Experimental investigation and population balance modeling of aggregation and breakage of polymer colloids in turbulent flow

Abstract

The focus of this thesis is to provide deeper understanding of flow-induced aggregation of dilute colloidal suspensions and other processes that accompany it, i.e., breakage of aggregates. For this purpose we designed an experimental setup where aggregation in a stirred tank (the coagulator) is monitored on-line using small angle static light scattering (SASLS). From SASLS measurements we obtain two independent moments of the cluster mass distribution (CMD) as well as information about the aggregate structure. Furthermore, our experimental setup allows for the introduction of controlled stimuli in the rotation speed and solid volume fraction to the coagulator. In all studies, surfactant-free polystyrene latex particles were used as a model system because polystyrene has a specific gravity of 1.05, which allows us to isolate flowinduced aggregation from aggregation due to differential settling. Moreover, solid recovery from polymer latexes is one of the most important industrial processes where flow-induced aggregation is the centrepiece. This makes understanding of flow-induced aggregation of polymer colloids very valuable and industrially relevant. Throughout this thesis, we considered only aggregation at salt concentration above the critical coagulation concentration (CCC). In the first part we focused on the effect of the solid volume fraction on flow induced aggregation and breakage. This allowed us to elucidate the mechanism behind the attainment of a steady state CMD. The experiments showed that the steady state CMD in the system studied is controlled by dynamic equilibrium between aggregation (with second order kinetics in cluster concentration) and breakage (with first order kinetics in cluster concentration). The reversibility of this equilibrium was verified and later exploited to collect data about the dependency of the steady state CMD over a range of solid volume fraction in one experiment. This was done by diluting the coagulator after steady state CMD is attained. The rate of dilution was designed using time scale analysis to be slow enough such that equilibrium is maintained throughout the dilution. It was observed that the aggregate size reached an asymptotic value during this dilution, below which breakage is negligible. The existence of such a critical size is an important aspect of the breakage phenomenon that must be taken into account in any models of breakage rate. In the second part the effect of the rotation speed, i.e., the volume average shear rate, G, on size and structure of the aggregates was investigated, as well as the response of the CMD to different stimuli in the shear rate. For this purpose, batch and dilution experiments were conducted at various values of the rotation speed. Detailed investigation of the initial kinetics