461
cobalt- based catalysts are highly active toward FTS reaction, and presence of water
in the reaction does not strongly inhibit the FTS reaction. However, the cobalt-based
catalyst is not active toward water gas shift reaction, hence requiring a stoichiometric ratio of H 2 /CO (molar ratio ~ 2.0) for efficient conversion of syngas over the
cobalt- based catalysts. The water-gas shift reaction (Eq. 6) is important in FTS synthesis, and the extent of reaction affects the overall distribution of H 2 and CO in the
reactor. In the gas-to-liquid process, the molar ratio of H 2 /CO of syngas is significant for efficient syngas conversion; therefore, the H 2 produced via water gas shift
reaction is undesirable. On the contrary, syngas coming from carbon-rich sources,
viz., coal, biomass, have a low H 2 /CO ratio, and therefore, a H 2 booster reaction
(WGS) is required in the reactor to maintain the required stoichiometric ratio. Thus,
many commercial FTS plants based on natural gas (Shell and Sasol in Qatar) are
using cobalt-based catalysts and coal-based plants are using iron-based catalysts [1,
2]. The selection of iron and cobalt catalysts also depends on the temperature of the
FT reaction. Both iron and cobalt catalysts have been implemented in a lowtemperature (180–260 °C) FT (LTFT) process and their major products are longchain hydrocarbons and wax, whereas the products of a high-temperature
(290–360 °C) FT (HTFT) process are short-chain hydrocarbons and gases. Iron is
frequently used as a commercial catalyst for HTFT.
Kinetic modeling of the FTS process is an integral part of the analysis, and it is
essential for reactor design, scale-up, and catalyst design for the process. Several
kinetic models of the FTS reaction on iron and cobalt catalysts have been proposed
and gained serious attention. The models were either empirical or based on different
proposed mechanisms. However, the ambiguity was always there as the several rate
expressions were conflicting and unacceptable. The reason was a multiple proposed
mechanism and a complex reaction system. As discussed, the FT process has a large
number of products wherein multiple reactions occur simultaneously and are sensitive to the reaction temperature, pressure, H 2 /CO ratio, and catalyst selection. The
detailed review of the kinetic models of the FTS reaction reported till date is essential to understand the ambiguity and to figure out the root cause of the prevailing
conflict in the modeling path. Mainly, two approaches are chosen by various authors
while performing FT kinetics. The first approach describes only the rate of consumption of reactants (syngas) either by implementing some empirical model or by
applying mechanism derived lumped modeling [3–11]. The other approach aimed at
the rate of formation of hydrocarbon or selectivity of products. In that regard, various selectivity models were proposed, and the term “chain growth probability” was
introduced [12–19]. However, even though some researchers were able to predict
the product distribution successfully, some classical models lacked in the accuracy
or quantitative prediction with respect to actual experimental data. Overall, the reaction mechanism plays a vital role in the kinetic modeling of the FTS reaction.
The Fischer-Tropsch synthesis is considered as a polymerization reaction, in
which the monomers in the form of CH x (x = 1, 2 or 3) are produced as intermediate
species. The proposed reaction pathways as found in literature mainly consisted of
three different reactions, i.e., CO activation for the formation of chain initiator,
chain growth or propagation, and chain-growth termination or desorption [20]. The
Recent Advancements and Detailed Understanding of Kinetics for Synthesis Gas…
cobalt- based catalysts are highly active toward FTS reaction, and presence of water
in the reaction does not strongly inhibit the FTS reaction. However, the cobalt-based
catalyst is not active toward water gas shift reaction, hence requiring a stoichiometric ratio of H 2 /CO (molar ratio ~ 2.0) for efficient conversion of syngas over the
cobalt- based catalysts. The water-gas shift reaction (Eq. 6) is important in FTS synthesis, and the extent of reaction affects the overall distribution of H 2 and CO in the
reactor. In the gas-to-liquid process, the molar ratio of H 2 /CO of syngas is significant for efficient syngas conversion; therefore, the H 2 produced via water gas shift
reaction is undesirable. On the contrary, syngas coming from carbon-rich sources,
viz., coal, biomass, have a low H 2 /CO ratio, and therefore, a H 2 booster reaction
(WGS) is required in the reactor to maintain the required stoichiometric ratio. Thus,
many commercial FTS plants based on natural gas (Shell and Sasol in Qatar) are
using cobalt-based catalysts and coal-based plants are using iron-based catalysts [1,
2]. The selection of iron and cobalt catalysts also depends on the temperature of the
FT reaction. Both iron and cobalt catalysts have been implemented in a lowtemperature (180–260 °C) FT (LTFT) process and their major products are longchain hydrocarbons and wax, whereas the products of a high-temperature
(290–360 °C) FT (HTFT) process are short-chain hydrocarbons and gases. Iron is
frequently used as a commercial catalyst for HTFT.
Kinetic modeling of the FTS process is an integral part of the analysis, and it is
essential for reactor design, scale-up, and catalyst design for the process. Several
kinetic models of the FTS reaction on iron and cobalt catalysts have been proposed
and gained serious attention. The models were either empirical or based on different
proposed mechanisms. However, the ambiguity was always there as the several rate
expressions were conflicting and unacceptable. The reason was a multiple proposed
mechanism and a complex reaction system. As discussed, the FT process has a large
number of products wherein multiple reactions occur simultaneously and are sensitive to the reaction temperature, pressure, H 2 /CO ratio, and catalyst selection. The
detailed review of the kinetic models of the FTS reaction reported till date is essential to understand the ambiguity and to figure out the root cause of the prevailing
conflict in the modeling path. Mainly, two approaches are chosen by various authors
while performing FT kinetics. The first approach describes only the rate of consumption of reactants (syngas) either by implementing some empirical model or by
applying mechanism derived lumped modeling [3–11]. The other approach aimed at
the rate of formation of hydrocarbon or selectivity of products. In that regard, various selectivity models were proposed, and the term “chain growth probability” was
introduced [12–19]. However, even though some researchers were able to predict
the product distribution successfully, some classical models lacked in the accuracy
or quantitative prediction with respect to actual experimental data. Overall, the reaction mechanism plays a vital role in the kinetic modeling of the FTS reaction.
The Fischer-Tropsch synthesis is considered as a polymerization reaction, in
which the monomers in the form of CH x (x = 1, 2 or 3) are produced as intermediate
species. The proposed reaction pathways as found in literature mainly consisted of
three different reactions, i.e., CO activation for the formation of chain initiator,
chain growth or propagation, and chain-growth termination or desorption [20]. The
Recent Advancements and Detailed Understanding of Kinetics for Synthesis Gas…