Nozzle zone agglomeration in spray dryers: Process dependency of the fines mass flow and its importance for agglomerate formation

Agglomeration in multi-stage spray dryers for food powder production is typically implemented by recycling fine material into the spray cone. The improved flow and reconstitution properties are offset by a process without direct control options. This study shows that the proportion of recycled fines allows for a more direct characterization of agglomerate formation during the process. With a measurement system based on isokinetic sampling, it was demonstrated that a total solids content of the feed solution of 30 to 50 %(w/w) resulted in a relative fines mass flow from 5.8 to 4.3 at constant process parameters. Increasing the main drying air mass flow had no effect on the relative mass flow of fines. As the fluidized bed air mass flow rate was increased, more particles were recycled, the relative mass flow rate of fines and the extent of agglomeration increased. Correlation to agglomerate size, porosity and shape confirmed that fines mass flow is a fundamental parameter for controlling and scaling agglomeration in the nozzle zone.