Catalysts containing ruthenium and cobalt-enhanced catalysts are characterized by higher activity per volume compared to the conventional iron-based catalysts which are used for ammonia synthesis. Main benefits are lower catalyst volumes, lower operating pressure, and a higher conversion rate per pass. While there are energy benefits offered by these catalysts (UNIDO, 1979 p.180; IPTS/EC, 2007 p.81), the energy savings could be offset by increased energy requirements for ammonia refrigeration (IPTS/EC, 2007 p.81). At high pressures most of the ammonia can be condensed by cooling water while at lower pressures (15-20 MPa) the refrigeration requirements are high (UNIDO, 1979 p.173).
Some examples of different catalyst types are (Ullmann’s, 2011 p.169):
Cobalt-modified magnetite catalyst. The activity per volume is twice that of conventional iron-based catalysts (Ullmann’s, 2011 p.170). The use of cobalt was introduced in 1984 by ICI. It was initially used in an ammonia plant in Canada using the ICI Catalco’s AMV process. Later on other ammonia plants using the AMV and LCA process.
Iron-based catalyst using wustite instead of magnetite. This type of catalyst is used in seven ammonia plants (total capacity 5,600 t/d).
Ruthenium on a graphite support catalyst. The ruthenium catalyst used in the Kellogg Advanced Ammonia Process (KAAP) has a considerably higher surface area than conventional iron-based catalysts, and is claimed to be 10-20 times more reactivity than iron-based catalysts. There are three plants in Trinidad using the KAAP ruthenium-based catalyst. Tøpsoe has also developed a ruthenium-based catalyst with no current industrial application.
In order for ruthenium-based catalysts to be widely adopted the benefits from their use will have to justify the higher costs. In 2000, ruthenium prices increased by a factor of four as ruthenium found new applications in the electronic industry (Ullmann’s, 2011 p.172).
This measure is applicable to both new and existing steam reforming plants and partial oxidation plants (IPTS/EC, 2007 p.81).