journal of membrane science

Journal of Membrane Science 353 (2010) 192200

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Journal of Membrane Science

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Effect o andthroug tac

A. Mansoa Advanced Me udai, Jb Industrial Me sity of

a r t i c l

Article history:Received 7 DeReceived in reAccepted 19 FAvailable onlin

Keywords:Porous PVDF hCO2 absorptionOperating parameters

hollombrah as aof the posas abrptiorane

increasing CO2 pressure and decreasing the absorbent temperature. However, in case of chemical absorp-tion with NaOH (1M), the reaction rate was dominant, where the CO2 ux was signicantly increased byincreasing the absorbent temperature. It was also found that when the liquid contacted the outer skinlayer of the hollow bermembrane, the operationwas stable at the same gas and liquid pressurewithoutbubble formation in the liquid phase. In addition, the results of long-term study demonstrated that the

1. Introdu

Separatitant procesnatural gasnomical anremoval ofhouse gasescombustionassociated wCO2 in natuheating valuable to solidthe gas streas packed, bbased gas athe acid gasibility and athese devic

CorresponE-mail add

0376-7388/$ doi:10.1016/j.membrane performance was maintained constant for long-term operation. The initial ux reduction wasfound to be about 30% within the 23h of physical absorption, and 20% of gradual reduction within 80hof chemical absorption.

2010 Elsevier B.V. All rights reserved.

ction

onofacidgases fromthegas streams isoneof the impor-ses in many industrial areas such as ue gas treatment,processing, biogaspurication, etc. Environmental, eco-d operational problems are the main reasons for thethe acid gases. CO2 is the primary component of green-,whichmore than one-third of its emission comes fromof fossil fuels in power plants worldwide. It has beenith global climate change [1]. In addition, presence of

ral gas can cause pipeline corrosion, reduction in thee, occupying the volume in the pipeline and CO2 is alsoify in cryogenic process. In order to separate CO2 fromams, several conventional gas absorption systems suchubble and spray towers have been used. Alkanolaminebsorption systems have gained approximately 90% oftreating processes in operation, because of their ex-bility to remove CO2 to very low levels [2]. Although,es have attained signicant success in the industries,

ding author. Tel.: +60 7 5505392; fax: +60 7 5581463.ress: [email protected] (A.F. Ismail).

they suffer fromsomeoperational problems suchas foaming, ood-ing, channeling and entrainment. Furthermore, because of theirrelatively smaller mass transfer coefcient, they tend to be largeand costly to build. Hollow ber gasliquid membrane contactoris a promising alternative to conventional gas absorption systemsfor CO2 capture from gas streams. The porous hydrophobic mem-brane acts as a xed interface between the gas and the liquid phasewithout dispersing one phase into another. The membrane offers aexiblemodular energy efcient devicewith a high specic surfacearea. The absorption process can offer a very high selectivity anda high driving force for mass transfer even at very low concentra-tions [3]. It was reported by Yan et al. [4] that a removal efciencyofmore than 90%was attained for chemical absorption of CO2 usingthe gas streams containing 820% of CO2. The potassium glycinatesolution as the liquid absorbent was used through the commercialpolypropylene (PP) hollow ber membrane contactor module.

Esato and Eiseman [5] were the rst to employ the microporousmembrane as a gasliquid contacting device using hydrophobicat Gore-Tex membranes made of polytetrauoroethylene (PTFE)for oxygenation of blood. Removal of CO2 from gas streams bya membrane contactor has been a research focus since 1980s.In order to improve CO2 removal efciency, several factors suchas membrane materials and properties, absorption solutions and

see front matter 2010 Elsevier B.V. All rights reserved.memsci.2010.02.054f operating conditions on the physicalh the PVDF hollow ber membrane con

urizadeha, A.F. Ismail a,, T. Matsuurab

mbrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 Skmbrane Research Institute, Department of Chemical and Biological Engineering, Univer

e i n f o

cember 2009vised form 12 February 2010ebruary 2010e 25 February 2010

ollow ber membrane

a b s t r a c t

Porouspolyvinylideneuoride (PVDF)process and used in the gasliquid meof different operating parameters suclong-term operation on the CO2 uxshowed that the prepared membranresistance, which are favorable for grevealed that in case of physical absowhich signicantly affected thememb/ locate /memsci

chemical CO2 absorptiontor

ohor, MalaysiaOttawa, 161 Louis Pasteur St., Ottawa, ON, K1N 6N5, Canada

wbermembraneswere fabricatedvia awetphase-inversionne contactor for physical and chemical CO2 absorption. Effectbsorbent temperature, CO2 pressure, absorbent ow rate ande membrane were investigated. The characterization resultssess small pore size with high surface porosity and wettingsorption application. Results of CO2 absorption experimentsn with distilled water, CO2 solubility was the key parameterCO2 ux. A signicant increase in theCO2 uxwasobservedby

