Mixed alcohol synthesis (MAS) from
syngas involves an overall reduction in the number of moles (i.e.,
volume reduction) and is highly exothermic. As such, the high-pressure,
dense-phase solvent conditions of a supercritical reaction medium
afford opportunities to enhance the performance of these reactions.
In this paper, the effect of supercritical hexanes (a mixture of hexane
isomers) on the reaction performance of MAS was studied in a fixed
bed reactor. Experiments were carried out over a 0.5 wt % K promoted
Cu based mixed metal oxide catalyst in the temperature range of 200
°C–300
°C, a partial pressure of syngas of 4.5 MPa, and the hexanes/syngas
molar ratio ranging from 0 to 3. The results of mixed alcohol synthesis
under supercritical hexanes phase conditions demonstrated remarkable
enhancement in CO conversion and methanol productivity, while the
CH4 and CO2 selectivity was notably reduced.
In addition, the effect of temperature has also been investigated
on conversion, selectivity, and alcohol productivities for operation
under both gas phase and supercritical phase conditions.
A vertically aligned fixed-bed reactor system with a cascade of three sequential catalyst beds has been used to incorporate Fischer-Tropsch synthesis (FTS) in the first bed, oligomerization (O) in the second bed, and hydrocracking/isomerization (HC or C) in the third bed (FTOC). Compared to gas phase FTS (GP-FT) alone, gas phase FTS with the subsequent upgrading beds (GP-FTOC) is demonstrated to result in a reduction in the olefin selectivity, a reduction in the C 261 selectivity, and a marked enhancement in the production of branched paraffins and aromatics. Utilization of supercritical hexane as the reaction medium in supercritical FTS (SC-FT) and supercritical phase FTOC (SC-FTOC) resulted in a significant reduction in both CH 4 selectivity and CO 2 selectivity. Interestingly, significant amounts of aldehydes and cyclo-paraffins were collected in SC-FT and in SC-FTOC, respectively, while not being observed in traditional gas phase operation (both GP-FT and GP-FTOC).
Fischer–Tropsch synthesis (FTS) allows for the
synthesis
of fuels and chemicals from syngas. The reaction is highly exothermic,
with the removal of the generated heat being a central consideration
in the reactor design. Traditionally, low temperature Fischer–Tropsch
synthesis (LTFT) has been carried out under gas-phase conditions (GP-FTS)
in a fixed bed reactor or a slurry phase reactor (SP-FTS). Supercritical
Fischer–Tropsch (SC-FTS) offers an interesting alternative
to traditional Fischer–Tropsch operation. In addition to the
many performance benefits of SC-FTS that have been previously documented,
the presence of the supercritical solvent provides a diluent effect
that greatly reduces the adiabatic temperature rise of the process,
making adiabatic operation possible under supercritical fluid conditions.
In this work, the technical viability of adiabatic operation for supercritical
Fischer–Tropsch is examined. Additionally, preliminary designs
of supercritical adiabatic reactor (SCAR) systems, a series of modules
each consisting of an adiabatic reactor and a heat exchanger, have
been completed, and their capital costs were estimated. The capital
cost of this reactor design concept compares very favorably with conventional
reactor designs and allows for much easier catalyst replacement and
greater reactor control/flexibility than the traditional ARGE reactor.
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