(77c) Electrostatic Charging and Fluidization Behaviors of Dielectric Particles in the Presence of Noble Gases

In dynamic gas-solid particulate systems such as a fluidized bed reactor, particle-particle and particle-wall interactions can generate and accumulate significant amount of static charges and the particle surface charge can significantly influence the hydrodynamics of such particulate systems. There has been a number of interesting studies focusing on the measurement of static charges in fluidized beds and the means of reducing charge accumulation in those reactors [1,2]. One of the well-studied method of reducing static charges in multiphase reactors is the increase of humidity. In our recent study, we have found that the electrostatic charge accumulation on particles fluidized by noble gases is significantly smaller than that in air, even under dry conditions. Considering that dielectric properties of noble gases are similar to commonly used gases (e.g., air, N₂and CO₂), it was a surprising result. Thus, this study is designed to provide a first insight into electrostatic charge generation and dissipation phenomenon in the presence of noble gases (Ar and He) and its effect on fluidization behavior. Specifically, experiments on fluidization of glass beads (300 μm) by dry air, He and Ar are conducted in a fluidized bed equipped with an electrostatic ball probe. It is found that the particle charge dissipation rate in noble gases is significantly faster than in air, resulting in improved fluidization behaviors. Also, particle sheeting on the reactor wall completely disappears in the case of fluidization with noble gases. This interesting static reduction phenomenon by noble gases is attributed to a low breakdown potential of a noble gas. The breakdown of charges in gas-solid systems are closely linked the collision behaviors of objects (particle-particle, particle-wall and particle-probe), and a conceptual model is derived based on Matsuyama’s approach [3,4] to illustrate how particles lose charges as the potential between surfaces increases to the breakdown limit of the gas phase.

[1] Guardiola J., Rojo V., Ramos G., Influence of particle size, fluidization velocity and relative humidity on fluidized bed electrostatics, Journal of Electrostatics, 37 (1996) 1-20.

[2] Yao L., Bi H. T., Park A-H., Characterization of electrostatic charges in freely bubbling fluidized beds with dielectric particles, Journal of Electrostatics, 56 (2002) 183-197.

[3] Matsuyama T., Yamamoto H., Charge relaxation process dominates contact charging of a particle in atmospheric conditions. Journal of Physics D: Applied Physics, 28 (1995) 2418-2423.

[4] Matsuyama, T. A Discussion on Maximum Charge Held by a Single Particle due to Gas Discharge Limitation. AIP Conference Proceedings 1927, (2018).