A. Mansourizadeh et al. / Journal of Membrane Science 353 (2010) 192200 193

membrane modules have been considered in the literature [613].Physical absorption of CO2 in propylene carbonate at elevated pres-sures using polypropylene (PP) hollow ber membrane contactorwas studied byDindore et al. [14]. They found thatwetting problemin long-term application can be avoided by applying over pres-sure in gas side, and the membrane contactor can be applied forhigh pressure application. Feron and Jensen [15] employed porouspolyolen membranes with the novel absorbent liquids (CORAL)based on mixtures of salts and amino-acids for chemical absorp-tion of carbon dioxide fromvarious feed gases. The novel absorbentshowed an excellent performance in terms of system stability andmass transfer. Ren et al. [16] prepared asymmetric polyvinylideneuoride (PVDF) hollow ber membranes for CO2 capture in themembrane contactor. They studied rheological characteristics ofthe dope solution on the membrane structure and the system per-formance for CO2 absorption. They found that for 20wt.% polymerdope, increasing the shear rate in the spinning process resulted inbigger geometric mean diameter, larger molecular weight cut off(MWCO) and higher CO2 absorption ux.

So far, most of the studies have been conducted on the fab-rication and characterization of porous hydrophobic membrane.In addition, the effects of different liquid absorbents and differ-ent membrane materials on the performance of CO2 absorption inmembraneabout the emance of mthe open litber membating paramabsorbent in the memphysical COoperation.

2. Theory

Membraacross a hydfer betweenother. In thiand membr

In ordershell side tomembrane,the mass trcontactor.

Fig. 1. Concenhollow ber m

The overall mass transfer process consists of three steps: rst,transfer of the gas from the bulk gas phase to the membrane sur-face; second, transfer through the membrane pores; last, transferfrom the membraneliquid interface into the bulk of the liquid.Therefore, for a hydrophobic hollowbermembranewith gas lledpores, the overall transfer coefcient based on the liquid phase (Ko)can be expressed by resistance in series model [17,18]:

1Ko

= mkgdo/di

+ mkm dlm/di

+ 1Ekl

(1)

where kg is the gas side mass transfer coefcient (m/s); km is themembrane mass transfer coefcient (m/s); kl is the liquid phasemass transfer coefcient (m/s); do is the outer diameter of hollowber membrane (m); di is the inner diameter of hollow ber mem-brane (m); m is the distribution coefcient between gas and liquidphase whicthe enhanc

2.1. Physica

In generthe followin

2H2O

the vall,

ed ping tw in

1.62

Ul is(m2/ametpre

peneaxisent ice, thgth

ivingers. Ttain

lmasacromoly (se

z =

al CO

(mCA

Fig.contactors arewell documented. However, informationffect of different operating parameters on the perfor-embrane contactor for CO2 absorption is still rare inerature. In this study, a laboratory made PVDF hollowranes was used to study the effects of the main oper-eters such as gas pressure, absorbent temperature andow rate on the physical and chemical CO2 absorptionbrane contactor. In addition, a long-term chemical and2 absorption was conducted around 150h of prolonged

ne gas absorption is based on a gasliquid contactrophobic porousmembranewhich permitsmass trans-the two phases without dispersing one phase into the

s section, the basic aspects ofmass transfer in gas, liquidane phase are discussed.to describe mass transfer from the gas phase on thethe liquid phase on the lumen side of a hollow ber

the resistance in series model can be used. Fig. 1 showsansfer process in a hollow ber gasliquid membrane

tration prole of gas absorption from shell side to lumen side of aembrane.

CO2 +Sincevery smabsorbaccordside o

kldiDl

=

whereliquidside diusuallyof theto theabsorb

Sinthe lenthe drthe bis mainoveralof CO2

Thegiven b

Jzndid

The loc

Jz = kzh can be replaced by Henrys law constant (H); and E isement factor due to chemical reaction.

l absorption

al, absorption of CO2 in aqueous media is presented byg reaction:

HCO3 +H3O+ (2)alue of the equilibrium constant of the reaction isthe formation of bicarbonate is negligible and CO2 is

hysically. In this case, the value of kl is widely predictedo the GraetzLvqu correlation developed for lumen-hollow ber modules [18]:(

d2iUl

DlL

)0.33= 1.62 (Gz)0.33 Gz > 20 (3)

the liquid velocity (m/s), Dl is diffusivity of CO2 in as), and L is the length of the ber (m). As the lumen-er of the hollow ber is very small the Gz>20 conditionvails. Gz is the Graetz number which shows the ratiotration time of the solute gas from gasliquid interfaceof the ber to the average residence time of the liquidn the hollow ber.e CO2 concentration in the liquid phase increases acrossof the hollow bers at continuous mode of operation,force of the mass transfer reduces with the length ofhe CO2 concentration in the gas and membrane phaseed constant by applying pure CO2 as the feed gas. Thes transfer coefcient canbedeterminedbymolebalancess the length of the hollow bers.e balance across a segment (dz) of the hollow ber ise Fig. 2):

