AM2(m3 mol−1 s−1)

k2,AM2kH2O/k−1(m6 mol−2 s−1)

k2,AM2kAM2/k−1(m6 mol−2 s−1)

k2,AM2kAM1/k−1(m6 mol−2 s−1) ref

MEA (AM1) +H2O

1.74 × 101 1.675 × 10−5 5.33 × 10−3 25

MEA (AM1)/MDEA (AM2) +H2O

1.74 × 101 1.675 × 10−5 5.33 × 10−3 7.80 × 10−4 1.29 × 10−2 25,26

DEA (AM1) + H2O 1.51 2.45 × 10−6 2.46 × 10−3 27

AMP (AM1) +H2O

1.25 5.82 × 10−3 3.16 28

DEA (AM1)/AMP(AM2) + H2O

1.60 2.51 × 10−6 2.62 × 10−3 7.17 × 10−3 1.20 3.29 × 10−5 1.00 × 10−3 2.80 × 10−2 27

MDEA (AM1) +H2O

1.29 × 10−2 26

MDEA (AM1)/PZ(AM2) + H2O

1.29 × 10−2 1.51 × 102 29,30

Table 3. Physicochemical Parameters of Single Amines Used for Prediction of Mass Transfer Coefficient at 313 K (Cin = 14 vol%, ω0 = 10 wt %)

param units MEA DEA MDEA

ρL kg m−3 1000.03 1007.81 1002.35DCO2,L m2 s−1 2.38 × 10−9 1.08 × 10−9 9.88 × 10−10

DCO2,G m2 s−1 1.87 × 10−5 1.87 × 10−5 1.87 × 10−5

Damine m2 s−1 1.05 × 10−9 3.71 × 10−10 3.08 × 10−10

Digm m2 s−1 8.72 × 10−6 8.72 × 10−6 8.72 × 10−6

Digk m2 s−1 1.29 × 10−6 1.29 × 10−6 1.29 × 10−6

HCO2,L kPa m3 kmol−1 4065.84 4471.47 4156.79

ηL Pa s 8.25 × 10−4 3.06 × 10−3 3.56 × 10−3

μG Pa s 1.79 × 10−5 1.79 × 10−5 1.79 × 10−5

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the liquid outlet for preventing gas bubble formation in theliquid. Two gas stream inputs, CO2 and N2, are mixed by massflow controllers at around ambient pressure (107 kPa) toproduce a simulated flue gas with 14 vol % CO2. The gasmixture flows into the shell side of the hollow-fiber membranecontactor after being heated to the desired temperature. Duringthe course of the experiment, the entire membrane contactorsystem is kept at a constant temperature of 40 °C. The CO2inlet and outlet concentrations in the gas phase were measuredusing a gas analyzer (Rosemount NGA-2000), and CO2loadings in solvents after absorption were measured using atitration method which was described in our previous work.38

3.3. Experimental Overall Mass Transfer Coefficient.The experimental overall mass transfer coefficient was used toassess the membrane’s CO2 absorption performance andcompared with the theoretical mass transfer coefficient. It canbe calculated by the following equation:

=−Δ

KV C V C

A COEXP g,in g,in g,out g,out

(19)

where Vg,in and Vg,out are the inlet and outlet gas flow rates,respectively; Cg,in and Cg,out represent the gas phase CO2concentrations in mol/m3 at the inlet and outlet, respectively;A denotes the contact area of membrane in m2. ΔC is the

logarithmic mean driving force based on gas-phase concen-tration, and can be described as

Δ =− * − − *

− * − *CC C C C

C C C C

( ) ( )

ln[( )/( )]g,in g,in g,out g,out

g,in g,in g,out g,out (20)

where Cg,in* and Cg,out* are the inlet and outlet gas phaseconcentrations in equilibrium with the corresponding liquidphase CO2 concentrations, respectively. In this study, Cg,in* andCg,out* can be assumed to be zero, due to very low CO2 loadingsin the solution.

