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Zeolite NaA membranes supported on alumina hollow fibers: Effect of support resistances on pervaporation performance
Journal of Membrane Science 451 (2014) 10–17
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Zeolite NaA membranes supported on alumina hollow fibers: Effect of support resistances on pervaporation performance Jia Shao a,b, Zhiying Zhan a, Jiangang Li a, Zhengbao Wang a,n, Kang Li b, Yushan Yan a,c a Department of Chemical and Biological Engineering, MOE Engineering Research Center of Membrane and Water Treatment Technology, Zhejiang University, Hangzhou 310027, PR China b Department of Chemical Engineering and Chemical Technology, Imperial College London, London SW7 2AZ, UK c Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 31 July 2013 Received in revised form 21 September 2013 Accepted 24 September 2013 Available online 1 October 2013
Porous Al2O3 hollow fibers with different asymmetric macrostructures, i.e. single or double finger-like layer, have been prepared by a phase inversion/sintering technique. Such hollow fibers are used as supports to synthesize zeolite NaA membranes with high separation factors ( 410,000) in the pervaporation of ethanol/water solution (90 wt%, 75 1C). Different zeolite thicknesses are achieved by means of extending the crystallization time and increasing the number of synthesis cycles. Influences of both the zeolite layer thickness and support layer macrostructures on pervaporation properties of zeolite NaA/Al2O3 supported membranes are investigated. The results indicate that variations of support macrostructures have a more noticeable effect on the pervaporation flux than zeolite layer thickness. There is an approximate linear dependence of the pervaporation flux of the supported membrane on the support porosity despite the fact that these hollow fibers are produced with different preparation conditions. When the support porosity increases to 69%, a pervaporation flux of 11.1 kg m 2 h 1 is achieved. & 2013 Elsevier B.V. All rights reserved.
Keywords: Zeolite membrane Hollow fiber support Pervaporation flux Support resistance
1. Introduction Under non-acidic conditions, zeolite NaA membranes can be used in the dehydration of organic solvents with dual benefits of high separation factor (410,000) and high water flux. These attributes make zeolite NaA membranes an attractive alternative for dehydrating solvents at an industrial scale [1]. In 1999, Mitsui Engineering and Shipbuilding Co. Ltd. brought the zeolite NaA membranes to the market, which was widely considered a milestone in the commercialization of zeolite membranes [2]. Since then supported zeolite membranes have attracted considerable research interest. Zeolite NaA membranes have been prepared by in-situ/seeded hydrothermal synthesis on an internal/external surface of porous α-Al2O3 tubular/hollow fiber/sheet supports. Pina et al. [3] reported that the membranes obtained on the external surface of α-Al2O3 tubular supports by a semi-continuous system, where fresh gel was periodically supplied to the synthesis vessel, displayed a good separation performance (e.g., a separation factor of 3600 and water permeation flux of 3.8 kg m 2 h 1) in the pervaporation of ethanol/water mixtures. Wang et al. [4] demonstrated that, by a novel seeding method of dipcoating-wiping and
n
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a single hydrothermal synthesis, a zeolite NaA membrane with a high flux of 9.0 kg m 2 h 1 and high water/ethanol separation factor of 10,000 could be obtained on an α-Al2O3 hollow fiber support. They showed the potential in fabricating a compact module, as hollow fiber supports provide very high surface area per volume [5]. Pera-Titus et al. [6] developed a cross-flow filtration technique to allow a controlled seeding of zeolite NaA crystals on the inner surface of an α-Al2O3 tube support. With this procedure, zeolite membranes with a selectivity up to 600 and a flux of 0.50 kg m 2 h 1 were obtained in the pervaporation of an ethanol/water (92/8 wt%) mixture at 50 1C. Despite the successful commercialization of supported NaA membranes, the fundamental understanding of the membrane separation performance remained limited, and the production of NaA membranes with high flux still seems to rely on a synthesis-and-test process to find a suitable membrane system under given synthesis conditions [7]. Ceramic hollow fibers, used as supports of zeolite membranes, have attracted wide attention, not only because of their relatively high resistance to abrasion and to chemical and thermal degradation, but also because of their higher surface/volume ratio and thinner walls compared to their counterparts of tubular supports. For industrial applications, the hollow fiber membranes must be incorporated into a module. The use of a bundle of ceramic hollow fibers in place of ceramic tubes (about 10 mm in diameter) or flat discs used in conventional inorganic membrane systems offers
J. Shao et al. / Journal of Membrane Science 451 (2014) 10–17
major process engineering advantages. Also, both the smooth surface of sponge-like structure and the low permeation resistance of the finger-like sub-layer [8] make ceramic hollow fibers, prepared by immersion-induced phase inversion/sintering technique, attractive supports for zeolite membranes. In order to develop commercially viable membranes with high permeation fluxes, it is essential to understand the fluid transport characteristics through the membrane as well as the support. So far, numerous academic articles [9–14] have been focused on the effect of zeolite membrane thickness on its permselectivity, especially in pervaporation studies, and the role of support layer has been largely ignored [15]. In fact, the control of morphologies of porous asymmetrical supports is of importance in preparing a supported membrane with high permselectivity and mechanical strength [16]. Zhou et al. [13] studied ultrathin zeolite X films with a total thickness of about 1 μm grown from a seed monolayer by hydrothermal treatment in clear synthesis solutions, one of which showed a total flux of 3.377 0.08 kg m 2 h 1 and a separation factor of 296 74 for dehydration of a 90/10 wt% ethanol/water mixture by pervaporation at 65 1C. Although the film thickness is much less than that of the best membranes (thickness 5–30 μm) in previous reports, the membranes prepared in their work do not display proportionally high fluxes, probably due to the high resistances of the support layers employed. de Bruijn et al. [15] calculated the pressure drop over the support layer by a simple modeling approach from the data available in literature and concluded that the flux resistance was mainly located in the support layer for the dehydration separation of ceramic membranes. Hasegawa et al. [17] reported that the thickness and porosity of a porous support affected the dehydration PV performance of a CHA type zeolite membrane. It thus follows that to promote industrial applications and process intensification of zeolite-based membranes, the support layer properties should be further improved. In this study, α-Al2O3 hollow fibers with different asymmetric macrostructures were prepared by a phase inversion/sintering technique. Such hollow fibers were used as supports to synthesize zeolite NaA membranes that have high separation factors (4 10,000) in pervaporation of ethanol/water solution (90 wt%, 75 1C). Influences of both the zeolite layer thickness and support layer macrostructures on pervaporation properties of zeolite NaA/ Al2O3 supported membranes were investigated in detail.
2. Experimental 2.1. Materials Commercially available alumina particles (100 nm in diameter, JR-L100, An'hui Xuancheng Jingrui Nano-materials Co. Ltd.), polyethersulfone (PES) (Gafone-3000p, Solvay Advanced Polymers), N-methyl-2-pyrrolidone (NMP) (Sinopharm Chemical Reagent Co. Ltd.), polyvinylpyrrolidone (PVP) (Sinopharm) were used to fabricate alumina hollow fiber supports, while sodium aluminate (molar ratio of Al/NaOH ¼0.81, Wako Pure Chemical Industries Ltd.), sodium silicate (Wako) and sodium hydroxide (Sinopharm) were employed to prepare zeolite membranes. 2.2. Preparation of
α-Al2O3 hollow fibers
The precursors of alumina hollow fibers can be prepared using an immersion-induced phase inversion method first described by Loeb and Sourirajan [18] for the formation of asymmetric polymeric membranes. Dehydrated PES was first dissolved in NMP to form a homogeneous polymer solution. Then a desired amount of dehydrated alumina particles was added into the polymer solution
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Table 1 Spinning conditions of hollow fiber precursor. Spinning parameters Slurry composition (wt%) NMP:PES:Al2O3:PVP Diameter of tube-in-orifice spinneret (mm) Slurry temperature (1C) Coagulation bath (tap water) temperature (1C) Injection rate of internal coagulant (deionized water) (mL h 1) Nitrogen pressure (MPa) Air gap (cm)
I: 38.0:8.8:52.7:0.5 II: 38.0:6.8:54.7:0.5 2.0/1.0 30 25 1000 0.10–0.20 3
slowly under agitation. If well mixed, the ceramic/polymer/solvent/additive (Al2O3/PES/NMP/PVP) system can be seen as slurry of polymer-coated ceramic particles. The detailed slurry composition is shown in Table 1. After being stirred for 40 h in a threenecked flask by a mixing stirrer, the slurry was degassed using a water-circulation vacuum pump for 3 h to remove the air bubbles present in it. Once immersed in a non-solvent for the polymer which is miscible with the solvent, solvent/nonsolvent exchange takes place leading to precipitation of the polymer phase. Alumina particles are immobilized once precipitation has taken place and the membrane macrostructure can be largely determined at that point by manipulating and adjusting the various parameters of the phase inversion process [8]. The detailed diagram of the hollow fiber spinning process is given elsewhere [19]. The detailed spinning conditions of the α-Al2O3 hollow fibers are also given in Table 1. The degassed spinning slurry was transferred to a stainless steel tank and pressurized to 0.10–0.20 MPa using nitrogen. A tube-in-orifice spinneret with orifice diameter/inner diameter of the tube of 2.0/1.0 mm was used to obtain hollow fiber precursors. The air gap was kept at 3 cm for all spinning runs. The forming hollow fiber precursor went into a coagulant (water) bath to complete the solidification process. The fiber precursors were left in the external coagulant for 24 h to complete the phase inversion, and then dried in an oven at 60 1C for 6 h to fulfill the dehydration and setting process. The formed hollow fiber precursors were first heated in an electric resistance furnace at about 600 1C for 2 h and 1000 1C for 2 h, and then were calcined at a high temperature (1450–1600 1C) for 4 h to allow binding of alumina particles. The detailed sintering profile can be found elsewhere [5]. The sintered hollow fibers were then taken out for characterization and zeolite membrane synthesis.
