Determination of intraparticle diffusion coefficients in catalyst pellets—a comparative study of measuring methods

Abstract

Effective intraparticle diffusion coefficients in different industrial catalysts were determined with three different measuring methods. On the one hand by steady state counterdiffusion measurements and dynamic pulse response measurements on single pellets in a diffusion cell, on the other hand by the gas chromatographic method employing the “single pellet string reactor” (S.P.S.R.). In general the steady state method and the dynamic pulse response measurements on single pellets yielded similar values for the effective diffusion coefficients. Within limits, the S.P.S.R. gave comparable diffusion coefficients, however, a statistical consideration indicates that with this method experimental uncertainties may lead to significantly broader confidence limits for the diffusion coefficients than with measurements on single pellets in a diffusion cell. Different pore models are employed for the prediction of the intraparticle diffusivities in the commercial catalyst pellets. The diffusivities predicted with the random pore model are about 1.5 times larger than the experimentally determined values. With measurements carried out on single pressed pellets in the diffusion cell no axial anisotropy effects caused by pellet pressing could be observed.