4. RESULTS AND DISCUSSION4.1. CO2 Membrane Absorption in Single Amines.

Figure 3 describes the experimental and theoretical CO2 overall

mass transfer coefficients, KO, of single amines when the amineconcentrations were varied from 5 to 40 wt %. A good

Table 4. Specifications of Hollow Fiber and MembraneModule

item parameter units value

PP membrane fiber fiber i.d. mm 0.344fiber o.d. mm 0.424pore size μm 0.2 × 0.02porosity % 40

membrane module effective length mm 200module i.d. mm 20no. of fibers 500contact areaa m2 0.108

aContact area is calculated based on the inner diameter of the fiber.

Figure 2. Schematic diagram of the experimental setup for CO2 absorption membrane absorption.

Figure 3. Influences of amine concentration on overall mass transfercoefficient for single amines (VG = 120 L/h, VL = 8 L/h, T = 40 °C,CCO2,in = 14 vol %, PL = 135 kPa, PG = 107 kPa, ΔPL = 6 kPa).

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agreement between the experimental and theoretical KO valueswas achieved within the range of amine concentrationsinvestigated, which confirms the validity of the theoreticalmodel. KO values in MEA solvent are much higher than thosein DEA and MDEA. This can easily be explained by thedifference in reaction rate constants for the reaction of CO2with different amines. As listed in Table 2, MEA (as a primaryamine) possesses the highest second-order forward reactionconstant, k2, to form the zwitterions. This is 1 order ofmagnitude higher than that for DEA and 3 orders of magnitudehigher than that for MDEA, respectively.Figure 3 also shows that, as the amine concentration

increases, KO varies following different patterns in the threedifferent amines. The KO in MEA increases greatly in the low-MEA-content region and then slowly increases at high MEAconcentrations. Therefore, there is an optimal value for MEAconcentration, beyond which KO does not increase noticeablywith an increase in concentration. This transitional MEAconcentration is around 30 wt %, as shown in Figure 3.Aqueous DEA also presents a critical concentration of 20 wt %,beyond which KO reduces with higher DEA concentrations.Similar results can also be found in Jamal’s work.39 In the caseof MDEA, there is no obvious improvement of KO with risingMDEA concentration, due to its low reactivity and highviscosity.These phenomena were further analyzed by investigating the

effects of amine concentration on the Graetz number (Gz); theresults are presented in Figure 4. The dimensionless Gz

indicates the ratio of the penetration time of solute CO2 toreach the axis of the hollow fiber from the gas−liquid interface,to the average residence time of the liquid in the fiber. A higherGz number means that the penetration depth of the CO2diffusing from the gas−liquid interface is smaller than the fiberradius at a fixed gas−liquid contact time (i.e., constant liquidflow rate). Consequently, the free amine far from the interfaceis essentially undisturbed. Based on eq 15, Gz is inverselyproportional to the CO2 diffusion coefficient DCO2,L, which is astrong function of the viscosity of amine solutions. In Figure 4,aqueous MEA has the lowest Gz number, which increasesslowly as the concentration rises. This indicates that the CO2

reaction zone may extend up to the axis of the fiber, and that anincrease in MEA concentration has a positive influence on KO.For DEA, Gz increases almost 10-fold, from 25 to 286, as theDEA concentration increases from 10 to 90 wt %. Thisdemonstrates that CO2 has difficulty diffusing into the liquidphase, in particular, in the high DEA concentration region.MDEA solution has an intermediate Gz number, whichincreases slightly as the concentration rises. Varying theMDEA concentration therefore has a relatively insignificanteffect on KO.

4.2. Effects of Additive Concentration for BlendedAmines. Figure 5 presents the simulated and experimental KO

values for three different blended amine systems (MEA/MDEA, DEA/AMP, and MDEA/PZ) as a function of additive(MDEA, AMP, and PZ) content. In this study, the base amineconcentrations, ω0, remain constant at 10 wt % for all blendedamines. As shown in Figure 5, in the MEA/MDEA blendedamine, addition of MDEA weakly influences KO. A similar weakinfluence of additives was found in blended DEA/AMPsolution, in which an increase in AMP content only slightlyincreases KO. Although MDEA and AMP are not goodpromoters of CO2 absorption, they can increase the CO2capture capacity and are beneficial to CO2 desorption, due totheir low CO2 reaction heats. The addition of PZ stronglyincreases KO in blended MDEA/PZ solution, which indicatesthat PZ is an effective absorbent to activate MDEA in CO2membrane absorption.To further optimize blended amines, the effects of additive