2.3. Preparation of zeolite NaA membranes Zeolite NaA membranes were prepared by seeded hydrothermal synthesis on the external surface of alumina hollow fiber supports. The dipcoating-wiping seeding method [4] was employed to deposit home-made NaA seeds with an average particle size of about 190 nm (DLS, Malvern, ZEN 3600) on hollow fiber supports. The concentration of the seed suspension was 1 wt%. Two types of the synthesis solutions with different molar ratios of Na2O/Al2O3/SiO2/H2O ¼3.4:1.0:2.0:155.0 and 2.0:1.0:2.0:130.0 were used. The detailed preparation condition was described elsewhere [20,21]. The milk-like synthesis solution was obtained by slow addition of sodium aluminate to the sodium silicate solution under stirring. After aging for 30 min at ambient temperature, the synthesis solution (40 g) was carefully poured into a Teflon-lined stainless steel autoclave (50 mL).
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J. Shao et al. / Journal of Membrane Science 451 (2014) 10–17
The seeded support was placed vertically in the autoclave to ensure the complete immersion of the support, and then the autoclave was sealed. The crystallization was carried out under autogenous pressure in an electrically controlled convection oven at 100 1C for 4 h. After each synthesis, the obtained membranes were rinsed with deionized water and dried at 60 1C for more than 4 h. 2.4. Characterization 2.4.1. Hollow fiber support The viscosity data (Pa s) of spinning slurries were collected using a digital viscometer (SNB-1, Shanghai Precision & Scientific Instrument Co. Ltd.) at 30 1C. The slurry was placed in a beaker, and its liquid level was adjusted according to the chosen spindle. Before testing, sufficient time was allowed to reach thermal equilibrium. The rotational speed was set at 6 rpm according to the viscosity range of the spinning slurry under test. The top and cross-sectional morphology of hollow fibers were observed using a scanning electronic microscopy (SEM, Hitachi TM-1000). The hollow fiber porosity ε (%) is defined as the volume of the pores divided by the total volume of the porous material. It was determined by the gravimetric method, based on the weight of liquid water contained in the membrane pores as
ε ¼ ðmH2 O =ρH2 O Þ=ðmH2 O =ρH2 O þ mAl2 O3 =ρAl2 O3 Þ100%
ð1Þ
where mAl2 O3 is the weight of hollow fiber support per unit length (g cm 1), mH2 O is the weight of water adsorbed by the support per unit length (g cm 1), ρH2 O is the water density (1.0 g cm 3), and ρAl2 O3 is the alumina density (3.95 g cm 3). The mechanical strength of ceramic hollow fibers was determined by the three-point bending technique using testing equipment (Sundoo Instrument Co. Ltd.). The span of the sample was 6 cm. Each hollow fiber was tested for 3 times and then the average bending force was obtained. The bending strength, sF, is calculated from sF ¼ ð8FLDÞ=ðπ ðD4 d ÞÞ 4
where PWF is the permeation flux of membrane for pure water (kg m 2 h 1), Q is the mass of the permeate (kg), A is the permeation area of membrane (m2), calculated by the outer diameter of hollow fibers, and t is the permeation time (h). It should be noted that all the pure water fluxes were obtained at the operating pressure difference of 1 atm at the most. The pore size distribution (PSD) of hollow fiber supports was determined by a mercury porosimeter (AutoPore IV 9510, Micrometrics Inc.). Before intruding mercury in step-wise pressure increments in the range from 3.4 kPa to 69.0 MPa, the hollow fibers with open pore structures were oven-dried at 60 1C. Mercury porosimetry is based on the capillary law governing liquid penetration into small pores. In the case of a non-wetting liquid like mercury and cylindrical pores, the pore diameter r is expressed by [23] P ¼ ð2γ Hg cos θÞ=r
ð4Þ
where P is the external pressure (Pa) applied in the chamber, γHg is the surface tension of mercury (0.485 J m 2), θ is the contact angle of mercury (1301), and r is the pore radius of pore aperture (m) for a cylindrical pore. PSD was presented in the form of differential pore volume curves versus logarithmical pore radius. 2.4.2. Zeolite membrane Microstructure and morphology of zeolite layers were examined using scanning electronic microscopy (SEM, Hitachi TM1000) with an accelerating voltage of 15 kV. The pervaporation experiments of 90 wt% ethanol aqueous solution were carried out in a laboratory scale setup at 75 1C [4]. The flux (kg m 2 h 1) was measured by weighing the cold trap before and after the pervaporation experiment and the trapping time. The water/ethanol separation factor (α) of the membrane was calculated as the proportion between the ratios of the weight fractions of water and alcohol at the permeate and feed sides, respectively.
3. Results and discussion
ð2Þ
which can be derived from the principles given by Wachtman [22]. Here, F is the measured force at which fracture takes place (N); L, D, and d are the length (in this case, 0.06 m), the outside diameter (m), and the inner side diameter (m), respectively. The hollow fiber permeation property was characterized by pure water flux (PWF) and measured using a dead-end flowing cell connected to a water-circulation vacuum pump. The permeation experiments were carried out at room temperature and PWF is obtained as PWF ¼ Q =ðAtÞ
ð3Þ
3.1. Al2O3 hollow fiber supports The Al2O3 hollow fiber supports prepared by the phase inversion and sintering technique usually possess an asymmetric structure simultaneously containing finger-like and sponge-like layers. Several factors can be adjusted to control the asymmetric macrostructure of the ceramic hollow fibers such as the composition of spinning slurry, extrusion rate and calcination temperature. The influences of these parameters on the wall thickness, wall density, porosity, water permeation and mechanical strength of the α-Al2O3 hollow fibers are given in Table 2.
Table 2 Spinning conditions and structure data of the hollow fiber supports. Support no.
Al2O3/PES ratio
Extruding pressure (MPa)
Calcination temp. (1C)
Wall density (g cm 3)
Wall thickness (mm)
Porosity (%)
Pure water flux (kg m 2 h 1)
Bending strength (MPa)
S1 S2 S3 S4 S5 S6 S7 S8 S9
6a 6 6 6 6 6 8b 8 8
0.10 0.15 0.20 0.20 0.20 0.20 0.10 0.15 0.20
1500 1500 1500 1450 1550 1600 1500 1500 1500
1.2 1.5 1.6 1.5 2.0 2.2 2.0 2.2 2.3
0.38 0.38 0.37 0.38 0.36 0.33 0.31 0.31 0.33
69 61 55 62 36 30 54 46 39
5142 2256 1568 1832 429 45 929 704 344
21 64 99 69 145 187 117 129 138
a b
Spinning slurry: Composition (S1–S6): NMP:PES:Al2O3:PVP ¼38.0:8.8:52.7:0.5; viscosity: η ¼ 52 Pa s. Spinning slurry: Composition (S7–S9): NMP:PES:Al2O3:PVP ¼ 38.0:6.8:54.7:0.5; viscosity: η ¼35 Pa s.
