High temperature Fischer-Tropsch synthesis

Steynberg, A. P., Espinoza, R. L., Jager, B., and Vosloo, A. C. 1999. High temperature Fischer-Tropsch synthesis in commercial practice. Appl. Catal. A 186 41-54. 

Figure 1.16 High-temperature Fischer-Tropsch synthesis reactors, (a) Sasol Synthol circulating fluidized-bed reactor.

Figure 6.11.9 XTL plant (X to liquid with X as natural gas, coal, biomass) with low or high temperature Fischer-Tropsch synthesis.

Change occurred in high-temperature Fischer-Tropsch reactor technology. The circulating fluidized bed Sasol Synthol reactors were replaced by fixed fluidized bed Sasol Advanced Synthol (SAS) reactors.44 This did not meaningfully affect the Fe-HTFT syncrude composition, but it reduced the operating cost of HTFT synthesis. 

The synthetic fuels that can be produced by low-temperature Fischer-Tropsch synthesis inherently have a high quality (being sulfur- and aromatics-free) and can therefore be used as quality improvers with conventional components. 

Fischer-Tropsch Technology FTS can be carried out in several different reactor types fixed bed, fluidized bed, or slurry phase and at different temperatures. The high-temperature Fischer-Tropsch (HTFT) synthesis runs at 320°C-350°C, at which temperatures typically all products are in the gas phase [22], HTFT is operated in fluidized-bed reactors, with iron catalysts. Selectivities correspond to chain-growth probabilities in the range of 0.70-0.75 and are ideal for gasoline production, but olefins and oxygenates are formed as well and are used as chemicals. 

Although not as widely used as a gas-sofid PBR, FBR remains as the only choice for processes such as FCC and high-temperature Fischer-Tropsch (HTFT) synthesis, both of which have key roles in the petroleum processing and petrochemical industries. FCC is a critical step in petroleum refining and involves catalytic breakdown of heavy gas oil molecules into... 

In principle, solids found in nature can display activity as heterogeneous catalysts (e.g., iron ores are applied in high temperature Fischer-Tropsch catalysis). However, after 100 years of intense research in catalysis it has been found that in most cases synthetic materials are more suitable as the reproducibility of their synthesis is higher and their specific properties can be adjusted by the synthetic procedure. 

Heat Release and Reactor Stability. Highly exothermic reactions, such as with phthaHc anhydride manufacture or Fischer-Tropsch synthesis, compounded with the low thermal conductivity of catalyst peUets, make fixed-bed reactors vulnerable to temperature excursions and mnaways. The larger fixed-bed reactors are more difficult to control and thus may limit the reactions to jacketed bundles of tubes with diameters under - 5 cm. The concerns may even be sufficiently large to favor the more complex but back-mixed slurry reactors. 

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. 

The quadrupole doublet has an isomer shift corresponding to iron in the ferric or Fe " state. After reduction in H2 at 675 K the catalyst consists mainly of metallic iron, as evidenced by the sextet, along with some unreduced iron, which gives rise to two doublet contributions of Fe " and Fe " in the centre. The overall degree of iron reduction, as reflected by the relative area under the bcc ion sextet, is high. Fischer-Tropsch synthesis at 575 K in CO and FI2 converts the metallic iron into the Flagg carbide, Fe5C2. The unreduced iron is mainly present as Fe ". Exposure of the carburized catalyst to the air at room temperature leaves most of the carbide phase unaltered but oxidizes the ferrous to ferric iron. 

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... 

In many respects the SMDS process (Figure 18.8) precipitated a change in the Fischer-Tropsch community with respect to the preferred catalyst for Fischer-Tropsch synthesis and the approach to product workup. It is therefore instructive to understand why Shell moved away from iron-based Fischer-Tropsch catalysts (and as a consequence also high-temperature synthesis) and opted for a Co-LTFT process with an uncomplicated refinery design that does not produce... 

The methanation reaction is a highly exothermic process (AH = —49.2 kcal/ mol). The high reaction heat does not cause problems in the purification of hydrogen for ammonia synthesis since only low amounts of residual CO is involved. In methanation of synthesis gas, however, specially designed reactors, cooling systems and highly diluted reactants must be applied. In adiabatic operation less than 3% of CO is allowed in the feed.214 Temperature control is also important to prevent carbon deposition and catalyst sintering. The mechanism of methanation is believed to follow the same pathway as that of Fischer-Tropsch synthesis. 

Possible inter relationships of natural substances are important. Similarities of the low molecular weight alkane isomers from crude oil and Fischer-Tropsch synthesis product have been reported. A similar composition for high temperature coal carbonization has been found. The C4 to C7 alkane isomers from these sources can be calculated quantitatively with equations developed for Fischer-Tropsch products. A reversal of the concentrations of the monomethyl isomers from CG (2 Me > 3 Me) to C7 (3 Me > 2 Me) occurs in all three products comparisons at higher carbon numbers indicate some dissimilarities. Naphthene isomers for crude oil and high temperature coal carbonization also have similar compositions. Aliphatic hydrocarbons from low temperature coal processes are considerably different. The C1 isotopic composition of pure compounds from the various sources are being compared in order to provide information on their origin. 

The higher hydrocarbon formation from syngas has long been industrialized as the Fischer-Tropsch synthesis [27]. Yet, syngas production from the C02 reforming of methane is an endothermic reaction, and requires a high temperature (ca. 1073 K) for a favorable equilibrium ... 

The adsorption of CO is probably the most extensively investigated surface process. CO is a reactant in many catalytic processes (methanol synthesis and methanation, Fischer-Tropsch synthesis, water gas shift, CO oxidation for pollution control, etc. (1,3-5,249,250)), and CO has long been used as a probe molecule to titrate the number of exposed metal atoms and determine the types of adsorption sites in catalysts (27,251). However, even for the simplest elementary step of these reactions, CO adsorption, the relevance of surface science results for heterogeneous catalysis has been questioned (43,44). Are CO adsorbate structures produced under typical UHV conditions (i.e., by exposure of a few Langmuirs (1 L = 10 Torrs) at 100—200 K) at all representative of CO structures present under reaction conditions How good are extrapolations over 10 or more orders of magnitude in pressure Such questions are justified, because there are several scenarios that may account for differences between UHV and high-pressure conditions. Apart from pressure, attention must also be paid to the temperature. 

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