amine concentration on CO2 outlet concentration in blendedamines were also investigated and are shown in Figure 6. ForMDEA/PZ blends, the CO2 removal ratio improves signifi-cantly with an increase of ωPZ when ωPZ/ωMDEA is smaller than0.4. At ωPZ/ωMDEA above 0.4, the room for an increase of theCO2 removal ratio is small, since the CO2 removal ratio is closeto 0.9. In the blended MEA/MDEA solution, an increase ofωMDEA/ωMEA has little influence on the improvement of CO2removal efficiency, which remains above 0.9 at all additiveconcentrations. In the DEA/AMP system, AMP promoted CO2absorption, but the maximum CO2 removal ratio theoreticallyachieved in this study is only 75% as ωAMP/ωDEA increases to 1.

Figure 4. Change of Gz number with amine concentration (VG = 120L/h, VL = 8 L/h, T = 40 °C, CCO2,in = 14 vol %, PL = 135 kPa, PG = 107kPa, ΔPL = 6 kPa).

Figure 5. Effects of additive amine concentration on overall masstransfer coefficient (VG = 120 L/h, VL = 8 L/h, T = 40 °C, CCO2,in = 14

vol %, ω0 = 10 wt %, PL = 135 kPa, PG = 107 kPa, ΔPL = 6 kPa).

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DEA/AMP may therefore not be a suitable blended amine forCO2 absorption due to its low KO and CO2 removal rate. Thesame conclusion was drawn by Paul et al.18 However, DEA/AMP is still competitive, considering its relatively lowregeneration energy demand.4.3. Effects of Liquid Flow Velocity. Figure 7 shows the

variation of KO with liquid velocity in single and blendedamines. The ratio of additive amine to base amine, ωadditive/ω0,was fixed at 0.2 with ω0 being 5 wt % for all blended aminesystems. KO rises with an increase in liquid velocity, which isexpected, due to decreasing liquid-phase resistance. In practice,howeverconsidering the high power consumption andprobability of membrane wetting as a result of high liquidpressureit may not be attractive to elevate the liquid velocityto a very high level (e.g., turbulent flow) to improve CO2absorption in a membrane contactor with narrow fibers.

In terms of KO, amine systems differ in their sensitivity toliquid velocity. Generally, KO is more sensitive to the liquid flowrate in amines with a higher CO2 absorption rate than inamines with lower CO2 absorption rate. For example, withincreasing liquid velocity, the extent of increase of KO in MEAand MEA/MDEA is larger than that in DEA, DEA/AMP, andMDEA/PZ, and there is almost no increase of KO in MDEAsolution. This is due to the difference in individual masstransfer coefficients in the liquid phase for amines. The liquidmass transfer coefficient is quite small in MDEA solution, andconsequently, the size of the overall mass transfer coefficient isrelatively limited by increasing liquid velocity.

4.4. Optimization of Blended Amine System. Toachieve a greater CO2 absorption rate with the minimum useof amines, the profiles of the overall mass transfer coefficient asa function of base amine concentration and additive proportion,ωadditive/ω0, were plotted for three different blended aminesystems. Figure 8 depicts the profile of KO with changing ωMEAfrom 5 to 50 wt % and ωMDEA/ωMEA from 0 to 1. In MEA/MDEA blends, a small ωMDEA/ωMEA ratio improves the KOmore effectively than a large ωMDEA/ωMEA ratio does. This isespecially the case at ωMEA greater than 35 wt % and ωMDEA/ωMEA greater than 0.15, when KO begins to decrease with risingMDEA content. This phenomenon can be attributed to thecombined influences of MDEA’s high viscosity and low CO2reaction activity. Although a higher ωMEA could increase theoverall mass transfer coefficient, this effect becomes weaker athigh ωMEA regions, due to a higher Gz number. A higher ωMEAcould also raise the solution’s viscosity. Therefore, an optimalωMEA is recommended to be approximately 30 wt %, withωMDEA/ωMEA being 0.1−0.3 for blended MEA/MDEA.Figure 9 shows the profiles of KO in DEA/AMP blends as a

function of DEA and AMP concentrations. Like MDEA inMEA/MDEA blends, adding more AMP will not alwaysimprove KO. When the value of ωDEA exceeds 35 wt %, KOdeclines with the increase of ωAMP/ωDEA, but when ωDEA isbelow 20 wt %, KO has an approximately linear dependence onωAMP/ωDEA. Hence, a smaller ωAMP/ωDEA is recommended in

