Xylenes reactions and diffusions in ZSM-5 zeolite-based catalyst

Xylenes transformations have been investigated over ZSM-5 zeolite in a riser simulator that mimics closely the operation of commercial fluidized-bed reactors. p-Xylene was found to be more reactive than the other isomers followed by o-xylene, and m-xylene was found to be the least reactive. The order of reactivity is consistent with the order of diffusivity. At low temperature (400 degrees C), isomerization and disproportionation yields for all xylenes were found to be equal at constant conversion. However, as reaction temperature increases to 500 degrees C, m-xylene reaction became more selective to disproportionation than that of the other xylene isomers. The effectiveness factor for the reactions of the xylene isomers was found based on two different diffusivities obtained from the literature. The m-xylene effectiveness factor was estimated to be less than 0.1, indicating a high diffusional resistance inside ZSM-5 zeolite. While o-xylene was found to have an effectiveness factor in the range of 0.35-0.45, the p-xylene effectiveness factor was found to be close to unity, indicating a negligible diffusional resistance.