Electrostatic Charging of Powders during Pneumatic Conveying

Electrostatic charge generation which is mainly observed during particle handling, transport (e.g., pneumatic conveying) and processing (e.g., gas-solid fluidized beds), is an unavoidable and, in some cases, an undesired phenomenon that can result in operational challenges. Particle-wall adhesion, electrostatic discharge, particle agglomeration and segregation are problems associated with electrostatic charging that have been reported in gas-solid processes such as gas-solid fluidized beds [1]. As an example, electrostatic charging has been clearly observed in catalytic gas-solid fluidized bed reactors in polyethylene production plants. Adhesion of the charged polyethylene and catalyst particles to the reactor wall, leads to a problem known as “sheeting” [2]. This is a major drawback since particle sheets can break off and block the reactor, causing production and economical losses. A potential source for the sheet formation is continuous collisions between the polyethylene and catalyst particles as well as collisions with the reactor wall. The electrostatic charge generated on the surface of these particles as a result of the collisions attracts a portion of them towards the reactor wall due to formed image and electrostatic forces and in turn forming a sheet. It is also anticipated that the catalyst particles, which are pneumatically conveyed into these reactors through narrow tubes, to be highly charged upon entering the reactor and therefore contribute significantly to the bed electrification and subsequently the sheeting formation. Thus, the aim of this work was to investigate electrostatic charging behaviour of various powders through their pneumatic conveying. The effects of conveying tube length and its pathway (addition of elbow and bend), conveying gas velocity, and solids loading ratio are also investigated. Two charge measurement techniques were employed in this work. A Faraday cage containing a filter bag was used to directly measure the powder electrostatic charge as they exited the conveying line. Current was also measured from the conveying line as this technique can be employed as an alternative charge measurement method where implementing the Faraday cage technique is not feasible in a commercial process.

References

[1] P. Mehrani, M. Murtomaa, D.J. Lacks, An overview of advances in understanding electrostatic charge buildup in gas-solid fluidized beds, J. Electrostat. 87 (2017) 64–78.

[2] B.D. Fulks, S.P. Sawin, C.D. Aikman, J.M. Jenkins, Process for reducing sheeting during polymerization of alpha-olefins, US 4532311, 1985.