J. Shao et al. / Journal of Membrane Science 451 (2014) 10–17
During the spinning process, the extrusion pressure has a significant influence on the extrusion rate of the spinning slurry. The high extrusion pressure restricts not only the extension of the inner finger-like voids, but also the solvent evaporation in the surface of nascent hollow fibers. As shown in Fig. 1(I), the fingerlike voids originating from the inner surface decreased in length when the extrusion pressure was increased. The high extrusion pressure, i.e., high extrusion speed, provided less time for the solvent/non-solvent exchange on the inner interface between the fiber and the internal coagulant before the precipitation of hollow fiber precursor was completed in the external coagulation bath. Furthermore, the increased slurry viscosity resulted in the rapid external solidification, which limits the growth of the inner fingerlike voids. The percentage of the finger length in the cross section is reduced from approximately 90% to 75% and 60%, when the extrusion pressure is increased from 0.10 MPa to 0.15 and 0.20 MPa, respectively. Fig. 1(II) displays two types of hollow fiber morphologies, i.e., single finger-like structure and double fingerlike structure. To be more specific, the finger-like voids are initiated from the inner surface alone or from both the inner and outer surfaces simultaneously. Once the amount of PES is decreased, the low viscosity results in fast extrusion rate and there would be less time for the inner voids to extend to the center of the fiber cross-section. Besides, more alumina particles in the spinning slurry would probably slow the exchange rate of the
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solvent and the non-solvent. Therefore, under the same spinning conditions, slurry with high viscosity tends to produce hollow fibers with single finger-like voids, while less viscous slurry offers greater potential for the production of hollow fibers with double finger-like voids. As shown in Table 2, the influence of extrusion pressure on the structure parameters of α-Al2O3 hollow fibers suggests that the wall density increases, while the porosity decreases, as the extrusion pressure is increased from 0.10 to 0.20 MPa (support S1–S3). During the heat treatment process, the pores in hollow fiber precursors (support S3–S6) decreased when the sintering temperature is elevated. The shrinkage of both the finger-like and sponge-like regions results in the decrease in the porosity and the increase in the wall density. The wall thickness of all the hollow fibers is in the range of 0.31–0.38 mm. Fig. 2 depicts the effect of extrusion pressure on the water permeation and mechanical strength of the α-Al2O3 hollow fibers prepared with different Al2O3/PES ratios. When the extrusion pressure was elevated from 0.10 to 0.20 MPa, the pure water flux of hollow fibers prepared from the Al2O3/PES ¼6 slurry decreased substantially from 5142 (S1) to 1568 (S3) kg m 2 h 1 while their bending strength rose from 20 to 100 MPa. There is a similar trend in the pure water flux and bending strength of hollow fibers prepared from Al2O3/PES ¼8 slurry, though they have stronger bending strength and lower pure water permeation than those
Fig. 1. Typical cross-sectional SEM images of α-alumina hollow fiber supports prepared with Al2O3/PES ratios, I ¼6 and II ¼8 at the extrusion pressures of (a) 0.10 MPa (S1, S7), (b) 0.15 MPa (S2, S8), and (c) 0.20 MPa (S3, S9) and sintered at 1500 1C.
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Fig. 4. Pore size distribution of the hollow fiber support S1. Fig. 2. Permeation flux for pure water and mechanical strength of Al2O3 hollow fibers prepared with different extrusion pressures and Al2O3/PES ratio.
Fig. 3. Permeation flux for pure water and mechanical strength of Al2O3 hollow fibers S3–S6 prepared from different calcination temperatures.
from the A2O3/PES¼6 slurry. The differences in the permeation and mechanical strength are consistent with the variation of structure parameters shown in Table 2 and morphology shown in Fig. 1. Fig. 3 illustrates the effect of calcination temperature on the water permeation and mechanical strength of the α-Al2O3 hollow fibers prepared. When the calcination temperature was increased from 1450 to 1600 1C, the pure water flux decreased from 1832 (S4) to 45 (S6) kg m 2 h 1, while the bending strength increased from 70 (S4) to 187 (S6) MPa. It thus follows that the water permeation of hollow fibers is increased at the expense of support bending strength. Among the hollow fibers prepared in this study, the hollow fiber S1 has the highest water permeation. It is likely to achieve the highest pervaporation flux when being used as the support of zeolite NaA membrane. Fig. 4 shows the pore size distribution of the hollow fiber support S1. The finger-like voids lie in 9000 nm, and the sponge-like pores in the surface layer are in the range of 150–300 nm. The sponge-like pores would probably affect the permeate performance of the zeolite/Al2O3 supported membrane more significantly than the finger-like voids. 3.2. Zeolite NaA membranes supported on Al2O3 hollow fibers The seeded hydrothermal synthesis was employed to prepare zeolite NaA membranes on hollow fibers. The synthesis composition
was 3.4Na2O:Al2O3:2SiO2:155H2O. All zeolite membranes synthesized at 100 1C for Z4 h showed high separation factors (410,000) in the pervaporation of 90 wt% ethanol/water solution, which essentially is free from defects for practical separation purposes. Such zeolite membranes are suitable for investigating effects of zeolite layer and support layer on the flux of water without the abnormal fluctuation of pervaporation flux resulting from the defects of the zeolite membranes. Before the effect of support resistances on pervaporation properties is studied, different crystallization times and multiple syntheses are introduced to investigate effects of zeolite layer on the pervaporation performance of supported membranes. All the supported membranes for studying the effects of zeolite layer were synthesized on the hollow fiber support S1.
3.2.1. Effect of zeolite layer on pervaporation flux Nikolakis et al. [10] reported the time–variation curve of zeolite layer thickness based on SEM and XRD data of zeolite faujasite membranes synthesized from different crystallization times, and found that the zeolite layer thickness initially appeared to increase linearly with time reaching to a value of 30 μm after 15 days, and then tended to be stable. Navajas et al. [14] studied the influence of synthesis time on the weight gain, water/ethanol separation factor and water flux of zeolite mordenite membranes. Initially, the weight gain increased with the synthesis time until it reached a constant value at approximately 24 h probably due to the consumption of the reactants. However, in the case of zeolite NaA, the time required for the synthesis of uniform and dense zeolite NaA layer on the alumina support was shorter (e.g., 4 h) with remarkable performance. Probably due to the nutrient depletion, the thickness of zeolite layers prepared from the synthesis solution of 3.4Na2O:Al2O3:2SiO2:155H2O was found with no significant difference with the increase of the crystallization time from 4 h to 6 h. Therefore, the synthesis solution of 2.0Na2O: Al2O3:2SiO2:130H2O was used to investigate the effect of synthesis time on the zeolite layer thickness. SEM images of cross-sectional morphology of zeolite NaA membranes after 4 h, 5 h, and 6 h of hydrothermal treatment from the synthesis solution of 2.0Na2O: Al2O3:2SiO2:130H2O are presented in Fig. 5. It shows that the zeolite layer thickness increased slightly with the prolongation of the crystallization time at a constant crystallization temperature of 100 1C. The average thickness of M1, M2, and M3 were 3.1, 3.9, and 4.1 μm, respectively. This indicates that it is difficult to increase the zeolite layer thickness just by increasing the synthesis time. Xu et al. [24] reported that when the synthesis time was extended too much, XRD diffraction patterns of impurities, such as zeolite X and
J. Shao et al. / Journal of Membrane Science 451 (2014) 10–17
hydroxyl-sodalite, appeared. Therefore, multiple syntheses were used to increase the zeolite layer thickness instead of extending the crystallization time to more than 6 h.
Fig. 5. Cross-sectional SEM images of zeolite NaA membranes on the support S1 with different crystallization times. (a) M1, 4 h, (b) M2, 5 h, and (c) M3, 6 h.
