Fluidization effect of walnut shell particles in a fluidized bed with slotted gas distributor

Abstract: Gas distributor is a key component in fluidized beds. Various gas distributors for practical application have been successfully developed and studied in depth. Fluidization of coarse particles is still a challenge for good gas-solid contacting. In this study, a slotted gas distributor with simple structure was proposed for the fluidized bed processing coarse particles. The fluidization characteristics of the Geldart D type large particles (2-2.8 mm walnut shell) were investigated experimentally in a 285 mm × 190 mm × 700mm fluidized bed with the slotted gas distributor whose opening rate was 5.6%. The experimental results were compared with the ones obtained from the traditional perforated distributor. It was found that the bed expansion ratio increased by about 5% and the minimum fluidization velocity reduced by about 8% when using the slotted gas distributor, indicating that the slotted gas distributor had better fluidization characteristics for the large-sized particles. To explain the better fluidization performance of the slotted gas distributor, a computational fluid dynamics (CFD) model was developed based on the Eulerian-Eulerian model, the particle kinetic theory, the standard k-? turbulence model and the SIMPLE algorithm. The CFD model was used to simulate the behaviors of the gas particle flow in the fluidized bed of walnut shell particles with slotted gas distributor or traditional perforated distributor, and the results of experiment and simulation were agreed well. The simulations were conducted under such operation conditions: The physical two-dimensional model of the rectangular fluidized bed was 285 mm × 700 mm with slotted gas distributor or traditional perforated distributor in which the opening rate was 5.6%. Particles packed in the fluidized bed had the volume fraction of 0.6 and the height of the bed was 200 mm. The superficial gas velocity of inlet was selected as operation parameter, which ranged from 0.615 to 1.128 m/s. The simulation results were compared on the bed pressure fluctuations due to bubble formation, the coalescence and eruption, the maximum bed expansion radio, the transient particle volume fraction distribution, the gas/particle velocity vector distribution and so on between the fluidized beds of these two distributors. It was observed that small bubbles first formed close to the gas distributor, coalesced when rising up, and finally erupted near the bed surface, which resulted in periodic fluctuations of bed pressure with different amplitudes and frequencies. Compared with the traditional perforated distributor, the frequency and amplitude of the bed pressure drop fluctuation caused by the slotted gas distributor were bigger and the bed expansion ratio was higher. The change of the particle concentration in the bed was scattered in the fluidized bed with slotted gas distributor, which was beneficial to the fluidization of particles near the bed wall. Also, the dead zone and recirculation area formed near the wall area were reduced. The V-shaped structure of the slotted gas distributor generated strong upward flow jets. The strong jets could deeply reach the material particle bed and easily bring the formation of bigger bubbles. The bigger bubbles caused more intensive disturbance inside the material bed, and thereby the bed pressure drop fluctuation and the bed expansion ratio were improved. Four small circulations were observed to form within the fluidized bed with slotted gas distributor, which was beneficial to the better mixing of gas and particle. Therefore, coarse particles of walnut shell can be well fluidized with the slotted gas distributor. The study provides a reference for the design and selection of fluidized bed distributor in the process of coarse particle processing in fluidized bed.