Yanmei Liu,Xuerui Wang,Yuting Zhang,Yong He,Xuehong Gu*

State Key Laboratory of Materials-Oriented Chemical Engineering,College of Chemistry and Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

Keywords:NaA zeolite membrane Vacuum seeding method Hollow fiber Pervaporation

ABSTRACT NaA zeolite membranes were prepared by secondary growth method on the outer surface of α-Al2O3 hollow fiber supports.Vacuum seeding method was used for planting zeolite seeds on the support surfaces.Hydrothermal crystallization was then carried out in a synthesis solution with molar ratio of Al2O3:SiO2:Na2O:H2O=1:2:2:120 at 100°C for 4 h.Effects of seeding conditions on preparation of hollow fiber NaA zeolite membranes were extensively investigated.Moreover,hollow fiber membrane modules with packing membrane areas of ca.0.1 and 0.2 m2 were fabricated to separate ethanol/water mixture.It is found that the thickness of seed layer is obviously affected by seed suspension concentration,coating time and vacuum degree.Close-packing seed layer is required to obtain high-quality membranes.The optimized seeding conditions(seed suspension mass concentration of 0.5%–0.7%,coating time of 5 s and vacuum degree of 10 kPa)lead to dense NaA zeolite layer with a thickness of 6–8 μm.Typically,an as-synthesized hollow fiber NaA zeolite membrane exhibits good pervaporation performance with a permeation flux of 7.02 kg·m-2·h-1 and separation factor＞10000 for separation of 90%(by mass)ethanol/water mixture at 75°C.High reproducibility has been achieved for batch-scale production of hollow fiber NaA zeolite membranes by the hydrothermal synthesis approach.

1.Introduction

Zeolite membranes have uniform microporous structure and extraordinary thermochemical stability,presenting excellent performance in the separation of gas or liquid mixtures[1–3].NaA zeolite membrane has an effective pore size of 0.42 nm and strong hydrophilicity,with high selectivity for dehydration of organics by pervaporation(PV).Since Kita et al.[4]reported NaA zeolite membranes on porous α-Al2O3tubes,many efforts have been made to improve zeolite membrane performance for applications[5–7].Industrial PV facilities based on NaA zeolite membranes have been constructed for organic solvent dehydration by Mitsui Shipbuilding[8],GFT-Inocermic[9]and our group in cooperation with Jiangsu Nine-Heaven Hi-tech Co.Ltd.[10].

Currently,commercial NaA zeolite membranes are mainly synthesized on single-channel or four-channel tubular ceramic supports.The membrane modules provide limited packing density of 30–250 m2·m-3.Because of the high transfer resistance through the supports,the as-synthesized NaA zeolite membranes show moderate permeation fluxes,usually between 2–3 kg·m-2·h-1for separation of 90%(by mass)ethanol/water mixture at 75°C[11,12].High fabrication cost is the main obstacle for large-scale applications of separation apparatuses based on NaA zeolite membranes.Recently,NaA zeolite membranes have been successfully prepared on porous ceramic hollow fibers[13,14].Such membrane configuration provides a very high packing density ＞1000 m2·m-3.Moreover,small thickness and finger-like pore structure of ceramic hollow fibers significantly reduce the transport resistance,improving the permeation flux up to 9.0 kg·m-2·h-1for separation of 90%(by mass)ethanol/water at 75°C[14].

Generally,NaA zeolite membranes are synthesized by secondary growth method,in which support surfaces are first coated with a seed layer prior to hydrothermal crystallization.The distribution of seeds on the support surface is a key factor for the preparation of NaA zeolite membranes.Several seeding methods such as dip-coating[6],rubcoating[15],cross- flow filtration[16]and combined seeding[17]have been successfully used for synthesis of tubular NaA zeolite membranes.Hollow fiber supports are featured with low capacity for water suction,which is negative for dip-coating seeding method.To obtain high separation performance,Wang et al.[14]have developed a dipcoating wiping method to prepare a continuous seed layer on the outer surface of hollow fiber support.However,the manual operation is not suitable for large-scale production of hollow fiber NaA zeolite membranes since the supports are really thin and frangible.

