Axial dispersion of spheres fluidized with liquids

Abstract

Solids dispersion due to the simultaneous diffusion (random particle motion) and classification (segregation) of fluidized spheres has been investigated. The model developed permits calculation of the mean concentration of spheres of both sizes in a mixture of two sizes of fluidized spheres os a function of bed length. Fick's law has been applied to the diffusional phenomenon.A hypothesis has been advanced and confirmed which permits calculation [Equation (511 of the classification velocity for each size of sphere in a fluidized mixture. Calculation of the classification velocities in a mixture of spheres is based on the relation of void fraction to superficial velocity for the individual sizes of spheres. To facilitate carrying out these calculations, a generalized equation has been developed for the relation of void fraction to superficial velocity in terms of the Galileo number (d3gAppf/p2) and sphere-to-column diameter ratio. Measurement of the bead size grodients (change in bead size with bed length) at steady state fluidization was used to investigate dispersion. Closely sized glass beads of 0.1 and 0.2 cm. diameters were fluidized with liquids of 1 and 15 cps. a t void fractions of 0.5 to 0.8 in 2.5 and 5.0 cm. columns. Lead beads of 0.12 cm. diameter were also used. Further, several experiments were conducted by a method which is analogous to that used in molecular diffusion cells.The unique transport behavior which is observed in a fluidized bed is, of course, due to the mobility of the solid phase in addition to that of the fluid phase. The work reported here is an investigation of the diffusion and classification of spheres which have been fluidized with a liquid. Diffusion is used to describe the random mixing of the particles which is observed in fluidized beds. Classifcation refers to the tendency of a mixture of fluidized particles of different sizes or densities to separate into layers which are of the same size or density. These two mechanisms are the sources of dispersion which are considered here. As will be explained in more detail, since only the average concentration variations in the direction of fluid flow were measured, consideration is given only to dispersion in the axial direction. PREVIOUS WORKRichards ( 1 ) has shown in a remarkable photograph the importance of both classification and diffusion in the dispersion occurring in a fluidized bed of quartz and galena. Classification is clear in that the larger and denser particles tend to move to the bottom of the bed. Diffusion is observed since a wider range of particles is found in each section of the bed than would be possible if classification were not offset by mixing of the particles in the axial direction. In his work it is pointed out that the greater the terminal velocity of a single particle in an infinite media, the lower is the equilibrium level to which the particle tends to move, as a result of classification. This observation was observed regardless of the void fraction of the fluidized bed.Brotz ( 2 ) used a method somewha...

Particle dispersion in a binary solid‐liquid fluidized bed

“…The presence of the two counteracting mechanisms of convection and dispersion in fluidized beds containing binary-size mixture of solid particles was recognized first by Kennedy and Bretton (1966). In the literature, this phenomenon is commonly described as:…”

“…It is worthwhile to point out that most previous studies basically followed the approach of Kennedy and Bretton (1966) as represented by Eq. 2.…”

“…For example, Al-Dobbin and Garside (1979), like Kennedy and Bretton (1966), used the correlation of Richardson and Zaki (1954) to define the voidage function, f(e), for the evaluation of Up8 using Eq. 1.…”

Fluidized bed photoreactors using composites of titania CVD-coated onto quartz sand as photocatalyst: Assessment of photochemical efficiency

“…As regards the dynamics of the solid-liquid media in the FB, using beads with just two different sizes Kennedy and Bretton [5] proposed that two opposing mechanisms are simultaneously interacting in the system: a 'diffusion mechanism' which promotes random mixing versus a 'classification mechanism', as a result of dissimilar sedimentation velocities. Along these lines Van der Meer et al [6] worked on liquid FB using particles of different densities, but quite similar sizing, while Dutta et al [7] and Di Felice et al [8] investigated on liquid FB constituted by binary systems of particles with different size and/or density.…”

Dynamic model of multi‐species segregation and dispersion in liquid fluidized beds

“…Kennedy and Bretton (1966) introduced the concept of a segregation velocity to account for the effects of dispersion on the distribution of particles in a liquid-fluidized bed. Their basic formulation continues to underpin all models referred to in this article including the one we propose.…”