Scale-up in Turbula? mixers based on the principle of similarities

Many processes from a wide range of industries use powders as raw materials. In most of them, mixing of dry particles is a critical step. However, the complexity of granular materials leads to difficulties in predicting behavior of powders inside a blender. Cost and amount of raw materials needed to achieve these studies imply that such work is often performed at the laboratory scale. The main challenge then lies in the extrapolation of results from lab to pilot or industrial scale with minimum testing in order to optimize costs. Defining reliable scale-up laws for powder mixers remains one of the main industrial issues. The work presented here concerns the scaling-up of Turbula (R) mixers that are commonly used both in industry and research. Mixtures obtained in different mixer sizes have been compared on the basis of kinematic and dynamic similarities and discussed according to the different flow regimes involved. The dynamic similarities appear to lead to mixture qualities at different scales closer to each other than those obtained on the basis of kinematic similarities probably because of similar flow regimes. However, some other parameters must be taken into consideration, such as the free surface of powder related to the filling ratio.