Adsorption equilibria and kinetics of propane and propylene on zeolite 13X pellets

Abstract The adsorption equilibria and kinetics of propane and propylene on zeolite 13X pellets were investigated by a piezometric sorption method at 283–363 K and vacuum to 650 kPa for propane and 750 kPa for propylene. The difference of the adsorption capacity between the two gases was not so great, the saturation capacity being measured at 2.95 mol/kg for propane and 3.34 mol/kg for propylene. However, it was observed clearly that propylene was more strongly adsorbed on 13X than propane upon looking into the isotherms and comparing the isosteric heats of adsorption. Dual-site Langmuir model could fit the experimental adsorption isotherm data well. Subsequently, its temperature-dependent parameters were extracted, enabling to make use of the isotherm model in designing a non-isothermal adsorption separation process. To extract the apparent reciprocal diffusion time constants, two different kinetic models were applied to the experimental uptake curves: a piezometric model suitable for the first uptake measured at the very low pressure close to zero and a non-isothermal sorption model for uptakes in the range of 10–40 kPa. The resultant apparent reciprocal diffusion time constants exhibited a strong dependency on the pressure, corresponding to the effective intercrystalline reciprocal diffusion time constants theoretically estimated by Knudsen diffusivity and the secant of isotherm. This confirmed the presumption that diffusions of propane and propylene into the 13X pellet were controlled by the intercrystalline diffusion.