Analysis of powder flow and in-system rheology in a horizontal convective mixer with reclining blades

ABSTRACT

A prototypal convective mixer, designed and built for this work, allows investigating powder rheology under various geometrical configurations. The configuration chosen here is a horizontal vessel with four rectangular blades. Two blade inclinations (0–33°) and three filling ratios are studied for two powders of different kind: a free-flowing powder (semolina) and a cohesive powder (lactose). For the smaller agitation speeds, the flow regime of the powder is rolling and is characterized by surface powder avalanches. For greater agitation speeds, the flow regime is cataracting, with particle projections that follow the blade movement. These flow regimes are identified for both powders and do not depends on the filling ratio. Rheological measurements evidence that the blade inclination has little impact on the mechanical power needed to stir the free-flowing powder. It has an impact observable on cohesive powders, especially for high filling ratios. A correlation between the power number and the Froude number is established and compared to previous results obtained on a different technology. It is of the form: N_p = a.F_r^b. The dependencies of the coefficients a and b on the powder type and on the flow regime are quantified.

KEYWORDS:

• Cohesive powders
• convective blender
• flow regimes
• powder mixing
• rheology

Acknowledgments

We would like to thank Asma Toualoua and Luana Grillet, internship students in our laboratory, for their help during rheology measurements that are presented in this work. Many thanks to Pierre Bertorelle for his technical ideas and dedication, both necessary for the elaboration of the blending prototype.