Comparatively,vacuum seeding is a simple and effective method for coating seeds on hollow fiber supports.With vacuum suction,zeolite seeds can readily adhere onto the surface of porous hollow fiber supports.The method does not involve complex manual operation,which could be more suitable for the production of hollow fiber zeolite membranes.We have adopted vacuum method for seeding yttrium stabilized zirconia hollow fibers and prepared T-type zeolite membranes[18].More recently,we have investigated preliminarily the seeding approach for the preparation of hollow fiber NaA zeolite membranes[19],with high-quality membranes synthesized by hydrothermal crystallization twice.It is possible to seed multi- fibers in batch by the seeding approach,which is critical for large-scale production of hollow fiber NaA zeolite membranes.In this work,we make an extensive investigation on influences of vacuum coating conditions for the preparation of hollow fiber NaA zeolite membranes in one hydrothermal crystallization process.Optimized coating conditions are used for batch-scale production of membranes.Hollow- fiber membrane modules are also designed and evaluated for dehydration of ethanol/water mixture.

2.Experimental

2.1.Membrane preparation

Home-made porous ceramic hollow fibers were fabricated by a drywet spinning technique.The hollow fibers had an outer diameter/inner diameter of 1.8 mm/0.9 mm,an average pore size of～0.65 μm and a porosity of about48%.Ahollow fiber support with 10–40 cm in length was cleaned by deionized water and then coated with NaA zeolite seeds with an average particle size of about 0.3 μm.The seeds were obtained by milling regular NaA zeolite crystals(average particle size ～2.8 μm)for 3 h with a planetary ball mill(PM-100,Retsch,Germany).

Vacuum seeding method was used to plant NaA zeolite seeds on the outer surface of porous hollow fiber.The schematic diagram of the apparatus for vacuum seeding is shown in Fig.1.Prior to seeding,one end of the support was sealed with a silicone cap,and the other end was connected to a vacuum line.The pores of hollow fiber supports were first filled with deionized water prior to vacuum seeding.After the hollow fibers were immersed in aqueous seed suspensions,vacuuming operation was carried out for 5–30 s while the inner side was extracted through the vacuum line at a vacuum degree of 5–20 kPa.Dip-coating and rubbing seeding methods were also used for comparison.For dip-coating method,the hollow fibers were immersed vertically into 1.0%(by mass)seed suspension for 5 s.For rubbing method,NaA seed paste was first prepared by mixing ball-milled NaA seeds with deionized water at the mass ratio of 1:10.The seed paste was rubbed onto the support surface along axial direction with a brush[17].The seeded hollow fibers were dried at 60°C overnight before membrane synthesis.

NaA zeolite membranes were hydrothermally synthesized on seeded α-Al2O3hollow fiber supports.The synthesis gel was prepared by dissolving sodiumaluminate,sodium hydroxide and waterglass into deionized water with a molar ratio of Al2O3:SiO2:Na2O:H2O=1:2:2:120,as reported in our previous work[20].All the chemicals were industrial grade purchased from commercial companies in China.After being vigorously stirred for 30 min,homogeneous synthesis solution was poured into a Teflon-lined stainless steelautoclave containing a couple of seeded hollow fibers held vertically.Hydrothermal crystallization of NaA zeolite membranes was carried out at 100°C for 4 h.After crystallization,the hollow fiber membranes were washed with deionized water and dried at 60°C overnight.

