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Fischer Tropsch synthesis of syngas to liquid hydrocarbons 221

formation. FT products formation depends on the used catalysts and the
reaction conditions. Therefore the FT synthesis step directly influences
the product qualitatively and quantitatively. The FT synthesis is kinetically
and mechanically controlled reaction. The kinetic and mechanical factors
affect the products formation during the FT synthesis [25,50 52].

7.3 Roles of catalyst during Fischer Tropsch synthesis

The heterogeneous catalysts play a major role in FT process, which
should have the optimum hydrogenation activity to catalyze the hydro-
genation of CO to higher hydrocarbons. The selectivity of FT synthesis
primarily depends on transition metals used during the reaction. There
are some metals of VIII transition group, which show optimum hydro-
genation activity for FT synthesis, which are cobalt (Co), iron (Fe), nickel
(Ni), and ruthenium (Ru) [25,27,47,53 56]. However, other elements,
such as rhodium (Rh), iridium (Ir), palladium (Pd), and platinum (Pt), are
also used in FT synthesis, which have higher selectivity compared to Ru,
Ni, Co, and Fe but not considered as industrial catalysts due to high cost.
Among all catalysts, Fe is the most commonly applied catalyst, due to
low cost and easily availability in respect to other catalysts. However,
ruthenium shows maximum catalytic reaction during FT synthesis, but it
is an extremely expensive catalyst. Nickel has very high activity for
hydrogenation but higher CH 4 selectivity and is more costly compared
to Co or Fe [27,49,55,57]. Therefore Co and Fe are the favorable
catalysts and possible options for FT synthesis that could be used for the
operation of FT plants. Overall, Co is considered to be more active than
Fe, and the economics of operation plant during FT synthesis with Co
catalysts are significantly higher than on Fe catalysts. Several salient
features of Co and Fe catalysts for FT synthesis are summarized in
Table 7.1. Moreover, the physico-chemical properties of the catalyst,
specifically the rate of diffusion, influence the overall FT synthesis and
diffusion rate [6,7,54,58,59]. As observed in the Thiele Wheeler graph,
shown in Fig. 7.2, the catalyst efficiency drops less than one as the Thiele
modulus, ϕ, and increases above unity. It is reflected that intraparticle
diffusion plays a major role for FT catalyst particle with diameters rela-
tively greater than 500 μm. Therefore creating intraparticle diffusion is
crucial for selecting catalyst particle size and shape for a fixed-bed FT
process [6,60].