CH4 conversion with

As can be seen in Figure 1, the CH4 conversions percentage at different reaction temperatures were in the order of Ni/SA>Ni/Si02>Ni.NPs. For Ni/SA there was a linear increase in CH4 conversion with increase in reaction temperature. Similarly CH4 conversion over Ni/Si02 increased until TSO C however with further increase in reaction temperature CH4 conversion decreased, whereas for Ni.NPs with increase in reaction temperature there was a slight increase in CH4 conversion in temperature range between 700-750 C and remained almost constant in rest of the tested temperature ranges. 

The C2 + yield which can be derived from the plots of selectivity and methane-conversion versus temperature first rises with increasing temperature due to an increase in C2+ selectivity and hence, in CH4 conversion. With ftirther increasing temperature the yield drops. This is caused by the decrease in C2+ selectivity due to non-selective catalytic and homogeneous gas-phase reactions. The yield increases with increasing oxygen partial pressure due to the increasing methane conversion which overcomes the decreasing selectivity (see also above). With... 

Thus indeed CH4 oxidation in a SOFC with a Ni/YSZ anode results into partial oxidation and the production of synthesis gas, instead of generation of C02 and H20 as originally believed. The latter happens only at near-complete CH4 conversion. However the partial oxidation overall reaction (3.12) is not the result of a partial oxidation electrocatalyst but rather the result of the catalytic reactions (3.9) to (3.11) coupled with the electrocatalytic reaction (3.8). From a thermodynamic viewpoint the partial oxidation reaction (3.12) is at least as attractive as complete oxidation to C02 and H20. 

As shown on Fig. 8.49 one can influence dramatically both the total CH4 conversion as well as product selectivity by varying the Ag catalyst potential. Thus under open-circuit conditions (Uwr=U r ) the CH4 conversion is near 0.02 with a C2 selectivity (methane molecules reacting to form C2H4 and C2H6 per total reacting CH4 molecules) near 0.5. Increasing Uwr increases the methane conversion to 0.3 and decreases the selectivity to 0.23, while decreasing Uwr decreases the conversion to 0.01 and increases the... 

Figure 8,49. Effect of Ag/YSZ catalyst potential on CH4 conversion and on selectivity to C2 hydrocarbons. T=800°C, pO2=0.25 kPa, pCH4=lO.I3 kPa, U R=-0.45 V.2,54 Open symbols correspond to open-circuit. Reprinted from ref. 2 with permission from Elsevier Science.

Effects of Li content on the catalytic behaviors and structures of LiNiLaOx catalysts The dpendence of performance of LiNiLaOx catalysts on Li content at 1073K was shown in Fig.l. When D/Ni mole ratio was 0, the relatively acidic LaNiOx had the highest CH4 conversion(92.0%), but no C2 yielded. The products were CO, CO2 and H2, and CO selectivity was 98.3%. It is not an OCM catalyst but a good catalyst for partial oxidation of methane(POM). With Li content and the baric property of LiNiLaOx catalysts increasing, CH4 conversion and CO selectivity decreased, but there was still no C2 formed imtil Li/Ni mole ratio was 0.4. There was a tumpoint of catalytic behavior between 0.2 and 0.4 (Li/Ni mole... 

Consequently, in the early 1990s, interest in the direct processes decreased markedly, and the emphasis in research on CH4 conversion returned to the indirect processes giving synthesis gas (13). In 1990, Ashcroft et al. (13) reported some effective noble metal catalysts for the reaction about 90% conversion of methane and more than 90% selectivity to CO and H2 were achieved with a lanthanide ruthenium oxide catalyst (L2Ru207, where L = Pr, Eu, Gd, Dy, Yb or Lu) at a temperature of about 1048 K, atmospheric pressure, and a GHSV of 4 X 104 mL (mL catalyst)-1 h-1. This space velocity is much higher than that employed by Prettre et al. (3). Schmidt et al. (14-16) and Choudhary et al. (17) used even higher space velocities (with reactor residence times close to 10-3 s). 

In 1989, Gadalla and Sommer (252) reported that a solid-solution NiO/MgO (1 1.35) catalyst prepared by precipitation can inhibit the carbon deposition in the CO2 reforming of methane however, they obtained a low CO2 conversion (66%), a low H2 selectivity (79%), and a low CO selectivity (77%), even at the very low WHSV of 3714 cm3 (g catalyst)-1 h-1 with a CH4/CO2 (1/1, molar) feed gas and the high temperature of 1200 K. Their relatively high CH4 conversion was partly a consequence of homogeneous gas-phase reactions that occurred under their conditions. Indeed, the authors found extensive carbon deposits plugging the reactor upstream and downstream of the reaction zone. 

