* Corresponding author. Chemical Engineering Science 54 (1999) 5451}5460 Suspension densities in a high-density circulating #uidized bed riser A.S. Issangya*, D. Bai, H.T. Bi, K.S. Lim, J. Zhu, J.R. Grace Department of Chemical and Bio-Resource Engineering, University of British Columbia, 2216 Main Mall, Vancouver, Canada V6T 1Z4 Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Canada N6A 5B9 Abstract Flow behaviour of #uid catalytic cracking particles were investigated in a riser with volumetric solids concentrations of 20% and more. The air velocity and solids circulation #ux had only a small in#uence on the solids hold-up once high-density conditions were attained. At these high-density conditions, re#uxing of solids near the riser wall, commonly observed in low-density CFB risers, disappeared and was replaced by a more homogeneous #ow structure. The slip velocity increased with solids hold-up and, for constant super"cial gas velocity, there appears to be a unique relationship between the two. Slip factors as high as 10 were obtained in the developed region of the riser, compared to values of about 2 to 5 reported in the literature for more dilute fully developed #ows in the fast #uidization #ow regime. Di!erential pressure #uctuations increased with increasing suspension density and were not signi"cantly a!ected by super"cial gas velocity and height. A transition point is de"ned to distinguish dilute from high density conditions in the CFB riser.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Fluidization; High-density CFB reactors; Gas}solids #ow dynamics; Solids distribution; Hydrodynamics; Dual-loop CFB 1. Introduction The industrial application of circulating #uidized bed (CFB) technology has rapidly expanded over the past two decades. Berruti, Chaouki, Godfroy, Pugsley and Patience (1995), Dry and Beeby (1997), Avidan (1997) and Matsen (1997) discuss a large number of industrial pro- cesses that utilize CFB technology, as well as processes under development. These processes can be divided into gas}solids reaction processes and gas-phase catalytic re- action processes. The former, which include combustion of low-grade fuels (e.g. coal) and alumina calcination, usually have low reaction rates and therefore do not necessarily require high gas velocities or high solids cir- culation rates. Catalytic gas-phase reactions, which in- clude #uid catalytic cracking of petroleum, Fischer} Tropsch synthesis and oxidation of butane, require a relatively high gas velocity in the riser to promote plug #ow, thus minimizing gas backmixing. Higher gas vel- ocities are also possible because of the short contact times needed between the gas and solids. Circulating #uidized bed combustion (CFBC) and #uid catalytic cracking (FCC) units di!er greatly in design and operating characteristics (Grace, 1990; Werther, 1994; Berruti et al., 1995). Solids circulation rates in combustors are generally less than 100 kg/m s and the gas velocity is typically 5}9 m/s, resulting in low solids hold-ups in the riser, usually less than 1% in the developed upper part. For FCC units, on the other hand, solids mass #uxes above 250 kg/m s are typical, super"cial gas velocities are as high as 20 m/s, and mean solids hold-ups often exceeds 10%. Geldart group A powders are used in FCC, while group B particles are employed in CFBC. In addition, there are signi"cant di!erences in column geometry, solids feeding devices and solids inventory. Circulating #uidized beds operating under high solids #ux and/or high solids hold-up conditions can be referred to as high density circula- ting #uidized beds (HDCFB), while those operating at low solids #ux ((200 kg/m s) and low sus- pension densities ((3% in the developed region) are low density circulating #uidized beds (LDCFB) (Zhu & Bi, 1995). Almost all reported CFB data are for LDCFB systems representative of CFB combus- tors. There is, therefore, a need to study the hydrodynam- ics of high density/high solids #ux systems to improve 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 8 3 - 3