Figure 6. Effects of additive amine concentration on CO2 outletconcentration ratio in blended amines (VG = 120 L/h, VL = 8 L/h, T =40 °C, CCO2,in = 14 vol %, ω0 = 10 wt %, PL = 135 kPa, PG = 107 kPa,ΔPL = 6 kPa).

Figure 7. Effects of liquid velocity for single and blended amines (VG = 120 L/h, T = 40 °C, CCO2,in = 14 vol %, ω0= 5 wt %, ωadditive/ω0= 0.2, PL =

135 kPa, PG = 107 kPa, ΔPL = 6−10 kPa).

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the high ωDEA region, whereas a larger ωAMP/ωDEA is better forlow ωDEA conditions. The optimal ωDEA is around 15 wt %,where the highest KO can be achieved. For DEA/AMP blends,the specific DEA concentration ωDEA is therefore recom-mended to be approximately 15 wt % with a ωAMP/ωDEA ratioof 0.5−0.8.The influence of ωMDEA and ωPZ/ωMDEA on KO for blended

MDEA/PZ is shown in Figure 10. As ωMDEA increases beyond10 wt %, the whole range of ωPZ/ωMDEA from 0 to 1 cannot befully covered, due to the limited solubility of PZ in water at 40°C. PZ clearly improves the CO2 absorption rate, although thisinfluence becomes relatively weaker as ωPZ/ωMDEA increases. Inaddition, as a result of MDEA’s high viscosity, the extent ofincrease of KO reduces as the value of ωMDEA becomes larger.Thus, for an aqueous MDEA/PZ system, the most effectiveconcentration of MDEA is nearly 20 wt %, and ωPZ/ωMDEA isoptimal around 0.3.

4.5. Mass Transfer Resistance Analysis and Compar-ison. The analysis of individual mass transfer resistances forMEA/MDEA, DEA/AMP, and MDEA/PZ amine systems withthe variation of ωadditive/ω0 is presented in Figure 11. The baseamine concentration ω0 was kept constant at 10 wt %. ForMEA/MDEA blends, the gas-side mass transfer resistance iscomparable to the liquid-side resistance, but the membrane-sideresistance is quite small. Therefore, the CO2 absorption processin MEA/MDEA blends is controlled by both gas-phase andliquid-phase mass transfer. In DEA/AMP blends, althoughincreasing the AMP concentration can weaken the contributionof the liquid-side resistance, the liquid-phase resistance is stilldominant; hence, CO2 membrane absorption is controlled byliquid-phase mass transfer. In MDEA/PZ blends, the liquidresistance declines sharply with an increase in PZ content, as aresult of PZ’s high CO2 reaction activity; consequently, theCO2 absorption process gradually changes from liquid-phase

Figure 8. Profiles of overall mass transfer coefficient in MEA/MDEA blended amine as a function of MEA and MDEA concentrations (VG = 120 L/h, VL = 8 L/h, T = 40 °C, CCO2,in = 14 vol %, PL = 135 kPa, PG = 107 kPa, ΔPL = 6 kPa).

Figure 9. Overall mass transfer coefficient profile of DEA/AMP blended amine as a function of AMP and DEA concentrations (VG = 120 L/h, VL =8 L/h, T = 40 °C, CCO2,in = 14 vol %, PL = 135 kPa, PG = 107 kPa, ΔPL = 6 kPa).

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controlled to combined liquid−gas controlled. In addition, themembrane-phase resistance in this study is relatively smallcompared to liquid-side resistance, and can usually be ignored ifa nonwetted membrane is assumed. However, if the absorbentswet the membrane, CO2 absorption can deteriorate, due todramatically increased membrane resistance.

