Methanal Synthesis

As mentioned in Chapter 2, methane is a one-carhon paraffinic hydrocarbon that is not very reactive under normal conditions. Only a few chemicals can he produced directly from methane under relatively severe conditions. Chlorination of methane is only possible by thermal or photochemical initiation. Methane can be partially oxidized with a limited amount of oxygen or in presence of steam to a synthesis gas mixture. Many chemicals can be produced from methane via the more reactive synthesis gas mixture. Synthesis gas is the precursor for two major chemicals, ammonia and methanol. Both compounds are the hosts for many important petrochemical products. Figure 5-1 shows the important chemicals based on methane, synthesis gas, methanol, and ammonia. ... 

Figure 5-1. Important chemicals based on methane, synthesis gas, ammonia, and methanol. ...

Kinetics. Extensive studies of the kinetics of methane synthesis were reported by White and co-workers (10,11, 12, 13, 14, 15). They studied the reaction between CO and hydrogen over a reduced nickel catalyst on kieselguhr at 1 atm and 300°-350°C (10). They correlated the rate of methane formation by the equation ... 

The preceeding discussion was confined mostly to the carbon deposition curves as a function of temperature, pressure, and initial composition. Also of interest, especially for methane synthesis, is the composition and heating value of the equilibrium gas mixture. It is desirable to produce a gas with a high heating value which implies a high concentration of CH4 and low concentrations of the other species. Of particular interest are the concentrations of H2 and CO since these are generally the valuable raw materials. Also, by custom it is desirable to maintain a CO concentration of less than 0.1%. The calculated heating values are reported as is customary in the gas industry on the basis of one cubic foot at 30 in. Hg and 15.6°C (60°F) when saturated with water vapor (II). Furthermore, calculations are made and reported for a C02- and H20-free gas since these components may be removed from the mixture after the final chemical reaction. Concentrations of CH4, CO, and H2 are also reported on a C02 and H20-free basis. 

Various schemes have been proposed for carrying out the methane synthesis reaction some of these are now in use (6,12,13,14). 

The findings from two long term test runs in the SASOL plant relevant to catalyst life under design conditions in a commercial methane synthesis plant have already been published (3). This paper reports further test results from both demonstration units concerning the effect of certain reaction parameters which are the basis for flexibility and operability of the Lurgi methanation scheme. 

The scheme of commercial methane synthesis includes a multistage reaction system and recycle of product gas. Adiabatic reactors connected with waste heat boilers are used to remove the heat in the form of high pressure steam. In designing the pilot plants, major emphasis was placed on the design of the catalytic reactor system. Thermodynamic parameters (composition of feed gas, temperature, temperature rise, pressure, etc.) as well as hydrodynamic parameters (bed depth, linear velocity, catalyst pellet size, etc.) are identical to those in a commercial methana-tion plant. This permits direct upscaling of test results to commercial size reactors because radial gradients are not present in an adiabatic shift reactor. 

Residual C02 Content. The feed gas to Rectisol gas purification contains 29-36 vol % C02 depending on the rate of shift conversion. The rate of C02 to be washed out will be determined by the requirements of methane synthesis and by the need to minimize the cost of Rectisol purification. 

Operability of methane synthesis 117 Operation of catalytic methanation in the Hygas pilot plant, de-... 

T.M. Giir, and R.A. Huggins, Methane Synthesis on Nickel by a Solid-State Ionic Method, Science 219, 967-969 (1983). 

Figure 2. A comparison of the rate (turn-over frequency) of methane synthesis over single crystal and supported ruthenium catalysts. Total reactant pressure for the single crystal studies was 120 Torr.

Figure 3. An Arrhenius plot of methane synthesis on a Ru(llO) catalyst at total reactant pressures of 1, 10 and 120 Torr. H2/CO 4/1.

This symbiotic relationship between H2-evolver (S-organism) and H2-utilizer (M.O.H.-organism) provides close interdependence in which M. 0. H.-functions in methane synthesis. Cell extracts of both the... 

Horita and Berndt (1999) studied the abiogenic formation of methane under conditions present at hydrothermal vents. Solutions of bicarbonate (HCO3 ) were subjected to temperatures of 470-670 K and a pressure of 40 MPa. Under these conditions, CO2 was reduced only very slowly to methane. Addition of a nickel-iron alloy, which corresponds closely to the minerals in the Earth s crust, led to a clear increase in the reaction rate of methane synthesis. The following reaction is assumed to occur ... 

RM [Ralph M. Parsons] A process for methanating synthesis gas, i.e. converting a mixture of carbon monoxide and hydrogen to mainly methane and carbon dioxide. Six adiabatic reactors are used in series, and steam is injected at the inlet. Under development by the R. M. Parsons Company in 1975. 

Figure 10.1 A comparison of the rate of methane synthesis overtwo different nickel single-crystal catalysts and supported Ni/alumina catalysts at 120 torr total reactant pressure. (Reprinted from Goodman, D.W., J. Vac. Sci. Technol., 20, 522-526, 1982. With permission from the American Institute of Physics.)...

Fig. 22. A comparison of the rate of methane synthesis over a clean single crystal Ni(100) catalyst with the corresponding rate over a potassium-doped catalyst. Total reactant pressure is 120 torr, Hj/CO = 4/1. (From Ref. 148.)...

Gur T.M. and Huggins R.A., Methane synthesis over transition metal electrodes in a solid state ionic cell, J. Catal. 102 443 (1986). 

Although much attention is given to Fe, Co, and Ni for these reactions, it is interesting to note that W, when atomically clean, rivals Ni in methane synthesis. 

Figure 9. Comparison of the turnover frequency (TOF) for methane synthesis over a clean Ni and a titania-containing Ni surface based on model calculations at a total pressure of 120 Torr and an H /CO ratio of 4.

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