| Zeolite NaA with high Al content and small pore size (4.2A), has been found to be suitable for dehydrating water/organic mixtures by pervaporation. Despite the successful commercialization of supported zeolite NaA membranes (Mitsui Engineering&Shipbuilding Co. Ltd.), the fundamental understanding of membrane formation remains limited. Also, the separation performance of zeolite membranes needs to be further improved, and the manufacture cost could be further reduced. Ceramic hollow fibers, used as supports of zeolite membranes, have attracted wide attentions, not only because of their relatively high resistance to abrasion and, to chemical and thermal degradation, but also their higher surface/volume ratio and thinner walls compared with their counterparts of tubular supports. This research focuses on the preparation of zeolite NaA membranes on alumina hollow fiber supports. The induction nucleation of pre-coated zeolite seeds during the secondary growth process is investigated. The in situ hydrothermal synthesis is developed by placing the support in Si source before mixing. The contribution of the support layer to the overall mass-transfer resistance is determined. Accordingly, the permeation property and mechanical strength of alumina hollow fibers is improved. And finally, the dense zeolite NaA membranes with high pervaporation performance are obtained on the suitable alumina hollow fiber supports.(1) Influences of seeds on the properties of zeolite NaA membranesZeolite NaA membranes were prepared by seeded growth on the external surface of alumina hollow fiber supports. Based on the morphology and performance of zeolite NaA membranes grown from nano-and micro-sized seeds, seed-assisted membrane formation process is briefly discussed. It is easier for nano-sized seeds to uniformly cover the surface of the support with a low concentration while micro-sized seeds need unreasonably large concentration to get the same coverage. Initial nuclei generated from hydrogel surround the precoated nano-sized seeds everywhere and grow up together with them, leading to a relatively continuous layer at the early crystallization stage. For the micro-sized seeds, the inadequate coverage of seed crystals leads to the embedding of less intergrown cubic crystals and defects. When the seed concentration is as low as1wt.%, the separation factor of as-synthesized membrane from90-nm-sized seeds was higher than10.000with high reproducibility, while for1500-nm-sized seeds, separation factors are even below10. Furthermore, the influence of crystal fragments (～50nm) obtained by ball-milling available crystals (e.g.,1500nm) on the formation of zeolite membranes is also studied. Initially, a rise in the seed concentration leads to an increase of the separation factor. When the concentration reaches a specific value, the separation factors maintain more than10,000. In order to ensure uniform and adequate coverage of seeds, the amount of seeds implanted in the support by dip-coating has to exceed a certain limit. The lowest seed concentration is～0.1wt.%.(2) Improvement of in situ hydrothermal synthesisThe in situ hydrothermal synthesis is developed by placing support in Si source before mixing. The detailed preparation process is as follows. At first, the hollow fiber support is placed in Si solution before mixing with the Al solution; then, the synthesis mixture is aged together with the support; after that, the support is taken out and a bottom-up rotary wiping is applied; finally, a centrifugal clear solution from the synthesis mixture is used to accomplish the zeolite crystallization. As Si ingredient encounters the Al near support, the initial zeolite nuclei start to grow from the surface of the support. Due to the elimination of crowding effect, the dispersion density of nucleation sites for the zeolite membrane increases greatly. As a result, we demonstrate for the first time that, by one single in situ hydrothermal synthesis, zeolite membranes with high pervaporation performance (separation factor>10,000and flux is5.0～6.5kg m-2h-1) and high reproducibility on ceramic hollow fiber supports can be obtained using a placing-support-in-Si source method.(3) Effects of support resistances on pervaporation performance of zeolite membranesPorous Al2O3hollow fibers with different asymmetric macrostructures, i.e. single or double finger-like layer, have been prepared by a phase inversion/sintering technique. Such hollow fibers are used as supports to synthesize zeolite NaA membranes with high separation factors (>10,000) in the pervaporation of ethanol/water solution (90wt.%,75℃). By means of extending the crystallization time (4～6h) and increasing the number of synthesis cycles (single/double/triple syntheses), zeolite membranes with different thicknesses are synthesized. Extrusion pressure, Al2O3/PES ratio, and calcination temperataure are adjusted to control the asymmetric macrostructure of the ceramic hollow fibers. Influences of both the zeolite layer thickness and support layer macrostructures on pervaporation properties of zeolite NaA/Al2O3supported membranes are investigated. It is indicated that the variation of the support macrostructure rather than zeolite layer thickness results in a greater difference in the pervaporation flux of the supported membrane. There is an approximate linear dependence of the pervaporation flux of the supported membrane on the support porosity despite the fact that these hollow fibers are produced with different preparation conditions. Using a hollow fiber with the porosity of69%as a support, a flux of11.1kg m-2h-1could be achieved. When the wall thickness of the support is reduced by half, the flux could increase by～35%. Furthermore, a mathematical model is used to calculate the pressure drop in the support layer. Using hollow fibers with different wall thickness as supports, the contribution of the support layer to the overall mass-transfer resistance is over50%. In addition, when the alumina hollow fiber with through finger-like pores is used as the support, the flux could further increase by～15%. Therefore, in order to obtain the zeolite NaA/Al2O3supported membranes with the high pervaporation performance and mechanical strength, it is probably effective to focus on the improvement and development of the support properties.(4) Improvement in the mechanical strength of ceramic hollow fibersWith the addition of zeolite NaA to Al2O3/PES spinning slurry, the alumina hollow fibers with high mechanical strength are prepared by a combined phase-inversion and sintering method. Effects of zeolite NaA content, dope composition, extrusion pressure and calcination temperature on the morphology and structure of the ceramic hollow fibers are investigated. Improvement in the mechanical strength of hollow fibers resulting from the addition of zeolite NaA crystals is analysed. During the calcination process, zeolite NaA crystals probably experience framework collapse, crystalline phase transition, dissolution and solidification. Finally the zeolite NaA remains play a role as rivets and lead to the strong adhesive of alumina particles. |
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