Journal of Membrane Science 372 (2011) 154–164
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Journal of Membrane
Science
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m e m s c
i
Preparation,characterization and permeation property of Al 2O 3,Al 2O 3–SiO 2and Al 2O 3–kaolin hollow fiber membranes
Ling-Feng Han a ,b ,Zhen-Liang Xu a ,b ,∗,Yue Cao b ,Yong-Ming Wei b ,Hai-Tao Xu b
a State Key Laboratory of Chemical Engineering,East China University of Science and Technology (ECUST),130Meilong Road,Shanghai 200237,China b
Membrane Science and Engineering R&D Lab,Chemical Engineering Rearch Center,ECUST,130Meilong Road,Shanghai 200237,China
a r t i c l e i n f o Article history:
Received 20November 2010
Received in revid form 29January 2011Accepted 31January 2011
Available online 24 February 2011Keywords:Alumina Silica Kaolin
Preparation
Hollow fiber membrane
a b s t r a c t
Al 2O 3,Al 2O 3–SiO 2and Al 2O 3–kaolin hollow fiber precursors were prepared by a wet-spinning method using polyethersulfone (PES),Al 2O 3,SiO 2and kaolin.The hollow fiber membranes were obtained after a preheating and sintering combining process.The inorganic hollow fiber membranes had been characterized by SEM,XRD,porosity,density,mechanical property,permeation property and the mean/maximum pore size as well as pore size distribution,to investigate the effects of SiO 2and kaolin on ␣-Al 2O 3hollow fiber membranes.The morphology of precursors depended on the particle contents and PES concentrations in the dispersion,and the final structure of sintered membranes were controlled by the precursors.XRD results showed that Al 2O 3remained as ␣-Al 2O 3before and after being sintered while SiO 2changed from tridymite to cristobalite from ambient temp
erature to 1450◦C,and at higher temper-ature it would react with Al 2O 3to produce the stoichiometric 3:2mullite (3Al 2O 3·2SiO 2,or Al 6Si 2O 13)by solid state reaction.The effect of SiO 2and kaolin on the membrane properties depended on both the ratio of Al 2O 3:SiO 2/kaolin and the sintering temperature.Moreover,when sintering temperature was 1600◦C,Al 2O 3–kaolin-5(Al 2O 3:kaolin =1:1)achieved a mean pore size of about 0.5m.
© 2011 Elvier B.V. All rights rerved.
1.Introduction
Inorganic membranes have attracted more and more attentions becau of its endurance to harsh environment such as high tem-perature,strong acid and alkali,as well as corrosive systems [1–3].Among all kinds of inorganic membranes,Al 2O 3membranes are the most widely ud substrate for its stability and availability [4–6].In the various geometric shapes of membranes,hollow fibers are con-tinuously concerned due to their exceptionally high surface area per volume,making membrane units have a much higher pack-ing density in comparison to flat-sheet and tubular membranes [7–9].
The most widely ud method to prepare Al 2O 3hollow fiber membrane is a combination of pha inversion method and sin-tering process.For instance,Li et al.(UK,using 0.01,0.3and 1m Al 2O 3po教师工资多少钱一个月
wder as inorganic materials)[10–14],Jong et al.(Netherlands,using 0.7m and 0.29m Al 2O 3powder)[15],Tan et al.(China,using 0.6m LSCF powder)[16],and Lee et al.(Korea,using 8–15m Ni powder)[17]prepared ceramic hollow fiber
∗Corresponding author at:Membrane Science and Engineering R&D Lab,Chemi-cal Engineering Rearch Center,ECUST,130Meilong Road,Shanghai 200237,China.Tel.:+862164252989;fax:+862164252989.
E-mail address:chemxuzl@ecust.edu (Z.-L.Xu).membranes through this method.Besides,Li et al.[4]adopted a pha inversion method combined with the reaction bonded alu-minum oxide (RBAO)process using 0.5m Al 2O 3and 5–50m Al powder as raw materials,suggesting the lower melting point and oxidation reaction of Al during sintering process improved the property of sintered hollow fibers.All the works indicated the application of different particle sizes and reactions between the raw materials would be beneficial for membrane preparation.
