Methane ratio

Instead of conversion, some producers prefer to use other identifications of severity, including coil outlet temperature, propylene to methane ratio, propylene to ethylene ratio, or cracking severity index (33). Of course, all these definitions are somewhat dependent on feed properties, and most also depend on the operating conditions. 

The rate is near first order in methane and zero order in oxygen for oxygen to methane ratios higher than 1. Also, the reaction kinetics remain unaffected upon polarization conditions. The kinetic data indicate weak bonding of methane and strong bonding of oxygen on the catalyst surface. 

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. 

The distribution of hydrocarbon products found with the Ir4(CO),2/ A1C13 system is unusual in that of the four hydrocarbons detected, i.e., methane, ethane, propane, and isobutane, ethane is the major product. At the initial stage of the reaction, the ethane-to-methane ratio is 10 1, decreasing to 2 1 after 0.5-3 days. The reason for this variation with reaction time is not clear, although it was felt unlikely that it resulted from ethane being cracked to methane during the course of the reaction (59). 

Ding and Alpay also studied sorption-enhanced reforming with K-HTC as sorbent [28], using a commercial Ni-based catalyst. They found that the SER process benefits from higher pressures and that lower steam to methane ratios can be used than in ordinary reforming. Reijers et al. [25] have shown that K-HTC is an effective sorbent between 400 and 500 °C, with an C02 uptake of approx. 0.2 mmol g 1. This capacity is low compared with calcium oxides and lithium zirconates. Above 500 °C, the C02 sorption capacity of K-HTC decreases rapidly to zero [36]. 

Vanpee and Grard133 made a quantitative study of the formation of saturates (mainly ethane) in the photolysis of CH2CO with added methane (ratios CH4/CH2CO = 1 to 7) at 28 to 250 °C. and found that the results could be explained by a mechanism involving competition between CH4 and CH2CO for methylene by reactions of the first order in methylene. The rate of reaction of CH2 with CH4 was found to be 0.183 that of the reaction with ketene. Decomposition of excited ethane by the reaction... 

Methylene dichloride and chloroform may be produced by modified methods using a mixture of chlorine, methane, and methyl chloride as feed. Chlorination is run at 350-400°C reactor temperature at slightly above atmospheric pressure. A 2.6 1 chlorine methane ratio results in an optimal yield of chloroform. Alternatively, excess methane is reacted with chlorine at 485-510°C to produce methylene dichloride as the main product.181 The predominant method, however, still is the chlorination of methyl chloride manufactured by the reaction of methyl alcohol and hydrogen chloride.181... 

The thermodynamics of the above reactions are illustrated in Figures 5.6 and 5.7174. Both figures assume a steam-to-methane ratio of 1.0. Figure 5.6 illustrates how the feed and product gases interact when the product gas has a hydrogen-to-carbon monoxide ratio of 3.0. Figure 5.7 illustrates the effects of temperature and pressure on the reactions. As pressure increases, lower conversion can be expected and more methane will not be converted and will be found in the reformer discharge stream. 

Figure 5.6. Steam Reforming Thermodynamics with Steam-to-Methane Ratio = 1.0 174.

FlgmeS.7. Steam Reforming Thenuodynamic - Effict of Temperature and Pressure with Steam-to-Methane Ratio 1. 0... 

To avoid carbon deposition, the steam-to-carbon ratio is normally kept between 2.5 -3.0, but processes exist too where a steam to methane ratio is used, as high as 4.0 [6],... 

The quantities of coke deposited on the catalyst were determined by oxidation of coke to CO2, and adsorption on Ascarlte. The experimental minimum ratios were obtained graphically from these data. The quantities of coke obtained experimentally were less than the theoretical values, whereas the experimental minimum steam to methane ratios were higher than the theoretical. A simple model of the Voorhies type described the coking data reasonably well. In the course of the coking runs the catalyst did not deactivate to a great extent, the conversion decreasing by not more than 15 percent. 

In the feed Is sufficiently high so that carbon cannot be present at equilibrium, the equilibrium composition can be calculated from consideration of only Chemical Reactions I and II. Accordingly, the activities of each of the species can be calculated If the equilibrium constants, Kj and KII and value of n are known provided the activity ratio Inequalities given In Equation 1 are met. If the Inequalities of Equation 1 are not met, coke deposition Is possible. Values of the n at which the equalities of Equation 1 are met represent the minimum steam/methane ratio, n., at which no carbon deposition will take place at equilibrium. Figure 1 displays n versus temperature, T. Extending the analysis to Include Chemical Reaction III permits the calculation of the equilibrium coke laydown as a function of n and T shown In Figure 2. These curves will be used later to compare with the corresponding kinetic curves obtained experimentally. 

Figure 1. Minimum thermodynamic steam-methane ratio vs. temperature.

It is also of interest to observe that the coke laydown observed experimentally is more than an order of magnitude, less than would be predicted from equilibrium calculations. That is, the amount of coke on the catalyst per liter of methane on Figure 4 at a given temperature and steam methane ratio is about 5% of that shown on Figure 2 formed under equilibrium conditions suggesting that coke formation is rate limited. 

With adjustment of the steam/methane ratio, the reactor can produce a synthesis gas with CO/H2 = 1/2, the stoichiometric proportions needed for methanol production. This mixture at approximately 200 atm pressure is fed to the methanol unit where the reaction then proceeds at 350°C. Per pass conversions range from 30 to 50 over the catalyst— typically a supported copper oxide with a zinc, chromium, or manganese oxide promoter 3... 

The process is controlled by design of the reactor used for the reforming process, by input mixture (typical steam/methane ratios are 2 to 3, i.e. above the stoichiometric requirement) and, as mentioned, by reaction temperature and catalysts. Other reactions may take place in the reformer, such as the in-... 

Petit-Maire N., Fontugne M., and Rouland C. (1991) Atmospheric methane ratio and environmental changes in the Sahara and Sahel during the last 130 kyrs. Palaeogeogr. Palaeoclimatol. Palaeoecol. 86, 197-204. 

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