Conductive thin film drying kinetics relevant to drum drying

Abstract

Direct assessment of the kinetics of drum drying operation has been a difficult task as the mass and temperature profiles are hard to monitor. Still, developing better understanding of conductive drying would help to identify new operating windows for this technology. The drying kinetics was investigated by drying maltodextrin and starch suspensions with a novel custom-built laboratory-scale apparatus, which allows on-line monitoring of mass and temperatures. During drying, three separate periods were identified: the heating, the boiling and the conductive drying (declining rate) periods. The duration of the initial heating period was proportional to the film thickness and was responsible for a relatively small amount of water evaporated due to natural convection. During the boiling period, the drying rate kept constant while bubble formation impeded the heat transfer. Larger bubbles were observed for starch suspensions due to its viscoelastic properties. Thus, large temperature gradients between the heating pan and the film were observed for starch suspensions. During the conductive drying period, the initial amount of dry solids per surface area determines the drying rate as it determines the thickness of the semi-moist layer subjected to conductive drying. Application of a thin film is preferred to avoid boiling, especially at increasing solids content. This situation also better approaches double drum drying processes, where boiling occurs in the pool and conductive drying occurs on the drum.