(457d) DEM Simulation On the Effect of Particle Size Distribution On the Bubble Size

    Fluidized bed has been widely used for many applications such as coal combustion, fluidized catalytic cracking and biomass gasification. The advantages of fluidized beds are its simplicity, high heat and mass transfer and continuous operation. Most of these applications involve particles with different size distribution (PSD) and its effects on the hydrodynamics are critical. Previous studies showed that the particles with wide PSD or particles with fines added offer better heat transfer and mass transfer characteristics. Beetstra et al.[1] analyzed the effect of PSD for Geldart A particles and showed that with increase in PSD width, bubble size decreases at higher velocities and increases at lower velocities. Chew et al.[2] observed a monotonic increase in bubble frequency, velocity and chord length with PSD width for Geldart B particles.

    The mechanism behind the enhanced performance due to addition of fines is least understood. One popular surmise is that the fines added produces lubrication effect which leads to low granular viscosity and better fluidization. Alternate suggestions proposed are increased porosity in the dense phase resulting in high flow through dense phase and higher solids holdup in voids which alters the bubble characteristics. The current study aims at analyzing the effect of the PSD using Discrete Element Method (DEM). DEM resolves particle-particle interaction and particle-wall collisions and provides detailed view of the bubble dynamics.

    The current study analyzes two-dimensional bubbling fluidized bed with cross section 20cm by 15cm height. The simulation is carried for Geldart B particles with mean diameter 500 mm. The analysis is carried out for monodisperse, bidisperse and a continuous PSD with standard deviation to the mean diameter ratio equal to 15%. The results will be analyzed to estimate effect of particle size on the bubble frequency, speed and size. The granular temperature will be computed which will identify the effect of the fines over the granular viscosity.

 1. Beetstra, R., Nijenhuis, J., Ellis, N., and van Ommen, J. R. "The influence of the particle size distribution on fluidized bed hydrodynamics using high-throughput experimentation," AIChE Journal Vol. 55, No. 8, 2009, pp. 2013-2023.

2. Chew, J. W., and Hrenya, C. M. "Link between bubbling and segregation patterns in gas-fluidized beds with continuous size distributions," AIChE Journal, 2011.