Segregation in a fluidised powder of a continuous size distribution

The state of mixing in a gas fluidised bed of sand of a continuous size distribution has been investigated at various fluidisation velocities. The results are shown mainly as axial concentration profiles of the individual size fractions obtained by sieving. It was found that the local size distribution of the bed material was strongly non-uniform axially due to segregation up to fluidisation velocities significantly higher than the minimum fluidisation velocity. Moreover, the segregated bed exhibited a pattern which can be described roughly as two superimposed layers, each of approximately constant powder composition axially. This is similar to what has been found in beds containing binary mixtures of powders. In contrast to binary systems, however, defluidisation of the bottom of the bed was not seen under any of the conditions investigated. An attempt at a qualitative, and in some measure quantitative, explanation of the obtained results has been made.