Flight unloading in rotary sugar dryers

Abstract

The goal of all rotary sugar dryers is to present wet, crystalline sugar to the oncoming air stream, in order to maximise heat and mass transfer between these phases. Flight design plays a critical role in this process. This paper presents a simple analysis of flight discharge, based on geometric considerations and measured material flow properties. Emphasis is placed on the importance of the active or airborne sugar mass, resulting in the ability to investigate the effects of material moisture content and flight geometry on the magnitude of airborne sugar. Material dynamic repose angles for wetted and dry sugar are measured using a rotating drum apparatus. The rotary transport model is applied to a case study dryer configuration in order to determine the percentage of airborne sugar during steady state operation. The effect of sugar moisture content on flight capacity is also investigated, providing insight into geometry requirements for uniform sugar hold up along the length of the case study dryer.