Separation and Purication Technology 74 (2010) 242252

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Separation and Purication Technology

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Study o rolNaAlg PA

Tianrong ingCollege of Chem na

a r t i c l

Article history:Received 1 ApReceived in reAccepted 14 Ju

Keywords:PervaporationCaprolactamComposite mePoly(vinyl pyrrolidone)Permeance

olactestigyrroliranememP coincrein thes wit

323K, for 50wt.% caprolactam. Besides, operating temperature and feed composition on pervaporationperformances were investigated. The results showed that the membranes permeation ux increasedwith increasing feed temperature and the water concentration, while separation factor decreased. Thenormalized permeation uxes in terms of water permeance, caprolactam permeance and selectivitywere also introduced to evaluate the membranes performance. The evaluated results revealed that theseparation performances of NaAlgPVP composite membranes were strongly related to their intrinsic

1. Introdu

Pervapoeco-friendlyconsideredseparationfusion propstudied formixtures, iscompoundsused to desoverall mastop layer aPV mass tra(a) diffusiontion into thspecies acrolayer; and (ate side of

CorresponE-mail add

1383-5866/$ doi:10.1016/j.hydrophilic/hydrophobic nature as well as the operating parameters. 2010 Elsevier B.V. All rights reserved.

ction

ration (PV) is one such type of cost effective andclean membrane separation technology, which is

as a prospective industrial separation process. As itsefciency relies on the differences in sorption and dif-erties of the permeating molecules, PV has been widelyseparation aqueousorganic azeotropes, closely boilingomers, aswell as being safe to handle the heat-sensitive[1,2]. The solutiondiffusion model has been widely

cribe the PV separation process [35]. Fig. 1 shows thes transfer resistance composed of the resistance of thend the resistance of the support layer. It is held thatnsport process consists of ve fundamental processes:through the feed boundary layer; (b) selectively sorp-e membrane top layer; (c) diffusion of the dissolvedss the membrane matrix; (d) desorption out of the tope) diffusion through the porous support to the perme-the membrane using a vacuum on the permeate side.

ding author. Tel.: +86 27 68772263; fax: +86 27 68776726.ress: yuping@whu.edu.cn (P. Yu).

The mass transfer resistance on the feed and vapor side is usuallyconsidered to be negligible. Thus, in a composite membrane, onlythe active layer plays a role in selecting components and control-ling permeation. Compare with all other membrane processes thephase change occurred during material transport [6].

Caprolactam (C6H11NO) is one of the most important inter-mediates in the polymer industry, which is synthesized via thecyclohexanone oximation and Beckmann rearrangement routeusing highly concentrated sulfuric acid by neutralizing the reactionmixture with aqueous ammonia [7]. The crude caprolactam phaseconsists of 6570wt.% caprolactam, 11.5wt.% ammonium sulfateand water. The industrial processes utilized for ber productionare sensitive to quality uctuations; so high purity of commer-cial caprolactam (>99.8%) is most often required by articial berindustry. Water is the most important impurity in the nal capro-lactam purication because the existence of water can hinder thegrowth of molecular weight [8]. However, since caprolactam isvery heat-sensitive substance, to prevent decomposition, crystal-lization under a reduced pressure distillation through triple-effectevaporation sets has been thoroughly investigated and used forthe purication in recent years, but which suffers from high oper-ating costs, high energy consumption, and the pollutants beingtransferred to a second phase. It is hence difcult to purify the

see front matter 2010 Elsevier B.V. All rights reserved.seppur.2010.06.012f pervaporation for dehydration of cappoly(vinyl pyrrolidone) membranes on

Zhu, Yunbai Luo, Yanwen Lin, Qin Li, Ping Yu , Mistry and Molecular Sciences, Wuhan University, Luojia Street, Wuhan 430072, PR Chi

e i n f o

ril 2010vised form 12 June 2010ne 2010

water solutionsmbranes

a b s t r a c t

To improve or develop a new caprcaprolactamwater solutions was invsodium alginate (NaAlg)poly(vinyl ptrile (PAN) ultraltration (UF) membcrosslinked with glutaraldehyde. Theangle measurements. The effect of PVmancewas investigated. The uxwasincreased when the PVP content wasPVP exhibited excellent PV propertie/ locate /seppur

actam through blendN supports

Zeng

am dehydration process, pervaporation (PV) separation ofated using a composite membrane consisting of a selectivedone) (PVP) blendmembrane as top layer and a polyacryloni-as substrate. The selective layer was physically blended andbranes were characterized by SEM, FTIR, XRD, and contact

ntent in the blend membranes on the pervaporation perfor-ased by the addition of PVP and the separation factor was alsorange of 020wt.%. The blend membrane containing 20wt.%

h a ux of 1634.4 g/(m2 h) and separation factor of 1610.6 at

T. Zhu et al. / Separation and Purication Technology 74 (2010) 242252 243

Nomenclature

Ji 2

JDikipi,feedpi,permeatpi,activeQipi,permeat

xiyip0i,feed

ppermeateJ0

EJEQ

RTAWt

Greek letmembran i

Subscriptiactivesupport12

caprolactamtallization bstudied bycaprolactambrane crosswas obtaineexhibited amance.

