Scale-up of bottom-bed dynamics and axial solids-distribution in circulating fluidized beds of Geldart-B particles

In this paper it is shown that the gas flow pattern in the dense bottom-bed of circulating fluidized beds, operated with Geldart-B type of solids, exh ibits three different types of dynamic behavior, dependent on the width of the riser and on the aspect ratio of the dense bottom-bed. These types of d ynamic behavior are defined as: (a) 'exploding bubble with wall effect' in risers with a diameter smaller than 0.7 m and with a bed aspect ratio small er than 3.5; (b)'exploding bubble without wall effect' in risers with a dia meter larger than 0.7 m and with a bed aspect ratio smaller than 3.5; (c)'c irculating slugging' in narrow risers at bed aspect ratios larger than 5. T he term 'wall effect' indicates that the wall of the riser imposes limitati ons to the growth of the exploding bubbles and to the solids flow in the de nse bottom-bed. Non-dimensional correlations are presented, for each of the se types of dynamic behavior, for the Kolmogorov entropy, quantifying the d ynamic flow behavior, and for the decay-parameter a, describing the decay o f the solids volume fraction in the splash zone above the bottom-bed. These correlations were developed using pressure fluctuation and time-averaged p ressure-drop data that were measured in five CFB risers varying in size fro m 0.083 m LD to 1.2 x 0.8 m using the same type of Geldart-B sand. (C) 1999 Elsevier Science Ltd. All rights reserved.