(87a) Computation of Turbulence and Low Sherwood Numbers in Fluidized Beds

It was known for half a century that the Sherwood and Nusselt numbers in fluidized beds are often three orders of magnitude lower than the classical diffusion controlled limit of two. We have shown that our kinetic theory based computer codes correctly compute low Sherwood numbers in agreement with published experimental data. For tall fluidized bed risers the computed behavior is similar to that for convective diffusion in a channel, but with a greatly reduced mass transfer. The Sherwood numbers are low due to formation of clusters, consistent with our measurements of granular temperature.

The mass transfer coefficients and Sherwood numbers were estimated by scaling the Sherwood numbers with the cluster sizes. The Sherwood numbers were of the order of 0.01, and the mass transfer coefficients were of the order of 0.001 m/s.

Also, the mass transfer coefficients were computed using the ozone decomposition reaction. The Sherwood numbers of the order of 0.001, and the mass transfer coefficients of the order of 0.001 m/s, were in agreement with those estimated from the cluster sizes.

We observed from the CFD simulations that the Sherwood number and the mass transfer coefficient decrease with the height in the riser. The trend was in reasonable agreement with that obtained by Kato et al. (1970) experiments.

References:

1. Kato, K., H. Kubota, C.Y. Wen, “Mass transfer in fixed and fluidized beds”, Chemical Engineering Progress Symposium Series 105 (1970) 66, 87-99