Flow Properties of Moisturized Powders in a Couette Fluidized Bed Rheometer

This work investigates on the effect of air humidity on the flow properties of powders. The moisture of a powder sample (50 µm glass beads) was conditioned by fluidization with humid air. Air humidity was kept between 0 and 70% at ambient temperature. A Schulze shear cell was used to measure the bulk flow properties of the moisturized samples. A Couette fluidized bed rheometer was used to measure the torque necessary for the rotation of the inner cylinder when the fluidized powder had been moisturized with the same procedure. These experiments show a certain continuity of the results below and above the minimum fluidization velocity, suggesting a similar continuity of the role that interparticle interactions play in the fixed and in the fluidized bed. Experiments below the minimum fluidization velocity were interpreted with a rheological model in which the variable load along the vertical direction in the Couette was calculated with a modified Janssen equation. In this approach the apparent weight of the powder is given by the difference between the gravity and the upward body force determined by the rising gas flow. The agreement between the model and the experiments supports the proposed approach.