Fischer-Tropsch studies in the slurry phase favouring wax production
Van Berge, Peter Jacobus
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The Fischer-Tropsch synthesis over iron and cobalt based catalysts has been investigated in the slurry phase. Reaction conditions were restricted to those conducive for the optimum production of synthetic wax (i.e. high molecular weight straigth chain hydrocarbons). In this field of application of Fischer-Tropsch, strong indications exist to believe that cobalt does possess the potential of threatening to replace the current commercial iron based catalyst. This possibility is, however, limited to low temperature Fischer- Tropsch and is therefore not applicable in the case of high temperature Fischer-Tropsch (i.e. Synthol, where the optimum production of gasoline is paramount). The main objective of this investigation was thus the comparison between promising cobalt catalysts and the current Sasol commercial low temperature precipitated iron catalyst (i.e. Arge catalyst) in the slurry phase reactor. The slurry reactor was preferred because of the conviction that this mode of operation had all the attributes for replacing the mature tubular fixed bed reactor technology, a belief since then proven correct with the recent successful commercialization of the Sasol Slurry Bed Process (i.e. SSBP). In this comparative study, the emphasis was placed on selectivity and activity (in the absence of mass transfer limitations) as synthesis parameters. This approach guaranteed comparisons solely based upon intrinsic catalyst performance. Care was further exercised not to compare the various selected catalyst in a simplistic manner, but to rather have concentrated on the strengh of each individual catalyst. A distinction was therefore drawn between reaction conditions accentuating these strong characteristics, information believed to be of importance in the design of grass-roots plants. The kinetic investigations into ,the commercial iron based catalyst, revealed its complexity. The reason for this being that aspects such as catalyst oxidation and carbon deposition (restricted to reactor temperatures higher than 250°C), renders the catalyst itself a variable. In certain instances (e.g. bulk phase oxidation resulting in the loss of surface area as a consequence of sintering) these catalyst changes are irreversible. The conclusion was reached that the existence of a generally valid iron based Fischer-Tropsch rate equation could be placed in doubt. In this regard published claims were re-examined, and through statistical analyses of these published data, the conclusion was reached that these published claims are not beyond criticism. As a result of this statistical approach the conclusion was made that H,O, and not in combination with CO,, inhibits the iron based Fischer-Tropsch reaction rate. The cobalt based Fischer-Tropsch kinetic investigation supported the conception that the absence of phenomena such as catalyst oxidation and carbon deposition, strenghtens the likelikhood of a generally valid rate equation. It is believed that the proposed equation , published by Satterfield, could satisfy this need. This equation supports the absence of Fischer- Tropsch reaction rate inhibition by water, the single biggest difference between iron and cobalt based Fischer-Tropsch. From a direct comparative kinetic study, the conclusion was made that supported cobalt catalysts could be prepared with sufficient specific (per unit catalyst mass) activity, in order to compete favourably with the current commercial iron based catalyst employed by Sasol. With respect to the product selectivity investigation, the emphasis was placed on wax (i.e. 1 in the case of reactor wax, and 2 C3,,, in the case of hard wax). The published "double a" model was selected as a reliable tool for accurately extrapolating from the readily quantitative analysable C, to C,, product slate to the desired wax cuts. This technique resulted in the deduction that similar wax selectivities are attainable with cobalt and iron. Speculatively, indications exist to believe that cobalt based wax selectivities can further be improved (and tailored) through fine tuning reactor pressures in combination with space velocities, an avenue of anticipated limited impact in the case of iron. Published literature is also inclined to create the impression that relative high product branching degrees are inherent to cobalt based Fischer-Tropsch, thus negatively influencing wax quality. This perception stems from normal pressure Fischer-Tropsch operations. This investigation, however, underlined the notion that high branching degrees are only associated with normal pressure operation, and that branching degrees attained during medium pressure (- 20 bar) cobalt based operation compares well with that of iron. The improved version of the published "non-trivial-surfacepolymerization" model was used in support of this claim. This model was also successfully used in the elucidation of the Fischer-Tropsch mechanism, in the sense that it provided an ideal tool for the consolidation of published individual mechanistic observations. In conclusion it can be stated that enough information has been generated in order to assume that cobalt based Fischer-Tropsch (low temperature and medium pressure application, preferably in the slurry phase), is a viable commercial alternative to the Arge process.
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