Axial hydrodynamic studies in a gas–liquid–solid circulating fluidized bed riser S.A. Razzak, S. Barghi, J.-X. Zhu ⁎ Department of Chemical and Biochemical Engineering, University of Western Ontario, London, ON, Canada N6A 5B9 abstract article info Article history: Received 13 February 2009 Received in revised form 30 April 2009 Accepted 23 May 2009 Available online 28 May 2009 Keywords: Three-phase circulating fluidized bed Electrical resistance tomography Fiber optics Phase holdups Axial distribution of phase holdups was studied in the riser of a gas–liquid–solid circulating fluidized bed (GLSCFB). The effects of gas and liquid superficial velocities as well as solids circulation rate on radial distribution of phase holdups at different axial locations were investigated. Electrical resistance tomography (ERT) and optical fiber probe were employed online in the experiments for a precise determination of phase holdups. An empirical model was developed for the determination of gas bubbles in analysis of data obtained by fiber optic sensor. Gas holdup was higher at the central region of the riser and increased axially due to coalescence of small bubbles and decrease of hydrostatic pressure at higher levels in the riser. This led to an increase in solids holdup in regions close to the wall which was slightly higher than the solids holdup at the wall. Both solids and liquid holdups were lower in the central region and increased radially towards the wall. Gas holdup decreased with increasing solids circulation rate but opposite trend was observed for solids holdup. Solids circulation rate had negligible effect on liquid holdup at lower axial locations compared to top of the riser. Cross-sectional average of solids, gas and liquid holdups did not change significantly at higher liquid superficial velocities. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Flow distribution in different axial locations is an important aspect of study in gas–liquid–solid circulating fluidized (GLSCFB) beds and gas–liquid–solid three phase fluidized beds due to its wide range of applications in chemical, biochemical, petrochemical and environ- mental process [1,2]. The development of GLSCFB would benefit wastewater treatment [3–5], desulphurization of petroleum products and other biochemical industries [6,7] where solids particles are used as catalyst and carrier. Most of the studies in three phase reactors focused on conventional system [2]. Conventional fluidized beds are limited in operating parameters such as superficial gas, liquid velocities, solids density and size etc. Three-phase circulating fluidized beds do not suffer from such constraints and can operate in wide ranges of operating parameters and also enhancing mass and heat transfer efficiency. GLSCFB system is quite new and little work has been done in the study of hydrodynamics in tall risers, which can lead to more efficient operation. Information of flow structure helps in the design and operation of the GLSCFB reactors. Different measurement methods such as direct sampling, optical fiber probe, electrical conductive probe, process tomography, ultrasound etc. are employed in the study of hydrodynamics [8]. Liang et al. [9] studied the macroscopic axial flow behavior in GLSCFB riser using conductive probe. They found an S-shaped average solids distribution in different axial locations under different superficial liquid velocities ranging between 6.4 and 9.5 cm/s. Uchida et al. [10] developed a technique to measure solids holdup using ultrasound technique. Later Vatankul et al. [11] measured cross-sectional average gas and solid holdup in two different axial locations using similar techniques. Process tomography technique has experienced a significant interest in the study of multiphase flow systems due to its non-intrusive nature. The only limitation of tomography techniques is its application to conductive phase. However, such a technique is not available for the study of three phase systems in real time [12]. George et al. [13] combined measurement technique with electrical impedance tomography (EIT) and gamma-densitometry tomography (GDT) to measure distribution of phases in a vertical flow system. Razzak et al. [15] measured cross- sectional average phase holdup using combination of electrical resistance tomography (ERT) and pressure transducers (PT). Razzak et al. [14,15] developed a combined system of ERT and optical fiber probe to measure local radial distribution holdups in a GLSCFB riser. In this study axial flow profile measured in four different axial locations (H =1.01 m, 2.02 m, 3.03 m and 3.82 m) in the riser of GLSCFB system. A newly developed technique combined with ERT and optical fiber probe was used to distinguish phases in 7 dimensionless radial positions (r/R =0, 0.2034, 0.492, 0.6396, 0.7615, 0.8614, 0.9514). In these experiments water was used as conductive and continuous phase, air as gas phase and glass beads with 500 μm ranges as solids phase. Average and local phase holdups were obtained using this measurement technique. 2. Experimental setup GLSCFB systems shown in a schematic diagram in Fig. 1 consists of two main sections, riser and downer, made of Plexiglas. Total setup is Powder Technology 199 (2010) 77–86 ⁎ Corresponding author. Tel.: +1 519 661 3807; fax: +1 519 661 3498. E-mail address: jzhu@uwo.ca (J.-X. Zhu). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.05.014 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec