Characteristics of gas pockets in fluidized beds



Some characteristics of gas pockets rising through beds of fluidized solids have been measured directly with a light probe technique. The vertical thickness, numerical frequency, and rate of rise of the bubbles were obtained from oscillographs of dual probe signals. Room-temperature air at 1-atm. pressure was used in 4- and 6-in. columns. Glass beads, a crushed rock, commerical cracking catalysts, coal, and hollow resin spheres were studied in particle sizes from 12-µ fluid cracking catalyst to 450-µ coal.

Vertical bubble thickness was found to increase with particle size, distance above the bed support, and gas velocity. Size growth upon rising was mainly the result of coalescence of bubbles. The rise velocity ranged from 1 to 2 ft./sec., relatively unaffected by operating conditions. Bubble frequency decreased with height above the bed support owing to coalescence. Total bed-depth variation from 1.0 to 2.5 ft. did not significantly influence the results, which should be of interest in studying gas by-passing in fluidized beds and predicting the slugging behavior and fluidization uniformity.