Methanation process

Catalytic methanation processes include (/) fixed or fluidized catalyst-bed reactors where temperature rise is controlled by heat exchange or by direct cooling using product gas recycle (2) through wall-cooled reactor where temperature is controlled by heat removal through the walls of catalyst-filled tubes (J) tube-wall reactors where a nickel—aluminum alloy is flame-sprayed and treated to form a Raney-nickel catalyst bonded to the reactor tube heat-exchange surface and (4) slurry or Hquid-phase (oil) methanation. 

Catalytic reactions at somewhat lower temperatures also produce ethylene and other olefins. When coupled with a methane process to methyl chloride, this reaction results ia a new route to the light hydrocarbons that is of considerable interest. 

Figure 3. Shift and methanation process flow diagram. (Used with permission of Simulation Sciences Inc.)...

The chapter by Haynes et al. describes the pilot work using Raney nickel catalysts with gas recycle for reactor temperature control. Gas recycle provides dilution of the carbon oxides in the feed gas to the methanator, hence simulating methanation of dilute CO-containing gases which under adiabatic conditions gives a permissible temperature rise. This and the next two papers basically treat this approach, the hallmark of first-generation methanation processes. 

The chapter by Eisenlohr et al. deals with the results of large scale pilot operations using a newly developed high-nickel catalyst with hot-gas recycle for temperature control. This and other work, conducted by Lurgi Mineraloeltechnik GmbH, with South African Coal and Oil Limited (SASOL), are the bases of the methanation process which Lurgi is proposing for commercial plants. 

This paper presents a review of the chemistry of the methanation process, the mechanisms of the key reactions, the kinetics of the reactions over various catalysts, and the application of this background to commercial processes. 

Chemistry and Thermodynamics. The principal reactions which occur in the methanation process are ... 

Carbon laydown cannot be tolerated in a commercial methanation process since it could lead to rapid plugging and shutdown of the catalyst bed. Fortunately there are catalysts which avoid laydown in the first zone (33, 37), as opposed to catalysts which do not (21, 22, 23, 24). 

Methanation Processes, Amer. Chem. Soc., Div. Fuel Chem., Prepr. 14 (3), 104-163 (September, 1970). 

The methanation process commonly operates at pressures up to 30 atm, and, with the nickel catalyst which is almost universally used for the process, the inlet temperature is about 300°C ( 570°F). Almost complete conversion of the oxides of carbon occurs giving a product synthesis gas containing less than 5 ppm CO + C02. The temperature rise for the exothermic methanation reactions is typically 35 °C (63°F). 

The IGT multistage cold-gas recycle methanation process has a built-in temperature control mechanism and a low recycle ratio. Much less recycle gas is used in this multistage operation than would be required if the same amount of conversion were achieved in a single vessel with all fresh feed and recycle gas added at the inlet to one large catalytic reactor. 

Various design and operating problems have been experienced by most developers of methanation systems. Specifically, carbon formation and catalyst sintering are two of the more common problems in methanation processes. Carbon formation refers to the potential production of carbon from carbon oxides and methane by the following reactions. 

Steam Utilization. Less steam is used in the RMProcess than is required for conventional shift conversion even though in other methanation processes as little as one-half of the total syngas is processed through shift conversion in order to achieve a near-stoichiometric balance of hydrogen and carbon monoxide for methanation. 

Figure 1. Schematic of liquid-phase methanation process...

On the basis of this past work and ongoing experiments, we feel that the liquid-phase methanation process promises to become an economic, reliable, and versatile means of converting synthesis gas mixtures to high Btu gas. Chem Systems believes that this technology is a key step in the transformation of fossil feeds into pipeline gas, and we look forward to its successful application in commercial coal gasification plants. 

L. Seglin We have heard today two rather novel approaches to methanation the steam-moderated RMProcess and the slurry methanation process. These are the results of new or recent R D. What can we visualize beyond that Would it be some other exotic process ... 

D. Blum Just going a bit further, the liquid-phase methanation process now uses one reactor. You can or you cannot use a polishing reactor as the economics dictate. You can actually go right to pipeline quality gas in one reactor, which is equivalent to about 99.8% conversion of a 20% CO feed gas. We envision at this moment that combined shift-methanation could be done in the same single reactor. It would obviously require lower feed gas rates so you may need two of these reactors. We don t exactly have the numbers yet. I think that s one of the areas that deserves future work. 