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In the available literatures [12,24,25], the multiple synthesis method for the preparation of zeolite NaA membrane on the support was used to decrease defects from the first incomplete synthesis. The general steps were seeding, crystallization, and re-crystallization. In this study, to assure each single layer was continuous and dense, that is, to increase the effective thickness of separation membrane, the synthesis steps were seeding, crystallization, re-seeding, and re-crystallization. When seeds were planted, not only the formation of zeolite NaA crystals was accelerated, but also the transformation of zeolite NaA into other types of zeolites was inhibited [11]. The first seeding method was dipcoating-wiping, while the second one was rubbing zeolite powders on the first zeolite layer. The cross-sectional SEM images of zeolite NaA membranes synthesized from double and triple hydrothermal syntheses are presented in Fig. 6. The zeolite layer thickness increased after multiple syntheses. Table 3 lists the zeolite layer thickness and pervaporation performance of zeolite membranes. As for a single synthesis, when the crystallization time increased from 4 to 6 h, the thickness of zeolite layer rose slightly and pervaporation flux decreased to a minor extent. After double syntheses (membrane M4), the zeolite layer thickness had been doubled from 3.1 to 7.7 μm, but the flux loss was less than 20%. After triple syntheses (membrane M5), the zeolite layer thickness increased by 4 μm, whereas the flux was almost the same with the membrane M4. It follows that the pervaporation flux of supported zeolite NaA membranes was only slightly influenced by the zeolite layer thickness. Therefore, studying from the support aspect may be meaningful to search for possible influential factors of pervaporation flux of zeolite NaA membranes. 3.2.2. Effect of hollow fiber support on pervaporation flux In Fig. 7a, the pervaporation flux data of zeolite NaA/Al2O3 supported membranes by single synthesis are plotted versus the porosity of the whole hollow fiber support. Several spinning parameters were adjusted, including extrusion pressure, calcination temperature, and spinning solution composition, to extend the range of porosity of whole hollow fiber supports to 70%. Experimental data showed that the pervaporation flux of the dense zeolite NaA membrane (separation factor 410,000) could reach 11.1 kg m 2 h 1 on the hollow fiber with the porosity of 69% (S1). Although hollow fibers are produced with different preparation conditions, the same porosity of supports corresponds to a similar flux of zeolite NaA membranes. This is consistent with our previous report [4]. As shown in Fig. 7b, there is a downward trend in the mechanical strength of hollow fibers with the increase of their porosity. When hollow fibers are used as the supports of zeolite NaA membranes, the Al2O3/PES¼ 8 hollow fibers tend to obtain higher mechanical strength than the Al2O3/PES ¼6 hollow fibers with similar pervaporation performance. Specifically, hollow fibers S3 and S7 had the same morphology of single finger-like region,
Fig. 6. Cross-sectional SEM images of zeolite NaA membranes on the support S1 with multiple hydrothermal syntheses. (a) M4, 4 h þ 4 h and (b) M5, 4 hþ 4 h þ 4 h.
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Table 3 Thickness and pervaporation performance of zeolite membranes on the support S1. No.
Crystallization time (h)
Zeolite layer thickness Flux (μm) (kg m 2 h 1)
Separation factor
M1 M2 M3 M4a M5a
4 5 6 4 þ4 4 þ4 þ4
3.17 0.2 3.9 7 0.2 4.17 0.2 7.7 7 0.2 12.0 7 0.2
410,000 410,000 410,000 410,000 410,000
a
11.17 0.1 10.2 7 0.1 9.6 7 0.1 9.0 7 0.1 8.9 7 0.1
Multiple syntheses. Pervaporation conditions: 90 wt% ethanol aq. at 75 1C.
on the external surface of the home-made hollow fibers. By means of extending the crystallization time (4–6 h) and increasing the number of synthesis cycles (single/double/triple syntheses), zeolite membranes with different thicknesses were synthesized. It is indicated that the variation of the support macrostructure rather than zeolite layer thickness results in a greater difference in the pervaporation flux of the supported membrane. Though hollow fiber supports were produced from different preparation conditions, there was an approximate linear relationship between the pervaporation flux of dense zeolite NaA membrane (separation factor 410,000) and the porosity of hollow fiber support. When the support porosity increased to 69%, a flux of 11.1 kg m 2 h 1 was achieved. The alumina hollow fibers prepared from higher ratio of Al2O3/polymer slurry tended to achieve similar pervaporation separation performance with a higher mechanical strength. Therefore, to obtain the zeolite NaA/Al2O3 supported membranes with high pervaporation performance and mechanical strength, it is probably effective to focus on the improvement and development of the support properties.
Acknowledgments The financial supports provided by the National Natural Science Foundation of China (21028002 and 21236006), the Natural Science Foundation of Zhejiang Province (ZJNSF) (R4090099), the Science and Technology Department of Zhejiang Province (2009R 50020), and the Hengyi Foundation at Zhejiang University are gratefully acknowledged. J. Shao thanks the China Scholarship Council (201206320079).
Nomenclature mAl2 O3 mH 2 O
Fig. 7. (a) Pervaporation fluxes of zeolite NaA/Al2O3 supported membranes prepared by the single hydrothermal synthesis as a function of support porosity, (b) mechanical strengths of hollow fiber supports as a function of their porosity. P¼ 0.10–0.20 MPa, Al2O3/PES ¼ 6, T ¼1500 1C; P ¼0.10–0.20 MPa, Al2O3/PES ¼8, T ¼1500 1C; P ¼0.20 MPa, Al2O3/PES ¼ 6, T ¼ 1450 and 1550 1C.
as well as the ratio of finger/sponge-like pores (Fig. 1(I)c and (II)a). Since they have the similar porosity at around 55%, the pervaporation fluxes of supported membranes are in the range of 9.0– 9.6 kg m 2 h 1. However, their bending strengths are 99 and 117 MPa. With similar porosity, scattered distribution of pores in sponge-like layer tends to improve mechanical performance of hollow fibers. High ratio of Al2O3/PES means increase in the number of alumina particles, and results in a closer particle distance, that is, the pores in sponge-like layer are distributed effectively. This indicates that the mechanical strength of hollow fiber supports with the same supported membrane flux can be improved by modifying their structures.
F L D d Q A t PWF P r
weight of support per unit length (g cm 1) weight of water adsorbed by the support per unit length (g cm 1) bending force of support (N) length of support (m) outside diameter of support (m) inside diameter of support (m) permeation water mass (kg) permeation area of membrane (m2) permeation time (h) permeation flux for pure water (kg m 2 h 1) mercury intrusion pressure (Pa) pore radius (m)
Greek letters
η ρ γHg θ ε sF
α
viscosity of spinning slurry (Pa s) density (g cm 3) surface tension of mercury (J m 2) contact angle of mercury (deg) support porosity (%) bending strength of support (MPa) pervaporation separation factor
4. Conclusions Asymmetric Al2O3 hollow fibers with different macrostructures were prepared by varying the spinning conditions (extrusion pressure, Al2O3/PES ratio) and sintering temperature. Zeolite membranes with high separation factor (410,000) were synthesized
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J. Shao et al. / Journal of Membrane Science 451 (2014) 10–17 [2] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Separation and Purification Technology 25 (2001) 251–260. [3] M.P. Pina, M. Arruebo, A. Felipe, F. Fleta, M.P. Bernal, J. Coronas, M. Menendez, J. Santamaria, A semi-continuous method for the synthesis of NaA zeolite membranes on tubular supports, Journal of Membrane Science 244 (2004) 141–150. [4] Z.B. Wang, Q.Q. Ge, J. Shao, Y.S. Yan, High performance zeolite LTA pervaporation membranes on ceramic hollow fibers by dipcoating-wiping seed deposition, Journal of the American Chemical Society 131 (2009) 6910–6911. [5] S. Koonaphaptleelert, K. Li, Preparation and characterization of hydrophobic ceramic hollow fibre membrane, Journal of Membrane Science 291 (2007) 70–76. [6] M. Pera-Titus, J. Llorens, F. Cunill, R. Mallada, J. Santamaria, Preparation of zeolite NaA membranes on the inner side of tubular supports by means of a controlled seeding technique, Catalysis Today 104 (2005) 281–287. [7] J. Zah, H.M. Krieg, J.C. Breytenbach, Layer development and growth history of polycrystalline zeolite A membranes synthesised from a clear solution, Microporous and Mesoporous Materials 93 (2006) 141–150. [8] B.F.K. Kingsbury, K. Li, A morphological study of ceramic hollow fibre membranes, Journal of Membrane Science 328 (2009) 134–140. [9] D. Shen, W. Xiao, J.H. Yang, N.B. Chu, J.M. Lu, D.H. Yin, J.Q. Wang, Synthesis of silicalite-1 membrane with two silicon source by secondary growth method and its pervaporation performance, Separation and Purification Technology 76 (2011) 308–315. [10] V. Nikolakis, G. Xomeritakis, A. Abibi, M. Dickson, M. Tsapatsis, D.G. Vlachos, Growth of a faujasite-type zeolite membrane and its application in the separation of saturated/unsaturated hydrocarbon mixtures, Journal of Membrane Science 184 (2001) 209–219. [11] A. Malekpour, M.R. Millani, M. Kheirkhah, Synthesis and characterization of a NaA zeolite membrane and its applications for desalination of radioactive solutions, Desalination 225 (2008) 199–208. [12] D. Kunnakorn, T. Rirksomboon, P. Aungkavattana, N. Kuanchertchoo, D. Atong, K. Hemra, S. Kulprathipanja, S. Wongkasemjit, Optimization of synthesis time for high performance of NaA zeolite membranes synthesized via autoclave for water–ethanol separation, Desalination 280 (2011) 259–265.
