In this paper the bed collapsing properties of three kinds of FCC particles with different average sizes are studied by a new isolated freeboard bed collapsing technique. The bed emulsion phase properties were investigated at high superficial gas velocities. With a video camera the bed collapsing process was recorded and the collapsing curves were obtained. From the collapsing curves the properties of the emulsion phase and the bubble phase were investigated and the effect of the particle average size on thecollapsing properties was analyzed further. The experimental results showed that dp = 30μm particles had the longest collapsing time

the smallest emulsion phase density and the larger bubble phase fraction.The conclusion can be drawn that the dp = 30μm FCC particles possess the most desirable fluidization properties.