Transient measurements of adsorption and diffusion in H‐ZSM‐5 membranes

Abstract

AbstractA transient permeation method presented here not only determines the adsorption and diffusion properties of the pores that are the transport pathways through zeolite membranes, but nondestructively estimates the effective thickness of the membrane. Transient responses of the permeate concentration to step changes in the feed were measured on two H‐ZSM‐5 tubular membranes and modeled assuming Maxwell‐Stefan diffusion and Langmuir adsorption. The adsorption isotherms determined from these transient measurements at 298 K of N2 and CO2 were nearly identical to those measured by calorimetry on H‐ZSM‐5 powders. The CH4 isotherm at 298 K was similar to isotherms measured by calorimetry and gravimetric techniques on Na‐ZSM‐5 and silicalite powders. The similarity of the isotherms indicates that transport of these light gases occurs mainly through zeolite pores. The Maxwell‐Stefan diffusion coefficients DMS depended on concentration and were higher for higher feed partial pressures. Average DMS values for the two membranes were 7.5, 5 and 1.5×10−10 m2/s for N2, CH4, and CO2, respectively; these are in the same range and order as diffusion coefficients measured in zeolite crystals.