Fischer-Tropsch synthesis rate equation

Fischer-Tropsch synthesis can be regarded as a surface polymerization reaction since monomer units are produced from the reagents hydrogen and carbon monoxide in situ on the surface of the catalyst. Hence, a variety of hydrocarbons (mainly n-paraffines) are formed from hydrogen and carbon monoxide by successive addition of C, units to hydrocarbon chains on the catalyst surface (Equation 12.1). Additionally, carbon dioxide (Equation 12.3) and steam (Equations 12.1 and 12.2) are produced C02 affects the reaction just a little, whereas H20 shows a strong inhibiting effect on the reaction rate when iron catalysts are used. 

In case of Fischer-Tropsch synthesis, we have to consider that the first-order reaction rate constant is related to the concentration in the gas phase (e.g., ce2), and that the diffusive flux in the liquid-filled pores is related to the concentration in the liquid (ce21). Thus, instead of Equation 12.10, we have to use... 

Studies of the Fischer-Tropsch synthesis on nitrided catalysts at the Bureau of Mines have been described (4,5,23). These experiments were made in laboratory-scale, fixed-bed testing units (24). In reference 5, the catalyst activity was expressed as cubic centimeters of synthesis gas converted per gram of iron per hour at 240°C. and at a constant conversion of 65%. Actually, the experiments were not conducted at 240°C., but the activity was corrected to this temperature by the use of an empirical rate equation (25). Conditions of catalyst pretreatment for one precipitated and two fused catalysts are given in Table IV. 

Because the expressions for methanation are easier to interpret than those for Fischer-Tropsch synthesis, we first present the equations for the former reaction. The rate of methanation is given by Equation (1)... 

To simulate the dynamic behavior of the Fischer-Tropsch synthesis a reactor description and a set of detailed kinetic equations and constants are needed. In literature much is known about Fischer-Tropsch reactors (e.g. [1]), but the detailed kinetics is lacking. For calculation of conversions or selectivities towards certain (light) products or fi actions rather simple reaction kinetics is enough, but the description of the reaction rates of both reactants and products requires more detailed information about the reaction mechanism and the constants in the rate equations. 

Anderson BJ3, Karn FS A rate equation for the Fischer-Tropsch synthesis on iron catalysts, JPhys Chem 64(6) 805-808, 1960. 

Since the discovery of the synthesis at the Kaiser Wilhelm Institute (Germany) in 1923 by Franz Fischer and Hans Tropsch, the kinetics of the Fischer-Tropsch synthesis have been studied extensively and many attempts have been made to describe the rate of reaction, either by using power law rate equations or equations based on certain mechanistic assumptions. In most cases, the rate of H2 and CO consumption is correlated with the (measurable) gas phase concentrations or partial pressures of H2, CO, and/or H2O. An overview of rate equations for iron catalysts is given by Huff and Satterfield (1984a) and for cobalt catalysts by Yates and Satterfield (1991). Details on the kinetics and reaction mechanism are, for example, discussed by Donnelly and Satterfield (1989), Dry (1982), Fernandes (2005), Huff and Satterfield (1984b), Post et al. (1989), Riedel et al. (1999), Schulz and Claeys (1999), Schulz et al. (1999), Van Steen and Schulz (1999), and Van Steen (1993). 

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