Partial methane oxidation

The peak power density was enhanced when the operating temperature increased, but was limited to 650 C. The reason for the performance decrease in the cell operated over 650 C may result from increase in catalytic activity of LSM for methane partial oxidation. 

Yamada, Y., Ueda, A., Shioyama, H. et al. (2003) High throughput experiments on methane partial oxidation using molecular oxygen over silica doped with various elements. Appl. Catal. A Gen., 254, 45. 

They used a Ni-containing catalyst. In contrast to steam reforming of methane, methane partial oxidation is exothermic. However, the partial oxidation requires pure oxygen, which is produced in expensive air separation units that are responsible for up to 40% of the cost of a synthesis gas plant (2) (in contrast, the steam reforming process does not require pure oxygen). Therefore, the catalytic partial oxidation of methane did not attract much interest for nearly half a century, and steam reforming of methane remained the main commercial process for synthesis gas manufacture. 

Fig. 1. Relationship between catalyst temperature and reaction time in methane partial oxidation catalyzed by Ni/Si02 (temperature of the gas phase (a) 1019 K, (b) 899 K, (c) 809 K, (d) 625 K). The reaction was carried out in a fixed-bed reactor (a quartz tube of 2 mm inside diameter) at atmospheric pressure. Before reaction, the feed gas was allowed to flow through the catalyst undergoing heating of the reactor from room temperature to 1073 K at a rate of 25 K min-1 to ignite the reaction, and then the reactant gas temperature was decreased to the selected value. Reaction conditions pressure, 1 atm catalyst mass, 0.04 g feed gas molar ratio, CH4/O2 = 2/1 GHSV, 90,000 mL (g catalyst)-1 h-1) (25).

Recently, such a temperature oscillation was also observed by Zhang et al (27,28) with nickel foils. Furthermore, Basile et al (29) used IR thermography to monitor the surface temperature of the nickel foil during the methane partial oxidation reaction by following its changes with the residence time and reactant concentration. Their results demonstrate that the surface temperature profile was strongly dependent on the catalyst composition and the tendency of nickel to be oxidized. Simulations of the kinetics (30) indicated that the effective thermal conductivity of the catalyst bed influences the hot-spot temperature. 

Transition metal carbide catalysts have also been explored as methane partial oxidation catalysts [110] promising results were obtained over M02C systems and enhancements were reported with the addition of transition metal promoters. 

The direct reaction of methane partial oxidation always competes with total oxidation reactions, which are also responsible for O2 consumption, whereas steam and dry reforming and C-forming reactions are also to be considered. All reactions are catalyzed by the materials which are active in partial oxidation, but different scales of reactivity for the catalysts can be estimated from the experimental data. Total oxidation prevails at the light-off of the fuel-rich stream over most catalysts, but precious metals are more active than transition metals. 

Concerning the operation of catalysts under adiabatic conditions, Basini et al. [156] reported the results of methane partial oxidation runs in a pilot-scale reactor operating at high pressure and short contact times, showing stable activity (almost complete conversion of methane and over 90% selectivity to CO and H2) during more than 500 h on-stream. In addition, operability for 20 000 h bench-scale testing has been claimed recently by the same group [157]. 

Table Activity and Selectivity of Si02, 4% Mo03/Si02 and 5% 203/8102 Catalysts in Methane Partial Oxidation. Batch reactor data ...

Table II. Methane Partial Oxidation over Si02 Catalyst in Batch and Continuous Flow Reactors...

Moreover, as neither the concept of surface initiated homogeneous-heterogeneous reaction (11) can be invoked to explain our results, it can be stated that the methane partial oxidation reaction proceeds via a surface catalysed process which likely involves specific catalyst requirements. However, by comparing the HCHO productivity of the different catalytic systems previously proposed (9) with that of our 5% V205/Si02 catalyst, it emerges that our findings constitute a relevant advancement in this area (23),... 

Figure 1. Batch reactor test of methane partial oxidation on Si02 catalyst at 600 C.

Figure 2. Methane Partial Oxidation. Rate of HCHO formation on unpromoted Si02, 5%V205/Si02 and 4%Mo03/Si02 catalysts.

Table m. Influence of the Gas Phase Oxygen on the Products Formation in Methane Partial Oxidation over Si02> 4% Mo03/Si02 and 5% V205/Si02 Catalysts. Pulse Reactor Data ... 

The stronger acidity of 4%Mo03/Si02 and 5%V2 l8i02 catalysts with respect to the unpromoted Si02 support seems to exert no direct influence on the reaction pathway of the methane partial oxidation. 

In addition to importance of the catalyst composition and temperature, we have shown that methane partial oxidation selectivity is strongly affected by the mass transfer rate. Our experiments show that increasing the linear velocity of the gases or choosing a catalyst geometry that gives thinner boundary layers enhances the selectivity of formation of H2 and CO. Since H2 and CO are essentially intermediate... 

The viability of one particular use of a membrane reactor for partial oxidation reactions has been studied through mathematical modeling. The partial oxidation of methane has been used as a model selective oxidation reaction, where the intermediate product is much more reactive than the reactant. Kinetic data for V205/Si02 catalysts for methane partial oxidation are available in the literature and have been used in the modeling. Values have been selected for the other key parameters which appear in the dimensionless form of the reactor design equations based upon the physical properties of commercially available membrane materials. This parametric study has identified which parameters are most important, and what the values of these parameters must be to realize a performance enhancement over a plug-flow reactor. 

It is expected that the conclusions reached in the analysis of the series reaction will also be valid for methane partial oxidation. The first objective of this study was to verify this expectation. The second objective of the study was to determine how much faster than methane formaldehyde must permeate for the membrane reactor to begin to outperform a plug-flow reactor. 

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