X-ray tomography of a fluidized bed Effect of pressure and fines on bubbles in a fluidized bed

Using a fast X-ray tomography setup measurements have been done on a 25 cm diameter f luidized bed of Geldart B pow^der and a 24 cm bed of Geldart A powder. The average bubble sizes have been determined over a measurement period of 60 seconds. The resolution of this setup is about 4.5 m m per pixel at a rate of 250 reconstructions per second. I t is possible to detect bubbles as small as 2.5 cm. The Geldart B powder consisted of polystyrene particles w i t h an average diameter of 607 fim and a bulk density of 625 k g / m ^ . This bed was studied at pressures ranging f r o m 1 to 5 haVabsThis was done at superficial gas velocities f rom 12 to 32 cm/s for the atmospheric pressure measurements. For the highest pressure (5 bar^bs) this was 10 to 15 cm/s. These measurements showed that the normal gas f low can be increased f r o m 500 l / m i n at atmospheric pressure to 1700 l / m i n at 5 barabs before bubbles start appearing. The bubble size is significantly reduced at higher pressures for similar gas flows. The Geldart A powder came in the fo rm of several mixtures. These mixtures consist of a base of aluminum oxide particles w i t h an average diameter of 76 pm and a bulk density of 680 k g / m ^ . A different amount of fines was added to these base parficles for the different mixtures. The fines consist of aluminum oxide particles w i t h an average diameter of 38 pm and a bulk density of 620 k g / m ^ . The fines contents varied f rom 0 to 50%u,ei<,/itA n increase i n fines content showed a clear reduction i n average bubble size. I f the fines content is increased f r o m 0 to 50%u, the average spherical equivalent bubble diameter is reduced by 20%.