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Black plate (263,1) ethanolywater mixtures. The PV performance of the as-synthesized membranes was explored at 303–363K for water contents of 1– 60wt% and for feed and permeate pressures in the range 0–7 bar and 2–70 mbar, respectively. The selectivity was found to show a maximum at a water feed composition in the range 5–10wt% and to rise with temperature. Water and ethanol fluxes were successfully fitted to an adsorption-solution model derived from the Maxwell- Stefan theory, where the ethanol flux is coupled with the water flux. It appears that the PV performance of zeolite NaA membranes is determined mainly by the adsorption of water and ethanol135. ZSM-5 zeolite membranes were synthesized by means of the secondary growth procedure over porous alpha-alumina supports from clear template-free precursor solution and their permeation properties were compared with membranes prepared using templates. The templated membrane (M1) was shown to contain smaller crystals with a greater number of faces than their template- free counterpart (M2). The expected size selectivity could not be achieved for M1 as this membrane possessed some micro-defects formed during the calcination procedure. The required size selec- tivity was however achieved for M2 at ambient temperatures and shape selectivity found at 200C. A selectivity ratio of 44 was achieved for butane isomers through M2 with n-butane molecules experiencing a maximum activation energy of 43 kJ mol 1, resulting from an activated diffusion mechanism. The permeation values measured for M2 membranes were normally an order lower than those for M1 membranes136. Two methods were compared for the preparation of zeolite beta membranes with a thickness of around 1000 nm on beta seeded, commercial Trumem disk supports. The first method produced an alkali-free zeolite beta layer with a SiyAl ratio of 40. By means of the second method, a Na-containing zeolite beta layer with a SiyAl ratio of 13 was obtained. The mechanism for permeation was controlled by surface diffusion. Significantly, only those membranes prepared by the first method were effective in separating di- from mono-branched alkanes, according to PV experiments at 303 K with a 50 : 50 wt% 2- methylpentane (2MP) and 2,2-dimethylbutane (22DMB) mixture, giving a selectivity of 1.5 in favour of 2 MP which is greater than that expected on the basis of Knudsen diffusion. Respective adsorption energies for 2 MP and 22 DMB of 48 kJ mol 1 and 42kJ mol1 in zeolite beta were determined137. Asymmetrically structured zeolite membranes of a hydrophobic MFI (ZSM-5) type and hydrophilic type, NaA were prepared as thin layers of thickness 10 – 30 microns on ceramic substrates and www.scienceprogress.co.uk Properties and applications of zeolites 263PDF Image | Properties and applications of zeolites
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