Exit effect on hydrodynamics of the internal circulating fluidized bed riser R. Mabrouk ⁎ , J. Chaouki, C. Guy Department of Chemical Engineering, Ecole Polytechnique de Montreal, C.P. 6078, succ. Centre-Ville, Montreal, Quebec, Canada H3C 3A7 Received 28 April 2006; received in revised form 16 February 2007; accepted 5 July 2007 Available online 13 July 2007 Abstract This is the first time an extensive investigation has been carried out regarding the effects of riser exit geometry on pressure drop and solid behaviour inside the Internal Circulating Fluidized Bed (ICFB) riser, using different riser exit geometries at several operating conditions. The Radioactive Particle-Tracking (RPT) technique was used for solid concentration measurements and solid residence time distribution at the exit zone. Experiments were conducted using Geldart B particles, in the gas superficial velocity range of 4 to 10 m/s. Axial solid hold-up, solid residence time distribution in the exit zone, and the reflux ratio factor k m , (defined earlier by [E.H. Van der Meer, R.B. Thorpe, J.F. Davidson, Flow patterns in the square cross-section riser of a circulating fluidized bed and the effect of riser exit design, Chem. Eng. Sc. 55 (19) (2000) 4079–4099]), were the main criteria used to investigate the impact of gas–solid separator devices implemented at the ICFB riser exit. Solid residence time distribution results and axial solid hold-up profiles provided clear evidence that the separator device at the riser exit strongly influences the hydrodynamic structure of the ICFB riser. The V-shaped riser exit geometry was found to be the optimum of all the configurations studied. © 2007 Elsevier B.V. All rights reserved. Keywords: Gas solid separator; Exit geometry; Internal circulating fluidized bed 1. Introduction Hydrodynamic characterization of two-phase flow is extremely complex. Axial voidage distribution is found to be dependent not only on gas velocity, solid flux and solid properties, but also on the configuration and design of the system. Gas–solid separation, for instance, is one of the most important and difficult tasks in circulating systems affecting the exit region hydrodynamic structure and solid residence time. This subsequently affects heat transfer efficiency at high temperature transformations [2]. However, the hydrodynamics of circulating systems can be better understood once the effects of the exit zone are taken into account. Many studies have been done regarding the efficiency (separation quality and separation time) of gas–solid separation, but, unfortunately, the effects of the separator device and its design on the behaviour of the flow at the exit region before entering the separator are usually omitted or neglected. Therefore, studying the impact of the separator device on riser hydrodynamics is necessary for a better understanding of solid distribution along the riser. In Circulating Fluidized Beds (CFBs), numerous studies have examined the influence of exit geometry. In 1992 D.-R. Bai et al. [3] found that an abrupt exit affects the last few meters of the riser (1 to 2 m in an industrial CFB unit). The same observation was later reported by Zheng and Zhang [4], while Jin et al. [5] and Grace [6] among others concluded that the exit design may affect flow behaviour in zones far from the exit region, (this might be the whole riser length in certain conditions.). Senior and Brereton [7] also examined the effect of different geometries on solid concentration in the CFB riser. He introduced a new parameter, R f (Reflection coefficient), defined as the ratio of solid flows descending and ascending. Senior showed that the solid concentration in the riser was more significant with an abrupt exit, than with an elbow exit. Using the same parameter R f , Mickal et al. [8] explained the effect of “T” exit geometry on hydrodynamic structure. “T” exit geometry was also studied by Juray De Wilde et al. [9] using computational fluid dynamics. They found that the opening area at the exit region directly affected the behaviour of the solid in the riser. Van der Meer (2000) introduced another parameter, the reflux ratio factor k m , defined as the ratio of downward solid flow to external circulating solid flow. Using the parameter k m , Available online at www.sciencedirect.com Powder Technology 182 (2008) 406 – 414 www.elsevier.com/locate/powtec ⁎ Corresponding author. Fax: +1 514 340 4159. E-mail address: rachid.mabrouk@polymtl.ca (R. Mabrouk). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.07.008