F-T synthesis

One of the most important, and perhaps the best studied, applications of three-phase fluidization is for the hydrogenation of carbon monoxide by the Fischer-Tropsch (F-T) process in the liquid phase. In this process, synthesis gas of relatively low hydrogen to carbon monoxide ratio (0.6 0.7) is bubbled through a slurry of precipitated catalyst suspended in a heavy oil medium. The F-T synthesis forms saturated and unsaturated hydrocarbon compounds ranging from methane to high-melting paraffin waxes (MW > 20,000) via the following two-step reaction ... 

Selectivity to desired products including light hydrocarbons, gasoline, or diesel fuel depends upon the catalyst employed, the reactor temperature, and the type of process employed. Products of the F-T synthesis are suitable for further chemical processing because of their predominantly straight chain structure and the position of the double bond at the end of the chain. By-products formed on a lesser scale include alcohols, ketones, acids, esters, and aromatics. 

Transport properties and reaction engineering aspects of F-T synthesis have been the subject of comprehensive reviews, e.g., Kolbel and Ralek (1980), Anderson (1984), and Saxena et al. (1986). Readers are referred to these references for details on the F-T synthesis and the extensive literature listings contained in them. 

Li, C., Cao, F.H., Ying, W.W., and Fang, D.Y. 2006. Intrinsic kinetics of Zr02 modified Co-Ru/y-Al203 catalyst for F-T synthesis. Huadong Ligong Daxue Xuebao Ziran Kexueban 32 1253-57. 

It is now superflous to point out the renewed interest for the Fischer-Tropsch (F-T) synthesis (j) i. . the conversion of CO+H2 mixtures into a broad range of products including alkanes, alkenes, alcohols. Recent reviews (292.9k ) emphasized the central problem in F-T synthesis1 selectivity or more precisely chain-length control. 

As already mentioned, the acidity of the HY zeolite precludes its use as a support for Fe3(C0)- 29 then iron particles. The same behaviour is observed for Fe3(C0) 2 silica alumina system. This material, when decomposed at 200°C is not an efficient catalyst for F-T synthesis and only C-j-C products are... 

These adducts are more active than the iron ones in the conversion of syngas. At 250°C, a higher yield of methane is observed (Table U) and carbon dioxide is produced in smaller amounts. Inspection of Table 5 summarizing the influence of the H2/CO ratio on products selectivity also indicates a higher production of saturated hydrocarbons. This behavior is typical for cobalt catalysts in F-T synthesis (j2,25). The chain-length distribution is similar to that observed for catalysts derived... 

After the catalytic runs no modification of mean particle size is observed for this last system. Conversly, Ru CO) deposited on silica-alumina is readily decomposed at 200°C to metallic particles of 1 nm mean size which are also catalysts for the F-T synthesis. The catalytic activity at 200°C is C i one tenth of the Y zeolite supported ones and methane is practically the only hydrocarbon formed. Electron microscopy examination of the catalyst after reaction reveals a drastic sintering of the... 

Fischer-Tropsch Catalysts. - It is well known that all Group VIII transition metals are active for F-T synthesis. However, the only F-T catalysts, which have sufficient CO hydrogenation activity for commercial application, are composed of Ni, Co, Fe or Ru as the active metal phase. These metals are orders-of-magnitude more active than the other Group VIII metals and some characteristics of Ni-, Fe-, Co- and Ru-based F-T catalysts are summarized in Table 2. 

Supported Co-Mn Fischer-Tropsch Catalysts. F-T synthesis of lower hydrocarbons on silicalite-1 supported Co and Co-Mn catalysts was reported by Das et C03O4 was found to be the only phase present in Mn-free... 

Recently, Martinez et al. reported the use of mesoporous Co/SBA-15 catalysts promoted with Mn for the F-T synthesis. They observed that Mn favored the formation of long-chain n-paraffins (Cio+), while decreasing the selectivity towards methane. The Mn-promoted catalysts, however, turned out to be less active than the unpromoted ones. 

Modification of the zeolite appears to have affected the selectivity of Ru in these hydrogenation reactions. Exchange of K cations for Na cations in Y zeolite increases the basicity of the support (ref. 9). In Fischer-Tropsch reactions over similar catalysts, Ru/Y catalysts so modified yielded significant increases in the olefinic product fraction at the expense of paraffins. Olefins are believed to be primary products in F-T synthesis, with paraffins being produced from olefins in secondary hydrogenation reactions. In an analogous fashion, the Ru/KY catalyst used in the present study might also be expected to... 

Although chain growth is not a feature relevant to methanation, the initiation and termination steps of the Anderson model for F-T synthesis are believed by at least some workers in the field to be applicable to the mechanism of the highly specific methanation reaction (71). The formation of methane is proposed to follow from the surface bound hydroxycarbene species by (19). 

With the established success of heterogeneous catalysts in the hydrogenation of CO via methanol synthesis, methanation, and F-T synthesis, it is justifiable to question the interest in investigating these reactions under... 

The CO-hydrogenation reaction, or Fischer-Tropsch (F-T) synthesis reaction, has been thoroughly investigated since its discovery fn the 1920 s [1]. A range of catalysts has been shown to be active for hydrocarbon synthesis and iron [2] and cobalt [3] have found commercial applications in this field. A variety of reactors have been developed to optimize the synthesis reaction [4]. Variations of reactor conditions have been shown to maximize specific products from the broad range of products produced in the reaction [5). 

Fischer-Tropsch. The process most frequently considered for indirect coal liquefaction is the Fischer-Tropsch (F-T) synthesis, developed in 1925 by German chemists Franz Fischer and Hans Tropsch. In the F-T process, synthesis gas is reacted over a catalyst, typically iron or cobalt based, at 1-30 atm and 200-350°C to produce a wide range of mainly aliphatic hydrocarbons, including gas, LPG, gasoline, jet fuel, diesel oil, middle distillates, heavy oil, and waxes. Germany used this technology during World War II to produce nearly 15,000 barrels/day of military fuels. 

The F-T synthesis is basically a polymerization reaction in which carbon monoxide molecules are integrated one at a time into a growing chain, followed by hydrogenation. 

Depending on the hydrogenation activity of the catalyst, the product may be predominantly paraffinic, or it may contain appreciable amounts of olefins and alcohols. The basic reactions in the F-T synthesis are ... 

The F-T synthesis typically follows polymerization kinetics. The Anderson-Schulz-Flory equation describes the product distribution ... 

When the mass fraction of the long-chain hydrocarbon products of the F-T synthesis (W) is plotted against the carbon number (TSf) it is found that W decreases approximately monotonically with molecular size. Thus the major product is the Ci, methane, followed by the C2 hydrocarbons (ethylene and ethane), the C3 hydrocarbons, and so forth, as shown in Figure 15. This distribution follows Schultz-Flory statistics for a polymerization involving the sequential addition of Ci units to a chain, given by the dotted line in Figure 15. Further and more detailed consideration of the mechanisms is in Annex 1. 

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