To imprare often revarious moto be an efsynergisticare often pavailabilityalginate (Nweed, whicBut very offrom the intivity [18].membraneor inorgani

artialration

aAlgpermeation ux of component i (g/(m h))pervaporation permeation ux (g/(m2 h))diffusion coefcient of component i (m2/s)adsorption coefcient of component i (mol/(m3 Pa))vapor pressure of component i in feed (kPa)

e vapor pressure of component i in permeate (kPa)vapor pressure of component i in active layer (kPa)membrane permeance component i (g/(m2 hkPa))

e vapor pressure of component i in support layer(kPa)mole fraction of component i in the feedmole fraction of component i in the permeatesaturation vapor pressure of pure i in the feed liquidtemperature (kPa)permeate pressure (kPa)pre-exponential factor of the permeation ux fromthe Arrhenius equation (g/(m2 h))apparent activation energy of the ux (kJ/mol)apparent activation energy of the permeance(kJ/mol)gas constant (J/(molK))absolute temperature (K)effective membrane area (m2)

Fig. 1. Ppervapo

with N

weight of penetrant (g)permeation measuring time (h)

ters

e membrane selectivitypervaporation selectivityactivity coefcient of component i in the feedmembrane thickness (m)

scomponent i, either water or caprolactamactive layersupport layerwatercaprolactam

just by thin-lmdistillation, crystallization,melt crys-y suspension [9]. So its purication by PV has been

Zhang et al. [10], who tried to remove the water fromwater mixtures through a poly(vinyl alcohol) mem-linked with glutaraldehyde. A good separation factord though the permeation uxwas not very good,whichn obvious trade-off behavior in pervaporation perfor-

ove its separation efciency, membrane modicationsquiredwith both higher ux and selectivity. Among thedication methods [1113], polymer blending provesfective way to fabricate membranes with a favorableeffect of the two polymers [14,15]. Natural polymersreferred to synthetic polymers due to the abundant, biocompatibility, and commercial viability. SodiumaAlg) is a natural polysaccharide extracted from sea-h shows excellent water sorption properties [16,17].ten membranes made from a single polymer sufferingherent drawback of trade-off between ux and selec-Poly(vinyl pyrrolidone) (PVP) is an idea hydrophilicmaterial, which can easily blend with other organicc compounds. PVP is chosen as the additive to blend

out appreciexamples ureported insolvents an[19,20], PVreduce exccrosslinkingmeability o

PV compon a porousby achievinstrength. Ning the uusing NaAlpaper, a neis providedthe NaAlgand selectiindicates thprogress inthe normalcaprolactamthe membr

2. Experim

2.1. Materi

Sodiumdone) (PVPSinopharmin water) wtam (chemLtd (SINOPacrylonitril500 l/m2 hbTreatmentout furtheraqueous feepressure, concentration proles and mass transport in the compositemembrane.

, with the aim of increasing the permeation ux with-ably reducing the selectivity. Recently, many successfulsing blend membranes based on the PVP have beenthe eld of pervaporation for dehydration of organicd separation of azeotropic mixtures, such as CSPVPAPVP [21] and CAPVP blend membranes [22]. Toessive swelling, polymer blending is accompanied byand annealing which can all strongly inuence per-

r selectivity [23].osite membranes consist of thinner skin layer coatedsupport layer, which are often widely used in industryg a higher permeation rate and sufcient mechanicalowadays, no studies are available by both improv-x and separation factor for caprolactam dehydrationgPVP/PAN composite membrane. Therefore, in thisw composite membrane for caprolactam dehydrationand characterized. Experimental data showed that

PVP/PAN composite membranes had both higher uxvity for dehydration of caprolactamwater mixture. Itat our work shown in this manuscript makes some

separation of caprolactamwater mixture. Particularly,ized permeation uxes in terms of water permeance,permeance and selectivitywere calculated to evaluate

anes performance.

ental

als

alginate (NaAlg, Mw 204,000) and poly(vinyl pyrroli-, K-30, Mw 30,600) were both purchased fromChemical Reagent Co., Ltd. Glutaraldehyde (GA, 25wt.%as supplied by Aldrich Chemicals (USA). Caprolac-

ical pure) was obtained from Baling Petrochemical Co.EC, China). Porous ultraltration membrane of poly-e (PAN) (cut-off Mw 5104) with pure water ux ofarwas obtained from theDevelopmentCenter ofWaterTechnology (China). All the chemicals were used with-purication. Deionizedwaterwas used in preparing thed solutions for the pervaporation experiments.

