| Zeolite membranes,due to their uniform pore structures and unique hydrophilicity/hydrophobicity,are suitable for the separation of gas mixtures and liquid mixtures.Especially,zeolite NaA membranes,because of its uniform pore size(0.42 nm)and strong hydrophilicity,are suitable for the dehydration of organic mixtures,such as alcohols,lipids and nitriles.At present,the zeolite NaA membranes have been industrialized by Mitsui Engineering&Shipbuilding Co.Ltd.,Dalian HST Technology Co.Ltd.and Jiangsu Nine Heaven High-Tech Co.Ltd.Compared with the conventional distillation,extraction and adsorption separation,membrane separation does not need to introduce the third component,and the device is compact and low energy-consumption.Compared with the organic membranes,zeolite membranes show good separation performance,long life and good temperature resistance.But their applications are greatly limited due to the high cost.This paper aims to reduce the cost of zeolite membranes.On the one hand,by manipulating the membrane microstructure and reducing the resistance of the support side,the flux of the zeolite membranes is improved,thereby increasing the separation efficiency.On the other hand,the alumina hollow fibers containing mullite phase and the alumina hollow fibers containing camegieite(NaA zeolite transformation at high temperature)are prepared separately.The interaction between the support and zeolite is improved,leading to the formation of dense zeolite membranes without seeding in a clear solution.It simplifies the synthesis method,thereby reducing the synthesis cost of the zeolite membranes.(1)Manipulation of the surface microstructure of zeolite membranes and the influence on its pervaporation fluxBy adding CDAs(crystallization improving agents)to the gel synthesis solution(2.2Na2O:2SiO2:Al2O3:150H2O),the structure of zeolite NaA membranes on the the macroporous alumina tubes is manipulated,thereby improving the pervaporation flux of the membranes.The influence of synthesis time and the amount of the CDAs addition on the synthesis process,membrane surface morphology and pervaporation performance of zeolite membranes have been investigated.It is found when the synthesis time is 2 h,the weight and the flux of the membrane increase with the increase of the amount of CD As addition,but the separation factor of the membrane decreases.When the synthesis time is 3 h,the weight of the membrane decreases with the increase of the amount of CDAs addition though the separation factor is over 10000.When the amount of CD As addition is 10 wt%,the zeolite NaA membrane has the highest flux of 4.53 kg m-2 h-1 with the separation factor over 10000.The flux of the zeolite NaA membrane is increased by 14%compared with that of the zeolite NaA membrane without CDAs addition.When the synthesis time is 4 h,the weight and the flux of the membrane decreases with the increase of the amount of CDAs addition though the separation factor is over 10000.By characterization of SEM and contact angle measurements,it is found that the addition of the CDAs changes the adhesion of the bulk gel particles on the membrane surface during the synthesis process,thereby changing the surface roughness of the zeolite membranes.Then the hydrophilicity of zeolite membranes has been affected,leading to the change of its pervaporation flux.The zeolite membrane synthesized for 3 h(10 wt%CDAs addition)has the roughest surface,thus the flux is highest.At the same time,the influence of the surface roughness of the zeolite membrane on its pervaporation flux is verified by polishing and short-time re-synthesis after polishing.(2)Preparation of alumina disks by phase inversion-sintering method and the pervaporation performance of the zeolite membranesA series of alumina disks with different morphology,porosity,thickness and pemeability are prepared.The effects of the solvent amount,the coagulation bath termperature,the alumina particle size,and the support/casting solution thickness on the phase inversion process are investigated.It is found that the solvent amount has little effect on the morphology and structure datas of the disks,leading to the small influence on the flux of zeolite NaA membranes.The higher phase inversion termperature inhibits the formation of finger-like macropores,leading to disk with higher resistance which is not suitable for the preparation of zeolite membranes with higher flux.Large alumina particles(2 ?m)produce the disks with large pore size and high permeability.But due to their large pore size and crack defects,the dense zeolite NaA membranes can not be prepared by the one-step secondary growth.A portion of 2 ?m alumina particles is replaced with 0.1 ?m alumina particles which act