(7f) Agglomerate Shrinkage and Bubbling Stimulation on Vibrated Apf Beds

In the first part of our work we derive a theoretical equation that predicts the expansion of a fluidized bed of fine powder in the Agglomerate Particulate Fluidization (APF) regime when it is subjected to vibration. This equation is based on the modified Richardson-Zaki equation [1] for APF and the scaling law [2] on the agglomerate limit size in the APF regime. The predicted expansion of the bed has been successfully compared to experimental data on fluidized beds of fine agglomerated particles. Vertical vibrations induce a progressive expansion of the fluidized bed as vibration amplitude is increased in agreement with our prediction. The physical mechanism responsible for bed expansion is the reduction of agglomerate size due to the increased effective gravity acceleration. A side effect of the induced bed expansion and agglomerate shrinkage is the increase of the concentration and size of local voids. As a consequence visible bubbles are developed and expansion is curtailed at a critical effective acceleration. According to our estimations bubbling stimulation by vibration occurs for a similar agglomerate volume fraction that the volume fraction of agglomerates at incipient bubbling for a sufficiently large gas velocity in the nonvibrated bed. This coincidence suggests that the physical mechanisms for the formation of bubbles are similar in both cases. The dynamical equilibrium between splitting and coalescence of voids prevents the formation of large gas bubbles but at the critical vibration amplitude (or gas velocity in the absence of vibration) this balance is shifted to coalescence into large gas bubbles.

[1] Valverde JM and Castellanos A. Fluidization of nanoparticles: A modified Richardson-Zaki law. AIChE 2006;52:838-842. Valverde JM et al. The settling of fine cohesive powders. Europhys. Lett. 2001;54:329-334.

[2] Castellanos A, Valverde JM and Quintanilla MAS. Physics of Compaction of fine cohesive powders. Phys. Rev. Lett. 2005;94:75501.