Reforming kinetics equilibria

To model the performance of the autothermal reformer, kinetics from the literature that had been determined for the catalytic combustion of methane over a platinum-based catalyst and for steam reforming over nickel-based catalyst were combined and fitted to the experimental data of Flytzani-Stephanopoulos et al. [153]. The water-gas shift reaction was assumed to reach thermodynamic equilibrium under all conditions in the reformer reactor, which is usually the case in reformers. Methane formation was not considered. Because catalyst pellets had been used for the determination of the kinetics, diffusion limitations were to be expected. They had been lumped into the kinetic models. The hot spot formation usually observed at... 

Kreutz, T., Steinbugler, M., and Ogden, J., Onboard fuel reformers for fuel cell vehicles Equilibrium, kinetic and system modeling, Proc. 1996 Fuel Cell Seminar, Orlando, FL, 714, 1996. 

The reformate gas contains up to 12% CO for SR and 6 to 8% CO for ATR, which can be converted to H2 through the WGS reaction. The shift reactions are thermodynamically favored at low temperatures. The equilibrium CO conversion is 100% at temperatures below 200°C. However, the kinetics is very slow, requiring space velocities less than 2000 hr1. The commercial Fe-Cr high-temperature shift (HTS) and Cu-Zn low-temperature shift (LTS) catalysts are pyrophoric and therefore impractical and dangerous for fuel cell applications. A Cu/CeOz catalyst was demonstrated to have better thermal stability than the commercial Cu-Zn LTS catalyst [37], However, it had lower activity and had to be operated at higher temperature. New catalysts are needed that will have higher activity and tolerance to flooding and sulfur. 

Downstream of the reformer the CO is converted into hydrogen by two subsequent water-gas shift sections a high-temperature shift (HTS) followed by a low-temperature shift (LTS). This is done because the equilibrium of the WGS reaction lies at the product side at lower temperatures (around 200 °C), but the reaction kinetics are faster at increasing temperature. Therefore, to reach high CO conversions, most of the CO is converted in a HTS section and the remainder is converted within a LTS section. 

The direct detection of a complex from an equilibrium mixture is certainly the most obvious evidence of specific molecular interactions between components. Electrophoresis of an equilibrium mixture is an easily performed experiment, enabling the determination of complex formation parameters. When the dissociation kinetics of the complex is slow, the complex gives rise to a new peak in the electropherogram, in addition to the peaks of the free component molecules. Since the separation of the free components prevents the reformation of the complex inside the capillary, the complex peak should decrease in size during electrophoresis. The extent of this decrease depends on dissociation kinetics and separation time. In view of that fact, short analysis times, as obtained in CE, are required to detect less stable complexes, which would hardly be detected using previous formats of electrophoresis with longer separation times. 

Steam reformers are used industrially to produce syngas, i.e., synthetic gas formed of CO, CO2, 11-2, and/or hydrogen. In this section we present models for both top-fired and side-fired industrial steam reformers by using three different diffusion-reaction models for the catalyst pellet. The dusty gas model gives the simplest effective method to describe the intermediate region of diffusion and reaction in the reformer, where all modes of transport are significant. This model can predict the behavior of the catalyst pellet in difficult circumstances. Two simplified models (A) and (B) can also be used, as well as a kinetic model for both steam reforming and methanation. The results obtained for these models are compared with industrial results near the thermodynamic equilibrium as well as far from it. 

K3 is the equilibrium constant for reaction (42), which is the product of the equilibrium constants for reactions (41) and (37) K4] Kiy. For the ratio C02/C0 the authors assume only a slight deviation from the equilibrium and use an empirical relation without a kinetic term C02/C0=/(CH4 conversion, S/C ratio, K,7). Other kinetic expressions may be found in [362], [418], [422], For the reaction mechanism [422] of steam reforming of methane, the following scheme (Eqs. 51-55) was proposed ... 

A. Methane Reforming.—Several of the kinetic equations in Table 4 relating to CH4 reforming include a term (1 — K jK), where K is the equilibrium constant for the reforming reaction, e.g. (5) or (6), and K is pco. Ph IPch. Ph o for example for reaction (5), pco, etc. being the actual partial pressures of the gaseous components. The term is important under conditions near equilibrium because it takes into account the back reaction, the rate of which increases as equilibrium... 

Such thermodynamic conclusions are only relevant when the system is completely at equilibrium for reactions (4), (5), and, say, (9), but in an open system, such as a catalyst zone in a reformer where the gas is not yet at equilibrium, reaction between the components of that non-equilibrated gas can produce carbon even when the equilibrated gas shows no affinity for carbon formation. This is particularly so when higher hydrocarbons are involved and reaction (7) is possible. Whether carbon is deposited in that zone depends upon the kinetics of the carbon-forming and carbon-removing reactions, which can be influenced... 