For WGS, commercial catalysts are only operated up to 550 °C and no catalysts are available for higher temperatures, because adverse equilibrium conversion makes the process impractical in the absence of a CO2 sorbent. Han and Harrison [38] have shown that, at 550 °C, dolomite and limestone have a sufficiently high WGS activity. For SMR a conventional Ni SMR catalyst is used in a 1 1 ratio with CaO [30]. Meyer et al. [32] have also used a Ni-based catalyst in combination with limestone and dolomite, and achieved CH4 conversions of 95% at 675 °C while the CH4 conversion at equilibrium was 75%. 

Natural gas is reacted with steam on an Ni-based catalyst in a primary reformer to produce syngas at a residence time of several seconds, with an H2 CO ratio of 3 according to reaction (9.1). Reformed gas is obtained at about 930 °C and pressures of 15-30 bar. The CH4 conversion is typically 90-92% and the composition of the primary reformer outlet stream approaches that predicted by thermodynamic equilibrium for a CH4 H20 = 1 3 feed. A secondary autothermal reformer is placed just at the exit of the primary reformer in which the unconverted CH4 is reacted with O2 at the top of a refractory lined tube. The mixture is then equilibrated on an Ni catalyst located below the oxidation zone [21]. The main limit of the SR reaction is thermodynamics, which determines very high conversions only at temperatures above 900 °C. The catalyst activity is important but not decisive, with the heat transfer coefficient of the internal tube wall being the rate-limiting parameter [19, 20]. 

Both CH4 conversion and adiabatic temperature are significantly affected by the 02 CH4 and H20 CH4 feed ratios. The CH4 conversion increases with the 02 CH4 ratio up to the value 0.6, at which total conversion is reached. For values greater than 0.6, the adiabatic temperature continues to increase although the CH4 conversion remains at 100%. This is due to the oxidation reaction of H2 and CO to H2O and CO2 favored by the excess O2 supply. On the other hand, on increasing the H20 CH4 ratio at a fixed 02 CH4 value, the adiabatic temperature decreases. 

The first step is relevant to the start-up phase, which in this particular case we chose to extend for up to 1 h in order to verify the reactor stability also in these conditions, where water is not present and while there is a higher oxygen concentration in the feed gas with respect to the ATR conditions. By lowering the 02 CH4 ratio, the H2 concentration at the reactor outlet increases, approaching the value expected by thermodynamic evaluation and CH4 conversion is still complete. A further decrease in the 02 CH4 feed ratio to values lower than 1.16 corresponds to an abrupt decrease in temperature in the lower section and a simultaneous temperature increase in the catalytic reforming section. 

The influence of preheating on H2 concentration is more pronounced at lower 02 CH4 ratios. Up to an 02 CH4 value of 0.84 (not reported), H2 concentrations were almost the same, and very high CH4 conversions were reached due to the oxidation reaction, which proceeds to a major extent. When the 02 CH4 ratio reaches 0.7, preheating of the reactants generally leads to an enhancement of ATR reactor performance with respect to both CH4 conversion and reactor outlet H2 concentration. 

CO2 and H2O instead of forming only H2 and CO, leaving no O2 to react with the remaining CH4. As the ideal feed composition (29.6% CH4) is approached from leaner compositions, the CH4 conversion decreases drastically. In fact, for feed compositions richer than the CH4 breakthrough point, die O2/CH4 conversion ratio actually increases as the O2/CH4 feed ratio decreases. Thus, the H2 and CO selectivities are optimal near the breakthrough point. 

Run the calculation again with TS 2 = 1290 K. Plot the CH4 mole fraction against height at distances x = 1, 2, 3,4, and 5 cm. Make similar plots for CO2. Use the CO2 plot at x = 1 cm to estimate the boundary layer thickness at this point downstream. Re-run the problem with average velocity set to 5000 cm/s, and make plots as above. How does the boundary layer thickness <5 scale with velocity That is, if 5 scales as vn, find the approximate value of n. How did the CH4 conversion efficiency change as the velocity was increased ... 

Discussion of Results and Conclusions. The results of regression analysis show that a chemical reaction model, first order with respect to fractional carbon conversion, with a production and a decomposition step for each of CH4, C2H5, BTX and Oils, satisfactorily describes the dilute phase flash hydrogenation of both lignite and subbltumlnous coal. 

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