Axial dispersion in beds of small particles

Abstract

Axial dispersion coefficients (E A) in beds of small (0.1–1 mm) particles are often needed to interpret reaction rate data. A chromatographic technique was used to evaluate such coefficients for beds of porous catalyst particles and beds of non-porous glass beads over a wide range of velocities (Re p = 0. 00237 to 11.9). Two gaseous systems (helium in hydrogen and oxygen in nitrogen) were used to provide different molecular diffusivities. The results can be correlated by adding two terms: one due to molecular diffusion, which is dominant at low velocities, and a second term due to convective mixing, which is dominant at high velocities. The unusually high values of E A for porous particles at low velocities could be explained by including a contribution due to intraparticle diffusion. At high velocities Peclet numbers approached constant values which were proportional to particle diameter. Channeling in the bed due, for example, to shrinkage of catalyst particles as a result of reduction in situ, or to movement of particles, can cause a large increase in axial dispersion.