Methanation Fischer-Tropsch synthesis

Sulfur poisoning is a key problem in hydrocarbon synthesis from coal-derived synthesis gas. The most important hydrocarbon synthesis reactions include methanation, Fischer-Tropsch synthesis, and methanol synthesis, which occur typically on nickel, iron, or cobalt, and ZnO-Cu catalysts, respectively. Madon and Shaw (2) reviewed much of the early work dealing with effects of sulfur in Fischer-Tropsch synthesis. Only the most important conclusions of their review will be summarized here. 

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. 

Ethylene epoxidation Carbon dioxide methanation Fischer-Tropsch synthesis... 

Fischer-Tropsch Synthesis The best-known technology for producing hydrocarbons from synthesis gas is the Fischer-Tropsch synthesis. This technology was first demonstrated in Germany in 1902 by Sabatier and Senderens when they hydrogenated carbon monoxide (CO) to methane, using a nickel catalyst. In 1926 Fischer and Tropsch were awarded a patent for the discovery of a catalytic technique to convert synthesis gas to liquid hydrocarbons similar to petroleum. 

Synthesis gas consists of a nonhydrocarhon mixture (H2,CO) ohtain-ahle from more than one source. It is included in this chapter and is further noted in Chapter 5 in relation to methane as a major feedstock for this mixture. This chapter discusses the use of synthesis gas obtained from coal gasification and from different petroleum sources for producing gaseous as well as liquid hydrocarbons (Fischer Tropsch synthesis). 

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... 

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 catalytic partial oxidation of methane for the production of synthesis gas is an interesting alternative to steam reforming which is currently practiced in industry [1]. Significant research efforts have been exerted worldwide in recent years to develop a viable process based on the partial oxidation route [2-9]. This process would offer many advantages over steam reforming, namely (a) the formation of a suitable H2/CO ratio for use in the Fischer-Tropsch synthesis network, (b) the requirement of less energy input due to its exothermic nature, (c) high activity and selectivity for synthesis gas formation. 

Ruthenium is a known active catalyst for the hydrogenation of carbon monoxide to hydrocarbons (the Fischer-Tropsch synthesis). It was shown that on rathenized electrodes, methane can form in the electroreduction of carbon dioxide as weU. At temperatures of 45 to 80°C in acidihed solutions of Na2S04 (pH 3 to 4), faradaic yields for methane formation up to 40% were reported. On a molybdenium electrode in a similar solution, a yield of 50% for methanol formation was observed, but the yield dropped sharply during electrolysis, due to progressive poisoning of the electrode. 

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. 

Iron-based Fischer-Tropsch synthesis (FTS) catalysts are preferred for synthesis gas with a low H2/CO ratio (e.g., 0.7) because of their excellent activity for the water-gas shift reaction, lower cost, lower methane selectivity, high olefin... 

Methane is always one of the products of any Fischer-Tropsch synthesis and its production from CO may be schematically represented as the formal addition of three moles of hydrogen to a metal carbonyl giving,... 

Kim42,43 found higher CO conversion and higher C5+ and lower methane selectivities when external water was added (20 parts per 100 parts of 2 1 H2 CO feed) during Fischer-Tropsch synthesis. However, when the water concentration in the feed was higher than 28% the CO conversion decreased. 

If the gasifier product stream is intended for downstream use as the feedstock for further upgrading such as methanation, methanol or Fischer Tropsch synthesis, very thorough desulphuri-sation is essential since the catalysts in these upgrading processes are highly sensitive to sulphur poisoning. The methanation catalysts normally cannot tolerate more than 0.05 ppm of sulphur in the feedstock. In addition to H2S sulphur values in the gasifier product it may contain COS, CS2, mercaptans and thiophenes. These are normally removed by activated carbon or zinc oxide filters ahead of the sensitive synthesis catalyst beds. 

Stage 3 shifts the source of hydrocarbon for transportation and chemicals to methane. Methane currently is reformed at elevated temperatures and pressures to synthesis gas. This mixture of hydrogen and carbon monoxide can then be converted via the well-known technologies of methanol synthesis and Fischer-Tropsch synthesis to eventually produce a variety of chemicals and fuels. In this stage, focus... 

Group II The activity drops more than the Ni surface concentration (Fig. 13), i.e., at least about 20 times. However, for several reactions this drop is two or more orders of magnitude. The reactions included in this group are methanation and Fischer-Tropsch synthesis, isomerization, de-hydrocyclization or hydrogenolysis of alkanes, ether formation from alcohols, metathesis of alkylamines, and possibly other reactions. 

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. 

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