Reforming kinetics temperature effects

It is critical to determine and control the steam-to-carbon (S/C) and/or oxygen-tointernal reforming) to avoid carbon deposition. Thermodynamic analysis is commonly used to estimate the minimum ratios. For example. Figure 33.18 shows the equilibrium number of moles of carbon per mole of methane introduced into an ATR as a function of S/C and O/C at two reformer inlet temperatures of 150 and 400 °C [8]. It can be seen that for aU values of O/C between 0 and 1.5, carbon deposition should not be a concern if an S/C > 1.2 is maintained in the fuel gas mixture entering the ATR (fiiUy mixed inlet stream). It should be noted that many thermodynamic calculations (as in this example) assume adiabatic equilibrium reactions and do not take into account reaction kinetic effects. The inclusion of reaction kinetics in the analysis may lead to different results. 

Fhosphoric acid does not have all the properties of an ideal fuel cell electrolyte. Because it is chemically stable, relatively nonvolatile at temperatures above 200 C, and rejects carbon dioxide, it is useful in electric utility fuel cell power plants that use fuel cell waste heat to raise steam for reforming natural gas and liquid fuels. Although phosphoric acid is the only common acid combining the above properties, it does exhibit a deleterious effect on air electrode kinetics when compared with other electrolytes ( ) including such materials as sulfuric and perchloric acids, whose chemical instability at T > 120 C render them unsuitable for utility fuel cell use. In the second part of this paper, we will review progress towards the development of new acid electrolytes for fuel cells. 

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 ... 

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. 

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... 

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. 

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