244 T. Zhu et al. / Separation and Purication Technology 74 (2010) 242252

2.2. Preparation of crosslinked NaAlgPVP composite membranes

The technique of NaAlgPVP composite membrane preparationfollows a procedure reported in Ref. [24]. The casting procedure isas follows: rstly, formulation of casting solution, NaAlg (2 g) wasdissolved in 100ml of water with constant stirring. PVP particleswere dispersed in water, sonicated for 30min, added to the previ-ously prepared NaAlg solution. To this solution, HCl as a catalystand a certain amount of crosslinking agent (glutaraldehyde) wasadded and the reactionwas started. Blend solutions were preparedby mixing these solutions in different ratios (w/w), stirred for 24hat room temperature. After being ltered and keep overnight toremove the non-dissolved solids and bubbles, the polymer solutionwas cast on a clean Plexiglas using a casting knife.

Secondly, the preparation of composite membranes was done.Phase inversion is theusualprocedure for fabricationof asymmetricPAN at-sheet membranes. PAN porous ultraltration membranesas supported membranes, which had been treated with about4wt.% 1N sodium hydroxide solution for 24h, were washed andrinsed by about 4wt.% 1N hydrogen chloride solution and deion-izedwater till neutrality and air-dried. Then, theprepared solutionswere casted onto PAN porous ultraltration membranes held on aglass plate with the aid of a casting knife made in our laboratory.Themembranes in the gelatination statewere allowed to evaporateslowly till dried at room temperature. Finally, themembraneswereannealed in vacuum at temperature 80100 C for 1h for thermalcrosslinking.

2.3. Charac

2.3.1. ScannSEMwas

membranesof sputteredcomposite mHolland).

2.3.2. FouriThe inte

tion of blenNicolet AVArecorded w

2.3.3. X-ray diffraction (XRD)The XRD patterns of themembrane samples were characterized

by a Germany Bruker D8Advance X-ray diffractometer using Cu Kradiation. The angle of diffraction was varied from 8to 50 using astep size of 0.02.

2.3.4. Contact angle measurementsThe relative hydrophilicity of a surface can be qualitatively

determined by measuring the contact angle of a water drop (5l)depositedonto themembrane surface. Contact anglewasmeasuredby DSA100 instrument using static sessile drop method with goniometer (Germany, Kruss Company). To reduce evaporation effect,measurements were made as quickly as possible (less than 10 s).Furthermore, membranes were dried under vacuum desiccatorsbefore being tested.

2.4. Swelling experiments

The dry NaAlgPVP blend membranes with different massratios were weighed before being immersed in feed mixtures ofcaprolactamwater at 40 C in a thermostatic bath for 48h. Theswollenmembrane sample was taken out from the solution, wipedwith lter paper to remove the surface liquid, and then quicklyweighed. All experiments were repeated at least three times. Theresults were averaged.

The degree of swelling (DS, %) was calculated by:

= Ws W

Wd a, resp

rvap

2 reoratembt witfrom

am sa puredwt.%

Fig. 2. Schem heatercollecting bott ; T: teterization of membranes

ing electron microscopy (SEM)used to study themorphology of the various composite. All specimens were coated with a conductive layergold. The morphologies of the crosslinked NaAlgPVPembranes were observed with SEM (FEI Quanta 200,

er transform infrared (FTIR) spectroscopyraction between NaAlg and PVP, the crosslinking reac-d membrane with GA were both conrmed using theTAR 360 FTIR Spectrophotometer. FTIR spectra were

ithin the range of 4000500 cm1.

DS (%)

wherebranes

2.5. Pe

Fig.pervapsteel mcontacculatedupstreture bymonito3070

atic diagram of pervaporation apparatus. (1) Feed tank; (2) liquid level meter; (3)le; (8) liquid nitrogen cold trap; (9) buffer vessel; (10) vacuum pump; V15: valvesd

Wd 100 (1)

ndWs were theweights of the dried and swollenmem-ectively.

oration experiments

presents the schematic diagram of the experimentalion set-up. The membrane was installed in a stainless-rane cell with the effective surface area of 72.35 cm2 inh feed mixture. The feed solution was continuously cir-

a feed tank at a relatively high ow rate 200 l/h to theide of the membrane in the cell at the desired tempera-mp. The feed temperature in the range of 4060 C wasby a digital vacuometer and the feed solution containedcaprolactam. Pervaporation experiments were carried

; (4) circulation pump; (5) rotor ow meter; (6) membrane cell; (7)mperature control; P: vacuum pressure gauge.

