Methane temperature profile

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

This approach was applied to data obtained by Hausberger, Atwood, and Knight (17). Figure 9 shows the basic temperature profile and feed gas data and the derived composition profiles. Application of the Hougen and Watson approach (16) and the method of least squares to the calculated profiles in Figure 9 gave the following methane rate equation ... 

The total reactor volume required is independent of the number of beds in the series. This is evident because (a) all the beds operate with the same temperature profile and essentially the same pressure, (b) the inlet gas composition is the same for all the beds, and (c) the outlet gas composition is the same for all beds. Hence, the average driving force is the same for all beds, and the catalyst volume is simply related to the total production of methane. 

The main disadvantage of this technique is that it relies on very accurate temperature measurement, particularly near the top of the temperature profile, so that the position of the 5°F point can be established and the tangent accurately constructed. Also, the end of the bed is predicted only from kinetic considerations when, in fact, other factors may be more important. In practice, however, although this introduces some scatter into successive measurements—as does variation in the duty required of the methanator—the technique has proved very satisfactory. 

Figure 10. Axial temperature profiles during methanation Top, experiment HGR-13 and bottom, experiment HGR-14...

Temperature profiles for methane-air mixture with = 0.50 in 50 mm tube diameter (a) neglecting and (b) considering radiation heat transfer. 

Recently, such a temperature oscillation was also observed by Zhang et al (27,28) with nickel foils. Furthermore, Basile et al (29) used IR thermography to monitor the surface temperature of the nickel foil during the methane partial oxidation reaction by following its changes with the residence time and reactant concentration. Their results demonstrate that the surface temperature profile was strongly dependent on the catalyst composition and the tendency of nickel to be oxidized. Simulations of the kinetics (30) indicated that the effective thermal conductivity of the catalyst bed influences the hot-spot temperature. 

Figure 2 shows the typical methane uptake profile (Lee et al., 2005b). An initial increase in methane consumption is observed due to the gas dissolution in water. Once the liquid is saturated with the gas, no more gas consumption is seen. The hydrate nuclei start to form at this point until it reaches a critical size as indicated by the arrow and an increase in the temperature profile due to heat released during the hydrate formation (exothermic). The curve... 

Figure 28 shows comparisons of the transient gas and solid axial temperature profiles for a step-input change with the full model and the reduced models. The figure shows negligible differences between the profiles at times as short as 10 sec. Concentration results (not shown) show even smaller discrepancies between the profiles. Additional simulations are not shown since all showed minimal differences between the solutions using the different linear models. Thus for the methanation system, Marshall s model reduction provides an accurate 2Nth-order reduced state-space representation of the original 5/Vth-order linear model. 

Use GRI-Mech (GRIM3 0. mec) and a laminar premixed flame code to calculate species and temperatures profiles for a stoichiometric, burner-stabilized methane-air flame at 20 Torr and a unbumed gas velocity of 1.0 m/s. 

Fig. 17.4 Simulation of stoichiometric methane-air flames approaching a stagnation surface. The top panels show the axial velocity and temperature profiles. The lower panels show details of the species composition with the thin flame-front.

Kikas et al. [47] operated a micro structured autothermal reformer for methane in the conventional and reverse flow mode. Owing to the reverse flow conditions, a more uniform temperature profile was achieved in the reactor (Figure 2.21). 

The theoretical and experimental results for a fuel-lean methane-air flame are given in Figures 5-7. These results include temperature and major species compositions. The experimental and theoretical results are compared by matching the abcissas of the temperature profiles. The model very accurately predicts the slope of the temperature profile but predicts a larger final flame temperature than is measured. This is a consequence of heat lost to the cooled, gold-coated burner wall that is 1.5 mm away from the positions where data were taken. 

Since the methanation reaction is strongly exothermic, a sharp temperature rise can be measured across the reaction zone in the catalyst bed. Most methanation reactors are designed with a number of thermocouples that monitor the position of the exotherm. A strong indicator of the amount and rate of methanation catalyst deactivation is the position of the temperature profile in the catalyst bed and its rate of movement over time. A record of the temperature profile should be kept to detect any movement during the first one to two years of operation. An estimate of future life can then be made. ... 

Taking into account the fact that methane steam reforming is a rapid reaction and that the local conversion is determined mainly by the catalyst temperature (Xref = Xref(T)), the evolution of the temperature profile can be estimated through a simplified procedure based on geometric considerations. It can be shown, that... 

Fig. 1.27. Methane steam reformer with cocurrent flow in the reaction zone, (a) Flow scheme, (b) Simulated and measured temperature profiles in the reaction zone of the three-channel reactor, (c) Sketch of the three-channel...

Fig. 7.18. Adsorptive reactor with fixed-bed macrostructuring and temperature profiling for steam reforming of methane according to Xiu et al. [28],...

The outside tubeskin temperature was taken to be identical to that generated in the previous simulation. The input data were also identical. Radial process temperature profiles are given in Figure 7. The ATg between the bed centerline and the wall amounts to 33°C, which is not excessive and permits the radially averaged temperature to be accurately simulated by means of the one dimensional model with "equivalent" heat transfer parameters, as discussed above. The methane conversion at the wall never differed more them 2% absolute from that in the centerline of the bed. The more detailed description which is possible by the two dimensional model would only be required if thermodynamic s predict possible carbon formation, and therefore catalyst deactivation, at locations different from those simulated by the one dimensional model. 

As measured by temperature profiles, catalysts A and B showed very little deactivation over a lifetime of 1000 hours (Figure 3.1.). However, catalyst C underwent severe deactivation over 200 hours. This was reflected in the increasingly shallow nature of the profile, as seen in Figure 3.2., and a rise in the methane content of the product gas. 

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