Granular size and shape effect on electrostatics in pneumatic conveying systems

Abstract

In solid processing systems, electrostatic problems are commonly observed for granules composed of various sizes and shapes. However, complete understanding about the functional dependence of electrostatic charge generation and transfer on the particle shape and size distributions has yet to be established. This observation has motivated the present study where novel methods are proposed to examine the effect of particle size and shape distributions on electrostatics. In this work, polyvinyl chloride (PVC) granule (original diameter 3.35-4.1 mm, in the shape of cylinders) was first discharged to remove any residual charges and subsequently its electrostatic charging was studied. Granular size and shape were varied by mechanical attrition conducted in a rotary valve jointly with a pneumatic conveying system. Characterised by induced current, particle charge density and equivalent current of the charged granular flow, granular electrostatics was found to increase with the extent of granular attrition in a continuous recycled pneumatic conveying process. In a separate setup, single particles (collected from the attrited granules formed in the rotary valve) were examined by correlating the extent of charge variation with size/shape. It was found that a dimensionless group, defined by the ratio of charge variation to size variation, is useful in describing the particle attrition process as this parameter increased with decreasing granule size. Smaller granules were found to be the main contributors in the enhancement of electrostatics charge density in bulk particles. By a separate shape analysis, it was uncovered that face shape requiring more shearing actions for its formation tended to give rise to a higher charge variation and so did column geometry. In this fashion, charge variation evaluated for whole attrited granules exhibited good agreement with the temporal variation of attrition weight; this applied for all air flow rates used in the conveying system. Furthermore, there is a reasonably good matching between results obtained by shape and size analyses. By the correlations presented above for single particle electrostatics either by size or shape analysis, charge variation of granular flow matched very well with that measured in the conveying system as well as the attrition process in the rotary valve. As such, the joint granule size and shape analysis has proven to be useful for characterisation of electrostatics in conveying systems where granules are made up of complex combinations of different particle sizes and shapes. © 2006 Elsevier Ltd. All rights reserved.