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Catalysts
After almost a hundred years of using iron catalysts for ammonia synthesis, ruthenium based materials seem to be challenging that role and are believed to be the second-generation ammonia synthesis catalysts. Their higher activity, that allows the operation under milder conditions in temperature and pressure while maintaining higher conversions than conventional systems, led to their first use in a commercial NH3 synthesis plant in 19921. Especially their improved performance at low temperatures is of interest in terms of an application in the solar thermal ammonia dissociation. Lower operating temperatures of solar receivers will result in lower reradiation losses and thermal stresses. Moreover, if a sufficient reaction rate can be obtained at about 250°C, the use of solar troughs becomes feasible. By that means, no three dimensional tracking system is used and tubular solar receivers would simply be placed in the focal line of reflecting troughs. The cost of such a concentrating system would almost certainly be lower than for dishes. Nevertheless, the pressure drop induced by long tubular receivers would have to be minimised by the use of some sort of ceramic foam support for the catalyst2.
The major drawback of ruthenium catalysts is hydrogen poisoning. Hydrogen thereby retards the dissociative adsorption of nitrogen which is the rate determining step in ammonia synthesis3. This is especially true for high pressures and has also led to the application of sub-stoichiometric H2/N2 mixtures in commercial ammonia synthesis plants using ruthenium catalysts1. Energy efficiency considerations however favour the operation under high pressures in a solar closed loop ammonia system and rules out non-stoichiometric synthesis gas mixtures4. Therefore other ways of dealing with hydrogen poisoning are sought. Resent catalyst developments have led to promising results concerning this drawback: no hydrogen retardation in ammonia synthesis was observed when using samarium oxide as a promoter material for alumina supported ruthenium catalysts at pressures up to 4MPa3. Following this work, we synthesised alumina and magnesia supported ruthenium catalysts, each promoted with caesium oxide/hydroxide or samarium oxide. The ammonia synthesis activity of caesium promoted ruthenium catalysts decreases strongly at high pressures while for samarium promoters it does not. We now investigate the performance of these catalysts in the thermocatalytic ammonia dissociation. For that purpose, a plug flow microreactor operating at pressures up to 30MPa and temperatures up to 700°C was built. Products can be analysed on-line by a non-diffusive infrared spectrometer and batch-wise with a gas chromatograph.
- Czuppon, T. A.; Knez, S. A.; Schneider, R. V. The 1995 Kellogg Advance Ammonia Technologies Symposium, New Delhi, India, 1995; p 1.
- Richardson, J. T. 7th International Symposium on Solar Thermal Concentrating Technologies, Moscow, 1994; p 896-909.
- Kadowaki, Y.; Aika, K. Journal of Catalysis 1996, 161, 178-185.
- Lovegrove, K. International Journal of Energy Research 1993, 17, 831-845.
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