Measuring Mixture Adsorption by Temperature-Programmed Desorption

Temperature-programmed desorption (TPD) was used to measure single-component and mixture adsorption of CO 2 and C 3 H 8 in zeolite 13X and SAPO-34 crystals. The TPD was carried out with the adsorbing gases in the feed stream so that adsorption could be determined for weakly adsorbing molecules. The single-component loadings measured by TPD and by volumetric adsorption were within 5% of each other. Carbon dioxide preferentially adsorbs on zeolite 13X from CO 2 /C 3 H 8 mixtures, whereas C 3 H 8 preferentially adsorbs on SAPO-34. Adsorption equilibrium was quickly reached on zeolite 13X. However, more than 12 h were required to obtain equilibrium for CO 2 /C 3 H 8 mixtures in SAPO-34 because C 3 H 8 diffuses much slower than CO 2 in SAPO-34 pores, but adsorbs more strongly and thus displaces half of the CO 2 that initially adsorbs. This behavior helps explain the slow approach to equilibrium observed for CO 2 /CH 4 separations with SAPO-34 membranes when C 3 H 8 was added to the feed.