An experimental analysis of coffee beans dynamics in a rotary drum

Abstract

Brazil is the largest producer and exporter of coffee in the world, one of the main global commodities. During the processing of coffee beans, rotary drums are used in the drying and roasting phases. This equipment is widely used in many industrial sectors. According to characteristics of the drum, particulate material, and operating conditions, different flow regimes may arise. Such regimes have a direct influence on the performance of these processes and therefore their characterization is of great importance. The present work developed an experimental study about the dynamics of coffee beans, under green and roasted conditions, in a rotary drum. Transitions between rolling, cascading, cataracting, and centrifuging regimes were evaluated, for both green and roasted coffee beans. For the centrifuging regime, a comparison between experimental critical rotation speeds and the ones obtained by correlations found in literature was performed. Additionally, a new expression to predict the centrifuging rotation speed was proposed by the authors. Further analyses regarding the rolling regime were also carried out. For this regime, velocity profiles of the particles were evaluated, as well as the thickness of the active layer for different filling degrees and drum rotation speeds. Great similarity was found between the behaviour of the active region for coffee beans before and after the roasting process.