Fischer-Tropsch conversion catalysts

Iron-based Fischer-Tropsch (FT) catalysts undergo a series of phase transformations during activation and use (1). Activation with carbon monoxide or syngas typically results in the conversion of Fe O to Fe O and ultimately to one or more iron carbides (2). During FT synthesis, iron carbides can be oxidized to Fe O if the or COj/CO ratios are high... 

These results make it clear that relating the catalyst composition to the catalytic activity of an iron Fischer-Tropsch synthesis catalyst will be a demanding task. It appears that the carbide phase is more active than the oxide phase. Furthermore, the data for the promoted and unpromoted show that the initial activity of the x-Fe5C2 phase is as great as the e or e -carbide (Fe2C to Fe2 2C) phases. The present data do not allow us to decide whether it is the presence of potassium or the stability of the iron carbide phases that allows the e or e -carbide (Fe2C to Fe2.2C) phases to remain as the conversion declines to a low level while the x-FesC2 oxidizes as the conversion decreases. 

Ternary composites have also been used comprising a Fischer-Tropsch catalyst, a methanol synthesis catalyst, and a zeolite [100]. Two Fe-based catalysts (ie, one promoted by K and the other by Ru), two HY zeolites with different acidities, a commercial HZSM-5, and Cu/ZnO/AljOj (methanol synthesis catalyst) were tested in these composites. Dimethyl ether (DME), methanol, and hydrocarbons were formed. Addition of the Cu/ZnO/Al Oj catalyst to a binary mixture of a Fischer-Tropsch catalyst and HZSM-5 results in the increase of the CO conversion by more than 20 times. The DME selectivity decreases as the conversion increases. Y zeolites and the Fischer-Tropsch synthesis catalyst promoted by Ru generated the most active composites. The role of zeolites in the ternary composite is assumed with the DME synthesis. First, methanol is synthesized from syngas on Cu/ZnO/Al Oj then it is dehydrated by an acid catalyst to produce DME and finally, DME initiates FT synthesis, which is then propagated by CO. 

Sasol Fischer-Tropsch Process. 1-Propanol is one of the products from Sasol s Fischer-Tropsch process (7). Coal (qv) is gasified ia Lurgi reactors to produce synthesis gas (H2/CO). After separation from gas Hquids and purification, the synthesis gas is fed iato the Sasol Synthol plant where it is entrained with a powdered iron-based catalyst within the fluid-bed reactors. The exothermic Fischer-Tropsch reaction produces a mixture of hydrocarbons (qv) and oxygenates. The condensation products from the process consist of hydrocarbon Hquids and an aqueous stream that contains a mixture of ketones (qv) and alcohols. The ketones and alcohols are recovered and most of the alcohols are used for the blending of high octane gasoline. Some of the alcohol streams are further purified by distillation to yield pure 1-propanol and ethanol ia a multiunit plant, which has a total capacity of 25,000-30,000 t/yr (see Coal conversion processes, gasification). 

The first demonstration of catalytic conversion of synthesis gas to hydrocarbons was accompHshed ia 1902 usiag a nickel catalyst (42). The fundamental research and process development on the catalytic reduction of carbon monoxide was carried out by Fischer, Tropsch, and Pichler (43). Whereas the chemistry of the Fischer-Tropsch synthesis is complex, generalized stoichiometric relationships are often used to represent the fundamental aspects ... 

The Fischer-Tropsch reaction is highly exothermic. Therefore, adequate heat removal is critical. High temperatures residt in high yields of methane, as well as coking and sintering of the catalyst. Three types of reac tors (tubular fixed bed, fluidized bed, and slurry) provide good temperature control, and all three types are being used for synthesis gas conversion. The first plants used tubular or plate-type fixed-bed reactors. Later, SASOL, in South Africa, used fluidized-bed reactors, and most recently, slurry reactors have come into use. 

The Fischer-Tropsch process is of considerable economic interest because it is the basis of conversion of carbon monoxide to synthetic hydrocarbon fuels, and extensive work has been done on optimization of catalyst systems. 

