Model development for conductive thin film drying processes

Abstract

A heat-transfer governed model is proposed to describe drying in a lab-scale conductive thin film dryer, which was designed to investigate the drying kinetics relevant to drum drying. The model calculations were compared to experimental data from drying experiments with maltodextrin DE12 and potato starch, considering the three distinct periods (heating, boiling and conductive drying) of the lab-scale process. The model uses measured temperatures and evaporation rate during the boiling period as input to calculate the decrease in moisture content during the drying process. Model calculations were evaluated by determining the root-mean-square-error (RMSE) values. The RMSE were very small (<0.24) indicating that the model was successful in describing the film drying process. During the last drying period, the starch films exhibited a higher initial heat transfer resistance (~0.0004 (m2∙K)/W) compared to maltodextrin (~0.0002 (m2∙K)/W). This reflects the formation of larger vapour bubbles in the boiling period impeding the heat transfer for starch films. Subsequently, the model was modified to describe a pilot-scale drum drying process for maltodextrin suspensions. The initial heat transfer coefficient for drum drying of maltodextrin was obtained from the lab-scale experiments. The simulations indicated residual moisture contents and optimal drying times for different drying conditions.