Chemical Engineering Science 59 (2004) 167 – 186 www.elsevier.com/locate/ces CFD simulation of dilute phase gas–solid riser reactors: Part I—a new solution method and ow model validation A.K. Das a , J. De Wilde a , G.J. Heynderickx a , G.B. Marin a ; * , J. Vierendeels b , E. Dick b a Laboratorium voor Petrochemische Techniek, Ghent University, Krijgslaan 281, S5, 9000 Gent, Belgium b Department of Flow, Heat and Combustion Mechanics, Ghent University, St. Pietersnieuwstraat 41, 9000 Gent, Belgium Abstract A three-dimensional simulation of a dilute phase riser reactor (solid mass ux: 2 kg m -2 s -1 ) is performed using a novel density based solution algorithm. The model equations consisting of continuity, momentum, energy and species balances for both phases, are formulated following the Eulerian–Eulerian approach. The kinetic theory of granular ow is applied. The gas phase turbulence is accounted for via a k – model. An extra transport equation describes the correlation between the gas and solid phase uctuating motion. The solution algorithm allows a simultaneous integration of all the model equations in contrast to the sequential multi-loop solution in the conventional pressure based algorithms, used so far in riser simulations. The simulations show an unsteady behaviour of the ow, but a core-annulus ow pattern emerges on a time-averaged basis. The abrupt nature of the T type outlets causes a signicant recirculation of gas and solid from the top of the riser. The ow near the outlets is highly non-symmetric and has a three-dimensional character. A signicant decrease of the gas phase turbulence and particle granular temperature across the riser length is attributed to the presence of small particles, which is qualitatively consistent with the experimental data from literature. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Multiphase ow; Multiphase reactors; Fluidization; Hydrodynamics; Simulation; Turbulence 1. Introduction Gas–solid riser reactors are used in many important pro- cesses e.g. uid catalytic cracking (FCC), uidised com- bustion of coal, adsorption of SO 2 –NO x from ue gases, etc. The main interest for studying the detailed hydrody- namics of gas–solid ow in risers is the accurate prediction of their performance. A riser usually operates in the tur- bulent regime, with uctuations in velocity, pressure and concentration elds at the corresponding length and time scales. The presence of the solid phase also results in uc- tuations at other length and time scales. For example, the macro-scale ow non-uniformities occur at a scale com- parable to the riser diameter/height. Core-annulus ow (Bader et al., 1988) and density inversion waves (Dry and Christensen, 1988) are examples of such macroscale phe- nomena. Such complex ow patterns are essentially 3D and ∗ Corresponding author. Tel.: +32-9-264-45-16; fax: +32-9-264-49-99. E-mail addresses: guy.marin@ugent.be, guy.marin@rug.ac.be (G.B. Marin). have a signicant eect on the ow and reaction vari- ables. The complete 3D simulation of riser reactors is thus desirable. In this work, a three-dimensional simulation is performed for a riser used in the simultaneous adsorption of SO 2 –NO x from ue gases over a Na–-Al 2 O 3 sorbent. Two main fea- tures of this riser are the very low solid ux, 2 kg m -2 s -1 , and a particle diameter of 60 m. A gas–solid uctuating motion correlation model (Simonin, 1996) and the inter- phase transfer of the uctuating kinetic energy between the two phases, are included in the simulation to account for the attenuation of the gas phase turbulence by such small parti- cles. The simulation is based on a new solution algorithm, discussed along with its convergence behaviour and the simulation results. It is the aim of this work to demonstrate the applicability of the new density based solution methods (De Wilde et al., 2002) for both the transient and steady simulation of gas–solid ow described with the Eulerian– Eulerian approach and the kinetic theory of granular ow. The grid sizes being used in this work are certainly not ne enough to resolve all the uctuations that occur in a riser, in particular the uctuations due to meso-scale clusters 0009-2509/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2003.09.016