2.2.Membrane characterization

Morphologies of seeded supports and NaA zeolite membranes were observed by field emission scanning electron microscopy(FE-SEM,S-4800,Hitachi,Japan).The PV performance of as-synthesized hollow fiber membranes was evaluated by dehydration of 90%(by mass)ethanol/water mixture at 75°C.The schematic diagram of the experimental apparatus is shown in Fig.2.A membrane cell containing single or multi hollow fiber membranes was used for performance evaluation.For single membrane test,silicon O-rings were used to seal both ends of the membrane in a stainless cell.For multi-membrane test,a silicon sealant was used to fix a bundle of hollow fibers onto a stainless plate.One end of each membrane was sealed with silicone and the other end was connected to vacuum line.In PV experiments,ethanol/water mixture was introduced through the outside of hollow fiber membrane at atmospheric pressure.The inside of the hollow fiber was extracted with a vacuum pump through a vacuum line,maintained at a downstream pressure below 200 Pa throughout the operation.The permeated vapor mixture was collected by two liquid nitrogen traps in parallel.All the hollow fiber membranes were tested for about 3–5 h to obtain stable results.

Fig.1.Schematic diagram of the apparatus for vacuum seeding.

Fig.2.Schematic diagram of the experimental apparatus for pervaporation.(1)Feed tank;(2)constant- flow pump;(3)temperature indicator;(4)membrane module;(5)vacuometer;(6)cold trap;(7)drying tower;(8)vacuum pump.

Compositions of both feed and permeate were analyzed by a gas chromatograph(GC-2014,Shimadzu,Japan)equipped with a thermal conductivity detector and a packed column of Parapak-Q.The permeation flux(J)through hollow fiber NaA zeolite membranes and separation factor(α)for water over ethanol are respectively defined as

where m is the total mass of permeate,kg;A is the effective membrane area,m2;t is permeation time,h;and yW/yEand xW/xEare mass ratios of water to ethanol in permeate and feed,respectively.

3.Results and Discussion

3.1.Membrane synthesis and PV performance

It has been documented that the quality of seed layer has a significant effect on the separation performance of as-induced zeolite membrane[21].In the vacuum seeding operation,water was extracted through a porous support wall by pressure-driven force,meanwhile the seed particles quickly accumulated on the outer surface of support.To obtain high-performance membranes,we investigated effects of seed suspension concentration,coating time and vacuumdegree on the quality of as-induced membranes.

3.1.1.The effect of seed suspension concentration

The suspensions containing 0.3%–0.9%(by mass)ball-milled seed particles were used for vacuum seeding.The operating pressure for the inner side of all supports was controlled at a vacuum degree of 10 kPa with a coating time of 5 s.Fig.3 shows SEM images of the seeded supports coated with 0.3%,0.5%,0.7%and 0.9%seed suspensions.It can be seen from Fig.3(a)that a low coverage seed layer forms with the low suspension mass concentration of 0.3%.Some α-Al2O3grains of support are observable on the discontinuous seed layer.It is interesting to find that the seed particles preferentially deposit in the dents or pinholes over the support surface,where the resistance is relatively weak.The thickness of seed layer seems much thinner in Fig.3(b).At the seed mass concentration of 0.5%,the coverage is improved,giving a seed layer with a thickness of about 0.8 μm.Some small flaws are on the seed layer after drying,which seems to have no obvious effect on membrane formation,as evidenced for tubular membrane synthesis in our previous work[20].The thickness of seed layer increases significantly,as the seed suspension mass concentration increases to 0.7%–0.9%,indicating that seed particles are apt to accumulate on the support surface at high suspension concentration.Meanwhile,we notice that the cracks on the seed layers are larger at higher seed suspension concentration,which could have a vital influence on the growth of zeolite membranes.

