Fischer-Tropsch synthesis reaction rate

This case occurs rarely. It usually happens at the cases of considerably small catalyst particles, less external mass transfer coefficient and relative large reaction rate constant. This will happen when the porousless netlike materials are used for catalyst as platinum-net for ammonia oxidation, or thin layer distribution of active component at external surface of catalysts. Besides, it is commonly considered that Fischer-Tropsch synthesis reaction to produce hydrocarbons is also controlled by external diffusion on iron catalysts. 

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... 

Iglesia, E., Soled, S.L., and Fiato, R.A. 1992. Fischer-Tropsch synthesis on cobalt and ruthenium. Metal dispersion and support effects on reaction rate and selectivity. J. Catal. 137 212-24. 

The most difficult problem to solve in the design of a Fischer-Tropsch reactor is its very high exothermicity combined with a high sensitivity of product selectivity to temperature. On an industrial scale, multitubular and bubble column reactors have been widely accepted for this highly exothermic reaction.6 In case of a fixed bed reactor, it is desirable that the catalyst particles are in the millimeter size range to avoid excessive pressure drops. During Fischer-Tropsch synthesis the catalyst pores are filled with liquid FT products (mainly waxes) that may result in a fundamental decrease of the reaction rate caused by pore diffusion processes. Post et al. showed that for catalyst particle diameters in excess of only about 1 mm, the catalyst activity is seriously limited by intraparticle diffusion in both iron and cobalt catalysts.1... 

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... 

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. 

Figure 5.9 Left Mossbauer spectra of a metallic iron catalyst after different periods of Fischer-Tropsch synthesis in CO+H2 at 240°C, showing the conversion of metallic iron (visible by the outer two lines in the upper two spectra) into iron carbides all spectra were recorded at room temperature. Right reaction rate of the Fischer-Tropsch synthesis (upper curve) and the relative contributions of metallic iron and various carbides to the Mossbauer spectra (from [22]).

According to the International Union of Pure and Applied Chemistry (IUPAC O)) the turnover frequency of a catalytic reac tion is defined as the number of molecules reacting per active site in unit time. The term active sites is applied to those sites for adsorption which are effective sites for a particular heterogeneous catalytic reaction. Because it is often impossible to measure the amount of active sites, some indirect method is needed to express the rate data in terms of turnover frequencies In some cases a realistic measure of the number of active sites may be the number of molecules of some compound that can be adsorbed on the catalyst. This measure is frequently used in the literature of the Fischer-Tropsch synthesis, where the amount of adsorption sites is determined by carbon monoxide adsorption on the reduced catalyst. However, it is questionable whether the number of adsorption sites on the reduced catalyst is really an indication of the number of sites on the catalyst active during the synthesis, because the metallic phase of the Fischer-Tropsch catalysts is often carbided or oxidized during the process. 

According to the Sachtler-Biloen mechanism, the Fischer-Tropsch reaction is initiated through CO adsorption followed by CO dissociation. Experimental evidence for the involvement of an oxygen-free intermediate exists it was observed that predeposited C is incorporated into the product during Fischer-Tropsch synthesis when CO was included in the feed gas (3). It is important to distinguish whether during the Fischer-Tropsch s)mthesis CO dissociation is strictly monomolecular or instead involves a reaction with Hads to produce an intermediate "HCO" formyl species that in a subsequent reaction decomposes to "CH" and Oads-Another question is how the rates of CO dissociation, chain growth, and termination depend on the catalyst surface structure. Thus, it is essential to know the relative values of the rate constants for these three elementary reactions. 

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)... 

Available reaction-transport models describe the second regime (reactant transport), which only requires material balances for CO and H2. Recently, we reported preliminary results on a transport-reaction model of hydrocarbon synthesis selectivity that describes intraparticle (diffusion) and interparticle (convection) transport processes (4, 5). The model clearly demonstrates how diffusive and convective restrictions dramatically affect the rate of primary and secondary reactions during Fischer-Tropsch synthesis. Here, we use an extended version of this model to illustrate its use in the design of catalyst pellets for the synthesis of various desired products and for the tailoring of product functionality and molecular weight distribution. 

CO reactants and the H2O product of the synthesis step inhibit many of these secondary reactions. As a result, their rates are often higher near the reactor inlet, near the exit of high conversion reactors, and within transport-limited pellets. On the other hand, larger olefins that are selectively retained within transport-limited pellets preferentially react in secondary steps, whether these merely reverse chain termination or lead to products not usually formed in the FT synthesis. In later sections, we discuss the effects of olefin hydrogenation, oligomerization, and acid-type cracking on the carbon number distribution and on the functionality of Fischer-Tropsch synthesis products. We also show the dramatic effects of CO depletion and of low water concentrations on the rate and selectivity of secondary reactions during FT synthesis. 

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