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In this study, we present the results of a preliminary investigation on the influence of operating variables (temperature, sweep gas flow rate, and total feed vapor pressure) on xylene vapor mixture separation using tubular nanocomposite MFI-alumina zeolite membrane prepared by the pore-plugging synthesis technique. Within the detection limit of our analytical system, neither m- nor o-xylene was detected in the permeate stream, the membranes displaying therefore “infinite” p-xylene selectivity. The mixture's p-xylene flux displayed a maximum value of ca. 3.5 µmol·m-2·s-1, corresponding to a mixture permeance of 11 nmol·m-2·s-1·Pa-1, at 473 K and for a feed composition 0.63 kPa p-xylene/0.27 kPa m-xylene/0.32 kPa o-xylene, being almost unchanged for sweep gas flow rates (N2) higher than 20 mL(STP)/min and increasing with the total xylene vapor pressure at 1 : 1 : 1-3 p/m/o-xylene composition. The experimental p-xylene fluxes can be well predicted by a Maxwell-Stefan model, as expected for a mass transfer process driven by competitive adsorption / surface diffusion. Unlike film-like MFI membranes, the membranes presented here preserved their selectivity to p-xylene for total xylene pressures as high as 150 kPa. This behavior is attributed to the intimate contact between the alumina confining pores and MFI nanoparticles, reducing long-term stresses and thus preventing distortion of the MFI framework during p-xylene adsorption. These results open up potential applications of nanocomposite MFI-alumina for selective p-xylene separations at high loadings, for instance in pervaporation, where the use of film-like MFI membranes is discouraged. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we present m-xylene isomerization in an extractor-type catalytic membrane reactor using nanocomposite MFI–alumina membrane tubes packed with a Pt-HZSM-5 catalyst. The catalytic performance of the membrane reactor was compared with that of a conventional fixed-bed reactor with similar dimensions and at comparable operation conditions. The influence of the reaction temperature, gas hourly space velocity, reaction time and reactor configuration on the performance of membrane reactor was explored. The membrane reactor displayed a maximum p-xylene yield of 5.1% at 523K when computed at permeate-only mode, decreasing with temperature. At combined mode, the p-xylene yield was ca. 28%, showing an increase of about 18% compared to an equivalent fixed-bed reactor. The catalytic membrane reactor showed 100% selectivity towards p-xylene at permeate-only mode, displaying a maximum value of ca. 42% at 523K at combined mode. Higher performance was obtained when the catalyst packing was close to the inner top layer of the membrane support. In this configuration, xylene permeate compositions higher than 95% could be achieved on the basis of the high p/o and p/m-xylene separation factors offered by the membrane. [ABSTRACT FROM AUTHOR]

Abstract: Nanocomposite MFI–alumina hollow fibres and membrane tubes have been prepared by pore-plugging hydrothermal synthesis and tested in the separation of xylene vapour mixtures. Single component and ternary xylene isomer mixtures diluted in N2 gas have been fed into the membrane lumen, while N2 gas swept over the outer membrane surface. The fibres showed a maximum p-xylene flux of 4.5μmolm2 s−1 at 573K for p/m/o-xylene partial pressures 0.62kPa/0.27kPa/0.32kPa, the pxylene to o-xylene (p/o) separation factor showing a value of 107. As expected for a nanocomposite material, xylene fluxes show no indication of flux increase beyond 573K due to pore opening. When comparing MFI–alumina hollow fibres to membrane tubes, the surface-to-volume ratios can be increased by one order of magnitude, allowing for more compact and cost-effective units. [Copyright &y& Elsevier]