Any vessel in which solids are transported upward by a gas stream and then recycled to the bottom may be classified as a Circulating Fluidized Bed (CFB). We describe possible CFB operating regimes in the context of this broad classification and highlight commercial processes that employ CFB technology and potential applications. Process design and development require a fundamental understanding of gas and solids hydrodynamicssolids hold-up, mixing and velocity distribution. We discuss techniques used to measure solids mass flux, which is a critical parameter for both design and control. In the last decade, significant research efforts have been devoted to new experimental techniques to measure both gas and solids spatial and temporal distribution. We list these techniques and detail the different modelling approaches that have emerged based on the new data. Characterization of the data is still incomplete and the available models require further refinement to reliably predict the effect of scale, operating conditions and particle characteristics on hydrodynamics. N'importe quel contenant dans lequel des particules sont emportks dans la direction ascendante par un courant gazeux puis recyclkes par le bas peut Stre class6 c o m e un Lit Fluidisk Circulant (LFC). Nous dkrivons les rkgimes possibles des op6rations dans ces LFC en tenant compte de cette dkfinition ktendue et les prockddcs industriels existants et potentiels qui utilisent cette technologie des LFC. La conception et le dkveloppement des proctklks exigent une comprkhension fondamentale des hydrodynamics du gaz et des particulesrktention de solides, degrk de mklange et distribution des vitesses. Nous discutons les diffkrentes techniques utiliskes pour mesurer le taux de circulation de solide qui est un paramttre critique pour aussi bien le design que le contr6le. Durant le dernitres mks, un effort significatif'de recherche a kt6 consenti au dkveloppement de nouvelles techniques exp6rimentales pour mesurer les distributions temporelle et spatiale des diffkrentes caractkristiques du gaz et du solide. Nous reportons ces techniques et nous dktaillons le diffkrentes approches de modklisation qui ont kt6 introduites en se basant sur ces nouvelles donnks.La caradrisation de ces donnks est encore incompltte et les modkles existants doivent Stre encore raflids afin de pddire ad6quatement les effets d'khelle, des conditions @ratoires et des mactkristiques des particules sur I'hydrodynamique.

The flow behavior of the solids phase in the fully developed region of a laboratory-scale circulating fluidized bed riser was studied using an assembly of sixteen NaI detectors to determine the position of a 500 pm radioactive particle, 100 timeds. The particle location was inferred from the number of y-rays recorded by the assembly. The knowledge of the instantaneous positions enabled the determination of the instantaneous and mean velocity fields. Tests were conducted in a 0.082 m diameter, 7 m tall riser using 150 p silica sand particles. Data were obtained at a gas superficial velocity of 4 m/s and solids mass fluxes from 23 to 75 kg/m2.s. Radial profiles of axial particle velocity showed that the solids velocity decreased with increasing solids circulation rates. Correspondingly, turbulent particle velocities and solids dispersion coefficient in the longitudinal direction were found to decrease as the solids circulation rate increased. The crosssectional area where, on average, solids downflow took place, increased with increasing solids circulation rate.On a etudie le comportement en koulement de la phase des solides dans la region pleinement developpee d'une colonne montante a lit fluidise circulant a I'echelle de laboratoire en utilisant un assemblage de seize detecteurs NaI afin de determiner la position d'une particule radioactive de 500 pm, a une frhuence de 100 foids. La localisation de la particule a ete estimee a partir du nombre de rayons y et la connaissance des positions instantanees a permis de determiner les champs de vitesse instantane et moyen. Nous avons utilid une colonne de laboratoire montante de 0,082 m de diametre et 7 m de hauteur avec des particules de sable de d i c e de 150 p. Des donnees ont ete obtenues a une vitesse de gaz superficielle de 4 m/s et a des taux de circulation de solides de 23 a 75 kg/m2.s. Des profils radiaux de la vitesse axiale des particules montrent que la vitesse des solides diminue avec I'augmentation des taux de circulation des solides. De la mBme maniere, on a trouve que les vitesses de particules turbulentes et le coefficient de dispersion des solides dans la direction longitudinale diminuaient avec I'augmentation du taux de circulation des solides. La region transversale a la paroi oh, en moyenne, se produit I'ecoulement de solides descendant, augmente avec la circulation des solides. Quantification of the flow patterns and backmixing is essential for proper design and scale-up of CFB risers. Existing models often suffer from a lack of pertinent experimental data for model validation. The gadsolid flow in the CFB riser is characterized by a strong heterogeneity. Yet, experimental investigations have demonstrated that the solids distribution is nonhomogeneous, in both axial and radial directions in the CFB riser. For example, the nonhomogeneous character of the solids distribution in the axial direction was attributed to the acceleration of particles entering with low velocities at the bottom of the riser. Harris et al. (1994) and Grace (1996) have also sh...

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On the basis of the benchmark modeling exercise at Fluidization VIII, predicting riser hydrodynamics continues to be more of an art than a science. Ten different hydrodynamic models were compared with a set of experimental data that covered a wide range of operating conditions and showed reasonable to poor overall agreement. Herein, we describe the model that gave the best overall agreement with the experimental data. Density is calculated by a correlation based on slip factor, and the radial voidage profile depends solely on the cross-sectional average void fraction. Both the gas and velocity profile follows a power law type expression; the gas velocity at the wall is zero. The model predictions agree well with experiments conducted with sand but not as well as those conducted with fluidized catalytic cracking catalyst.