The Gas Solid Vortex Reactor (GSVR) is a novel type of reactor, which surpasses major limitations of conventional gravitational Fluidized Bed (FB) reactors by replacing the gravitational field with a centrifugal field1, 2. In the GSVR, the process gas flows through tangentially inlet slots positioned along the circumferential wall of a static cylindrical chamber. The gas is forced to leave the chamber via the centrally placed exhaust. The solid particles, which are fed into the chamber via a solids inlet, are entrained by the gas and set to rotation. When the centrifugal force exerted on the rotating particles outweighs the drag force exerted by the flowing gas, a rotating solids bed is formed near the circumferential wall of the GSVR (Fig. 1). Since the centrifugal force can exceed gravitational force significantly, high gas-solid slip velocities are developed. Hence, the GSVR can achieve higher heat and mass transfer as compared to conventional FB, leading to Process Intensification (PI)3. Moreover, the GSVR differs from Rotating Fluidized Bed reactors (RFB), as it enables centrifugal fluidization in a static geometry, thus minimizing mechanical abrasion caused by moving parts. Among the several possible applications of the GSVR, drying of biomass4, effluent SO2/NOx adsorption5, even nuclear rocket fuel propulsion6 have been investigated in literature.
In the Laboratory for Chemical Technology, an experimental semi-batch GSVR setup has been constructed and tested under various operational conditions7-9. However, the need for non-intrusive measurement techniques8, has limited the information that the experiments alone can provide. This necessitates the use of Computational Fluid Dynamics (CFD) simulation of the hydrodynamics inside the GSVR.
In the present work, the commercial CFD software, FLUENT® 14.0 is used to numerically study the detailed three-dimensional (3D) cold flow hydrodynamics in the GSVR. Transient CFD calculations are performed on 1/9th section of the whole geometry to reduce computational cost. The multiphase flow is simulated in an Eulerian-Eulerian framework, where the two phases, air and polymer particles, are treated as interpenetrating continua. Kinetic Theory of Granular Flow (KTGF) modeling is used to close the solid phase equations. As the GSVR can give rise to the formation of dense beds, separate turbulence modeling is considered per phase, to account for four-way coupling of phase momentum equations. Since the main interest is focused on extracting global fluidization behavior inside the GSVR, only time-averaged data are considered.
First, a detailed sensitivity study is carried out to identify the key numerical parameters influencing the simulation results. The simulation data is found to be highly sensitive to the values of the particle-particle restitution coefficient and the particle-wall specularity coefficient. Particle-particle friction modeling is also found to become important at higher solids capacity, for accurate prediction of maximum solids content that the GSVR can hold at a given gas flow rate. Optimal parameter values have been determined by comparing the simulation data with their experimental counterpart9 over a wide range of operating conditions (gas flow rate, solids capacity) and particle properties (density, size). The validated model is then used to investigate channeling, slugging inside the GSVR. This helps identify the full range of operability of the reactor. Next the degree of fluidization is investigated. The solids are found to exhibit a complex pattern of bed behavior, ranging from almost packed to fluidized regime. The simulation results are used to construct a fluidization regime map for the GSVR (Fig. 2). This fluidization regime map can be applied to compare the GSVR performance with the traditional FB and RFB technologies and assess the extent of PI achieved in the GSVR.
The computational work was carried out using the STEVIN Supercomputer Infrastructure at Ghent University, funded by Ghent University, the Flemish Supercomputer Center (VSC), the Hercules Foundation and the Flemish Government – department EWI. The authors acknowledge the financial support from the European Research Council under the European Union's Seventh Framework Programme FP7/2007-2013/ERC grant agreement n° 290793.
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Fig. 1. A schematic representation of the Gas-Solid Vortex Reactor
Fig. 2. Field of solids volume fraction in an azimuthal GSVR plane. Fluidization regimes with increasing solids capacity