Solids suspension behaviour in profiled bottom and flat bottom mixing tanks

Abstract

Solids suspension in the classical flat bottom mixing tank was analysed phenomenologically in detail. Abrupt re-direction of the flow in the centre and the periphery of the flat bottom tank gives rise to induced recirculation loops which account for the formation of central and peripheral fillets of unsuspended solids. The extent of these recirculation loops has a controlling influence on the attainment of complete off-bottom suspension in this tank geometry. Therefore the use of the complete off-bottom suspension condition in the flat bottom tank as the sole criterion for scale-up is likely to lead to scale-up parameters which do not represent the behaviour of the bulk of the tank.A fully profiled tank bottom and its approximation, a “cone and fillet” bottom design were used as alternative geometries to mitigate recirculation loops. Streamlining of the mixing tank substantially improved solid and suspension efficiency and also produced a more homogeneous hydrodynamic regime in the bottom zone of the tank. This latter effect potentially simplifies theoretical modelling of solids suspension and further justifies the preference for profiled bottom mixing tanks rather than the flat bottom tank.