Elutriation characteristics of multi-component mixtures in a fluidized bed pyrolysis process

Abstract

Multi-component mixtures were prepared to simulate a bed of walnut shells undergoing pyrolysis in acold fluidized bed. Four mixtures with different ratios of walnut shell, semi-chars, and clean char were used. Batch elutriation experiments using these mixtures were performed in a transparent fluidized bed to allow direct observation of the flow patterns and particle composition. The results indicated that the bubble wake rather than bubble nose was primarily responsible for particle entrainment at higher gas velocities, and that coarser particles would be “transformed” into elutriable particles. Dimensionless gas velocities ranged from 0.2 to 1.0 and an exponential decrease in particle concentration with respect to bed height was observed. The transport disengagement height (TDH) gradually increased with the gas velocity until the entire bed layer enters a pneumatic transport state. Notably, larger TDHs were required when the bed contained a larger fraction of light components. Two characteristic parameters were used to evaluate particle elutriation: the elutriation rate constant (K), and the residual volatile content of the elutriated particles (v′). These parameters were used to optimize operating gas velocity for the fluidized bed.