NOTES zyxwvutsrqp Modelling the Internal Flow Structure of Circulating Fluidized Beds zyx FRANC0 BERRUTIt and NICOLAS KALOGERAKIS Department of Chemical and Petroleum Engineering, The University of Calgary, Calgary, Alberta, Canada T2N IN4 A simple hydrodynamic model for Circulating Fluidized Beds has been developed. The mathematical model, based on the core-annulus flow structure, is shown to be able to predict the two-phase flow characteristics and it requires only two measurable steady-state parameters, namely the experimental average voidage profile along the riser, or equiva- lently the pressure distribution, and the net solids circulation rate. The model has been successfully tested using recently obtained literature data covering a variety of reactor configurations and operating conditions. Un modkle hydrodynamique simple a ktk rnis au point pour des lits fluidisis circulants. On montre que le modkle mathkmatique, bask sur la structure d’kcoulement coeur-espace annulaire, est capable de prkdire les caractkristiques d’kcoulement biphasique; en outre, il ne nCcessite que deux parambtres zyxwv B I’ktat permanent mesurables, soit le profil de dksaturation moyen expkrimental le long de la colonne, ou de mani2re kquivalente la distribution de pression, soit la vitesse de circulation des solides nette. Ce modkle a ktC test6 avec succ&s B I’aide de donnkes publikes rkcemment couvrant une variktk de configurations de rkacteurs et de conditions de fonctionnement. Keywords: circulating fluidized bed, modelling fluidization, fluidization hydrodynamics, fast fluidization. Circulating Fluidized Bed (CFB) is a gas-solid A contactor in which fine solid particles are transported vertically in a riser by a high velocity gas stream. After exiting the top of the riser, the solids are separated from the gas and recirculated to the base. The riser normally operates in the so-called “fast fluidized” regime. The hydrodynamic behavior of a fast fluidized bed falls between that of a con- ventional bubbling fluid bed and of a pneumatic conveyor (Hirsch et al., 1986). Chemical reacting systems that require high specific transfer rates, high solids throughput and thermal uniformity within the reactor are excellent candidates for the use of CFB tech- nology (Hartge et al., 1986). In particular, combustion of low- grade fossil fuels and process residues for energy production under strict environmental control represents one of the most successful applications of CFBs. Furthermore, industrial processes such zyxwvuts as low temperature adsorption (dry scrubbing), biomass and coal pyrolysis and gasification, Fisher-Tropsch synthesis and other catalytic and non-catalytic reactions have been examined for CFB applications (Reh, 1986). The commercialization of CFB processes has outpaced fun- damental research and a number of very important gaps in the understanding of the behavior of these reactors still exist. In order to develop realistic reaction models, a clear picture of the characteristics of the mephase flow zyxwvut within the CFB is required. Several hydrodynamic studies have been reported in the literature where the gas-solid flow patterns have been inves- tigated using laboratory units of various sizes. Rhodes (1986) proposed a model for a fast fluidized bed riser based on the assumption that it consists of a conven- tional fluidized bed being rapidly elutriated with a freeboard of varying particle concentration above it. Recently, Horio et al. (1988) have reported experimental data on the axial and radial solids distributions, solid velocity profiles and solids circulation rates. Rhodes et al. (1988) measured radial and axial variations in solid flux whereas Bolton and Davidson (1988) concentrated their investigations on the behavior of particles flowing close to the wall of the riser. They modelled the flow in the riser of a CFB by considering its similarity to the freeboard region of a zyxwvu ?To whom correspondence should be addressed. conventional fluidized bed. Experimental results suggest an exponential decrease of the solids flowrate at the wall moving upward along the riser. The results reported by the above investigators and by many others (Weinstein et al., 1986; Monceaux et al., 1986; Brereton et al., 1988; Bader et al., 1988; Hartge et al., 1988) clearly indicate the existence of a characteristic core-annulus type of flow structure, with very significant radial density gradients and with the boundary between core and annulus not clearly defined. Li and Kwauk (1980) and Kwauk et al. (1986) developed a mathematical model to describe the average voidage pro- file along a CFB based on the existence of particle clusters moving upward and downward at rates depending upon the particle properties and the operating variables. Their work was based on the observation of two characteristic regions in a circulating fluidized bed when operated under certain conditions: a dilute phase region towards the top of the riser and a dense-phase region towards the bottom. Between the two regions, a transition zone was identified in which the inflection point of the average axial voidage profile is located. Therefore, the resulting voidage profiles reported followed a characteristic sigmidal distribution. The location of the inflection point has been found to depend on the imposed pressure drop across the unit (Weinstein et al., 1983; Rhodes and Geldart, 1986). In a circulating fluidized bed, the imposed pressure drop can be altered by changing the solids inventory or by adjusting the solid rate-control valve. By increasing the solids inventory, the inflection point in the axial solids voidage profile moves upward until it may pass beyond the top of the riser (dense-phase operation). On the other hand, by decreasing the inventory, the inflection point moves downward until it may pass beyond the bottom of the riser and the resulting flow can be expected to be of the type of a dilute-phase transport (Li et al., 1988). Despite the extension experimental work conducted in this area, axial voidage distribution in fast fluidization is far from being well understood, in particular when related to the mul- titude of interdependent factors (Li et al., 1988) as well as the influence of geometry and system design configuration (Schnitzlein and Weinstein, 1988). 1010 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 67, DECEMBER, 1989