Steam-to-carbon ratio

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 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. 

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. 

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... 

The aim of this study is to convert as much as the hydrogen in the fuel into hydrogen gas while decreasing CO and CH4 formation. Process parameters of fuel preparation steps have been determined considering the limitations set by the catalysts and hydrocarbons involved. Lower S/C (steam to carbon) ratios favor soot and coke formation, which is not desired in catalytic steam and autothermal reforming processes. A considerably wide S/C ratio range has been selected to see the effect on hydrogen yield and CO formation. 

The reactor temperature required to prevent coke formation varies considerably for the different processes. Table 2.1 summarizes the values calculated assuming thermodynamic equilibrium for 2,2,4-trimethylpentane reforming. Generally, the coking tendency increases in the following order at constant O/C ratio SR > ATR > POx. These calculations demonstrate that at steam to carbon ratios (S/C) > 2 and reaction temperatures > 600 °C, which is very common for hydrocarbon fuel processors, coke seems to be an unstable species especially under the conditions of steam reforming. 

Steam and propane were fed to the reactor with a low steam to carbon ratio (S/C) of 1.4, which corresponded to 5 Nml min-1 propane and 21 Nml min-1 steam balance nitrogen. The experiments were performed with three temperature ramps of 1 h each at 450, 550 and 650 °C [52],... 

Steam to carbon ratio Sensor analytical manager Split-and-recombine Sodium dodecyl sulphate Scanning electron microscopy Simultaneous gradient-sputtering Staggered herringbone mixer Static mixing elements... 

Intermediate Duty catalysts are for feeds with a significant content of components from ethanes up to liquid petroleum gas (LPG). The heavier feedstock increases the tendency for catalyst deactivation through carbon laydown and requires a special catalyst in the top 30% to 50% of the reformer tubes. This tendency also occurs when light feeds are run at low steam-to-carbon ratios and/or at a high heat flux. 

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