Residence Time Distribution of Segregating Sand Particles in a Rotary Drum

Abstract

The residence time distribution of single quartz sand fractions in a continuously operated rotary drum (D = 0.32 m, L = 2.6 m) was measured with the pulse method within the cascading regime at rotation speeds n ≤ 10 min−1. The particle size distribution (PSD) was composed of single fractions within the particle size range of 0.6 to 1.5 mm to obtain an RRSB-PSD (Rosin, Rammler, Sperling, and Bennet particle size distribution, see, e.g., Stieß, 2009) with a size parameter of x′ = 1.0 mm and a scatter parameter of nRRSB = 3.5. The particles were dosed in a mixed state at flow rates , collected at the drum outlet in samples, and evaluated by a color identification method. From the response curves, the mean residence time of single fractions could be obtained as well as the axial dispersion coefficient. The mean residence time was found to increase for particles with smaller diameters, as these particles are forced inward into the particle layer due to segregation with lower axial transportation displacements per revolution. Typically, the largest fraction was found to move about 1.4 times faster through the drum than the smallest fraction. Reducing the feed flow rate as well as the rotation speed of the drum, the overall residence time increases. Fractions at the border of the PSD exhibit much smaller scatter of residence time and show smaller dispersion coefficients than particles in the middle of the PSD, preferably moving at median trajectories within the layer. The dispersion coefficients were found to be within the range of 1.0 × 10−6 m2/s < Dax < 2.5 × 10−5 m2/s.