It can be seen clearly from the two pictures that as superficial air velocity goes up, bubble number and size increase. Since
U=0.081 m/s, the fluidized bed is filled with ascending air flow and the fluidized bed density can be adjusted by different
superficial air velocities (from 0.081 m/s to 0.119 m/s). After the superficial air velocity of 0.119 m/s, the solid particles performs
serious back-mixing and fluidized bed is full of large bubbles, indicating the fluidized bed became invalid for coal separation.
4.4 The separation performance of particle bed.