QlCA,lz+dz QlCA,l

z

(4)

2 absorption ux is shown by:

,g CA,l) (5)

2. Mole balance across the hollow ber membrane length.

194 A. Mansourizadeh et al. / Journal of Membrane Science 353 (2010) 192200

where kz is a local mass transfer coefcient, which can be consid-ered as a combination of gas side, liquid side and membrane masstransfer coefcients. m is the distribution coefcient which can besubstituted byHenry constant (H) for CO2-water system. Accordingto Eqs. (4) a

dCA,ldz

= nQ

Integratingoverall mas CA,lo

0 (HCA

Ko = QlndiL

where CA,loside concenHenrys law

2.2. Chemic

When aabsorbent, Cas follows:

CO2 +OH

HCO3 +OHReaction

bonizationimportant icentration Ocan be conspseudo-rsrate of CO2

NA = E kl(CAE is the enhdened as ttion ux:

E = JchemJphys

It must be mthe drivingtransfer provelocity ofpredicted a

2.3. Gas an

For lamicient (kg) vaCussler [20]

kgdeDg

= 1.8

where ug isow area (mDg is the gashell dene

de =D2

i n

Di + n

where Di is the inner diameter of the shell and n is the number ofthe bers. The validity of Eq. (13) depends on the packing densityof hollow bers in the module.

However, by using pure gas or high velocity feed gas stream inell sir promem), in]:

(1 H D

x isare trane,dditily ine eff24].ht-hed aon w

m

mdm

on Eis knaccoras anh then byed by

erim

ateri

merema), wapurd froP sobath

d wator.

brica

his sber resprop. Forre foto fa5,26

neutrize mPVD

or 24PVD

by viglymeg bynd (5):

dikz

l(HCA,g CA,l) (6)

Eq. (6) over the effective length of the hollow bers, thes transfer coefcient can be obtained as:

1

,g CA,l)dCA,l =

ndiQl

L0

kzdz (7)

ln

(1 CA,lo

HCA,g

)(8)

is the module liquid outlet concentration. CA,g is the gastration and it is constant for all the experiments. Theconstant (H) for CO2water is 0.85.

al absorption

n aqueous alkaline solution is used as the liquidO2 reacts with the hydroxyl ions within the liquid lm

HCO3 (9) CO32 (10)(9) is the secondorder in the forwarddirection. Thecar-

reaction (10) is much faster than reaction (9) and is notn determining the rate of CO2 transfer. Using high con-Hsolutions, the CO2 concentration in the liquid sideidered to be constant and the reaction (9) approachest order [19]. Therefore, based on this assumption, thetransfer can be written as:

,li CA,lb) (11)ancement factor due to the chemical reaction, which ishe ratio of chemical absorption ux to physical absorp-

(12)

entioned that Eq. (12) is based on the assumption thatforce is identical for both chemical and physical masscess. This condition can be achieved by applying highthe liquid absorbent. The enhancement factor can benalytically using traditional mass transfer theories.

d membrane phase mass transfer resistance

nar gas ow in the shell side, the mass transfer coef-lue is estimated by the correlation derived by Yang and:

(deL

)0.93(deugg

)0.93(gDg

)0.33(13)

the volumetric ow rate divided by the cross-sectional/s), g is the kinematic viscosity of the gas (m2/s), and

s diffusivity (m2/s). de is the equivalent diameter of thed as:

d2odo

(14)

the shtransfe

TheEq. (15[21,22

1km

=

whereand memb

In atitativealso thtance [the rigwas usequati

1Ko

=k

Basedwhich0.330.33througis giveobtain

3. Exp

3.1. M

Comby Ark>99.5%furthersupplie90% NMulationdistillecontac

3.2. Fa

In thollowtransfeThosecationsas a poknownsizes [2as theminim

Theoven f17wt.%60 C)The pospinninde the gas side resistance can be avoided in the masscess.brane mass transfer resistance, km, is often given by

which the pores are considered to be partially wetted

x)

g+ x

Dl(15)

the fraction of the pore lled with liquid. Here, , ,he surface porosity, tortuosity, and pore length of therespectively.on, theWilson plotmethod [23] can be applied to quan-vestigate the membrane mass transfer resistance andect of liquid velocity on the overall mass transfer resis-The gas side mass transfer resistance, the rst term onand side of Eq. (1), can be ignored because pure CO2s feed gas. By combining Eq. (1) and (3), the followingas obtained:

