(24d) Homogeneous Gas Fluidization: Kinematic and Dynamic Wave Velocities, Particle Mobility and Apparent Suspension Viscosity
Homogeneous fluidization-quality of gas fluidized beds has received little attention in the research literature but has grown in importance in recent years, in part as a result of the observed behaviour of fluidized nanoparticle agglomerates in investigations into processing technologies for dealing with large quantities of these materials for numerous state-of-the-art applications. These agglomerates assume diameters which correspond to conventional fluidized bed inventories, but are of very high porosity and correspondingly very low density; they have been found to fluidize homogeneously with ambient air, displaying expansion characteristics in good quantitative accord with the Richardson-Zaki law. This behaviour turns out to be good agreement with predictions of a fluid-dynamic model for the fluidized state (P U Foscolo and L G Gibilaro 1987. Fluid dynamic stability of fluidized suspensions: the Particle Bed Model. Chemical Engineering Science 42. 1489; L G Gibilaro 2001. Fluidization-Dynamics, Butterworth Heinemann, Oxford), which provides a theoretical map for fluidization of any particle species by any fluid, thereby generalising the empirical Geldart map for fluidization by ambient air, and contains a region of fully homogeneous behaviour into which most nanoparticle agglomerates fall. Ambient air-fluidized particles of low density, having diameters of 1 mm or even more, are predicted, and have been found, to lead to expansion in a fully homogeneous manner: for example, expanded polymer particles, which can have densities of around 10 kg/m3, which is even lower than that of reported nanoparticle agglomerates. There exists therefore a considerable range of particle species for which homogeneous gas-fluidization can occur. Homogeneous fluidization-quality may be characterised in terms of the velocities of kinematic and dynamic particle-concentration waves, the 'bulk mobility' of the particles and the apparent viscosity of the suspension. The bulk mobility parameter was first proposed by Batchelor (1988. A new theory for the instability of a uniform fluidized bed. Journal of Fluid Mechanics 183. 75), defined in terms of the small change in particle velocity brought about as a result of a small applied force. It may be readily predicted by means of the Particle Bed Model, and may be used to explain the observed differences between gas and liquid homogeneous fluidization: tightly held together gas-fluidized suspensions and considerable random particle motion for liquid fluidization. It would also appear to be closely linked to the concept of an apparent suspension viscosity, a simple inverse proportional relation existing between these two parameters for the limiting case of a single particle swept by a Newtonian fluid under low Reynolds number conditions. In the present study it becomes a key parameter in attempts at characterising quantitatively the observed fluidization-quality in various homogeneous gas-fluidized beds. Results are reported for homogeneous gas-fluidization using particles of small size and/or low density, which display significant regions of homogeneous behaviour and which include zones effected by hysteresis phenomena (S C Tsinontides and R Jackson 1993. The mechanics of gas fluidized beds with an interval of stable fluidization. Journal of Fluid Mechanics 255. 37). Ambient and moderate pressure operation conditions have been employed, using a fluidization rig which may be operated at high gas pressure and still permit for visual observation of the bed through narrow perspex windows. The velocities of kinematic and dynamic shocks have been measured (via respectively the bed surface velocity following a gas flux step change, and the 'raining down' interface for a bed packed against a top porous plate). Such measurements have been previously performed for liquid fluidized beds (L G Gibilaro, R Di Felice, I Hossain, P U Foscolo 1989. The experimental determination of one-dimensional wave velocities in liquid fluidized beds. Chemical Engineering Science 44. 101) but not for gas ones. Initial results from this research suggest that, in the absence of hysteresis effects, the kinematic and dynamic wave velocities conform well to fluid dynamic model predictions and, together with particle mobility, provide a comprehensive basis for the characterisation of homogeneous gas-fluidization quality. Results will also be presented for apparent viscosity estimates based on the rise velocity of small low density spheres, and attempts to relate these to predictions of the bulk mobility of the fluidized particles.
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