An analysis of choking and fast fluidization

摘要

Choking in gas-particle systems and its relationship to fast fluidization is still an area of much confusion and debate. This paper presents an analysis which may help clarify the situation. We consider a tall vessel with imposed gas velocity and solids feed rate such that there is dilute phase flow throughout. Increasing the solids feed rate whilst maintaining the gas velocity will cause an increase in suspension concentration at the exit. If solids feed rate is further increased, we reach the maximum exit suspension concentration that can be sustained (saturation carrying capacity) whilst maintaining dilute phase flow throughout. Any further increase in solids feed rate causes solids accumulation at the base of the vessel as a dense phase. In general, the upward solids entrainment flux in the dilute phase decreases with height. The effect of accumulation of dense phase is to push the dilute phase upwards until the upward solids entrainment rate at the exit matches the solids feed rate. The rate of decrease in upwards solids entrainment flux with height is described by a decay coefficient, which in practice, varies from a low value of 0.5 to a high value of 3. Decay coefficient increases with decreasing gas velocity, with decreasing vessel diameter and with increasing particle size. The time required for a system to respond to a change in feed flux decreases with increasing decay coefficient and increasing feed flux. In the light of this analysis, fast fluidization, choking and turbulent fluidization are defined and the structure of the dilute and dense phases are considered.