Fischer-Tropsch Reaction Kinetics By: Matt Spielman, Hannah Ham, Yusuf Khan, Trey Crump

Fischer-Tropsch Introduction The Fischer-Tropsch process is a set of reactions that converts carbon monoxide and hydrogen gas into liquid hydrocarbons General reaction formula for alkane synthesis via FT reaction kinetics (2n + 1)H2 + (n)CO → CnH2n+2+ (n)H2O Diesel fuel synthesis reactants derived from natural gas, coal, and biomass FT kinetics used worldwide in large scale gas-liquid and coal-liquid facilities

Reaction Mechanisms for FT synthesis Mechanism 1 rejected due to unexpected behavior. Equilibrium constant for water increased with increasing temp but kinetic constant kch increased while absorption equilibrium constant for hydrogen decreased. Mechanism 2 showed the same behavior Image adapted from http://www.aidic.it/cet/11/25/118.pdf

Reaction Mechanisms for FT synthesis cont. Mechanism 3 is valid if the effect of water on the FT reaction kinetics is removed. Assume that water does not adsorb strongly on the catalyst surface. Mechanism 4 is invalid because the kinetic constant increased with temperature and absorption constant for H2CO decreased while the absorption constants of CO and water increased Many other models exist but we will focus on M3 because it had R2 values of 0.0021 and 0.45 when compared to experimental data in this study Image adapted from http://www.aidic.it/cet/11/25/118.pdf

Kinetic Constant & Activation Energy Lumped kinetic constant values Kinetic constants each component extracted from the top table M3 was found to yield the most physically significant result so was mainly focused on the statistical testing Activation energy for M3 found to be 90 to 120 kJ/mol Heat of adsorption of CO 70 to 185 kJ/mol Heat of adsorption of H2 20 to 105 kJ/mol Image adapted from http://www.aidic.it/cet/11/25/118.pdf

Formation of Alkanes & Side Reactions (2n + 1)H2 + (n)CO → CnH2n+2+ (n)H2O Alkenes: (2n)H2 + (n)CO → CnH2n +(n)H2O Alcohol: (2n)H2 + (n)CO → CnH2n+2 + (n-1)H2O Boudouard: 2CO → C+ CO2 Alkanes ideal formula of CnH2n+2 desired Typically n ranges from 10-20 Produced alkanes are likely to be straight chain, used as diesel fuel Alkane formation needs stronger hydrogenating catalyst than alkene Reactions adapted from http://onlinelibrary.wiley.com/doi/10.1002/14356007.a07_197/full

H2/CO feed ratio Reaction rate decreases slightly at lower H2/CO ratios Significantly lower below 2 due to inhibition of CO2

Product Distribution As feed ratio is increased selectivity to methane, light gases, and gasoline increases Increasing feed ratio also decreases selectivity to diesel/wax

Effect of Temperature/Pressure Increasing temperature increased the reaction rate constant (k) Increasing feed pressure of CO increased the reaction rate, while increasing feed pressure of H2 decreased reaction rate. Increasing CO feed pressure decreases water pressure in the reactor, while increasing H2 pressure increases water pressure in the reactor. Water is a strong inhibitor on the catalyst and reduces reaction rate of FT reaction.

Catalysts Usually cobalt or iron based Cobalt is used for natural gas, while iron is used for biomass and coal Iron- biomass or coal, more water-gas-shift activity than cobalt Ruthenium- very active, very expensive Nickel- promotes methane formation, generally not desirable Iron has more water-gas shift activity than cobalt, so it is better for biomass or coal feeds due to their low H2/CO ratio. Natural gas is more hydrogen rich so cobalt is used Cobalt more expensive than iron, but operates at lower pressure and lasts longer, so costs offset.

Diesel Fuel Fischer-Tropsch processes produce a synthetic diesel fuel Low sulfur-content fuel, less harmful to environment As opposed to traditional petroleum-derived diesel

Conclusion Mechanism 3 was valid when using the assumption that water does not adsorb strongly on the catalyst surface Ideal formula of CnH2n+2 desired for diesel fuel. Typically n ranges from 10-20. Reaction rate decreases slightly at lower H2/CO ratios and it is significantly lower below a ratio of 2 due to inhibition of CO2 Increasing feed ratio increased selectivity to shorter chains Increasing temperature increased the reaction rate constant Increasing feed pressure of CO increased the reaction rate, while increasing feed pressure of H2 decreased reaction rate.

Questions?

Reference Links http://www.rug.nl/research/portal/files/9883465/thesis.pdf (pg.150) http://www.aidic.it/cet/11/25/118.pdf https://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_process https://www.netl.doe.gov/research/coal/energy-systems/gasification/gasifipedia/ftsynthesis http://onlinelibrary.wiley.com/doi/10.1002/14356007.a07_197/abstract