A fluid-bed continuous classifier of polydisperse granular solids Giuseppe Olivieri, Antonio Marzocchella, Piero Salatino * Dipartimento di Ingegneria Chimica, Universita ` degli Studi di Napoli Federico II, Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, P.le Tecchio 80, 80125 Napoli, Italy 1. Introduction Polydisperse solids mixtures are produced in different fields of the chemical and processing industries or result from a variety of mining and mineral ore processing. Operations aimed at the fractionation/separation of polydisperse mixtures of granular solids may be based on a variety of concepts: classification from the liquid phase (Chen et al., 2002; Doroodchi et al., 2006), classification from the gas phase (Adham, 2001; Kolaitis and Founti, 2002; Savage et al., 1996; Shapiro and Galperin, 2005; Smith, 1991; Yang et al., 2002), sieving (static or vibrated), screening, to cite a few. Despite their widespread application, these techniques may suffer from some disadvantages and drawbacks. Solids classifications carried out in the presence of a liquid phase are restricted to low temperatures to reduce liquid evaporation. Moreover, liquid classification may be lengthy, a feature that impacts the maximum throughput of continuous classifiers. On the other hand dry powder classification processes may be character- ized by significant mechanical stresses imparted to the granular material which may induce solids comminution. It has been suggested that separation processes based on fluidization may at least partly overcome some or all of the above mentioned problems (Adham, 2001; Gel’perin et al., 1964; Shapiro and Galperin, 2005; Yang et al., 2002). Occasionally, classification in fluidized beds may be assisted by devices as sieves (Bosna and Hoffmann, 2003; Chyang et al., 2002), mechanical stirrers (Beeck- mans and Yu, 1992) and mechanical vibrators (Yang et al., 2002; Thomas et al., 2000). Classification in the fast fluidization regime has recently been considered (Palappan and Sai, 2008a,b). Gas-fluidization of polydisperse granular solids gas has been the subject of extensive experimental investigation, comprehen- sively surveyed by Nienow and Chiba (1985), Fan et al. (1990), Huilin et al. (2003), Marzocchella et al. (2000), Olivieri et al. (2004), Joseph et al. (2007) and Formisani et al. (2008). Computational studies have also recently contributed to understanding mechan- isms underlying mixing/segregation of polydisperse solids in gas fluidized beds (Beetstra et al., 2007; Di Renzo et al., 2008; Fan and Fox, 2008; Owoyemi et al., 2007). Fluidization of polydisperse solids may promote either segregation or mixing, depending on the operating conditions. Typically, large gas superficial velocities (U) enhance solids mixing whereas smaller values of U, close to incipient fluidization, promote solids segregation. The segregated state becomes apparent as the stabilization of an underlying fluidized bed enriched in jetsam (consisting of heavy/coarse Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 638–644 ARTICLE INFO Article history: Received 11 March 2009 Received in revised form 18 May 2009 Accepted 21 May 2009 Keywords: Classifier Dissimilar solids Fluidization Segregation ABSTRACT The use of a tapered fluidization column to continuously classify streams of dissimilar solids has been investigated. Solids classification is based on the different propensity of dissimilar solids to segregate when operated in the transient fluidization regime. A test apparatus based on this concept was set up and operated with mixtures of solids having equal density (about 2500 kg/m 3 ) and different diameter (flotsam = 125 mm, jetsam = 500 mm). The jetsam mass fraction of the fed mixture ranged between 0.25 and 0.75. The mass flow rate of the solids mixture ranged between 2.4 and 5.8 kg/h. Results showed that the jetsam fractional mass in the jetsam-enriched stream was nearly 100% and that in the flotsam- enriched stream was as small as 2–8%, under the operating conditions investigated. The overall classification effectiveness was as high as 90%. Operation of the classifier was successfully carried out for more than 6 h, with excellent reliability and steadiness of the performance. The proposed classification technique features several advantages over other fractionation methods: absence of moving or easily wearable mechanical parts in the classifier; no need for a liquid medium; limited solids attrition; possibility to operate the classifier at high temperatures and/or under controlled atmospheres; possibility to perform fractionation at the same time as other physico-chemical processes (e.g. drying and calcination). ß 2009 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +39 081 7682258; fax: +39 081 5936936. E-mail address: salatino@unina.it (P. Salatino). Contents lists available at ScienceDirect Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice 1876-1070/$ – see front matter ß 2009 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jtice.2009.05.011