A. Vannice ( Exxon Research and Engineering Co.) Would a more sulfur-tolerant catalyst be a significant improvement in the overall methanation process If so, what would be the maximum tolerable sulfur concentrations in the feed stream ... 

The addition of steam to the CH4/C02 feedstock to avoid excessive carbon formation is a widely used technique in practical systems [3]. The resulting C02-steam gasification of methane process can be described by the following chemical equation ... 

Medium heating-value gas (10 to 20 MJ/m3 or 270 to 540 Btu/ft3) can be used as fuel gas for gas turbines in IGCC applications, for substitute natural gas (SNG) in combination with methanation process, for hydrogen production, for fuel cell feed, and for chemical and fuel synthesis. 

The ability of bimetallic systems to enhance various reactions, by increasing the activity, selectivity, or both, has produced a great deal of interest in understanding the different roles and relative importance of ensemble and electronic effects. Deposition of one metal onto the single-crystal face of another provides an advantage by which the electronic and chemical properties of a well-defined bimetallic surface can be correlated with the atomic structure.5 22 23 Besenbacher et al.24 used this method to study steam reforming (the reverse of the CO methanation process) on Ni(l 11) surfaces... 

The reaction in its basic form is illustrated by the conversion of 274 into 275. This transformation shows the control exercised within the two biradicals A and B on the final outcome of the reaction151. Another typical di-jr-methane process, this time with electron-withdrawing substituents on one of the vinyl moieties, arises on acetophenone-sensitized irradiation of the diene 276. This yields the cyclopropane 277 in 35% yield. The... 

Benzobarrelenes are an important class of molecule where the di-jr-mcthanc rearrangement is operative. Clearly, within this class there is a possibility that benzo-vinyl interactions can be in competition with vinyl-vinyl processes. The direct irradiation (X > 330 nm) of the benzobarrelene 313 in a variety of solvents (benzene, acetonitrile, methanol or hexane) affords two principal products 314 and 315 in a ratio of 1 1. The products are formed via a di-7r-methane process involving vinyl-vinyl bridging which... 

When one of the components of the molecule undergoing reaction is a double bond, an alternative reaction mode is operative. Thus it can be seen that 417 follows the usual path involving aryl-aryl bridging affording biphenyl and a carbene that is trapped by solvent. However, in addition a vinyl-aryl di-jr-methane process is also operative and affords the... 

The byproduct obtained from the overall reaction is carbon disulfide (CS ). The reaction between CH and H S given with Eq. 5.47 is the well-known methane process for production of CSj. Most commercial CH -snlfur processes employ silica gel/aluminum catalyst for CSj production. The reaction of CH with sulfur is thermodynamically favorable for CSj formation, and conversion is usually in the range of 90 to 95% with respect to methane (Arpe, 1989). The industrial CH -sulfur pro-... 

The increase in efficiency between the first- and second-generation reactors was attributed to less water in the feed and lower operating temperatures. Reactor models indicated that the major source of heat loss was by thermal conduction. The selective methanation reactor lowered the carbon monoxide levels to below 100 ppm, but at the cost of some efficiency. The lower efficiency was attributed to slightly higher operating temperatures and to hydrogen consumption by the methanation process. Typical methane levels in the product stream were 5-6.2%. ... 

We believe that the rearrangements of the di-ir-methane type observed in the DCA-sensitized irradiations of 1-aza- and 2-aza-1,4-dienes are important because the di-ir-methane process has been considered until now a paradigm of reactions that take place in the excited-state manifold only. Our results show that rearrangements of this type can also occur in the ground states of radical-cation intermediates. This opens the possibility of promoting di-ir-methane-type rearrangements by alternative thermal means. 

The observed effect of the increased concentration of the metals matches the role of metals in enzymes. Among the three metal ions, nickel has the greatest effect (compare Figure 8.4a-c). These results suggest that the controlled addition of Fe, Ni and Co could be beneficial for improving the methanation process of waste. 

Valstar, J. M., Van Den Berg, P. J., and Oyserman, J., Chem. Eng. Sci. 30,723-728 (1975). Vatcha, S. R., Analysis and Design of Methanation Processes in the Production of Substitute Natural Gas from Coal, Ph.D. thesis, California Institute of Technology, Pasadena (1976). Villadsen, J. V., and Michelsen, M. L., Solution of Differential Equation Models by Polynomial Approximation." Prentice-Hall, New York, 1978. 

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