Characterization of Natural Zeolite Membranes for H₂/CO₂ Separations by Single Gas Permeation

Abstract

Natural zeolite membranes can be used as a model for the development of robust molecular sieve membranes with superior separation characteristics. We describe the characterization of natural clinoptilolite membranes made from dense mineral deposits by single gas H 2 and CO 2 permeation. Permeability values as a function of temperature and pressure were analyzed on the basis of mass transport fundamentals of gas permeation through zeolite and nonzeolite pathways. H 2 and CO 2 fluxes through the membranes were fitted with a model based on a combination of zeolitic, Knudsen, and viscous transports so that the selective and nonselective flux fractions could be quantified. An increase in feed pressure increased the total permeance especially at low temperatures. The membranes were also characterized by XRD, SEM, and EDX analysis.

Preparation and Evaluation of Nanocomposite Sodalite/α-Al2O3 Tubular Membranes for H2/CO2 Separation

“…For example, a study by Kanezashi and Lin [47] where H2 and CO2 permeation through MFI membranes synthesized on alumina support via secondary seeded growth method was measured shows that the permeance of H2 and CO2 decreased with increasing temperature. The decrease in permeance was attributed to the Knudsen-type temperature dependency display of the membrane up to 500 °C and this agrees with studies reported elsewhere [48][49][50]. At the same time, these studies [48][49][50] suggested that increase in H2 and CO2 permeance with an increase temperature indicates that permeation is controlled by activated diffusion instead by Knudsen diffusion.…”

“…The decrease in permeance was attributed to the Knudsen-type temperature dependency display of the membrane up to 500 °C and this agrees with studies reported elsewhere [48][49][50]. At the same time, these studies [48][49][50] suggested that increase in H2 and CO2 permeance with an increase temperature indicates that permeation is controlled by activated diffusion instead by Knudsen diffusion. On the other hand, constant CO2 permeance at increasing temperature observed by Farjoo and coworker [51] was attributed to the combined contributions from activated process and non-zeolitic flux.…”

“…When these defects (non-zeolitic pores) are relatively large, they provide non-selective pathways, and viscous flow becomes the predominant mechanism with an increase in pressure. Also, presence of small defects will result in Knudsen mechanism controlling the separation, thus making the membrane semi-selective with flux remaining almost constant as pressure increased [50,51]. Constant flux at increasing feed pressure, while keeping the permeate pressure constant, will result in the decrease of permeance as a function of pressure as observed in this study and could indicate presence of defects in the membranes as well.…”

“…Similarly, CO2 permeance and N2 permeance decreased as the pressure was increased. This is According to Hosseinzadeh Hejazi et al [50], a constant permeance or a slight reduction in permeance as a function of pressure should be observed if the zeolitic pores are greater than the kinetic diameter of the permeated molecules and the permeation is only through the zeolitic pores. Gas can permeate through zeolitic pores and non-zeolitic pores in zeolite membranes.…”

“…Hence, membrane M4 possesses more zeolitic pores that contribute to selective separation with very little non-zeolitic pores. It is noteworthy to mention that a decrease in ideal selectivity with increasing pressure suggests gas transport flow through non-zeolitic regions and macroporous defects (viscous flow contribution) [28,50]. At high temperatures, CO 2 adsorption on the surface of the zeolite pores of M2 and M3 weakened dramatically and its mobility increased.…”