Abstract
Gravity transport of granular materials in vertical pipes is one of the most fundamental steps in bulk powder handling and processing. Presented study investigates powder flow characteristics in vertical pipes with open and closed outlets and condition of free powder fall. Powder flow of pharmaceutical grade powders was observed in transparent, vertical pipe model. Description of flow structures was performed. Powder volume flow rate, acceleration, and dilatation were quantified and correlated with powder properties. The results show that in pipes with a closed outlet the escaping air slows down the powder flow, resulting in a much slower flow than in pipes with an open outlet. A dense granular flow was detected in an open outlet condition, whereas in a closed outlet condition two concurrent flow regimes were observed: a slow moving, dense powder bed, and a fast dilute powder flow. Differences in flow regimes may promote segregation, with important implications to industrial processes.
Highlights
The gravity-driven flow of granular matter in vertical and inclined pipes is widely used for granular material transport between containers, production phases or it is integrated in a complex industrial processes such as circulating fluid bed reactors.[1]
Detailed measurements for powders with a broad particle size distribution was performed in order to evaluate the effects of smaller particle size on the pressure gradient at the pipe outlet, and on mass flow rates in steady state of powder discharge
The motivation behind our study is to investigate the interaction between the granular flow and the air back-flow in a vertical pipe with an airtightly closed outlet (CO) and to investigate the same powder materials for granular flow characteristics in condition of open pipe outlet (OO), which is most often encountered condition in literature
Summary
The gravity-driven flow of granular matter in vertical and inclined pipes is widely used for granular material transport between containers, production phases or it is integrated in a complex industrial processes such as circulating fluid bed reactors.[1]. Observations[5,6,7,8] of granular flow released into vertical pipes with an air-tightly closed outlet (e.g. in pharmaceutical manufacturing) reveal air counter flow and an abrupt exit of air that causes extensive disturbances in the powder bed in the container and is a source of unwanted phenomena such as segregation Another area of research of granular material flow involves narrow vertical tubes (a tube-to-particle diameter ratio of ∼10), where intermittent flow of particles is studied.[9,10,11] Horikawa[11] reported a formation of density waves due to the back-flow of air rising through falling powder. A link between powder flow rates and bulk powder properties has been assessed
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