Behaviour of gas-fluidized beds of fine powders part II. Voidage of the dense phase in bubbling beds

Abstract

The collapse rate technique has been used to evaluate the average dense phase properties in vigorously bubbling beds of fine powders. The results of experiments on 13 air/solid systems are used to correlate the average dense phase voidage, ϵ D , in terms of the physical properties of the gas and powder, and our predictive equations fit literature data. ϵ D increases as the particle density and mean particle size decrease, and as the fraction of fine <45 μm, gas viscosity and gas density increase. The gas velocity, U D , through the dense phase of a bubbling fluidized bed has been calculated from ϵ D assuming Darcy's law and can now be predicted. Since it is less than the minimum bubbling velocity, the improved performance of fluidized bed reactors when the gas/solid properties are changed so as to increase ϵ D can be attributed more to smaller bubbles splitting and coalescing frequently rather than to the small amount of extra gas passing through the dense phase.