Hydrodynamic study of tapered BED

Abstract

Hydrodynamic characteristics of fluidization in conical or tapered beds differ from those in columnar beds due to the variation of superficial velocity in the axial direction of the beds. In the former, fixed and fluidized regions could coexist and the sharp peaking of the pressure drop could occur, thereby giving rise to a remarkable pressure drop- flow rate hysteresis loop at incipient fluidization. To explore these unique properties, a series of experiments was carried out in gas-solid tapered fluidized beds with various tapering angles. Detailed visual observations of fluid and Particle behavior and measurements of the pressure drops have led to the identification of five flow regimes. The tapering angle of the beds has been found to dramatically affect the beds' behavior. Other hydrodynamic characteristics determined experimentally included the maximum pressure drop, minimum velocity of partial fluidization and minimum velocity of full fluidization. Models have been proposed to quantify the hydrodynamic characteristics of gas-solid tapered fluidized beds. The results predicted by the models compare favorably with experimental data. Naturally, the models are applicable to gassolid columnar fluidized beds corresponding to the tapered beds with a tapering angle of zero. Fluidization operations in cylindrical columns are extensively used in process industries. But, in a number of these operations, the particles are generally not of uniform size or there may be reduction in size due to chemical reactions like combustion or gasification, or attrition. In cylindrical beds, the particle size reduction results in entrainment, limitation of operating velocity in addition to the other demerits like slugging, non-uniform fluidization generally associated with such beds. These disadvantages are overcome by use of tapered beds to conduct fluidization operation. This is due to conduct fluidization operation. This is due to the gradual reductions of superficial velocity of the fluid because of increase in cross sectional area with height. However, in general the particles are not of uniform size in the feed itself and also there may be particle size reduction during operation in fluidized beds. Hence, the study of hydrodynamic characteristics of such beds is essential in the operation of such beds. Based on dimensional analysis, correlations have been developed with the system parameters viz., geometry of tapered bed, particle diameter, static bed height, density of solid and gas and superficial velocity of the fluidizing medium. Correlations have been developed for some characteristics specially, critical fluidization velocities and maximum bed pressure drops of gas – solid tapered fluidized bed of binary mixture of regular particles. Experimental values of critical fluidization velocities and maximum bed pressure drops have been compared with the developed correlations.