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.