Dj. Bochniak et B. Subramaniam, FISCHER-TROPSCH SYNTHESIS IN NEAR-CRITICAL N-HEXANE - PRESSURE-TUNINGEFFECTS, AIChE journal, 44(8), 1998, pp. 1889-1896
For Fe-catalyzed Fischer-Tropsch (FT) synthesis with near-critical n-h
exane (P-c= 29.7 bar; T-c=233.7 degrees C) as the reaction medium, iso
thermal pressure tuning from 1.2-24 P-c (for n-hexane) at the reaction
temperature (240 degrees C) significantly changes syngas conversion a
nd product selectivity For fixed feed rates of syngas (H-2/CO = 0.5; 5
0 std. cm(3)/g catalyst) and n-hexane (I mL/min), syngas conversion at
tains a steady state at all pressures, increasing roughly threefold in
this pressure range. Effective rate constants, estimated assuming a f
irst-order dependence of syngas conversion on hydrogen, reveal that th
e catalyst effectiveness increases with pressure implying the alleviat
ion of pore-diffusion limitations. Pore accessibilities increase at hi
gher pressures because the extraction of heavier hydrocarbons from the
catalyst pores is enhanced by the liquid-like densities, yet better-t
han-liquid transport properties, of n-hexane. This explanation is cons
istent with the single alpha ( = 0.78) Anderson-Schulz-Flory product d
istribution the constant chain termination probability, and the higher
primaly product (1-olefin) selectivities ( similar to 80%) observed a
t the higher pressures. Our results indicate that the pressure tunabil
ity of the density and transport properties of near-critical reaction
media offers a powerful tool to optimize catalyst activity and product
selectivity during FT reactions on supported catalysts.