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Experimental characterizing the residence time distribution of large spherical objects immersed in a fluidized bed

Cai, Rongrong, Zhang, Yanguo, Li, Qinghai, Meng, Aihong
Powder technology 2014 v.254 pp. 22-29
air, capacitance, emulsions, fluidized beds, mechanical models, normal distribution, powders, tomography
In many applications of fluidized bed, large objects are coexisting with small emulsion solids. The object motion patterns and residence time distribution (RTD) in the bed have a paramount effect in the performance of the reactor. In this paper, a series of experiments were conducted to study the influence of the superficial gas velocity and the constitutive properties of large spherical objects on their RTD in a fluidized bed with inclined air distributor using Electrical Capacitance Tomography (ECT) tracing technique. The objects were larger and denser than the fine particles of the dense phase, with the ratio of object diameter to bed characteristic length approximately 0.14 and the ratio of object density to static bed density ranging from 1.5 to 5.0. Experimental results show that the RTD curves of large objects have a relatively large discrepancy from the ideal normal distribution. As an object's size and density increases, the mean residence time (MRT) of the object decreases initially and increases subsequently. With the increasing of the superficial gas velocity, the MRT decreases with a trend of falling down fast at first then getting slow. Based on a mechanical model of the object on an inclined air distributor, the behaviors of the object were properly explained. Finally, an empirical correlation was derived for the MRT using some of experimental data and dimensionless analysis. The validations of the correlation by other independent experimental data show that the predicted values of MRT are well in accordance with the experimental values, and most of the relative errors are within ±30%.