Steam to carbon

Naphtha desulfurization is conducted in the vapor phase as described for natural gas. Raw naphtha is preheated and vaporized in a separate furnace. If the sulfur content of the naphtha is very high, after Co—Mo hydrotreating, the naphtha is condensed, H2S is stripped out, and the residual H2S is adsorbed on ZnO. The primary reformer operates at conditions similar to those used with natural gas feed. The nickel catalyst, however, requires a promoter such as potassium in order to avoid carbon deposition at the practical levels of steam-to-carbon ratios of 3.5—5.0. Deposition of carbon from hydrocarbons cracking on the particles of the catalyst reduces the activity of the catalyst for the reforming and results in local uneven heating of the reformer tubes because the firing heat is not removed by the reforming reaction. 

This reaction is affected by the steam-to-carbon ratio, temperature, and pressure, as well as catalyst activity. 

Carbon produced by these latter reactions is formed in the catalyst pores, making it much more difficult to remove, and potentially causing physical breakage. Operating steam to carbon ratios are chosen above the minimum required in order to make carbon formation by these reactions thermodynamically impossible (3). Steam is another potential source of contaminants. Chemicals from the boiler feedwater or the cooling system are poisons to the reformer catalyst, so steam quality must be carefully monitored. 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

D. Kitchen, A. Pinto, and H. Van-Praag, "ICI s Operating Experience with Shift Catalyst at Low Steam to Carbon Ratios," AIChE Ammonia Safety Symposium, Denver, Colo., Aug. 22—24, 1988, American Institute of Chemical Engineers, New York, 1989. 

A promoted nickel type catalyst contained in the reactor tubes is used at temperature and pressure ranges of 700-800°C and 30-50 atmospheres, respectively. The reforming reaction is equilibrium limited. It is favored at high temperatures, low pressures, and a high steam to carbon ratio. These conditions minimize methane slip at the reformer outlet and yield an equilibrium mixture that is rich in hydrogen. ... 

The inlet methanol molar concentration was determined by the mass of catalyst, S/C ratio, and W/F ratio. Here, steam-to-carbon (S/C) ratio is defined as the ratio of steam molecules per carbon atom in the reactant feed and W/F ratio as the amount of catalyst loading into the channel divided by the amount of methanol molar flow rate. For more information on the design parameters, physical properties, and operating conditions, refer to Jung et al. [12]. 

At 900 °F the equilibrium constant for this reaction is 5.62 when the standard states for all species are taken as unit fugacity. If the reaction is carried out at 75 atm, what molal ratio of steam to carbon monoxide is required to produce a product mixture in which 90% of the inlet CO is converted to C02 ... 

The hydrocarbon feed must contain sufficient steam to avoid carbon formation on the catalyst. The steam-to-carbon ratio is defined as moles of steam per mole of carbon in the hydrocarbon. The steam-to-carbon ratios are about 3.0 for hydrocarbon feedstocks but lower values can be used for some feedstocks. Carbon formation is more likely with heavier feedstocks. An alkali-based catalyst can be used to repress carbon formation. 

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]. 

Whether a driving force for carbon formation exists is dictated by thermodynamics, and so it is dependant on reaction temperature and pressure, and the H/C and O/C ratios in the system (Fig. 14.6). By removing hydrogen from the reaction zone, the H/C ratio decreases and the system moves closer to the thermodynamic area where carbon formation is likely. The arrow in Fig. 14.6 labeled SMR is equal to a reforming mixture with a steam-to-carbon ratio of 3. When hydrogen... 

The reactor was tested using a range of methanol and water concentrations, and researchers found the best results using a water and methanol mixture with a steam-to-carbon ratio (S C) of 1.1 1. They were able to achieve 90% conversion at 260 °C with a reactant liquid flow rate of 12 cmYh. Assuming a fuel cell efficiency of 60% and 80% hydrogen utilization, they estimated the output power to be 15 W. Eventually the complete system will include a cata-... 

Figure 7. Carbon formation temperature for n-octane fuel as a function of steam-to-carbon ratio.

Other important parameters in the steam reforming process are temperature, which depends on the type of oxygenate, the steam-to-carbon ratio and the catalyst-to-feed ratio. For instance, methanol and acetic acid, which are simple oxygenated organic compounds, can be reformed at temperatures lower than 800 °C. On the other hand, more complex biomass-derived liquids may need higher temperatures and a large amount of steam to gasify efficiently the carbonaceous deposits formed by thermal decomposition. 

Table 6.1 lists the stoichiometric yields of hydrogen and percentage yields by weight from steam reforming of some representative model compounds present in biomass pyrolysis oils, and also several biomass and related materials. The table also shows the equilibrium yield of H2, as a percentage of the stoichiometric yield, predicted by thermodynamic calculations at 750 °C and vdth a steam-to-carbon (S/C) ratio of 5 [32]. 

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]. 

By proper adjustment of the oxygen-to-carbon and steam-to-carbon ratios, the partial combustion in the thermal zone [reaction (9.8)] supplies the heat for the subsequent endothermic SR reaction (9.1) [24]. The CO shift reaction (9.2) also takes place in the catalytic zone. 

A structured ruthenium catalyst (metal monolith supported) was investigated by Rabe et al. [70] in the ATR of methane using pure oxygen as oxidant. The catalytic activity tests were carried out at low temperature (<800 ° C) and high steam-to-carbon ratios (between 1.3 and 4). It was found that the lower operating temperature reduced the overall methane conversion and thus the reforming efficiency. However, the catalyst was stable during time on-stream tests without apparent carbon formation. 

Lenz and Aicher reported the experimental results obtained with an autothermal reformer fed with desulfurized kerosene employing a metallic monolith coated with alumina washcoat supporting precious metal catalysts (Pt and Rh) [78]. The experiments were performed at steam-to-carbon ratios S/C = 1.5-2.5 and... 

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