Solids distribution and rising velocity of buoyant solid particles in a tank stirred with multiple turbines: a simplified approach for the study of sparged gas-liquid reactors (Paper 7.4)

The distribution of buoyant solids in agitated suspensions has been studied. The investigation was carried out in a baffled vessel characterised by an aspect ratio equal to four and stirred with four radial impellers. Dilute suspensions of single-sized spherical particles of expanded polystyrene (density equal to 90.7 kg/m3) in water were used. Solid concentration was measured with a non-intrusive optical technique. Measurements were performed along the axis of the reactor to obtain steady-state vertical profiles as well as at fixed elevations to determine the concentration transient after a pulse of solids injected at the bottom. Both the transient concentration curves and the steady-state profiles were interpreted by means of the axial dispersion model with sedimentation. The particle rising velocity in the agitated system was determined by data treatment; it proved to be significantly smaller than the free-rising velocity, i.e. the rising velocity in a still liquid. The ratio of these two velocities is in reasonable agreement with a correlation of the ratio of the settling velocities of heavy particles with the ratio of the Kolmogorov microscale to particle diameter established previously.