Steam-methane ratio, kinetic minimum

In the feed Is sufficiently high so that carbon cannot be present at equilibrium, the equilibrium composition can be calculated from consideration of only Chemical Reactions I and II. Accordingly, the activities of each of the species can be calculated If the equilibrium constants, Kj and KII and value of n are known provided the activity ratio Inequalities given In Equation 1 are met. If the Inequalities of Equation 1 are not met, coke deposition Is possible. Values of the n at which the equalities of Equation 1 are met represent the minimum steam/methane ratio, n., at which no carbon deposition will take place at equilibrium. Figure 1 displays n versus temperature, T. Extending the analysis to Include Chemical Reaction III permits the calculation of the equilibrium coke laydown as a function of n and T shown In Figure 2. These curves will be used later to compare with the corresponding kinetic curves obtained experimentally. 

Steam reforming of hydrocarbons has become the most widely used process for producing hydrogen. One of the chief problems In the process Is the deposition of coke on the catalyst. To control coke deposition, high steam to hydrocarbon ratios, n, are used. However, excess steam must be recycled and It Is desirable to minimize the magnitude of the recycle stream for economy. Most of the research on this reaction has focused mainly on kinetic and mechanistic considerations of the steam-methane reaction at high values of n to avoid carbon deposition ( L 4). Therefore, the primary objective of this studyis to determine experimentally the minimum value of n for the coke-free operation at various temperatures for a commercial catalyst. 

The reason why the minimum steam ratio goes down with temperature is not known with certainty. One possibility is that the competing reactions of carbon production and consumption have such kinetics that the rate of coke consumption increases faster with temperature than the rate of coke generation, which suggests that the carbon-steam reaction has a higher activation energy than the methane cracking and carbon monoxide disproportionation reaction. 

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. 

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