Mechanism of the Initial Phenomena of Defluidization Caused by Switching Fluidizing Gases

Abstract

In a uidized bed, particle agglomeration and channeling have been observed when the uidizing gas is switched from lower to higher density. This de uidization is a transient phenomenon, and uidization is restored after several minutes. In this paper, we assume this behavior can be explained by non-equimolar di usion, which causes pressure gradients in the bed and subsequently leads to viscous ow. To verify this assumption, pressure changes were measured for several binary di usion systems in a packed bed. After the packed bed was lled with a gas, another gas was supplied to the bed's outer surface to replace the rst gas. These gases were exchanged by di usion, and the pressure at the closed end of the column either increased or decreased due to di erences in the gases' molecular weights. The pressure changes were compared with pressure changes calculated from a model based on both an unsteady-state di usion equation and the Kozeny-Carman equation. The measured pressure changes in the packed beds could be correlated with the calculated values without any adjustable parameters. In a uidized bed, after the gas switched from lower to higher density, di erences in di usion rates due to the gases' various molecular weights caused a net molecular ow from the emulsion phase to the bubble phase. This ow caused a decrease in gas velocity in the emulsion phase and a contraction of the emulsion phase; channeling and de uidization were then observed.

A Model for Non-Equimolar Binary Gas Diffusion through Pores in the Region of Molecular Diffusion

“…Comparison of maximum pressure change during isovolumetric diffusion: Observations byKai et al (2014) and calcu-…”

JCEJ Outstanding Paper Award of 2014

“…"Mechanism of the Initial Phenomena of De uidization Caused by Switching Fluidizing Gases" by Takami Kai, Yuki Hirano, Tsutomu Nakazato, and Kei Mizuta, Kagoshima University (Kai et al, 2014) Citation: In this paper, the authors have proposed a model using the unsteady-state diffusion equation and KozenyCarman equation to study transient defluidization that occurs after the fluidizing gas is switched to another gas, and showed that experimental pressure changes for a binary diffusion system in a packed bed could be well predicted by the model without the inclusion of any adjustable parameters. This result confirms that the defluidization is caused by non-equimolar gas exchange between the bubble and the emulsion phases.…”