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.