The 7th World Congress on Particle Technology (WCPT7) Operation diagram of Circulating Fluidized Beds (CFBs)

CFBs are widely used in the chemical, mineral, environmental and energy process industries. Several authors stressed the need for a clear identification of the different operation regimes in th e riser of a CFB, to ensure a better comprehension of the hydrodynamic context, and thus better define the operation and design parameters.. First approaches to develop a "work map" of the riser operation, were presented by e.g. Grace[1], Yerushalmi and Avidan[2], Bai et a l.[3]. It was further developed by Chan et al.[4] and Mahmoudi et al.[5,6] for both Geldart A- and B-type powders, in terms of the operating gas velocity (U) and the solids circulatio n flux (G), which jointly delineate different regimes, called respectiv ely Dilute Riser Flow (DRF), Core-Annulus Flow (CAF) (possibly with a bottom Turbulent Fluidized Bed, TFBB), an d Dense Riser Upflow (DRU). For a given powder and its associated transport velocity, UTR, the combination of U and G will determine the flow regime encountered. Experiments in CFB r isers of 0.05 (2.5 m high), 0.1 and 0.15 m I.D. (both 6.5 m high), have demonstrated that common riser operations can be hampered by a specific (U,G) range where choking occurs. Angular sand, rounded sand, and spent FCC (all A-type powders) w ere used as bed material. Gas velocities were varied between 2 and 10 m/s, for solids circulation fluxes between 10 and 260 kg/m²s. Choking is understood as the phenomenon where a small change in gas or solids flow rate prompts a large change in the pr essure drop and/or solids holdup during the gas-solid flow: the stable riser upflow regime is no longer maintained when Gvalues exceed a certain limit for a given gas velocity. Experimental resul ts were empirically correlated, and proved to be about 30 % lower than predicted by the correlation of Bi and Fan[7], but largely exceeding other predictions. Introducing the findings in to the available operation diagram [5,6], adds a region where stable riser operation is impossible. The adapted diagram enables CFB d esigners to better delineate the operating characteristics.