Temperature Influence on Fluid Dynamics at the Transition from Bubbling to Turbulent Fluidization for Geldart's Group B Particles
Generally, oxygen carriers used for CLC are in the range of group B according to Geldartâs classification. Thus, a thorough investigation of the turbulent fluidized bed regime for this kind of particles, starting at superficial gas velocities in the bubbling state and ending at fast fluidization is the aim of this work. Temperature influences on fluid dynamics are investigated in a range between ambient conditions and temperatures of 800Â°C.
Three fluidized bed plants in pilot scale are used for this investigation. Two of them are heated electrically to reach the temperatures mentioned above. The first one has a diameter of 100 mm and the second one a diameter of 150 mm with heights of 17 m and 9 m, respectively. The third plant, having a diameter of 400 mm and a height of 17 m, is operated at ambient conditions.
In addition, the measurements are compared to results carried out in two laboratory scale plants with diameters of 50 mm and 100 mm.
A superficial gas velocity range between 0.2 m/s and 3 m/s is investigated. Aspect ratios (static bed height divided by bed diameter) are varied between 1 and 4. Two quartz sand fractions of different sizes (Sauter mean diameter: 188 Î¼m and 348 Î¼m) and ilmenite (Sauter mean diameter: 146 Î¼m) are used as bed material.
Pressure signals are recorded and analyzed related to their standard deviation and frequency spectra to get information about the transition from bubbling to turbulent fluidization and thus the flow structure. As local online measurement technique, capacitance probes are used for the determination of local solids concentrations and local velocities of bubbles/voids in the dense zone of the bed.
Static bed height and plant diameter are found to have a significant influence on the transition velocity from bubbling to turbulent fluidization for the range investigated. Higher static bed heights lead to higher transition velocities. With increasing temperature of the fluidizing gas, its viscosity and density change. Due to this change, in contrast to group A particles according to Geldartâs classification, for group B particles an increase in the transition velocity with increasing temperature can be observed. Finally, a correlation for the determination of the transition velocity from bubbling to turbulent fluidization is introduced by taking into account the static bed height, bed diameter and temperature.