(457e) Numerical Study of the Segregation of Binary Mixture In Dense Gas-Solid Fluidized Bed

N’dri A. Konan, E. David Huckaby and Thomas J. O’Brien

National Energy Technology Laboratory, Morgantown, WV 26505, USA

n’dri.konan@or.netl.doe.gov, E.David.Huckaby@netl.doe.gov

Keywords: Polydispersion, kinetic theory, segregation, drag law, restitution coefficient

Powders involved in the engineering flow systems consist of particle mixtures of various densities and especially of a wide distribution of the size of the particles. Depending on the flow conditions, the behavior of the larger/heavier particles assimilates to a migration towards the lower region of the system, while the upper region populates of the smaller/lighter particles. That typically results in local (or spatial) concentration of the species or type of the particles significantly higher than the other particle phases within the different regions of the system, in the situation of the severe segregation. 

Successful operation of chemical looping using solid fuels (e.g. coal) is dependent on designing a fuel reactor in which the oxygen carrier is returned to the air reactor with minimal unburned fuel and ash. One proposed approach (e.g. Berguerand & Lyngfelt, 2008 and Shen et al., 2009) is to design the oxygen carrier, fuel reactor and fuel grind such that the denser and larger oxygen carrier settles to bottom of the bed, the solid fuel moves toward the upper portion of the bed and eventual is entrained in the exhaust after the fuel is consumed. Accurate simulation segregation is critical for predicting the operation of a reactor and associated chemical looping system based on this design.

The origin of such a segregation phenomenon, although extensively investigated numerically as well as in the experiments in the frame of the dense gas-particle fluidized beds, is not yet completely understood. Numerically, the pertinence of the predictions is mainly attributed to the accurate description of:

• ·      the exchange of the momentum between the gas and the particles, i.e. the drag force, which acts differently on the inclusions of a cloud of polydisperse or monodisperse particles (in size), at both the same porosity and particle Reynolds number (see e.g. Beetstra et al., 2007; Leboreiro et al., 2008, Sarkar et al., 2009,…);
• ·      the energy dissipation rate due to the inelastic collisions between the particles via the restitution coefficient. Indeed, the local decrease of the particle kinetic energy that can be expected because of the higher dissipation will cause the accumulation of the particles at those locations. That consistently gives rise to denser regions within the bed and thus the emergence of the regions where the volume fractions of the particles are rather small, i.e. of the bubbles, whose dynamics dominates the hydrodynamics of the bed (see e.g. Goldschmidt et al., 2001, Dahl & Hrenya, 2005, …);
• ·      the momentum transfer between the particle phases through contact. Following Gera et al. (2004), the solid-solid momentum transfer term consists of a collisional term derived from kinetic theory and an additional term to account for the hindrance effect.  This term is to account for the fact that the different particle phases will begin to move as a single phase while approaching the packing limit.  Although the model successfully predicted the segregation rate (Gera et al., 2004; Fan & Fox, 2008, Azizi et. al. 2010), the modeling of the hindrance effect requires of the case to simulate for setting of the coefficient relevant to that effect. However, other authors have had similar success using kinetic theory models without an explicit model for non-collisional solid-solid momentum transfer (see e.g. Gourdel et al., 1999; Lathouwers & Bellan, 2001; Iddir et al., 2005, Garzo et al., 2007, …).  It should be noted that these studies employ radial distribution functions which diverge at the packing limit in contrast to the studies using the hinderance model which use plastic flow models and non-divergent radial distribution functions.

The present paper investigates the segregation occurring within the dense laboratory-scale fluidized bed of binary mixture of Geldart D-type particles studied by Goldschmidt et al. (2003). The hydrodynamics of the bed (considered as 2D) is modeled using the n-fluid Eulerian approach (Simonin, 1996). The polydisperse model relies upon the relative velocity between the particle phases and their kinetic energies as the main mechanisms that drive the collisions between phases (Gourdel et al., 1999). Two polydisperse gas-particle drag models are assessed: another combination of the drag models of Ergun and Wen & Yu based on their minimum for porosity smaller than 70% (Gobin et al., 2003) and the model of Beestrat et al. corrected by an effect of size polydispersity (Beestrat et al., 2007). Different fluidization conditions are simulated (ranging from smaller, slightly above and sufficiently larger than the minimum fluidization velocity of the largest particles) to investigate the influence of drag law on the segregation in terms of the fluidization velocity. Various compositions of the mixture and the influence of the restitution coefficients are also considered. The predictions of the segregation rates are compared against the experiments and discussed.

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