Effects of Fines on Fluidization

It is well known that addition of fines improves quality of fluidization [1,2]. However, the mechanism through which the fines affect fluidization quality is not fully understood. Addition of fines not only introduces particle size distribution, but also makes the system more cohesive. It is unclear which property of fines is responsible for its beneficial effect. Here, we probe this question through numerical simulations.

We have performed Euler-Lagrange simulations of gas-fluidization of Group A particles (d_p= 75 μm) containing different levels of fines (d_p= 25 μm). The simulations were performed in small periodic domains using ambient air as the fluidizing gas and various particle volume fractions. Inhomogeneous microstructures that take the form of clusters or bubble-like voids readily form when no fines were added. Introducing fines did not lead to appreciable improvement in the quality of fluidization, when van der Waals force of interaction between particles was not included. In contrast, the addition of cohesion due to van der Waals force produced significant changes under some flow conditions. At high volume fractions of the 75 μm particles, the addition of a small amount of fines was found to be sufficient to eliminate the inhomogeneous microstructure almost completely. The amount of fines required to homogenize the flow increased with decreasing volume fraction of the large particles. It is found that when fines managed to homogenize the flow, the contact coordination number between particles is very large; this is consistent with previous findings [3,4] regarding the role of cohesion on delay of the onset of bubbling in fluidized beds.

This presentation will summarize the results of this simulation study, discussing the nature and extent of inhomogeneous microstructure under various conditions.

[1] J.R. Grace, G. Sun, “Influence of particle size distribution on the performance of fluidized bed reactors”, Can. J. Chem. Eng.. 69, 1126 (1991).

[2] J.C. Agarwal, W.L. Davis, “The dynamics of fluidization of iron and its ores”, Chem. Eng. Prog. Symp. Ser. No. 67. 62, 101 (1966).

[3] Q.F. Hou, Z.Y. Zhou, A.B. Yu, “Micromechanical modeling and analysis of different flow regimes in gas fluidization”, Chem. Eng. Sci.. 84, 449 (2012).

[4] J.E. Galvin, S. Benyahia, “The effect of cohesive forces on the fluidization of aeratable powders”, AIChE J.. 60, 473 (2014).