KEYWORDS
Fischer-Tropsch synthesis, Pellet simulation, Hierarchical structure,
Internal diffusion limitation, Mass transfer enhancement
INTRODUCTION
Fischer-Tropsch synthesis (FTS), playing an essential role in the
production of liquid fuels and/or chemicals from non-petroleum fossil
fuels (coal, natural gas, biomass etc.), has drawn great attention due
to the environmental and political considerations, the changes in world
fossil energy reserves and the improvements to the FTS technology1-3 .
Fixed-bed reactors have been widely adopted in low-temperature
cobalt-based FTS process, because of the easy separation of catalysts
from heavy waxy products, easy to scale-up, low maintenance and low
losses due to attrition 2. Although the fixed-bed FTS
technology has been commercially applied worldwide, the trade-off
between diffusion length in one single pellet and the pressure drop on
the bed length is still a substantial issue for current fixed-bed
reactor study 4. For instance, fixed-bed reactors
generally require large catalyst pellets (1-3 mm) to ensure proper
pressure drop at specific capacity per reactor 5,
while such millimeter-scale catalyst pellets are proved to suffer from
severe internal diffusion limitation and the resulting inefficient
utilization of active cobalt species and excessive methane formation5-9. Therefore, a great number of studies have been
devoted to developing a viable catalyst that can balance high product
yield and reasonable pressure drop simultaneously.
How to effectively enhance intraparticle mass transfer is the kernel to
engineering FTS catalyst pellet and achieving this balance. According to
the definition of Thiele modulus \(\Phi=L{(k\rho_{s}/D_{e})}^{1/2}\)10, decreasing characteristic diffusion distance (L)
could help alleviate the mass transfer restrictions in catalyst pellet.
The eggshell catalyst with active species only deposited in the outer
portion of the pellet while keeping the size of the pellet unchanged is
an attractive solution to shorten the diffusion length independently.
The eggshell catalysts have been applied in FTS through both
experimental 6,11,12 and modeling studies5-7. However, the egg-shell structured pellet could
commonly possess a low inventory of active phase and thereby a decrease
in volumetric yield 13. The other approach is to alter
the geometry of pellets for reducing the pressure drop of catalyst bed
and enhancing the transport process. The simulation studies4,5,8,14 indicated that in comparison to spherical
pellets, more complex shapes, i.e., trilobes or hollow cylindrical
pellets, exhibit excellent hydrodynamic, transport characteristics under
industrial conditions, whereas, the low mechanical strength for hollow
cylinder pellet would limit its industrial application5.
The approaches mentioned above, either the nonuniform distribution of
active components or the adjustment on the geometry of pellets, mainly
focus on how to shorten the diffusion distance. Besides, designing the
pore spatial structure (bimodal pore or fractal-like structures) is also
a practical approach to enhance mass transfer by increasing the
effective diffusivity. In catalysis, the hierarchical structured
catalyst pellet could simultaneously meet the need of high internal
surface for active phase distribution and high mass transfer efficiency15,16. Coppens and his coworkers15,17-21 widely studied the general features of the
reaction-diffusion process in spatially distributed pore structure
catalyst by simulation. In recent years, the hierarchical pore
structured catalysts have been extensively applied in adsorption22,23 and catalysis processes 24-30.
However, contrary to plenty of reports in FTS focusing on the novel
preparation methods of the hierarchical structure and the
structure-activity relationships 31-37, little
attention has been devoted to the detailed reaction-diffusion process in
a hierarchical structured catalyst pellet. Xu et al.38 prepared a bimodal catalyst and elucidated the
reaction-diffusion process inside the pellet using a numerical
simulation. Nevertheless, the elaborate investigation on the
relationship between pore structure parameters and performances has not
been reported in literatures.
In comparison to experimental studies, which can only offer discrete
experimental points and trends, the numerical modeling study provides
the continuous variation of the apparent reaction performances with
structural parameters efficiently and conveniently. At present, steady
state continuum pellet models have been widely used to simulate the FTS
pellet via coupling reaction and transfer processes4,5,7,8,14,39,40. Still, such simulation studies were
basically limited to the optimization of the shape or dimension of the
FTS catalyst pellet based on the assumption of wax fully filled pellet.
In our previous study by Li 32, one kind of
hierarchical structured Co/SiO2 catalyst with the
macropore size of 1074 nm and mesopore size of 4 and 36 nm was prepared
and compared with a catalyst having only mesopores. The results proved
the effectiveness of hierarchical structure on improving mass transfer.
However, the question why the hierarchical structure enhances mass
transfer of FTS and how the pore structure parameters quantitatively
influence the catalysis performances remains mysterious, which hinders
the rational design of FTS catalyst engineering pellet.
Hence, in this work, on one hand, a series of meso-macroporous catalyst
pellets (10-20 mesh) with the equivalent mesopore size but various
macropore size were synthesized and evaluated for a detailed
relationship between macropore size and FTS performances. On the other
hand, a 1-dimension steady state continuum model was established to
simulate the meso-macroporous catalysts. The simulation results of the
pellet with the size of 10-20 mesh reasonably explained the enhancement
on mass transfer with increasing macropore size observed in experiments.
In addition, the simulation results of the 2 mm pellet give more
insights into the effects of pore structure parameters on FTS
performances at different operation conditions. A Langmuir-Hinshelwood
type kinetics for cobalt-based FTS by Yates and Satterfield41 corrected by our experiment data and a chain growth
probability model 42 were combined to calculate the
C5+ space-time yield which was used as an index to
evaluate the FTS performances. The experimental and simulated results
regarding to the intraparticle mass transfer enhancement could provide
useful guidance for rational design of industrial FTS catalyst
engineering pellet.