T. Zhu et al. / Separation and Purication Technology 74 (2010) 242252 245

urfac

out bymainthe permeapump. Upoafter aboutcollected inweighted toate was detthe composgas chromafollowing ctemperatur[10]. The comeasuring tusing high-which can bgraph of refrefractomet

The perm

J (g/m2 h) =

where W isarea and t i

The sepa

= ywater/xwater/

where xwateof water an

In orderand the inmalized pe

ropoof thndiFig. 3. Morphology of NaAlgPVP composite membranes: (a) top s

taining atmospheric pressure on the feed side while onte side about 10 mbar within 1mbar with a vacuumn reaching steady-state conditions which was obtained

were pnaturesolutio1h throughout the experiments, permeate vapor wasliquid nitrogen traps with certain intervals (1h), andcalculate thepermeateux. Thecompositionofperme-ermined by GC to calculate the separation factor. Hereition of the condensed liquid was analyzed by a SP3400tographywith a FID detector (made in China) under theonditions: PEG-20M capillary column, 2m6mm i.d.;e, 170 C; carrier gas, nitrogen; ow rate, 30mlmin1

mpositions of the liquid feedmixtureswere analyzedbyhe refractive indexwithin an accuracy of0.0001 unitsprecision Abbe Refractometer (Atago NAR-3T, Japan),e calculated by using previously established standardractive index versus known mixture composition. Theer prism was maintained at 250.1 C.eation ux (J, g/m2 h) was dened as follows:

W (g)A (m2) t (h) (2)

the weight of penetrant, A is the effective membranes the measuring time.ration factor was calculated by:

ycaprolactamxcaprolactam

(3)

r, xcaprolactam and ywater, ycaprolactam are themole fractiond caprolactam in the feed and permeate, respectively.to distinguish between intrinsic membrane propertiesuence of the experimental operation conditions, nor-rmeation ux (permeance) and membrane selectivity

was comporesistancelayer. Theical potentexperimentThe permeapressures afor pervapo

Ji =Diki

(pi

On the bmeate side)

Ji = Qi(xiip

where Ji isbrane for ithemembrand the perthe feed liqin the feede, (b)(d) cross-section of membranes.

sed and evaluated to clarify the contribution by thee membrane to separation performance. Following theffusionmechanism, the overallmass transfer resistance

sed of the resistance of the whole membrane or theof the active layer and the resistance of the supportdriving force in pervaporation is a gradient in chem-ial across the membrane, which can be expressed inally measurable quantities such as partial pressures.te side is considered to be negligible if the downstreampplied are close to vacuum. Thebasic transport equationration can be written as [25]:

,feed pi,permeate) = Qi(pi,feed pi,permeate)

= Qi,active(pi,active pi,permeate)

= Qi,support(pi,permeate pi,permeate)(4)

asis of Raoults law (feed side) and Daltons law (per-it is equal:

0i,feed yippermeate) (5)

the permeation ux, Qi is the permeance of the mem-(which equals the permeability coefcient divided byane thickness), xi, yi are themole fraction of i in the feedmeate, respectively, i is the activity coefcient of i inuid and p0i,feed is the saturation vapor pressure of pure iliquid temperature, ppermeate is the permeate pressure.

246 T. Zhu et al. / Separation and Purication Technology 74 (2010) 242252

The activbe calculate

ln(1) = 1

ln(2) = 1

The p01,feedthe Antoine

log(p01,feed)

log(p02,feed)

where T inThemem

the membrpermeance

membrane =Fig. 4. Interaction between the blended polymers and the crosslink

ity coefcient of water (1) and caprolactam (2) cand by using Van Laar equation:

.5810[ 1.0429x1

1.0429x1 1.5810x2

]2(6)

.0429[ 1.5810x2

1.0429x1 1.5810x2

]2(7)

(water) and p02,feed (caprolactam) are determined fromequation:

= 8.07131 1730.63T + 233.426 (8)

= 6.78000 2344.00T + 273.150 (9)

degree Celsius (C).brane selectivity (membrane) is an intrinsic property of

ane material, which is dened as the ratio of the waterover the caprolactam permeance.

QwaterQcaprolactam

(10)

3. Results

3.1. Membr

3.1.1. SEMSEM im

branes arethe crosslinsurface andwith the PVpatibility bthe multilaclearly: anThe total thfor pervapothat a unifo35m is pand (d).

3.1.2. FTIRThe hom

tions werewas observing mechanism of NaAlg with GA.

and discussions

ane characterization

analysisages of the crosslinked NaAlgPVP composite mem-presented in Fig. 3. Fig. 3(a) shows the top surface ofked NaAlgPVP membrane. There are no aws in thethe cross-section of the NaAlgPVP blend membraneP content of 10wt.%, which indicates that the com-

etween NaAlg and PVP is quite good. From Fig. 3(b)yer structure of composite membrane is observed veryactive layer, a supported porous layer and a substrate.ickness of dry composite membrane NaAlgPVP/PANration is found to be about 80100m. It is observedrm NaAlgPVP thin dense layer with thickness of aboutroperly cast on the top of the PAN substrate in Fig. 3(c)

analysisopolymer solutions of NaAlg, PVP and their blend solu-optically clear. No phase separation or precipitationed even after keeping the mixture for a longer time at

T. Zhu et al. / Separation and Purication Technology 74 (2010) 242252 247

Fig. 5. FTIR of

ambient temblended poGA.

