Steam-methane ratio, kinetic

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. 

A tube reactor with dimensions given in Table 11.9 was simulated. A high ratio between C02-acceptor and catalyst was used because the reforming kinetics are fast compared to the sorption rate. The reactor was filled with steam (97 mole%) and a small amount of hydrogen at the desired temperature, 848 K, at startup. The input to the reactor was methane and steam, in which the steam to methane ratio is set to 6. A high steam to carbon ratio is necesarry... 

The problem of the kinetics of coke formation is a very Important especially with the Increasing demand for the use of low steam to methane ratios [ 10]. Kinetic rate expressions for the coke formation need to be developed. These rate equations should give the rate of coke formation 1n terms of the partial pressure of the various components and not only 1n terms of the carbon deposition and time it should also take Into consideration pore blockage as well as active site coverage by coke. 

Ross and Steel (1973) studied the mechanism of the steam reforming of methane over a coprecipitated nickel alumina catalyst (75% Ni) in a powder form (250-335 pm). The reaction was carried out at low total pressures (0-1 mm Hg), in the temperature range 500-600 C and at a low steam to methane ratio (2.0-0.2). The kinetics are summarized by the following expression ... 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

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. 

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 ... 

This comparison shows that there is a potential to form carbon from the methane-cracking reaction in the inside of the reformer wall at this location in the reformer tube. Detailed, proprietary kinetic expressions for the reactions 8-10 indicate that while reaction 9 will form carbon, the coke will be gasified by steam and CO2 (reactions 8 and 10), so there is no net accumulation of carbon in this example. However, as the steam-to-hydrocarbon feed ratio is reduced further there will be a point where there is an accumulation of carbon because the coking rate of reaction 9 will be greater than the combined gasification rates of reactions 8 and 10. 

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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