Detection of Bubbling Onset in Fluidized Beds Using the Standard Deviation of Bed Pressure Drop
Estimates of minimum fluidizing velocity obtained from plots of standard deviation of bed pressure drop, σ, against superficial velocity can be in excellent agreement with values obtained by the classical pressure drop method. However, the standard deviation method measures the onset of bubbling rather than minimum fluidizing velocity and results should be interpreted with caution in systems where minimum bubbling velocity, Umbσ, and minimum fluidizing velocity are not the same, such as for Geldart Group A particles.
Measurements have been made on batches of silica sand that span a range of particle diameters in the vicinity of the Geldart Classification B/A boundary. Plots of σ against superficial velocity, U, suggest that for each sand batch there could be a step change, or very rapid change, in σ at a critical superficial velocity, followed by a slower linear increase. This is supported by an analysis using the ratio of total variance to high speed variance,{VTOTP/VHIP}, and also from plots of the Hurst exponent calculated for each pressure drop data set. A consequence of assuming a monotonic relationship between σ and U and extrapolating to σ = 0 is that Umbσ is underestimated.