Based onNaAlgPVPFig. 5 showNaAlgPVPtaining diffepeaksarounstretching o(symmetricshowed peing the streand CN, rcrosslinkedstretching vtheweakenthe interactin the spectthe increasepronouncedbonyl groupsimultaneosuggest thabonding bePVP. The prcompatibili

3.1.3. XRDFig. 6 sh

membranesbroad peaktion peaksdispersive dthe NaAlg mtion of PVP,than the puthe polymesmall moleincrease inover the Na

RD of pure NaAlg membrane, PVP powder and NaAlgPVP blend mem-

Contact angle resultspervaporation performance of a composite membrane isto7, tPVPt haolarshowg wi

ellin

mbrabra

memen inbleng. Tincreof PVs expto swNaAlg membrane, PVP powder and NaAlgPVP blend membranes.

perature. Fig. 4 represents the interaction between thelymers and the crosslinking mechanism of NaAlg with

the above related reaction, the results of crosslinkedblend membranes can be analyzed by FTIR spectra.s the spectra of pure PVP, pure NaAlg, uncrosslinkedand GA crosslinked NaAlgPVP blend membranes con-rent PVP contents. The spectrum of NaAlg showed thed3389, 2934, 1605,1415, and1036 cm1, indicating thef OH, aliphatic CH, OC O (asymmetric), OC O), and COC, respectively. The spectrum of the PVPaks around 3446, 2967, 1650 and 1385 cm1 indicat-tching of OH, aliphatic CH, NC O (asymmetric),espectively. By comparing uncrosslinked with that ofblend membrane, it is clearly displayed that COCibrations around 1036 cm1 boost up connected withing of the relative intensity of theOHbands, indicatingion of GA with the blend membrane. It can be observedra of the crosslinked NaAlgPVP membranes that withof PVP content, the band at 3383 cm1 becomes morewith a slight shift to higherwave numbers and the car-at 1650 cm1 is shifted to higher wave numbers and

usly the peak intensity increased. These phenomena

Fig. 6. Xbranes.

3.1.4.The

relatedin Fig.NaAlgcontenmore pas it isbondin

3.2. Sw

Meof memblendare givin theswellinblendgroupbrane iis easyt NaAlg and PVP could form intermolecular hydrogentween the OH groups of NaAlg and the C O groups ofesence of such OH O C interactions implied a goodty of NaAlg and PVP in the blend membranes.

resultsows the effect of PVP content on the crystallinity of the. The XRD spectrum of the NaAlg membrane shows aat a diffraction angle (2) of 13.5 and two sharp diffrac-at 2 of 22.4 and 29. The PVP powder exhibits twoiffraction peaks at 2 of 12.4 and 21. It is found thatembrane has the highest crystallinity. With the addi-the blend membranes exhibit less crystalline domainsreNaAlg. This indicates that the amorphous regions andr chains exibility increase, therebymaking it easier forcules to transport through and possibly resulting in anpermeation ux of the NaAlgPVP blend membranesAlg membrane. Fig. 7. Effect othe hydrophilicity of the separation layer. As shownhe contact angles decrease with PVP content in theblend, which means that the membrane of higher PVPs a higher relative hydrophilicity, mainly because ofgroups of the unreacted carbonyl in blend membrane,n in FTIR. These free carbonyl groups form hydrogen

th H2O.

g results

ne swelling controls PVperformance.Hence, thedegreene swelling (DS) is important. The results of differentbranes in different feed mixtures (5070wt.% water)Fig. 8. It can be seen that the higher content of PVP

d membrane leads to the higher degree of membranehis suggests that the hydrophilicity of the NaAlgPVPased with increasing PVP content. The free carbonylP forms hydrogen bonding with H2O when the mem-osed to caprolactamwatermixtures, so themembraneell. Furthermore, the XRD results indicate a more ex-f PVP content on the contact angle of NaAlgPVP blend membranes.

248 T. Zhu et al. / Separation and Purication Technology 74 (2010) 242252

Fig. 8. Effect obranes for 50,

Table 1Effect of PVPperformance oat 323K).

NaAlgPVP m

NaAlg/PVP m

100/090/1080/2070/30

ible membrincreases, tswollen.

