Development of a kinetic model for low temperature Fischer-Tropsch synthesis

Abstract

Globally, there is a need to replace our dependence on fossil fuels as the main source of energy.This requires a shift towards renewable and sustainable alternatives. The well-established Fischer-Tropsch (FT) synthesis is a potential process route to produce liquid fuels and speciality chemicals and address this challenge. FT synthesis is a polymerisation reaction in which syngas, a mixture of CO and H 2 , is converted to hydrocarbon products ranging from methane to wax when low temperature conditions are used. Subsequent product upgrading steps allow high quality liquid fuels to be obtained which are clean burning. This will help to mitigate the impact of human activity on the environment.The versatility of this process route is attributed to the ability of syngas to be generated from any carbon-containing feed such as coal, natural gas or biomass. The latter is attractive to enable a shift to a more sustainable way of living. Particularly for biomass-to-liquid plants, the high cost of syngas generation means that FT synthesis should use syngas as efficiently as possible. This requires an effective description of the FT reaction kinetics. This study therefore focuses on the development of a kinetic model for low temperature FT (LTFT) synthesis to improve understanding of the reaction behaviour and aid in the development of a biomass-to-liquid process route.Although the kinetics of the FT reactions under the low temperature conditions of 180-260 • C and 20-30 bar(a) have been extensively studied, the challenge to kinetic model development is the large number of possible reaction products. A common simplification is to consider the formation of the main products only, which are linear n-paraffins and 1-olefins. The polymerisation character of FT synthesis means that its product distribution could ideally be described using models based on probability theory. Deviations from probability theory distribution, however, occur especially at the conditions of LTFT synthesis. These deviations are a high methane yield, low ethene yield and the change from mainly 1-olefins at low carbon number to mainly n-paraffins at high carbon number. Comprehensive kinetic models in literature focus on finding a kinetic explanation for these deviations. These kinetic models, however, cannot easily be used with few being extended to include the formation of products of higher carbon number.An aspect ignored in current kinetic model development is that FT synthesis shares many aspects of an equilibrium-controlled process. This is since CO hydrogenation which leads to monomer formation is the rate-determining step for the FT reactions. Consequently, the rate of chain growth is rapid in comparison. This leads to the distribution of n-paraffins and 1-olefins being controlled by equilibrium. By modelling FT synthesis as an equilibrium-controlled process, the kinetic model formulation could be simplified, consist of fewer rate expressions and contain the minimum number of model parameters without compromising on prediction quality. At the condi...

Promoting jet fuel production by utilizing a Ru-doped Co-based catalyst of Ru-Co@C(Z-d)@Void@CeO2 in Fischer Tropsch synthesis

“…Hence, it was predicted that the selectivity towards jet fuel (C 8 –C 16 ) at the RNC catalyst would be greater in comparison to the FTS products, in particular, liquid fuels. Besides, from a thermodynamic point of view, methane is the most probable product in FTS, which is according to the reports by Davies et al and Zaffran et al , 47,48 and the Δ G results that were extracted from MD computations confirm this. The MD simulation results illustrate that Δ G for methane formation, 0.29 and 0.26 kcal mol −1 in NC and RNC catalysts, respectively, have the lowest values and Ru boosts its formation.…”

Fischer–Tropsch Synthesis Model Involving the Living Growth of Carbon Chains

“…This remark is important for proper understanding of the proposed living growth mechanism, as diffusion can control the chain length, as the effective-constants fall with the chain size. Particularly, Davies and Möller observed that the product distribution might be controlled by the chemical equilibrium, while the hydrocarbon yield was kinetically controlled. In this case, the reduction of the kinetic rate constants might indicate the occurrence of other parallel reactions besides chain growth.…”

An Overview on Dynamic Phase Transformation and Surface Reconstruction of Iron Catalysts for Catalytic Hydrogenation of COx for Hydrocarbons

“…On the other hand, WGS and Boudouard reactions are less exothermic compared to the hydrogenation reactions. Based on the Gibbs free energy analysis, it can be speculated that higher temperatures favor the WGS and Boudouard reactions and the formation of short-chain products. , Davies et al . developed an equilibrium-constrained kinetic model to predict the hydrocarbon distribution (Figure a)–d).…”

“…Based on the Gibbs free energy analysis, it can be speculated that higher temperatures favor the WGS and Boudouard reactions and the formation of short-chain products. 147,148 Davies et al 149 developed an equilibrium-constrained kinetic model to predict the hydrocarbon distribution (Figure 25a)−d). It was found that the product distribution was equilibrium-controlled, while the yield of hydrocarbon products was kinetically controlled.…”

“…(a–d): Carbon distribution of paraffins and olefins from CO hydrogenation as a function of reaction temperature and pressure; (e) and (f): Selectivity of hydrocarbon products and CO 2 conversion at different temperatures and pressures; (g) Δ r G of RWGS and FTS reactions during CO 2 hydrogenation; (h) Δ r G of CO and hydrocarbon products formation during CO 2 hydrogenation. Reproduced from refs − with permissions from Elsevier and Wiley-VCH.…”