Fischer-Tropsch process mechanism

Foreseeable improvements that will increase operability and decrease operating costs of Fischer-Tropsch processes are the development for the fluidized-iron process of a catalyst that will not accelerate the reaction 2CO = C02 + C and will not be appreciably oxidized during the steady-state life of the catalyst and the development of a more active and mechanically stable catalyst for the oil-circulation process so as further to reduce Ci + C2 production. The hot-gas recycle process could be made operable by use of a catalyst that will be less active but more resistant to thermal shock which occurs during regeneration to remove carbon deposits, and during operation at lower end-gas recycle rates. The powdered catalyst-oil slurry process recently has been satisfactorily operated in a pilot plant by K6lbel and Ackerman (21). Although the space-time yield in this operation was low (10 to 20 kg. of C3+ per cubic meter of slurry per hour), the Ci + C2 production was less than one third of that... 

Good evidence has been obtained that heterogeneous iron, ruthenium, cobalt, and nickel catalysts which convert synthesis gas to methane or higher alkanes (Fischer-Tropsch process) effect the initial dissociation of CO to a catalyst-bound carbide (8-13). The carbide is subsequently reduced by H2to a catalyst-bound methylidene, which under reaction conditions is either polymerized or further hydrogenated 13). This is essentially identical to the hydrocarbon synthesis mechanism advanced by Fischer and Tropsch in 1926 14). For these reactions, formyl intermediates seem all but excluded. 

In a Fischer-Tropsch process, some of the elementary steps in the reaction mechanisms that are characterized by "CH2" as the key growth intermediate may occur in parallel to give chain growth. These parallel... 

Complexes of the type [Cp Rh(R)(/4-CH2)]2 have been studied to probe details of the coupling of alkyl and methylene groups that could be relevant to mechanisms for the Fischer-Tropsch process (see Organic Synthesis Using Metal-mediated Coupling Reactions) Reaction... 

Another system in this class of mechanisms that was also modeled mathematically is the technically important Fischer-Tropsch process on Fe catalysts investigated by Caldwell (217,218). Oscillation had been previously observed in this system by Tsotsis and co-workers (279). Caldwell reduced the mechanism of the Fischer-Tropsch reaction to three reactions ... 

Since the 1980s, SSITKA has been widely used to understand the formation mechanism of methane as the first paraffin in the chain. The study of the dynamics of the entire complex of reactions involved in the Fischer-Tropsch process became possible only after the development of the GC-MS technique with high resolution time. A review of field suggests that the cycle of papers by van Dijk et al. [18-21] describes the results that were obtained using the full potential of the SSITKA technique. First, a comparison of C, O, and H labeling on different Co-based catalyst formulations and in different conditions was made. For the first time, a substantial part of the product spectrum (both hydrocarbons and alcohols) was included in the isotopic transient analysis. After the qualitative interpretation of the experimental data, extensive mathematical modeling was performed for the identification and discrimination of reaction mechanisms. 

Methane and Higher Hydrocarbon Formation A combination of and D labeling was used to elucidate the mechanism of methane and higher hydrocarbon formation. Figure 51.9A shows the data on in CO and CFL, tracing that are typical of all Cobased catalysts used in the Fischer-Tropsch process. 

The reaction is done with a metal catalyst. Catalysts based upon iron or cobalt have been used commercially for hydrocarbon synthesis [5], The mechanism involves adsorption of hydrogen and carbon monoxide on the metal surface [6]. The Fischer-Tropsch process enables natural gas to be converted to liquid synthetic fuel. First, the natural gas is oxidized to syn gas which is then converted by the Fischer-Tropsch process to the liquid hydrocarbon mixture, which is useful as fuel. 

The Fischer-Tropsch process was discovered nearly 90 years ago. It converts Hz and CO into hydrocarbons by means of a heterogeneous catalyst, normally cobalt based. This is a commercially important process with several plants in South Africa and Malaysia and many more starting up around the world. The exact mechanism for this transformation is most certainly quite complex, but ultimately organic radicals on the surface must undergo a reductive elimination reaction. In the next couple of problems we shall examine some of the mechanistic issues that arise. Consider the (III) surface of an fee metal (see 23.10). 

Davis, B. H. 2001. Fischer-Tropsch synthesis Current mechanism and futuristic needs. Fuel Process. Technol. 71 157-66. 

In this work, a detailed kinetic model for the Fischer-Tropsch synthesis (FTS) has been developed. Based on the analysis of the literature data concerning the FT reaction mechanism and on the results we obtained from chemical enrichment experiments, we have first defined a detailed FT mechanism for a cobalt-based catalyst, explaining the synthesis of each product through the evolution of adsorbed reaction intermediates. Moreover, appropriate rate laws have been attributed to each reaction step and the resulting kinetic scheme fitted to a comprehensive set of FT data describing the effect of process conditions on catalyst activity and selectivity in the range of process conditions typical of industrial operations. 

In the case of ethane, this mechanism cannot occur since the resulting metal-ethyl intermediate does not display any alkyl group in the P-position. Consequently, with tantalum hydride(s), 3, which cleave ethane, another process must take place, involving only one carbon atom at a time. Among various reasonable possibilities, we assume a carbene deinsertion from a tantalum-ethyl species because the reverse step is known in organometallic chemistry (Scheme 3.4) [22]. Note that this reverse step has been postulated as the key step in Fischer-Tropsch synthesis [23]. 

The question of the mechanism of Fischer-Tropsch reaction is of considerable controversy. Three principal routes for product formation have been proposed the carbide mechanism, the hydroxymethylene mechanism, and the CO insertion mechanism. Numerous modifications were also introduced in attempts to account for some details in the complex chemistry of the process.205 207 208 211 229-233... 

The methanation reaction is a highly exothermic process (AH = —49.2 kcal/ mol). The high reaction heat does not cause problems in the purification of hydrogen for ammonia synthesis since only low amounts of residual CO is involved. In methanation of synthesis gas, however, specially designed reactors, cooling systems and highly diluted reactants must be applied. In adiabatic operation less than 3% of CO is allowed in the feed.214 Temperature control is also important to prevent carbon deposition and catalyst sintering. The mechanism of methanation is believed to follow the same pathway as that of Fischer-Tropsch synthesis. 

---