Separation of fine particles from liquid is commonly achieved by a coagulation step, where the particles, after destabilization through addition of coagulant, are aggregated to from larger clusters. In a stirred vessel, besides Brownian aggregation, which is the controlling mechanism for sub-micron particles or clusters, shear-induced aggregation is an additional mechanism promoting cluster growth. As the forces acting on the clusters increase with their size, when the clusters become large enough, eventually, breakage sets in and ceases further growth. It is well known that the rates of both processes, aggregation and breakage, are nontrivial functions of the shear rate [1]. Therefore, process dynamics as well as steady-state characteristics will denpend strongly on the shear rate distribution inside a coagulation unit. In order to investigate the effect of flow field heterogeneity, we have investigated the aggregation of a model system consisting of surfactant-free polystyrene particles with diameter equal to 610nm in a stirred tank and three different types of Taylor-Couette device, which exhibit distinctly different shear rate distributions as calculated by computational fluid dynamics [2]. The time evolution of the cluster mass distribution, characterized by the root-mean-square radius of gyration, <R_g>, and the zero-angle intensity of scattered light, I(0), was measured by in-loop small-angle static light scattering. When these measured integral quantities, <R_g> and I(0), were plotted as a function of the volume average shear rate, they were significantly smaller for the stirred tank than for the Taylor-Couette-type devices, proving the direct impact of the shear rate distribution, which differs substantially for the given devices [2], on aggregation and breakage processes. Additionally, it was found that the aggregate structure as analyzed by image analysis is independent of the unit type. Present results confirm our previous consideration that the steady-state aggregate size cannot be simulated just by the volume average shear rate even for dilute conditions and complete information about shear rate distribution has to be included in the analysis.

References:

[1] D.L. Marchisio, M. Soos, J. Sefcik, M. Morbidelli, Role of turbulent shear distribution in aggregation and breakage processes. AIChE J. 2006, 52, 158-173.

[2] M. Soos, H. Wu, M. Morbidelli, Taylor-Couette unit with a lobed inner cylinder cross section. AIChE J. 2007, 53, 1109-1120.