Figure 3. Snapshots of solids concentration distribution at different
temperature.
4.2 Minimum bubbling velocity
(\(\mathbf{U}_{\mathbf{\text{mb}}}\))
It is widely accepted that the minimum bubbling velocity
(\(U_{\text{mb}}\)) is regarded as the superficial gas velocity when
bubbles first occur8, 37-39. This definition is
apparently vague and general, because it largely depends on the methods
of visualization. For instance, video camera is normally used to obtain
images in 2D fluidized beds. The first occurrence of bubbles in a 2D
fluidized bed would be different from that in a 3D fluidized bed with
comparable dimension. As shown above, we can distinguish the occurrence
of first bubble in 3D fluidized bed via ECT images. However, it would be
more precise to quantify \(U_{\text{mb}}\) by plotting the standard
deviation of average solids concentration of the bed as the function of
superficial gas velocity.28, 40 Figure 4 shows the
standard deviation of average solids concentration versus the
superficial gas velocity. As can be seen, there exists an apparent jump
of standard deviation for each temperature. We found that the
superficial gas velocity corresponding to the jump agrees well with our
observation on the appearance of first bubble in the fluidized bed. In
this sense, we can determine that \(U_{\text{mb}}\) = 3.6, 3.4, 3.0, and
2.9 cm/s for temperature T= 20, 200, 400, and 600°C, respectively.