as bridges in the sintering process,improving sintering and reducing the pore size of the support.As the amount of small alumina particles addition increases,the resistance of the disk increases,resulting in a decrease in the flux of the membrane.When the mass ratio of the small particles(0.1 ?m)and large particles(2 ?m)is 5:95,the dense zeolite NaA membranes can be obtained on the prepared disks(thickness:2.2 mm)by the secondary growth with the separation factor of 8350 and the flux of 16.4 kg m'2 h-1.When the mass ratio of small particles(0.1 ?m)to large particles(2 ?m)is 20:80,the dense NaA zeolite membrane can be obtained on the prepared disks(thickness:2.0 mm)by the secondary growth with the separation factor of 9957 and the flux of 13.0kg m-2 h-1.Because the thickness of the finger-like macropores layer increases with the increase of the thickness of the casting solution in a certain range,the influence of the support thickness is offset,leading to the small decrease on the flux of the NaA zeolite membranes.(3)Preparation of zeolite NaA membranes by pre-dipping clear solution on thed alumina hollow fibers containing mullite phaseAdding silica powders to the conventional alumina spinning precursor to change the surface property of the prepared hollow fibers has been changed.The hollow fibers are pretreated by dipping a clear solution aged for short time and zeolite NaA membranes are synthesized in the clear solution.It is found that with the increase of the content of silica addition,the mullite content,pore size and the pure water flux of the hollow fiber increases;the porosity increases firstly and then remains basically the same;the separation factor of the membranes increases firstly and then decreases.When silica:silica+alumina(mass ratio)is 4.78%,the hollow fibers can be obtained with the porosity of 67.2%and average pore size of 282 nm.Dense zeolite NaA membranes are obtained with separation factor over 10000 and high flux of 10.8 ± 0.2 kg m-2 h-1.When the proportion of silica reaches 28.18%,the hollow fibers with the large pore size of 834 nm are composed of pure mullite phase.And it is difficult to obtain a dense membrane,so the separation factor is only a few hundred.The method presented in this paper has two main advantages.On the one hand,the nuclei of the pre-dipping clear solution play roles like seeds,which avoids the synthesis of nanoseeds and the dip-coating rubbing step,then simplifies the membrane-preparation process.On the other hand,the amount of active hydroxyl groups of the mullite phase is more than that of the alpha-alumina phase.The abundant hydroxyl groups improve the interactions between zeolite and the support,leading to the migration of nutrition in the bulk solution to the support surface spontaneously.Therefore,the dense zeolite NaA membranes have been prepared.(4)In situ hydrothermal synthesis of zeolite membranes in clear solution on the modified supportsWhen adding appropriate amount of NaA zeolite crystals into the conventional alumina spinning precursor,the zeolite crystals collapse and change into carnegieite after phase-inversion and sintering.The camegieite can improve the density of the zeolite membrane prepared in clear solution by in-situ hydrothermal synthesis.The influences of the amount of crystals,the calcination time,the calcination temperature and the size of crystals on the microstructure,porosity,permeability and membrane properties of the hollow fibers are investigated.It is found that adding NaA zeolite crystals can improve the mechanical strength of the prepared hollow fibers.When 10 wt%1.5 ?m NaA crystals are added and calcined at 1400 ? for 4 h,the prepared hollow fibers are the most suitable supports for zeolite membrane preparation.Small NaA zeolite crystals addition is not conducive to the formation of dense membranes,and high calcination temperature/long calcination time leads to the significant decrease of the membrane flux.The separation factor of zeolite NaA membranes by in-situ hydrothermal synthesis at 80 ? for 4 h is over 10000 and the flux is 9.8 kg m-2 h-1.The separation factor of zeolite FAU membranes by in-situ hydrothermal synthesis at 75 ? for 24 h is over 1800 and the flux is 8.8 kg m-2 h-1.The separation factor of zeolite MFI membranes by in-situ hydrothermal synthesis at 175 0C for 20 h is 57 and the flux is 4.7 kg m-2 h-1.We speculate that the carnegieite facilitates the formation of zeolite nucleation sites on the support surface,making it easier to obtain dense zeolite membranes by in-situ hydrothermal synthesis.This method has universality and provides new ideas for in-situ hydrothermal synthesis of zeolite membranes. |
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