Effect of interparticle forces on the conical spouted bed behavior of wet particles with size distribution M.S. Bacelos, M.L. Passos, J.T. Freire ⁎ Universidade Federal de São Carlos, Centro de Ciências Exatas e de Tecnologia, Departamento de Engenharia Química, Centro de Secagem de Pastas, Suspensões e Sementes, Rod. Washington Luiz, Km 235, 13565-905, CP 676, São Carlos, SP, Brazil Received 5 July 2005; received in revised form 23 November 2006; accepted 19 January 2007 Available online 30 January 2007 Abstract This paper aims to analyze air–solid flow behavior in conical spouted beds composed of glass bead mixtures coated by glycerol. Four mixtures of glass beads are used as the solid phase. Although these mixtures have the same mean Sauter diameter, each one is characterized by a different size distribution function (mono-sized; flat, Gaussian or binary size distribution). When glycerol is added to the bed of these particles, which are spouted by air, the gas–solid flow characteristics are changed due to the growth of interparticle forces; however, the trends of these changes are affected by the glass bead mixture type as well as by the concentration of glycerol. For beds of mono-sized particles, the minimum spouting velocity is maintained almost unchanged as the glycerol concentration rises; while, for beds of inert particle mixtures, this velocity increases, becoming greater for flat and binary size distribution particles. Conversely, the minimum spouting pressure drop decreases as the glycerol concentration rises for all beds of particles used. Based on theoretical prediction of interparticle forces, it is shown that these changes in the minimum spouting conditions can be explained by the magnitude of these forces. © 2007 Elsevier B.V. All rights reserved. Keywords: Conical spouted beds; Liquid bridges; Particle size distribution; Interparticle forces; Fluid and solid flows 1. Introduction The spouted bed is a fluid–particle contact technique commonly used for processing coarse particles, which have a mean diameter from 1 to 6 mm [1]. Because of the reduced level of particle segregation [2,3], a contactor in conical column known as the conical spouted bed, is a potential alternative device to the conventional cylindrical spouted bed of conical base for drying pasty materials [4–7] or for coating seeds and tablets [8–10]. During these operations, interparticle forces are produced by the liquid bridges formed between particles coated by paste or suspensions [11–16]. Applications of this technique in industrial drying of pasty materials are limited by spout stability, powder wall adherence and particle agglomeration. To overcome and control particle agglomeration and spout stability, a good knowledge of inter- particle forces and their effect on the spouted bed flow behavior is required. Several works reported in the literature [8–17] have inves- tigated the gas–particle flow behavior on minimum spouting conditions and flow stability in spouted beds of wet mono-sized particles, by analyzing the effects of interparticle forces, along with suspension viscosity, temperature and bed geometry. Although these works have sought to understand, describe and model the interparticle forces between two particles, only a few have tried to extend their model to describe the behavior of the wet particle assembly [11,12,14,17]. Note that Passos and Mujumdar [17] have modeled the capillary force components arising from liquid bridges, relating them to stress tensor applied to the solid phase assembly in fluidized and spouted beds. A later study by Spitzner-Neto et al. [11,12], has related the drag force to the capillary force to predict the minimum spouting velocity. Thus, both studies have obtained semi-empirical models with adjustable parameters to describe the fluid flow variables in spouted beds of wet particles. This corroborates the need to understand the sig- nificance of each interparticle force as a component acting on beds of wet particles, and to be able to correlate such force components to the operational variables of spouted beds. Powder Technology 174 (2007) 114 – 126 www.elsevier.com/locate/powtec ⁎ Corresponding author. E-mail address: freire@power.ufscar.br (J.T. Freire). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.01.023