Species segregation of binary mixtures and a continuous size distribution of Group B particles in riser flow Jia Wei Chew a , Roy Hays b , John G. Findlay b , S.B. Reddy Karri b , Ted M. Knowlton b , Ray A. Cocco b , Christine M. Hrenya a,n a Department of Chemical and Biological Engineering, University of Colorado at Boulder, Boulder, CO 80309, USA b Particulate Solid Research, Inc. Chicago, IL 60632, USA article info Article history: Received 16 February 2011 Received in revised form 24 May 2011 Accepted 14 June 2011 Available online 23 June 2011 Keywords: Fluidization Multiphase flow Gas–solids circulating fluidized bed riser Binary and continuous distributions Species segregation Geldart Group B abstract Experiments involving a gas–solid, pilot-scale circulating fluidized bed (CFB) have been carried out, with a focus on species segregation measurements in a riser. Three mixtures were considered: (i) a binary mixture with particles of different sizes (d ave ) but same material density (r s ), (ii) a binary mixture with particles of different material densities (r s ) but same size (d ave ), and (iii) a continuous particle size distribution (PSD). Local measurements of the composition (i.e., species segregation) of each mixture were obtained over a range of operating conditions. Similar to previous works, the results show that the more massive species (i.e., greater d ave or r s ) preferentially segregates toward the wall in all cases. Several new trends were also observed. First, for the binary mixtures, composition of the more massive species increases with riser height at the wall under some operating conditions. The operating conditions that cause this phenomenon are mutually exclusive for the size-difference and density- difference systems. Second, for the continuous PSD, radial segregation is observed even when there is a net positive flux in the annular region, contrary to previous findings which indicated segregation only for conditions leading to a net downward flux in the annular region. Finally, two qualitative differences between the binary and continuous mixtures were noted: (i) a monotonic decrease in species segregation is observed for the binary mixtures with an increase in the solid loading (m), while a non-monotonic trend is observed for the continuous PSD, and (ii) while the shape of the radial segregation profile is flattest at the riser bottom for the binary mixtures, the flattest radial profile is at the riser top for the continuous PSD. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Polydispersity is prevalent in flows involving solids, and species segregation (or de-mixing) is ubiquitous in systems comprising of various particle sizes (d ave ) and/or particle material densities (r s ). Regardless of operational parameter space, both experiments and simulations have indicated that particles segre- gate according to species in granular systems (negligible gas phase), as well as in fluidized systems (Fan et al., 1990; Hrenya, 2011; Kunii and Levenspiel, 1991b; Lim et al., 1995; Muzzio et al., 2002; Nienow and Chiba, 1985; Ottino and Khakhar, 2000; Rhodes, 1998; Rowe and Nienow, 1976; Sundaresan, 2001; van Deemter, 1980; Werther and Hirschberg, 1997). An accurate predictive understanding of the effects of polydispersity remains elusive (Curtis and van Wachem, 2004; Hrenya, 2011; Muzzio et al., 2002; Sundaresan, 2001), and hence empirical approaches in the design and scale-up of unit operations is often employed, leading to subpar performance and undesirable flow behavior (Merrow, 1985, 1988). The focus of this work is on the riser portion of a gas–solid circulating fluidized bed (CFB) of polydisperse Geldart (1973) Group B particles. The CFB represents an important gas–solids unit operation due to its high heat and mass transfer rates, capability of continuous solids mobility, and applicability over a wide range of particle properties (Fan and Zhu, 1998; Kunii and Levenspiel, 1991a). Examples of CFB applications include gasifica- tion of solid feedstock like coal or biomass, both of which are polydisperse by nature. Because species segregation varies along the riser, the gas–solid contact times of each species may vary and hence impact the overall efficiency of the unit. Although investi- gation into species segregation behavior in low-velocity bubbling fluidized beds has been fairly extensive (Joseph et al., 2007; Kunii and Levenspiel, 1991b; Lim et al., 1995; Nienow and Chiba, 1985; Rowe and Nienow, 1976), a similar effort for CFBs is lacking. The current work is divided into two categories: binary mixtures (two species with particle size (d ave ) or material density (r s ) Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2011.06.031 n Corresponding author. Tel.: þ1 303 492 7689; fax: þ1 303 492 4341. E-mail address: hrenya@colorado.edu (C.M. Hrenya). Chemical Engineering Science 66 (2011) 4595–4604