New continuous tumble mixer characterization Miguel Florian, Carlos Velázquez, Rafael Méndez ⁎ Chemical Engineering Department, University of Puerto Rico, PO Box 9046, Mayagüez 00681, Puerto Rico abstract article info Article history: Received 9 October 2013 Received in revised form 23 December 2013 Accepted 7 February 2014 Available online 15 February 2014 Keywords: Continuous mixing Tumble mixer Powder behavior Mixing uniformity The pharmaceutical industry is in the midst of a major transition in manufacturing, which is focused on contin- uous operation. One of the transitioning operations is powder mixing, arguably the principal process in the prod- uct preparation. This work examined the performance of a new continuous tumble mixer. The study focused on the mixing capabilities and powder flow phenomena inside the mixer as a function of the inlet flow rate, mixer rotation speed, and material properties. The continuous tumble mixer achieved good mixing levels when consid- er the obtained relative standard deviation (b 5% in most cases) of the outlet concentration. A map of the material compressibility index was developed to relate the effect of the material properties and operating conditions to the powder behavior inside the mixer. The rolling, cascading, and, principally, cataracting regimes were obtained, which ensured blend uniformity. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The number of studies focused on the flow and mixing of particulate material using both computational and experimental approaches has increased during the last years, especially for batch systems. This increase is observed because granular material mixing is a major step in diverse industries, including the pharmaceutical industry. On the other hand, continuous mixing has been applied in other industries [1–3] with excellent results, including powder processing in chemical and food industries. 1.1. Batch drum mixers Drum mixers are characterized by the folding or avalanche behavior the particulate material exhibits inside the vessel as it rotates. These rotating drums, one of the most common mixers, yield six different material behaviors, namely, slipping, slumping, rolling, cascading, cat- aracting, and centrifuging [4]. Several authors have mentioned a sev- enth behavior, denominated surging [5–7], which is situated between slipping and slumping. In the slipping regime, the material slides as a solid bed as the mixer rotates; in slumping behavior, the material on the top of the accumulation flows to the bottom; rolling behavior is characterized by a flat surface; in the cascading regime, the flat surface disappears to produce a curve; cataracting occurs at high rotational ve- locities when the material reaches the superior part of the mixer and falls back due to the gravitational force; in the centrifuging regime, the material remains on the mixer walls due to the centrifugal force. The rolling regime is the most extensively studied behavior because it provides the highest mixing uniformity [8]. Two principal zones can be identified: the undergoing solid-body, which is the layer near the wall of the mixer (stagnant layer), and the active layer surface [9,10], which is the top section of the material in the mixer. Mixing primarily occurs in this top section [11]. The effects of the density, friction coefficient, surface quality, and particle geometry on the final uniformity of granular mixtures have been previously studied [11]. This study found that the use of additional parameters is essential to characterize the particle flow regimes. Typi- cally, the Froude number (Eq. (1)) is used to describe the flow regimes inside a rotary cylinder mixer, where R represents the radius of the drum mixer, w represents the rotational velocity, and g represents the gravitational force: Fr ¼ Rw 2 g ð1Þ This number represents the ratio of the centrifugal to the gravita- tional forces in a drum system and does not include other factors, such as the material properties. Due to this definition, Aissa et al. [11] con- cluded that this value must be complemented with the filling ratio. Continuous mixing processes are an alternative to batch mixing processes. Continuous processes have been used in different industries, such as the food, chemical, and petrochemical industry [12]. This mode has several advantages compared to batch mixing, including lower costs, the possibility of implementing online analysis, the elimination of scale-up process, and the easy measuring of uniformity at the outlet of the system [13]. Powder Technology 256 (2014) 188–195 ⁎ Corresponding author. Tel.: +1 787 832 4040x3585. E-mail address: rafael.mendez1@upr.edu (R. Méndez). http://dx.doi.org/10.1016/j.powtec.2014.02.023 0032-5910/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec