Steam hydrocarbon reforming catalysts

Hydrocarbon steam reforming catalysts are classified into natural gas steam reforming catalysts and light-oil steam reforming catalysts according to the feedstock, and primary- and secondary- steam reforming catalysts according to the processes. 

Although the CO shift reaction, i.e., (1.16), is a moderate exothermic reaction, the reactions of (1.14), (1.15) and (1.17) are strongly endothermic. Therefore, methane steam reforming is a strongly endothermic reaction. Tube-type reactor with external heat-supplier is usually applied in industry. 

It is known from the above reactions that low-pressures and high-temperatures are beneficial to methane steam reforming. Pressurized reactions are adopted in industry for the sake of economy. 

With liquid-hydrocarbon such as naphtha as feedstock, the steam reforming reactions are very complicated, and the reactions can be summarized as follows  

Reaction (1.18) is strongly endothermic and its reaction heat is higher than the whole heat released from reactions (1.19) and (1.16). Therefore, the overall process is endothermic, and low-pressure and high-temperature are favorable. Reaction (1.18) is irreversible at normal temperatures. Other than CO, CO2, H2, and residual steam, there is no hydrocarbon in the outlet gases. The reaction equilibration is dependent on the reactions (1.19) and (1.16). The outlet gases equilibrium compositions are related to temperature, pressure and ratio of H2O/C, as well as molar ratio of H/C in raw hydrocarbons. Excess of steam is beneficial to the conversion of naphtha. 

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HYDROCARBON STEAM REFORMING CATALYSTS - ALKALI INDUCED RESISTANCE TO CARBON FORMATION... 

Composition of Some Industrial Steam Reforming Catalysts (NG = natural gas, HC = hydrocarbon, PR = prereforming, LPG = liquefied petroleum gas, SEC = secondary reforming)... 

Here we shall briefly summarize the effects of individual poisons on various catalytic reactions taking place on automotive catalysts. There are three main catalytic processes oxidation of carbon monoxide and hydrocarbons and reduction of nitric oxide. Among secondary reactions there are undesirable ones which may produce small amounts of unregulated emissions, such as NH3, S03 (6), HCN (76, 77), or H2S under certain operating conditions. Among other secondary processes which are important for overall performance, in particular of three-way catalysts, there are water-gas shift, hydrocarbon-steam reforming, and oxygen transfer reactions. Specific information on the effect of poisons on these secondary processes is scarce. 

Reaction (1) is the primary reforming reaction and is endothermic. Reaction (2) is the water-gas shift reaction and is exothermic. Both these reactions are limited by thermodynamic equilibrium. The overall reaction is endothermic and hence requires that additional fuel be combusted to supply heat. The conventional steam reformer is a fired furnace containing catalyst-filled tubes. The hydrocarbon and steam mixture is processed in the catalyst-filled tubes while external burners heat the tubes. Nickel supported on a ceramic matrix is the most common steam reforming catalyst. 

Sulphur Poisoning. Sulphur is the most common poison for steam reforming catalysts. Sulphur is a natural component of all hydrocarbon feedstocks, but the sulphur contents of the feed is reduced to a few ppb by hydro-desulphurization followed by absorption over zinc oxide. The remaining sulphur, normally below the analytical detection limit, will slowly poison the catalyst (7). The mechanisms of sulphur poisoning are described in detail in the literature 2,8), The sulphur compounds are chemisorbed dissociatively on the nickel surface equation 4. 

Gum formation, which is a polymerization of hydrocarbons (especially aromatic compounds) on the catalyst surface, is a deactivation phenomenon that takes place at low temperature. Therefore, an investigation of the appearance of gum on steam reforming catalysts used at prereforming conditions is very relevant. Deactivation by gum formation can proceed several times faster than ordinary sulphur poisoning. 

R.M. Yarrington, I.R. Feins, and H.S. Hwang, Evaluation of steam reforming catalysts for use in autothermal reforming of hydrocarbon feed stocks. Proceedings of National Fuel Cell Seminar, San Diego, CA (1980). 

Coking resistant Nickel Catalysts for Hydrocarbon Steam Reforming Song Ruojun, Zhang Liangqu and Guo Shendu... 

The effect of the addition of a potassium promoter to a nickel steam reforming catalyst has been probed in terms of the propensity of the catalyst to resist carbon formation. It has been found that potassium facilitates a reduced accumulation of carbon by decreasing the rate of hydrocarbon decomposition on the catalyst and by increasing the rate of steam gasification of filamentary carbon from the catalyst. The effect of the promoter on the carbon removal reaction is evident in an enhancement of the pre-exponential factor in the rate equation by promotion of water adsorption on the catalyst surface. 

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. 

COKING-RESISTANT NICKEL CATALYSTS FOR HYDROCARBON STEAM REFORMING... 

With commercial methane steam reforming catalyst very good results are being obtained- Not only the methane in the exit gas but also higher hydrocarbons and tars can be eliminated by steam reforming, fig.4,... 

One of the main problems associated with hydrocarbon steam reforming over Ni is the deactivation of the Ni catalyst as a result of the formation of carbon deposits on Ni. The C-induced deactivation of Ni has been studied extensively [10,18,28-35], For example, Rostrup-Nielsen reported that steam reforming of various hquid fuels on Ni leads to the formation of encapsulating, whisker-like, or pyrolytic carbon on the catalyst [18, 30], To illustrate the problem of carbon poisoning, in Fig. 13.1 we show a transmission electron micrograph (TEM) of a Ni particle taken after steam reforming of propane at steam to carbon ratio of 1.5. The micrograph shows that carbon deposits are formed on Ni [16],... 

Zhuang, Q., Qin, Y., and Chang, L. Promoting effect of cerium oxide in supported nickel catalyst for hydrocarbon steam-reforming. Applied Catalysis, 1991, 70 (1), 1. 

Chin, Y.-H., King, D.L., Roh, H.-S., Wang, Y., and Heald, S.M. Structure and reactivity investigations on supported bimetallic AuNi catalysts used for hydrocarbon steam reforming. Journal of Catalysis, 2006, 244 (2), 153. 

Although copper-based catalysts have long been known to have good WGS activities, sensitivity of those catalysts to poisons that were present in the coal-derived gas precluded them from being employed industrially. It is only due to a massive change from coal gasification to hydrocarbon steam reforming which produces much purer synthesis gas, that copper-based catalysts entered the scene of WGS processes. Since... 

Carbon formation on steam reforming catalysts takes place in three different forms whisker-like carbon, encapsulated carbon, and pyrolytic carbon as described in Table 2.2 [1]. Whisker-like carbon grows as a fiber from the catalyst surface with a pear-shaped nickel crystal on the end. Strong fibers can even break down catalyst particles increasing the pressure drop across the reformer tubes [4], The carbon for whisker formation is formed by the reaction of hydrocarbons as well as CO over transition metal catalysts [1], The whisker growth is a result of diffusion through the catalyst and nucleation to form a long carbonaceous fiber. 

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