Fischer-Tropsch rates

The power law kinetic equation could be a simplified form of a mechanistic scheme. A summary of some of the reported reaction orders for the partial pressure of hydrogen and carbon monoxide which have been obtained from power law fits by different groups are listed in Table 9. The partial pressure dependencies vary rather widely. The power law fits were obtained for different cobalt catalysts prepared using different supports and methods. The data in Table 9 show that there is not one best power law equation that would provide a good fit for all cobalt catalysts. Brotz [10], Yang et al. [12] and Pannell et al. [13] defined the Fischer-Tropsch rate as the moles of hydrogen plus carbon monoxide converted per time per mass of catalyst (r g+Hj) Wang... 

Fischer-Tropsch Process. The Hterature on the hydrogenation of carbon monoxide dates back to 1902 when the synthesis of methane from synthesis gas over a nickel catalyst was reported (17). In 1923, F. Fischer and H. Tropsch reported the formation of a mixture of organic compounds they called synthol by reaction of synthesis gas over alkalized iron turnings at 10—15 MPa (99—150 atm) and 400—450°C (18). This mixture contained mostly oxygenated compounds, but also contained a small amount of alkanes and alkenes. Further study of the reaction at 0.7 MPa (6.9 atm) revealed that low pressure favored olefinic and paraffinic hydrocarbons and minimized oxygenates, but at this pressure the reaction rate was very low. Because of their pioneering work on catalytic hydrocarbon synthesis, this class of reactions became known as the Fischer-Tropsch (FT) synthesis. 

The principal advance ia technology for SASOL I relative to the German Fischer-Tropsch plants was the development of a fluidized-bed reactor/regenerator system designed by M. W. Kellogg for the synthesis reaction. The reactor consists of an entrained-flow reactor ia series with a fluidized-bed regenerator (Fig. 14). Each fluidized-bed reactor processes 80,000 m /h of feed at a temperature of 320 to 330°C and 2.2 MPa (22 atm), and produces approximately 300 m (2000 barrels) per day of Hquid hydrocarbon product with a catalyst circulation rate of over 6000 t/h (49). 

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... 

Instinctively it would seem that Step C would be rate controlling and the slowest. In the case of the Fischer-Tropsch reaction, one would postulate that the surface would offer more methylene groups for chain polymerization. This mechanism differs from that of Vlasenko and Uzefo-vich (4) essentially in the concept that the whole molecule interacts with the surface. Furthermore, the HCOH intermediate is wholly horizontal to the surface rather than perpendicular. 

Calderbank et al. (C6) studied the Fischer-Tropsch reaction in slurry reactors of 2- and 10-in. diameters, at pressures of 11 and 22 atm, and at a temperature of 265°C. It was assumed that the liquid-film diffusion of hydrogen from the gas-liquid interface is a rate-determining step, whereas the mass transfer of hydrogen from the bulk liquid to the catalyst was believed to be rapid because of the high ratio between catalyst exterior surface area and bubble surface area. The experimental data were not in complete agreement with a theoretical model based on these assumptions. 

FIGURE 2.2 TPR profiles of carbon nanomaterial Fischer-Tropsch catalysts (gas mixture 10% H2 in Ar heating rate 10 K/min). 

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. 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The TPR-XAFS technique confirmed that doping Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) remarkably promotes the carburization rate relative to the undoped catalyst. The EXAFS results suggest that either the Hagg or e-carbides were formed during the reduction process over the cementite form. A correlation is observed between the a-value of the product distribution and the carburization rate. 

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 reality, not only the main reaction (the Fischer-Tropsch reaction) leading to the formation of higher hydrocarbons (Equation 12.1), but also methane formation (Equation 12.2) and the water-gas shift reaction (Equation 12.3) have to be considered. The rate equations for these three reactions on a commercial Fe-catalyst were determined by Popp8 and Raak2 and summarized by Jess et al.9 However, to simplify matters, just the Fischer-Tropsch reaction forms the basis of the approach presented here ... 

A continuous cross-flow filtration process has been utilized to investigate the effectiveness in the separation of nano sized (3-5 nm) iron-based catalyst particles from simulated Fischer-Tropsch (FT) catalyst/wax slurry in a pilot-scale slurry bubble column reactor (SBCR). A prototype stainless steel cross-flow filtration module (nominal pore opening of 0.1 pm) was used. A series of cross-flow filtration experiments were initiated to study the effect of mono-olefins and aliphatic alcohol on the filtration flux and membrane performance. 1-hexadecene and 1-dodecanol were doped into activated iron catalyst slurry (with Polywax 500 and 655 as simulated FT wax) to evaluate the effect of their presence on filtration performance. The 1-hexadecene concentrations were varied from 5 to 25 wt% and 1-dodecanol concentrations were varied from 6 to 17 wt% to simulate a range of FT reactor slurries reported in literature. The addition of 1-dodecanol was found to decrease the permeation rate, while the addition of 1-hexadecene was found to have an insignificant or no effect on the permeation rate. 

At this point, the system was tested with catalyst for activation and FTS, in the hopes that the seal leak rates would be impeded by the presence of small catalyst particles. The FTFE 20-B catalyst (L-3950) (Fe, 50.2% Cu, 4.2% K, 1.5% and Si, 2.4%) was utilized. This is part of the batch used for LaPorte FTS run II.20 The catalyst was activated at 543 K with CO at a space velocity (SV) of 9 sl/h/g catalyst for 48 h. A total of 1,100 g of catalyst was taken and 7.9 L of C30 oil was used as the start-up solvent. At the end of the activation period, an attempt was made for Fischer-Tropsch synthesis at 503 K, 175 psig, syngas SV = 9 sl/h/g catalyst, and H2/CO = 0.7. However, the catalyst was found to be completely inactive for Fischer-Tropsch synthesis. Potential reasons for catalyst poisoning under present experimental conditions were investigated. Sulfur and fluorine are known to poison iron-based Fischer-Tropsch catalysts.21,22 Since the stator of the pump is... 

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. 

The data available for heterogeneous Fischer-Tropsch catalysts indicate that with cobalt-based catalysts the rate of the water gas-shift reaction is very slow under the synthesis conditions (5). Thus, water is formed together with the hydrocarbon products [Eq. (14)]. The iron-based catalysts show some shift activity, but even with these catalysts, considerable quantities of water are produced. 

The extremely low turnover rate (0.011 mmol CO/nmol Co/h, production of 0.003 mmol CH /mmol Co/h average), stimulated attempts to increase the efficiency of the reaction. Indeed, pretreatment involving removal of CO via vacuum thermolysis of CpCo(C0)2 5 to yield " CpCo", followed by its use in a Fischer-Tropsch reaction, led to improved activity (turnover of 0.130 mmol CO/irmol Co/h, production of 0.053 mmol CH /mmol Co/h average). It appears that decarbonylation is necessary for formation of the catalytically active species. 

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]).

S. Krishnamoorthy, M. Tu, M. P. Ojeda, D. Pinna and E. Iglesia, An Investigation of the Effects of Water on Rate and Selectivity for the Fischer-Tropsch synthesis on Cobalt-Based Catalysts, J. Catal., 2002, 211, 422-433. 

---