Diffusivities of light alkanes in a silicalite-1 membrane layer

Diffusivities of methane, ethane, propane and n-butane for silicalite-l mem branes, consisting of intergrown crystals, were determined from permeation measurements. Experiments were conducted according to a batch method and th e Wicke-Kallenbach method. The temperature of the experiments was varied be tween 273 and 673 K, and (partial) pressures from 10 to 425 kPa were applie d. The model used was based on two contributions to transport through the memb rane, viz., intracrystalline diffusion and activated gaseous diffusion. The Darken thermodynamic correction factor was applied to obtain the corrected transport diffusivity for intracrystalline diffusion. The single-component fluxes through the membrane could adequately be descri bed using this two-contribution model. Diffusivities obtained from the Wick e-Kallenbach method at lower loadings were lower than those obtained from t he batch method, owing to the back-permeation of the sweep gas in the Wicke -Kallenbach method. A slight occupancy dependence is observed for the intra crystalline diffusivities below 0.9. Above an occupancy of 0.9 these values decrease. For methane and ethane the temperature dependence of the intracrystalline d iffusivity was in agreement with literature data for self-diffusion. For pr opane and n-butane up to 100 times lower values for the diffusivity were fo und, and for these molecules a higher activation energy was observed than t hat for the self-diffusivity. These lower values and higher activation ener gy of the membrane diffusivities are attributed to additional energy barrie rs in the membrane. Diffusion through a layer of intergrown zeolite crystal s is not identical to diffusion in single zeolite crystals and must be dete rmined individually. There is considerable room for improvement of the memb rane, both by optimizing the zeolite layer and the mesoscopic structure of the support. (C) 2000 Elsevier Science B.V. All rights reserved.