Fischer-Tropsch selectivity

H. Combined Effects of Structural Catalyst Parameters on Fischer-Tropsch Selectivity... 

Fig. 10 Fischer-Tropsch chain growth mechanism (Fischer-Tropsch selectivity). (View this art in color at www.dekker.com.)...

A topic of current interest is that of methane activation to give ethane or selected oxidation products such as methanol or formaldehyde. Oxide catalysts are used, and there may be mechanistic connections with the Fischer-Tropsch system (see Ref. 285). 

Synthetic Fuels. Hydrocarbon Hquids made from nonpetroleum sources can be used in steam crackers to produce olefins. Fischer-Tropsch Hquids, oil-shale Hquids, and coal-Hquefaction products are examples (61) (see Fuels, synthetic). Work using Fischer-Tropsch catalysts indicates that olefins can be made directly from synthesis gas—carbon monoxide and hydrogen (62,63). Shape-selective molecular sieves (qv) also are being evaluated (64). 

Fischer Tropsch synthesis is catalyzed by a variety of transition metals such as iron, nickel, and cobalt. Iron is the preferred catalyst due to its higher activity and lower cost. Nickel produces large amounts of methane, while cobalt has a lower reaction rate and lower selectivity than iron. By comparing cobalt and iron catalysts, it was found that cobalt promotes more middle-distillate products. In FTS, cobalt produces... 

L. Seglin Why has Lurgi selected the hot gas recycle process for methanation rather than the isothermal reactor (ARGE) design which they used for the Fischer-Tropsch plant in SASOL s plant in South Africa ... 

Dr. Moeller A methanation plant does not have a problem of selectivity. Whether you operate at low or high temperature, when using a nickel catalyst you will form only methane and no higher hydrocarbon. But with the Fischer-Tropsch synthesis, you have a wide range of possible products which can be formed. If you want to have a certain product, you must keep your temperature at a certain constant value. 

This you cannot do in an adiabatic reactor unless you go to extremely high mixing ratios of fresh feed and recycle gas. In summary, it is a question of selectivity, which is the reason for using the isothermal reactor for Fischer-Tropsch. An adiabatic reactor with a waste heat boiler is cheaper than an isothermal feactor, and hence it is used for methanation. 

Epoxides such as ethylene oxide and higher olefin oxides may be produced by the catalytic oxidation of olefins in gas-liquid-particle operations of the slurry type (S7). The finely divided catalyst (for example, silver oxide on silica gel carrier) is suspended in a chemically inactive liquid, such as dibutyl-phthalate. The liquid functions as a heat sink and a heat-transfer medium, as in the three-phase Fischer-Tropsch processes. It is claimed that the process, because of the superior heat-transfer properties of the slurry reactor, may be operated at high olefin concentrations in the gaseous process stream without loss with respect to yield and selectivity, and that propylene oxide and higher... 

Fischer-Tropsch synthesis products contain also high quantities of n-a-olefins that can be recovered by selective sorption processes with suitable molecular sieves [19]. A large-scale Fischer-Tropsch synthesis plant operates in South Africa [20]. Another plant was started in Indonesia in 1993 [21]. 

Alkali promoters are often used for altering the catalytic activity and selectivity in Fischer-Tropsch synthesis and the water-gas shift reaction, where C02 adsorption plays a significant role. Numerous studies have investigated the effect of alkalis on C02 adsorption and dissociation on Cu, Fe, Rh, Pd, A1 and Ag6,52 As expected, C02 always behaves as an electron acceptor. 

The catalytic partial oxidation of methane for the production of synthesis gas is an interesting alternative to steam reforming which is currently practiced in industry [1]. Significant research efforts have been exerted worldwide in recent years to develop a viable process based on the partial oxidation route [2-9]. This process would offer many advantages over steam reforming, namely (a) the formation of a suitable H2/CO ratio for use in the Fischer-Tropsch synthesis network, (b) the requirement of less energy input due to its exothermic nature, (c) high activity and selectivity for synthesis gas formation. 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

Catalysts were tested for activity in the Fischer-Tropsch reaction using a fixed-bed reactor. The catalyst (0.4 g) was reduced in situ in flowing hydrogen at 425°C for 7 h prior to testing. The test was performed under 2/1 H2/CO at 20 bar total pressure. The initial flow was 64 ml/min, but this was reduced after 24 h to increase the conversion. A final reading of activity and selectivity was taken after 100 h on stream. 

The mannitol-modified catalyst showed significantly increased activity in the Fischer-Tropsch reaction (Table 1.3). After 20 h on stream, the mannitol-modified catalyst is 286% as active as the unmodified catalyst, and 262% as active after 100 h. The selectivity values of the catalysts are similar. Hence, the characteristics of a mannitol-modified catalyst are that it has a higher activity than but the same selectivity as an unmodified catalyst. Increased activity for the FT reaction... 

The potential of carbon nanomaterials for the Fischer-Tropsch synthesis was investigated by employing three different nanomaterials as catalyst supports. Herringbone (HB) and platelet (PL) type nanofibers as well as multiwalled (MW) nanotubes were examined in terms of stability, activity, and selectivity for Fischer-Tropsch synthesis (FTS). 

Yu, Z., Borg, 0., Chen, D., Enger, B. C., Frpseth, V., Rytter, E., Wigum, H., and Holmen, A. 2006. Carbon nanofiber supported cobalt catalysts for Fischer-Tropsch synthesis with high activity and selectivity. Catalysis Letters 109 43 -7. 

Schulz, H., Claeys, M., and Harms, S. 1997. Effect of water partial pressure on steady state Fischer-Tropsch activity and selectivity of a promoted cobalt catalyst. Stud. Surf. Sci. Catal. 107 193-200. 

Krishnamoorthy, S., Tu, M., Ojeda, M. P., Pinna, D., and Iglesia, E. 2002. An investigation of the effects of water on rate and selectivity for the Fischer-Tropsch synthesis on cobalt-based catalysts. J. Catal. 211 422-33. 

Bertole, C. J., Kiss, G., and Mims, C. A. 2004. The effect of surface-active carbon on hydrocarbon selectivity in the cobalt-catalyzed Fischer-Tropsch synthesis. J. Catal. 223 309-18. 

Iglesia, E., Reyes, S. C., Madon, R. J., and Soled, S. L. 1993. Selectivity control and catalyst design in the Fischer-Tropsch synthesis Sites, pellets, and reactors. Adv. Catal. 39 221-302. 

Summary of Catalyst Composition and Reaction Conditions Selected for Comparing Fischer-Tropsch Synthesis Activity in CSTR... 

Espinoza, R.L., Shingles, T., Duvenhage, D.J., and Langenhoven, P.L., Method of modifying and controlling catalyst selectivity in a Fischer-Tropsch process. U.S. patent 6,653,357, Sasol Technology, Nov. 25, 2003. 

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