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[13] H. Zhou, D. Korelskiy, T. Leppajarvi, M. Grahn, J. Tanskanen, J. Hedlund, Ultrathin zeolite X membranes for pervaporation dehydration of ethanol, Journal of Membrane Science 399 (2012) 106–111. [14] A. Navajas, R. Mallada, C. Tellez, J. Coronas, M. Menendez, J. Santamaria, Preparation of mordenite membranes for pervaporation of water–ethanol mixtures, Desalination 148 (2002) 25–29. [15] F.T. de Bruijn, L. Sun, Z. Olujic, P.J. Jansens, F. Kapteijn, Influence of the support layer on the flux limitation in pervaporation, Journal of Membrane Science (2003)141–156. [16] S.S. Madaeni, R. Pourghorbani, V. Vatanpour, Investigation of parameters affecting the flux of microfiltration poly(vinylidenefluoride) membranes for particulate removal, Advances in Polymer Technology 31 (2012) 29–40. [17] Y. Hasegawa, C. Abe, M. Nishioka, K. Sato, T. Nagase, T. Hanaoka, Formation of high flux CHA-type zeolite membranes and their application to the dehydration of alcohol solutions, Journal of Membrane Science 364 (2010) 318–324. [18] S. Loeb, S. Sourirajan, Sea water demineralization by means of an osmotic membrane, Saline Water Conversion II 38 (1963) 117–132. [19] J.G. Li, J. Shao, Q.Q. Ge, G.L. Wang, Z.B. Wang, Y.S. Yan, Influences of the zeolite loading and particle size in composite hollow fiber supports on properties of zeolite NaA membranes, Microporous and Mesoporous Materials 160 (2012) 10–17. [20] J. Shao, Q.Q. Ge, L.J. Shan, Z.B. Wang, Y.S. Yan, Influences of seeds on the properties of zeolite NaA membranes on alumina hollow fibers, Industrial & Engineering Chemistry Research 50 (2011) 9718–9726. [21] Q.Q. Ge, J. Shao, Z.B. Wang, Y.S. Yan, Effects of the synthesis hydrogel on the formation of zeolite LTA membranes, Microporous and Mesoporous Materials 151 (2012) 303–310. [22] J.B. Wachtman, Mechanical Properties of Ceramics, Wiley, New York, 1996. [23] E.W. Washburn, Note on a method of determining the distribution of pore sizes in a porous material, Proceedings of the National Academy of Sciences of the United States of America 7 (1921) 115–116. [24] X.C. Xu, W.S. Yang, J. Liu, L.W. Lin, Synthesis of NaA zeolite membranes from clear solution, Microporous and Mesoporous Materials 43 (2001) 299–311. [25] X.C. Xu, W.S. Yang, J. Liu, L.W. Lin, Synthesis of NaA zeolite membrane by microwave heating, Separation and Purification Technology 25 (2001) 241–249.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Zeolite NaA membranes supported on alumina hollow fibers: Effect of support resistances on pervaporation performance Jia Shao a,b, Zhiying Zhan a, Jiangang Li a, Zhengbao Wang a,n, Kang Li b, Yushan Yan a,c a Department of Chemical and Biological Engineering, MOE Engineering Research Center of Membrane and Water Treatment Technology, Zhejiang University, Hangzhou 310027, PR China b Department of Chemical Engineering and Chemical Technology, Imperial College London, London SW7 2AZ, UK c Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 31 July 2013 Received in revised form 21 September 2013 Accepted 24 September 2013 Available online 1 October 2013
Porous Al2O3 hollow fibers with different asymmetric macrostructures, i.e. single or double finger-like layer, have been prepared by a phase inversion/sintering technique. Such hollow fibers are used as supports to synthesize zeolite NaA membranes with high separation factors ( 410,000) in the pervaporation of ethanol/water solution (90 wt%, 75 1C). Different zeolite thicknesses are achieved by means of extending the crystallization time and increasing the number of synthesis cycles. Influences of both the zeolite layer thickness and support layer macrostructures on pervaporation properties of zeolite NaA/Al2O3 supported membranes are investigated. The results indicate that variations of support macrostructures have a more noticeable effect on the pervaporation flux than zeolite layer thickness. There is an approximate linear dependence of the pervaporation flux of the supported membrane on the support porosity despite the fact that these hollow fibers are produced with different preparation conditions. When the support porosity increases to 69%, a pervaporation flux of 11.1 kg m 2 h 1 is achieved. & 2013 Elsevier B.V. All rights reserved.
Keywords: Zeolite membrane Hollow fiber support Pervaporation flux Support resistance
1. Introduction Under non-acidic conditions, zeolite NaA membranes can be used in the dehydration of organic solvents with dual benefits of high separation factor (410,000) and high water flux. These attributes make zeolite NaA membranes an attractive alternative for dehydrating solvents at an industrial scale [1]. In 1999, Mitsui Engineering and Shipbuilding Co. Ltd. brought the zeolite NaA membranes to the market, which was widely considered a milestone in the commercialization of zeolite membranes [2]. Since then supported zeolite membranes have attracted considerable research interest. Zeolite NaA membranes have been prepared by in-situ/seeded hydrothermal synthesis on an internal/external surface of porous α-Al2O3 tubular/hollow fiber/sheet supports. Pina et al. [3] reported that the membranes obtained on the external surface of α-Al2O3 tubular supports by a semi-continuous system, where fresh gel was periodically supplied to the synthesis vessel, displayed a good separation performance (e.g., a separation factor of 3600 and water permeation flux of 3.8 kg m 2 h 1) in the pervaporation of ethanol/water mixtures. Wang et al. [4] demonstrated that, by a novel seeding method of dipcoating-wiping and
n
Corresponding author. Tel.: þ 86 57187952391. E-mail address: [email protected] (Z. Wang).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.09.049
a single hydrothermal synthesis, a zeolite NaA membrane with a high flux of 9.0 kg m 2 h 1 and high water/ethanol separation factor of 10,000 could be obtained on an α-Al2O3 hollow fiber support. They showed the potential in fabricating a compact module, as hollow fiber supports provide very high surface area per volume [5]. Pera-Titus et al. [6] developed a cross-flow filtration technique to allow a controlled seeding of zeolite NaA crystals on the inner surface of an α-Al2O3 tube support. With this procedure, zeolite membranes with a selectivity up to 600 and a flux of 0.50 kg m 2 h 1 were obtained in the pervaporation of an ethanol/water (92/8 wt%) mixture at 50 1C. Despite the successful commercialization of supported NaA membranes, the fundamental understanding of the membrane separation performance remained limited, and the production of NaA membranes with high flux still seems to rely on a synthesis-and-test process to find a suitable membrane system under given synthesis conditions [7]. Ceramic hollow fibers, used as supports of zeolite membranes, have attracted wide attention, not only because of their relatively high resistance to abrasion and to chemical and thermal degradation, but also because of their higher surface/volume ratio and thinner walls compared to their counterparts of tubular supports. For industrial applications, the hollow fiber membranes must be incorporated into a module. The use of a bundle of ceramic hollow fibers in place of ceramic tubes (about 10 mm in diameter) or flat discs used in conventional inorganic membrane systems offers
J. Shao et al. / Journal of Membrane Science 451 (2014) 10–17
major process engineering advantages. Also, both the smooth surface of sponge-like structure and the low permeation resistance of the finger-like sub-layer [8] make ceramic hollow fibers, prepared by immersion-induced phase inversion/sintering technique, attractive supports for zeolite membranes. In order to develop commercially viable membranes with high permeation fluxes, it is essential to understand the fluid transport characteristics through the membrane as well as the support. So far, numerous academic articles [9–14] have been focused on the effect of zeolite membrane thickness on its permselectivity, especially in pervaporation studies, and the role of support layer has been largely ignored [15]. In fact, the control of morphologies of porous asymmetrical supports is of importance in preparing a supported membrane with high permselectivity and mechanical strength [16]. Zhou et al. [13] studied ultrathin zeolite X films with a total thickness of about 1 μm grown from a seed monolayer by hydrothermal treatment in clear synthesis solutions, one of which showed a total flux of 3.377 0.08 kg m 2 h 1 and a separation factor of 296 74 for dehydration of a 90/10 wt% ethanol/water mixture by pervaporation at 65 1C. Although the film thickness is much less than that of the best membranes (thickness 5–30 μm) in previous reports, the membranes prepared in their work do not display proportionally high fluxes, probably due to the high resistances of the support layers employed. de Bruijn et al. [15] calculated the pressure drop over the support layer by a simple modeling approach from the data available in literature and concluded that the flux resistance was mainly located in the support layer for the dehydration separation of ceramic membranes. Hasegawa et al. [17] reported that the thickness and porosity of a porous support affected the dehydration PV performance of a CHA type zeolite membrane. It thus follows that to promote industrial applications and process intensification of zeolite-based membranes, the support layer properties should be further improved. In this study, α-Al2O3 hollow fibers with different asymmetric macrostructures were prepared by a phase inversion/sintering technique. Such hollow fibers were used as supports to synthesize zeolite NaA membranes that have high separation factors (4 10,000) in pervaporation of ethanol/water solution (90 wt%, 75 1C). Influences of both the zeolite layer thickness and support layer macrostructures on pervaporation properties of zeolite NaA/ Al2O3 supported membranes were investigated in detail.
