Reforming kinetics pressure effects

Note that in Fig. 18, KINPTR s prediction of C5- falls below the data points. However, when one considers the large temperature and pressure effect on activity in the C6 system and the fact that these same C6 kinetics are used in KINPTR to make predictions for all reforming feedstocks (full-range naphthas, pure components, etc.), the predictions are certainly acceptable. 

While the 13 hydrocarbon lumps accurately represent the hydrocarbon conversion kinetics, they must be delumped for the deactivation kinetics. In addition, delumping is necessary to estimate many of the product properties and process conditions important to an effective reformer process model. These include H2 consumption, recycle gas H2 purity, and key reformate properties such as octane number and vapor pressure. The following three lump types had to be delumped the C5- kinetic lump into Cl to C5 light gas components, the paraffin kinetic lumps into isoparaffin and n-paraffin components, and the Cg+ kinetic lumps into Cg, C9, C10, and Cn components by molecular type. 

R16H selectivity and activity kinetics were fit over a wide range of temperature and pressure. Reforming selectivity is shown in Figs. 16 and 17, where benzene and hexane are plotted against C5-, the extent of reaction parameter. The effect of pressure on reforming a 50/50 mixture of benzene and cyclohexane at 756 K is shown in Fig. 16. Selectivity to benzene improves significantly when pressure is decreased from 2620 to 1220 kPa. In fact, at 2620 kPa, hexane is favored over benzene when the C5 yield exceeds 10%. This selectivity behavior can be seen in the selectivity rate constants ... 

Sinfelt and associates (S6) for a 0.3% platinum on alumina catalyst. At these temperatures diffusional effects are much less important than at the usual reforming temperatures. Over the range of methylcyclohexane and hydrogen partial pressures investigated, 0.07 to 2.2 atm. and 1.1 to 4.1 atm., respectively, the reaction was found to be zero order with respect to hydrogen and nearly zero order with respect to methylcyclohexane (Table III). The kinetic data were found to obey a rate law of the form... 

The TEOM results presented in Figure 9.4 showed that CO in the product stream of C02 reforming might represent another major component contributing to carbon formation. Consequently, in an effort to further elucidate the contribution of CH4 and CO to carbon formation, a kinetics study on the effect of the partial pressure of CH4 or CO (PCH4 or PCo) on the carbon formation rate was conducted under conditions of 0.1 MPa and 923 K. 

Meanwhile, a number of different promoters have been tested with respect to their effects on the kinetics. NaAlH4 can be readily reformed within 2 to 10 min at around 100 °C, but at rather high pressures of 80 bar or more [20-22]. 

Today contractors and licensors use sophisticated computerized mathematical models which take into account the many variables involved in the physical, chemical, geometrical and mechanical properties of the system. ICI, for example, was one of the first to develop a very versatile and effective model of the primary reformer. The program REFORM [361], [430], [439] can simulate all major types of reformers (see below) top-fired, side-fired, terraced-wall, concentric round configurations, the exchanger reformers (GHR, for example), and so on. The program is based on reaction kinetics, correlations with experimental heat transfer data, pressure drop functions, advanced furnace calculation methods, and a kinetic model of carbon formation [419],... 

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. 

To evaluate the potential of carbon formation in a steam reformer, it is therefore essential to have a rigorous computer model, which contains kinetic models for the process side (reactor), as well as heat transfer models for the combustion side (furnace). The process and combustion models must be coupled together to accurately calculate the process composition, pressure, and temperature profiles, which result from the complex interaction between reaction kinetics and heat transfer. There may also be a temperature difference between bulk fluid, catalyst surface, and catalyst interior. Lee and Luss (7) have derived formulas for this temperature difference in terms of directly observable quantities The Weisz modulus and the effective Sherwood and Nusselt numbers based on external values (8). 

CO, a partial pressure of 10 vol.% CO in a reformate gas (typical CO volume fraction directly at the reformer exhaust [39]) reduces the amount of corroded carbon up to 40 % for start and around 80 % for stop events (Fig. 14.7 [40]). Therefore, it is not only possible but desirable to use CO-containing fuel gas to improve the overall lifetime of the fuel cell. Conceptually, this positive effect of the presence of CO on the corrosion during start/stop processes might be even more pronounced in LT-PEMFC due to very fast CO adsorption kinetics on Pt and significantly higher CO saturation coverages [15-19]. 

The retarding effect of sulphur is a dynamic phenomenon. This means that carbon may be formed at certain conditions in spite of sulphur passivation - although at strongly reduced rates and by a mechanism different from the formation of whisker carbon. Therefore, it was important to develop design criteria to make sure that the kinetic balance is in favour of no carbon formation at all positions in the reformer tube. This work was carried out mainly in the full-size monotube process demonstration plant (Dibbern et al., 1986). The influence of various process parameters (pressure, heat flux, sulphur content in feed, etc) was studied. It was demonstrated that the impact of variations in sulphur content in the feedstream on tube wall temperature and exit gas composition was completely reversible. 

It is commonly accepted that CO exchange in (diphosphine)Rh(CO>2H complexes proceeds via the dissociative pathway. The decay of the carbonyl resonances of the (diphosphine)Rh( CO)2H complexes indeed follows simple first-order kinetics. This clearly confirms that reformation of the (diphosphine)Rh( CO)2H complex via pathway is suppressed effectively, and all dissociated CO is replaced by CO. The experiments with ligand 35 at different CO partial pressure show that the rate of CO displacement is independent of the CO pressure. Furthermore, the rate is also independent of the (diphosphine)Rh( CO)2H complex concentration, as demonstrated by the experiments with ligand 33. It can therefore be concluded that CO dissociation for these complexes proceeds by a purely dissociative mechanism and obeys a first-order rate-law. 

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