Heat transfer in three-phase fluidized beds containing low-density particles

Abstract

The complex behavior of heat transfer in three-phase fluidized beds has been attributed to the interaction between phases which varies with the particle size, particle density, gas and liquid velocities. Three-phase fluidization with low-density particles is of considerable interest to biochemical applications. However, the understanding of heat transfer behavior in low-density systems is very limited. A special heat transfer probe is immersed in a three-phase fluidized bed and local instantaneous and time-averaged heat transfer coefficients are measured for calcium alginate beads and other low-density particles. In liquid—solid systems with low-density particles, the instantaneous heat transfer due to single-bubble passage exhibits a local maximum in the primary wake region caused by the enhanced surface renewal at the heat transfer surface. Enhancement in heat transfer due to bubble wake increases with bubble size but decreases with increasing particle density. Accordingly, relative enhancement in time-averaged heat transfer in three-phase systems over liquid—solid fluidized beds is higher for low-density particles compared to heavier particles under identical gas and liquid velocities. Local time-averaged heat transfer in low-density beds is found to be relatively independent of liquid velocity for gas velocity higher than 3 cm/s. Distributor effects on the flow regime and thereby the heat transfer are significant for low-density particles. The heat transfer is observed to be higher in the coalesced flow regime compared to that in the dispersed flow regime primarily due to significant bubble-wake effects. Axial variations in heat transfer coefficients are also observed for systems containing intermediate-density particles due to axial nonuniformities in solids holdup.