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How the carriers for ruthenium-based ammonia catalysts are upgraded and modernised

Sep. 15, 2023

KBR's KAAP ammonia synthesis process uses activated carbon based ruthenium catalysts in three of the four beds, which are much more active than standard ferrous iron catalysts. The higher activity allows the reaction to be carried out at 9 MPa compared to 20 MPa with conventional technology, thus reducing investment and energy consumption. The use of ruthenium catalysts in the last three beds increases ammonia yields to more than 18%.

The conventional ammonia feedstock hydrogen/nitrogen stoichiometry ratio is 3B1. Due to the properties of the ruthenium catalyst with accelerator, a lower feedstock hydrogen/nitrogen ratio is required, and the operating pressure of the internals is lower, resulting in a higher yield. However, from a thermodynamic point of view, graphite is very unstable under industrial conditions in ammonia synthesis and ruthenium has a catalytic effect on the hydrogenation of graphite to methane. Barium inhibits the formation of methane to a greater extent, but it cannot be completely avoided.

After a more detailed understanding of ruthenium catalysts without and with promoters, several improved ruthenium catalysts have emerged, e.g. Ba-Ru/MgO catalysts with high activity and stability developed by Ruhler et al. and ruthenium-containing catalysts for ammonia synthesis developed by Tops<e, Denmark. In these studies, ruthenium catalysts based on magnesium-aluminium spinel and high surface area graphite showed good activity. However, their stability under industrial conditions is still problematic.

Recently, ruthenium catalysts based on Ba-promoted BN (boron nitride) have been developed with unprecedented activity and stability. BN (known as white graphite) is a promising carrier material for ruthenium catalysts for ammonia synthesis, which is almost structurally similar to graphite (except for a slight difference in stacking of layers on top of the monolayers), and unlike graphite, it is stable in all hydrogenation reactions. At the same time, it is known for its high-temperature resistance and, in contrast to graphite, it is an insulating material. The developed Ba-Ru/ BN catalyst containing 5.6% Ba and 6.7% Ru on 81 m2/g carrier has been shown to be stable under equilibrium conditions of 5,000 h, 10 MPa, 550 e, and a hydrogen to nitrogen ratio of 3B1 .

The appropriate surface area of BN, ruthenium concentration, additives and concentration, particle size and density can be selected under specific reaction conditions (temperature, pressure, hydrogen/nitrogen ratio, ammonia concentration, etc.) to obtain the optimum catalytic activity of the Ru/BN catalysts. Furthermore, the useful Ba, Ru and B mixtures in the Ba-Ru/BN catalysts can be recovered in a similar manner to the Ba-Ru/MgO catalysts.

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