5. CONCLUSION

The CO2 absorption performance of three single amines (MEA,DEA, and MDEA) and three typical blended amines (MEA/MDEA, DEA/AMP, and MDEA/PZ) was investigatedexperimentally and theoretically in a PP hollow-fiber membranecontactor. The predicted mass transfer coefficients and CO2

removal ratio results agree very well with those obtainedexperimentally.Amine concentration has diverse influences on the overall

mass transfer coefficient, KO, of single amines. To achieve the

highest KO and minimize amine use, the optimal concentrationfor DEA is 15 wt %, and the optimal concentration for MEA isaround 30 wt %. The concentration of MDEA has little impacton KO. For all amines tested, increasing the liquid velocity leadsto a slight improvement in KO due to the reduction of liquid-side mass transfer resistance.For blended amines, solvent formulation optimization was

studied intensively by theoretical analysis. The recommendedoptimal blended amine compositions are the following: forMEA/MDEA, 30 wt % MEA with a MDEA proportion of 0.1−0.3; for DEA/AMP, 15 wt % DEA with an AMP proportion of0.5−0.8; and for MDEA/PZ, 20 wt % MDEA with a PZproportion of 0.3. The analysis of individual mass transferresistances shows that CO2 absorption is controlled by theliquid side for DEA/AMP blends and by combined liquid−gasphases for MEA/MDEA blends. For MDEA/PZ blends, controlof CO2 absorption is characterized by a gradual transition fromliquid-side controlled to liquid−gas combined controlled as theconcentration of PZ increases.

■ APPENDIX

A.1. Viscosities of Blended Amine and Mixture GasesThe viscosity of aqueous blended amines was estimated by themodel of Grunberg and Nissan:40

∑ ∑ ∑η η= +y yy Gln lni i i j ijm (A.1)

where ηi is the viscosity of the ith pure fluid and yi is the molefraction of the ith component in the mixture; Gij aretemperature-dependent parameters, which are provided byMandal et al.41

For a mixed gas (binary system), the viscosity can beestimated by Wilke’s method:

μμ μ

=+

++

y

y y M M

y

y y M M/ /G1 1

1 2 2 1

2 2

2 1 1 2 (A.2)

in which Mi is the mole weight of the ith component.

Figure 10. Overall mass transfer coefficient profile of MDEA/PZ blended amine as a function of MDEA and PZ concentrations (VG = 120 L/h, VL =8L/h, T = 40 °C, CCO2,in = 14 vol %, PL = 135 kPa, PG = 107 kPa, ΔPL = 6 kPa).

Figure 11. Individual mass transfer resistance comparison of differentblended amines (VG = 120 L/h, VL = 8 L/h, T = 40 °C, CCO2,in = 14

vol %, ω0 = 10 wt %, PL = 135 kPa, PG = 107 kPa, ΔPL = 6 kPa).

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A.2. CO2 Diffusivity in Liquid PhaseThe CO2 diffusivity in blended amine solutions can bedetermined by the analogy of N2O diffusivity in solution.42 Itcan be described as follows:

=⎛⎝⎜⎜

⎞⎠⎟⎟D D

D

DCO ,L N O,LCO ,W

N O,W2 2

2

2 (A.3)

The CO2 diffusivity DCO2,W and N2O diffusivity DN2O,W inwater can be expressed in the following equations,42

respectively:

= × − −D (2.35 10 )e TCO ,W

6 ( 2119/ )2 (A.4)

= × − −D (5.07 10 )e TN O,W

6 ( 2371/ )2 (A.5)

The diffusion coefficients of N2O in the amine solution arecalculated according to the modified Stokes−Einstein relation:

η η=D DN O,L L0.6

N O,W W0.6

2 2 (A.6)

where ηL and ηW are the viscosities of the amine solution andwater, respectively.A.3. Amine Diffusivity in Liquid PhaseThe diffusion coefficients of mixed amines in water can bedetermined by the following correlation:43

