Methane-steam reaction

Two important methods presently are used to produce the gas mixture from methane. The first is the methane-steam reaction, where methane and steam at about 900°C are passed through a tubular reactor packed with a promoted iron oxide catalyst. Two reactions are... 

Methane-steam reaction Hydrogenation of benzene to cyclohexane Dehydrogenation of ethylbenzene to styrene Tarhan Catalytic Reactor Design, McGraw-Hill, 1983) has computer programs and results for these cases ... 

The products of these endothermic reactions, which are not necessarily primary products, can be equilibrated by the water-gas shift reaction (4) and methane-steam reactions (5) and (6) ... 

From a consideration of the equilibrium of reaction (3) as a function of temperature (Fig. 14), it is noticed that this reaction occurs readily to the right at temperatures of 900° to 1000° C. The (COa - - CH4) reaction is catalyzed by substances similar to those used for the methane-steam reaction, i.e., 90 per cent nickel oxide-10 per cent thoria, etc. A combination of reaction (2) and reactions (3) and (4) may, hence, be considered as a means of producing hydrogen and carbon monoxide mixtures or by use of the water-gas catalytic reaction, (6), of producing hydrogen. A combination of these reactions thus becomes ... 

The methane-steam reaction requires an active catalyst even at temperatures of 1000° to 1100° C. as has been pointed out already. With a suitable catalyst and time of contact it is possible to obtain practically complete conversion of the methane to hydrogen and carbon monoxide at this temperature. Excess steam above that required for suppression of carbon deposition, about 60 mols total steam to 40 mols of methane, forces the water-gas reaction and carbon dioxide formation may result. 

CH4 reactions with CO2 or H2O on group VIII or noble metals (Ru, Rh, Pd, Ir, Pt) [1] form synthesis gas which is the precursor to valuable fuels and chemical compounds, as lirst shown by Fischer and Tropsch [2]. Due to the cost and availability of the nickel, compared to noble metals, Ni catalysts are used industrially. However, Ni-based catalysts tend to form inactive carbon residues that bloek the pores as well as the active sites of catalyst, and whose main activity is die formation of carbon filaments [3]. Therefore, the industrial methane steam reaction is usually performed under an excess of water to maintain the catalyst activity. Another alternative is the modification of the composition of the catalyst (generally Ni/Al203) by addition of a basic compound like MgO [4]. It is well known that the formation of NiO-MgO solid solution is easily favoured by calcining the mixed oxide at high temperatures [5] and much attention was devoted to its specific properties [6]. Parmaliana and al. 

Akers WW, Camp DP (1955) Kinetics of the methane-steam reactions. AIChE J 1 471-475 Abanades JC, Anthony EJ, Lu DY, Salvador C, Alvarez D (2004) Capture og CO2 from combustion gases in a fluidized bed of CaO. AIChE J 50 1614—1622 Allen DW, Gerhard ER, Likins MR (1975) Kinetics of the methane-steam reaction. Ind Eng Chem Proc Des Dev 14 256-259... 

Na.tura.1 Ga.s Reforma.tion. In the United States, most hydrogen is presently produced by natural gas reformation or methane—steam reforming. In this process, methane mixed with steam is typically passed over a nickel oxide catalyst at an elevated temperature. The reforming reaction is... 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

In a number of publications, Rostrup-Nielsen discusses different mechanism of methane steam reforming over Ni catalysts [17]. The proposed simplified reaction sequence for reforming of methane is as follows ... 

Ethanol and methane steam reforming reactions were studied assuming that the exit composition of the ethanol reformer depends on the steam reforming of methane. The competition for the same active site for ethanol and methane reforming maximizes the H2 and C02 production and minimizes the CO formation Catalysts were prepared by incipient wet impregnation. 20 wt% Ni supported on ZnO exhibited better performance compared to that supported on La203, MgO and A1203... 

Unlike the methane steam reformer, the autothermal reformer requires no external heat source and no indirect heat exchangers. This makes autothermal reformers simpler and more compact than steam reformers, resulting in lower capital cost. In an autothermal reformer, the heat generated by the POX reaction is fully utilized to drive the SR reaction. Thus, autothermal reformers typically offer higher system efficiency than POX systems, where excess heat is not easily recovered. 

Figures 1 to 3 present calculated equilibrium molar ratios of products to reactants as a function of temperature and total pressure of 1 and 100 atm. for the gas-carbon reactions (4), (7), and (5), (6), (4), (7), respectively. Up to 100 atm. over the temperature range involved, the fugacity coefficients of the gases are close to 1 therefore, pressures can be calculated directly from the equilibrium constant. From Fig. 1, it is seen that at temperatures above 1200°K. and at atmospheric pressure, the conversion of carbon dioxide to carbon monoxide by the reaction C - - COj 2CO essentially is unrestricted by equilibrium considerations. At elevated pressures, the possible conversion markedly decreases hence, high pressure has little utility for this reaction, since increased reaction rate can easily be obtained by increasing reaction temperature. On the other hand, for the reaction C -t- 2H2 CH4, the production of methane is seriously limited at one atmosphere pressure and practical operating temperatures, as seen in Fig. 2. Obviously, this reaction must be conducted at elevated pressures to realize a satisfactory yield of methane. For the carbon-steam reaction.

The same catalyst compositions used in the more important methane steam reforming [Eq. (3.1), forward reaction], may be used in methanation, too.222 All Group VIE metals, and molybdenum and silver exhibit methanation activity. Ruthenium is the most active but not very selective since it is a good Fischer-Tropsch catalyst as well. The most widely used metal is nickel usually supported on alumina or in the form of alloys272,276,277 operating in the temperature range of 300-400°C. 

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