Spray drying of detergents in counter current towers : a study of turbulent swirling flows, fouling and agglomeration

This thesis is concerned with the manufacture of granular laundry detergents in counter-current spray dryers making use of turbulent swirling flows. It contributes to a comprehensive description of this process by providing a) an experimental study of the swirling fluid dynamics in various scales and designs, introducing the use of sonic anemometry to this context, b) the description of a complex fouling dynamics at the walls, quantified by rates of deposition and re-entrainment of material, and c) the analysis of the sources of particle-droplet agglomeration in different sections of the dryer, under the operation of one or multiple sprays. This thesis demonstrates that the interaction with the walls is a central part of the process. In the one hand, friction affects the structure of the flow: it reduces its angular momentum and destabilizes the swirl, which causes recirculation in the dryer and the production of turbulence. In the other, particles deposit at the walls forming a multi-layer that continuously builds and breaks up. A tracer experiment has revealed that this equilibrium in part governs the product residence time, and ultimately how particles dry and aggregate. The data provided here constitute the first evidence of such a behavior in spray dryers.