3.3. Pervap

3.3.1. Effectperformance

Table 1on permeat323K. The ieffect on bocontent incwhichwas cin the feedmthe changinmer chainsof PVP. Whseparation fto 930.5whtor rstly inhad a morecould signi

as it has strong polarity and hydrophilic groups. The formation ofhydrogen bonding between the membranes and water was hencepromoted. Therefore the afnity between water and membranesincreased accordingly, which was conrmed by the contact anglemeasurement. The dropped separation factor may be caused bythe exceeding swell membranes and increase of free volume. Apromising result achieved for the NaAlgPVP blend membraneswas that both the permeation ux and selectivity went up simulta-neously with the increase of PVP content in the range of 020wt.%,which seemed not to be in accordancewith the trade-off rule thatcommonly existed between the selectivity and the ux [23]. Whenthe blend membrane contains 20wt.% PVP and 80wt.% NaAlg, themembrane shows the highest separation factor of 1610.6 and a per-meate ux of 1634.4 g/(m2 h). The reason should be that the effect

ogence sttotcap

site. Altd prsslinhow

Effecteffeanc

of 20ly then the0wtanges sontencaning f/(m2

Table 2Comparison of

Membranes

PVA crosslinPVA/PAN coPVA/PES com

GA crosslinkf PVP content on the degree of swelling of NaAlgPVP blend mem-60, 70wt.% caprolactam aqueous solution.

concentration in the NaAlgPVP coating solution on pervaporationf the NaAlgPVP/PAN compositemembrane (for 50wt.% caprolactam

embranes Pervaporation results

ass ratio GA content (wt.%) J (g/(m2 h))

0.5 534.7 1123.70.5 1473.6 1509.30.5 1610.6 1634.40.5 930.5 2012.4

ane structure. As the water concentration in the feedhe amorphous regions of the membrane become more

of hydra balan

The50wt.%compoTable 2rials anthe crostudy s

3.3.2.The

performbranenot ondent o30 to 7tion chaqueoutam co[10]. Ittion ris2552goration performances

of PVP content in blend membranes on pervaporationsshows the effect of PVP content in blend membranese ux and separation factor for 50wt.% caprolactam atncorporation of PVP into NaAlg matrix had a signicantth permeation ux and separation factor.When the PVPreased from 0 to 30wt.%, the permeation ux increasedonsistentwith the swelling behavior of themembranesixture. Another important factor affecting the uxwasg trend of crystallinity. The free volume and the poly-exibilityof themembranes increasedwith theadditionereas the PVP content increased from 0 to 20wt.%, theactor increased from 534.7 to 1610.6 and then droppeden the PVP content reached 30wt.%. The separation fac-creased because the membrane of higher PVP contentcompact network. Furthermore, introduction of PVPcantly enhance the hydrophilicity of the membranes

The higherthe blend mwhich hasthe blend min the feedare more swresulting indiffusion thwereboth idecreased.processes [

3.3.3. EffectTempera

poration beHence it caThe effect ois revealedwater conce

composite membrane separation performance with literatures.

Thickness(m)

Caprolactam infeed (wt.%)

Temp

ked with 0.5wt.% Gal 2535 50 50mposite membranes 80100 50 55posite membranes 1105 50 55ed NaAlgPVP composite membrane 80100

50 5050 55bonding and swelling on separation factor could reachate within this range.al ux and selectivity from the present study (forrolactam at 50, 55 C) were compared with othermembranes reported in literatures and presented inhough the membranes are made from different mate-eparation techniques, the dehydration performance ofked NaAlgPVP composite membrane prepared in thiss a comparable, good ux and selectivity.

of feed compositionct of caprolactamwater composition on pervaporationes tested at 323K by using the NaAlgPVP blend mem-wt.% PVP is shown in Fig. 9. These gures indicate thatux but also the separation factor are strongly depen-feed composition. The feed concentration range from.% of water was chosen by considering the concentra-s from the triple-effect evaporation sets. In addition,lution becomes stiff and saturated when the caprolac-t in feed solution is above 70wt.% at room temperaturebe seen from Fig. 9(a) that with the water concentra-rom 30 to 70wt.% the total ux increased from 843 toh) while the selectivity decreased from 2895 to 721.uxes can be explained by the stronger swelling inatrix due to the strong afnity of PVP toward water,

been observed in the swelling experiments. Althoughembranes are crosslinked, as the water concentrationincreases, the amorphous regions of the membraneollen and the polymer chains become more exible,both water and caprolactam molecules more easily

rough membranes. So the water and caprolactam uxncreased (as shown inFig. 9(b)) but the separation factorThis trade-off was generally observed in pervaporation28,29].

of feed temperatureture is an important operating parameter in perva-cause it affects both the sorption and diffusion rates.n signicantly affect the performance of membranes.f feed temperature on the pervaporation performancesin Fig. 10 for the different PVP content and the feedntration of 50wt.%. When temperature increased from

erature (C) Total uxg/(m2 h)

Separationfactor

Reference

900 575 [26]1802 890 [10]790 200 [27]

1634.4 1610.6Present work2220.5 1354.7

T. Zhu et al. / Separation and Purication Technology 74 (2010) 242252 249

Fig. 9. Effect of the feed concentration on the pervaporation performances through crosslinked NaAlgPVP composite membranes with 20wt.% PVP at 323K.

