5. CONCLUSIONS
A series of meso-macropore structured cobalt-loading catalyst with the same mesopore size (~8 nm) and different macropore size (50-6000 nm) were prepared, characterized and evaluated. It was found that macropore diameter had remarkable influences on FTS activity and selectivity. The underlying reason was explored by a 1-dimensional steady state continuum model of the pellet. The simulation results found that the enhancement of mass transfer was primarily contributed by the decrease of filling degree and the increase of macroporosity with increasing macropore size, and less contributed by the reduction of Knudsen diffusion resistance.
The further simulation results of a 2 mm meso-macroporous pellet demonstrated that wax filling degree had a significant influence on mass transfer, and the optimal \(\text{STY}_{C_{5+}}\) can be attained by increasing macropore size and adjusting porosity at different operation conditions. The enhancement on mass transfer by optimizing pore structure was more significant at high temperature. Besides, increasing pressure had a significant positive effect on \(\text{STY}_{C_{5+}}\)from 1.0 to 3.0 MPa and then improved slightly with further increasing pressure to 4.0 MPa.
This work confirmed the mass transfer advantage of hierarchical structured cobalt-based catalyst pellets for FTS via elaborate adjustment of pore parameters, and thus the optimized hierarchical structured engineering pellets via simulation would be further validated and used in the industrial FTS. The research method presented in this work could be extended to pellet design for other gas-liquid-solid reactions.