2. Experimental 2.1. Materials Commercially available alumina particles (100 nm in diameter, JR-L100, An'hui Xuancheng Jingrui Nano-materials Co. Ltd.), polyethersulfone (PES) (Gafone-3000p, Solvay Advanced Polymers), N-methyl-2-pyrrolidone (NMP) (Sinopharm Chemical Reagent Co. Ltd.), polyvinylpyrrolidone (PVP) (Sinopharm) were used to fabricate alumina hollow fiber supports, while sodium aluminate (molar ratio of Al/NaOH ¼0.81, Wako Pure Chemical Industries Ltd.), sodium silicate (Wako) and sodium hydroxide (Sinopharm) were employed to prepare zeolite membranes. 2.2. Preparation of
α-Al2O3 hollow fibers
The precursors of alumina hollow fibers can be prepared using an immersion-induced phase inversion method first described by Loeb and Sourirajan [18] for the formation of asymmetric polymeric membranes. Dehydrated PES was first dissolved in NMP to form a homogeneous polymer solution. Then a desired amount of dehydrated alumina particles was added into the polymer solution
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Table 1 Spinning conditions of hollow fiber precursor. Spinning parameters Slurry composition (wt%) NMP:PES:Al2O3:PVP Diameter of tube-in-orifice spinneret (mm) Slurry temperature (1C) Coagulation bath (tap water) temperature (1C) Injection rate of internal coagulant (deionized water) (mL h 1) Nitrogen pressure (MPa) Air gap (cm)
I: 38.0:8.8:52.7:0.5 II: 38.0:6.8:54.7:0.5 2.0/1.0 30 25 1000 0.10–0.20 3
slowly under agitation. If well mixed, the ceramic/polymer/solvent/additive (Al2O3/PES/NMP/PVP) system can be seen as slurry of polymer-coated ceramic particles. The detailed slurry composition is shown in Table 1. After being stirred for 40 h in a threenecked flask by a mixing stirrer, the slurry was degassed using a water-circulation vacuum pump for 3 h to remove the air bubbles present in it. Once immersed in a non-solvent for the polymer which is miscible with the solvent, solvent/nonsolvent exchange takes place leading to precipitation of the polymer phase. Alumina particles are immobilized once precipitation has taken place and the membrane macrostructure can be largely determined at that point by manipulating and adjusting the various parameters of the phase inversion process [8]. The detailed diagram of the hollow fiber spinning process is given elsewhere [19]. The detailed spinning conditions of the α-Al2O3 hollow fibers are also given in Table 1. The degassed spinning slurry was transferred to a stainless steel tank and pressurized to 0.10–0.20 MPa using nitrogen. A tube-in-orifice spinneret with orifice diameter/inner diameter of the tube of 2.0/1.0 mm was used to obtain hollow fiber precursors. The air gap was kept at 3 cm for all spinning runs. The forming hollow fiber precursor went into a coagulant (water) bath to complete the solidification process. The fiber precursors were left in the external coagulant for 24 h to complete the phase inversion, and then dried in an oven at 60 1C for 6 h to fulfill the dehydration and setting process. The formed hollow fiber precursors were first heated in an electric resistance furnace at about 600 1C for 2 h and 1000 1C for 2 h, and then were calcined at a high temperature (1450–1600 1C) for 4 h to allow binding of alumina particles. The detailed sintering profile can be found elsewhere [5]. The sintered hollow fibers were then taken out for characterization and zeolite membrane synthesis.
2.3. Preparation of zeolite NaA membranes Zeolite NaA membranes were prepared by seeded hydrothermal synthesis on the external surface of alumina hollow fiber supports. The dipcoating-wiping seeding method [4] was employed to deposit home-made NaA seeds with an average particle size of about 190 nm (DLS, Malvern, ZEN 3600) on hollow fiber supports. The concentration of the seed suspension was 1 wt%. Two types of the synthesis solutions with different molar ratios of Na2O/Al2O3/SiO2/H2O ¼3.4:1.0:2.0:155.0 and 2.0:1.0:2.0:130.0 were used. The detailed preparation condition was described elsewhere [20,21]. The milk-like synthesis solution was obtained by slow addition of sodium aluminate to the sodium silicate solution under stirring. After aging for 30 min at ambient temperature, the synthesis solution (40 g) was carefully poured into a Teflon-lined stainless steel autoclave (50 mL).
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J. Shao et al. / Journal of Membrane Science 451 (2014) 10–17
The seeded support was placed vertically in the autoclave to ensure the complete immersion of the support, and then the autoclave was sealed. The crystallization was carried out under autogenous pressure in an electrically controlled convection oven at 100 1C for 4 h. After each synthesis, the obtained membranes were rinsed with deionized water and dried at 60 1C for more than 4 h. 2.4. Characterization 2.4.1. Hollow fiber support The viscosity data (Pa s) of spinning slurries were collected using a digital viscometer (SNB-1, Shanghai Precision & Scientific Instrument Co. Ltd.) at 30 1C. The slurry was placed in a beaker, and its liquid level was adjusted according to the chosen spindle. Before testing, sufficient time was allowed to reach thermal equilibrium. The rotational speed was set at 6 rpm according to the viscosity range of the spinning slurry under test. The top and cross-sectional morphology of hollow fibers were observed using a scanning electronic microscopy (SEM, Hitachi TM-1000). The hollow fiber porosity ε (%) is defined as the volume of the pores divided by the total volume of the porous material. It was determined by the gravimetric method, based on the weight of liquid water contained in the membrane pores as
ε ¼ ðmH2 O =ρH2 O Þ=ðmH2 O =ρH2 O þ mAl2 O3 =ρAl2 O3 Þ100%
ð1Þ
where mAl2 O3 is the weight of hollow fiber support per unit length (g cm 1), mH2 O is the weight of water adsorbed by the support per unit length (g cm 1), ρH2 O is the water density (1.0 g cm 3), and ρAl2 O3 is the alumina density (3.95 g cm 3). The mechanical strength of ceramic hollow fibers was determined by the three-point bending technique using testing equipment (Sundoo Instrument Co. Ltd.). The span of the sample was 6 cm. Each hollow fiber was tested for 3 times and then the average bending force was obtained. The bending strength, sF, is calculated from sF ¼ ð8FLDÞ=ðπ ðD4 d ÞÞ 4
where PWF is the permeation flux of membrane for pure water (kg m 2 h 1), Q is the mass of the permeate (kg), A is the permeation area of membrane (m2), calculated by the outer diameter of hollow fibers, and t is the permeation time (h). It should be noted that all the pure water fluxes were obtained at the operating pressure difference of 1 atm at the most. The pore size distribution (PSD) of hollow fiber supports was determined by a mercury porosimeter (AutoPore IV 9510, Micrometrics Inc.). Before intruding mercury in step-wise pressure increments in the range from 3.4 kPa to 69.0 MPa, the hollow fibers with open pore structures were oven-dried at 60 1C. Mercury porosimetry is based on the capillary law governing liquid penetration into small pores. In the case of a non-wetting liquid like mercury and cylindrical pores, the pore diameter r is expressed by [23] P ¼ ð2γ Hg cos θÞ=r
ð4Þ
where P is the external pressure (Pa) applied in the chamber, γHg is the surface tension of mercury (0.485 J m 2), θ is the contact angle of mercury (1301), and r is the pore radius of pore aperture (m) for a cylindrical pore. PSD was presented in the form of differential pore volume curves versus logarithmical pore radius. 2.4.2. Zeolite membrane Microstructure and morphology of zeolite layers were examined using scanning electronic microscopy (SEM, Hitachi TM1000) with an accelerating voltage of 15 kV. The pervaporation experiments of 90 wt% ethanol aqueous solution were carried out in a laboratory scale setup at 75 1C [4]. The flux (kg m 2 h 1) was measured by weighing the cold trap before and after the pervaporation experiment and the trapping time. The water/ethanol separation factor (α) of the membrane was calculated as the proportion between the ratios of the weight fractions of water and alcohol at the permeate and feed sides, respectively.
