Methane steam reforming conversion

Increasing the inert gas flow on the retenate side increased the driving force for the steam reforming and water-gas shift reactions due to the thermodynamic equilibrium of methane steam reforming and water-gas shift. Consequently, conversion increased. While increasing pressure decreases (equilibrium) conversion in conventional methane steam reformers, conversion could be increased at elevated pressure in a membrane reactor under certain conditions. In a similar way, the water-gas shift reaction is pushed in a favourable dhection in a membrane reactor, while the reaction is pressure independent in a conventional reactor (see Section 3.10.1). 

Figure 2.68 Results from numerical calculations for combustion-assisted methane steam reforming, (a) Outlet conversion dependence on channel half-height (b) wall temperature as a function of dimensionless reactor length. Calculation results determined at constant inlet velocity [108]...

Taking into account the fact that methane steam reforming is a rapid reaction and that the local conversion is determined mainly by the catalyst temperature (Xref = Xref(T)), the evolution of the temperature profile can be estimated through a simplified procedure based on geometric considerations. It can be shown, that... 

A. Andersen, J.M. Dahl, KJ. Jens. E. Rytter, A. Slagtem, and A. Solbakken, Hydrogen acceptor and membrane concept for direct methane conversion, Catal. Today 4 389 (1989). S. Uemiya, N. Sato, H. Ando, T. Matsuda, and E. Kikuchi, Promotion of methane steam reforming by use of palladium membrane, Sekiyu Gakkaishi JJ 418 (1990). 

The performance of a CPO reactor is, in the literature, often characterized by the hydrocarbon conversion and selectivities to carbon monoxide and hydrogen. Methane conversion and selectivities are often reported to be more than 80-90%. This corresponds in general to conditions at which the exit gas is close to equilibrium for the shift reaction and the methane steam-reforming reaction with a low value of ATr in Eq. (5). The most likely reaction sequence is total oxidation in the initial part of the catalyst zone followed by other reactions including steam-reforming, shift, and possibly partial oxidation. 

Finally, it should be noted that FT should be considered as only one of the three steps in the conversion of natural gas into liquids, the other two being syngas generation and hydroprocessing.44 However, new concepts such as the combination of methane steam reforming and FT synthesis, in order to convert methane directly to hydrocarbons, have been explored.45 The idea seems to be operative at 573 K, using Ru and Co catalysts, but very low conversions are achieved.45 Thus, higher performance catalysts will have to be developed and the combination with alternative reaction conditions (such as supercritical fluids) merits consideration. 

Thorium and uranium are used in cotmnercial catalytic systems. Industrially, thorium is used in the catalytic production of hydrocarbons for motor fuel. The direct conversion of synthetic gas to liquid fuel is accomplished by a Ni-Th02/Al203 catalyst that oxidatively cracks hydrocarbons with steam. The primary benefit to the incorporation of thorium is the increased resistance to coke deactivation. Industrially, UsOs also has been shown to be active in the decomposition of organics, including benzene and butanes and as supports for methane steam reforming catalysts. Uranium nitrides have also been used as a catalyst for the cracking of NH3 at 550 °C, which results in high yields of H2. 

E. Johannessen, K. Jordal. Study of a H2 separating membrane reactor for methane steam reforming at conditions relevant for power process with CO2 capture. Energy Conversion Management 2005, 46(7-8), 1059 1071. 

FIGURE 1.3 Equilihrium CO conversions of a typical reformate stream from a methane steam reforming process at various steam to dry gas (S/G) ratios. (Taken from D. Mendes, A. Mendes, L.M. Maderia, A. Lidianelli, JM. Sousa, A. Basile, Asia-Pac. J. Chem. Eng. 5 (2010) 111.)... 

Lin Y, Liu S, Chuang C, Chu Y (2003) Effect of incipient removal of hydrogen through palladium membrane on the conversion of methane steam reforming experimental and modeling. Catal Today 82 127-139... 

S. Kara, G. Barbieri and E. Drioli, Limit conversion of a palladium membrane reactor using counter-current sweep gas on methane steam reforming, Desalination, 2006, 200, 708-709. 

Find et al. [25] developed a nickel-based catalyst for methane steam reforming. As material for the microstructured plates, AluchromY steel, which is an FeCrAl alloy, was applied. This alloy forms a thin layer of alumina on its surface, which is less than 1 tm thick. This layer was used as an adhesion interface for the catalyst, a method which is also used in automotive exhaust systems based on metallic monoliths. Its formation was achieved by thermal treatment of microstructured plates for 4h at 1000 °C. The catalyst itself was based on a nickel spinel (NiAl204), which stabUizes the catalyst structure. The sol-gel technique was then used to coat the plates with the catalyst slurry. Good catalyst adhesion was proven by mechanical stress and thermal shock tests. Catalyst testing was performed in packed beds at a S/C ratio of 3 and reaction temperatures between 527 and 750 °C. The feed was composed of 12.5 vol.% methane and 37.5 vol.% steam balance argon. At a reaction temperature of 700°C and 32 h space velocity, conversion dose to the thermodynamic equilibrium could be achieved. During 96 h of operation the catalyst showed no detectable deactivation, which was not the case for a commercial nickel catalyst serving as a base for comparison. 

---