/di+ 1

1.62

(d2

i

DlL

)0.33U1

0.33 (16)

q. (16), a plot of Ko1 vs. Ul results in a straight lineown as the Wilson plot. The value of is supposed to beding to Eq. (16). However, it is allowed to deviate fromempirical constant that provides the best straight linedata points. The overall mass transfer coefcient (Ko)

Eq. (8). The membrane mass transfer resistance can bethe intercept of the Wilson plot.

ental

als

cial PVDF polymer pellets (Kynar 740) were suppliedInc., Philadelphia, USA. N-methyl-1-pyrrolidone (NMP,s supplied by MERCK and used as the solvent withoutication. Lithium chloride monohydrate (LiClH2O) wasm QRC and used as the nonsolvent additive. Aqueouslution was used as the bore uid and tap water as coag-in the spinning process. NaOH solution (0.2, 1M) and

ter were used as liquid absorbents in the membrane

tion of asymmetric PVDF hollow ber membrane

tudy, an attempt was made to prepare hydrophobicr membranes with improved permeability (low massistance) and small pore sizes (high wetting resistance).erties are considered favorable for gas absorption appli-this purpose, an inorganic salt (LiClH2O)was employedrming additive in the spinning dope, a method which isbricate PVDF hollow ber membranes with small pore]. In addition, an aqueous 90% NMP solution was usedal bore uid in order to remove the inner skin layer andembrane mass transfer resistance.F polymer pellets were dried at 602 C in a vacuumh to remove moisture content. The spinning dope ofF, 5wt.% LiClH2O and 78wt.% NMP was prepared (atorous stirring until the solution becamehomogeneous.r dope was degassed before spinning. The hollow berthe dry-jet wet phase-inversion process was described


Table 1Spinning conditions of fabricating PVDF hollow ber membranes.

Dope extrusion rate (mL/min) 4.0Bore uid coBore uid oExternal coaAir gap distaSpinneret o.CoagulationRoom relativ

elsewhere [eters.

The sputhe residualbranes wermethanol acollapse be

3.3. Gas per

The meaporosity ovtransfer in tthe pore sizcan be regaow [28]. Lito determinover the eff

Byassummembranes

JA =2 rp3RTLp

where JA isand effectivis gas constmolecular w(Pa).

By plotteffective sufrom the in

rp = 5.333(

Lp= 8RT

r2p

In the gaThe test appThe testmoof about 10upstream pN2 was suprate was mow meter.the outer d

3.4. Porosit

To measlength of 50weighed. Th

Table 2Characteristics of the gasliquid membrane contactor.

Module i.d. (mm) 14le length (mm) 270.d. (m.d. (mve ber of g den

only(1

f aer d

4

(d2o L is te out1.77

itica

ical wg reso thagml. Atconsed inssurbe

2 ab

aslirmin

ect obe

e. Sp.e COd wuiduids phe. Fogas

d, COas ms, thy stiagramposition (wt.%) NMP/H2O 90:10w rate (mL/min) 1.50gulant Tap waternce (cm) 0.0d./i.d. (mm) 1.20/0.60temperature (C) 25e humidity (%) 7075

27]. Table 1 summarizes the detailed spinning param-

n bers were immersed in water for 3 days to removeNMPand the additive. The preparedhollowbermem-e post-treated by the solvent exchange method usingnd n-hexane to minimize ber deformation and poresfore drying at room temperature.

meation test

surement of gas permeability, pore size and surfaceer effective pore length is important in studying masshe porous membranes for gas absorption. Consideringe, the overall gas permeation through the membranerded as the combination of Poiseuille ow and Knudsenet al. [7] introduced amodied gas permeationmethode the mean pore size and the effective surface porosityective pore length of the asymmetric membrane.ing cylindrical pores in the skin layerof theasymmetric, gas permeance can be given by:

(8RTM

)0.5+ r

2p

8RTLpP or JA = K0 + P0P (17)

gas permeance (mol/m2 s Pa); rp and Lp are pore radiuse pore length, respectively (m); is surface porosity; Rant 8.314 (J/molK); is gas viscosity (kg/ms); M is gaseight; T is gas temperature (K); and P is mean pressure

ing JA vs. P, according to Eq. (17), mean pore size andrface porosity over pore length, /Lp, can be calculatedtercept (K0) and the slope (P0) as follows:

P0K0

)(8RTM

)0.5 (18)

P0 (19)

s permeation method, pure N2 was used as the test gas.aratuswas based on the volume displacementmethod.dulewhich contained two hollowberswith the lengthcm, was used to determine the gas permeance. The

ressure was in a range from 0.5105 to 4105 Pa. Theplied to the shell side of the module and permeationeasured at 25 C in the lumen side using soap-bubbleThe gas permeability was then calculated according to