Aksay and Pask [18]studied the equilibria in Al 2O 3–SiO 2sys-tem,indicating after sintered at high temperature mullite was the only stable compound of this system.Moreover,mullite grown by a solid state reaction appeared to be in the right order and of the 3:2type (3Al 2O 3·2SiO 2)and tho forme
d in the liquid pha by exsolution appeared not to,and is of 2:1type (2Al 2O 3·SiO 2).Kaolin is one of the most popular raw materials for aluminosilicate-bad ceramics,due to its common occurrence and good availability.It’s well-known that kaolin will undergo some reactions during the sin-tering process.The reaction of kaolin with alumina is of interest in the production of mullite by reaction sintering [19–22].
However,few papers reported the effects of the employed SiO 2and kaolin on Al 2O 3hollow fiber membrane properties,such as mechanical strength and pore size distribution.Therefore,the vast studied reaction of ␣-Al 2O 3–SiO 2[18]and ␣-Al 2O 3–kaolin [19,20]system in ceramic field was adopted in this work,to make some improvements of the Al 2O 3hollow fiber membranes,bad on the
0376-7388/$–e front matter © 2011 Elvier B.V. All rights rerved.doi:10.sci.2011.01.065
L.-F.Han et al./Journal of Membrane Science372 (2011) 154–164155 following two reactions[21,22]:
2SiO2·Al2O3·2H2O+2Al2O3>1050◦C
−→3Al2O3·2SiO2(mullite)
+2H2O(R1)怎么砍价
3Al2O3+2SiO2>1300◦C
−→3Al2O3·2SiO2(mullite)(R2) This rearch work introduced the above reactions in the prepa-ration process of Al2O3hollowfiber membrane.Series of Al2O3, Al2O3–SiO2and Al2O3–kaolin hollowfiber membranes were pre-pared and characterized to e the effects of SiO2and kaolin on the Al2O3hollowfiber membrane and also how the components of membrane dope solution,particle suspension as well as sintering temperature would affect the membrane property.
1.1.Particle suspensions
Membrane dope solution is very important for membrane for-mation.In this work,it could be approximately considered as a dispersion of Al2O3,SiO2,and kaolin.Dispersion system,a state with higher free energy than aggregated state,will occur without a substantial energy barrier preventing aggregation.With such a barrier,the system could be stable for a certain time.Generally,the dispersion will remain stable when the particle interaction energy barrier is big enough[23].Among the four interaction energies van der Waals interaction energy,electrostatic interaction energy, steric interaction energy and depletion interaction energy,Lu[24] found the van der Waals attraction was th
马蹄雪梨汤e dominant mechanism in destabilizing the dispersion system,while the steric repulsion was a more effective stabilization factor than the electrostatic repul-sion.One way to enhance the particle–particle repulsion is the introduction of polymer chains.However,too little polymer will cau bridgingflocculation while too much will cau depletion flocculation[25,26].
Ogden and Lewis[27]and Li and Lewis[28]studied the adsorbed and non-adsorbed polymer effects on particle suspension and obtained better stability at very low free polymer concentrations. Ideally,the adsorbed polymer layer should be just thick enough to hold the van der Waals attraction.In this work,the allowed addition evidently became more difficult with more SiO2particles.However, the particle size difference between Al2O3and SiO2will reinforce the suspension(as shown in Fig.1),due to the incread particle distance of Al2O3.When introducing kaolin powder,the stability and rheology property of the membrane dope would be reduced due to the higher specific gravity of kaolin powder.Actually,Liu et al.[10]found membranes with different sizes of Al2O3particles (0.01,0.3,and1m)prented much higher bend strength and lower gas permeability.
