Magnesia reforming

Supported nickel catalysts catalyze steam-methane reforming and the concurrent shift reaction. The catalyst contains 15-25 wt% nickel oxide on a mineral carrier. Carrier materials are alumina, aluminosilicates, cement, and magnesia. Before start-up, nickel oxide must be reduced to metallic nickel with hydrogen but also with natural gas or even with the feed gas itself. 

Reaction 1.1 is known as steam reforming. The reaction conditions are fairly severe (>1000°C),and the structural strength of the catalyst is an important point of consideration. The catalyst employed is nickel on alumina, or magnesia, or a mixture of them. Other non-transition metal oxides such as CaO, Si02, and K20 are also added. 

Development of active and stable nickel-magnesia solid solution catalysts for COj reforming of methane... 

Sidjabat, O. and Trimm, D.L. Nickel-magnesia catalysts for the steam reforming of light hydrocarbons. Topics in Catalysis, 2000, 11-12 (1), 279. 

Figure 5 Relationship between the catalytic activity for heptane reforming and the amount of hydrogen uptake numerals on the circles give the metal concentration. Catalyst supports were O, y-alumina 3> silica , magnesia (Reproduced by permission from Bull. Japan Pet. Inst., 1975, 17, 3)...

The metal catalysts active for steam reforming of methane are the group VIII metals, usually nickel. Although other group VIII metals are active, they have drawbacks for example, iron rapidly oxidizes, cobalt cannot withstand the partial pressures of steam, and the precious metals (rhodium, ruthenium, platinum, and palladium) are too expensive for commercial operation. Rhodium and ruthenium are ten times more active than nickel, platinum, and palladium. However, the selectivity of platinum and palladium are better than rhodium [1]. The supports for most industrial catalysts are based on ceramic oxides or oxides stabilized by hydraulic cement. The commonly-used ceramic supports include a-alumina, magnesia, calcium-aluminate, or magnesium-alu-minate [4,8]. Supports used for low temperature reforming (< 770 K) are... 

The positive impact of alkali and magnesia on carbon -free steam reforming of liquid hydrocarbons is well-known. It is explained (1,2) by enhanced steam adsorption and spillover to the nickel surface, as also reflected by negative reactions orders with respect to steam. 

Ethanol steam reforming catalysts were developed by Men et al. [24]. Nickel, rhodium and ruthenium catalysts on different carrier materials such as alumina, silica, magnesia and zinc oxide were tested at a S/C ratio of 1.5 and WHSV 90 Lh g J in the temperature range 400-600 °C. All the monometallic catalysts were mainly selective for acetaldehyde and ethylene. Over the rhodium catalyst, a reaction temperature of 600 °C was required to achieve 80% hydrogen selectivity. 

Tomishige, K., Chen, Y. G., Fujimoto, K. (1999). Studies on carbon deposition in CO2 reforming of CH4 over nNickel—magnesia solid solution catalysts. Journal of Catalysis, 181(1), 91-103. 

While resistant to high temperature, catalysts based on magnesia are sensitive to steaming at low temperatures because of the risk of hydration (refer to Reaction R45 in Table 4.1) The reaction may result in breakdown of the catalyst because it involves an expansion of the molecular volume. The equilibrium constant for the reaction is plotted in Figure 4.1 [381], It is seen that at the pressures typical of tubular reformers, hydration cannot take place at temperatures above approximately 350 C. Kinetic studies of the hydration reaction have shown that magnesia reacts via liquid phase reaction in which water condenses in the pores. Therefore, the rate depends on the relative humidity of the atmosphere, and in practice hydration is a problem only when the magnesia-based catalyst is exposed to liquid water or is operated close to the condensation temperature [381]. Hydration is eliminated for carriers where magnesia has reacted with alumina to form... 

Nickel-based catalysts on various carriers such as alumina, lanthana, magnesia and zinc oxide have been studied intensively for ethanol steam reforming [196]. 

Similar results were reported by Frusteri et of. for their nickel/magnesia and nickel/ceria catalysts [198]. At a reaction temperature of 650 °C, coke formation was significant under conditions of steam reforming. In the nickel/magnesia catalyst, which contained 15wt.% nickel, less than O.lwt.% carbon was formed within a 20-h test duration when operated under autothermal conditions. Consequently, no deactivation of the catalyst was observed. However, the 0/C ratio that was required to achieve this stable performance was rather high at 1.2, and the S/C ratio of 4.2 was also very high. Besides methane, small amounts of acetaldehyde were formed as a by-product. 

Methane or natural gas steam reforming performed on an industrial scale over nickel catalysts is described above. Nickel catalysts are also used in large scale productions for the partial oxidation and autothermal reforming of natural gas [216]. They contain between 7 and 80 wt.% nickel on various carriers such as a-alumina, magnesia, zirconia and spinels. Calcium aluminate, 10-13 wt.%, frequently serves as a binder and a combination of up to 7 wt.% potassium and up to 16 wt.% silica is added to suppress coke formation, which is a major issue for nickel catalysts under conditions of partial oxidation [216]. Novel formulations contain 10 wt.% nickel and 5 wt.% sulfur on an alumina carrier [217]. The reaction is usually performed at temperatures exceeding 700 °C. Perovskite catalysts based upon nickel and lanthanide allow high nickel dispersion, which reduces coke formation. In addition, the perovskite structure is temperature resistant. 

Table 5.4 Experimental results of methane steam reforming in a gap of 76- j,m height over a rhodium/magnesia/alumina catalyst coating at different contact time [385].

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