3. Results and discussion
ð2Þ
which can be derived from the principles given by Wachtman [22]. Here, F is the measured force at which fracture takes place (N); L, D, and d are the length (in this case, 0.06 m), the outside diameter (m), and the inner side diameter (m), respectively. The hollow fiber permeation property was characterized by pure water flux (PWF) and measured using a dead-end flowing cell connected to a water-circulation vacuum pump. The permeation experiments were carried out at room temperature and PWF is obtained as PWF ¼ Q =ðAtÞ
ð3Þ
3.1. Al2O3 hollow fiber supports The Al2O3 hollow fiber supports prepared by the phase inversion and sintering technique usually possess an asymmetric structure simultaneously containing finger-like and sponge-like layers. Several factors can be adjusted to control the asymmetric macrostructure of the ceramic hollow fibers such as the composition of spinning slurry, extrusion rate and calcination temperature. The influences of these parameters on the wall thickness, wall density, porosity, water permeation and mechanical strength of the α-Al2O3 hollow fibers are given in Table 2.
Table 2 Spinning conditions and structure data of the hollow fiber supports. Support no.
Al2O3/PES ratio
Extruding pressure (MPa)
Calcination temp. (1C)
Wall density (g cm 3)
Wall thickness (mm)
Porosity (%)
Pure water flux (kg m 2 h 1)
Bending strength (MPa)
S1 S2 S3 S4 S5 S6 S7 S8 S9
6a 6 6 6 6 6 8b 8 8
0.10 0.15 0.20 0.20 0.20 0.20 0.10 0.15 0.20
1500 1500 1500 1450 1550 1600 1500 1500 1500
1.2 1.5 1.6 1.5 2.0 2.2 2.0 2.2 2.3
0.38 0.38 0.37 0.38 0.36 0.33 0.31 0.31 0.33
69 61 55 62 36 30 54 46 39
5142 2256 1568 1832 429 45 929 704 344
21 64 99 69 145 187 117 129 138
a b
Spinning slurry: Composition (S1–S6): NMP:PES:Al2O3:PVP ¼38.0:8.8:52.7:0.5; viscosity: η ¼ 52 Pa s. Spinning slurry: Composition (S7–S9): NMP:PES:Al2O3:PVP ¼ 38.0:6.8:54.7:0.5; viscosity: η ¼35 Pa s.
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During the spinning process, the extrusion pressure has a significant influence on the extrusion rate of the spinning slurry. The high extrusion pressure restricts not only the extension of the inner finger-like voids, but also the solvent evaporation in the surface of nascent hollow fibers. As shown in Fig. 1(I), the fingerlike voids originating from the inner surface decreased in length when the extrusion pressure was increased. The high extrusion pressure, i.e., high extrusion speed, provided less time for the solvent/non-solvent exchange on the inner interface between the fiber and the internal coagulant before the precipitation of hollow fiber precursor was completed in the external coagulation bath. Furthermore, the increased slurry viscosity resulted in the rapid external solidification, which limits the growth of the inner fingerlike voids. The percentage of the finger length in the cross section is reduced from approximately 90% to 75% and 60%, when the extrusion pressure is increased from 0.10 MPa to 0.15 and 0.20 MPa, respectively. Fig. 1(II) displays two types of hollow fiber morphologies, i.e., single finger-like structure and double fingerlike structure. To be more specific, the finger-like voids are initiated from the inner surface alone or from both the inner and outer surfaces simultaneously. Once the amount of PES is decreased, the low viscosity results in fast extrusion rate and there would be less time for the inner voids to extend to the center of the fiber cross-section. Besides, more alumina particles in the spinning slurry would probably slow the exchange rate of the
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solvent and the non-solvent. Therefore, under the same spinning conditions, slurry with high viscosity tends to produce hollow fibers with single finger-like voids, while less viscous slurry offers greater potential for the production of hollow fibers with double finger-like voids. As shown in Table 2, the influence of extrusion pressure on the structure parameters of α-Al2O3 hollow fibers suggests that the wall density increases, while the porosity decreases, as the extrusion pressure is increased from 0.10 to 0.20 MPa (support S1–S3). During the heat treatment process, the pores in hollow fiber precursors (support S3–S6) decreased when the sintering temperature is elevated. The shrinkage of both the finger-like and sponge-like regions results in the decrease in the porosity and the increase in the wall density. The wall thickness of all the hollow fibers is in the range of 0.31–0.38 mm. Fig. 2 depicts the effect of extrusion pressure on the water permeation and mechanical strength of the α-Al2O3 hollow fibers prepared with different Al2O3/PES ratios. When the extrusion pressure was elevated from 0.10 to 0.20 MPa, the pure water flux of hollow fibers prepared from the Al2O3/PES ¼6 slurry decreased substantially from 5142 (S1) to 1568 (S3) kg m 2 h 1 while their bending strength rose from 20 to 100 MPa. There is a similar trend in the pure water flux and bending strength of hollow fibers prepared from Al2O3/PES ¼8 slurry, though they have stronger bending strength and lower pure water permeation than those
Fig. 1. Typical cross-sectional SEM images of α-alumina hollow fiber supports prepared with Al2O3/PES ratios, I ¼6 and II ¼8 at the extrusion pressures of (a) 0.10 MPa (S1, S7), (b) 0.15 MPa (S2, S8), and (c) 0.20 MPa (S3, S9) and sintered at 1500 1C.
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Fig. 4. Pore size distribution of the hollow fiber support S1. Fig. 2. Permeation flux for pure water and mechanical strength of Al2O3 hollow fibers prepared with different extrusion pressures and Al2O3/PES ratio.
Fig. 3. Permeation flux for pure water and mechanical strength of Al2O3 hollow fibers S3–S6 prepared from different calcination temperatures.
from the A2O3/PES¼6 slurry. The differences in the permeation and mechanical strength are consistent with the variation of structure parameters shown in Table 2 and morphology shown in Fig. 1. Fig. 3 illustrates the effect of calcination temperature on the water permeation and mechanical strength of the α-Al2O3 hollow fibers prepared. When the calcination temperature was increased from 1450 to 1600 1C, the pure water flux decreased from 1832 (S4) to 45 (S6) kg m 2 h 1, while the bending strength increased from 70 (S4) to 187 (S6) MPa. It thus follows that the water permeation of hollow fibers is increased at the expense of support bending strength. Among the hollow fibers prepared in this study, the hollow fiber S1 has the highest water permeation. It is likely to achieve the highest pervaporation flux when being used as the support of zeolite NaA membrane. Fig. 4 shows the pore size distribution of the hollow fiber support S1. The finger-like voids lie in 9000 nm, and the sponge-like pores in the surface layer are in the range of 150–300 nm. The sponge-like pores would probably affect the permeate performance of the zeolite/Al2O3 supported membrane more significantly than the finger-like voids. 3.2. Zeolite NaA membranes supported on Al2O3 hollow fibers The seeded hydrothermal synthesis was employed to prepare zeolite NaA membranes on hollow fibers. The synthesis composition
was 3.4Na2O:Al2O3:2SiO2:155H2O. All zeolite membranes synthesized at 100 1C for Z4 h showed high separation factors (410,000) in the pervaporation of 90 wt% ethanol/water solution, which essentially is free from defects for practical separation purposes. Such zeolite membranes are suitable for investigating effects of zeolite layer and support layer on the flux of water without the abnormal fluctuation of pervaporation flux resulting from the defects of the zeolite membranes. Before the effect of support resistances on pervaporation properties is studied, different crystallization times and multiple syntheses are introduced to investigate effects of zeolite layer on the pervaporation performance of supported membranes. All the supported membranes for studying the effects of zeolite layer were synthesized on the hollow fiber support S1.
