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The mechanics of gas fluidized beds with an interval of stable fluidization

Published online by Cambridge University Press:  26 April 2006

S. C. Tsinontides
Affiliation:
Department of Chemical Engineering, Princeton University, Princeton, NJ 08544, USA
R. Jackson
Affiliation:
Department of Chemical Engineering, Princeton University, Princeton, NJ 08544, USA

Abstract

When small, light solid particles are fluidized by gases it is well known that stable expansion occurs over a finite interval of gas flow beyond the point of minimum fluidization. The existence of such an interval can be predicted from linear stability theory provided the momentum equation for the particles contains a sufficiently large term representing an effective pressure that increases with the concentration of the particles. There is at present some controversy regarding the physical origin of such a term. Some workers attribute it to forces exerted between particles at points of solid–solid contact, while others invoke hydrodynamic mechanisms related to the interaction between the particles and the fluid. In this paper the processes of fluidization and defluidization for fine particles are followed very carefully round complete cycles, starting from zero gas flow and extending to a value at which bubbles appear, then back to zero. The depth of the bed and the pressure drop in the gas traversing it are recorded at each stage, and vertical profiles of the volume fraction of particulate material are determined with a high-resolution gamma-ray densitometer. Similar information is also obtained for sub-cycles extending over more restricted intervals of the gas flow rate. The particles studied are cracking catalyst, with mean diameter 75 μm, and Ottawa sand with mean diameter 154 μm. The results lead to the conclusion that the particle assemblies exhibit yield stresses throughout the range of stable behaviour, and thus are not truly fluidized beds, in the accepted sense. The phenomena observed are such that it is most unlikely that their origin is hydrodynamic. For the particular systems studied we therefore conclude that contact forces are responsible for stabilization.

Type
Research Article
Copyright
© 1993 Cambridge University Press

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References

Abrahamsen, A. R. & Geldart, D. 1980 Behavior of gas fluidized beds of fine powders. Part I. Homogeneous expansion. Powder Technol. 26, 3546.Google Scholar
Anderson, T. B. & Jackson, R. 1968 Fluid mechanical description of fluidized beds. Stability of the state of uniform fluidization. Ind. Engng Chem. Fundam. 7, 1221.Google Scholar
Batchelor, G. K. 1988 A new theory of the instability of a uniform fluidized bed. J. Fluid Mech. 193, 75110.Google Scholar
El-Kaissy, M. M. & Homsy, G. M. 1976 Instability waves and the origin of bubbles in fluidized beds. Intl J. Multiphase Flow 2, 379395.Google Scholar
Foscolo, P. U. & Gibilaro, L. G. 1984 A fully predictive criterion for the transition between particulate and aggregative fluidization. Chem. Engng Sci. 39, 16671675.Google Scholar
Garg, S. K. & Pritchett, J. W. 1975 Dynamics of gas fluidized beds. J. Appl. Phys. 46, 44934500.Google Scholar
Geldart, D. 1973 Types of gas fluidization. Powder Technol. 7, 285292.Google Scholar
Ham, J. M., Thomas, S., Guazzelli, E., Homsy, G. M. & Anselmet, M.-C. 1990 An experimental study of the stability of liquid fluidized beds. Intl. J. Multiphase Flow 16, 171185.Google Scholar
Homsy, G. M., El-Kaissy, M. M. & Didwania, A. 1980 Instability waves and the origin of bubbles in fluidized beds. Part 2. Comparison with theory. Intl. J. Multiphase Flow 6, 305318.Google Scholar
Jackson, R. 1963 The mechanics of fluidized beds. Trans. Inst. Chem. Engrs 41, 1328.Google Scholar
Janssen, H. A. 1895 Versuche uber getreidedruck in silozellen. Ver. Deutsch. Ing. Zeit. 39, 10451049.Google Scholar
Kunii, D. & Levenspiel, O. 1969 Fluidization Engineering, 1st edn. Wiley.
Medlin, J. & Jackson, R. 1975 Fluid mechanical description of fluidized beds. The effect of distributor thickness on convective instabilities. Ind. Engng Chem. Fundam. 14, 315321.Google Scholar
Medlin, J., Wong, H. W. & Jackson, R. 1974 Fluid Mechanical description of fluidized beds. Convective instabilities in bounded beds. Ind. Engng Chem. Fundam. 13, 247259.Google Scholar
Mutsers, S. M. P. & Rietema, K. 1977a The effect of inter-particle forces on the expansion of a homogeneous gas fluidized bed. Powder Technol. 18, 239248.Google Scholar
Mutsers, S. M. P. & Rietema, K. 1977b Gas–solids fluids fluidization in a centrifugal field. The effect of gravity upon bed expansion. Powder Technol. 18, 249256.Google Scholar
Richardson, J. F. & Zaki, W. N. 1954 Sedimentation and fluidization. Trans. Inst. Chem. Engrs 32, 3553.Google Scholar
Rietema, K. 1973 The effect of interparticle forces on the expansion of a homogeneous gas-fluidized bed. Chem. Engng Sci. 28, 14931497.Google Scholar
Rietema, K. & Piepers, H. W. 1990 The effect of interparticle forces on the stability of gas-fluidized beds — I. Experimental evidence. Chem. Engng Sci. 45, 16271639.Google Scholar
Tsinontides, S. C. 1992 A theoretical and experimental investigation of the mechanics of fluidized gas-particle systems. Ph.D. dissertation, Princeton University.
Verloop, J. & Heertjes, P. M. 1970 Shock waves as a criterion for the transition from homogeneous to heterogeneous fluidization. Chem. Engng. Sci. 25, 825832.Google Scholar
Wallis, G. B. 1969 One-Dimensional Two-Phase Flow. McGraw-Hill.