(136d) Parametric Study for the Development of a Particle-Particle Collisional Charging Model for Use in the CFD Simulation of Electrostatic Effects in Gas-Solid Fluidized Beds

An operational challenge faced in some gas-solid fluidized beds is the generation of electrostatic charge, which is an inherent and unavoidable issue in this field. Triboelectric charging in gas-solid fluidized beds occur due to the continuous particle-wall and particle-particle contacts during the fluidization process. For instance, in polyethylene reactors, polyethylene and catalyst particles can become highly charged, which results in them adhering to the reactor wall and forming large fused polyethylene sheets. These sheets can break off the reactor walls and obstruct the distributor plate, forcing a reactor shut-down even within a few hours of operation for clean-up. The downtime due to the clean-up, which can last up to 30 days, can result in significant economic loss due to reduced production.

A good understanding of triboelectric charging and its effects in gas-solid fluidized beds allows the development of techniques to reduce or control the charging in the bed. It also gives rise to the development of electrostatic charging models for use in computer simulations. Simulations are a very cost-effective alternative to running experiments, especially for industrial-scale test runs. Thus, it would be beneficial to build an effective Computational Fluid Dynamic (CFD) simulation model to study the electrostatic effects of charge generation in industrial-scale gas-solid fluidized beds. Since triboelectric charging in gas-solid fluidized beds occurs due to both particle-wall and particle-particle charging, the charging model must carefully consider the mechanisms for both types of charging within the fluidized bed. The overall focus of this work is specifically on particle-particle charging, which has received very limited attention.

One important charging phenomenon observed in gas-solid fluidized beds is bipolar charging. Bipolar charging occurs when particles of the same material, but different sizes, become oppositely charged during fluidization. It has been observed that the wall fouling thickness in a fluidized bed depends on both the magnitude of charge in the bed and the presence of both positively and negatively charged particles [1]. Once a layer of charged particles adheres to the wall, the oppositely charged particles are attracted to it to form a second layer. Multiple layers can then form depending on the magnitude of the electrostatic force in the fluidized bed [2]. The mechanism to describe how bipolar charging occurs is still unclear, since the studies on poly-sized particle systems have shown conflicting results; some noted that the smaller particles charged negatively and the larger particles charged positively [3], while others have reported the opposite [4]. By identifying the mechanism for bipolar charging, and the impact of various parameters on this phenomenon, it would be possible to develop a charge transfer model that reflects bipolar charging due to particle-particle collisions. This model can be used to simulate the change in charge of particles in a poly-dispersed gas-solid fluidized bed system. The successful development of this model would bring this research group one step closer to the final objective of this project: to develop a CFD model to simulate triboelectric charging and its effects on an industrial-scale gas-solid fluidized bed.

Thus, the specific objective of this work is to propose a mechanism for bipolar charging by performing systematic particle-particle collision experiments. A novel apparatus enabling particle-particle collisions was designed and fabricated in this work. The apparatus consisted of two Faraday cups situated above a wind-box. Single particles were dropped through the two Faraday Cups to measure their individual initial charges. The particles then went through the wind-box, which was designed to use dry air to push the two particles towards each other and cause a collision. After the collision, the two particles dropped into another set of separate Faraday Cups, located below the wind box, to measure their final charges. The collision and charge measurements were conducted in this way to minimize any external influence on the two particles; the charge transferred was solely due to the collision between the two particles. Since every test run may not experience a successful collision in the wind-box, the collisions were observed using a video recording device linked to a particle-tracking program. Parametric studies of this provide some conclusion on whether larger particles charge negatively or positively when colliding with smaller particles of the same material. Some parameters studied, which may affect the direction and magnitude of charge transfer, included impact velocity, particle size and/or size difference, particle material, and initial charge.

[1] A. Sowinski, A. Mayne, P. Mehrani, Effect of fluidizing particle size on electrostatic charge generation and reactor wall fouling in gas–solid fluidized beds, Chem. Eng. Sci. 71 (2012) 552–563.

[2] D. Song, P. Mehrani, Mechanism of particle build-up on gas-solid fluidization column wall due to electrostatic charge generation, Powder Technol. 316 (2017) 166–170.

[3] K.M. Forward, D.J. Lacks, R. Mohan Sankaran, Methodology for studying particle–particle triboelectrification in granular materials, J. Electrostat. 67 (2009) 178–183.

[4] A. Sowinski, L. Miller, P. Mehrani, Investigation of electrostatic charge distribution in gas–solid fluidized beds, Chem. Eng. Sci. 65 (2010) 2771–2781.