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.