Experimental investigation of solid mixing and segregation in a tetrapodal blender Ebrahim Alizadeh, Habibollah Hajhashemi, François Bertrand n,1 , Jamal Chaouki n,2 Chemical Engineering Department, École Polytechnique de Montréal, C.P. 6079 succ. Centre-Ville, Montréal, Québec, Canada H3C 3A7 AUTHOR-HIGHLIGHTS  The performance of the tetrapodal blender is compared to that of the V-blender.  The mixing mechanisms are more efficient in the tetrapodal blender than the V-blender.  The tetrapodal blender is less prone to granular segregation than the V-blender.  A criterion is provided for the scale-up of tumbling blenders. article info Article history: Received 26 March 2013 Received in revised form 10 April 2013 Accepted 15 April 2013 Available online 24 April 2013 Keywords: Tetrapodal blender V-blender Solid mixing Segregation Scale-up Thief sampling abstract Known limitations of tumbling blenders (weak diffusive axial mixing and segregation of free flowing granules) have provided the motivation to investigate the flow and mixing of granules inside a tetrapodal blender. This blender can be thought of as two V-shaped pairs of arms connected and twisted at their bottom ends. In this work, more than 100 experiments were carried out under a wide set of operating conditions and geometrical configurations. Compared to the conventional V-blender, this geometry is shown to provide shorter mixing times and better axial and radial mixing efficiency, especially when its upper or lower V-shaped part is twisted by 451 with respect to the rotation axis. Segregation of granules with different sizes and densities was investigated for varying rotational speeds (5–30 RPM) and fill levels (35–65%V). It is observed that the segregation intensity is far less important in the tetrapodal blender than in the V-blender, and that it decreases significantly with an increase in rotational speed, the effect of the fill level being insignificant. It is also shown that kinetic sieving is the main governing mechanism for the segregation of granules. Finally, a criterion is proposed for the scale-up of the tetrapodal blender and the V-blender so that they may operate efficiently, without pronounced segregation. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Tumbling blenders, available for the purpose of granular mixing, have various applications in many industries including ceramic, metallurgical, chemical, food, cosmetics, plastics and pharmaceutical. These blenders are essentially a hollow vessel horizontally attached to a slow rotating shaft, rendering them ideal for shear sensitive materials. Among these types of blenders, rotating drums, V-blenders, double cone and bin blenders (tote and bohle) are the most common. In such blenders, active agents are generally mixed with excipients, and their concentration must be strictly maintained within very small intervals to meet the quality and performance goals. In tumbling blenders, mixing happens in both axial and radial directions. Axial direction is along the rotation axis of the blender (axis X 1 in Fig. 1a) and radial direction is perpendicular to the rotation axis (plane X 2 X 3 in Fig. 1a). The mixing mechanism is diffusive in the axial direction and a combination of convection and diffusion in the radial direction. Given that diffusive mixing is an order of magnitude slower than convective mixing, tumbling blenders always suffer from weak mixing in the axial direction (Arratia et al., 2006; Brone and Muzzio, 2000; Brone et al., 1997; Lemieux et al., 2007; Moakher et al., 2000). Several attempts have been made to increase axial mixing efficiency (e.g. Manickam et al., 2010; Mehrotra and Muzzio, 2009). In addition, these blenders have a tendency to segregate granules differing in size, shape or density into isolated regions. Several studies have been carried out to investigate segregation in a rotating drum (Alizadeh et al., in press; Chen et al., 2010), V-blender (Adams and Baker, 1956; Alexander et al., 2003, 2004b; Harnby, 1967; Lemieux et al., 2008; Samyn and Murthy, 1974), double cone blender (Alexander et al., Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ces.2013.04.035 n Corresponding authors. E-mail addresses: francois.bertrand@polymtl.ca (F. Bertrand), jamal.chaouki@polymtl.ca (J. Chaouki). 1 Tel.: +1 514 340 4711x5773. 2 Tel.: +1 514 340 4711x4034. Chemical Engineering Science 97 (2013) 354–365