ρ= × −

−⎛⎝⎜

⎞⎠⎟D

M2.5 10amine,W

100.54

(A.7)

in which M is the molar mass of the amine and ρ is the densityof the amine.The diffusion coefficients were correlated for viscosity and

temperature using the modified Stokes−Einstein relation:41

ηη

=⎛⎝⎜⎜

⎞⎠⎟⎟D D

T273amine,L amine,W

W

L

0.6

(A.8)

A.4. CO2 Diffusivity in Gas PhaseWhen gas B is diffused in gas F, the diffusivity of gas B can becalculated by the Maxwell−Gilliland equation:44

ν ν=

× +

+

−

DT

P

(4.36 10 )

( )

M MB,F

5G

3/2 1 1

F1/3

B1/3 2

B F

(A.9)

where TG is the gas phase temperature, MB and MF are themolecular weights of gases B and F (g/mol), P is the pressurein the gas phase (kPa), and νB and νF are the molecularvolumes of gases B and F (cm3/mol), respectively.A.5. Molecular and Knudsen Diffusion Coefficients of CO2

As introduced previously, the effective diffusion coefficient ofCO2 in the membrane’s pores is the combination of molecularand Knudsen diffusion coefficients. The molecular self-diffusioncoefficient of CO2 is calculated from the kinetic gas theory:45

μ=ΩΩ

μDRTMP

1200D

igm

(A.10)

where Digm is the molecular self-diffusion coefficient, M is the gas

molecular weight, and μ is the gas dynamic viscosity in Pa s. P isthe gas pressure in kPa. Ωμ and ΩD are the viscosity collisionintegral and diffusion collision integral, respectively:

Ω =̅

+̅

+̅

+̅

T T T

T

1.6036( )

0.193exp(0.47635 )

1.03587exp(1.52996 )

1.76474exp(3.89411 )

D 0.1561

(A.11)

Ω =̅

+̅

+̅μ T T T

1.16145( )

0.52487exp(0.7732 )

2.16178exp(2.43787 )0.14875

(A.12)

T̅ is dimensionless temperature: T̅ = κT/ε, where κ is theBoltzmann constant and ε is the parameter of the Stockmayerpotential, which can be estimated by the critical temperature.The Knudsen diffusion coefficient can be calculated by the

following equation:46

=D dTM

48.5igk

pG

(A.13)

where TG is the gas temperature, M is the molecular weight ofthe gas, and dp is the pore mean diameter.

A.6. Henry’s ConstantHenry’s constant, H, of CO2 in MEA solution also can becalculated by the N2O analogy:

=H H H H( / )CO ,L N O,L CO ,W N O,W2 2 2 2 (A.14)

in which HN2O,L is the Henry’s constant of N2O in aqueous

amine. Henry’s constants of CO2 and N2O in water, HCO2,W and

HN2O,W, can be determined in the following equations providedby Versteeg et al.:42

= × −H (2.82 10 )e TCO ,W

6 ( 2044/ )2

L(A.15)

= × −H (8.55 10 )e TN O,W

6 ( 2284/ )2

L(A.16)

Wang et al.47 proposed the solubility of N2O in pure aminesolvent as follows:

=−H b b Texp( / )N O pure amine 1 22 (A.17)

in which b1 and b2 are the parameters for calculating thesolubility of N2O in pure amine.The solubility of N2O in aqueous mixed amines can be

described by a semi-empirical model, by correlating the excessHenry’s coefficient presented as the following:48

∑= − Φ=

X H Hln lni

n

i iN O,L1

N O,2 2(A.18)

where X is the excess Henry’s coefficient and Φi is the volumefraction of the ith solvent.In addition, the relationship of dimensionless Henry’s

constant, H, and the distribution coefficient, m, can bedetermined by

=mRTH (A.19)

in which R is the ideal gas constant, 8.314 J/(mol K).

