Synthesis processing, methanation

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. 

Two synthesis processes account for most of the hydrogen cyanide produced. The dominant commercial process for direct production of hydrogen cyanide is based on classic technology (23—32) involving the reaction of ammonia, methane (natural gas), and air over a platinum catalyst it is called the Andmssow process. The second process involves the reaction of ammonia and methane and is called the BlausAure-Methan-Ammoniak (BMA) process (30,33—35) it was developed by Degussa in Germany. Hydrogen cyanide is also obtained as a by-product in the manufacture of acrylonitrile (qv) by the ammoxidation of propjiene (Sohio process). 

When the gasified coal is to be used for synthesis of methane methanol, or hydrogen, part or all of it is subjected to the water-gas shift reaction, converting CO and water to CO2 and H2. Sulfur must be removed completely. The acid gases H2S and CO2 are first extracted from the gas before or after the shift conversion these acid gases may be processed in a second step in a Claus unit. The acid gas composition depends on each part of the sequence preceding the Claus unit. 

The normal approach to the chemistry of the post shock gas is to add gas phase reactions which, while prohibited from occurring under ambient conditions, can now occur at the elevated temperatures present. These processes include exothermic reactions with small activation energy barriers (e.g. neutral-neutral reactions) as well as slightly endothermic reactions. For example, a neutral-neutral synthesis of methane can occur (Mitchell 1984) via the following reactions in which the activation energy in terms of temperature is included ... 

Let us note once again that comparison of the results on methanol oxidation with hydrogen peroxide with methane oxidation data under atmospheric pressure (refer to Table 4.3, Figures 4.10 and 4.11) indicates significant differences in these processes. Methane is oxidized to formaldehyde at a higher rate and higher selectivity than at methanol oxidation. Low methanol yields at methane oxidation compared with formaldehyde confirm parallel proceeding of formaldehyde and methanol synthesis from methane. 

Shieh, J. H. and Fan, L. T., "Multiobjective Optimal Synthesis of Methanation Process," AIChE Annual Meeting, Chicago, IL, Nov. (1980). 

Syngas composition, most importantly the H2/CO ratio, varies as a function of production technology and feedstock. Steam methane reforming yields H2/CO ratios of three to one whereas coal and biomass gasification yields ratios closer to unity or lower. Conversely, the required properties of the syngas are a function of the synthesis process. Fewer moles of product almost always occur when H2 and CO are converted to fuels and chemicals. Consequently, syngas conversion processes are more thermodynamically favorable at higher H2 and CO partial pressures. The optimum pressures depend on the specific synthesis process. 

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. 

The application of monolith catalysts to a variety of commercial synthesis processes has been investigated because of the potentially smaller size and lower pressure drop through the chemical reactors. One of the earliest of these investigations was for methanation, the chemical reaction between carbon monoxide and hydrogen to produce methane selectively. In a detailed study [14] a comparative evaluation involved the use of nickel catalyst on (1) spherical alumina pellets (0.32-cm diameter), (2) alumina washcoated (10-20% by weight) cordierite monoliths with 31- and 46-cells (square)/cm density, (3) an alumina... 

One of the ways in which natural gas could be converted to liquid products is by Fischer-Tropsch synthesis. In this process, methane is reformed with steam and oxygen to produce a synthesis gas that is a mixture of carbon monoxide and hydrogen. The synthesis gas is then reacted over a catalyst to produce a variety of fuels. However, recently the most emphasis has been on the production of high-cetane, sulfur-free diesel fuel. Fischer-Tropsch fuels can be produced at the equivalent of 14 to 20 a barrel of oil, and plants with capacities of 10 to 100,000 barrels a day have either been built or designed.1... 

The fundamental advantage of the use of fluidized catalysts for the highly exothermic hydrocarbon synthesis consists of a radical solution of the crucial heat transfer problem, which limited the yield per space and time in the case of fixed catalyst beds. The fluidized system presents the possibility of going to higher synthesis temperatures which means higher conversions with cheaper catalysts and more efficient heat recovery. This can be done without producing excessive amounts of carbon or methane. The yields of valuable olefinic hydrocarbons are very high in comparison with other hydrocarbon synthesis processes. 

In the methane-methanobmethanal process, the major steps are methane splitting, methanol synthesis, methanal synthesis, and methanal electrolysis, for which the chemical reactions are [1] ... 

For example, in methane the bond energy of the carbon to hydrogen bond is one quarter of the Enthalpy of the synthesis process. 

In connection with the synthesis of methane and other organic substances plasma reduced mixtures of CO2 and H2, that is, CO and H2O have been passed over a number of mixed catalysts previously reduced in a stream of H2 for several hours at 420-440 °C. Figure 37 shows a typical kinetic curve from such an experiment. The upper section ab of the curve corresponds, to the plasma reaction, the horizontal section be corresponds to the conditioning at 150 °C of the catalyst, and the vertical section de corresponds to the afterglow stage of the reaction which is rapid. Note that an appreciable acceleration begins immediately after the temperatures was raised to 185-190 °C (Section cd of the curve). Since the process is highly exothermic, much heat is evolved, the temperature rises rapidly and the catalyst is heated. To stop the reaction the catalytic reactor was cooled to 185 °C (Section ef of the curve). 

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