(265b) Numerical Hydrodynamics Study of Gas Solid Vortex Reactor for Process Intensification in Fluidization Technology

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 field^{1, 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/NO_x 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 conditions^7-9. 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 1/9^th 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.

Acknowledgment

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