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Particle size segregation in particulately fluidised beds. Pruden, Barry Blythe

Abstract

A paper by Jottrand which deals with particle size segregation in liquid-fluidised beds is examined critically, and an attempt has been made to extend and complement his work. It is assumed that the driving force for segregation of two groups of particles in a particulately fluidised bed is the difference in bulk density of the beds formed by each group of particles separately when subjected, to the same superficial velocity of the fluid. Accordingly the following equation for the bulk density difference between two groups of particles with unequal diameters was derived and studied experimentally: [ Equation omitted ] In the above equation, the subscripts L and S refer to large and small particles respectively, m is the index of fluid régime (m = 1 in Stokes region, m = 2 in Newton region), and n is the exponent in a Richardson-Zaki type expansion equation. It was found that for equal density particles the segregation tendency increased as the overall porosity, and hence ɛL was increased and as the diameter ratio (r) was increased. The tendency either to mix or to segregate was found to be very sensitive to changes in ϒ (density difference between large particles and fluid divided by the density difference between small particles and fluid), as is the bulk density difference in the above equation. It was observed that a bed could be either mixed or completely segregated, with one or the other size of particles occupying the upper portion of the bed, depending on the relative magnitudes of r and ϒ. It was also found that, while an increase in the factor ( ρpL-ρ) increased the bulk density difference, it did not increase the tendency of the particles to segregate, provided r and ϒ were kept constant. The values of m and n were found to change sufficiently, as the fluid régime was changed from the Stokes region to the Newton region, to account for the corresponding changes in segregation tendencies. Finally it was concluded that the bulk density difference equation predicted the right trends, insofar as mixing or segregation are concerned, but that the absolute magnitude of the predicted bulk density difference was not uniquely correlated with the observed phenomena, when the particle density was varied at constant r and ϒ . The applicability of a reduced bulk density difference was suggested by the limited data available.

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