Higher Hydrocarbon Reforming

Mori et al. concluded that n-butane reformed over a Ni/Al203 catalyst at 450 °C by a single-site mechanism with the hydrocarbon and the steam competing for the nickel surface when the amount of H2O adsorbed was not 

Takami, A. Igarashi, and Y. Ogino, Bull. Japan Pet. Inst., 1977, 19, 37. 

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Similar expressions have been found to be applicable to the steam reforming of higher hydrocarbons. For example, it has been shown that if it is assumed that ethane, CaHg is adsorbed on two neighbouring sites, the overall reaction rate can be expressed by the equation... 

Natural gas consists mainly of methane together with some higher hydrocarbons (Tab. 8.1). Sulfur, if present, must be removed to a level of about 0.2 ppm prior to the steam reforming process as it poisons the catalyst. This is typically done by cata-lytically converting the sulfur present as thiols, thiophenes or COS into H2S, which is then adsorbed stochiometrically by ZnO, at 400 °C, upstream of the reactor. 

The bulk of the naphtha was hydrotreated and catalytically reformed over a chlorided Pt/Al203-based catalyst to produce an aromatic motor gasoline. However, the hydrotreated Fischer-Tropsch naphtha is a poor feed for standard catalytic reforming on account of its high linear hydrocarbon content (>75%).37 In order to limit liquid yield loss, typical operation resulted in a reformate with quite low octane value (Table 18.10). Higher octane reformate could be produced, but at the expense of significant liquid yield loss. 

A "mild reformer" is assumedto eliminate of the higher hydrocarbons prior to entering the fuel cell to prevent sooting. This reformer is called a "mild reformer" to indicate that the reforming reactions are not pushed to completion, for it is desired that the methane be reformed in the fuel cell for better temperature management. Some of the methane, however, will be reformed with the higher hydrocarbons in the mild reformer. 

It can be seen that the thermodynamic driving force for carbon formation decreases as temperature increases. Carbon formation from the Boudouard reaction is thermodynamically favored at lower temperatures because this reaction is exothermic. This kind of carbon formation usually dominates at the reactor inlet (or feed lines) where the temperature is lower. However, higher temperatures favor the cracking reaction (7). Therefore it is often desirable to conduct the hydrocarbon reforming at an intermediate temperature where the thermodynamic driving force for carbon formation is minimal. 

In addition to Ni catalysts, Lee and Park explored some unconventional catalysts, such as limestone, dolomite, and iron ore, in a fluidized bed reactor to carry out SR of kerosene and bunker oil. H2 yields from SR of bunker oil over various catalysts (temperature = 800°C, bed height = 10 cm, superficial gas velocity = 20 cm/sec, and S/C = 1.6) were sand (20%), iron ore (29%), commercial Ni catalyst (89%), limestone (93%), and dolomite (76%). Limestone as a SR catalyst looked very promising, but H2 yields over a limestone catalyst decreased over time due to elutriation of fines during the reaction. A fluidized-bed reactor was advantageous for reforming of higher hydrocarbons, due to its ability to replace coked catalyst with fresh catalyst during operation. 

Recently, the use of Rh supported on washcoated alumina monoliths has attracted interest for ATR of higher hydrocarbons.Reyes et al carried out ATR of n-C6 in monolithic catalysts containing Rh as an active component. A maximum H2 yield of 170% was obtained from the reforming of n-C6 at an O/C ratio of 1, a S/C of 1, preheat temperature of 700°C, and GHSV of 68,000 h Brandmair et al. also carried out ATR of n-C6 over Rh supported on ceramic monoliths at similar conditions, and reported that the Rh catalyst provided better performance over time. 

The probability of carbon formation on the reformer catalyst is significantly decreased since no higher hydrocarbons are present in the feed. 

Reactivity of Hydrocarbons. - Each homologous series in a liquid fuel can exhibit different kinetics upon reforming under similar reaction conditions. For example, aromatic compounds are the most difficult to reform and require higher temperatures and lower space velocities. Aromatics also contribute significantly to carbon formation, compared to paraffins and naphthenes. At the same reaction conditions, the H2 production rates are typically in the order aromatics naphthenes. ° The relative reactivities of various higher hydrocarbons are summarized in Table 12. 

From the practical point of view, this is probably the most important reaction related to the metallic component of the reforming catalysts (despite the fact that a part of aromatization is acid catalyzed). There are certainly several pathways which can, at least in principle, lead to the aromatic products. Let us mention here the most relevant facts on aromatization of hexanes and higher hydrocarbons. 

The higher hydrocarbon formation from syngas has long been industrialized as the Fischer-Tropsch synthesis [27]. Yet, syngas production from the C02 reforming of methane is an endothermic reaction, and requires a high temperature (ca. 1073 K) for a favorable equilibrium ... 

Higher Hydrocarbons. - A number of papers describing the steam reforming of higher hydrocarbons are particularly concerned with the subject of carbon deposition on the catalysts. The subject of carbon deposition on nickel catalysts is considered to be somewhat outside the subject of this review, especially as the subject is covered by two excellent recent discussions of papers on carbon deposition and coking during steam reforming, methanation, and other reactions.202 203... 

The steam reforming of hydrocarbons is in principle a reduction of water with the carbon of the organic starting material. In the case ofmethane, / of the hydrogen is supplied by water. This share increases with the higher hydrocarbons. Other reactions that proceed at the same time as the reforming... 

In addition to the capital cost for a new reactor, the pre-reformer does add complexity to the system - particularly in the requirements for startup and shutdown. It is most attractive in cases 1) where more capacity is needed without increased firing and 2) where the feedstock contains a significant amount of C3 or higher hydrocarbons, which could crack and form coke in the preheat coil86. 

Avoiding carbon deposition on the catalyst is a major challenge [2, 3]. Carbon can be present as graphite-like coke and in the form of whiskers, or carbon nanofibers. The latter lead to detachment of the nickel crystallites from the support and breaking of the catalyst pellets. This may cause blockage of the reformer reactor tubes and the formation of hot spots. Higher hydrocarbons exhibit a larger tendency to form... 

Description Natural gas or another hydrocarbon feedstock is compressed (if required), desulfurized, mixed with steam and then converted into synthesis gas. The reforming section comprises a prereformer (optional, but gives particular benefits when the feedstock is higher hydrocarbons or naphtha), a fired tubular reformer and a secondary reformer, where process air is added. The amount of air is adjusted to obtain an H2/N2 ratio of 3.0 as required by the ammonia synthesis reaction. The tubular steam reformer is Topsoe s proprietary side-wall-fired design. After the reforming section, the synthesis gas undergoes high- and low-temperature shift conversion, carbon dioxide removal and methanation. 

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