1.2.Sintering process
Sintering is a densification technology of powder green body by thermal energy,during which process the macroporous pre-cursor will be changed into ceramic material with smaller surface area and porosity,and the mechanical property will be excessively improved.Generally,the sintering technology is divided into two types,viz.solid state sintering and liquid pha sintering.In the Al2O3–SiO2system,cristobalite could be formed and the formation of this intermediate cristobalite is detrimental to thefinal densifi-cation of mullite at around1600◦C.Liu’s work[19]revealed that the cristobalite was formed when silica in kaolin was heated to 1300◦C,but it was retarded between1250◦C and1350◦C and was totally prohibited above1380◦C due to the reaction of kaolin with ␣-Al2O3to form condary mullite.The reaction of kaolin with␣-Al2O3was initial at about1250◦C.It became quite extensive
above Fig.1.Schematic prentation of the effect of the addition of SiO2on Al2O3disper-sion and sintered membranes.
1380◦C and was extremely fast at1600◦C and above.Moreover,it is well-accepted that mullite crystalsfirst appeared at980◦C with the exsolution of silica.
However,there were still some limitations in the preparation process,primarily rearcher could not add a large amount of SiO2and kaolin to the membrane dope.This was becau:(1)the decrea of particle size resulted in some new problem of mem-brane dope such as agglomeration and aggregation;(2)the reaction of SiO2/kaolin and Al2O3reduced the amount of effective parti-cles;and(3)due to the technological limitation in spinning process, excessive addition of kaolin might not be allowed.Therefore,the maximal ratio of SiO2and Al2O3was only2:3,and the maximal ratio of kaolin and Al2O3was1:1in this work.
2.Experiment
描写节日的诗句2.1.Materials
The polymer binder,PES(characteristic viscosity:Á=0.48dL g−1, =1.370g cm−3)was produced by Jilin
Jida High Performance Materials Co.,Ltd.(China).The solvent,N-methyl-2-pyrrolidone(NMP)was purchad from Shanghai Chemical Reagent Company(China).Polyvinylpyrrolidone(PVP)(K90)was supplied by BASF Auxiliary Chemicals Co.,Ltd.(Germany).␣-Al2O3 powder(MC2A,150–300nm)was bought from Hongsheng Mater Sci and Tech Co.,Ltd.(China).SiO2(Aerosil R972,about16nm) and kaolin(300–500nm,containing47%O,26%Al,24%Si,1% S and2%K by EDS results)were produced by Evonik Degussa Chemicals(Germany)and Shanghai Nahui Desiccant Reagent Company(China),respectively.All water ud in this work was lf-prepared deionized water.
2.2.Membrane precursor preparation
PES/NMP solution was prepared by adding5.0–9.0wt.%PES slowly to38.5–56.5wt.%NMP in a500mL three-neck reactionflask. After the polymer solution was homogeneous,PVP as an additive was introduced into the solution.Then a given amount of Al2O3, SiO2or kaolin powder was added into the polymer solution very
156L.-F.Han et al./Journal of Membrane Science 372 (2011) 154–164
Table 1
Composition of Al 2O 3,Al 2O 3–SiO 2,and Al 2O 3–kaolin hollow fiber precursors.Precursor no.PES (wt.%)NMP (wt.%)PVP (K90)(wt.%)Al 2O 3(wt.%)SiO 2(wt.%)Kaolin (wt.%)Total particle (wt.%)Al 2O 3:SiO 2or Al 2O 3:kaolin Al 2O 3-19.055.50.534––34–Al 2O 3-28.050.50.541––41–Al 2O 3-37.044.50.548––48–Al 2O 3-4 6.038.50.555––55–Al 2O 3-5
5.044.50.550––50–Al 2O 3–SiO 2-1 5.044.50.5455–509:1Al 2O 3–SiO 2-2 5.04
般若波罗蜜心经6.50.5408–485:1Al 2O 3–SiO 2-3 6.055.50.52315–383:2Al 2O 3–kaolin-1 5.044.50.545–5509:1Al 2O 3–kaolin-2 5.044.50.540–10508:2Al 2O 3–kaolin-3 5.044.50.535–15507:3Al 2O 3–kaolin-4 5.044.50.530–20506:4Al 2O 3–kaolin-5
5.0
44.5
0.5
25
–
25
50
5:5
slowly,with a mechanical stirring at a suitable speed to ensure all the particles well-disperd in the polymer solution.Noticeably,for membrane dopes with SiO 2or kaolin particles,to obtain homoge-neous suspension,the content of PES was decread to 5.0–6.0wt.%in the polymer solution,due to the much smaller particle size of SiO 2and higher specific gravity of kaolin.