3.2.1. Effect of zeolite layer on pervaporation flux Nikolakis et al. [10] reported the time–variation curve of zeolite layer thickness based on SEM and XRD data of zeolite faujasite membranes synthesized from different crystallization times, and found that the zeolite layer thickness initially appeared to increase linearly with time reaching to a value of 30 μm after 15 days, and then tended to be stable. Navajas et al. [14] studied the influence of synthesis time on the weight gain, water/ethanol separation factor and water flux of zeolite mordenite membranes. Initially, the weight gain increased with the synthesis time until it reached a constant value at approximately 24 h probably due to the consumption of the reactants. However, in the case of zeolite NaA, the time required for the synthesis of uniform and dense zeolite NaA layer on the alumina support was shorter (e.g., 4 h) with remarkable performance. Probably due to the nutrient depletion, the thickness of zeolite layers prepared from the synthesis solution of 3.4Na2O:Al2O3:2SiO2:155H2O was found with no significant difference with the increase of the crystallization time from 4 h to 6 h. Therefore, the synthesis solution of 2.0Na2O: Al2O3:2SiO2:130H2O was used to investigate the effect of synthesis time on the zeolite layer thickness. SEM images of cross-sectional morphology of zeolite NaA membranes after 4 h, 5 h, and 6 h of hydrothermal treatment from the synthesis solution of 2.0Na2O: Al2O3:2SiO2:130H2O are presented in Fig. 5. It shows that the zeolite layer thickness increased slightly with the prolongation of the crystallization time at a constant crystallization temperature of 100 1C. The average thickness of M1, M2, and M3 were 3.1, 3.9, and 4.1 μm, respectively. This indicates that it is difficult to increase the zeolite layer thickness just by increasing the synthesis time. Xu et al. [24] reported that when the synthesis time was extended too much, XRD diffraction patterns of impurities, such as zeolite X and
J. Shao et al. / Journal of Membrane Science 451 (2014) 10–17
hydroxyl-sodalite, appeared. Therefore, multiple syntheses were used to increase the zeolite layer thickness instead of extending the crystallization time to more than 6 h.
Fig. 5. Cross-sectional SEM images of zeolite NaA membranes on the support S1 with different crystallization times. (a) M1, 4 h, (b) M2, 5 h, and (c) M3, 6 h.
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In the available literatures [12,24,25], the multiple synthesis method for the preparation of zeolite NaA membrane on the support was used to decrease defects from the first incomplete synthesis. The general steps were seeding, crystallization, and re-crystallization. In this study, to assure each single layer was continuous and dense, that is, to increase the effective thickness of separation membrane, the synthesis steps were seeding, crystallization, re-seeding, and re-crystallization. When seeds were planted, not only the formation of zeolite NaA crystals was accelerated, but also the transformation of zeolite NaA into other types of zeolites was inhibited [11]. The first seeding method was dipcoating-wiping, while the second one was rubbing zeolite powders on the first zeolite layer. The cross-sectional SEM images of zeolite NaA membranes synthesized from double and triple hydrothermal syntheses are presented in Fig. 6. The zeolite layer thickness increased after multiple syntheses. Table 3 lists the zeolite layer thickness and pervaporation performance of zeolite membranes. As for a single synthesis, when the crystallization time increased from 4 to 6 h, the thickness of zeolite layer rose slightly and pervaporation flux decreased to a minor extent. After double syntheses (membrane M4), the zeolite layer thickness had been doubled from 3.1 to 7.7 μm, but the flux loss was less than 20%. After triple syntheses (membrane M5), the zeolite layer thickness increased by 4 μm, whereas the flux was almost the same with the membrane M4. It follows that the pervaporation flux of supported zeolite NaA membranes was only slightly influenced by the zeolite layer thickness. Therefore, studying from the support aspect may be meaningful to search for possible influential factors of pervaporation flux of zeolite NaA membranes. 3.2.2. Effect of hollow fiber support on pervaporation flux In Fig. 7a, the pervaporation flux data of zeolite NaA/Al2O3 supported membranes by single synthesis are plotted versus the porosity of the whole hollow fiber support. Several spinning parameters were adjusted, including extrusion pressure, calcination temperature, and spinning solution composition, to extend the range of porosity of whole hollow fiber supports to 70%. Experimental data showed that the pervaporation flux of the dense zeolite NaA membrane (separation factor 410,000) could reach 11.1 kg m 2 h 1 on the hollow fiber with the porosity of 69% (S1). Although hollow fibers are produced with different preparation conditions, the same porosity of supports corresponds to a similar flux of zeolite NaA membranes. This is consistent with our previous report [4]. As shown in Fig. 7b, there is a downward trend in the mechanical strength of hollow fibers with the increase of their porosity. When hollow fibers are used as the supports of zeolite NaA membranes, the Al2O3/PES¼ 8 hollow fibers tend to obtain higher mechanical strength than the Al2O3/PES ¼6 hollow fibers with similar pervaporation performance. Specifically, hollow fibers S3 and S7 had the same morphology of single finger-like region,
Fig. 6. Cross-sectional SEM images of zeolite NaA membranes on the support S1 with multiple hydrothermal syntheses. (a) M4, 4 h þ 4 h and (b) M5, 4 hþ 4 h þ 4 h.
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J. Shao et al. / Journal of Membrane Science 451 (2014) 10–17
Table 3 Thickness and pervaporation performance of zeolite membranes on the support S1. No.
Crystallization time (h)
Zeolite layer thickness Flux (μm) (kg m 2 h 1)
Separation factor
M1 M2 M3 M4a M5a
4 5 6 4 þ4 4 þ4 þ4
3.17 0.2 3.9 7 0.2 4.17 0.2 7.7 7 0.2 12.0 7 0.2
410,000 410,000 410,000 410,000 410,000
a
11.17 0.1 10.2 7 0.1 9.6 7 0.1 9.0 7 0.1 8.9 7 0.1
Multiple syntheses. Pervaporation conditions: 90 wt% ethanol aq. at 75 1C.
on the external surface of the home-made hollow fibers. By means of extending the crystallization time (4–6 h) and increasing the number of synthesis cycles (single/double/triple syntheses), zeolite membranes with different thicknesses were synthesized. It is indicated that the variation of the support macrostructure rather than zeolite layer thickness results in a greater difference in the pervaporation flux of the supported membrane. Though hollow fiber supports were produced from different preparation conditions, there was an approximate linear relationship between the pervaporation flux of dense zeolite NaA membrane (separation factor 410,000) and the porosity of hollow fiber support. When the support porosity increased to 69%, a flux of 11.1 kg m 2 h 1 was achieved. The alumina hollow fibers prepared from higher ratio of Al2O3/polymer slurry tended to achieve similar pervaporation separation performance with a higher mechanical strength. Therefore, to obtain the zeolite NaA/Al2O3 supported membranes with high pervaporation performance and mechanical strength, it is probably effective to focus on the improvement and development of the support properties.
Acknowledgments The financial supports provided by the National Natural Science Foundation of China (21028002 and 21236006), the Natural Science Foundation of Zhejiang Province (ZJNSF) (R4090099), the Science and Technology Department of Zhejiang Province (2009R 50020), and the Hengyi Foundation at Zhejiang University are gratefully acknowledged. J. Shao thanks the China Scholarship Council (201206320079).
Nomenclature mAl2 O3 mH 2 O
Fig. 7. (a) Pervaporation fluxes of zeolite NaA/Al2O3 supported membranes prepared by the single hydrothermal synthesis as a function of support porosity, (b) mechanical strengths of hollow fiber supports as a function of their porosity. P¼ 0.10–0.20 MPa, Al2O3/PES ¼ 6, T ¼1500 1C; P ¼0.10–0.20 MPa, Al2O3/PES ¼8, T ¼1500 1C; P ¼0.20 MPa, Al2O3/PES ¼ 6, T ¼ 1450 and 1550 1C.
as well as the ratio of finger/sponge-like pores (Fig. 1(I)c and (II)a). Since they have the similar porosity at around 55%, the pervaporation fluxes of supported membranes are in the range of 9.0– 9.6 kg m 2 h 1. However, their bending strengths are 99 and 117 MPa. With similar porosity, scattered distribution of pores in sponge-like layer tends to improve mechanical performance of hollow fibers. High ratio of Al2O3/PES means increase in the number of alumina particles, and results in a closer particle distance, that is, the pores in sponge-like layer are distributed effectively. This indicates that the mechanical strength of hollow fiber supports with the same supported membrane flux can be improved by modifying their structures.
F L D d Q A t PWF P r
weight of support per unit length (g cm 1) weight of water adsorbed by the support per unit length (g cm 1) bending force of support (N) length of support (m) outside diameter of support (m) inside diameter of support (m) permeation water mass (kg) permeation area of membrane (m2) permeation time (h) permeation flux for pure water (kg m 2 h 1) mercury intrusion pressure (Pa) pore radius (m)
Greek letters
η ρ γHg θ ε sF
α
viscosity of spinning slurry (Pa s) density (g cm 3) surface tension of mercury (J m 2) contact angle of mercury (deg) support porosity (%) bending strength of support (MPa) pervaporation separation factor
4. Conclusions Asymmetric Al2O3 hollow fibers with different macrostructures were prepared by varying the spinning conditions (extrusion pressure, Al2O3/PES ratio) and sintering temperature. Zeolite membranes with high separation factor (410,000) were synthesized
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