ModuFiber oFiber iEffectiNumbPackin

comm

o (%) =

whereThe bas:

f =

whereand thmer is

3.5. Cr

Critwettinfed intdiaphrintervaat theappearthe prehollow

3.6. CO

A gto detethe effhollowmodulTable 2

PurDistillethe liqThe liqthe gauid sidfor themethorates wsamplea steadow d[31].iameter of the hollow ber.

y measurement

ure the overall porosity, ve hollow bers with thecm were dried for 2h at 105 C in a vacuum oven ande overall porosity (o) was calculated according to the

4. Results

4.1. Structu

It was pin more poto fabricatewas introdum) 1m) 0.55er length (mm) 150bers 10sity (m2/m3) 204

used method based on density measurements [29,30]:

fp

) 100 (20)

nd p are the ber and polymer density, respectively.ensity was calculated from the mass and volume ratio

w

d2i)L

(21)

he ber length; w is ber mass; di and do are the innerer diameters, respectively. Thedensity of the PVDFpoly-g/cm3.

l water entry pressure test

ater entry pressure (CEPw) was measured to know theistance of the prepared membrane. Distilled water wase lumen side of the hollow ber membranes using apump. Thepressurewas slowly increasedat0.5105 Paeach pressure interval, themembranemodulewas kepttant pressure for 30min to check if any water dropletthe outer surface of the ber. CEPw was considered as

e for the rst water droplet in the outer surface of ther.

sorption experiment

quid membrane contactor module was used in ordere the membrane mass transfer resistance and study

f operating parameters on CO2 absorption. A total of 10rs were packed randomly in a stainless steel membraneecics of the membrane contactor module are given in

2 was employed as the feed gas in all the experiments.ater and NaOH solutions (1 and 0.2M) were used asabsorbents in order to measure the absorption ux.phase pressure was controlled 0.2105 Pa more thanase in order to prevent bubble formation in the liq-r all the experiments, a counter-current ow was usedand the liquid absorbent. Using the chemical titration2 concentration in the liquid outow at various oweasured to determine the CO2 ux. Before taking thee experiments were carried out for 30min to achieveate condition. The membrane module as well as them of the experimental setup were shown elsewhere

and discussionre of the hollow ber membrane

roven that increasing phase-inversion rate could resultrous asymmetric membranes [32]. Therefore, in orderporous membranes with small pore sizes, LiClH2Oced into spinning dope as a phase-inversion promoter


n; (b)

additive. Thexamined t(FESEM). Tber memb1050m, iwall thicknthe cross-seof morpholnger-likecavities nea

Using taphase-inver(Fig. 3b anduid inducesurface witphology wa95wt.% NM

It seemsment in thea thin ngeother handhigh, a decrning dopesponge-likein the sponto the slowIn fact, theat slow solibined to fo[9].

As for thviscosity, itdiffuse out

tratiurfacof g

mallFig. 3. FESEM micrographs of the PVDF hollow ber membrane: (a) cross-sectio

e morphology of the prepared PVDF membrane washrough eld emission scanning electron microscopyhe micrographs are presented in Fig. 3. The hollowranes possess outer diameters ranging from 1000 to

concenouter sResultswith snner diameters ranging from 580 to 620m and theess ranging from 190 to 210m. As it can be seen fromction, the membrane structure consists of three kindsogies: a porous and thin outer skin layer with a thinlayer beneath, a sponge-like sublayer with drop shaper the inner surface, and an inner skinless layer.p water at 25 C as external coagulant induced fastsion which resulted in a thin and porous skin layerd). Meanwhile, using a 90wt.% NMP solution as bored delay phase-inversion and provided an inner skinlessh open microporous structure (Fig. 3c). The same mor-s also obtained for the polysulfone membranes usingP as the bore uid [33].that the nonsolvent additive resulted in an improve-precipitation rate of the spinning dope which providedr-like layer (520m) near the outer surface. On the, since viscosity of the spinning dope was obviouslyease of mutual diffusion between solvent in the spin-and nonsolvent in the coagulation bath resulted in asublayer. In addition, formation of drop shape cavitiesge-like layer near the inner surface can be associatedsolidication because of the bore uid composition.wall between small droplets is difcult to be formeddication process, thus many small droplets are com-rm larger droplets that generate drop shape cavities

e outer surface formation, due to the spinning dopeseems that the solvent andnonsolvent couldnot rapidlyof the polymer solution, hence prevented the polymer

pore sizes cFESEM test

The chaobtained as

As the reoverall porpolymer insoluble addon the memuble in NMsolventno5% of LiClHture with h

Results oa high surfato the use oincrease thehigh porosiare favorab

As for thhas a signi

Table 3Properties of t

N2 permeanMean pore sEffective surCritical wateOverall poroskin layer; (c) inner surface; and (d) outer surface.