The dope solution was stirred for at least 24h to obtain a homo-geneous suspension.Then,the dope solution was transferred to an air-proof steel container and degasd at moderate temperature (70◦C).The membranes were labeled as shown in Table 1.
Finally,the degasd dope solution was pressurized by nitrogen.A tube-in-orifice spinneret (with inner diameter 1.0mm and outer diameter 2.5mm)was ud to obtain hollow fiber precursors by wet-spinning method at room temperature.Deionized water was ud as both bore fluid solution and coagulation bath.The details of the wet-spinning process of hollow fiber spinning have been describe
d elwhere [29].The fabricated hollow fibers were kept in water for 24h to remove the residual solvents,and then stored in 40wt.%glycerol aqueous solution to prevent the damage of mem-brane structure during air-drying process [30].Then the precursors were dried at ambient and stored in a clean,dry and well-ventilated room.
2.3.Sintering process
The preheating and sintering process were carried out in an electric furnace.There were veral steps of heat prervation for 1–2h:to vaporize the residual water and solvent on the membrane surface at 100◦C;to remove the residual water and solvent in the membrane pore structure at 200◦C;to decompo PES chains at 500–600◦C;to preheat particles at 1200◦C;and all the five steps was carried out by a heating rate of 2◦C min −1.While a much slower rate 1◦C min −1was adopted to finish the crystal transformation and particle bonding at an aimed temperature,viz.1450◦C or 1600◦C in this work.Some inorganic hollow fiber membranes are shown in Fig.2.2.4.Characterizations
2.4.1.Morphology obrvation
The membrane morphology was examined by a scanning elec-tron microscopy (SEM)(JEOL Model
JSM-6360LV,Japan).
The
Fig.2.Some of the prepared inorganic hollow fiber membranes.
membranes were broken and deposited on a copper holder.All samples were coated with gold under vacuum before tested.2.4.2.X-ray diffraction (XRD)analys
To obrve the possible crystal differences of the precursors and membranes,the XRD patterns of the membrane were recorded on a D/max-rB diffractometer (Rigaku,Japan)equipped with graphite monochromated Cu K ␣radiation ( =0.15405nm)operated at 50mA and 50kV from 10◦to 40◦.
2.4.
3.Membrane density and porosity measurement
A wet membrane was weighed after immerd in pure water for 48h and wiped by tissue quickly.Then the samples were dried under vacuum oven above 100◦C until a constant weight in a dry state was achieved.The density of the hollow fiber membranes could be calculated using Eq.(1): =
m dry V
飞黄腾达什么意思
=
m dry
( /4)(D 2−d 2)l
(1)
where was defined as the density (g m −3),m dry was the weight of the dry hollow fiber samples (g),D was the outer diameter (m),d was the inner diameter (m),and l was the effective length of the samples (m).
The membrane porosity ε(%)was defined as the volume of the pores divided by the total volume of the porous membrane.It could usually be determined by gravimetric method,bad on the weight of liquid water contained in the membrane pores,as shown in Eq.(2)[31]:ε=
m
V · H 2O ×100%=m wet −m dry ( /4)(D 2−d 2)l H 2O
×100%
(2)
where m wet was the weight of wet membrane (g), was the water density (1.0g cm −3),while m dry ,D ,d and l were the same as in Eq.(1).
