Modelling zyxwv A Circulating Fluidized Bed Riser Reactor With Gas-Solids Downflow At The Wall zy DAVID M. J. PUCHYR, ANIL K. MEHROTRA and LEO A. BEHIE* Department of Chemical and Petroleum Engineering, The University of Calgary, Calgary, AB T2N 1N4, Canada and NICOLAS E. KALOGERAKIS Department of Chemical Engineering, State University of New York, Buffalo, NY 1422604200, U.S.A. A predictive model was developed for the fully developed zone of a circulating fluidized bed (CFB) riser reactor oper- ating in the fast fluidization regime that overcomes limitations of existing models. The model accounts for the upward flow of gas and solids in the core and downward flow of the two phases in the annulus. Additionally, a numerical solu- tion methodology for the simulation of a riser reactor employing the hydrodynamic model was devised. A simulation was performed using the fast, main Claw reaction to demonstrate the effects of backmixing in the fast fluidization regime. It was found that the molar flow rates of the reactants leaving a fast fluidized CFB riser reactor were signifi- cantly higher than those leaving an identical reactor operating in the pneumatic transport regime. Un modele predictif a ete mis au point pour la zone pleinement developpee d’un reacteur a colonne montante et a lit fluidise circulant fonctionnant dans un regime de fluidisation rapide qui depasse les limites des modeles existants. Le modele tient compte de I’ecoulement ascendant du gaz et des solides dans l’ecoulement au coeur et I’ecoulement ascen- dant des deux phases dans I’espace annulaire. De plus, on a conqu une methodologie de resolution numerique pour la simulation d’un reacteur a colonne montante employant le modele hydrodynamique. On a effectue une simulation a I’aide de la reaction principale de Claus zyxwvutsr - qui est rapide - afin de demontrer les effets du retromelange dans le regime de fluidisation rapide. Les debits molaires des kactifs quittant un reacteur a colonne montante et a lit fluidise circulant rapide se sont averees plus grands que dans le cas d’un reacteur fonctionnant en regime de transport pneumatique. Keywords: circulating fluidized bed, predictive model, core-annulus. gassolids downflow. ix distinct hydrodynamic regimes exist when a gas is S passed upward through a bed of Group B particles (Grace, 1986). Varying degrees of solids entrainment occur when the gas superficial velocity is greater than the incipient fluidiza- tion velocity. If the entrained solids are recirculated back to the bottom of the bed it can be classified as a “circulating flu- idized bed” (CFB) (Bermti et al., 1995). Throughout this study we consider the two cases in which a CFB is operating in either the fast fluidized or pneumatic transport regimes. CIRCULATING FLUIDIZED BED RISER REACTORS An advantage of using a conventional fluidized bed over a fixed bed reactor is that the excellent axial mixing of solids caused by the bubbles in a fluidized bed allows for near isothermal operation of the reactor when cooling tubes are employed. But, the bubbles are also the vehicle for gas by- passing, which reduces the gas-solids contacting, thereby reducing conversion. A CFB, which also allows for near isothermal operation via the relatively large heat capacity of the recirculated solids, provides outstanding gassolids con- tacting because of the slip factor zyxwvuts (w) between the two phases. The slip factor is defined as “the ratio of interstitial gas velocity over the solids velocity”. In the fully developed zone of a tall riser, the overall slip is approximately 2 (Bermti et al., 1995; Patience et al., 1992; Matsen, 1976). Therefore, the problem of gas by-passing present in a con- ventional fluidized bed is essentially eliminated in a CFB. *Author zyxwvutsrqpon to whom correspondence should be addressed. E-mail address: behie@acs.ucalgary.ca Contractor et al. (1994) clearly demonstrated that the pneumatic transport regime can justifiably be modelled as a plug flow reactor (PFR) as a good first approximation. The applicability of the PFR model is explained by the cocurrent flow of gas and solids and the large slip factor in a CFB oper- ating in this regime. Their experiments, performed on a large scale pilot plant used to oxidize n-butane to maleic anhy- dride, yielded gas phase residence time distributions which showed very little gas dispersion at various axial sampling locations. The lack of axial dispersion indicates near plug flow conditions. The use of a pre-reactor particle accelerator, a proprietary device, resulted in approximately uniform sus- pension densities for very large solids mass fluxes ranging from 290 to 690 kg/m2.s. The uniform suspension densities indicate that the acceleration zone in the reactor, the region at the base of the reactor where the solids are accelerated to their steady-state velocity, which is characterized by chaot- ic mixing of the two phases, can be effectively eliminated. In order to model a CFB as a plug flow reactor, the bed density must be calculated so that the reaction rate can be determined. The bed density is a function of voidage which, in turn, is a hnction of operating conditions and particle characteristics. Knowing the overall slip factor, the solids mass flux (GJ, particle density (ps), and gas superficial velocity zyxwv (ugo), the average “plug flow” voidage in the reac- tor is heuristically determined to be: THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 75, APRIL, 1997 317