Methane concentration profiles from

Kelley C. A. and Jeffrey W. H. (2002) Dissolved methane concentration profiles and air-sea fluxes from 41°S to 27°N. Global Biogeochem. Cycles 16, 10.1029/2001GB001809. 

Fig. 8.6 Pore-water concentration profiles from gravity core GeoB 3714-9 from the Benguela upwelling area (2060 m water depth), South Atlantic. The shaded bar marks the sulfate-methane transition zone. The methane sample labeled C.C. was taken from the core catcher immediately after core recovery. From Niewohner et al. (1998).

Figure 2.42. Relative steady-state methanation activity profiles for Ni ( ), Co (A), Fe ( ), and Ru (O) as a function of gas-phase H2S concentration. Reaction conditions 100 kPa, 400°C, 1% CO/99%H2 for Co, Fe, and Ru, 4% CO/96% H2 for Ni.131 Reprinted with permission from Academic Press.

The differences in reactions at different reactor positions was studied by Springmann et al. who reported product compositions for ATR of model compounds as a function of reactor length in a metal monolith coated with a proprietary noble metal containing Rh. As expected, the oxidation reactions take place at the reactor inlet, followed by the SR, shift, and methanation reactions. Figure 32 shows the product concentration profiles for a 1-hexene feed, which are typical results for all the fuels tested. These results show that steam, formed from the oxidation reactions, reaches a maximum shortly after the reactor inlet, after which it is consumed in the shift and reforming reactions. H2, CO and CO2 concentrations increase with reactor length and temperature. In this reactor, shift equilibrium is not reached, and the increase in CO with distance from the inlet is the net result of the shift and SR reactions. Methane is... 

Figure 4.3.6 Principle of two in situ cells that are optimized for the spectroscopic studies and the catalytic experiments the X-ray transmission image is recorded by an X-ray eye, and the calculated profile of the methane concentration is estimated for 1% CH.4/4% 02/He on a pellet of 1 mm thickness and catalyst particles of 100 pm at 500°C (taken from ref. [29]).

Figures 28 and 29 show the transient methanation activity of a Ni/Al203 flat-plate catalyst and the gas-phase H2S concentration profile, respectively. The presence of just 13-ppb H2S caused about a 200-fold loss in steady-state methanation activity. Increasing the H2S level to 62 ppb resulted in an additional tenfold activity loss an increase to 95 ppb lowered the activity further. However, increasing the H2S level above 95 ppm did not cause a significant additional decrease in activity (Fig. 30) and decreasing the H2S level from 95 to about 15 ppb reversibly restored the activity level originally observed at this latter concentration level, thereby demonstrating that sulfur adsorption and poisoning by sulfur are reversible, and that a truly dynamic...

From studies of the concentration profiles through dichlorodifluoro-methane/fluorine flames at low pressures, Homann and MacLean have proposed a chain mechanism involving fluorine and chlorine atoms as chain carriers [151(b)]. 

Fig. 9 (A) Major species concentration profiles and (B) Some of the polycyclic aromatic hydrocarbons (PAH) formed in a fuel-rich, premixed laminar methane flame the formation of a large number of intermediates and by-products are evident. Highly toxic benzo-fl-pyrene is the 3rd PAH from the bottom. (From Ref. " l)...

The coincidence of maxima in the methane oxidation rate and the sulfate reduction rate in Saanich Inlet strongly suggests that the methane oxidizing agent was sulfate, either via direct reaction, or coupled indirectly through reactions with other substrates (Devol, 1983). A methane-sulfate coupled reaction diffusion model was developed to describe the inverse relationship commonly observed between methane and sulfate concentrations in the pore waters of anoxic marine sediments. When fit to data from Saanich Inlet (B.C., Canada) and Skan Bay (Alaska), the model not only reproduces the observed methane and sulfate pore water concentration profiles but also accurately predicts the methane oxidation and sulfate reduction rates. In Saanich Inlet sediments, from 23 to 40% of the downward sulfate flux is consumed in methane oxidation while in Skan Bay this value is only about 12%. 

Fig. 3.14 Concentration profiles of pore water from anoxic sediments obtained from an upwelling area off Namibia at a water depth of approximately 1300 m. The analysis of sulfide and methane was carried out in samples that were punched out with syringes from small and quickly sawed-out windows in the fresh sediment core. As for sulfide, these syringe-drawn samples were brought into an alkaline environment, whilst for methane analysis the samples were stored in head space vials for subsequent gas-chromatography analysis. The arrow points to a methane sample that originated from a sealed sediment core obtained by using a sample from the core catcher (after Niewohner et al. 1998).

It is noticeable that the values in the profiles hardly scatter at the somewhat lower concentrations (about 4 nunol/1 for methane and 7 nunol/1 for sulfide, respectively). The values of the higher concentrations obtained in greater depths scatter more strongly and are altogether far too low. This became evident since the highest methane concentration is to be found in a sample that was not obtained from an extra sawed-out window , but from one that was previously taken directly from the core catcher at the lower open end of the core. As to the question concerning the potential... 

