【作者】 葛琴琴；

【导师】 王正宝；

【作者基本信息】 浙江大学 ， 化学工程， 2012， 博士

【摘要】 A型分子筛由于具有规整的孔道结构、优异的热稳定性和化学稳定性,广泛应用于吸附与分离领域。A型分子筛的有效孔径为0.42nm,与小分子气体和低碳烷烃分子的动力学直径接近,因此对气体分离具有很好的选择性。同时A型分子筛由于具有很低的硅铝比(硅铝比等于1),具有很强的亲水性,由它构成的分子筛膜可以通过渗透汽化将高浓度有机物中的水脱除,特别对于生物乙醇的脱水。正是基于这些优点,A型分子筛膜的研究受到国内外学者的广泛关注。1998年,日本三井造船公司首次建成了用于乙醇脱水的A型分子筛膜分离装置,迈出了分子筛膜工业化应用的第一步。由于载体成本高,制膜技术不成熟,导致分子筛膜的价格高,因而A型分子筛膜分离至今没有在全世界得到推广。因此,A型分子筛膜的研究具有重要的学术意义和实际应用价值。本文的研究目标为低成本高性能A型分子筛膜的制备与应用。通过采用廉价大孔氧化铝,设计和构建氧化铝中空纤维及分子筛／聚合物复合中空纤维为载体来降低分子筛膜的成本；通过创制新型涂晶技术来提高分子筛膜的成膜率。合成的A型分子筛膜在75℃下对浓度为90wt%的乙醇／水溶液进行渗透汽化表征,获得了优异的分离性能。主要研究内容和结果如下：(1)廉价大孔氧化铝管表面成膜人们通常在具有中间层(平均孔径小于1μm)的氧化铝管(外径12mm)表面制备分子筛膜,从而造成分子筛膜的成本提高。直接采用一次性制备、平均孔径大于1μm的廉价大孔氧化铝管为载体,可以降低分子筛膜的生产成本,但是该载体表面局部存在很大的缺陷孔(>10μm),所以在该类载体表面合成A型分子筛膜具有很大的挑战性。本文提出了一种新型晶种浆料涂晶法,将晶种与部分合成液(9:1)调成晶种浆料(固含量50wt%)后擦涂至载体表面。通过该涂晶方法不仅可以有效地填补载体表面存在的大孔缺陷,而且可以在载体表面覆盖一层均匀的晶种层。通过一次水热合成可以获得高分离因子(>10000)和高通量(3.5-4.0kg/m2.h)的A型分子筛膜。同时,通过滚擦晶种浆料法,也可在大孔氧化铝载体内表面成功合成高性能的A型分子筛膜,膜的分离因子大于10000,通量可达5.0kg/m2.h。考察了乙醇水溶液中NaOH对氧化铝管载体上合成的A型分子筛膜通量的影响。研究发现,乙醇水溶液中的NaOH对分子筛膜的分离通量有降低作用。研究还发现膜的洗涤对分离性能有重要影响。(2)氧化铝中空纤维载体(HF)表面成膜为了提高分子筛膜的表面积,减小膜组件的规模,采用氧化铝中空纤维(外径<2mm)为载体(表面积／体积比>1000m2/m3)合成具有高分离性能的A型分子筛膜。本文提出了一种新型浸涂-滚擦涂晶法,通过滚擦处理后载体表面覆盖均匀的晶种痕迹,使晶种进入载体缺陷,通过一次水热合成就可以高重复性地获得具有高分离性能的A型分子筛膜,其分离因子大于10000,通量高达9.0kg/m2.h,是日本三井造船公司工业化膜的4倍以上。同时研究载体孔隙率对A型分子筛膜通量的影响,研究发现分子筛膜的通量主要取决于载体的孔隙率,随着载体孔隙率的增大而提高,两者几乎成线性关系。(3)复合中空纤维载体(CHF)表面成膜设计构建了一种新型分子筛膜载体,即分子筛／聚合物(聚醚砜,PES)复合中空纤维(CHF)(外径<2mm)。CHF载体是将A型分子筛晶体与聚醚砜在有机溶剂中均匀地混合后,通过干-湿法纺丝技术制备而成,无需高温煅烧,极大地降低了分子筛膜载体的成本。复合中空纤维载体的内、外表面具有均匀分布的分子筛晶体,可作为分子筛膜生长的晶种,省略了制膜过程中复杂的预涂晶过程,直接通过一次水热合成,高重复性地获得了高分离性能的A型分子筛膜。其分离因子大于10000,通量高达8.0-9.0kg/m2.h。鉴于CHF载体无涂晶过程的影响,考察了合成液组成(硅源和碱度)对合成A型分子筛膜的影响。结果表明,硅源在合成液中的溶解度对合成A型分子筛膜至关重要。如当硅源以硅单体(如九水偏硅酸钠)的形式存在于合成液中时,合成液自身的晶化速度快,导致合成液营养快速消耗,无法为载体表面分子筛膜的生长提供充足的营养。提高碱度可以提高硅源在合成液中的溶解度,从而加速合成液自身的晶化速度,影响分子筛膜的生长,造成膜的上下段不均匀。使用动态合成可以使合成液中的营养均匀分布,获得膜厚均匀的分子筛膜,从而提高了分子筛膜的成膜率。更多还原

【Abstract】 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.更多还原