Fig.4 shows SEM images of the as-synthesized membranes.Well inter grown NaA zeolite crystals with clear boundary are observed on the membrane layers for the seed mass concentrations from 0.3%to 0.9%.The membrane layers are composed of cubic NaA zeolite crystals with different sizes because of the broad particle size distribution of ball-milled seeds.The morphologies are similar to those of tubular membranes induced by similar seeds[20].As shown in Fig.4(b),the membrane layer has a thickness of 4 μm at seed mass concentration of 0.3%.Some defects are embedded in the membrane layer(circled area),due to insufficient coating of seeds.At the seed suspension mass concentrations of 0.5%and 0.7%,the thickness of membrane layer increases to 6 and 8 μm,respectively.The membrane thickness increases to 13μm at seed mass concentration of0.9%,with the membrane surface being rough and covering with some isolated zeolite crystals.The phenomena confirm that the seed concentration for coating exerts a strong influence on the formation of membrane layer.It is observed that seed particles distribute sparsely at low seed suspension mass concentration(0.3%),inducing a thin membrane layer with some amorphous substance[Fig.4(a)].At higher seed concentration,a more uniform and denser membrane layer[Fig.4(c)]forms owing to better coverage of seed layer.However,high seed concentration leads to much thicker seed layers with larger cracks after drying,inducing thick zeolite membranes with a large amount of defects.More large zeolite crystals are on the outer surface of the membrane,since more big seeds accumulate on the seed layer surface[Fig.4(e)and(g)].

To further reveal the effect of seed suspension concentration,we synthesized a batch of hollow fiber NaA zeolite membranes under the fixed seeding conditions mentioned above.Five membranes for each seeding conditions were randomly chosen for PV performance evaluation by dehydration of 90%(by mass)ethanol/water mixture at 75°C.The PV separation results are shown in Fig.5.The permeation flux of as-synthesized membranes fluctuates slightly for each seeding condition.Overall,the permeation flux decreases with the increase of seed concentration,which is essentially due to the increased membrane thickness at larger thickness of seed layer.The separation factor increases first and then declines as the seed concentration increases.At the seed mass concentration of 0.2%,the permeation flux was as high as 8 kg·m-2·h-1,but the separation factor was less than 500.As the seed suspension mass concentration increased from 0.3%to 0.5%,the separation factor increased from～1000 to～10,000.For seed mass concentrations of 0.6%and 0.7%,almost all the separation factors were higher than 10,000 and the permeation fluxes decreased modestly.At seed mass concentrations of 0.8%and 0.9%,however,the separation factor was obviously less,probably due to the big cracks in seed layers.Therefore,a moderate seed mass concentration between 0.5%–0.7%is preferable to prepare hollow fiber NaA zeolite membranes via vacuum seeding approach.

Fig.3.SEM images of the supports seeded by seed suspension with different mass concentrations at 10 kPa vacuum degree for 5 s.(a,b)0.3%;(c,d)0.5%;(e,f)0.7%;(g,h)0.9%.

3.1.2.The effect of coating time

To investigate the effect of coating time,we extended the coating time at seed mass concentration of 0.5%and vacuum degree of 10 kPa.Fig.6(a–d)shows SEM images of the membranes with different coating time.The membrane thickness increases from 6 μm to 10 μm as the coating time increases from 5 s to 30 s,while the quality of membrane layers decreases obviously,with more defects on the surface.It may be due to the excessive coating amount of seed particles with long coating time and weakened adsorption affinity of vacuum suction for particles onto the support surface,resulting in loose and non-uniform seed layers.As shown in Fig.6(c),the crystal boundaries are unclear over the membrane with a coating time of 30 s,indicating incomplete crystallization.Large amount of zeolite seeds requires more nutrition compensated for crystallization,which needs more time to complete.

