Chemical Engineering Journal 164 (2010) 425–431 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Relationship between mechanical properties and shape descriptors of granules obtained by fluidized bed wet granulation Darío I. Téllez-Medina ∗ , Edmond Byrne, John Fitzpatrick, Muammer Catak, Kevin Cronin Department of Process and Chemical Engineering, University College Cork, College Road, Cork, Ireland article info Article history: Received 3 July 2009 Received in revised form 12 November 2009 Accepted 27 November 2009 Keywords: Wet granulation Coefficient of restitution Granule strength Fractal dimension Lacunarity abstract This paper focuses on two mechanical properties of granules, the coefficient of restitution and the strength, and analyzes their sensitivity to granule geometric parameters. The granules were obtained by fluid bed granulation of glass beads with an aqueous solution of PEG1500. Collisions were arranged between granules, and for granules against two glass plates, the first of them a non-covered plate, whereas the second was a plate covered with a thin film of PEG1500. The coefficient of restitution and the strength were measured for the granules, the former for individual particles also. In the case of individual glass particles the coefficient of restitution was around 0.61 for impacts on the flat glass, and 0.5 on the cov- ered glass; for the granules, this parameter was around 0.44 for both situations and for collisions between granules. Sphericity, lacunarity and fractal dimension of the granule projected area, as well as the gran- ule porosity, were determined. Granules giving the highest values for strength had the largest fractal dimension and the smallest lacunarity values regardless of their sphericity, porosity and coefficient of restitution. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Fluidized bed technology is employed to achieve granulation of particles with the net agglomeration process being defined by the nature of the inter-particle collisions. Either size enlargement or alternatively attrition and breakage can result from these collisions. In addition, collisions between granules and the walls of the equip- ment have a significant effect on the evolution of granule size [1]. To understand and predict the outcome of collisions, knowledge of the mechanical properties of the granular material is required [2]. This work focuses on two such properties, the coefficient of resti- tution and the granule strength. In particular, this work examines their sensitivity to geometric parameters of the granules. The system under analysis consists of the Würster granulation process based on fluidized bed bottom-spray granulation. The flu- idized granules travel up an inner tube and exit out the top into the main chamber, fall down the annular space between the tube and the chamber and then repeat the motion. The granules were assembled from glass beads by addition of PEG1500 into an aqueous solution. Geometric characteristics, e.g. sphericity, can influence the result of particle collisions, i.e. due to the particle shape and orien- tation it is possible to obtain different results for particle–particle ∗ Corresponding author. E-mail address: darioiker@gmail.com (D.I. Téllez-Medina). and wall–particle interactions [3,4]. Another geometric character- istic is the irregularity of particle surface which can be quantified by means of fractal dimension (D F ). Fractal dimension is based on the concepts first proposed by Mandelbrot [5] to characterize nat- ural shapes with mathematical patterns more close to reality than those extracted from Euclidean geometry, and it has been applied to different systems and phenomena to quantitatively describe their morphology [6–10]. The use of fractal dimension as a shape descrip- tor for granules has been reported by [6,10,11]. Lacunarity () is a complementary parameter to quantify the heterogeneity in the distribution of void spaces inside a figure or pattern [12]. Compara- ble to sphericity, D F and  can be determined with image analysis software, by applying the box counting and gliding box methods, respectively [5–14]. 2. Experimental Granules were obtained by fluidized bed wet granulation from 200 g of glass beads (Jencons-PLS, UK) with a mean diameter equal to 268 m, and 10 g of an aqueous 60% (w/w) dissolu- tion of polyethylene glycol (PEG) 1500 Da (Fluka, Germany). The granulation equipment employed was a Mini-AirPro (Pro-C-epT, Belgium) with the Würster configuration. In Table 1 are resumed the processing conditions. The total processing time was approxi- mately 25 min plus a drying period (1 min) to evaporate the water remaining in the binder liquid making contact with the individual particles. 1385-8947/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2009.11.035