Effects of pressure and fines content on bubble diameter in a fluidized bed studied using fast X-ray tomography

Using a fast X-ray tomography setup the bubble size and velocity has been measured in a 25 cm diameter fluidized bed of Geldart B powder and a 24 cm bed of Geldart A powder. The average bubble size has been determined for a measurement period of 60 s. The resolution of this setup is about 4.5 mm per pixel at a rate of 250 reconstructions per second. It is possible to detect bubbles as small as 2.2 cm.The Geldart B powder consists of polystyrene particles with an average diameter of 607 μm and a bulk density of 625 kg/m3. This bed was studied at pressures ranging from 1 to 5 bar. The superficial gas velocities varied from 12 to 32 cm/s for the atmospheric pressure measurements; for the highest pressure from 10 to 15 cm/s. The bubble size is significantly reduced at higher pressures for similar gas flows.The Geldart A powder consist of a base of aluminum oxide particles with an average diameter of 76 μm and a bulk density of 680 kg/m3. A varying amount of fines was added to these base particles for the different mixtures. The fines consist of aluminum oxide particles with an average diameter of 38 μm and a bulk density of 620 kg/m3. The fines contents varied from 0%w to 50%w. An increase in fines content results in a clear reduction of the average bubble size. If the fines content is increased from 0%w to 50%w the average spherical equivalent bubble diameter is reduced by 20%.

► Using our fast X-ray tomographic scanner we measure bubble size and velocity.
► Increasing the pressure from 1 to 5 bar reduces the bubble size significantly.
► The bubble size can be accurately described by Darton’s model.
► Adding 50% fines to a Geldart A powder reduces the bubble size by 20%.