Angesichts ausbleibender Erfolge bei der Reduktion der CO2‐Emission im Verkehr treten zunehmend auch synthetische Kraftstoffe aus CO2 und erneuerbarer elektrischer Energie in den Fokus. Für diesen sog. Power‐to‐Liquid(PtL)‐Ansatz werden neben konventionellen Technologien für Großanlagen auch intensivierte Technologien für dezentrale Anlagen in Betracht gezogen. Der Beitrag gibt einen Überblick über den Entwicklungsstand und die Perspektiven kompakter Anlagen für dezentrale PtL‐Verfahren auf Basis der Fischer‐Tropsch‐Synthese.
Hydrogenation
of
CO2 to long-chain hydrocarbons via
combined reverse water gas shift (RWGS) and Fischer–Tropsch
(FT) gained much attention in the last years as a way to produce sustainable
hydrocarbons for the chemical industry or fuel applications. Despite
the large amount of interest in the reaction, so far only a few studies
have been conducted regarding the kinetics. In this study we carefully
investigated the kinetics of an alumina supported iron catalyst at
280–320 °C, 10–20 bar, 900–120 000 mLN h–1 g–1, and a H2/CO2 molar inlet ratio of 2–4.
Special attention was focused toward the thermodynamic constraints
under reaction conditions. Based on elementary reaction steps according
to recent mechanistic investigations, we derived new Langmuir–Hinshelwood–Hougen–Watson
type kinetic expressions which allow an excellent reproduction of
the experimental data and outperform existent models. Possible model
combinations were discriminated against each other, and the best fit
was obtained for the assumption of H-assisted CO2 and H-assisted
CO dissociation mechanisms for RWGS and FT, respectively. Model uncertainties
that are introduced by the RWGS being close to equilibrium are discussed
in detail and are possibly a reason for strongly varying results for
activation energies between different studies. The detrimental effect
of water vapor on the reaction progression is analyzed numerically
and can be attributed to two parameters: kinetic inhibition via strong
adsorption of oxygen containing species and thermodynamic constraints
by shifting the equilibrium CO partial pressures to lower values.
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.
FT reaction kinetics were adopted to description of a pilot-scale microchannel reactor.• Product condensation was matched by non-ideal vapor-liquid equilibrium calculations.• Residence time distribution models were developed for all FTS plant components.• Time-dependent product composition under dynamic FTS operation are described.
Dedicated to Prof. Dr. Thomas Hirth on the occasion of his 60th birthday Synthesis gas (syngas) used for the production of synthetic fuels may contain significant amounts of CO 2 , depending on its source. For Fischer-Tropsch synthesis on cobalt, CO 2 can be considered as inert diluent. However, in the specific case of a coupled Fischer-Tropsch-hydrocracking (FT-HC) process, CO 2 could interact with the catalyst in the HC step. In this experimental study, HC product distributions obtained for FT-syngas compositions with and without CO 2 and N 2 are presented. The selected feed gas compositions result from an advanced syngas production route via plasma splitting of CO 2 . Main target product was kerosene, here being defined as C 10 -C 14 . It was found that the CO 2 presence is negligible with regard to adsorption or reaction on the HC catalyst. Further insights into possible impacts of CO 2 could be obtained from the analysis of alcohols in the aqueous phase.
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