Fischer–Tropsch Process
Coal and natural gas can be utilized as feedstock of the chemical industry
and the transportation fuels market. The conversion of natural gas to
hydrocarbons (Gas-To-Liquids route) is currently one of the most promising
topics in the energy industry due to economic utilization of remote natural gas to
environmentally clean fuels, specialty chemicals and waxes. Alternatively, coal
or heavy residues can be used on sites where these are available at low costs. The
resources of coal and natural gas are very large. Coal and natural gas can be
converted into synthesis gas, a mixture of predominantly CO and H2, by either
partial oxidation or steam reforming processes. Possible reactions of synthesis
gas are shown in
Natural gas and Coal  CO+H2  Hydrocarbons
T+
The conversion of the synthesis gas to aliphatic hydrocarbons over metal
catalysts was discovered by Franz Fischer and Hans Tropsch at the Kaiser
Wilhelm Institute for Coal Research in M¨ ullheim in 1923. They proved that CO
hydrogenation over iron, cobalt or nickel catalysts at 180-250 °C and atmospheric
pressure results in a product mixture of linear hydrocarbons. The Fischer-Tropsch
product spectrum consists of a complex multicomponent mixture of linear and
branched hydrocarbons and oxygenated products. Main products are linear
paraffins and -olefins. The hydrocarbon synthesis is catalyzed by metals such as
cobalt, iron, and ruthenium. Both iron and cobalt are used commercially these
days at a temperature of 200 to 300 °C and at 10 to 60 bar pressure. Thus the
Fischer-Tropsch synthesis (FTS) offers the potential of converting syngas into
high value chemicals.
Fischer–Tropsch liquids from natural gas and ethanol from biomass may
become widespread if there are large amounts of stranded natural gas selling for
very low prices at the same time that petroleum is expensive or extremely low
sulfur is required in diesel fuel. Ethanol could become the dominant fuel if
energy independence, sustainability, or very low carbon dioxide emissions
become important—or if petroleum prices double.
Synthesis Gas Production
Synthesis gas can be obtained by steam reforming or (catalytic) partial oxidation of
fossil fuels: coal, natural gas, refinery residues, biomass or industrial off-gases. Synthesis gas can be
obtained from reforming of natural gas with either steam or carbon dioxide, or by partial oxidation. The
most important reactions are:
Steam reforming
CH4 + H2O - CO + 3H2
CO2 reforming
CH4 + CO2  2CO + 2H2
Partial oxidation
CH4 + 1/2 O2  CO + 2H2
Water gas shift reaction CO + H2O  CO2 + H2
Although several metals are active for the FTS, only iron and cobalt
catalysts appear economically feasible on an industrial scale. The high water-gas
shift activity of iron makes it an ideal catalyst for converting hydrogen lean
syngas derived from coal. Controlling selectivity is an important aspect of FTS
catalyst development. Cobalt catalysts have a high activity for hydrogenation and
tend to produce linear alkanes. Iron catalysts are more versatile than cobalt
catalysts, produce less methane and can be geared for the production of alkenes,
oxygenates and branched hydrocarbons depending on promoters and the process
conditions employed. Sasol is currently using an iron-based catalyst in a
commercial slurry reactor; high alkene selectivity relative to fixed bed reactors is
reported. Several factors influence the selectivity and activity of FTS catalysts.
Product selectivity of iron catalysts is generally controlled by promoting with one
or more alkali metals. Potassium has long been known to increase wax and
alkene yields while decreasing the production of undesirable methane. Potassium
also has been implicated in increasing FTS and water-gas shift activity.
Activation procedures also can have a large affect on the selectivity and activity
of iron catalysts. Precipitated iron catalysts are generally activated with hydrogen,
carbon monoxide or syngas.
Copper has traditionally been added to precipitated iron catalysts to facilitate
reduction of iron oxide to metallic iron during hydrogen activations [7]. Copper
has been shown to minimize sintering of iron catalysts when activating with
hydrogen by lowering the reduction temperature. The effect of copper promotion
on iron catalysts activated with carbon monoxide or syngas is not as well
documented although Kölbel and Ralek have reported that only 0.1 wt% of copper
is necessary for successful activation of Fe/Cu/K catalysts with syngas. It has been
found that syngas activation at elevated pressure (>0.80 MPa) requires the catalyst
be promoted with copper in order to achieve reasonable activity. Previous work
has shown that the activity of syngas activated, Fe/Si/K catalysts is related to the
hydrogen partial pressure of the activation gas. Low hydrogen partial pressure
inhibits the formation of oxidizing water which enables active iron carbides to be
formed.
The active site for FTS is still under debate. Fischer initially proposed bulk
carbides were responsible for catalyst activity . Emmet later demonstrated that an
iron catalyst carbided with 14C labeled carbon monoxide produced hydrocarbons
with a lower radioactivity than the catalyst, thereby indicating that the bulk
carbide was not responsible for the activity; however, he did not rule out the
possibility that a surface carbide could be responsible for FTS activity. Biloen et
al., using 13C tracers, have proposed that a surface carbide species is responsible
for FTS activity on nickel, cobalt and ruthenium catalysts. Similarly, Stockwell et
al. have proposed that a CH species derived from surface carbon is the active
species on a supported iron catalyst.
Iglesia found that the contacting of Fe
oxide precursors with synthesis gas (H2/CO mixtures) leads to structural and
chemical changes and to the formation of the active sites required for the
Fischer-Tropsch synthesis (FTS). The local structure and oxidation state of the
starting Fe2O3 precursors promoted by Cu and/or K were probed using in situ
X-ray absorption spectroscopy during these processes. The activation of these
precursors occurs via reduction to Fe3O4 followed by carburization to form FeCx.
FTS reaction rates increased markedly during the initial stages of carburization,
suggesting that the conversion of near-surface layers of Fe3O4 to FeCx is
sufficient for the formation of the required active sites.
Figure 1 Carbon number product distribution according to extended
Anderson-Schulz-Flory model. Open squares are data points excluded from parameter
estimation.
Reference:
1. Senzi Li, George D. Meitzner,
and Enrique Iglesia, J. Phys. Chem. B 2001, 105, 5743.
2. DB Bukur, X Lang - Ind. Eng. Chem. Res, 1999,