Experimental observations on the different mechanisms for solid ejection in gas-fluidized beds

This work presents an experimental study of the ejection velocity for different mechanisms of solid ejection in fluidized beds. The experiments were carried out in a 2D fluidized bed, where the bubble eruptions were recorded with a frequency of 250 frames per second using a high speed video camera with a resolution of 1.3 Megapixels. The results show that in isolated bubble eruption, the dome velocity is significantly reduced by the effect of a group of raining particles in the form of stalactites within the bubble. Higher velocities are observed when bubble coalescence takes place. If bubbles coalesce before the leading bubble breaks, the momentum of the trailing bubble together with the increase in the throughflow accelerate the dome of the leading bubble. In contrast, when coalescence occurs after the breakage of the leading bubble, the wake of the trailing bubble is projected into the freeboard with a very high velocity (wake spike mechanism). The last observed mechanism, the jet spike mechanism, occurs when a stream of bubbles reaches the bed surface following the path opened by the previous bubbles. A cloud of particles moving upward is observed, although their velocities are not as high as in the wake spike mechanism due to the interchange of momentum during the collisions with other particles. Finally, an explanation for some of the patterns of gas release from erupting bubbles recently observed by Hartung et al. [G. Hartung, C.R. Muller, J. Hult, J.S. Dennis, C.F. Kaminski, Laser diagnostic investigation of the bubble eruption patterns in the freeboard of fluidized beds. 1. Optimization of acetone planer laser induced fluorescence measurements. Ind. Eng. Chem. Res. 47 (2008) 5686-5697] is proposed. (c) 2008 Elsevier B.V. All rights reserved.