Analysis of floc size distributions in a mixing tank

The aim of this work is to analyse the relation between floc size distribution and hydrodynamics in a mixing tank. The objectives are to answer the two following questions: (1) Does a steady state exist in flocculation? (2) Do the floc size distributions depend on the impeller? Flocculation experiments are realised for fixed physico-chemical conditions (pH 3.5) in a standardized tank with two different impellers at the same dissipated power: an axial impeller (Lightnin A310) and a radial impeller (Rushton turbine). To analyse the sensitivity of flocculation to hydrodynamics, a hydrodynamic sequencing is performed. Whatever the impeller, a steady state, characterised by a balance between agglomeration and rupture phenomena, is rapidly reached. The characteristic size of the floc is shown to be related to the Kolmogorov scale and thus, to the dissipated power. In the case of the Rushton turbine, the floc size distributions are monomodal and can be fitted by a log-normal law. In the case of the Lightnin A310, floc size distributions are rather bi-modal and can be fitted by a weighted combination of two log-normal distributions. Given a dissipated power, different impellers, with different power numbers, rotate at different velocities; the mixing efficiency of these different impellers is shown to generate different floc size distributions. Thus the dissipated power cannot be the single parameter sufficient to model aggregation.