Table 1 gives PV performance of as-synthesized hollow fiber NaAzeolite membranes with various coating time,with all the membranes tested with 90%(by mass)ethanol/water mixture at 75°C.The permeation flux decreases with the increase of coating time,essentially due to the increased membrane thickness.The extension of coating time could reduce the selectivity of membranes.At the same seed mass concentration of 0.5%and vacuum degree of 10 kPa,the separation factor decreased from 17020 to 1560 as the coating time increased from 5 s to 30 s.This could be due to a non-uniform and loose packed seed layer on the support surface with longer coating time.The seed layers may generate large cracks after drying and induce low quality zeolite membrane layers with many defects.In the preparation of hollow fiber T-type zeolite membranes by vacuum seeding approach,we have observed that the loose packed seed layer has a significant effect on the membrane quality[18].As shown in Fig.6(c),the crystal boundaries are unclear without cubic morphology,which could be due to the relatively loose packing seed layeras well.Consequently,a relatively short coating time of 5 s is sufficient for the formation of membrane layer.

Fig.4.SEM images of hollow fiber NaA zeolite membranes prepared with different seed suspension mass concentrations at10 kPa vacuum degree for 5 s.(a,b)0.3%;(c,d)0.5%;(e,f)0.7%;(g,h)0.9%;circled area:defects.

Fig.5.PV performance of hollow fiber NaA zeolite membranes synthesized with different concentrations of seed suspension at 10 kPa vacuum degree for 5 s.

3.1.3.The effect of vacuum degree

Vacuum operation is critical for providing driving force to adhere seeds on support surface.To reveal the effect of vacuum degree,we investigated seed coating under 5 and 20 kPa.The seed mass concentration and coating time were controlled at 0.5%and 5 s,respectively.Fig.6(e–h)shows SEM images of resultant membranes.The thickness of membrane layer increases with the vacuum degree.At lower vacuum degree(5 kPa),a thin membrane layer(5 μm)forms.However,some defects appear on the membrane layer,which could be related with loose packed seed layer.Higher vacuum degree could enhance seed suction,resulting in a compactseed layer.Meanwhile,the thickness of seed layer increases,generating a thicker membrane layer.As shown in Fig.6(h),a dense membrane layer with a thickness ＞10 μm was obtained at a vacuum degree of 20 kPa.

Fig.6.SEM images of hollow fiber NaAzeolite membranes prepared with different vacuum degree and coating time.(a,b)10 kPa,10 s;(c,d)10 kPa,30 s;(e,f)5 kPa,5 s;(g,h)20 kPa,5 s;seed suspension mass concentration:0.5%;circled area:defects.

Table 1 also presents PV performance of as-synthesized hollow fiber NaA zeolite membranes with various vacuum degrees,with all the membranes tested with 90%(by mass)ethanol/water mixture at75°C.Higher coating pressure is beneficial for the densification of NaA zeolite membranes,which could be due to the close packed seed layers.The separation factor maintains at a quite high level even at the coating vacuum degree of 20 kPa,while the permeation flux drops to some extent,due to the thicker seed layer.In order to obtain higher separation factor and permeation flux simultaneously,coating vacuum degree of 10 kPa is appropriate.

Table 1 PV performance of hollow fiber NaA zeolite membranes synthesized under different seeding conditions

In a word,seeding conditions present significant effect on the membrane performance.It is suggested that the optimized coating conditions for the vacuum seeding method be seed suspension mass concentration of 0.5%–0.7%,coating time of 5 s and vacuum degree of 10 kPa.The optimized vacuum conditions are consistent with those reported in our previous work[19],while in this work,due to well controlled seeding operation and support structure,high quality hollow fiber NaA zeolite membranes can be achieved by one hydrothermal crystallization process.The membranes synthesized once are relatively thin due to less crystallization time,which could be beneficial to improve the permeation flux.

Fig.7.SEM images of the support surfaces seeded by dip-coating(a)and rubbing(b)methods.

3.2.Scale-up of membrane preparation

To evaluate the feasibility of vacuum seeding method for large-scale production,longer hollow fiber NaA zeolite membranes were synthesized and their reproducibility was examined under the optimized coating conditions.The hollow fiber supports with lengths of 10,20 and 40 cm were employed for membrane synthesis.For each length,more than 20 hollow fibers were put together for vacuum seeding and then used for a hydrothermal crystallization in a stainless steel autoclave.

