Horng Yuan Saw

Tracking the Transitions in Gas-Fluidized Beds with Measurements of Pressure Fluctuations

When a packed bed of powder is fully fluidized to the bubbling state it undergoes two principal transitions. The first is at the point of incipient fluidization and the second is at the onset of bubbling. The superficial gas velocity at the point of incipient fluidization is the minimum fluidizing velocity, U mf , and minimum bubbling velocity, U mb , is the superficial velocity at the onset of bubbling. By convention, U mf is experimentally determined with a plot of bed pressure drop versus superficial gas velocity, and U mb is found by visual observation; both velocities can be crucial to the design and operation of fluidized beds. In this paper, we present experimental results for sand and lactose powders in an 80-mm fluidized bed. We have analyzed fluctuations in bed pressure drop signals, obtained at ambient conditions for increasing and decreasing gas flows, using signal standard deviation, σ, and von Neumann ratio, T, and plotted values of these parameters against superficial velocity, bed pressure drop, and bed height. Our findings show that T is more sensitive to changes in the internal state of a fluidized bed than σ, with distinct and abrupt changes in T occurring at superficial velocities consistent with the values of U mf and U mb determined by conventional methods. This work has provided further support for the use of T as an indicator of the transitions occurring in powder fluidization.