Transient permeation of butanes through ZSM‐5 and ZSM‐11 zeolite membranes

Abstract

AbstractButane isomer adsorption isotherms and diffusivities in the transport pathways through Al‐ZSM‐11 (MEL), Al‐ZSM‐5 (MFI), and B‐ZSM‐5 zeolite membranes were measured by a transient permeation technique. The permeate responses to feed step changes were measured between 383 and 473 K, and the diffusion was modeled as Maxwell–Stefan surface diffusion with dual‐site Langmuir adsorption. The effects of adsorption and diffusion on the n‐C4/i‐C4 ideal selectivities were quantified, and the selectivity differences between the membranes were explained based on the adsorption properties and the diffusivities. The resistance of the porous tubular supports affected the coverage in the zeolite at the permeate boundary, and the ratio of the n‐C4/i‐C4 diffusion driving forces consequently increased with increasing temperature. The n‐C4 diffusivities were almost the same in the three membranes, but the i‐C4 diffusivities in the MEL membrane were an order of magnitude higher than those in the MFI membranes. The bulkier i‐C4 molecules are apparently significantly less constrained in the straight, circular pores and slightly larger intersections of MEL zeolite, although the structural differences do not affect the mobility of n‐C4. Higher i‐C4 coverages in the Al‐ZSM‐11 and B‐ZSM‐5 membranes compared to the Al‐ZSM‐5 membrane were attributed to a smaller change in entropy upon adsorption in the channels of Al‐ZSM‐11 and to a higher heat of adsorption in the intersections of the more substituted B‐ZSM‐5. Initial transient flux overshoots and undershoots after i‐C4 feed concentration changes were apparently caused by a small amount of pressure‐driven flow (<10% of the total i‐C4 flux and <0.1% of the total n‐C4 flux) through parallel pathways. The effective membrane thicknesses were also estimated from the transient measurements. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2816–2834, 2004