Xylene Vapor Mixture Separation in Nanocomposite MFI-Alumina Tubular Membranes: Influence of Operating Variables
Résumé
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 oinfiniteo p-xylene selectivity. The mixture's p-xylene flux displayed a maximum value of ca. 3.5 mu mol center dot m-2 center dot s-1, corresponding to a mixture permeance of 11 nmol center dot m-2 center dot s-1 center dot Pa-1, at 473K and for a feed composition 0.63kPa p-xylene/0.27kPa m-xylene/0.32kPa o-xylene, being almost unchanged for sweep gas flow rates (N2) higher than 20mL(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 150kPa. 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.