9 - Dense-Phase Fluidized Beds

Summary

Introduction

Contact schemes of gas–solid systems can be classified on the basis of the state of solids motion. For a batch–solids system, the gas at a low velocity merely percolates through the voids between packed particles while the particles remain motionless. The solids in this case are in the fixed bed state. With an increase in the gas velocity, particles move apart and become suspended; the bed then enters the gas fluidization state. Under relatively low gas velocities, a dense gas–solid suspension characterizes the bed; on the bed surface, some solids entrainment occurs. Gas bubbles or voids of bubble shapes can usually be clearly distinguished in the dense suspension. Under relatively high gas velocities, significant solids entrainment occurs and bubbles or voids are much less distinguishable in the dense suspension. These states of the beds are referred to as dense-phase fluidization. Further increase in the gas velocity will intensify the solids entrainment and the dense bed surface gradually becomes undistinguishable. At high gas velocities, particles are fully entrained, and the whole bed transforms to a “lean” gas–solid suspension. This state of fluidization is maintained by means of external recirculation of the entrained particles. Such operation is termed lean-phase fluidization. This chapter focuses on dense-phase fluidization. Lean-phase fluidization or transport is discussed in Chapters 10 and 11. As indicated in the Preface, the correlation equations presented in the text, unless otherwise noted, are given in SI units.