Reconsideration of the hydrodynamic behavior of fluidized beds operated under reduced pressure

Abstract

Fluidized beds operated at sub-atmospheric pressure can be employed for granulation and drying of thermo-sensitive materials in the food and pharmaceutical industries. However, the hydrodynamics of vacuum fluidized beds has not been extensively investigated. Some authors argue that at low pressures, the slip flow of gas is the major factor influencing the hydrodynamic behavior. The influence of change in gas properties on the hydrodynamics due to reduction in pressure has not been clearly distinguished. In this contribution, the individual effects of gas properties and slip flow on the hydrodynamic behavior, particularly on the minimum fluidization velocity, of vacuum fluidized beds are quantified. This has been achieved by expanding the classical minimum fluidization velocity correlation, valid under atmospheric pressure, to include the slip flow term. The results obtained describe a critical Knudsen number which indicates when the slip term begins to significantly influence the flow behavior. The derived correlation is compared with correlations reported in literature as well as validated with experimental data.