2.4.4.Mechanical property measurement
The flexure strength was determined by the three-point bend-ing technique using testing equipment (ZWICK/Z010,Germany),in which process,the loading rate was 1mm min −1and the span was 30mm.Each membrane was tested for 3times and then averaged.2.4.5.Permeation property test
The membrane permeation property was characterized by pure water flux (PWF ).The PWF was measured using a dead-end flowing cell connected to compresd air to apply the feed pressure.The details of testing process were exhibited in Fig.3.The filtration experiments were carried out at room temperature and the hollow fiber membranes were pre-presd at 0.1MPa (1.0bar)using pure water for 1h before measurement,and PWF would be obtained by
L.-F.Han et al./Journal of Membrane Science372 (2011) 154–164
157
Fig.3.Dead-endflowing cell ud in pure waterflux measurement. Eq.(3).
PWF=
Q
A·t· P
(3)营业收入利润率计算公式
where PWF was the permeationflux of membrane for pure water (L h−1m−2bar−1),Q was the volume of the permeated pure water (L),A was the effective area of the membrane(m2),here calculated by the inner diameter of hollowfibers,t was the permeation time
(h)and P was the operation pressure,here it was1.0bar.
2.4.6.Pore size calculation and pore size distribution
measurement
The maximum pore size was characterized by the bubble point pressure(P,MPa)test.Hollowfibers with one end aled were immerd in pure water for24h and then presd by N2;when the first bubble came through the membrane wall,the instantaneous pressure was recorded,namely the bubble point pressure.Each membrane was tested for at least3times and the averaged value was obtained.Thus,the maximum pore size(r,m)was calculated by the following equation[32]:
r=2 cosÂ
P
(4)
where was the surface tension of water at20◦C, 72.75×10−3N m−1,Âwas the contact angle of water to the membrane and P was the bubble point pressure.
Mean pore size was determined by thefiltration velocity method.It was a method for the permeation and related withfil-tration velocity and other important structure parameters of the membrane.According to Guerout–Elford–Ferry equation[33],r m could be calculated by:
r m=
(2.9−1.75ε)×8ÁlQ
ε·A· P
(5)
whereÁwas water viscosity(8.9×10−4Pa s)and l was the mem-brane thickness(m).
SEM image analysis was adopted to obtain the pore size dis-tribution of hollowfiber membranes.This method has been ud in Maximous’s[34]work to describe the Al2O3distribution in the prepared membrane.In this work,Al2O3-5,Al2O3–SiO2-1,and Al2O3–kaolin-1was lected becau the only difference among them was5wt.%of Al2O3,SiO2and kaolin respectively.3.Results and discussion
3.1.Morphologies of precursors and membranes
From the images of the membrane cross-ctions shown in Fig.4,there were littlefinger-like structures in membrane wall. This was becau the multitude particles reduced the exchange rate between solvent and non-solvent.With further magnifica-tion of the cross-ction,linkage of polymer chains and particles could be evidently obrved,which indicated the particles were ttled in the membrane structure by the interaction of polymer chains.Moreover,the surfaces were varied accordi
ng to different membranes.
Al2O3-4possd less particle aggregations as well as surface defect on the cross-ction structure,though the solid loading was much higher than Al2O3-5.Becau the amount of PVP was very low,it could be assumed that the effect of PVP was negligible. The concentration of PES in Al2O3-4membrane dope was higher, and more polymer chains could have enhanced the steric repul-sion in the dispersion system.On the other hand,the polymer chains were adsorbable for particles ttling along the polymer chains,which would alleviate the aggregation.Moreover,parti-cles were well packed on the inner surfaces of both Al2O3-4and Al2O3-5polymer layers,though they were quantitatively more for Al2O3-4,due to the higher particle content.Nevertheless,the outer surface of Al2O3-4and Al2O3-5were comparatively dis-tinct.Al2O3-4possd much more naked particles than Al2O3-5, owing to higher solid loading as well as the effect of extrusion stress.
