Cavinato, M., Artoni, R., Bresciani, M., Canu, P., Santomaso, A.C. Scale-up effects on flow patterns in the high shear mixing of cohesive powders (2013) Chemical Engineering Science, 102, pp. 1-9. DOI: 10.1016/j.ces.2013.07.037 Scale-up effects on flow patterns in the high shear mixing of cohesive powders Mauro Cavinato, Riccardo Artoni b , Massimo Bresciani c , Paolo Canu a , Andrea C. Santomaso a, * a APTLab - Advanced Particle Technology Laboratory, Università di Padova, Dipartimento di Ingegneria Industriale, via Marzolo 9, 35131 Padova, Italy b L’UNAM, IFSTTAR Route de Bouaye, CS4, 44344 Bouguenais Cedex, France c Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria *Corresponding author: andrea.santomaso@unipd.it Abstract Processing of granular material often requires mixing steps in order to blend cohesive powders, distribute viscous liquids into powder beds or create agglomerates from a wet powder mass. For this reason, using bladed, high-speed mixers is frequently considered a good solution by many types of industry. However, despite the importance of such mixers in powder processing, the granular flow behavior inside the mixer bowl is generally not totally understood. In this work extensive experimentation was performed comparing the behavior of a lab-scale mixer (1.9 l vessel volume) to that of a pilot-scale mixer (65 l vessel volume) with a mixture of some pharmaceutical excipients (e.g. lactose, cellulose). The aim was to propose a new and more detailed method for describing the complex powder rheology inside an high shear mixer using impeller torque, current consumption and particle image velocimetry (PIV) analysis. Particularly, a new dimensionless torque number is proposed for the torque profile analysis in order to isolate the contributions of mass fill and blade clearance at the vessel base. Impeller torque and motor current consumption were integrated with PIV to obtain more detailed information about the surface velocity and flow pattern changes in the pilot-scale mixer. Mass fill resulted to be one of the most critical variables, as predicted by the torque model, strongly affecting the powder flow patterns. An additional mixing regimes was furthermore defined according to the observation of the surface velocity of the powder bed. Keywords: high shear mixer, cohesive powders, scale-up, mixing regimes 1 Introduction High shear mixers, also known as high intensity mixers (Paul et al., 2003), are widely used for the powder processing. They can be used for simple mixing, in particular of cohesive materials, since they exert a high local shear on the powder which breaks down the small aggregates (Harnby, 1997), or for more complex operations which involve both solids and liquids. For example in a wet granulation process, a high shear mixer can promote a good liquid dispersion and proper consolidation of the product, in order to obtain aggregates with useful structural forms, improved flow properties and reduced segregation propensity (Litster et al., 2004). They are constituted by a bowl and a centrally mounted impeller rotating about a vertical or an horizontal axis. For the case © 2013. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/