Methane conversion, pressure

Figure 3 illustrates the shift and methanation conversion. The resulting methane is inert and the water is condensed. Thus purified, the hydrogen-nitrogen mixture with the ratio of 3H2 pressed to the pressure selected for ammonia synthesis. 

Figure 6 shows typical results obtained with the plug-flow quartz reactor containing 0.5 g of Sr(lwt%)/La203 catalyst operated in the continuous flow recycle mode. The inlet CH partial pressure was 20 kPa (20% CH in He) at inlet flowrates of 7.1 and 14.3 cm STP/min. A 20% O2 in He mixture was supplied directly, at a flowrate Fog, in the recycle loop via a needle valve placed after the reactor (Fig. 1). The methane conversion was controlled by adjusting Fog, which was kept at appropriately low levels so that the oxygen conversion...

GP 8] [R 7] Given constant catalyst temperature and GHSV, methane conversion and CO and H2 selectivity decrease with increasing pressure at total oxygen consumption for a rhodium catalyst [CH4/O2 2.0 1-12 MPa 1.17 10 h (STP) 1200 °C] [3]. The decrease is larger than thermodynamically expected. 

GP 8[ [R 7[ Syngas generation with commercial Pt-Rh gauzes, metal-coated foam monoliths and extruded monoliths has been reported. For similar process pressure, process temperature, and reaction mixture composition, methane conversions are considerably lower in the conventional reactors (CH4/O2 2.0 22 vol.-% methane, 11 vol.-% oxygen, 66 vol.-% inert species 0.14—0.155 MPa 1100 °C) [3]. They amount to about 60%, whereas 90% was reached with the rhodium micro reactor. A much higher H2 selectivity is reached in the micro reactor the CO selectivity was comparable. The micro channels outlet temperatures dropped on increasing the amount of inert gas. 

The overall effect of catalyst pellet geometry on heat transfer and reformer performance is shown in the simulation results presented in Table 1. The performance of the traditional Raschig ring (now infrequently used) and a modern 4-hole geometry is compared. The benefits of improved catalyst design in terms of tube wall temperature, methane conversion and pressure drop are self-evident. 

Fig. 5. Methane conversion and oxygen flux during partial oxidation of methane in a ceramic membrane reactor. Reaction conditions pressure, 1 atm temperature, 1173 K, feed gas molar ratio, CH Ar = 80/20 feed flow rate, 20 mL min-1 (NTP) catalyst mass, 1.5 g membrane surface area, 8.4 cm2 (57).

Selective synthesis of acetylene (>90%) from methane was accomplished by microwave plasma reactions.568 Conversion of methane to acetylene by using direct current pulse discharge was performed under conditions of ambient temperature and atmospheric pressure.569 The selectivity of acetylene was >95% at methane conversion levels ranging from 16 to 52%. In this case oxygen was used to effectively remove deposited carbon and stabilize the state of discharge. Similar high... 

Fig. 14.11 Influence of residence time on methane conversion and product yield [63]. Reaction conditions Temperature 703 K, pressure 34 bar, CH4/02 ratio 16/1.

The figures discussed above indicate that the optimum temperature for the reforming step in the UMR process is 700-850°C, the optimum pressure is less than 7 bar and the optimum calcium-to-carbon molar ratio during the reforming step is around one. Under these optimum conditions the methane conversion varies from 82 to 100% and the hydrogen-to-carbon-monoxide molar ratio in the reformate stream varies from 6 to 100. 

Figure 6. Effect of temperature, pressure and calcium-to-carbon molar ratio on methane conversion. The calcium-to-carbon molar ratio is for the entire reforming step (steam-to-carbon molar ratio of 3).

High energy content, low vapor pressure (200 psi), and liquid state at ambient temperatures favors low-cost liquid pipeline transportation vs. high-pressure compressed gas—1000+ psig if methane conversion is done near the production site. 

Thermodynamically, the reforming reaction. Equation 3.5.1, shows that the reformer should be operated at die lowest pressure and highest temperature possible. The reforming reaction occurs on a nickel-oxide catalyst at 880 C (1620 "F) and 20 bar, which results in a 25 "C approach to the equihbrium temperature [25,29]. Methane conversion increases by reducing the pressure, but natural gas is available at a high pressure. It would be costly to reduce the reformer pressure and then recompress the synthesis gas later to 100 bar (98.7 atm) for the converter. The steam to carbon monoxide ratio is normally in the range of 2.5 to 3.0 [30]. The ratio favors both the conversion of methane to carbon monoxide and the carbon monoxide to carbon dioxide as indicated by Equations 3.5.1 and 3.5.3. If the ratio is decreased, the methane concentration increases in the reformed gas, but if the ratio is set at three, the unreacted methane is small. The methane is a diluent in the synthesis reaction given by Equation 3.5.2. 

Methane can be catalytically oxidized in the fuel cell mode to simultaneously generate electricity and C2 hydrocarbons by dimerization of methane using a yttria-stabilized zirconia membrane. A catalyst, used as the anode, is deposited on the side of the membrane that is exposed to methane and the cathode is coated on the other side of the membrane. When the catalyst Ag>Bi2C>3 is used as the anode for the reaction at 750> 900X and atmospheric total pressure, the selectivity to ethane and ethylene exceeds 90%. But this high selectivity is at the expense of low power output and low overall methane conversion (less than about 2%). 

The equilibrium methane conversions with increasing temperature calculated at different S/C ratios and pressure between 1 and 20 bar are shown in Figure 2.4. The methane conversion increases with higher S/C ratios (S/C varies from 1 to 5) and decreases with increasing pressures (1-20 bar pressures were studied). A complete... 

Figure 2.4. Equilibrium methane conversions at different temperatures, steam/carbon ratios, and pressures obtained by thermodynamic calculations.

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