Steam-carbon ratio, coke deposition

Figure 2. Thermodynamic coke deposition vs. steam-carbon ratio.

Carbon deposition is one of the luost serious problems of the steam reforming catalyst process (ref 1). The deposition of carbon on naphtha steam reforming catalysts depends ori the chemical composition of the hydrocarbon oil, the steam/carbon ratio in the feedstock, as well as the pi ocesa temperature and pressure, it is also affected by tlie presence of sulfur poisons Our past research of SNG catalysts ejiamined the nature of the carbon deposits as a function of the sulfur level on the catalyst (refs, 2 4). A small amount of sulfur was found to promote the formation of carbon that is non-reactive with steam and hydrogen under steam reforming reaction conditions. The continuous accumulation of this less reactive carbon [continuous carbon deposition (CCD)l on the catalyst surface leads to coke fouling Studies of the occurrence of CCD in our laboratory tests allow ua to predict, that coke fouling is likely to occur on the same catalyst used in real Indusl.rlal applications. 

Although the stoichiometry for reaction (9.1) suggests that one only needs 1 mol of water per mole of methane, excess steam must be used to favor the chemical equilibrium and reduce the formation of coke. Steam-to-carbon ratios of 2.5-3 are typical for natural gas feed. Carbon and soot formation in the combustion zone is an undesired reaction which leads to coke deposition on downstream tubes, causing equipment damage, pressure losses and heat transfer problems [21]. 

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. 

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. 

Coke deposition is a potential problem in steam reformers, and is favored by low steam to carbon ratio and high temperatures. High hydrogen composition also lowers the potential for carbon deposition. The following three reactions can deposit solid carbon, and the thermodynamic equilibrium relationships for these reactions, when carbon is graphite, are well loiown. ... 

Another cause of activity loss is carbon deposition, which can be avoided if a high steam to carbon (S/C) ratio is employed [45, 46], However, economic evaluations indicate that the optimum S/ C ratio tends to be low. The presence of tars in the reforming reactor enhances coking and it is the main cause of carbon formation in reforming a gas from biomass thermal conversion [29]. 

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. 

Pyrolytic carbon is formed mainly by three different reactions, namely, the reversible decomposition of methane (Reaction 2.5), the irreversible cracking of higher hydrocarbons (Reaction 2.6), and/or coke formation (Reaction 2.7). The formation of these carbon deposits leads to the breakdown of the catalyst and hot spots in the reactor. Pyrolytic carbon is usually found as dense shales on the reformer wall or encapsulating the catalyst particles. The process leads to the deactivation of the catalyst and increase of pressure drop across the reformer tubes. The thermal cracking of hydrocarbon occurs at high temperatures and at low steam to hydrocarbon ratios. 

In any case, one of the most important issues to be prevented in SOFC systems is carbon deposition (coke formation) from the fuels. Figure 6.21 shows the equilibrium products for (a) methane- and (b) methanol-based fuels with the steam-to-carbon (S/C) ratio of 1.5 at elevated temperatures [251]. Assuming thermochemical equilibrium, carbon deposition is not expected to occur within a wide temperature range. The calculated results for various other fuels mentioned above have been shown elsewhere [251]. The minimum amounts of H2O (water vapor) necessary to prevent carbon deposition are shown in Fig. 6.22 for hydrocarbon fuels. While S/C of 1.5 is enough for CH4, higher S/C is needed with increasing carbon number of hydrocarbons, especially at lower temperatures. Such dependencies have also been revealed for O2 (partial oxidation) and CO2 (CO2 reforming) [251] to prevent carbon deposition. 

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