(207f) Experimental Measurement of Minimum Bubbling Velocity

Despite the importance of minimum bubbling velocity, U_mb, in the design and modeling of fluidized bed systems, there continues to be debate as to the relative merits of approaches to measuring it. In this work, we compare measurements of U_mb obtained by direct visual observation, with estimates obtained indirectly from pressure fluctuations. We have previously reported observations suggesting that at or close to U_mb, the reciprocal of the von Neumann ratio, T, calculated for the fluctuations in the bed pressure drop signal, obtained at incremental changes in superficial velocity, U, rapidly rises to a maximum value, and the standard deviation, σ, of the pressure fluctuations, increases abruptly, though can subsequently exhibit some scatter. These features of plots of T^-1 vs U and σ vs U, have been used to obtain estimates of U_mb for 12 different powders; with surface-volume mean diameters, d₃₂, spanning the range 36 μm to 223 μm and particle densities, ρ_p, from 1540 kg m^-3 to 3010 kg m^-3. The accord between U_mb determined visually and the values obtained indirectly, by both the T^-1: U method and the σ vs U method was encouraging, with R² values of ~0.98. However, the use of T^-1 is easier to implement, with less potential for ambiguity.