The
process intensification possibilities of a gas–solid vortex
reactor have been studied for biomass fast pyrolysis using a combination
of experiments (particle image velocimetry) and non-reactive and reactive
three-dimensional computational fluid dynamics simulations. High centrifugal
forces (greater than 30g) are obtainable, which allows
for much higher slip velocities (>5 m s–1) and
more intense heat and mass transfer between phases, which could result
in higher selectivities of, for example, bio-oil production. Additionally,
the dense yet fluid nature of the bed allows for a relatively small
pressure drop across the bed (∼104 Pa). For the
reactive simulations, bio-oil yields of up to 70 wt % are achieved,
which is higher than reported in conventional fluidized beds across
the literature. Convective heat transfer coefficients between gas
and solid in the range of 600–700 W m–2 K–1 are observed, significantly higher than those obtained
in competitive reactor technologies. This is partly explained by reducing
undesirable gas–char contact times as a result of preferred
segregation of unwanted char particles toward the exhaust. Experimentally,
systematic char entrainment under simultaneous biomass–char
operation suggested possible process intensification and a so-called
“self-cleaning” tendency of vortex reactors.
The process intensification abilities of gas− solid vortex units (GSVU) are very promising for gas−solid processes. By working in a centrifugal force field, much higher gas−solid slip velocities can be obtained compared to gravitational fluidized beds, resulting in a significant increase in heat and mass transfer rates. In this work, local azimuthal and radial particle velocities for an experimental GSVU are simulated using the Euler−Euler framework in OpenFOAM and compared with particle image velocimetry measurements. With the validated model, the effect of the particle diameter, number of inlet slots and reactor length on the bed hydrodynamics is assessed. Starting from 1g-Geldart-B type particles, increasing the particle diameter or density, increasing the number of inlet slots or increasing the gas injection velocity leads to an increased bed stability and uniformity. However, a trade-off has to be made since increased bed stability and uniformity lead to higher shear stresses and attrition.
Using detailed kinetic
models in computational fluid dynamics (CFD)
simulations is extremely challenging because of the large number of
species that need to be considered and the stiffness of the associated
set of differential equations. The high computational cost associated
with using a detailed kinetic network in CFD simulations is why one-dimensional
simulations are still used, although this leads to substantial differences
compared to reference three-dimensional simulations. Therefore, a
methodology was developed that allows one to use detailed single-event
microkinetic models in CFD simulations by on the fly application of
the pseudo-steady-state assumption to the radical reaction intermediates.
Depending on the reaction model size, a speedup factor of more than
50 was obtained compared to the standard ANSYS Fluent routines without
losing accuracy. As proof of concept, propane steam cracking in a
conventional bare reactor and a helicoidally finned reactor was simulated
using a reaction model containing 85 species: 41 radicals and 44 transported
species. Next to a drastic speedup of the simulations due to the kinetic
network reduction technique, significant differences were observed
between the bare and the finned reactor in the three-dimensional simulations.
In particular, the ethene selectivity is reduced by 0.20% by application
of the helicoidally finned reactor. The one-dimensional simulations
were not able to correctly predict the selectivity effect of the different
reactor geometry.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.