A.7. Enhancement FactorThe enhancement factor, E, in a fast reaction regime is given byDeCoursey:49

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= −−

+−

+−

+∞ ∞

∞

∞E

HaE

HaE

E HaE2( 1) 4( 1) 1

12 2

2

2

(A.20)

where Ha is the Hatta number and E∞ is the asymptotic infiniteenhancement factor.For a system with a fast first-order irreversible chemical

reaction, the Hatta number can be described as

=Ha k D k/obs CO ,L L2 (A.21)

The asymptotic infinite enhancement factor, based on apenetration model, is described as

β= +∞

⎛⎝⎜⎜

⎞⎠⎟⎟⎛⎝⎜

⎞⎠⎟E

C DC D

D

D1 amine amine

CO ,L CO ,L

CO ,L

amine

0.5

2 2

2

(A.22)

where β is the stoichiometric coefficient of amine in thereaction.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: mxfang@zju.edu.cn. Tel.: 86-571-87952803. Fax: 86-571-87951616.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the National NaturalScience Foundation of China (Nos. 51076139, 51276161) andZhejiang Province Key Science Innovation Team Project (No.2009R50048). Z.W. is grateful for financial support from theAustralian government under the Australia−China JointCoordination Group on Clean Coal Technology PartnershipFund, which allows him to carry out part of his work at CSIRO.

■ NOMENCLATUREA = liquid−gas contact area [m2]b = parameter for calculating N2O solubility in pure amineC = CO2 concentration [mol m−3]di = inner diameter of hollow-fiber membrane [m]dm = log mean diameter of hollow-fiber membrane [m]do = outer diameter of hollow-fiber membrane [m]dh = hydraulic diameter [m]dp = pore mean diameter [m]D = diffusion coefficient [m2 s−1]DCO2

= diffusion coefficient of CO2 [m2 s−1]

DN2O = diffusion coefficient of N2O [m2 s−1]Dig

e = effective membrane diffusion coefficient [m2 s−1]Dig

m = molecular diffusion coefficient of CO2 [m2 s−1]

Digk = Knudsen diffusion coefficient of CO2 [m

2 s−1]E = enhancement factorE∞ = asymptotic infinite enhancement factorGij = temperature-dependent parameter for blended amineviscosity calculationH = Henry’s constant [kPa·m3 kmol−1]Ha = Hatta numberk2,amine = reaction rate constant for amine [m3 mol−1 s−1]k2,R1R2NH = second-order forward reaction rate constant forprimary or secondary amines [m3 mol−1 s−1]k2,PZ = second-order reaction rate constant for PZ [m3 mol−1

s−1]k−1 = reverse first-order reaction rate constant [s−1]

kb = second-order forward reaction rate constant for base b, b= H2O or R1R2NH [m3 mol−1 s−1]kobs = pseudo-first-order reaction rate constant [s−1]KO = overall mass transfer coefficient [m s−1]kL = mass transfer coefficient in liquid phase [m s−1]km = mass transfer coefficient in membrane phase [m s−1]kg = mass transfer coefficient in gas phase [m s−1]L = fiber length of membrane module [m]m = distribution coefficient between liquid and gasM = mole weight [g mol−1]P = pressure [kPa]ΔPL = pressure drop in liquid side along the membranecontactor [kPa]r = reaction rate [mol s−1]robs = overall reaction rate [mol s−1]R = ideal gas constant [J mol−1 K−1]X = excess Henry’s coefficientT = temperature [K]T̅ = dimensionless temperatureuL = liquid mean velocity [m s−1]yi = mole fraction of the ith componentGz = Graetz numberRe = Reynolds numberSh = Sherwood numberSc = Schmidt number

Greek Symbolsβ = stoichiometric coefficient of amine in the reactionδ = membrane thickness [m]τ = tortuosityς = membrane porosityφ = packing densityη = viscosity of liquid [mPa s]μ = viscosity of gas [Pa s]ν = molecular volume of gas [cm3 mol−1]ρ = density [kg m−3]Ωμ = viscosity collision integralΩD = diffusion collision integralΦi = volume fraction of ith solventκ = Boltzmann constant [J K−1]ε = Stockmayer potential parameter [J]ω = mass concentration [wt %]ω0 = mass concentration of base amine

Subscriptsin = inlet of membrane contactorout = outlet of membrane contactorW = waterL = liquid phase of membrane contactorG = gas phase of membrane contactor

Superscripts* = equilibriumEXP = experimental

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