Nozzle zone agglomeration in spray dryers: Determination of the agglomeration efficiency in the fines return by means of agglomerate properties and residence time distribution

In multi-stage spray dryers, agglomeration is forced by recirculating fine material into the nozzle zone. Drying droplets collide with dry particles and form agglomerates. The aim of this study was to achieve a more comprehensive understanding and control of agglomerate formation. Agglomerates with different properties were produced using a pilot spray drying plant. An increased air mass flow of the fluidized bed as well as a reduced solids content of the feed solution resulted in an increased number of primary particles per agglomerate. A residence time-based approach was then applied to determine the agglomeration efficiency, defined as the number of successfully adhered primary particles per recirculation. It ranged from 0.33 to 0.71 and was correlated to collision and sticking probabilities in the nozzle zone. This value is of importance both for process optimization and for the validation of simulation approaches.