A precipitated iron catalyst (100 Fe/5 Cu/4.2 K/25
SiO2 on mass basis) was tested in a fixed-bed
reactor under a variety of process conditions during conventional
Fischer−Tropsch synthesis
(FTS) and supercritical Fischer−Tropsch synthesis (SFTS). In
both modes of operation it was
found that: total olefin content decreases whereas 2-olefin content
increases with either increase
in conversion or H2/CO molar feed ratio. Total olefin
and 2-olefin selectivities were essentially
independent of reaction temperature. The effect of conversion was
more pronounced during
conventional FTS. Comparison of olefin selectivities in the two
modes of operation reveals that
total olefin content is greater while the 2-olefin content is smaller
during SFTS. Also, both the
decrease in total olefin content and the increase in 2-olefin content
with increase in carbon
number (i.e. molecular weight of hydrocarbon products) was
significantly less pronounced during
SFTS in comparison to the conventional FTS. The obtained results
suggest that 1-olefins, and
to a smaller extent n-paraffins, are the primary products of
FTS. Secondary reactions
(isomerization, hydrogenation, and readsorption) of high molecular
weight α-olefins occur to a
smaller extent during SFTS, due to higher diffusivities and desorption
rates of α-olefins in the
supercritical propane than in the liquid-filled catalyst pores
(conventional FTS).
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