476
note, Yates et al. [88] also examined the effect of CO 2 concentration on the kinetics
of the FT reaction by adding 20–50% CO 2 in the feed gas and observed that the data
is well fitted with a model consisting of the H 2 O inhibition term but not for CO 2 .
Botes et al. [42] also attempted to clarify the ambiguity related to the inhibition term
in the kinetic study over the iron catalyst. Their model consists of water, CO, and
vacant sites in the squared denominator, which implicated the assumption of a dual
site mechanism. However, while validating the model with historical data, the effect
of water was observed to be statistically insignificant.
Kinetic studies over cobalt-based catalysts have also received significant attention and are available in the open literature [10, 29, 50, 55–64, 73]. Most of the
kinetic rate expressions derived for cobalt came from regression analysis of simple
power-law expressions [9, 60, 61]. However, some of them were derived, incorporating the mechanistic pathway with the assumption made regarding the ratedetermining step. Unlike Fe catalysts where generally water is accepted as an
inhibitor, there is an ambiguity about the role of water over cobalt catalysts. In most
of the models (Table 2), the rate of consumption of CO is a function of the partial
pressures of CO and H 2 [9, 29, 62, 70–72, 81]. Sari et al. [29] tested five important
models (empirical and mechanistic) in which all have incorporated CO inhibition
term due to strong CO adsorption on Co–Ru/γ-Al 2 O 3 catalysts. The model developed by Sarup and Wojciechowski was found to be a best-fitted model among all the
five models and concluded that −R FT = −R co because of the trivial contribution of the
WGS reaction. While investigating the kinetics of FT synthesis on commercial
cobalt with syngas containing CO, CO 2 , H 2 , and H 2 O, Kaisar et al. [56] observed
that CO 2 and H 2 O showed negligible or minor influence on the reaction rate. Some
authors [10, 57, 73] noted that in the case of cobalt, water also plays an equally
important role as CO and H 2 . Keyser et al. [57] investigated to establish an integral
reactor model involving both the Fischer-Tropsch and the water-gas shift reaction
kinetics over Co-MnO catalysts. They claimed that the Co-MnO behaves like a
precipitated iron catalyst, and an empirical rate equation for the water-gas shift reaction showed the first-order dependence for CO 2 and H 2 O. Das et al. [73] investigated
the effects of water on the kinetics of the FT synthesis reaction by adding external
water with the feed (<25 vol%) over Co/Al 2 O 3 catalysts in a CSTR. Regarding the
negative reversible effect of water in the kinetics, their kinetic data was reasonably
fitted by a semi-empirical power-law expression with the water inhibition term.
Very recently, Ma et al. [74] reviewed the previous kinetic studies done in the last
three decades over iron and cobalt catalysts and proposed a generalized model.
They argued that the data point at low (<70%) and high conversion level (>70%)
was used to fit the models differently and should be treated separately. They tried to
fit their 83 sets of data points on the previous as well as this generalized kinetic
model and found that the different kinetic parameters were obtained for lower and
higher conversion data. Again, their reasoning for this was based on the inhibition
effects of CO 2 and H 2 O, which are different at two-levels of conversions. Also, the
strong water absorption and the presence of CO 2 as the most abundant species cannot be ignored. Other important and contradictory observations were made, and the
water inhibition on the FT synthesis rate was insignificant over the entire range of
Sonal et al.
note, Yates et al. [88] also examined the effect of CO 2 concentration on the kinetics
of the FT reaction by adding 20–50% CO 2 in the feed gas and observed that the data
is well fitted with a model consisting of the H 2 O inhibition term but not for CO 2 .
Botes et al. [42] also attempted to clarify the ambiguity related to the inhibition term
in the kinetic study over the iron catalyst. Their model consists of water, CO, and
vacant sites in the squared denominator, which implicated the assumption of a dual
site mechanism. However, while validating the model with historical data, the effect
of water was observed to be statistically insignificant.
Kinetic studies over cobalt-based catalysts have also received significant attention and are available in the open literature [10, 29, 50, 55–64, 73]. Most of the
kinetic rate expressions derived for cobalt came from regression analysis of simple
power-law expressions [9, 60, 61]. However, some of them were derived, incorporating the mechanistic pathway with the assumption made regarding the ratedetermining step. Unlike Fe catalysts where generally water is accepted as an
inhibitor, there is an ambiguity about the role of water over cobalt catalysts. In most
of the models (Table 2), the rate of consumption of CO is a function of the partial
pressures of CO and H 2 [9, 29, 62, 70–72, 81]. Sari et al. [29] tested five important
models (empirical and mechanistic) in which all have incorporated CO inhibition
term due to strong CO adsorption on Co–Ru/γ-Al 2 O 3 catalysts. The model developed by Sarup and Wojciechowski was found to be a best-fitted model among all the
five models and concluded that −R FT = −R co because of the trivial contribution of the
WGS reaction. While investigating the kinetics of FT synthesis on commercial
cobalt with syngas containing CO, CO 2 , H 2 , and H 2 O, Kaisar et al. [56] observed
that CO 2 and H 2 O showed negligible or minor influence on the reaction rate. Some
authors [10, 57, 73] noted that in the case of cobalt, water also plays an equally
important role as CO and H 2 . Keyser et al. [57] investigated to establish an integral
reactor model involving both the Fischer-Tropsch and the water-gas shift reaction
kinetics over Co-MnO catalysts. They claimed that the Co-MnO behaves like a
precipitated iron catalyst, and an empirical rate equation for the water-gas shift reaction showed the first-order dependence for CO 2 and H 2 O. Das et al. [73] investigated
the effects of water on the kinetics of the FT synthesis reaction by adding external
water with the feed (<25 vol%) over Co/Al 2 O 3 catalysts in a CSTR. Regarding the
negative reversible effect of water in the kinetics, their kinetic data was reasonably
fitted by a semi-empirical power-law expression with the water inhibition term.
Very recently, Ma et al. [74] reviewed the previous kinetic studies done in the last
three decades over iron and cobalt catalysts and proposed a generalized model.
They argued that the data point at low (<70%) and high conversion level (>70%)
was used to fit the models differently and should be treated separately. They tried to
fit their 83 sets of data points on the previous as well as this generalized kinetic
model and found that the different kinetic parameters were obtained for lower and
higher conversion data. Again, their reasoning for this was based on the inhibition
effects of CO 2 and H 2 O, which are different at two-levels of conversions. Also, the
strong water absorption and the presence of CO 2 as the most abundant species cannot be ignored. Other important and contradictory observations were made, and the
water inhibition on the FT synthesis rate was insignificant over the entire range of
Sonal et al.