Flow of Fine and Cohesive Powders under Controlled Air Pressure Conditions

Powder flow appears in many industrial processes, including pharmaceuticals, food, detergents etc. Understanding the parameters affecting powder flow is necessary for rational product and process design. This research examines powder-air interactions for three powder handling systems: bin discharge, linear shoe-die system and rotary paddle feeder. As a starting point the flow behaviour of powders was characterised using established procedures to provide basic set of properties for assessing flowability. Powder permeability and the influence of small levels of compaction on permeability were determined. The influence of differential pressure on flow initiation from arching state was examined for powders discharging from bins. A dimensional model was developed to predict the differential pressure required to initiate powder flow as a function of orifice diameter and height of the powder above the exit. The mass flow rate of the powders were measured under a range of differential pressures. A dimensional model was developed to predict the mass flow rate as a function of differential pressure and exit diameter. The effect of processing parameters on the mass of the powder delivered into the die in linear shoe-die systems under the gravity and suction fill mechanism were investigated. The system was accommodated with differential pressure transducers and the evolution of the pressure inside during the process was monitored. Dimensional models were developed to predict the mass delivered into the die as a function of powder differential pressure developed during gravity and suction fill, shoe velocity and the velocity of the punch in the die. The influence of paddle rotational speed, exit diameter and differential pressure on the mass flow rate of powders in rotary feeding system was examined. This research identified the dimensionless groups relevant for the understanding of the flow of fine and cohesive powders under differential air pressure conditions. The dimensional models developed account for air pressure effects for 1) flow initiation and 2) flow rate during bin discharge, 3) gravity and 4) suction fill mechanisms in linear shoe-die filling systems. These models can be used to aid the design of powder flow processes where air pressure effects are influential.