Experimental and CFD studies of floc growth dependence on baffle width in square stirred-tank reactors for flocculation

This work mainly focuses on the effect of baffle width (B) on floc growth within square stirred-tank reactors for flocculation. Firstly, a series of flocculation tests were carried out, using polyaluminum chloride (PAC1) as coagulant, to evaluate flocculation performance in each tank at three typical shear rates of G(ave) = 10, 30 and 70 s(-1). Experimental results were reported in terms of kaolin-floc average size, size distributions and perimeter based fractal dimension, derived from an in-situ recognition system for floc morphology. Then, the hydrodynamic environments (where the growth of floc took place) were characterized, by performing Computational Fluid Dynamics (CFD) simulations, with predicted results of area-weighted average turbulent kinetic energy and its dissipation rate in the plane containing the impeller, and circulation time (denoted as k(0.331) epsilon(0.3311) and t(c) respectively), followed by a detailed discussion based on experimental and numerical results. As expected, baffles with different widths caused distinct turbulent flow fields in corresponding stirred tanks at the same shear rates, thereby affecting the evolution of floc size and structure during flocculation. According to CFD predictions, for a constant G(aye), the baffle width of B = 0.10D (where D is the bottom width of tank) gave rise to the highest values of k(0.33H) and epsilon(0.331H), as well as the shortest t(c), followed by B = 0.13D, 0 (unbaffled) and 0.07D, and for B = 0.20D, the lowest k(0.33H) and epsilon(0.33H), together with the longest t(c), were produced. It was found that the floc growth dependence on baffle width appeared to be greatly related to the predominant growth mechanism(s), i.e., at the shear rates of Gave = 10 and 30 s(-1), where the rate of breakage was approximately zero (because little or unimportant breakage was observed), a higher epsilon(0.33H) and a short re corresponded to an increased rate of floc growth resulting from higher particle collision frequency, thus forming larger flocs under the same shear-rate conditions, whereas at Gave = 70 s(-1), where breakage dominated over aggregation, larger flocs seemed to be produced by a lower epsilon(0.3311) and a longer t(c), likely as a result of a lower breakage rate and more sufficient opportunity for broken aggregates to reform or restructure during water flow circulation. Moreover, the effect on floc size and structure during flocculation was somewhat compressed when breakage became pronounced, and the same was true of the restructuring behavior. To enhance floc growth process, some baffles should be installed to break water flow co-rotation with the impeller (to increase axial flow rate), and however, larger-area dead zones might be simultaneously formed. Therefore, an appropriate width (e.g., B = 0.10D or 0.13D) would be required to maintain a compromise. The present study may provide useful insights for optimizing the design and operation of baffled stirred-tank reactors for flocculation.