Fig. 8.5 Profiles of pore-water sulfate and methane concentrations and of rates of sulfate reduction and methane oxidation for a sediment core recovered from the Kattegat (Station B 65 m water depth). The broken horizontal line denotes the depth where sulfate and methane were at equimolar concentrations - indicating the peak of the sulfate/methane transition. From Iversen and Jorgensen (1985).

Fig. 8.10 Geochemical data for core GeoB 1023-4 recovered off north Angola (17°09.6 S, 10°59.9 E, 2047 m water depth). Barium and sulfate pore-water concentration profiles as well as the distribution of solid-phase barium indicate the precipitation of authigenic barite at a front slightly above the depth of complete sulfate consumption. Below the sulfate/methane transition barite becomes undersaturated and is thus subject to dissolution due to the total depletion of pore-water sulfate. Dissolved barium diffuses upwards into the sulfate zone where the mineral barite becomes supersaturated and so-called authigenic or diagenetic barite precipitates at a front at the base of the sulfate zone. Modified from Gingele et al. (1999), after Kolling (1991).

The Pd-O bond also varies with the extent of oxidation of Pd. During the methane combustion reaction, the catalyst surface is a non-equilibrium, kineti-cally controlled structure. The oxygen concentration profile in the particle results from a combination of particle reconstruction, oxygen adsorption, bulk diffusion, and oxygen removal. This concentration profile varies as a function of time, and as the oxygen content increases, the Pd-O bond strength decreases. This increase is accompanied by an increase in the specific activity. The most widely accepted reaction pathway is the Mars and van Krevelen redox mechanism, which involves lattice oxygen and uneoordinated Pd centers as active species. Inhibition by products (H2O and CO2) and impurities (SO2) is a major drawback for low temperature combustion. The effect of sulfur is particularly important for catalytic converters for NGV applications because it drastically reduces the methane combustion activity. 

Spahni et al. 2005). The concentrations of both gases follow the 8D values of ice in Fig. 17.32 between 380,000 and 650,000 years. The peaks and valleys of the 8D profile are identified by MIS numbers placed on top for warm intervals (peaks) and on the bottom for cold intervals (valleys). The concentrations of methane in this time interval range from 375 parts per billion by volume (ppbv) to 725 ppbv which amounts to an increase by a factor of 1.9. Since methane, like carbon dioxide, is a greenhouse gas that absorbs infrared radiadon, the variations of the methane concentrations correlate closely with variations in 5D which tracks the average annual temperature of the East Antarctic ice sheet at Dome C. 

The concentration profiles that result from solving the methane and the higher hydrocarbon reaction rates simultaneously agree well with the profiles reported by others, and the reactor effluent species concentrations agree well with those observed fi om industrial reformers (Appendix A). The effluent concentrations are fairly insensitive to reaction rates, since industrial reformers operate near equilibrium conditions. Effluent conditions will only be noticeably affected by reaction rates as the catalyst activity declines significantly. The temperature profile, especially in the first on third of the reactor which is furthest from equilibrium, is affected significantly by reaction rates, and therefore is the most affected by the catalyst activity. The case study results in Appendix B illustrate the concentration profiles, and the effects of catalyst activity on temperature profiles. 

FIGURE 8.2 Concentration-time profiles in the kinetic calculation of the methane-air reaction at an inlet temperature of 1000K. P2 = lOatm, [Pg.424]

As a consequence of this explanation the reaction runaway to total methanation is not a necessary condition for the observed phenomenon. Any simple exothermic two phase reaction in an adiabatic reactor ought to show the same behaviour provided that one phase with a high throughput is used to carry the heat out of the reactor and the flow is suddenly reduced. This will be shown in the following simulation results. Due to problems with the numerical stability of the solution (see Apendix) only a moderate reaction rate will be considered. Reaction parameters are chosen in such a way that in steady state the liquid concentration Cf drops from 4.42 to 3.11 kmol/m3 but the temperature rise is only 3°C (hydrogen in great excess). At t = 0 the uniform flow profile... 

In the anoxic water methane reaches about 16. iM. The vertical distribution of methane differs from that of hydrogen sulfide, ammonia, and phosphate its profile curve bends at 500-600 m and keeps similar concentrations deeper toward the bottom [73]. 

The temperature and density structure of the troposphere, along with the concentrations of major constituents, are well documented and altitude profiles have been measured over a wide range of seasons and latitudes for the minor species water, carbon dioxide, and ozone. A few profiles are available for carbon monoxide, nitrous oxide, methane, and molecular hydrogen, while only surface or low-altitude measurements have been made for nitric oxide, nitrogen dioxide, ammonia, sulfur dioxide, hydrogen sulfide, and nonmethane hydrocarbons. No direct measurements of nitric acid and formaldehyde are available, though indirect information does exist. The concentrations of a number of other important species, such as peroxides and oxy and peroxy radicals, have never been determined. Therefore, while considerable information concerning trace constituent concentrations is available, the picture is far from complete. 

After a certain residence time or reaction length the deposition profile shows a decrease in the layer thickness because of the consumption of carbon from the gas phase used in the initial range of the deposition zone. Carbon concentration, which is defined as gas-phase concentration, gradually decreases as CVD takes place from the time that the methane enters the deposition space. 

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