We compare the vacuum coating method with other seeding methods such as dip-coating and rubbing.Fig.7 shows SEM images of the support surfaces seeded by dip-coating and rubbing methods.The seeds are sparsely distributed over the support surface with dipcoating method,and rubbing method could not provide a uniform seed layer either.The PV performance of as-synthesized hollow fiber NaA zeolite membranes with 10 cm long is summarized in Table 2.For each synthesis condition, five membranes picked up randomly from a batch of hollow fiber membranes are used for comparison.It can be seen that the seeding method has a significant influence on the PV performances of membranes.The dip-coating method presents permeation fluxes higher than 10 kg·m-2·h-1and relatively low separation factors.Compared to tubular supports,the hollow fiber supports have low water suction capacity,which makes it difficult to achieve uniform seed layer by dip-coating.With rubbing method,the membranes show low reproducibility,indicating the difficulty in controlling quality of seed layers.With vacuum seeding method,all the membranes give high separation factors,indicating its high reproducibility.For 10 cm hollow fiber membranes synthesized in a batch,only 1 or 2 membranes exhibit lower separation factor between 500–1000.The membranes show relatively low permeation flux due to thicker seed layers compared to those with dip-coating and rubbing methods.It is also noted that some seed particles may be trapped into support pores,which could induce some large zeolite crystals and block the transport channels[Fig.4(d)].

Table 3 shows the PV performance of five hollow fiber membranes with lengths 20 and 40 cm randomly picked up for evaluation.For 20 cm membranes,separation factors are at a relatively high level(＞3000),while for 40 cm membranes,they are only 800–2400.It is speculated that it is more difficult to control the uniformity of seedlayer for long supports,resulting in some defects in membrane layers.More optimization on the seeding condition is necessary for the preparation oflong hollow fiber membranes.However,such separation factor is acceptable for industrial applications[22].Therefore,it is feasible to adopt the vacuum seeding method for large-scale production of hollow fiber NaA zeolite membranes.

Table 3 Reproducibility for synthesis of hollow fiber NaA zeolite membranes with different lengths

We noted that the permeation flux of as-synthesized membranes showed a decreasing trend with hollow fiber length,e.g.from about 7 kg·m-2·h-1for 10 cm to 4 kg·m-2·h-1for 40 cm.It was not clear if the reduced permeation flux was due to the increase of membrane thickness or mass transfer resistance through the lumen of hollow fiber.Thus we cut the membrane M28 to four pieces with 10 cm long,marked as M28-1,M28-2,M28-3 and M28-4 from top to bottom as stood in a synthesis autoclave.Table 4 lists the PV performance for these pieces.The permeation flux of M28-1 is very high,up to 8.13 kg·m-2·h-1,and that of the piece near the bottom is relatively low(5.78 kg·m-2·h-1),due to larger membrane thickness.Since the seed layer is well controlled,thicker membrane layer is attributed to higher nutrient concentration of the synthesis gelat the bottom because of amorphous particle sedimentation in the synthesis process.All pieces have higher permeation flux than that measured as a whole fiber.The result suggests that the mass transfer resistance through the lumen of hollow fibers could be significant,affecting the separation efficiency of the hollow fiber zeolite membranes.On the one hand,along the length of hollow fiber,the amount of permeate increases and the flow rate of vapor inside the hollow fiber increases.Higher vapor flow rate causes significant pressure drop,decreasing the driving force for pervaporation.

Table 2 Comparison of PV performance of as-synthesized hollow fiber NaA zeolite membranes prepared by secondary growth with different seeding methods

Sample α J/kg·m-2·h-1Membrane thickness/μmOn the other hand,in PV separation,the lumen of hollow fiber is operated under vacuum condition,where water molecules have large mean free patch.Frequent collisions between water molecules and support walls would occur in the lumen due to small inner diameter of hollow fibers,resulting in transfer resistance.Therefore,the lumen size should be optimized for matching the permeation performance of hollow fiber NaA zeolite membranes.

