Two-phase theory and fine powders

Abstract

The validity of the assumptions built into the two-phase theory has been re-examined for a range of fine powders. These long-standing but often criticised assumptions are that, in the dense phase of a bubbling fluidised bed, the voidage and velocity correspond to those at incipient fluidisation. Eleven different powders were used, and average particle size varied from 12 to 67 μm. Particle density ranged between 1300 and 5200 kg/m 3. Dense phase velocity and voidage were measured by the bed collapse technique, and the systematic deviations from the above assumptions demonstrated. It has been shown that the minimum fluidisation point loses meaning at high fines levels; a transitional class on the Geldart [6] powder classification diagram, termed class AC, is postulated. This class is characterised by the absence of a meaningful incipient fluidisation point, and the absence of a contraction of the bed when bubbles first begin to pass through. The dense phase voidage has been found to increase more strongly than expected, and correlates strongly with fines level. Particle density has been found to have an insignificant effect on dense phase voidage in the range covered, and the gas velocity in the dense phase has also been found to increase and correlate strongly with increasing fines level. Correlations are presented for dense phase voidage and velocity.