Drying characteristics of fine powders in a media fluidized bed

Abstract

Hydrodynamic properties (bed voidage, bubble chord length, chord length distribution, rising velocity, volume fraction) and the drying rates of batch drying and drying efficiencies of continuous drying have been determined in an inert medium fluidized bed (0.08 m I.D. $\times$ 1.2 m high). The drying mechanism in an inert medium fluidized bed can be classified into adhesion-dispersion, evaporation and disintegration steps. The mathematical model for an inert fluidized bed in batch and continuous drying processes has been developed based on the three phase (bubble, interstitial gas phase and solid phase) theory. To determine the bubble properties in the fluidized beds with a constant static bed height (0.2m), the measured bubble chord length distribution has been represented by a logarithmic normal distribution. The measured bubble chord length, volume fraction and rising velocity of bubble increase with increasing gas velocity. In an inert medium fluidized bed, individual phase holdups are calculated by the pressure drop and the material balance. Liquid phase holdup decreases linearly with time but gas phase holdup increases slightly with time. The drying rate in batch drying process increases with increasing inlet gas temperature and velocity and the rates are about 10 times those reported for an agitated pan dryer. However, the drying rate decreases slightly as the mass ratio of starch to inert particles increases. Also, the drying rate exhibits a maximum value at an optimum bed porosity condition. The drying efficiency of continuous drying process decreases with increasing gas velocity and inlet gas temperature. However, the drying efficiency increases with increasing latex and solid feed rates. The drying efficiency increases with increasing mass loading (solid to gas feed rates). At a constant moisture content, the mass loading increases with increasing inlet gas temperature. Heat and mass transfer coefficients in terms of Biot number which account the tra...