Hydrodynamics and heat transfer in the solid recycle system of circulating fluidized beds

This research covers two aspects of the circulating fluidized bed (CFB) solid recycle system: the hydrodynamics of solid flow through the loop seal and the heat transfer in the moving packed bed of the standpipe.; The flow of solids through the horizontal passage of the loop seal of a CFB was studied. The effect of the length of an un-aerated horizontal passage on the solid flow through the loop seal was examined. It was found that as the passage length increases, the resistance to solid flow through the passage increases as a result of increased frictional resistance and increased area of stationary solids at the end of the passage. The flow of gas from the supply chamber was found to be the driving force for solid flow through the loop seal as the majority of the supply chamber aeration flows horizontally through the passage to the recycle chamber and not upwards through into the standpipe. The flow of solids through the loop seal was found to begin only after a threshold of gas velocity was reached and increased with increasing aeration, to a maximum solid flow, beyond which further increases in loop seal aeration had no effect on the solid flow through the loop seal. This maximum was limited by factors such as passage length, riser aeration, solid inventory and particle properties.; To understand the solid flow through the horizontal passage better the mobility of the bed particles was studied. The 'spread angle' of an aerated heap of solids (angle the heap makes with the horizontal) of varying sizes and sphericities, was investigated. It was found that particles with a higher sphericity had a smaller spread angle and thus are more mobile than less spherical particles. The size of the particle had little effect on the initial spread angle of the heap, with a larger particle requiring a higher aeration velocity to reach the same spread angle as a smaller particle. The spread angle is zero at an aeration velocity equal to the minimum fluidization velocity for all particle shapes and sizes.; The heat transfer to cross tubes in a moving packed bed was studied to determine the heat transfer characteristics of solid flow in the standpipe of the CFB solid recycle system. The effect of tube diameter on heat transfer was studied. It was found that the heat transfer coefficient between the moving packed bed and the tube decreased with increasing tube diameter, due to the increased residence time of the particles on the tube. It was also confirmed that an increase in particle diameter caused a decrease in heat transfer due to an increase in the gas-gap annulus that exists adjacent to the tube surface. The empirical relation for heat transfer to a cross-flow tube in a moving packed bed developed by Obuskovic et al. (1991) was modified to fit the current experimental data. The model showed a good correlation for the present data as well as for that reported by Courbariaux et al. (1999).