Full Paper Effects of Draft Tubes on Particle Velocity Profiles in Spouted Beds The vertical particle velocity profiles in a full-column cylindrical conical spouted bed, with or without a draft tube, are measured using a fibre optic probe system. The profiles have different characteristics for a draft tube spouted bed (DTSB) than for a conventional spouted bed (CSB). The spout of a CSB consists of a cen- tral flow where particle velocities fit exponential distributions, and a boundary layer where particle velocities are nearly uniform. The spout of a DTSB has no boundary layer and its radial particle velocity profiles are approximately linear. The particle velocities in the spout of a DTSB increase when superficial gas velocity increases, draft tube diameter decreases, or when entrainment height decreases. A kinematic model has been used to simulate the granular flow in the annulus of a CSB and DTSB, and they are compared with the experiments. The particle velocities in the annulus of a DTSB are much lower than that of a CSB. Their radial profiles are also different with a CSB. The dependence of particle velocities in the annulus of a DTSB on superficial gas velocity, draft tube dia- meter, and entrainment height are also discussed. One concludes that the draft tube diameter and entrainment height are two key factors for the solid circulation rate of a DTSB. Keywords: Particles, Spouted beds, Tubes, Velocity profiles Received: March 5, 2006; revised: April 6, 2006; accepted: May 4, 2006 DOI: 10.1002/ceat.200600087 1 Introduction Spouted beds are used as efficient fluid-particle contactors for various physical and chemical processes involving coarse parti- cles (d p > 1 mm) [1]. In a conventional spouted bed (CSB) without an internal tube, the spout has direct contact with the annular solids over the entire height of the bed. At the same time, a continuous percolation of gas from the spout region to the annular region exists. If the operational bed height is larger than the maximum spoutable bed depth H m , the spout cannot support the bed, and a transition from spout to collapse occurs [2–3]. In addition, it is also reported that the elevated temper- ature causes a decrease in H m and the stable spout eventually disappears at temperatures above 420 °C despite the changing operating variables [4]. However, if a draft tube – which is a vertically aligned tube containing the spout – is included in the solid bed, the spout of the CSB efficiently becomes a vertical transport riser. Its H m becomes limited only by the energy of the gas stream entering the bottom of the draft tube [5, 6]. Besides overcoming the bed height limitation, a draft tube spouted bed (DTSB) has several other advantages. For example, the cases with small- sized fine particles, or at high temperature, etc., which is diffi- cult for a CSB to handle, can be used safely in a DTSB. In addi- tion, the use of a draft tube offers accurate control of gas and solid residence times and a greater design flexibility, which are valuable in some specific applications. The DTSB has been rapidly developed and used in a wide variety of industrial pro- cesses, e.g., drying [7], coal gasification [8], combustion [9], pyrolysis [10], fuel cell [11], coating [12], pharmaceuticals [13], and mixing [14]. There are quite a few studies on particle velocity profiles in a CSB by using various measuring techniques. The optic fiber probe is one of the most commonplace methods, which has been attempted by Benkrid and Caram, He et al., and Olazar et al., respectively, to measure the vertical particle velocities in both the annulus and spout of a CSB [15–17]. Some other techniques, such as the radioactive tracer technique [18], mag- netized marker particle with search coils [19], and the X-ray © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com Xiang-Long Zhao 1 Qiang Yao 1 Shui-Qing Li 1 1 Department of Thermal Engineering, Tsinghua University, Beijing, China. – Correspondence: Dr. S.-Q. Li (lishuiqing@mail.tsinghua.edu.cn), Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of ThermalEngineering, Tsinghua University, Beijing, 100084, China. 1) List of symbols at the end of the paper. Chem. Eng. Technol. 2006, 29, No. 7, 875–881 875