PERMEATION OF SINGLE GASES IN THIN ZEOLITE MFI MEMBRANES

The permeation of a series of gases with widely different Lennard-Jone s kinetic diameters and sorption properties is investigated as a funct ion of feed pressure (up to 100 kPa) and temperature (298-473 K) with two different methods. The membrane system studied consists of an MFI (silicalite) top-layer (thickness 3 mu m) directly grown on an a-alumi na support. Flux values of molecules with a kinetic diameter, d(k), <0 .45 nm (H-2, CO2 and n-alkanes (CH4 to n-butane)) are in the range of 1.5-15 mmol m(-2) s(-1). Size exclusion is found for o-xylene (d(k)=0. 68 nm) and 2,2-dimethylbutane (d(k)=0.60 nm). Flux values are obtained for iso-butane, SF6 and benzene, intermediate between those for small molecules and those for molecules with size exclusion. Maxima in the curve of flux versus temperature are found for gases with non-linear L angmuir-type adsorption curves as a function of pressure. The temperat ure of maxima at 100 kPa pressure increases in the order of CO2<ethane <propane<butane and shifts to higher values with increasing pressure. (C) 1998 Elsevier Science B.V.