Table 4 PV performance of four pieces for the hollow fiber NaA zeolite membrane M28

3.3.Hollow fiber membrane modules

Membrane modules packed with hollow fiber NaA zeolite membranes were designed for practical application.Fig.8 shows the photographs of the membrane modules with packing area of ca.0.1 and 0.2 m2.Both membrane modules have 105 pieces of hollow fiber membranes,divided equally into 7 bundles and sealed in a stainless steel cylinder with inner diameter of 5 cm.The effective membrane lengths are 18 and 35 cm for the membrane modules,with the same packing density of 300 m2·m-3.The membrane modules were used for dehydration of ethanol/water mixture with an initial mass composition of 70%ethanol/30%water at 75°C by cycling mode.The feed solution had an initial mass of about 3.5 kg and feed flow rate was maintained at 2.5 L·min-1.

Fig.9 shows time dependence of ethanol dehydration performance for the two modules.The initial permeation flux for the 0.1 m2module was 4.37 kg·m-2·h-1while that for 0.2 m2module was 3.05 kg·m-2·h-1.In the dehydration process,the water content on the feed side decreased continuously and permeation flux decreased as well,while that on the permeate side was larger than 90%(by mass)at feed water mass content＞2%.As the feed water content decreased further,the water content on the permeate side decreased to some extent.The permeation flux of 0.1 m2membrane module was always greater than that of 0.2 m2membrane module for the same feed water content.The average permeation flux of 0.1 m2membrane module was about 1.5 times that of 0.2 m2module.Typically,the average permeation fluxes were 2.67 and 1.76 kg·m-2·h-1,respectively,for 0.1 and 0.2 m2modules at feed water mass content of 10%.Although the hollow fiber membrane modules exhibit high separation efficiency,the performance may be affected by concentration polarization,with their average permeation fluxes only about half of those measured with single hollow fiber membranes.Due to the non-uniform flow distribution on the shell side of hollow fibers,the concentration polarization may occur easily on the feed side[23,24].The concentration polarization may be more serious for longer hollow fibers since more water is removed through the membranes.More efforts are required to optimize the configuration of hollow fiber membrane modules,which is ongoing in our laboratory.

Fig.9.PV performance of membrane modules for dehydration of ethanol/water mixture at 75°C by cycling mode.(a)0.1 m2 membrane module;(b)0.2 m2 membrane module;initial mass composition of feed mixture:70%ethanol/30%water;feed flow rate:2.5 L·min-1.

Fig.8.Photographs of hollow fiber NaA zeolite membranes and membrane modules.

4.Conclusions

NaA zeolite membranes were hydrothermally synthesized once on the outer surface of α-Al2O3hollow fiber supports pre-coated with ball-milled NaA seeds by vacuum seeding method.Qualities of as synthesized membranes are affected by seed suspension concentration,coating time and vacuum degree.High-performance membranes can be produced reproducibly in batch scale under the optimized coating conditions.Typically,an as-synthesized membrane exhibits a permeation flux of 7.02 kg·m-2·h-1and separation factor＞10,000 for separation of 90%(by mass)ethanol/water solution at 75°C.For longer hollow fiber membranes,the transfer resistance through lumen presents a significant effect on the permeation flux.Hollow fiber membrane modules fabricated in our laboratory have been successfully used for dehydration of ethanol/water mixture.Further optimization on configuration of membrane modules is required to eliminate the effect of concentration polarization.

Nomenclature

A effective area of membranes,m2

J permeation flux,kg·m-2·h-1

m total mass of permeate,kg

t permeate time,h

xW/xEmasst ratio of water to ethanol in feed

yW/yEmass ratio of water to ethanol in permeate

α separation factor

Subscripts

E ethanol

W water