494
empirical correlation for the overall rate model. Also, a large number of reported
studies have focused on lumped kinetics wherein the rate of CO or syngas consumption was modeled based on chain initiation reaction mechanism. The method
neglected the product distribution calculations, which consists of a large number of
products. Since the method did not involve long and complex FT reaction networks
or, in some cases, the reaction was lumped, the results seem to be more accurate and
robust. However, the differences among the rate expressions were evident due to the
change in reactor condition viz. present and absence of mass transfer limitation, and
the selection of catalyst. In the case of iron catalyst, most of the rate expressions
consist of CO 2 and H 2 O as inhibition terms due to its good activity toward water gas
shift reaction. It also suggests strong competition of CO and H 2 O over adsorption on
a single site. In the case of cobalt-based FT catalysts, there was no significant role
of CO 2 and H 2 O in the rate expression as the catalyst was inactive for the water gas
shift reactions. The difference between rate expression for iron- and cobalt-based
catalysts can also be attributed to the mechanistic approach selected for both. It was
observed that a cobalt-based catalyst favors direct dissociation and adsorption of
CO before interaction with H 2 , whereas iron favors hydrogen-assisted CO
dissociation.
In the parallel approach, hydrocarbon selectivity models were proposed, which
showed significant deviation from the observed experimental trends due to the presence of secondary reactions. Therefore, the segregation of the syngas conversion
model from the product formation rate model gives rise to a significant amount of
error in the results. However, this does not signify that the approach is erroneous.
Nevertheless, the product distribution models and syngas conversion models were
examined over a large number of catalysts by the various researchers. In recent studies, a detailed, comprehensive approach was undertaken to describe the product
formation rate considering each elementary reaction. The method was able to
explain the syngas conversion rate as well as the product formation rate simultaneously and was more accurate compared to the previous approach of segregated models. Although, the comprehensive modeling is a cumbersome approach wherein the
model development is a complicated task, and the solution is time taking, yet the
accuracy of the results made the approach more acceptable than the other approaches.
Hence, while selecting any of these approaches or the model for kinetic study of FT
reaction, one has to take care of the correlation that has been used to develop the
model, and the condition should be closest to the condition at which the simulation
has been done.
References
1. Liu Z, Shi S, Li Y (2010) Coal liquefaction technologies—development in China and challenges in chemical reaction engineering. Chem Eng Sci 65:12–17. https://doi.org/10.1016/j.
ces.2009.05.014
2. Wood DA, Nwaoha C, Towler BF (2012) Gas-to-liquids (GTL): a review of an industry
offering several routes for monetizing natural gas. J Nat Gas Sci Eng 9:196–208. https://doi.
org/10.1016/j.jngse.2012.07.001
Sonal et al.
empirical correlation for the overall rate model. Also, a large number of reported
studies have focused on lumped kinetics wherein the rate of CO or syngas consumption was modeled based on chain initiation reaction mechanism. The method
neglected the product distribution calculations, which consists of a large number of
products. Since the method did not involve long and complex FT reaction networks
or, in some cases, the reaction was lumped, the results seem to be more accurate and
robust. However, the differences among the rate expressions were evident due to the
change in reactor condition viz. present and absence of mass transfer limitation, and
the selection of catalyst. In the case of iron catalyst, most of the rate expressions
consist of CO 2 and H 2 O as inhibition terms due to its good activity toward water gas
shift reaction. It also suggests strong competition of CO and H 2 O over adsorption on
a single site. In the case of cobalt-based FT catalysts, there was no significant role
of CO 2 and H 2 O in the rate expression as the catalyst was inactive for the water gas
shift reactions. The difference between rate expression for iron- and cobalt-based
catalysts can also be attributed to the mechanistic approach selected for both. It was
observed that a cobalt-based catalyst favors direct dissociation and adsorption of
CO before interaction with H 2 , whereas iron favors hydrogen-assisted CO
dissociation.
In the parallel approach, hydrocarbon selectivity models were proposed, which
showed significant deviation from the observed experimental trends due to the presence of secondary reactions. Therefore, the segregation of the syngas conversion
model from the product formation rate model gives rise to a significant amount of
error in the results. However, this does not signify that the approach is erroneous.
Nevertheless, the product distribution models and syngas conversion models were
examined over a large number of catalysts by the various researchers. In recent studies, a detailed, comprehensive approach was undertaken to describe the product
formation rate considering each elementary reaction. The method was able to
explain the syngas conversion rate as well as the product formation rate simultaneously and was more accurate compared to the previous approach of segregated models. Although, the comprehensive modeling is a cumbersome approach wherein the
model development is a complicated task, and the solution is time taking, yet the
accuracy of the results made the approach more acceptable than the other approaches.
Hence, while selecting any of these approaches or the model for kinetic study of FT
reaction, one has to take care of the correlation that has been used to develop the
model, and the condition should be closest to the condition at which the simulation
has been done.
References
1. Liu Z, Shi S, Li Y (2010) Coal liquefaction technologies—development in China and challenges in chemical reaction engineering. Chem Eng Sci 65:12–17. https://doi.org/10.1016/j.
ces.2009.05.014
2. Wood DA, Nwaoha C, Towler BF (2012) Gas-to-liquids (GTL): a review of an industry
offering several routes for monetizing natural gas. J Nat Gas Sci Eng 9:196–208. https://doi.
org/10.1016/j.jngse.2012.07.001
Sonal et al.