| Zeolite membranes are a new type of inorganic membrane materials developed in recent years,which can be used in molecular sieving.Because of its unique sieving property,hydrophilicity/hydrophobicity and selective catalytic properties,it has broad application prospects in the field of removal of trace water in organics and separation of small molecules.NaA zeolite membrane is a hydrophilic membrane material,which is the most suitable for the separation of organic/water mixture at present.However,the synthesis of NaA zeolite membrane is mainly prepared on the outer surface of the tubular support,has the disadvantages of easy collision,small operating space,large permeation resistance and low permeation flux.Therefore,the synthesis of zeolite membrane on the inner surface of the tube is an important way to improve the separation performance and to realize the preparation of the zeolite membranes on the inner surface of the multi-channel tubes.In this paper,based on the analysis of the present research situation of zeolite NaA membranes,the route of preparing zeolite membrane on the inner surface of porous ?-Al2O3 ceramic tube was determined.The zeolite NaA membrane was prepared on the inner surface of nano porous ceramic tube,microporous ceramic tube and macroporous ceramic tube,and then investigated their dehydration performance in four kinds of organic compounds,the effects of temperature,pressure and concentration on the separation performance were studied.The main research contents and results are as follows:Firstly,the NaA zeolite membranes were synthesized on the inner surface of the ceramic tube(pore size?100 nm,tube inner diameter?3 mm)by static and dynamic in situ synthesis.A comparative study of the support placement(horizontal and vertical)by static in-situ synthesis process and the effects of the rotational speed,the synthesis temperature and the synthesis time on the microstructure and separation properties of the zeolite membrane by dynamic-in situ synthesis process,and the growth mechanism of zeolite membranes in different synthetic processes was discussed.The results show that the gravitational effect is the main factor affecting the microstructure and pervaporation performance of the membrane during the static synthesis process.When the support is placed horizontally,the film at the top of the inner wall of the fiber tube is dense as a single layer while the bottom is dense and stacked double-layer structure.When the carrier is placed vertically,the synthetic solution produces a concentration difference due to gravity,which leads to the thin and discontinuous film on the upper of the support,and the lower is thick and easy to form a larger intergranular pore.So it is impossible to obtain the NaA zeolite membrane with uniform structure and good performance.In order to overcome the influence of gravity,the rotational speed,synthesis temperature and synthesis time are the important factors that affect the structure and properties of zeolite membrane by dynamic in situ synthesis.When the speed is too small,the membrane structure and properties are similar to those of the static method,while the speed is too large,it can not form a continuous zeolite membrane.When the rotational speed is 10 r/min,the zeolite NaA membrane with smooth surface,dense continuity and thickness of 6 ?m is synthesized at 90? for 3 h.The pervaporation results showed that the permeationflux of as-synthesized membrane was 2.3 kg/m 2·h,and the separation factor was 5316,which showed a good separation performance for the concentration of 90 wt.%ethanol solution.Secondly,the NaA zeolite membrane was prepared on the inner surface of the microporous ceramic tube(pore size?1.4 ?m tube inner diameter?7 mm).In this paper,the porous ceramic tubes were prepared by dip coating method,and the microstructure,pore characteristics and application properties of the porous ceramic tubes were compared with that of the conventional extrusion method.The results show that the porous support were prepared by dipping method is more uniform,the average pore size is 1.4 ?m,which is more suitable for the support of NaA zeolite membrane.Then,the NaA zeolite membrane was prepared on the inner surface of the microporous ceramic tube by in situ synthesis and secondary growth method,respectively.The effects of different preparation methods on the structure and properties of zeolite membranes were studied.In addition,the effects of parameters such as NaA seed dispersant,the concentration of seed solution and the number of coating on the synthesis of zeolite membranes were studied.The results show that the in situ synthesis method cannot synthesize the dense NaA zeolite membrane on the surface of porous microporous.However,NaA zeolite membrane prepared by secondary growth method has no obvious defects and the separation performance is obviously improved.Under the condition of MC concentration of 0.8 wt.%and the seed concentration of 1 wt.