Segregation in water fluidized beds of sand particles

Fluidization experiments were conducted on four fractions of filtration (quartz) sand using columns of 144 and 64 mm i.d. The size distribution of the fractions obtained by sieving followed an approximately modified normal (Gaussian) particle size distribution with respect to the projected particle diameter, while the shape factor decreased linearly with increasing projected particle diameter. The expansion characteristics of the sand particles followed a Richardson-Zaki relationship. The intercept velocity, i.e., the extrapolated value of the fluid superficial velocity to epsilon=1 on the plot log (U) vs. log (epsilon), agreed quite well with the experimentally determined mean free settling velocity of the cloud of about 30 randomly selected particles. During fluidization, the beds were hydraulically separated into 10 sub-fractions, which are then analyzed. For the mixture d(p)=0.75-1.25 mm during fluidization at an overall bed expansion of 35%, the bed partially segregated since, remarkably, the smallest particles were concentrated at the top, while the largest particles were concentrated in the bottom zone. In the remaining part of the bed (about 80% of total mass), the particles were well mixed. Similar experiments were performed with sand mixtures d(p)=1.60-2.00 mm, d(p)=1.40-2.00 mm and d(p)=1.166-2.00 mm. By analyzing the particle size distribution at the bottom and at the top of the bed, it was concluded that the bed was well mixed if the sieving ratio of the largest to smallest sieve opening was less than about 1.5