onon theouter surface. Thisphenomenon resulted inane with high porosity and very small pore sizes (Fig. 3d).as permeation test also conrmed high surface porositypore sizes of the membrane. However, the nanometer

ould not be observed through high magnication of the.racterization results of the membrane which werethe average of ve samples are given in Table 3.sults show, the fabricated membrane possesses a goodosity, which can be a result of low concentration ofthe spinning dope. Using low molecular weight wateritive in the spinning dope may also result in an increasebrane porosity. LiClH2O as nonsolvent additive is sol-P and leaches out of the spinning dope during the

nsolvent exchange in the coagulation bath. Hence, using2O in the spinning dope resulted in an open pore struc-igh porosity.f gas permeation test revealed that the membrane hadce porosity with small pore sizes, which can be relatedf additive. Since LiClH2O is a strong nonsolvent, it canprecipitation rate of the spinning dope and results in a

ty skin layerwith small pore sizes [33]. These propertiesle for gasliquid contacting application.e critical water entry pressure (CEPw), the membranecantly higher CEPw than commercial PP and PTFEmem-

he PVDF hollow ber membrane.

ce at 1105 Pa (103 cm3/cm2 s cmHg) 1.62 0.05ize (nm) 2.33 0.51face porosity /Lp (102 m1) 1070 199r entry pressure (105 Pa) 5.33 0.47sity (%) 70.83 2.49


Fig. 4. EffectPg =1105 Pa

branes [13](Table 3). Twithstand wbrane contadesirable togas side (prto preventloss of gas cuid side canoperation.

4.2. Effect oresistance

Physicaldistilled waabsorbents.function ofthat CO2 uabsorbents.cal amines[35,36].

In caserelatively hthe hollowinlet liquidaround theity resultedwhich led toon Eq. (3).

In case otionwas reldue to the hsuch case, treach to zerthat resultsthedrivinging the absoOH deplet

Resultsux in chemabsorption(see Fig. 4).tion (due tochemical Ctension canand deterio

ilson plot of the prepared PVDF membrane (pure CO2-distilled water sys-

5 shows the Wilson plot of 1/Ko versus Ul0.93 for the pre-PVDF membrane using physical absorption. The value ofas found to give the best linear t to the experimental. Thiss theraneplot,r resliquiglectas 75y toed te liqy.

fect o

meaabsouid s. Rensideorbeed freasempeorptlow of absorbent ow rate on CO2 ux. (Qg =200mL/min, T=26 C,, Pl =1.2105 Pa).

, which can be attributed to the presence of small poresherefore, it seems that the prepared membrane canetting during the CO2 absorption process in the mem-ctor. In fact, for gasliquid membrane contactors, it isapply a higher pressure on the liquid side than the

essure difference ranging from0.2105 to 0.4105 Pa)bubble formation in liquid phase, which can result inomponents [34]. However, the higher pressure in liq-cause wetting of the porous membrane in a long-term

f absorbent ow rate on CO2 ux and mass transfer

and chemical CO2 absorption were conducted usingter and NaOH solution (1M), respectively as the liquidFig. 4 shows the experimental results of CO2 ux as aabsorbent ow rate on the lumen side. It was observedx increased with an increase in the ow rate of bothA similar trend was also reported for the use of typi-to absorb CO2 in PVDF and PTFE membrane contactor

of physical absorption, since the Graetz number wasigh (400


Fig. 7. Effect oQg =200mL/m

absorption,the liquid te

In generof the speciArrhenius eperature cain absorbenarenot favoture, watercondensatiotion, CO2 bleads to a lo

Apparenperature onspecies arein the porecal absorptand capillardecreasing teffect of temPTFE hollow

4.4. Effect ooperation st

The effeabsorptionillustrated ilumen at thsure had athe CO2 prux increasoverall massure [14], tto the increincrease incal absorptiincreasing Cow rate ispressure rethus higherthe CO2 ualmost indethe pseudo-

In orderstability ofCO2 and disformation i

ffect omL/m

n then froing05 tomatiher iat thin t

the hre atallCO2

am.ist wlly caperareasion is a ntor petric

ase oliqu

ery sable,hen

d 0.3to t

on thressunsiont preed inf CO2 pressure on the CO2 absorption performance. (Ql =120mL/min,in, Pl was controlled 0.2105 Pa more than Pg).