8g SiO2particles and2g NMP replaced10g Al2O3in the com-position of Al2O3–SiO2-2as compared with Al2O3-5.Although the solid loading was reduced,the particle volume greatly incread due to the much smaller particle size of SiO2.The particle dis-persion was quite homogeneous without visible defect across the membrane structure.However,small amount of aggregations eas-ily formed with nanoparticles with extremely high surface area and high surface energy.The aggregation formati
on would reduce the surface energy,conquently enhancing the system stability.
Morphology of Al2O3–kaolin-1was disparate with the former ones.Similarly,the polymer layer was much more compact on the inner and outer surfaces,with few naked particles.This might be caud by the inherent binder property of kaolin[20],which is fre-quently applied in ceramic preparation for its excellent molding behavior.Some amount of kaolin,took part in inorganic binding of the membranes,therefore promoted both particle dispersion and membrane formation.However,large quantity of kaolin would
158L.-F.Han et al./Journal of Membrane Science
372 (2011) 154–164
Fig.4.SEM photos of Al 2O 3,Al 2O 3–SiO 2and Al 2O 3–kaolin hollow fiber precursors prepared by wet-spinning method.越狱是什么
deteriorate the homogenous dispersion system and induce defects in membrane structure.
Obviously different from the precursors,there was no polymer chain residual in the membranes after sintering process as shown in Fig.5.Moreover,particles bonding with each other would provide the basic mechanical strength.The structure did not change much from the precursors,indicating that the membrane morphology was determined by the wet-spinning process.Similar conclusion was obtained by Kingsbury and Li [14].However,the morphologies were distinct due to different particle types and contents.
From the photos of Al 2O 3-1to Al 2O 3-4,scarcely difference in cross-ction structure could be obrved.However,the membrane appearance changed greatly with solid loading.The membrane inner surface became denr from Al 2O 3-1(34wt.%Al 2O 3)with many defects to Al 2O 3-4(55wt.%Al 2O 3)who inner surface con-sisted of well-bonding particles.For the outer surfaces,the same situations were found.
Difference between Al 2O 3-4(6.0wt.%PES,55wt.%Al 2O 3)and Al 2O 3-5(5.0wt.%PES,50wt.%Al 2O
3)could be easily obrved.Even with much higher solid loading,Al 2O 3-4possd less particle aggregations and defects,indicating better dispersion of Al 2O 3-4.This obrvation is consistent with the precursors,and also indicates that the precursor would determine the morphology of sintered membranes.
While comparing Al 2O 3-5with Al 2O 3–SiO 2-2,sheet-like struc-tures were clearly visible on the latter.They were formed from partly melted SiO 2particles and partly the reaction product of Al 2O 3and SiO 2,offering better linkage between residual SiO 2and Al 2O 3particles (if there was any).Moreover,the particle size as well as appearance of Al 2O 3particles was different from pure Al 2O 3membranes,possibly becau the surface of the particles consumed in the reaction.
Membrane Al 2O 3–kaolin-1was much denr and the particles were clearer,as shown in Fig.4.Owing to the multicomponent property,kaolin would have helped in the sintering of the mem-branes.When polymer chains is thermo-decompod and the temperature is not high enough to melt the particles,this can destroy the membrane structure of the particles but with the pres-ence of kaolin as the inorganic binder,the structure could be maintained and the particles would be fixed up by the reaction of Al 2O 3and silica in kaolin.
Comparing Fig.6with Fig.5,great change was obrved when the sintering temperature raid to 1600◦C.This indicated that higher temperature would enhance the specific character of mem-branes with different particles.
For pure Al 2O 3membranes,the combination of particles was largely enhanced at 1600◦C,with particles bonding more tightly with each other.When the content of Al 2O 3was lower,such as Al 2O 3-1(34wt.%)and Al 2O 3-2(41wt.%),the particles bonded along planar orientation,becau the distance between particles was enough to allow it.When there were more particles introduced,it resulted in three-dimensional structural formation to meet the needs of particle expansion.
For Al 2O 3–SiO 2membranes,the reaction product was visible in membrane Al 2O 3–SiO 2-2as shown in Fig.6.Clear particles could be hardly en,due to the reaction of Al 2O 3and SiO 2.Similarly,
