A generalized empirical description for particle slip velocities in liquid fluidized beds

An empirical equation for calculating the slip velocity of a species in any homogeneous suspension is proposed. The Richardson and Zaki (1954, Trans. Inst. Chem. Engng, 32, 35-53) and Lockett and Al-Habbooby (1973, Trans. Inst. Chem, Engng 51, 281-292; 1974, Powder Technol., 10, 67-71) equations are special cases of the proposed equation, and arise when all species are of the same density. Therefore, the main value of the proposed equation is in describing the slip velocities of particles in suspensions containing species of different density. In this short note results from one experimental system are presented, and shown to be consistent with the model. The model is also consistent with the explanation used by Moritomi et al. (1982) to account for phase inversion in fluidized beds. The model is appealing in its simplicity, and should find favour in the design and control of process equipment. The new model predicts the strong segregation effects observed in suspensions containing particles of different density, and hence represents an immediate improvement on the Lockett and Al-Habbooby equation. Its application is expected to cover all homogeneous suspensions, in which the particles are all more dense than the suspension. At this stage its validation has been limited to low concentrations of dense particles settling through a fluidized bed of low density particles as occurs in a Teetered Bed Separator, and to phase inversion conditions in fluidized beds. It is hoped that this note might lead to a much more extensive validation of the model by workers using vastly different particle species. (C) 1999 Elsevier Science Ltd. All rights reserved.