The effect of H : N ratio on the NH synthesis rate and on process economics over the CoMoN catalyst

In this study, the process economics of ammonia synthesis over Co 3 Mo 3 N was investigated by searching for an optimum feed stoichiometry. From ammonia synthesis rate measurements at atmospheric pressure and 400 °C over Co 3 Mo 3 N, it was found that the rate was independent of H 2 : N 2 stoichiometry for stoichiometries above 0.5 : 1. For H 2 : N 2 stoichiometries below 0.5 : 1, there was a linear dependency of ammonia synthesis rate on the H 2 : N 2 stoichiometry. Static measurements of hydrogen adsorption isotherms at 25, 50, and 100 °C revealed that the adsorbed amounts of strongly bound hydrogen over the Co 3 Mo 3 N surface were saturated at around 100 Torr hydrogen pressure. This pressure corresponds to the partial pressure of hydrogen when the H 2 : N 2 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. Ammonia synthesis rates are independent of H 2 : N 2 stoichiometry for H 2 : N 2 > 0.5 over the Co 3 Mo 3 N catalyst. Only strongly adsorbed hydrogen is involved in the reaction. Its coverage is independent of hydrogen pressure for H 2 : N 2 > 0.5.