| Zeolite membranes can be used in the separation of gas and liquid mixtures due to their unique pore structures, adsorption properties, strong mechanical strength, chemical stability and thermal stability. Pure silica MFI (Silicalite-1) zeolite membrane with hydrophobicity, can be applied to the recovery or concentration of organic from organic/water mixtures. The ethanol can be continuously removed from the fermentation broth using silicalite-1 membrane in preparing bioethanol from fermentation process, increasing the conversion rate and the overall production efficiency.At present, most membranes reported in literatures were synthesized on flat plates or large diameter tubular supports. The thick wall of the support leads to high resistance of the permeate transport through the support and the reduction of the flux of zeolite membrane, and thus limits its industrial applications. In this paper, silicalite-1 membranes are synthesized by the secondary (seeded) growth method onα-Al2O3 hollow fiber supports with thinner wall thickness (～150μm) and larger loading density (>1000 m2/m3). Separation performance measurement of zeolite membranes obtained is carried out in a laboratory scale setup for the pervaporation experiments of ethanol/water mixtures. The main experimental results and conclusions are summarized as follows:Influences of the content of the structure-directing agent (SDA, e.g. TPAOH) in the synthesis solution and the seed suspension concentration are investigated. It is found that high TPAOH content may lead to aluminum leaching from the alumina support seriously and caused the membrane less hydrophobic; low TPAOH content may lead to non-continuous zeolite layer, leading to poor separation performance. A continuous and dense zeolite seed layer is necessary to obtain the silicalite-1 membrane with high separation performance. Weak intergrowth can be found in the silicalite-1 layer and support leaching is serious due to the poor continuity of the seed layer for low seed concentration. However, too thick seed layer may cause peeling and cracking of the membrane and thus decrease the separation performance. The membrane prepared from the synthesis solution molar composition TPAOH:TEOS: H2O= 0.17:1:165 at 175℃for 12 and the seed suspension of 5 wt.% concentration showed the flux of 2.95 kg·m-2·h-1 and the separation factor of 66 for 3 wt.% ethanol feed at 60℃, and 5.40 kg·m-2·h-1 and 54 for 5 wt.% ethanol feed at 75℃, respectively. A linear relationship between the flux of silicalite-1 membranes and the support porosity was observed. The silicalite-1 zeolite membrane synthesized on theα-Al2O3 hollow fiber with the porosity of 69% showed the flux of 4.74 kg·m-2·h-1 for 3 wt.% ethanol feed at 60℃, which is the highest flux compared with data reported in literatures.The silicalite-1 zeolite membrane is also prepared by a two-step method from the synthesis solution molar composition TPAOH:TEOS:H2O= 0.17:1:165 and TPAOH: TEOS:H2O= 0.64:1:165. The two-step method can significantly improve the separation performance compared with the one-step method for a short crystallization time (2 h or 4 h). The separation performance of membrane increased significantly with increasing the second-step crystallization time, when the crystallization time extended from 2 h to 4 h, the separation factor increased from 20 to more than 40. However, longer second-step crystallization time leaded to a large number of extra crystals on the membrane surface, affecting the membrane performance. Smooth zeolite membrane surfaces were obtained by dynamic rotation method and reducing the template (TPA+) content. |
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