Why should the hydrogen-to-nitrogen ratio in ammonia synthesis be between 2.2 and 2? Between 8

As can be seen from the reaction rate of ammonia synthesis, under non-equilibrium conditions, appropriately increasing the nitrogen partial pressure is beneficial for the rate at which the catalyst adsorbs nitrogen, since the active adsorption of nitrogen is the controlling step in the ammonia synthesis process. Under normal production conditions, the ammonia yield can only reach 50%–70% of the equilibrium value; therefore, it is necessary to increase the proportion of nitrogen in the production process, with a hydrogen-to-nitrogen ratio in the recycle gas of around 2.2–2.8 being appropriate.

That’s quite comprehensive; I’m learning *learning*

It relates to the reaction kinetics of ammonia synthesis; the adsorption of nitrogen is the controlling step in the entire reaction, so increasing the partial pressure of nitrogen appropriately is necessary

According to the principle of equilibrium shift, if the concentration of an equilibrium system is changed, the equilibrium shifts in a direction that counteracts this change. The ammonia synthesis reaction proceeds by consuming the reactants in a ratio of H2/N2=3; therefore, increasing the partial pressures of hydrogen and nitrogen while maintaining H2/N2=3 can raise the equilibrium ammonia content. As can be seen from the reaction rate of ammonia synthesis, under non-equilibrium conditions, appropriately increasing the nitrogen partial pressure is beneficial for the rate at which the catalyst adsorbs nitrogen, since the active adsorption of nitrogen is the controlling step in the ammonia synthesis process. Under normal production conditions, the ammonia yield can only reach 50%-70% of the equilibrium value; therefore, it is necessary to increase the proportion of nitrogen in the production process, with a hydrogen-to-nitrogen ratio in the recycle gas of around 2.2–2.8 being appropriate.

Reply to 5# chen3jun: I’m sorry. I’m a bit confused about what you mentioned, as the active adsorption of nitrogen is the controlling step in the ammonia synthesis process. Could you explain it in more detail? Thank you.

Reaction mechanism of ammonia synthesis: Thermodynamic calculations show that low temperature and high pressure are favorable for the ammonia synthesis reaction; however, in the absence of a catalyst, the activation energy for this reaction is very high, resulting in almost no reaction occurring. When an iron catalyst is used, the reaction pathway is altered, reducing the activation energy and enabling the reaction to proceed at a significant rate. It is currently believed that a possible mechanism for the ammonia synthesis reaction is first the chemical adsorption of nitrogen molecules on the surface of the iron catalyst, which weakens the chemical bonds between nitrogen atoms. Next, the chemically adsorbed hydrogen atoms continuously interact with the nitrogen molecules on the surface, gradually forming –NH, –NH2, and NH3 on the catalyst surface; finally, the ammonia molecules desorb from the surface to form gaseous ammonia. The aforementioned reaction pathways can be simply expressed as follows:
xFe + N2 → FexN (ammonia synthesis)
FexN + [H] → FexNH
FexNH + [H] → FexNH2
FexNH2 + [H] → FexNH3
xFe + NH3. In the absence of a catalyst, the activation energy for the ammonia synthesis reaction is quite high, at around 335 kJ/mol. With the addition of an iron catalyst, the reaction proceeds in two stages: the formation of nitrides and hydronitrides. The activation energy for the reaction in the first stage is 126 kJ/mol to 167 kJ/mol, while the activation energy for the reaction in the second stage is 13 kJ/mol. Due to the change in the reaction pathway (formation of unstable intermediate compounds), the activation energy of the reaction is reduced, thereby increasing the reaction rate.

It’s very detailed, thank you to the friend above

Well, I suggest the original poster take a look at a book titled \"Questions and Answers on the Operation of Small and Medium-Sized Ammonia Synthesis Plants\", hehe

Reply to 1# Chemical Engineering 0855: It can be seen from the reaction rate of ammonia synthesis that, in a non-equilibrium state, appropriately increasing the nitrogen partial pressure is beneficial for the rate at which the catalyst adsorbs nitrogen, as nitrogen adsorption is the controlling step in the ammonia synthesis process. Under normal production conditions, the ammonia yield can only reach 50%-70% of the equilibrium value; therefore, it is appropriate to increase the proportion of nitrogen in production, with the hydrogen-to-nitrogen ratio in the recycle gas generally controlled between 2.2 and 2.8.

The optimal ratio for the ammonia synthesis reaction is 3:1, and current large-scale plants all operate at this ratio. The ratio of 2.2-2.8 is one used only in small and medium-sized towers; this ratio is more suitable for the operation of such towers. In this context, the hydrogen-to-nitrogen ratio must take into account not only production capacity but also operational factors. So everyone should not generalize from a few examples