| Due to unique pore structure, good thermal and chemical stability, A-type zeolite was wildely used in adsorption and separation field. A-type zeolite has a pore diameter of ca.0.42nm, which is close to the small molecule gases and light-alkane molecules. So it has good separation selectivity to gases. At the same time, owing to the low silicon-aluminum ratio (Si/Al=1), A-type zeolite has perfect hydrophilicity, and the A-type zeolite membranes can be used for dehydration of high concentration organics, especially for bioethanol dehydration. With these advantages, the perspectives of A-type zeolite membranes have received widespread attentions all over the world. In1998, Mitsui Engineering and Shipbuilding Co. Ltd. in Japan built the first large scale pervaporation (PV) plant using tubular A-type membranes for dehydration of bioethanol solvents, which made the first step of zeolite membranes in industrial application. Because of the high cost of the supports and the immaturity of membrane synthesis technology, the cost of A-type zeolite membrane is still very high, so the separation process by A-type zeolite membrane has not yet been used in the world. Therefore, the research of A-type zeolite membrane is very important in both academic and practical applications.Thus the purpose of this article is to synthesize low cost and high performance A-type membrane. By using inexpensive large-pore a-alumina support, designing and constructing alumina hollow fiber and zeolite/polymer composite hollow fiber support, the cost of zeolite membrane will be decreased; and by creating a new seeding method the reproducibility of zeolite membrane will be improved. The dehydration performance of as-synthesized membranes is studied by pervaporation of90wt%ethanol/water mixture under75℃. The main experimental results and conclusions are summarized as follows:(1) Preparation of zeolite A-type membranes on the surface of inexpensive large-pore α-alumina tube supportsAs people often synthesized zeolite membranes on the out surface of α-alumina tubular support (out diameter:12mm) with an intermediate layer (average pore size< 1μm), which leads to high price of zeolite membranes. The large-pore (>1μm) a-alumina tubular supports can be prepared at one time, which largely reduces the cost of the support. But due to the large defects (>10μm) on its surface, it is a great challenge to synthesize high performance A-type zeolite membranes on the surface of this kind of support. Here we presented a novel seeding method, rubbing a seed paste, in which a small amount of the synthesis hydrogel is mixed with small zeolite crystals (seeds)(crystals/gel=9:1) and used as a binding agent to paste the seeds to the surface of the support. By this new method, not only the large defects can be filled up, but a uniform seeding layer on the outer surface of the support can be obtained. A-type zeolite membranes with high separation factor (>10000) and high flux (3.5-4.0kg/m2.h) can be fabricated by once hydrothermal synthesis. At the same time, by wiping the seed paste, a high performance A-type zeolite membrane can be synthesized on the inner surface of the large-pore a-alumina tubular support. The separation factor of the membrane is more than10000, and its flux is as high as5.0kg/m2h. The effect of NaOH in the ethanol aqueous feed on the flux of A-type zeolite membrane was investigated. It is found that NaOH in the ethanol aqueous feed decreased the flux of the zeolite membrane. It is also found that the washing step has a great influence on the separation property of the membrane.(2) Preparation of zeolite A-type membranes on the surface of a-alumina hollow fiber (HF) supportIn order to improve the surface of zeolite membrane and reduce the size of membrane modules, high performance A-type zeolite membrane is synthesized on the outer surface of a-alumina hollow fiber support (outer diameter<2mm, surface area/volume ratio>1000m2/m3). Here we proposed a new seeding method: dipcoating-wiping seeding method. After wipping, a trace amount of "seeds" are obtained and seeds can enter into the defects on the surface of hollow fiber, and then A-type zeolite membranes with high performance and high reproducibility can be successfully obtained by one single in-situ hydrothermal synthesis. The separation factor of the membrane is more than10000, and the flux is as high as9.0kg/m2.h, which is four times higher than the membrane synthesized by Mitsui Engineering and shipbuilding Co. Ltd used in industrial application. And the relationship between the flux of A-type zeolite membrane and the porosity of the support is also investigated. It is found that the flux of the membrane was mainly associated with the porosity of the supports. The flux was almost increased linearly with the increase of the support porosity.(3) Preparation of zeolite A-type membrane on the surface of the composite hollow fiber (CHF)Zeolite/polymer (PES) composite hollow fiber (CHF, outer diameter less than2mm), a new kind of support is designed. A-type zeolite crystals are uniformly dispersed into the PES solution, and then the composite hollow fibers (CHF) were spun by a dry-wet spinning process. Without calcination process, it greatly cut the cost of the support. While zeolite crystals distribute homogeneously on the the inner and outer surface of the CHF, which serve as seeds for the zeolite membrane growth, a complex seeding process could be omitted, and a high performance A-type zeolite membrane can be directly synthesized by once hydrothermal synthesis. The separation factor of the membrane was more than10000, and the flux reached8.0to9.0kg/m2.h. With the advantage of no seeding process for CHF supports, we studied the influence of the synthesis hydrogel composition (silica sources and alkalinity) on the synthesis of zeolite membrane. It is found that the solubility of silica source is a crucial factor for the zeolite membrane synthesis. If the silica source existed as monomers (such as sodium metasilicate nonahydrate), the crystallization rate of the synthesis hydrogel is fast, the nutrition of the hydrogel is consumed fast by the synthesis hydrogel itself, leading to that the synthesis hydrogel has not enough nutrition for zeolite membrane growth. Increasing the alkalinity can increase the solubility of the hydrogel, and accelerate the crystallization rate of the synthesis hydrogel, leading to the bad growth and non-uniform of the membranes. Dynamic synthesis process can improve the nutrition distribution status of the synthesis hydrogel, and zeolite membranes with more uniform thickness can be synthesized, which improves the reproducibility of the synthesis of zeolite membranes. |
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