(592c) Hydrodynamics of Pulsed Gas-Solid Fluidized Beds with and without Internals

Hadi, B., Delft University of Technology
van Ommen, J. R., Delft University of Technology
Coppens, M. O., Rensselaer Polytechnic Institute

Fluidized beds are known for their excellent mixing and heat transfer. However, potential de-fluidization of cohesive particles, gas bypassing, and gas channeling lead to inefficient contact between the gas and the solid particles. Over the past years, we have been working on several methods to structure fluidized beds [1,2]. Oscillating the fluidizing gas is a method of structuring fluidized beds that strongly improves the particle mixing. This is crucial to overcome mass and heat transfer limitations, e.g., for fluidized bed driers [3].

A square fluidized bed was used in this study. The pulsation frequency was varied from 0.5 Hz up to 5.0 Hz. The effect of bed height was also studied for H/D ratio from 0.5 up to 2.0. Finally, the effect of the presence of internals has been examined. The hydrodynamics are investigated by measuring pressure fluctuations, from which average cycle frequencies and incoherent standard deviations are calculated to quantify changes in bubble size. In addition, optical probe measurements were used to determine the bubble fraction.

When a fraction of the total flow is periodically oscillated within an appropriate range of frequencies, a possible defluidization problem is prevented and the pulsed flow behavior is superimposed. A clear structuring pattern was observed when increasing pulsation frequency up to 3.0 Hz. Higher pulsation frequencies, beyond this value, have insignificant effect when compared to a conventional, constant flow. Pulsed flow significantly increases bubble size, increases bubble passage frequency and decreases bubble fraction. Moreover, the presence of internals slightly decreases the bubble size, slightly decreases the bubble passage frequency, significantly decreases the bubble fraction, and influences the bubble pathway especially at low gas velocity. Finally, the bubble size increases with an increase of the bed height, where it is more significant in pulsed bed. Also, increasing the bed height has a less clear trend on the bubble passage frequency.

[1] van Ommen, J.R., Nijenhuis, J., van den Bleek, C.M., and Coppens, M.-O., Ind. Eng. Chem. Res., 46, 4236-4244, 2007.

[2] van Ommen, J.R., Nijenhuis, J., and Coppens, M.-O., Chem. Eng. Prog., 105, 49-57, 2009.

[3] Akhavan, A., van Ommen, J.R., Nijenhuis, J., Wang, X.S., Coppens, M.-O., and Rhodes, M.J., Ind. Eng. Chem. Res., 48, 302-309, 2009.