%,a seed layer with a thickness of about 5?10?m and without defects could be formed.The seeded support was synthesized at 90? for 3 h and repeated 2-3 times to obtain a complete and dense NaA zeolite membrane.The pervaporation results showed that the as-synthesized membrane had good permeation selectivity,the flux is 1.95 kg/m2·h and the separation factor is 4624 for the mixture of 90 wt%ethanol solution.Thirdly,the NaA zeolite membranes were synthesized on the inner surface of the porous ceramic tubes(pore size-4.3?m,tube inner diameter-7 mm)by zeolite precursor dynamic modification method.In this paper,the modified layer was prepared on the surface of the tube,which was used to modify the pore and improve the smoothness of the support.Then the zeolite membrane was synthesized on the modified layer by hydrothermal synthesis.The effects of preparation conditions such as the nature of the modified solution,rotational speed,modification temperature,synthesis time and number of synthesis on the modification layer and membrane structure were studied.The latex-like modified solution with the ratio of Na2O:Al2O3:SiO2:H2O=2:1:2:120 can form a flat and continuous modification layer and synthesize the dense and continuous NaA zeolite membrane.The lower the temperature,the longer the formation of the modified layer,and will make the modified layer is subjected to high alkalinity solution erosion.Excessive temperature will cause the modified solution penetrate into the inside of the tube and cannot form a modified layer,thereby affecting the quality of the zeolite membrane.The optimum modification conditions were modified at 70? for 2 h.The rotational speed has a large effect on the thickness of the modified layer and the zeolite membrane,low rotating speed will lead to the modified layer is too thick,and high speed cannot form a modified layer.When the rotational speed is 10 r/min,a modified layer with uniform coverage and a thickness can be formed,and the NaA zeolite membrane synthesized on that modified layer is dense and without defects.In the hydrothermal synthesis of zeolite membrane,the synthesis time and the number of repetitive synthesis are the two most important factors.The synthesis time is too short,the crystallinity is low and the NaA zeolite membrane is poorly crosslinked.On the contrary,the synthesis time is too long is easy to become other crystal.After the first hydrothermal synthesis,the surface of the support cannot form pure phase NaA zeolite membrane.Too many times of repeated crystallization can easily lead to the increase of the membrane thickness and the cracks.By optimizing the synthesis parameters,the NaA zeolite membrane was synthesized twice or thrice with good separation performance at 90? for 3 h,the as-synthesized membrane had good permeation selectivity,and the flux and separation factor were 4.72 kg/m2·h and 529 for the mixture of 90 wt.%ethanol solution.Finally,the pervaporation performance of NaA zeolite membrane and its influencing factors were studied.Four mixtures of methanol/water,ethanol/water,acetone/water and dimethyl carbonate/water were selected as feed solution.The effects of feed concentration,operating temperature and pressure difference on the dehydration performance of NaA zeolite membranes were investigated.Then,the separation mechanisms of zeolite membrane were studied.The results show that the feed temperature,the concentration and the pressure difference between two sides of the membrane have a great influence on the permeation flux of the zeolite membrane,but the effect on the separation factor is small.Among them,the pressure difference on the separation factor is more obvious.Considering the influence of three factors,the separation of the methanol/water mixture by NaA zeolite membrane is suitable for operation at less than 70 ? and low pressure of 0.1 MPa,the ethanol/water and acetone/water mixture should be operated at 70? and below 0.5 MPa,while dimethyl carbonate/water has the best separation performance at suitable temperature(50?100?),ethanol concentrations(80?95 wt.%)and lower pressure difference(<0.5 MPa).Essentially,pervaporation separation is mainly controlled by the membrane structure,physical and chemical properties of the components to be separated,and the interaction between the separation system and the membrane.The driving force of the pervaporation process is due to the reduction of the chemical potential at the downstream side of the flow and therefore does not require high pressure.However,pervaporation is a phase transition from liquid to gas,so it needs to be heated to complete the process.In this paper,a continuous and dense NaA zeolite membrane was prepared on the inner surface of nanoporous ceramic tube,microporous ceramic tube and macroporous ceramic tube,which solves the problems existing in the preparation of the molecular sieve membrane on the outer surface of the tubular support.The as-synthesized membranes showed good separation performance when were applied to the dehydration of organics in various systems,which is of great significance to promote the industrial production of NaA zeolite membrane on the inner surface of tubular porous ceramics. |
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