SOLIDS TRANSPORT IN ROTARY SUGAR DRYERS

ROTARY drum sugar drying is the last important unit operation used in the manufacture of raw sugar. In order to improve their design, operation and control, it is helpful to model their dynamic behaviour. One important aspect involves predicting the rate at which solids are conveyed through the dryer, so that hold-up of crystalline material can be better predicted. It is also important to accurately predict the amount of sugar presented to the oncoming air stream in the falling curtain, so that meaningful heat and mass transfer relations can be developed. This paper presents a dynamic model of solids transport through a rotary sugar dryer. The model is developed by assuming a tanks-in-series/parallel arrangement, akin to chemical reaction engineering systems. The use of two tanks connected in parallel allows the separation of sugar undergoing drying from the sugar particles at rest in the flights. This approach allows the prediction of air phase and rolling sugar hold-up, and residence time distribution within the dryer. Correlations to account for overloaded dryers and the impact of airflow are included. The model is based on a tracer study performed on a 100 t/h industrial dryer.