Diffusion and adsorption of hydrocarbons from automotive engine exhaust in zeolitic adsorbents

Abstract

Diffusion characteristics of hydrocarbons typical of automotive engine exhaust were determined by the Zero Length Column (ZLC) chromatographic method in silicalite and ferrierite pellets. The data clearly show that intracrystalline diffusion is too fast to exert any significant influence on the sorption kinetics in adsorbent pellets, and the dominance of meso/macropore diffusional resistance was confirmed by replicate experiments with different particle size fractions. Surface diffusion makes a significant contribution to the meso/macropore diffusivity for toluene in silicalite pellets leading to large differences in effective diffusivities relative to other sorbates, in particular ethylene. Ethylene, iso-butane and iso-butene adsorption and diffusion in ferrierite pellets showed a peculiar behavior due to immobilization effects involving both zeolite crystals and possibly mesoporous walls of the composite particles. At low concentration levels sorbate species diffused as single species in their respective binary mixtures.