Plate grid distributors have been widely used in fluidized bed operation for over fifty years. A critical design parameter for grid distributors is the stability factor defined as the ratio of the grid pressure drop to the pressure drop within the fluidized bed (DPgrid/DPbed). Sufficiently large grid pressure drop is required to activate all the grid holes and uniformly fluidize the bed of solid particles. A low value for the stability factor can lead to defluidized regions resulting in large temperature and pressure fluctuations as well as adversely affecting the performance of cyclone diplegs returning entrained particles to the fluidized bed. Several literature studies have investigated the minimum value for the stability factor in order to achieve uniform fluidization.[1, 2, 3, 4] A key focus for these studies was the impact of the bed aspect ratio (diameter:height) on the quality of fluidization. However, these studies exclusively investigated plate grid distributors where only the fluidizing gas passes through the grid holes. Older designs of fluidized catalytic cracking units (FCCUs) employ grids where both gas and catalyst particles flow through the grid holes, and it has been typical practice to use the same minimum stability factor for these designs as for the air-only grids.
This study presents experimental results on the fluidization behavior for both air-only and air-catalyst plate grid designs. The investigation was conducted at Particulate Solid Research, Inc (PSRI) in a scaled-down circulating fluidized bed of FCC catalyst where the stability factor was varied by manipulating grid hole velocity, solids flux, and bed depth. A fluidization index is defined to account for variations in fluidization quality occurring in different regions of the catalyst bed as measured by high speed pressure transducers. Results demonstrate that the fluidization index varies with changes in the stability factor for the different grid designs. The findings from these experiments are shown to be reasonable based on an analysis of the kinetic energy of the gas and catalyst mixture flowing through the grid distributor.
1. Gregory, S. A., Proc. Int. Symp. Fluidization, Eindhoven, Neterhlands University Press, Amsterdam, 1967.
2. Zenz, F. A., Proc. of Tripartite Chem. Engr. Conf., Montreal, Inst. Chem. Engr., 1968
3. Geldart, D. and Baeyens, J., Powder Tech., 42 (1985)
4. Sathiyamoorthy, D. and Horio, M., Chem. Engr. J., 93 (2003)
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