Fluidization of cohesive materials is difficult to achieve. These materials tend to form channels during fluidization. They also form structures within the fluid bed that lift surround bulk material and can develop pressures and pressure gradients that are larger than the hydrostatic gradient normally experienced in fluid beds. We want to approach this phenomenon from a solid mechanics point of view. One can model the channel formed in a fluid bed as a pipe or a rathole in material that has strength and treat it as a slope stability problem – except that the bulk material has pressure gradients that modify the stability of the channel surface. The basis of these equations is part of a previous work. The size of stable ratholes with gas pressure gradients can be computed using this past work. However, the motion of material in the channel during fluidization can induce a shear that is not compatible with the traditional assumptions for calculation of rathole stability from a solids mechanics point of view. Traditional theory assumes an unconfined state at the surface of the rathole and implies unique conditions on principle stress at this location. This assumption is not valid when a shear stress exists on this surface. We have reformulated the rathole stability equations with the assumption of a local shear stress at this boundary and also included gas pressure gradients. The resulting equation was designed to predict the stability of a channel formed in a bulk solid subject to movement in the channel and local gas pressure gradients. This is closer to the conditions that one would expect in the fluidization of a cohesive material.
We constructed a fluid bed and measured the standard fluidization curves as well as the size of channels formed in the bed. We also measured the unconfined yield strength of the material at low solid contact pressures and then compared this data to the values predicted from the modified rathole theory, including movement of material. An FEM analysis of gas pressures and solids stresses surrounding the channel was coupled with the results of the rathole equations to predict the channel size. We also measured the settlement time of cohesive materials as a function of the unconfined yield strength, and then created a settlement model that included gas loss through channels that formed. Finally, we correlated this behavior with the expected aerated rathole stability predictions. The results are presented in this paper.
