Efficiency of different shear devices on flocculation

Various types of coagulation devices have been used to study aggregation, with the goal of understanding aggregation dynamics in natural and engineered systems. Three different devices were investigated here (paddle mixer, oscillating grid and Couette) to examine the effect of different laboratory devices on the particle size spectra over a wide range of shear range values (G = 4-102 s(-1)) at four different initial volume concentrations (phi(0), = 2 x 10(-5), 4 x 10(-5), 8 x 10(-5) and 10 x 10(-5)). For each device there was maximum aggregate size produced at each shear rate, with three distinct zones observed depending on the magnitude of G. At low shear rates (G < 20 s(-1)), mean particle diameter increased with G, showing that aggregation dominated over breakup. At intermediate shear rates (20s(-1)<30s(-1)), flocculation rates were maximized, producing the largest flocs. At higher shear rates (G > 30 s(-1)), the dominant effect of breakup was shown through reduced maximum floc sizes with increasing shear rates. Fractal dimensions of aggregates from the paddle mixer and oscillating grid, calculated using cumulative size spectra and assuming pseudo-steady-state conditions, were typical of aggregates formed by reaction-limited conditions or those compacted by aggregate reformation (D = 2.2 +/- 0.2). Fractal dimensions for the Couette flow device were extremely low and less than unity (D = 0.9 +/- 0.2), indicating that this method of calculating D could not be used for conditions produced in this device. The paddle mixer and the oscillating grid produced almost identical maximum diameters of the suspension under pseudo-steady-state conditions for all initial volume fractions, but the Couette device consistently produced larger diameter flocs at the same average shear rates and the maximum size of the aggregates increased with the volume concentration. These results indicate that even at low shear rates aggregate formation was influenced by breakup. It is therefore concluded that the Couette device is the most useful method for forming aggregates that are least influenced by breakup processes during aggregate formation. (C) 2007 Elsevier Ltd. All rights reserved.