Surface-to-Suspension Heat Transfer Model in Lean Gas-Solid Freeboard Flow zyx dc ML ZHAB H. FARAG” Chemical Engineering Department, University zyxwvu DCBA of New Hampshire, 255 Kingsbury Hall, Durham, NH zy KJI 03824-3591 and KUN-YUNG TSAI Refining zyxwvutsrq EDCBA & Manufacturing Research Center, Chinese Petroleum Corporation, Chia-Yi, Taiwan, Rep of China Heat transfer in dense fluidized beds have been extensively studied. However, there is not much detailed informa about the mechanism of surface-to-suspension heat transfer in the freeboard region. In the present work, a newly designed heating plate was used to measure the plate-surface-to-particle-suspension heat transfer coefficients in the freebo The experimental unit consisted of a 30 cm i.d. fluidized bed reactor packed with fluidized catalytic particles of mean particle size 90 pm. Three types of plate orientations were used to test directional effects of surface on heat transfer rate. Height of the freeboard was 171 cm, and the superficial gas velocity was varied from 0.28 to zyx IHG 0.64 m/s. Local solids concentrations in the freeboard were also obtained by a nozzle-type sampling probe. Data on axial distribution of solids concentration were used to find out the solids kinematics in the freeboard region. Finally, a surface-to-sus heat transfer model was developed to elucidate the surface to particle heat transfer mechanism in this lean phase system. The model is based on the transient gas-convective heating of single particles when sliding over the heating plate a the assumption of instantaneous attachment-detachment equilibrium between particles and the plate surface. On a mene une Ctude approfondie sur le transfert de chaleur en lits fluidisCs denses. Peu de donnkes sont toutefois disponibles concernant le mCcanisme du transfert de chaleur de la surface vers la suspension dans la zone libre. D le present travail, un plateau de chauffage de conception nouvelle est utilist pour mesurer les coefficients de tran de chaleur de la surface des plateaux vers la suspension des particules dans la zone libre. Le dispositif expkrimen comprend un rCacteur lit fluidis6 de 30 cm de diamktre intkrieur garni de particules catalytiques fluidisCes d’une taille moyenne de 90 pm. Trois types d’orientation des plateaux ont CtC utilisCs pour tester les effets directionnels de la sur- face sur le taux de transfert de chaleur. La hauteur de la zone libre est de 171 cm et on a fait varier la vitesse de gaz superficielle de 0,28 B 0,64 m/s. Les concentrations locales des solides dans la zone libre ont Cgalement CtC obtenues au moyen d’une sonde d’kchantillonnage h tuykre. On a utilisC la distribution axiale de la concentration des solides afin de trouver la cinkmatique des solides dans la zone libre. Enfin, un modtle de transfert de chaleur de la surfa vers la suspension a CtC mis au point pour expliquer le mCcanisme de transfert de chaleur de la surface vers les par- ticules dans ce systkme a phases pauvres. Ce modtle s’appuie sur deux CICments: le chauffage convectif transitoire de particules uniques lorsque celles-ci glissent au-dessus du plateau de chauffage et l’hypothkse d’un Cquilibre ins tan6 de type attachement-dktachement entre les particules et la surface des plateaux. Keywords: fluidization, heat transfer, freeboard, mechanistic models. ne of the most attractive features of a fluidized bed is 0 the favorable rate of heat transfer to or from immersed heat transfer surfaces. The heat transfer coefficient between gas-fluidjzed bed and an immersed tube or containing wall is generally of order 300-600 W/m2 . K. In the freeboard region of a fluidized bed reactor, the solids concentration is much lower than that in the dense bed. Nonetheless, heat transfer surfaces in the freeboard region can be used to extract significant heat from the gadsolid mixture. For example, in the freeboard of a fluidized bed coal combustor, it has been estimated that 1/2 to 1/3 of the in-bed heat transfer rate may be obtained (Adibhatla and Boggs, 1985). The total heat transfer in the freeboard is determined by energy balance and the desired bed temperature. The tube surface tempera- ture is set by the operating requirements for process heating. The temperature of the gadsolid mixture is set by the unit operating conditions. Clearly, accurate design of the free- board heat transfer surface and freeboard volume requires accurate prediction of the heat transfer coefficient. Experimental studies with fluidized beds tend to be difficult, expensive, and time consuming. Therefore, *To whom correspondence should be addressed mathematical models and simulation studies are particula attractive to investigate the heat transfer phenomena in a gas- solid fluidized bed system. Besides, such a model is essen tial for the explanation of the observed heattransfer phenomena and the prediction of the heat transfer rate from the gas-solids hydrodynamics in the freeboard. A numbe of correlations and models have been reported to describe the variables affecting the surface-suspension heat transf rate in dense fluidized beds and CFB’s (e.g., Mickley and Fairbanks, 1955; Fraley et al., 1983; Subbarao and Basu, 1986; Martin, 1981; Zabrodsky, 1966; Botterill, 1975; Zie- gler et al., 1964). However, the correlations are rather limited for dilute phases such as those in the freeboard (Hashimoto et al., 1990; Biyikli and Chen, 1982; Biyikli et al., 1987) and in solids-gas transfer lines (Wen and Miller, 1961). In this study, a suspension-to-surface heat transfer mod was developed to elucidate the surface to particle heat tr mechanism in the freeboard of a fluidized bed. The model is based on the transient gas-convective heating of single par- ticle when sliding over the heated plate and the assumpti of instantaneous attachment-detachment equilibrium betw particles and the plate surface. The data obtained from the 514 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 71. AUGUST, fYY3