Higher hydrocarbons do not exist at equilibrium and any risk of whisker formation from these compounds can be disregarded at these conditions. Nevertheless whiskers may still form from higher hydrocarbons because at nonequilibrium conditions a potential for the irreversible carbon formation [e.g.. Reaction (11) in Table 3] may exist. The formation of whisker carbon at these conditions depends on a kinetic balance between the rate of the carbon forming and steam-reforming reactions. A simplified reaction sequence outlining the kinetic balance is shown in Fig. 8. The key step is whether the adsorbed hydrocarbon species will react to form adsorbed carbon and whiskers or react with oxygen species to produce gas. ... 

The solid lines in Fig. 1.64b and c represent the fit of this mechanism to the experimental data. It equally well fits all other obtained kinetic data [33]. In this catal dic reaction cycle, Au202 reacts with CO to form Au2(C0)02, which will either redissociate to the oxide or further react with a second CO molecule to reform Au2 while liberating two CO2 molecules. It should be noted that the quality of the fit is very sensitive to the postulated reaction steps and that the kinetic evaluation procedure that was employed is clearly able to discriminate against alternative mechanisms, as has been demonstrated before [32,187,188]. The replacement of the equilibrium in reaction (1.65b), e.g., by a simple forward reaction will lead to a mechanism that yields an inadequate fit to the experimental data. The Au202 signal will then disappear at long reaction times, which is not the case as can be seen from Fig. 1.64c. 

The outlet from the secondary reformer contains about 10-14% CO (dry gas) which is fed to a high-temperature water gas shift (WGS) reactor (Fig. 2.2), typically loaded with Fe or Cr particulate catalyst at about 350°C. This further increase the H2 content lowering CO content to about 2% as governed by the thermodynamic and kinetics of the Eq. 2.3, that is an exothermic reaction. Water gas shift reaction equilibrium is sensitive to temperature with the tendency to shift towards products when temperature decreases. 

The H/C ratio in eq. (2) is 1, whereas the ratio in eq. (1) is 3. The use of C02 instead of steam represents no change in overall reaction kinetics (ref. 2). However, the presence of CO, in the feedstock results in more critical conditions for carbon because of lower H/C ratio. The ratio of H2/CO in the reformer exit gas can be estimated by thermodynamic calculations knowing the atomic ratio 0/C and H/C in the feed stream, and the pressure and temperature at the reformer exit. The results of the calculations are presented in the equilibrium chart (ref. 3) shown in Fig. 2 for a given pressure and temperature. 

Although thermodynamic calculations show appreciable quantities of di-methylbutanes at equilibrium (about 30-35% of the total hexane isomers at 500°C), such quantities are not observed in reforming. Equilibria are established readily between n-hexane and the methylpentanes, but not between these hydrocarbons and the dimethylbutanes. The reaction kinetics are not favorable for the rearrangement of singly branched to doubly branched isomers (11). This limitation apparently does not exist for the rearrangement of the normal structure to the singly branched structures. 

All these factors are functions of the concentration of the chemical species, temperature and pressure of the system. At constant diffu-sionai resistance, the increase in the rate of chemical reaction decreases the effectiveness factor while al a constant intrinsic rate of reaction, the increase of the diffusional resistances decreases the effectiveness factor. Elnashaie et al. (1989a) showed that the effect of the diffusional resistances and the intrinsic rate of reactions are not sufficient to explain the behaviour of the effectiveness factor for reversible reactions and that the effect of the equilibrium constant should be introduced. They found that the effectiveness factor increases with the increase of the equilibrium constants and hence the behaviour of the effectiveness factor should be explained by the interaction of the effective diffusivities, intrinsic rates of reaction as well as the equilibrium constants. The equations of the dusty gas model for the steam reforming of methane in the porous catalyst pellet, are solved accurately using the global orthogonal collocation technique given in Appendix B. Kinetics and other physico-chemical parameters for the steam reforming case are summarized in Appendix A. 

It is of interest and practical importance to show the situation where different feeds are introduced to the model with different steam partial pressures while the feed partial pressures of the other components are kept constant, which means of course, a change of the total pressure. The steam reformer tube chosen for simulation is 5 m long to illustrate the kinetic effects rather than the thermodynamic equilibrium effect since the assumption of constant temperature along the tube causes a fast approach to thermodynamic equilibrium of the mixture. 

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