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 FTS was conducted at varying temperatures (from 483 to 513 K) over approximately 50 h of reaction time in order to investigate the reaction kinetics achieved with the respective catalysts. A typical conversion curve using the Co/ HB catalyst as an example is shown in Figure 2.3. After a short settling phase (caused by the pore filling of liquid Fischer-Tropsch products) of only about 4 h, steady-state conditions were reached. In the observed synthesis period of 50 h no deactivation of the catalysts was detected. However, industrially relevant experiments over several weeks are still outstanding. 

Bezemer, G. L., van Laak, A., van Dillen, A. J., and de Jong, K. P. 2004. Cobalt supported on carbon nanofibers—A promising novel Fischer-Tropsch catalyst. Natural Gas Conversion 147 259-64. 

The data derived from modeling at different conversion degrees (X = 5, 40, and 80%) were also compared to the results obtained from the calculation of the classical Thiele modulus. The calculated (by the Thiele modulus) and modeled (by Presto Kinetics) effectiveness factors showed comparable values. Hence, the usage of simulation software is not required to get a first impression of the diffusion limitations in a Fischer-Tropsch catalyst pore. Nevertheless, modeling represents a valuable tool to better understand conditions within a catalyst pore. 

Sarup, B., and Wojciechowski, B.W. 1989. Studies of the Fischer-Tropsch synthesis on a cobalt catalyst. II. Kinetics of carbon monoxide conversion to methane and to higher hydrocarbons. Can. J. Chem. Eng. 67 62-74. 

Wang, Y., and Davis, B. H. 1999. Fischer-Tropsch synthesis Conversion of alcohols over iron oxide and iron carbide catalysts. Applied Catalysis A General 180 277-85. 

In view of the size of operation being contemplated, it is unlikely that homogeneous catalysts will play a primary role in the production of synthetic oil. However, from the standpoint of the chemical industry, the complex mixture of products obtained from the classical Fischer-Tropsch process is generally unattractive owing to the economic constraints imposed by costly separation/purification processes. What is needed is a catalyst system for the selective conversion of CO/H2 mixtures to added-... 

If the reaction in which the metallic fraction serves as a catalyst produces water as a by-product, it may well be that the catalyst converts back to an oxide. One should always be aware that in fundamental catalytic studies, where reactions are usually carried out under differential conditions (i.e. low conversions) the catalyst may be more reduced than is the case under industrial conditions. An example is the behavior of iron in the Fischer-Tropsch reaction, where the industrial iron catalyst at work contains substantial fractions of Fe304, while fundamental studies report that iron is entirely carbidic and in the zero-valent state when the reaction is run at low conversions [6],... 

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]).

The conversion of iron catalysts into iron carbide under Fischer-Tropsch conditions is well known and has been the subject of several studies [20-23], A fundamentally intriguing question is why the active iron Fischer-Tropsch catalyst consists of iron carbide, while cobalt, nickel and ruthenium are active as a metal. Figure 5.9 (left) shows how metallic iron particles convert to carbides in a mixture of CO and H2 at 515 K. After 0.5 and 1.1 h of reaction, the sharp six-line pattern of metallic iron is still clearly visible in addition to the complicated carbide spectra, but after 2.5 h the metallic iron has disappeared. At short reaction times, a rather broad spectral component appears - better visible in carburization experiments at lower temperatures - indicated as FexC. The eventually remaining pattern can be understood as the combination of two different carbides -Fe2.2C and %-Fe5C2. 

When the Fe-MnO catalyst is analyzed after use in the Fischer-Tropsch reaction (the synthesis of hydrocarbons from CO and H2), the XRD pattern in Fig. 6.2 reveals that all metallic iron has disappeared. Instead, a number of weak reflections are visible, which are consistent with the presence of iron carbides, as confirmed by Mossbauer spectroscopy [7]. The conversion of iron to carbides under Fischer-Tropsch conditions has been studied by many investigators and has been discussed in more detail in Chapter 5 on Mossbauer spectroscopy. 

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