Rathole stability analysis for aerated powder materials

Abstract

Forty years ago, Jenike [Gravity Flow of Bulk Solids, Bulletin 108, Utah Engineering Station, 1961] and Johanson [Flow Indices in the Prediction of Powder Behavior, Pharmaceutical Manufacturing International, pp. 159–164, 1995] developed the flow-no-flow equations used to predict stability of bulk solid structures in silos and hoppers. These equations were developed into a theory that has been used to design process equipment to handle cohesive materials. The basis of the theory is a limiting stress state analysis of a bulk material forming a cylindrical pipe (rathole) around or an arch across the hopper outlet. Reliable process operation requires that these two cohesive obstructions be avoided to achieve proper flow of bulk materials through process equipment. Today, industry uses a variety of flow aid devices to overcome these stable flow structures. One such device is aeration pads which are used to maintain fluidization of fine powders and decrease cohesive behavior of bulk materials. Alternatively, air blasters can inject a given quantity of gas into the bulk material creating large transient gas pressure gradients that may destroy these cohesive structures in process equipment. It is important to note that air blasters may destroy cohesive structures provided they are placed in close enough proximity to the stable rathole and with sufficient frequency along the axis of the bin or around the bin perimeter. Although these aeration techniques work, a full understanding of the reason is lacking. Currently, both the placement and required number of these flow aid devices are based on practical experience and not sound theoretical principles. This paper addresses this knowledge void by adding the gas pressure gradient terms to the rathole stability analysis performed by Jenike, thus extending the flow-no-flow rathole analysis to aerated conditions.