283601 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, Hi/D, was 1.35–1.80. Changes in bed height, H, bed surface and sides were observed visually. Three transition velocities were identified, viz Umf, minimum fluidizing velocity, Umb,v, visually determined minimum bubbling velocity, and Ubv, the velocity at which the bed was visually uniform, and vigorous bubbling began. Fluctuations in bed pressure drop, ΔPb, were analyzed using standard deviation, σ, and von Neumann ratio, T. Composite plots of normalized ΔPb, 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 Umf, Umb,v and Ubv. 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.