313 to 338K, the uxes increased continuously (Fig. 10(a)) but theseparation factors and water concentration in permeate side bothdecreased (Fig. 10(b) and (c)). According to free volume theory, anincrease in temperature increases thermal mobility of the poly-

mer chains, which generates extra free volume within the polymermatrix, resulting inmorewater and caprolactammolecules perme-ating to themembrane. In addition, the vapor pressure ofwater andcaprolactam in the feed mixture increased with increasing of the

Fig. 10. Effectcaprolactam.of the feed temperature on the pervaporation performances through crosslinked NaAlgPVP composite membranes with different PVP contents at 50wt.%

250 T. Zhu et al. / Separation and Purication Technology 74 (2010) 242252

aqu

feed tempenot affectedtemperaturmolecules wthese lead twith feed teseparationin permeate

The temmeance can

Ji = J0 exp(

Qi = Q0 exp

where J0, Qactivation emeance, resis absolute

The diffetion HV, e

HV = EJ

Fig. 11NaAlgPVPfound that tmeance agrvalues EJ acare present

As can bwhereas EQ

raneergyed. Aal anencant p

inTof wad areloweus reantsan bfrom

hanore, mort aactivintricompablyx andd teors con

paraondiin Ta

actam

Table 3Activation ene

NaAlgPVP m90/1070/30

Caprolactam6070Fig. 11. Temperature dependence of (a) permeance at 40wt.% caprolactam

rature, and the vapor pressure at the permeate sidewas. As a result, the driving force increased with the feede rising. The diffusion rate of water and caprolactamas enhanced, leading to high permeation ux. All of

o an increase in ux and a decrease in separation factormperature increasing. It is worth pointing out that thefactors decrease sharply but the water concentrationsside decrease slightly [30].

perature dependence of the pervaporation ux and per-be expressed by Arrhenius equations [31,32]:

EJRT

)(11)

(EQ

RT

)(12)

0, EJ and EQ are the pre-exponential factor, apparentnergy of the permeation ux and the membrane per-pectively. R indicates the gas constant (J/(molK)) and Ttemperature (K).rence between EJ and EQ is the molar heat of vaporiza-xpressed as follows.

EQ (13)

shows Arrhenius plots (lnQ versus 1/T) throughcomposite membranes according to Eq. (12). It can behe temperature dependence of the pervaporation per-ees well with the Arrhenius relationship. The evaluatedcording to Eq. (11), EQ and HV according to Eq. (13)ed in Table 3.

membthe enincreasmateridependpenetrPVP.

Alsouxesthe feetinctlyand thpenetr

As cculatedlower tThereftransplowerby the

Byremarkthe uthe feebehavidependand seating cshowncaprole seen in Table 3, EQ of water is observed to decreaseof caprolactam increases a litter, comparing the blend

membranesconsistent w

rgy data for blend membranes at 40wt.% caprolactam and for feed solutions at 30wt.% P

Activation energy (kJ/mol)

EQ EJ

Water Caprolactam Total Water

ass ratio32.45 44.66 55.12 55.1018.91 47.90 41.62 41.55

in feed (wt.%)16.24 53.86 32.90 32.8317.43 32.71 38.75 38.72eous solution, (b) permeance for 30wt.% PVP.

of 10wt.% PVP with 30wt.%. These results suggest thatbarrier for water has decreased while caprolactamnd then water can more readily transport though thed the permeance of water has become less temperaturee, and vice versa. Duo to the EQ decreases for water, theermeance should increase after increasing content of

able 3, the total activationenergies calculated fromtotalter and caprolactam for 60 and 70wt.% caprolactam in32.90 and 38.75kJ/mol, respectively. The former is dis-r than the latter, this is due to the membrane swelling,sults in a lowered permeation activation energy for theto transport through the membrane material.e seen in Table 3, the activation energies of water cal-

either the ux (EJ) or the permeance (EQ) are muchthoseof caprolactamfor thedifferentblendmembranes.

ore energy is required for caprolactam molecules tocross the membrane at the same conditions [33]. Theation energies of water than caprolactam are reectednsic properties of hydrophilic membrane materials.aring EJ with EQ, it can be found that the former arehigher than the latter, which in turn indicates thatthe separation factor are more strongly dependent on

mperature than the permeance and selectivity. Thesean be explained that permeance and selectivity onlymembrane intrinsic properties for evaporation but uxtion factor are also dependent on experimental oper-tions for both evaporation and solution/diffusion. Asble 3, the molar heat of vaporization HV for water orare almost the same between the two different blend

or caprolactam concentrations investigated, which isith the previous reports in the dehydration of aqueous

VP.

