Adsorption and diffusion properties of butanes in ZSM-5 zeolite membranes

Adsorption isotherms and diffusion coefficients for n-C-4 and i-C-4 in the transport pathways through tubular ZSM-5 zeolite membranes were determined over a range of temperatures by a transient permeation method. The permeate response to step changes in the feed were measured, and the transport was modeled as Maxwell-Stefan diffusion with single site Langmuir adsorption in the zeolite. The heats of adsorption and the Langmuir parameters were comparable to values reported for MFI powders. The similarity of the membrane and powder isotherms indicates that butanes diffuse mainly through zeolite pores. Maxwell-Stefan diffusion coefficients for n-C-4 and i-C-4 in the ZSM-5 membranes were similar to those measured by other macroscopic techniques for zeolite crystals and membranes, and diffusion coefficients increased with increasing feed partial pressure for a high-quality membrane. The effective membrane thickness was also estimated from these transient measurements.