No abstract

No abstract

No abstract

An experimental study was performed with an aged Co/Pt/Al 2 O 3 catalyst in a laboratory slurry reactor to develop a macrokinetic expression for the Fischer-Tropsch (FT) synthesis. A semiempirical model was found to be the preferred two-parameter rate equation of the reaction. However, it was shown that this model is virtually indistinguishable from a mechanistically derived three-parameter rate model that assumes the following kinetically relevant steps in the cobalt-FT synthesis: CO dissociation occurs without hydrogen interaction and is not a rate-limiting step; the first hydrogen addition to surface carbon and the second hydrogen addition to surface oxygen are the rate-determining steps.

A new product characterization model has been proposed for the iron-based low-temperature Fischer−Tropsch (Fe-LTFT) synthesis. The chain-length-dependent desorption model is based on the premise that the increase in chain-growth probability and decrease in the olefin/paraffin ratio with the carbon number in the Fe-LTFT synthesis is essentially a characteristic of the primary product spectrum. The model could successfully describe the olefin and paraffin distributions in the C3+ range. The ethylene/ethane ratio is overestimated by the model because of the high reactivity of ethylene for secondary hydrogenation. However, the total C2 formation rate was predicted almost perfectly, while the methane formation rate was described adequately, using parameter values that were obtained from the C3−C10 product fraction. This is a true extrapolation, because the C1 and C2 data were not used at all for the estimation of the parameter values. This may be the first product characterization model that can successfully be extrapolated to the C1 and C2 components without introducing additional (unique) parameter values for these products.

Based on the common belief that water inhibits the intrinsic Fischer−Tropsch (FT) reaction rate in the iron-FT synthesis, water is included in almost all iron-FT kinetic expressions. A new rate expression is now proposed where vacant sites, CO, and water are all included in the denominator. This model was evaluated with data from various historical experimental studies. In all cases it was found that the effect of water is not statistically significant and should therefore be omitted from the model. The new model describes the historical data more accurately than some other popular rate equations. To validate these conclusions, new experimental data were measured in a well-mixed slurry reactor in the absence of mass transfer limitations. The experimental methodology employed ensured that the iron-FT catalyst did not suffer measurable deactivation. It was confirmed that there is no basis for including water in the denominator of the new rate equation and that it is more accurate than the rival models considered.

No abstract

The influences of the water partial pressure and the syngas pressure on the reaction kinetics of a commercial alumina-supported cobalt Fischer-Tropsch (FT) catalyst were investigated. Both a fresh catalyst, as well as an aged catalyst from a demonstration reactor, were employed in the study. It was concluded that water has a negligible influence on the overall FT reaction rate but lowers the methane selectivity and increases the CO 2 selectivity. Longer term exposure to higher water partial pressures did not lead to step changes in the catalyst activity but may have increased the rate of gradual irreversible catalyst deactivation. Increasing the syngas pressure at constant H 2 /CO ratio significantly increased the FT reaction rate and decreased the methane selectivity, suggesting a substantial incentive to explore higher pressure operation of commercial cobalt-FT processes.