a decrease of 33.3% in the CO2 ux was observed whenmperature increased from 10 to 40 C.al, an increase in temperature can increase diffusivityes [14] and the reaction rate constant according to thexpression [37]. On the other hand, an increase in tem-n also cause a decrease in CO2 solubility [14], increaset evaporation and increase in CO2 back-pressure whichrable for the absorptionprocess. By increasing tempera-vapor lls themembrane pores, thus results in capillarynwhichcan increase themembrane resistance. Inaddi-ack-pressure may occur at high temperature, whichwer driving force for CO2 absorption.tly, in case of chemical absorption, the effect of tem-the increasing reaction rate and diffusion rate of the

more important than increased capillary condensations and CO2 back-pressure. However, in case of physi-ion, a decrease in the CO2 solubility with temperaturey condensation in the pores can be themain reasons forheCO2 ux.KimandYang [38]also reported the reverseperature on the physical CO2 absorption through theber membrane contactor.

f CO2 pressure on the absorption performance andability

ct of CO2 pressure on the physical and chemical CO2performance of the PVDF hollow ber membrane isn Fig. 7. The liquid absorbents passed through the berse constant ow rate. As it can be seen the CO2 pres-signicant effect on the physical absorption. When

essure increased from 1105 to 6105 Pa, the CO2ed from 1.25103 to 6.5103 mol/m2 s. Since thes transfer coefcient is independent of the systempres-he considerable increase in CO ux can be attributed

Fig. 8. EQg =200

ow obe seeincreas0.21ble forBy furtstablemationacrosspressuvery smat highlet strecan resgraduaterm ocan incformatthere icontacasymmsures.

In cgas andtains vwas steven w(arounrelatedBasedhigh pface tecase, aappear2

ase in driving force for absorption as a result of anCO2 concentration. On the other hand, in case of chemi-on, only a small increase in theCO2 uxwasobservedbyO2 pressure. As discussed earlier, since the absorbentconstant, an increase in the CO2 concentration with

sulted in the liquid saturation through the lumen side,increment of the CO2 pressure had no more effect onx. Therefore, it can be said that the reaction rate ispendent from CO2 concentration, which can conrmrst order reaction.to evaluate the effect of CO2 pressure on the operationthe hollow ber gasliquid membrane contactor, puretilled water were used as gas and liquid phases. Bubblen the liquid phase was considered at two cases: water

the membrside volumbubbles forbefore bein

4.5. Long-te

Fig. 9 shical CO2 abliquid absolumen sideperature. Frdecreased auntil the enf CO2 pressure on bubble formation in liquid phase. (Ql =120mL/min,in).

lumen side and water ow on the shell side. As it canm Fig. 8, in case of water ow on the lumen side, byCO2 pressure up to 3105 Pa, a higher pressure (from0.4105 Pa) in liquid sidewas required toprevent bub-on at the liquid outlet and keep the operation stable.ncrement in the CO2 pressure, the operation remainede same gas and liquid pressure. In fact, the bubble for-he liquid phase can be a result of liquid pressure dropollow ber, where the CO2 pressure exceeds the liquidthe end parts of the bers. In addition, it seems that thebubbles formed in the liquid phase dissolved in waterpressure, as they could not be observed at the out-

Although, hydrophobic PVDF hollow ber membraneetting, applying higher pressure in the liquid side canuse wetting in initial parts of hollow bers in the long-

tion. It is well known that partial wetting of membranee mass transfer resistance signicantly, and also bubblen liquid results in loss of gas components. Therefore,eed to control these issues in the gasliquid membranerocesses. It seems that gas absorption operation usingPVDF membrane can be more stable at higher pres-

f water ow on the shell side, the contact area betweenid is the outer skin layer of the hollow ber which con-mall pores in the range of nanometer. The operationwithout any bubble appearance in the outlet liquid,the liquid pressure was less than gas side pressure1050.7105 Pa) (Fig. 8). This phenomenon can behe membrane pore size and surface tension of water.e Laplace equation, for the small pore sizes a relativelyre difference (Pg Pl) is required to overcome the sur-force and form bubbles in the liquid. However, in this

ssure difference more than 0.8105 Pa, some bubblesthe outlet liquid. It indicates that the outer surface ofane also possess big pores. In addition, since the shelle is much higher than the lumen side, the very smallmed might have dissolved in liquid at high pressureg observed in the outlet liquid.

rm performance of CO2 absorption

ows the experimental results of the physical and chem-sorption in the membrane contractor over 150h. Therbents were owed in the shell side and CO2 in theof the PVDF hollow ber membrane at ambient tem-om Fig. 9, in case of physical absorption, the CO2 uxbout 30% in the initial 23h and then remained constantd of the operation. Similar performance deterioration


Fig. 9. Long-te150h. (Ql =80shell side).

wasobservemembraneshollow beface are verhas high rewhen the sthe partialsation of wstate was reIt is worth mcapillary cowhich dictain channels

As for chgradual peroperation,decline in ttension of Nis less readcaused byalkaline solreported fosolution (2

5. Conclus

Porous Pphase-inveterizedandin order toand chemicseveral sign

Using we(LiClH2Osolution opore size,

In case ofux was sthe reactiaffected t

In case ofabsorptiocould notsignicanwhich can

It was fouation can

bubble formation in the liquid outow. It can be attributed to theouter skin layer with small pore sizes of the membrane.

In caseof chemical absorption, the results showedthatby increas-ing the absorbent temperature, the CO2 ux increased due to

asing the reaction rate. However, in case of physical absorp-a decrease in the CO2 ux was observed by increasingerature, which can be related to the decrease of CO2 sol-ty.dition, the results of long-term operation showed that theal wetting was probably caused by capillary condensation ofr vapor in themembrane pores in physical absorption. How-, in chemical absorption it seems that pore enlargement wasain reason for gradual ux reduction after 80h operation.

wledgement

authors gratefully acknowledge the nancial support fromnistry of Science, Technology and Environment, Malaysia,e Grant number of 03-01-06-SF0282.

enclature

surface area (m2)concentration (mol/m3)diameter (m)diffusion coefcient (m2/s), shell side diameter (m)enhancement factorHenrys law constantgas permeance (mol/m2 s Pa)mass transfer coefcient (m/s)overall mass transfer coefcient (m/s)constanthollow ber length (m), pore length (m)distribution coefcientmolecular weight (kg/mol)number of bersmass transfer ux (mol/m2 s)mean pressure (Pa)constantow rate (m3/s)radius (m)gas constant (J/molK)temperature (K)velocity (m/s)ber mass (g)fraction of pore lled with liquid

scriptscomponentbulkrm performance of the PVDF hollow ber membrane contactor overmL/min, Qg =100mL/min, Pl =Pg =1105 Pa, liquid absorbents on the

d forCO2 absorptionbywater in thepolypropylene (PP), which was a result of initial wetting [21]. Since PVDFrs are hydrophobic and the pore sizes on the outer sur-y small, where the skin layer is formed, the membranesistance to wetting by water penetration. Therefore,ame pressure is applied on both gas and liquid sides,wetting was probably caused by the capillary conden-ater vapor in the membrane pores [39] and the steadyached within the initial short period of the operation.entioning that the smaller pore size is more prone to

ndensationofwater vaporbasedon theKelvin equation,tes that even under-saturated vapors can be condensedwith very small dimensions [40].emical CO2 absorption with a NaOH solution (0.2M),formance deterioration was observed after 80h of theand then the CO2 ux leveled off (Fig. 9). The initialhe CO2 ux was approximately 23%. Although, surfaceaOH solution is higher than water and the membrane

ily wetted, it seems likely that the partial wetting wasthe pore enlargement due to an interaction betweenution and PVDF membrane. A similar result was alsor CO2 absorption by aqueous diethanolamine (DEA)M) in a PP hollow ber membrane contactor [11].

ion

VDFhollowbermembraneswere fabricatedusingwetrsion process. The prepared membranes were charac-used in thehollowbergasliquidmembranecontactorstudy the effects of operating parameters on physicalal CO2 absorption. The conducted experiments led toicant conclusions:

t spinning process, phase-inversion promoter additive

incretion,tempubili

In adpartiwateeverthe m

Ackno

Thethe Miwith th

Nom

ACdDEHJkKK0LmMnNPP0QrRTUwx

SubAbe) in the spinning dope and neutral bore uid (aqueousf 90wt.% NMP) resulted in the membrane with smallhigh surface porosity and low mass transfer resistance.chemical absorptionwithNaOH (1M), although theCO2ignicantly more than physical absorption because ofon, the results showed that the liquid phase resistancehe mass transfer process.chemical absorption, the effect of CO2 pressure on thenuxwasnot signicant,meaning that the reaction ratebe related to CO2 concentration. However, the CO2 uxtly increased with the pressure in physical absorption,be a result of increasing driving force.

nd that in case of liquid ow on the shell side, the oper-be stable at the same gas and liquid pressure without

fgillmmop

Greek letequivalentbergasinner, interfaceliquidlog meanmembraneouter, overall, outletpolymer, pore

terspore length (m)surface porosity, porositygas viscosity (kg/ms)


density (g/cm3) tortuosity kinematic viscosity (m2/s)

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Effect of operating conditions on the physical and chemical CO2 absorption through the PVDF hollow fiber membrane contactorIntroductionTheoryPhysical absorptionChemical absorptionGas and membrane phase mass transfer resistance

ExperimentalMaterialsFabrication of asymmetric PVDF hollow fiber membraneGas permeation testPorosity measurementCritical water entry pressure testCO2 absorption experiment

Results and discussionStructure of the hollow fiber membraneEffect of absorbent flow rate on CO2 flux and mass transfer resistanceEffect of absorbent temperature on CO2 absorptionEffect of CO2 pressure on the absorption performance and operation stabilityLong-term performance of CO2 absorption

ConclusionAcknowledgementReferences

journal of membrane science

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