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Pöhlmann, Ferdinand ; Jess, Andreas:
Interplay of reaction and pore diffusion during cobalt-catalyzed Fischer–Tropsch synthesis with CO₂-rich syngas.
In: Catalysis Today.
Bd. 275
(2016)
.
- S. 172-182.
ISSN 0920-5861
DOI: https://doi.org/10.1016/j.cattod.2015.09.032
Abstract
For cobalt-catalyzed Fischer–Tropsch synthesis (FTS), a model was developed to analyze numerically the reaction–diffusion performance in catalyst particles. The model is based on a mesoporous particle and is valid for CO₂-free and -rich syngas. The kinetic parameters of all three relevant reactions, CO hydrogenation to C₂₊ hydrocarbons as well as CH₄ and CO₂ hydrogenation (mainly) to CH₄, were derived at intrinsic conditions (dp = 150 μm). Thereafter, the effective kinetics considering pore diffusion limitations were derived by using a homemade catalyst in 5 mm diameter. From the data measured, a kinetic approach for conversion of CO₂ inside the catalyst particle was developed. The experimentally derived Langmuir–Hinshelwood type rate expressions and a variable chain growth parameter α, dependent on temperature and syngas ratio, were implemented into the model. Furthermore, the change of the local reaction rates and selectivities, as a consequence of changing syngas ratio due to diffusion limitation is taken into account. The simulated data of effective kinetics and selectivities are in agreement with the measured data. The simulation predicts that CO₂ is only converted in the CO-free core region of large catalyst particles at high temperatures and strong pore diffusion limitations. For CO₂ converts mainly to CH₄ (selectivity 95%C), a slightly increased overall methane selectivity is expected indicating consumption of CO₂. However, this effect was not measurable even with 5 mm particles at high temperatures as methanation of CO₂ occurs only to a minor extent even at pronounced diffusion limited conditions and is negligible at industrial conditions.