(344a) Onset of Bubbling in Geldart Group A Powders

In this work, we have tracked transitions in shallow
beds aerated with incrementally increasing and decreasing gas flows, in an
80-mm column at ambient conditions. The test materials were six lactose
powders, as well as one sample of glass ballotini and two samples of sand. The
size and size distributions of the test powders were adjusted to provide powders
that spanned the Group A region and part of the Group B region of a Geldart
classification diagram. The ratio of loosely packed powder bed height to column
diameter, H_i/D, was 1.35?1.80. Changes in bed height, H,
bed surface and sides were observed visually. Three transition velocities were
identified, viz U_mf, minimum fluidizing velocity, U_mb,v,
visually determined minimum bubbling velocity, and U_bv, the
velocity at which the bed was visually uniform, and vigorous bubbling began.
Fluctuations in bed pressure drop, ΔP_b, were analyzed
using standard deviation, σ, and von Neumann ratio, T. Composite
plots of normalized ΔP_b, H, σ, and T
versus normalized superficial velocity, U, were constructed to assess
the trends exhibited by each bed parameter. Our experimental results and
observations indicate that for both increasing and decreasing superficial velocities,
there were break points -abrupt changes in slope- in plots of σ and
T versus normalised U, at or close to superficial velocities
consistent with the values of U_mf, U_mb,v
and U_bv. We discuss the relative merits of the use of each
index, which may have utility in fluidized bed diagnostics in cases where
direct observation of the bed is not feasible.