Measuring Mixture Adsorption by Temperature-Programmed Desorption

Temperature-programmed desorption (TPD) was used to measure single-component and mixture adsorption of CO2 and C3H8 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 CO2/C3H8 mixtures, whereas C3H8 preferentially adsorbs on SAPO-34. Adsorption equilibrium was quickly reached on zeolite 13X. However, more than 12 h were required to obtain equilibrium for CO2/C3H8 mixtures in SAPO-34 because C3H8 diffuses much slower than CO2 in SAPO-34 pores, but adsorbs more strongly and thus displaces half of the CO2 that initially adsorbs. This behavior helps explain the slow approach to equilibrium observed for CO2/CH4 separations with SAPO-34 membranes when C3H8 was added to the feed.