Preparation And Separation Performance Of NaA Zeolite Membranes On Composite Hollow Fibers

Zeolite NaA membrane has shown high efficient pervaporation separation performance in dehydration of organic solvents. While due to high cost of support and low reproducibility of membrane synthesis, its industrial application needs to be progressed in regard of the first industrial zeolite NaA membrane separation plant developed by Japan in 1998. As an emerging, economical and effective support of zeolite membranes, hollow fiber has high packing density and thus high membrane surface per module volume enabling to broaden zeolite NaA membrane’industrialization. As a kind of hollow fiber, zeolite/polymer composite hollow fiber support is more economical but has relatively lower mechanical properties. Therefore, this article aims at preparing blend composite hollow fiber as a new type support to improve mechanical properties, following characterization of pervaporation performance of zeolite NaA membrane on the surface of supports. Effects of blend modification and heat treatment method are studied, and modelling data are discussed to analyse influences of pervaporation process, and finally some researches have carried on tri-bore supports.（1） Research in blending modification of preparation blend composite hollow fiber （BCHF） and synthesizing zeolite membraneDuring phase inversion of BCHF preparation, effects of polyimide （PI） content and N₂ extrusion pressure on the structures of supports are investigated. Through blending polyimide with polyethersulfone （PES）, BCHF-ZO-85 （ZO-85 means 4 um NaA zeolite crystals with 85 wt%in the support） with much more spongy region can be prepared. On the other hand, BCHF with more spongy region can be obtained by raising N₂ extrusion pressure. It is said that spongy region is beneficial to the mechanical properties of the support which consists poor finger-like region, spongy region and rich finger-like region from external to internal in the cross section. It is found that mechanical properties are strengthened by blending PI with PES and increasing N₂ extrusion pressure. Moreover, the synthesis and pervaporation properties of NaA zeolite membranes on BCHFs are researched. It is found that NaA zeolite membrane synthesized on the inner surface of BCHF-10%PI-Z0-85 （10% means PI content in polymer） has excellent separation performance with flux above 10 kg m^-2 h^-1 and separation factor over 10,000. As a result, the appropriate conditions are PES/PI=9/1 with N₂ extrusion pressure from 0.07 to 0.09 MPa. In addition, BCHF-10%PI-Z1-75 （Z1-75 means 1.5 μm NaA zeolite crystals with 75 wt% in the support） with relative high mechanical strength can be prepared by reducing zeolite content in the support and choosing small zeolite crystals, and zeolite NaA membranes synthesized on the support are well inter-grown and shows high pervaporation separation performance.（2） Influences of heat treatment on separation properties of zeolite NaA membrane on the BCHFsHeat treatment method is applied to improve separation properties of zeolite NaA membrane on the blend composite hollow fiber. The structures of BCHFs are modified with temperature raising during heat treatment, leading to decrease of porosity and increase of pure water flux. The support investigated here is BCHF-10%PI-Zl-75 based on its high mechanical properties and excellent membrane sysnthesis which can be prepared on the inner and outer surface of support. For inner-side zeolite NaA membrane, it is suggested that heat treatment is carried out after membrane synthesis, and dehydration flux of 90/10 wt% ethanol/water mixture increases with increasing heat treatment temperature. While high heat treatment temperature decreases the pervaporation flux, and heat treatment at appropriate temperature for long time has the same influence, which is suggested by the blockage of polymers on the zeolite membrane channels. The suitable conditions of heat treatment for inner-side zeolite NaA membrane are heating at 240℃ for 6 h, and the separation flux increases from 5.14 to 11.05 kg m^-2 h^-1 after heat treatment. While the effects of heat treatment before or after membrane synthesis are investigated for outer-side zeolite NaA membrane, and the supports heated before membrane synthesis ensure high separation performances of membranes.（3） Modelling investigation of support effect on the separation performance of zeolite NaA membraneZeolite NaA membrane is synthesized on the surface of the support, therefore, the membrane layer and support layer influence pervaporation process. Through modelling simulation with the combination of experiment data, it is found that porosity and mean pore size of support layer to some extent determine the separation properties of zeolite membranes. When porosities are close to each other, the flux of zeolite NaA membranes increases with increment of pure wate flux, that is, the increment of mean pore size. It is certain that support layer affects the separation performances of zeolite membranes. Meanwhile, it is found that other factors have effects on the separation properties.（4） Preparation of tri-bore supports and research in membrane synthesis and its separation performanceMulti-channel composite hollow fibers are prepared by phase inversion method through a multi-channel spinneret. Effects of zeolite content, bore liquid composition, bore flow rate. air gap distance and PI blending on the morphology and structure of supports are investigated. The inner and outer zeolite membrane synthesis and separation performance are studied on these prepared supports. The PI blending shows better effect on preparation of supports and characterizaition of membranes.