The effect of shear rate on aggregate size distribution and structure at steady state: a comparison between a Taylor–Couette reactor and a mixing tank

The paper deals with the dependence of aggregate properties on the shear rate (G) at steady state. Aggregation of natural raw water and ferric sulphate was carried out in a laboratory Taylor–Couette reactor (TC) and continuous flow mixing tank (MT) with a paddle stirrer. Shear rates in the range of 20–350 s⁻¹ were used. Methods of image and fractal analyses were used to determine the aggregate size and structure, respectively. It was found that the aggregate size decreased with increasing shear rate. There was a small difference in sizes of aggregates formed in TC and MT at G < 150 s⁻¹, but at higher shear rates, almost no difference between TC and MT was observed. The D₂ fractal dimension increased with increasing shear rate indicating that aggregates became less porous and more compact. Moreover, a very close match in D₂ values was attained for both mixing devices. The D_pf fractal dimension decreased with increasing shear rate meaning that aggregates were more regular at higher shear rates, but jagged on the surface with irregular shape at lower shear rates. In contrast to D₂, aggregates formed in MT were much more irregular (jagged) than those formed in TC.