The effect of H₂ : N₂ ratio on the NH₃ synthesis rate and on process economics over the Co₃Mo₃N catalyst

In this study, the process economics of ammonia synthesis over Co₃Mo₃N was investigated by searching for an optimum feed stoichiometry. From ammonia synthesis rate measurements at atmospheric pressure and 400 °C over Co₃Mo₃N, it was found that the rate was independent of H₂ : N₂ stoichiometry for stoichiometries above 0.5 : 1. For H₂ : N₂ stoichiometries below 0.5 : 1, there was a linear dependency of ammonia synthesis rate on the H₂ : N₂ stoichiometry. Static measurements of hydrogen adsorption isotherms at 25, 50, and 100 °C revealed that the adsorbed amounts of strongly bound hydrogen over the Co₃Mo₃N surface were saturated at around 100 Torr hydrogen pressure. This pressure corresponds to the partial pressure of hydrogen when the H₂ : N₂ stoichiometry is around 0.5 : 1, confirming the role of strongly bound hydrogen in ammonia synthesis. These results were used to modify an existing kinetic expression to be used in a conceptual design, based on a late mixing strategy for the hydrogen stream. This conceptual design and its economic analysis revealed that using low hydrogen stoichiometries can cut the investment and operating costs by a factor of 2.