Radial Distributions of Phase Holdups and Phase Propagation Velocities in a Three-Phase Gas-Liquid-Solid Fluidized Bed (GLSCFB) Riser S. A. Razzak, J.-X. Zhu,* and S. Barghi Department of Chemical and Biochemical Engineering, UniVersity of Western Ontario, London, ON, Canada N6A 5B9 Electrical resistance tomography (ERT) and fiber optic were applied to investigate phase holdups and phase propagation velocities in a gas-liquid-solid circulating fluidized bed (GLSCFB). Since ERT is applicable only to conductive phase(s), e.g. the liquid phase in this study, a fiber optic probe was employed simultaneously to quantify all three phases. Saline water was used as the conductive and continuous phase. Glass beads and lava rocks constitute the solid phase and air as the gas phase. Glass beads were transparent and spherical in shape; however, lava rock particles were irregular in shape and opaque, which affected the signals obtained from the optical fiber probe. An empirical model was developed to measure the gas holdup using optical fiber probe data. Gas holdup was higher in the central region and decreased radially, while opposite trend was observed with solid holdup due to the drag forces imposed on solid particles by the gas and liquid flow in the riser. By applying cross-correlation between the data obtained at two different levels in the riser, nonconductive phase propagation velocity was obtained. The propagation velocity was higher in the central region compared to the wall region and increased with increasing liquid superficial velocity. 1. Introduction Good mixing and efficient heat and mass transfer have made fluidized bed reactors a unique choice in many processes in chemical, petrochemical, and biochemical industries. Recently, the liquid-solid circulating fluidized bed (LSCFB) and gas-liquid-solid circulating fluidized bed (GLSCFB) reactors have received growing interest on wastewater treatment, des- ulphurization of petroleum products, and in biochemical reac- tions. 1 Most of the studies on gas-liquid-solid fluidization systems have mainly focused on conventional expanded bed regime in the past decades. 2 Conventional fluidized beds also suffer from limitations such as liquid and gas velocities, solid particles size and density, etc. In GLSCFB, solid particles are circulated between the riser and the downer at higher velocities compared to conventional fluidized beds, which leads to formation of smaller bubbles and a better contact between phases and reduced backmixing. GLSCFB also offers great flexibility in terms of solid particles or catalyst regeneration in the downer. In spite of substantial work, the hydrodynamics of GLSCFB is not completely understood yet. The radial nonuniformity of phase holdup in liquid-solid circulating fluidized bed (LSCFB), using conductivity probe, was reported by Liang et al. 4 Zheng et al. 3 confirmed the radial nonuniformity using optical fiber probe. The solids holdup increased radially from the center to the wall. It was claimed that radial flow structure is affected significantly by operating conditions and particle properties. Zheng et al. 3 showed radial distribution of the solids holdup under a wide range of operating conditions and tested the effect of particle density on the flow structure. Radial distribution of local liquid velocity was measured using a dual conductivity probe, with two probes, 20 mm apart, placed in the riser and a pulse injection of saturated NaCl electrolyte solution below the probes. 5 Different methods have been employed in the study of hydrodynamics such as direct sampling, optical fiber, electric conductive probe, process tomography, static-pressure, ultra- sound, and iso-kinetic separation. Phase holdup as a main parameter was of major concern. Lee et al., 6 de Lasa et al., 7 and Yong et al. 8 used fiber optic probe to measure the gas holdup directly in a three-phase system. A single-core silica optical fiber of 400 μm U-shape probe was employed to detect gas bubbles. Wang et al. 9 studied bubble behavior in a fluidized bed using 62.5 μm diameter optical fiber probe. Uchida et al. 10 developed a new technique for solids holdup measurement in a three phase fluidized bed using ultrasonic sound waves. Later Vatankul et al. 5 used similar concept for flow detection. This technique is based on the change in speed and amplitude of ultrasonic wave incident on a surface. Similar to light beams, when ultrasonic waves strike at the interface between two media, they may be partially/totally reflected, scattered or transmitted. Liang et al. 11,12 used a horizontal probe for the measurement of solid holdup in the bubble wake and the emulsion phase. They also measured the solids holdup from the same signals using the conductivity of the pure liquid as the baseline. Process tomography is an area which has experienced a significant growth over the last ten years in the study of multiphase flow due to its nonintrusive technique. 13 However, there are no imaging techniques available for the study of three phase systems in real time. 14 George et al. 15 developed a combined system of electrical impedance tomography (EIT) and gamma-densitometry tomography (GDT) to measure distribution of phases in a vertical three-phase flow system simultaneously. Razzak et al. 16 measured phase holdups and velocities in a GLSCFB system by combining ERT and pressure transducers (PT). In this study, electrical resistance tomography (ERT), a newly developed method for the phase holdups measurement, is presented. However, ERT cannot measure phase holdups for all the phases, therefore an optical fiber probe and pressure transducers are used simultaneously to measure phase holdups for all three phases. In the experiments, water was used as the liquid (continuous and conductive) phase, air as the gas phase, and glass beads and lava rocks with 500 μm range as the solids phase. Combination of these measurement techniques provided * Corresponding author. Tel.: 1-519-661-3807. Fax: 1-519-661-3498. E-mail: jzhu@uwo.ca. Ind. Eng. Chem. Res. 2009, 48, 281–289 281 10.1021/ie800299w CCC: $40.75  2009 American Chemical Society Published on Web 07/08/2008