HV (kJ/mol)

Water Caprolactam

Caprolactam

91.78 22.65 47.1291.90 22.64 44.00

96.06 16.59 42.2077.62 21.29 44.91

T. Zhu et al. / Separation and Purication Technology 74 (2010) 242252 251

Fig. 12. Effectselectivity and

alcohol syst[34].

3.3.4. PermThe per

described inquantitiesobscures thcess [25]. Sodescribe thuxes are r(see Eq. (4))water andcentration iillustratedthe permeaously withhighly hydrin the highblend memTable 3), coIt is interesincrease ofreveals a cmeation thof the feed concentration and temperature on the pervaporation performances in termseparation factor through crosslinked NaAlgPVP composite membranes with 20wt.% P

ems through lled-hydrophilic polymeric membranes

eance and selectivityformance of pervaporation membranes is typicallyterms of permeation ux and separation factor. These

depend heavily on the operating conditions, whiche role of the driving force in the pervaporation pro-the permeance and membrane selectivity are used to

e intrinsic membranes performance. Partial permeateequired to calculate the permeance of the components. When the blend membrane contains 20wt.% PVP, thecaprolactam permeances as a function of water con-n the feed at the different operating temperatures arein Fig. 12. It can be seen from Fig. 12(a) and (b) thatnces of both water and caprolactam increase continu-increasing water feed concentration. This implies theophilic NaAlg and PVP membrane material, resultinger degree of swelling. The sorption of the NaAlgPVPbrane increases and the EQ decreases for water (fromrresponding to increase in the permeances of water.ting that caprolactam permeance also increases. Thiscaprolactam permeance with water feed concentrationoupling effect between caprolactam and water per-rough the membrane [33]. That is to say, membrane

swelling orties and rescomponentdependencfrom the EJmeance ispermeant increasing tture dependactivity coetemperaturtrates alsocompared wthe saturatimore to thances is dueffects menand Gmehlwater/isopr

The diffein Fig. 12(cperature radecreasingof both sepexcessive sws of (a) water permeance, (b) caprolactam permeance, (c) and (d)VP.

plasticization effect changes the membrane proper-ults in the facilitation of transport of the caprolactamthrough the membrane. But no obvious temperature

e is observed, which is in accord with the conclusionhigher than EQ. Because the temperature effect on per-complicated. The reason may arise from the fact thatux and permeant driving force are both increased withemperature. The driving force combines two tempera-ent factors: i and pi,feed. When temperatures rise, thefcients ( i) of penetrates are quite close at differentes but the saturation vapor pressure (pi,feed) of pene-increases. Since the downstream pressure is very lowith the upstream pressure, it can be neglected. Thus,

on vapor pressure of the feed composition contributese driving force. Consequently, the increase in perme-e to the combination of uxes and the driving forcetioned above. A similar behavior was observed by Sanzing [35] for the membrane PERVAP 2201 for binaryopanol mixtures.rence of separation factor and selectivity is compared) and (d). With the water concentration and feed tem-ising both the separation factor and selectivity revealtrend. The effect of water concentration on decreasesaration factor and selectivity is easily explained by theollenof this highlyhydrophilicmembrane, resulting in

252 T. Zhu et al. / Separation and Purication Technology 74 (2010) 242252

enlarging interstitial space of polymer chains and declining separa-tion performance. The effect of temperature may be caused by theinteractions between permeating molecules and membrane. Theswollen membrane matrix at higher temperature also facilitatesthe transport of caprolactam along with water molecules, therebythe separation factor decreases. Compared with separation factorversus selectivity from Fig. 12(c) and (d), in spite of the similaritybetween the separation factor and selectivity plots, some differ-ences still cmay be resuintrinsic meoperating cmembraneimental opreect intri

4. Conclus

The mepoly(vinylhyde. The icrosslinkingconcentratiNaAlgPVPhydrophiliction of thecaprolactamperformancsolutions. Itthe hydroppermeationincreases frOf all the bPVP was thand separatexcluding thationmadecontributioformance mindicated thdehydration

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Study of pervaporation for dehydration of caprolactam through blend NaAlgpoly(vinyl pyrrolidone) membranes on PAN supportsIntroductionExperimentalMaterialsPreparation of crosslinked NaAlgPVP composite membranesCharacterization of membranesScanning electron microscopy (SEM)Fourier transform infrared (FTIR) spectroscopyX-ray diffraction (XRD)Contact angle measurements

Swelling experimentsPervaporation experiments

Results and discussionsMembrane characterizationSEM analysisFTIR analysisXRD resultsContact angle results

Swelling resultsPervaporation performancesEffect of PVP content in blend membranes on pervaporation performancesEffect of feed compositionEffect of feed temperaturePermeance and selectivity

ConclusionReferences