Methane steady-state model

Samuelson, R. E. Mayo, L. A. (1997). Steady-state model for methane condensation in Titan s troposphere. Planetary and Space Science, 45,949-58. 

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

In combustion systems it is generally desirable to minimize the concentration of intermediates, since it is important to obtain complete oxidation of the fuel. Figure 13.5 shows modeling predictions for oxidation of methane in a batch reactor maintained at constant temperature and pressure. After an induction time the rate of CH4 consumption increases as a radical pool develops. The formaldehyde intermediate builds up at reaction times below 100 ms, but then reaches a pseudo-steady state, where CH2O formed is rapidly oxidized further to CO. Carbon monoxide oxidation is slow as long as CH4 is still present in the reaction system once CH4 is depleted, CO (and the remaining CH2O) is rapidly oxidized to CO2. 

G. Veser, J. Frauhammer, Modeling steady state and ignition during catalytic methane oxidation in a monolith reactor. Chem. Eng. Sci.,... 

The steady-state permeation model of in situ coal gasification is presented in an expanded formulation which includes the following reactions combustion, water-gas, water-gas shift, Boudouard, methanation and devolatilization. The model predicts that substantial quantities of unconsumed char will be left in the wake of the burn front under certain conditions, and this result is in qualitative agreement with postburn studies of the Hanna UCG tests. The problems encountered in the numerical solution of the system equations are discussed. 

The steady state temperature of the catalyst surface under mass-transport-limited conditions can exceed the adiabatic flame temperature if the rate of mass transport of fuel to the surface is faster than the rate of heat transport from the surfaee. The ratio of mass diffusivity to heat dilTusivity in a gas is known as the Lewis number. Reactor models [9] show that for gases with a Lewis number close to unity, such as carbon monoxide and methane, the catalyst surface temperature jumps to the adiabatic flame temperature of the fuel/air mixture on ignition. However, for gases with a Lewis number significantly larger than unity the rate of mass transport to the surface is much faster than the rate of heat transport from the surface, and so the wall temperature can exceed the adiabatic gas temperature. The extreme case is... 

A fairly general treatment of trace gases in the troposphere is based on the concept of the tropospheric reservoir introduced in Section 1.6. The abundance of most trace gases in the troposphere is determined by a balance between the supply of material to the atmosphere (sources) and its removal via chemical and biochemical transformation processes (sinks). The concept of a tropospheric reservoir with well-delineated boundaries then defines the mass content of any specific substance in, its mass flux through, and its residence time in the reservoir. For quantitative considerations it is necessary to identify the most important production and removal processes, to determine the associated yields, and to set up a detailed account of sources versus sinks. In the present chapter, these concepts are applied to the trace gases methane, carbon monoxide, and hydrogen. Initially, it will be useful to discuss a steady-state reservoir model and the importance of tropospheric OH radicals in the oxidation of methane and many other trace gases. 

Comparison with Fig. 4-6 shows that the steady-state prediction is in very good agreement with the observational data. This justifies the assumption that methane is the dominant source of formaldehyde in the marine atmosphere. Figure 4-6 includes the results from a two-dimensional model calculation (Derwent, 1982). The values are somewhat higher than the measured ones, but they confirm the constancy with latitude in the equatorial region. The decline of m(HCHO) toward high latitudes, which is evident... 

Example 3 also illustrates the modeling of a sulfate profile really measured, including a sulfate-methane transition zone in the deep part of the profile. However, in this case the profile displayed such an unusual course that its interpretation as a steady-state condition did not appear justified by any means. As a consequence, this profile rested among many other measurement data for almost ten years. Only after we understood that nonsteady-state conditions in marine sediment profiles - particularly in the continental slope region -are much more frequent than previously assumed were we able to understand this profile as well (Hensen et al. 2003). 

Figure 4 shows the model results (lines) along with e.xperimental data from steady state experiments (symbols) during methane partial oxidation with air on a Pt coated monolith for two feed temperatures. Obviousl . the model very closely reproduces both the catalyst temperature and methane conversion over the entire range of methane/air mixtures studied, while syngas selectivities are in less good agreement with the experiment. 

The good agreement between the model and both steady state and ignition experiments indicates that the surface reaction mechanism describes the essential steps of the catalytic reaction very accurately. The next step in developing this model further will now be to fill in the remaining gaps in the surface mechanism for methane oxidation and to extend the mechanism towards C2 chemistrv on the catalvst surface. 

Figure 6.4 Pure gas transport data at 25 °C of membranes AF1600 (O), AFl 6 350 30 fD), AF16 80 15 (A), AF16 80 30 (U), AF16 80 40 ( 0), silicalite-1 (O) as derived from literature data (see text), and predictions of the Maxwell model fora AF16/MFI30% membrane ( ) (a) Pure gas steady state permeability vs kinetic diameter of the permeating molecules (b) gas/methane separation factor (c) gas diffusion coefficients from time-lag experiments vs kinetic diameter (d) gas solubility vs the e/k Lennard-Jones parameter...

The kinetics of methane combustion over a perovskite catalyst (Lao.9Ceo.iCo03) has been studied in Micro-Berty and fixed bed reactors. Discrimination among twenty-three rival kinetic models from Eley-Rideal, LHHW and Mars-van Krevelen (MVK) types has been achieved by means of (a) the initial rate method as well as by (b) integral kinetic data analysis. Two MVK type models could be retained as a result of the two studies, with a steady-state assumption implying the equality of the rate of three elementary steps. 

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