Detailed Kinetic Modeling of CO₂-Based Fischer–Tropsch Synthesis

The direct hydrogenation of CO₂ to long-chain hydrocarbons, so called CO₂-based Fischer–Tropsch synthesis (FTS), is a viable future production route for various hydrocarbons used in the chemical industry or fuel applications. The detailed modeling of the reactant consumption and product distribution is very important for further process improvements but has gained only limited attention so far. We adapted proven modeling approaches from the traditional FTS and developed a detailed kinetic model for the CO₂-FTS based on experiments with an Fe based catalyst in a lab-scale tubular reactor. The model is based on a direct CO₂ dissociation mechanism for the reverse water gas shift and the alkyl mechanism with an H-assisted CO dissociation step for the FTS. The model is able to predict the reactant consumption, as well as the hydrocarbon distribution, reliably within the experimental range studied (10 bar, 280–320 °C, 900–120,000 mLN h−1 g−1 and H₂/CO₂ molar inlet ratios of 2–4) and demonstrates the applicability of traditional FTS models for the CO₂-based synthesis. Peculiarities of the fractions of individual hydrocarbon classes (1-alkenes, n-alkanes, and iso-alkenes) are accounted for with chain-length-dependent kinetic parameters for branching and dissociative desorption. However, the reliable modeling of class fractions for high carbon number products (>C₁₂) remains a challenge not only from a modeling perspective but also from product collection and analysis

Composition Modulation over Three-Way Catalysts

Compared to conventional internal combustion engines, modern hybrid electric vehicles (HEVs) can save fuel under urban driving conditions, which results in lower CO2 emissions. However, frequent stops and restart of the HEV engine go along with periods of low exhaust gas temperatures and therefore cause a decline in pollutant conversion over the three-way catalyst (TWC) system that is typically used for exhaust gas after-treatment [1]. Composition modulation is reported to increase pollutant conversion over the TWC at low temperatures and to be beneficial for cold start performance [2]. In this regard, dithering frequency, amplitude, temperature, and space velocity are the most important parameters influencing the rate enhancement from composition modulation. A synthetic gas bench with fast switching valves is used to conduct a comprehensive parameter study on the influence of dithering parameters on the TWC performance. For this, monolithic catalysts with Pd/Al2O3 and a commercially relevant Ce-based Pd catalyst are prepared by incipient wetness impregnation and subsequent dip coating. The application of a square wave signal to the catalytic converter remains a challenging task due to axial dispersion in the setup periphery. To further examine the phenomena and predict the behaviour of the TWC under periodic conditions, a detailed kinetic model is under development. Literature suggests that the dithering effect can be described by kinetic models with detailed chemistry considering the interaction among species adsorbed on surface sites [3]. Under the assumption of an ideally backmixed reactor, initial modelling results exploiting a detailed microkinetic model for CO oxidation [4] show an increased average CO conversion for all frequencies around the light-off and a decrease for higher temperature (Figure 1). Furthermore, an increase of optimal frequency with increasing temperature and amplitude was observed for constant amplitude and temperature respectively, which is in line with experimental data from literature [2]. Using transient data for model development will provide valuable insights on surface phenomena that are responsible for the dithering effect on three-way catalysts and will ultimately allow for reducing pollutant emissions from HEVs. [1] Y. Huang, N. Surawski, B. Organ, J. Zhou, O. Tang and E. Chan, "Fuel consumption and emission performance under real driving: Comparison between hybrid and conventional vehicles", Sci. Total Environ. 659, 275-282 (2019). [2] P. Silveston, "Automotive exhaust catalysis: Is periodic operation beneficial?", Chem. Eng. Sci. 51, 2419-2426 (1996). [3] P. Kočí, M. Kubíček, M. Marek, "Multifunctional aspects of Three-Way Catalyst: Effects of Complex Washcoat Composition", Chem. Eng. Res. Des. 82, 284-292 (2004). [4] D. Chan, S. Tischer, J. Heck, C. Diehm, O. Deutschmann, "Correlation between catalytic activity and catalytic surface area of a Pt/Al2O3 DOC: An experimental and microkinetic modelling study", Appl. Catal. B 156-157, 153-165 (2014)

Detailed Kinetic Modeling of CO2-Based Fischer–Tropsch Synthesis

The direct hydrogenation of CO2 to long-chain hydrocarbons, so called CO2-based Fischer–Tropsch synthesis (FTS), is a viable future production route for various hydrocarbons used in the chemical industry or fuel applications. The detailed modeling of the reactant consumption and product distribution is very important for further process improvements but has gained only limited attention so far. We adapted proven modeling approaches from the traditional FTS and developed a detailed kinetic model for the CO2-FTS based on experiments with an Fe based catalyst in a lab-scale tubular reactor. The model is based on a direct CO2 dissociation mechanism for the reverse water gas shift and the alkyl mechanism with an H-assisted CO dissociation step for the FTS. The model is able to predict the reactant consumption, as well as the hydrocarbon distribution, reliably within the experimental range studied (10 bar, 280–320 °C, 900–120,000 mLN h−1 g−1 and H2/CO2 molar inlet ratios of 2–4) and demonstrates the applicability of traditional FTS models for the CO2-based synthesis. Peculiarities of the fractions of individual hydrocarbon classes (1-alkenes, n-alkanes, and iso-alkenes) are accounted for with chain-length-dependent kinetic parameters for branching and dissociative desorption. However, the reliable modeling of class fractions for high carbon number products (>C12) remains a challenge not only from a modeling perspective but also from product collection and analysis