Page 275 - Lignocellulosic Biomass to Liquid Biofuels

Fischer Tropsch synthesis of syngas to liquid hydrocarbons 231

economic feasibility and efficiency in process, it is of primary importance to
accurately formulate the integral kinetic models of the appropriate catalyst
based on elementary steps, keeping in mind the influence of transport of
reactants on reaction kinetics. Therefore reaction kinetics is essential for opti-
mizing the catalyst design based on the knowledge of the rate-determining
steps and kinetic trends. In general, the composition of catalyst as well as
influence of transport of reactants onreactionconditioncancontrol the
numerical values of the intrinsic kinetic rate parameters. Still the equations
rate is not equivalent; thus k could be considered as combinations of differ-
ent adsorption and kinetic constants. Several studies showed that the FT syn-
thesis activity could be determined on the preparation method, metal
loading on the catalyst, and catalyst support [35,107].

7.4.1 Kinetics of Fe- and Co-based catalysts
The first kinetic equations for Fe-based catalysts were proposed by Huff
and Satterfield [108] and Zimmerman and Bukur [68], which are summa-
rized in Table 7.5. In general, the FT reaction rate increases with the H 2
partial pressure and decreases with the H 2 O partial pressure. It was
observed that addition of 27 mol.% water vapor to syngas decreased the
catalyst activity, and after the addition of 42 mol.% water vapor, the cata-
lyst did not regain its initial activity. Addition of water vapor decreased
CH 4 selectivity and increased oxygenate selectivity and the rate of the
WGS [117]. The mechanistic kinetic rate expression of Fe catalysts is
dependent on the construction of monomer and different models for the
formation of the monomer species are assumed in literature: (1) carbide
mechanism, where CO is adsorbed and dissociates on the metal surface
and the intermediate carbon species undergo to hydrogenation, forming
methylene species [111], (2) combined carbide mechanism, where methy-
lene groups are the result of water elimination from the enol [118].
However, it could be summarized that the development of FT kinetic
equations still requires additional research during the FT synthesis on iron
catalysts.
Few kinetic studies on Co catalysts were carried out and were summa-
rized in Table 7.5, and kinetic equations of Co- and Fe-based catalysts are
different. Commonly, the LHHW kinetic expression depends on a
rate-determining step, which involves uppermost layer reaction of double
molecule, while H 2 O inhibition rate on Co catalysts is not observed
because the WGS reaction hardly shows effect on Co catalysts and