The effect of shear rate on aggregate size distribution and structure at steady state in laboratory and pilot plant scale

0378884 - ÚH 2013 DE eng C - Conference Paper (international conference)
Bubáková, Petra - Pivokonský, Martin - Pivokonský, Radek
The effect of shear rate on aggregate size distribution and structure at steady state in laboratory and pilot plant scale.
Proceedings of the International Conference on Particle Separation. Berlin: IWA, 2012 - (Jekel, M.), s. 28-35
[IWA Specialist Conference on Particle Separation. Berlin (DE), 18.06.2012-20.06.2012]
R&D Projects: GA AV ČR IAA200600902
Institutional research plan: CEZ:AV0Z20600510
Keywords : aggregation * aggregate size distribution * flocculation * fractal dimension * shear rate
Subject RIV: BK - Fluid Dynamics

The paper deals with the dependence of aggregate properties on the shear rate (G) at steady state. Natural raw water and ferric sulphate were aggregated in a laboratory Taylor-Couette reactor (TC) and continuous flow mixing tank (MT) with a paddle stirrer (pilot plant). The shear rates in the range of approximately 20-350 s^-1 were used and the mixing time corresponded to 15 min (steady state). The methods of image and fractal analysis were used to determine the aggregate size and structure, respectively. It was found that the maximum steady state size of aggregates produced by both mixing devices was almost identical and ranged approximately from 7800 μm at G ~ 20 s^-1 to 245 μm at G - 300 s^-1. In the case of the average aggregate size, there was a little difference at G ~ 80 s^-1 when the aggregate size in MT was higher than in the TC device. A new relationship which describes the dependence of aggregate size on the shear rate was proposed. It takes into consideration the finite values of aggregate size at extreme values of shear rates. The D₂ fractal dimension was found to increase from 1.54 to 1.91 in TC and from 1.58 to 1.90 in MT with increasing shear rate indicating that the aggregates became less porous and more compact. Moreover, the very precise match in D2 values was attained for both mixing devices. The D _pf fractal dimension decreased from 1.37 to 1.10 in TC and from 1.36 to 1.12 in MT with increasing shear rate, which means that aggregates were more regular at higher shear rates but jagged on the surface with irregular shape at lower shear rates. On the contrary to D₂, aggregates formed in MT were much more irregular or jagged than those formed in TC.
Permanent Link: http://hdl.handle.net/11104/0210211