Numerical study of the hydrodynamics of fluidized beds operated under sub-atmospheric pressure

Fluidized beds operated under partial vacuum offer a promising alternative to the expensive freeze-drying technique for processing thermo-sensitive materials in the food and pharmaceutical industries. However, the hydrodynamics of vacuum fluidized beds has not been extensively investigated. In this paper, we apply a two-fluid Eulerian model to investigate the influence of sub-atmospheric pressure on the hydrodynamic characteristics in two-dimensional fluidized beds. The simulations are compared with experiments performed in a batch vacuum fluidized bed plant. A parametric study of the effect of pressure on bed hydrodynamics is performed for two fluidization conditions: (1) constant excess velocity and (2) three times the minimum fluidization velocity. The pressure is varied in the range 1000 mbar-100 mbar with fluidized particles belonging to Geldart group D. A detailed investigation is carried out on porosity distribution, bed expansion, bypass ratio, and bubble characteristics. At low pressure, the porosity distribution is non-homogeneous with increased bubble activity near walls. A decrease in bed porosity and bubble splitting is observed as the operating pressure is reduced. The simulation results are also compared with semi-empirical correlations developed for atmospheric pressure conditions. The applicability of these correlations at sub-atmospheric pressures is, thus, evaluated.