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The present paper describes measurements of time-averaged voidage distribution in a pilot-scale fluidized bed using γ-ray transmission technique. The voidage profiles in absence of side gas injection showed that the tracks of bubble flow change from one side of the wall to the center of the bed with increasing superficial gas velocity. The side gas injection can drastically alter the shape of the bed voidage distribution. At incipient fluidization the jet bubbles and distributor bubbles were found to be moving without interacting with each other. At 1.5u mf the jet bubbles and the distributor bubbles move from the wall to the central region of the bed with the increasing bed height above the nozzle plane. At 2u mf the jet bubbles and distributor bubbles follow the same track from the nozzle plane. The void fraction measurement below the nozzle showed that there was no back mixing of gas injected through the nozzle.

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Mixing times and holdup of solids were measured in a gas-solid fluidized bed using radiotracer technique. Sand and air were used as solid and gas phase, respectively in the fluidized bed. Gold-198 labeled sand particles were used as radiotracer for mixing time measurement at different operating conditions and 137 Cs sealed source was used for holdup measurement at different axial and radial positions. The experiments were conducted at different operating conditions. The measured mixing times ranged from 1.4 to 21 s at different conditions. It was observed that at a particular bed height, the mixing time initially decreases with increasing gas velocity and tend to become constant at higher gas velocities. However, mixing time increases with increasing bed height. The holdup fraction of solid was found to be more towards the wall compared to the centre of the column. The study provided inputs to improve the existing design, design of a new system and scale-up of the process.

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