Mixing Behaviors of Dry and Wet Particles in a Pulsating Fluidized Bed

Dry and wet particles exhibited different fluidization and mixing behaviors in a fluidized bed with or without the pulsation of gas velocity. At minimum fluidization velocity, pulsation of gas velocity led to some convective motion of dry particles within an unexpanded packed bed, and a small extent of mixing could be achieved. In contrast, a bed of wet particles became fluidized and fairly well-mixed with the pulsation of gas velocity at a low pulsating frequency. For example, the Lacey mixing index of a bed of wet particles reached a value of about 0.7 after 10 s of fluidization at a mean gas velocity of U (mf) with pulsation at 2 Hz compared with negligible increase in the Lacey mixing index value in the absence of gas pulsation. The periodic increase in the average magnitude of fluid drag forces beyond the weight of particles was effective in inducing large expansions and contractions of the bed, giving rise to large oscillations in average particle-particle collision forces, which in turn was effective in overcoming capillary liquid bridge forces between particles. Pulsation of the gas velocity would be beneficial for enhancing mixing behaviors of wet particles at lower mean gas velocities where mixing would normally be difficult to achieve.