Investigation of humidity effects on electrostatic behavior of a small cold model fluidized bed

Introduction: Fluidized beds are being used in wide variety of industrial applications ranging from oil cracking to pharmaceutical powder manufacturing. The ambient humidity of the fluidizing gas can have a complicated role in the fluidization and can cause varying inter-particle phenomena, such as liquid bridging (LaMarche et al., 2016) in highly humid systems or lead to triboelectric charging in lower humidities (Park, 2002). Triboelectric charging of granular matter taking place at low ambient humidity (less than 60%) can cause particles to adherefluidizing column walls (Hendrickson, 2006), lead to particle clustering (Lee et al., 2015), and affect particleentrainment (Fotovat et al., 2017). We have studied a small cold model fluidized bed both experimentally and numerically to shed light on the system changes in humidities ranging from 0% to 60% relative humidity.
Methodology: In this work we considered a small rectangular fluidized bed with soda lime glass walls, nitrogen as the fluidizing gas, and spherical nearly monodisperse polyethylene particles. The extent of particle charging was adjusted by changing the humidity of the fluidizing gas. The experimental model was fitted with a pressure transducer that allowed us to measure the pressure drop across the bed. In addition, we employed particle image velocimetry (PIV) to investigate the particle velocities seen at the column wall to determine the extent which particles had adhered to the column walls. Particle charges were measured using a faraday cup method (Fotovat et al., 2017) that allowed us to determine the mean particle charge in the system. The measured particle charge was used as an input for a numerical model (Kolehmainen et al., 2017) to calibrate an effective work function value (Laurentie et al., 2013) that was used to adjust the extent of charging seen in the numerical model. The wall layer velocities and pressure drop obtained from the simulations were compared with the experimental results to validate the numerical model.

Results: The charging behavior of the system studied here showed non-monotonic relation between the relative humidity and the mean charge. Around 30% relative humidity the mean particle charge diminished; above this level it was negative; and below the 30% relative humidity the measured particle charge was positive. The measured mean charge correlated with the wall adhesion, and downward particle velocities were observed at 30% relative humidity that is consistent with our charge measurement. The numerical model employed was able to capture the pressure drop results for small particle charge levels (less than 10nC/g), but failed to reproduce the results for higher charge levels. This inconsistency was accounted for the numerical model only forming a monolayer of particles at the wall, while the observed wall layer thickness in the experimental model was multiple particles thick.