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 CoMoN was investigated by searching for an optimum feed stoichiometry. From ammonia synthesis rate measurements at atmospheric pressure and 400 °C over CoMoN, it was found that the rate was independent of H : N stoichiometry for stoichiometries above 0.5 : 1. For H : N stoichiometries below 0.5 : 1, there was a linear dependency of ammonia synthesis rate on the H : N stoichiometry. Static measurements of hydrogen adsorption isotherms at 25, 50, and 100 °C revealed that the adsorbed amounts of strongly bound hydrogen over the CoMoN surface were saturated at around 100 Torr hydrogen pressure. This pressure corresponds to the partial pressure of hydrogen when the H : N 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.