Steam reforming of higher hydrocarbons

The steam reforming of higher hydrocarbons shows reaction orders with respect to hydrocarbons less than one [389] [497] as shown in Table 3.5. This reflects a stronger adsorption of the higher hydrocarbons than of methane. 

The conversion of higher hydrocarbons on nickel takes place by irreversible adsorption with only Ci components leaving the surface [389]. The first reaction step is probably related to hydrogenolysis  

The hydrogenolysis at low temperatures of hydrocarbons higher than ethane shows that nickel attacks selectively the ends of the chains [308] [389] by successive a-scission, in contrast to, for instance, platinum. This means that methane is the main product on nickel. 

Hydrocarbons show different reactivities for the steam reforming reaction. Literature [346] [389] reports different sequences of reactivities depending on temperature and the active metal. 

Results in Table 3.5 show that methane has a lower reactivity than the higher hydrocarbons. The apparent activation energy is higher for methane than for ethane and n-butane, the two latter showing nearly identical activation energies. On a molar basis and at a given steam-to-carbon ratio, butane is less reactive than ethane, whereas the reactivities remain similar on a carbon atom basis. 

---

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... 

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 mechanism of steam reforming of higher hydrocarbons over Ni-based catalysts has been reported to involve adsorption of the hydrocarbon on the catalyst surface followed by the formation of Ci species by the successive a-cleavage of the C-C bonds. Similar to that proposed for the SMR reaction (Eqs. 2.12-2.20), the adsorbed Ci species are then dehydrogenated stepwise into adsorbed carbon atoms, which may dissolve in the Ni crystal. When the concentration of carbon is above saturation, a carbon whisker will nucleate. These reactions compete with the reaction of Ci... 

From these two reactions it can be gathered that at higher temperatures less CH4 and more CO will be present in the equilibrium gas. However, by LeChatelier principle, increasing the pressure will increase the methane equilibrium content. Reaction 2.3 represents the steam reforming of higher hydrocarbons, which are present in small quantities in natural gas. 

For practical design, a conservative guideline for carbon-free operation would be to require that at no position in the reactor, there would be a thermodynamic potential for carbon formation (15) corresponding to carbon limit A . This approach does not apply for the steam reforming of higher hydrocarbons because the decomposition into carbon is irreversible. It means that it is important to control the parameters influencing carbon limit A. 

Chen, Y., Xu, H., Wang, Y. and Xiong, G. (2006) Application of Coprecipitated Nickel Catalyst to Steam Reforming of Higher Hydrocarbons in Membrane Reactor. Chinese Journal of Catalysis, 27, 772-777. 

Dreyer, BJ, Lee, IC, Krummenacher, JJ, Schmidt, LD. Autothermal steam reforming of higher hydrocarbons n-decane, n-hexadecane and JP-8. Appl. Catal. A Gen. 2006 307 184-194. 

Although all reactions may describe specific operating conditions, only two out of the first four reactions are independent from a thermodynamic point of view, since die other two can be established as linear combinations of the two selected ones. Catalytic studies indicate that it is steam reforming of methane to carbon monoxide and the water-gas-shift reactions that are the independent reactions in addition to the steam reforming of higher hydrocarbons as the last reaction. This set of reactions (Rl, R4, and R5 in Table 1.2) will consequently be used in the following. 

Table 3.5 Steam reforming of higher hydrocarbons. Ni/MgO catalyst. 1 bar abs, 500°C. Intrinsic rates. Power-law kinetic expression in partial pressure (atm.abs) [389] [497].

This approach does not apply to the steam reforming of higher hydrocarbons, because in this case the decomposition into carbon is irreversible. 

In general, steam reforming of higher hydrocarbons is usually performed at S/C ratios exceeding the stoichiometry (S/C = 2) in order to suppress coke formation. An S/C ratio of 3 may be required for higher hydrocarbons in the absence of oxygen in the feed as for autothermal reforming (see Section 3.3 below). However, excess steam reduces the overall effidency of the system (see Sections 2.2 and 5.4.3). 

It is widely accepted that steam reforming of higher hydrocarbons requires S/C ratios higher than 2.5 to prevent coke formation. Thus, coke formation might well have been the origin of catalyst deactivation observed by Villegas et cd. [71]. 

Numerous other rate expressions for steam reforming of higher hydrocarbons are listed in the literature (Reference 4, pages 55 57). Many of these are simplifications of the relationships shown here, with assumptions imposed on the relative contribution of the various terms. These assumptions may be valid for some operating conditions, or for some catalysts. Many of the rate expressions are simple power law relationships that are difficult to relate to mechanistic pathways, and to physical attributes of the catalyst or the reacting species. The rate expression chosen for the model is one that has a broad range of applicability, for different catalyst types, and wide operating conditions. 

---