The Effect of Bottom Roughness on the Minimum Agitator Speed Required to Just Fully Suspend Particles in a Stirred Vessel

In this work, the effect of the vessel bottom roughness on the suspension of solid particles in stirred tank reactors is investigated. The experiments were performed in a baffled vessel, which was mechanically stirred with a 45° pitched blade turbine. In order to evaluate the influence of the bottom roughness on particle suspension, four bottoms of different roughness and 8 different sets of spherical particles were used. The density of the solid particles, ρs, ranged from 2500 kg m−3 to 8743 kg m−3 and they were characterized by narrow size distributions with a mean diameter, dp, from 128 μm up to 1850 μm. Measurement of the minimum impeller speed for ‘just complete suspension’, Njs, showed that the roughness of the bottom had a significant influence. The precise effect depends on the particle size compared to the size of the roughness elements and to the Kolmogoroff microscale, λK.