Agglomerate particulate fluidization (APF), consisting of uniform bubbleless fluidization of agglomerated fine particles, has been observed in some fluidized beds of fine and ultrafine powders. Experiments performed in centrifugal fluidized beds (CFB) and vibrofluidized beds (VFB) show that the quality of fluidization is improved by increased effective acceleration. The Richardson-Zaki modified equation is used, and a proposed criterion for agglomerate size is presented in order to predict the effect of the increase of gravitational acceleration on the APF bed. Following, model predictions are compared with experimental data on VFB of xerographic toners (particle size approximate to 10 mu m) displaying APF in a wide interval of gas velocities. It is seen that the agglomerate size and agglomerate volume fraction phi* decrease as the effective acceleration is increased. The agreement with the model predictions is good, but at some critical acceleration visible bubbling is stimulated. At bubbling stimulation phi* is close to the value of phi* at the onset of bubbling in the absence of external vibrations when the gas velocity is increased up to the minimum bubbling velocity. This result indicates that the agglomerate size cannot be decreased indefinitely since at some critical point the concentration and size of local voids can be higher enough to yield the irreversible coalescence of voids into large bubbles. Particle agglomeration is, therefore, essential at some extent to the uniform fluidized state observed for fine particles. (c) 2006 American Institute of Chemical Engineers.