Chemical Engineering Science 54 (1999) 5383 } 5391 The e!ect of particle size and density on solids distribution along the riser of a circulating #uidized bed Maria Laura Mastellone, Umberto Arena* Department of Chemical Engineering, University **Federico II++ of Naples, Piazzale Tecchio 80, 80125 Napoli, Italy Department of Environmental Sciences, II University of Naples, Italy Abstract A cold model of a CFB having a 0.120 m-ID and 5.75 m-high riser, was operated under a relatively wide range of gas velocity and solids mass #ux. Four solids (two Ballotini, a silica sand and a spent FCC), belonging to groups A and B of Geldart's classi"cation, were selected in order to separately investigate the e!ect of particle size and density on axial pro"les of voidage and on radial pro"les of solids mass #ux. Attention was focused, in particular, on the suspension density at the bottom and exit regions of the riser.  1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Hydrodynamics plays a crucial role in de"ning the performance of circulating #uidized-bed (CFB) reactors. This awareness promoted a remarkable number of stud- ies, which appeared in the scienti"c literature in the last 15 years (Werther, 1993; Horio, 1997). It led to a satisfac- tory understanding of several aspects of gas}solids sus- pension behavior in risers of di!erent sizes and shapes, operated under a variety of conditions. There are, how- ever, some important areas where available information is limited, and the poor degree of knowledge hinders the design and/or the operation of the industrial CFB reac- tors. This is, in particular, the case of the e!ect of the properties of particles (size and density) on the solids distribution along the riser and on the collapse condition of dilute suspension. An investigation on the e!ects of particle size was carried out by Bai, Jin, Yu and Zhu, (1992) by using low-density particles (about 708 kg/m). That study pointed out that larger particles present axial voidage pro"les `with higher bed density in the bottom section and lower bed density in the top section of the beda and that these di!erences appeared to be smaller when super"cial gas velocity increased or solids mass #ux decreased. The authors utilized the data of Arena, * Corresponding author. Tel.: #39-81-7682267; fax: #39-81- 5936936. E-mail address: arena@irc.na.cnr.it (U. Arena) Cammarota, Massimilla and Pirozzi (1988) to a$rm that the same behavior could be found when particle density was increased. These conclusions were also reported by Lim, Zhu and Grace (1995) with reference to the same experiments. A careful analysis of these data suggests that there is not a clear-cut trend, particularly with reference to the e!ect of particle diameter. Kunii and Levenspiel (1996) took into account the role of particle size and density in the development of their hydrodynamic model for vertical distribution of solids in CFBs. Data obtained from several cold models, operated under a variety of conditions and with particles belonging to di!erent groups of Geldart's classi"cation were summarized. Un- fortunately, the scattering of data and the diversity of experimental equipment and procedures made a reliable comparison of results di$cult. It was merely possible to observe a tendency in the variation of voidage in the dense lower and the dilute upper regions of the riser. The present study aims to investigate the in#uence of both particle size and density on some aspects of CFB hydrodynamics. Four solids (two Ballotinis, a silica sand and a spent FCC), belonging to groups A and B of Geldart's classi"cation, were selected in order to separ- ately investigate the e!ect of particle size and density on axial pro"les of voidage and on radial pro"les of solids mass #ux. Attention was focused, in particular, on the bottom and exit regions of the riser. The experiments were carried in a single apparatus, so their validity is necessarily limited, particularly to the size of the cold model used. 0009-2509/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 2 7 2 - 9