Fluidized Bed Rheology: Direct Measurements of Apparent Viscosity through Stokesian and Bingham Approaches

Gas Fluidization as a process is an important part of many industrial processes, however compared to collapsed states the measurement of its intrinsic flow properties is in its infancy.

Our fluidized bed rheology setup consisting of a powder containing cylinder with a gas permeable bottom, through which a defined controlled gas flow is run into the powder sample, and a geometry placed in the powder. Attached to a rotational rheometer such a setup allows real rheological measurements over large ranges of shear rates and shear stresses on powder systems in a well-defined fluidization state. Being able to measure small torques in a reproducible manner enables a proper rheological study of fluidized beds similar to the rheology of complex fluids.

To obtain absolute viscosity values a linear velocity profile is required and slip needs to be prevented. Different concentric cylinders smooth, rough and profiled surfaces were tested. Viscosity curves of a glass bead sample in the un-fluidized and the fully fluidized state measured with two profiled cylinders with different sizes are in good agreement indicating that the no slip condition is met. Results of viscosity curves in different fluidization states are presented and are compared to the behavior of liquid suspensions. While exhibiting some unique effects, the shear rate dependent behavior of a gas fluidized bed can be for the most part be linked to that of particulate suspension.

Furthermore comparisons on multiple concentric cylinder geometries and a novel ball measurement system based upon stokesian surface viscosity that allows the measurement of more cohesive powders. We will illustrate the capabilities of this approach on multiple industrially relevant powders and applications and their applications for process design and control.