Why is the hazard greater when the hydrogen-to-nitrogen ratio is high compared to when it is low?

Since the stoichiometric ratio of hydrogen is 3, the negative impact of an excess of hydrogen is three times that of an excess of nitrogen; therefore, a high hydrogen-to-nitrogen ratio is more harmful than a low one!

1. From the perspective of the reaction mechanism, the active adsorption of nitrogen is an important controlling factor in the ammonia synthesis reaction; therefore, maintaining a slightly higher nitrogen content is beneficial for this reaction. Generally, the hydrogen-to-nitrogen ratio should be kept below 2.8. 2. Based on the reaction equation, hydrogen and nitrogen are used in a ratio of 3:1. Obviously, when there is an excess of hydrogen, accumulation occurs more rapidly, posing greater harm to the system.

1. First, in terms of the mechanism of ammonia synthesis, the adsorption of nitrogen is a key step in this reaction; therefore, increasing the nitrogen content appropriately can accelerate nitrogen adsorption and thus boost the ammonia synthesis reaction. 2. In the ammonia synthesis reaction, hydrogen and nitrogen are combined in a ratio of 3 to 1, and they are added to the recycle gas. Assuming a total ratio of hydrogen to nitrogen of 100%, H2 constitutes 75% while N2 accounts for 25%. When the hydrogen-to-nitrogen ratio is imbalanced, there are two scenarios. (1) When hydrogen is in excess: Suppose H2 increases by 3%; then the composition becomes 78% H2 and 22% N2. According to a 3:1 reaction ratio, 66% of H2 is required to react with 22% nitrogen, leaving 12% of H2, which continues to accumulate in the system. (2) When nitrogen is in excess: Suppose N2 increases by 3%; then the composition becomes 72% H2 and 28% N2. Based on a 3:1 reaction ratio, 24% of nitrogen is needed to react with 72% H2, leaving 4% of nitrogen, which also accumulates in the system. As can be seen from this, when the hydrogen-to-nitrogen ratio is high, the gases that do not participate in the reaction accumulate more rapidly, causing pressure to rise too quickly and leading to severe deterioration in the temperature of the catalyst layer. Last edited by snowdfr on 2009-3-24 12:40

As a supplementary note, high hydrogen levels are more detrimental to the compressor; they increase energy consumption and also raise the risk level.

It can’t be calculated that way either. In terms of the equilibrium of a synthesis reaction, when the concentration of the reactants increases, the chemical equilibrium changes accordingly. It’s just that when the hydrogen-to-nitrogen ratio is off, the equilibrium constant deviates from its optimal value; there is no direct proportional relationship here. In adjusting the hydrogen-to-nitrogen ratio, it is the amount of air supplied for conversion at the front end that plays a decisive role. In practice, neither the conversion rate of methane at the front end nor the amount of air supplied follows a 3:1 hydrogen-to-nitrogen ratio. Therefore, the cumulative effect of an imbalance in the hydrogen-to-nitrogen ratio and the speed at which pressure increases represent one aspect of the potential hazards associated with overpressure. Personally, I think it might be more appropriate to consider their hazards from the perspective of the chemical bond energies of hydrogen and nitrogen

The equilibrium constant is calculated using fugacity at high pressure; hydrogen varies as the cube of its fugacity, while nitrogen varies as the first power of its fugacity.

Harm of a high hydrogen-to-nitrogen ratio to the system: 1. A high hydrogen-to-nitrogen ratio necessarily results in insufficient air supply to the second-stage furnace; as a consequence, the methane content at the outlet of the second-stage furnace is high. Reducing the methane content by 0.1% increases ammonia production by 1.2–1.4%. Therefore, the ammonia production will definitely decrease by 2. Since hydrogen is less dense, a higher hydrogen-to-nitrogen ratio results in an increased rotation speed of the syngas compressor. If this is not detected or adjusted in time, the rotation speed may fluctuate; in severe cases, it could exceed the safe limit. 3. When the hydrogen density reaches a certain level, it will be carried into the CO2 gas phase by the decarburization liquid during the decarburization process, thereby causing damage to the CO2 compressor. 4. A high hydrogen-to-nitrogen ratio can cause a drop in bed temperature and an increase in system pressure

The hydrogen-nitrogen reaction takes place in a ratio of three to one. Suppose 100 parts of gas consist of 75 parts hydrogen and 25 parts nitrogen. If there is a 3% excess of hydrogen, then only 78 + 22 = 88% of it participates in the reaction (only 66% of the hydrogen does so). If there is a 3% excess of nitrogen, then 72 + 24 = 96% of it participates in the reaction. It can be seen from this that an excess of hydrogen is more harmful than an excess of nitrogen.

It should mainly be considered from two aspects: the cumulative balance and the degree of deviation from the optimal hydrogen-nitrogen ratio of 2.8.

Therefore, it is important to control the hydrogen-to-nitrogen ratio reasonably; the optimal range is between 57 and 59

The 4th floor provided a very detailed explanation; therefore, H/N is generally around (55 ± 2)%

This issue indeed needs to be analyzed carefully, as those responsible for adjusting the gas composition often have little understanding of the subsequent processing steps. This is the reality in the case of small-scale nitrogen fertilizer production. Moreover, the production process for such fertilizers is lengthy, there are few online analysis instruments available, there are numerous points where gas can be recovered, and the influence of associated reactions is significant – all of these factors make it difficult to adjust the gas composition. Furthermore, with several systems mixed together, it becomes even more difficult to adjust them. Be sure to pay attention to the dangers of high hydrogen levels!

Is this also related to the original gas type? Would natural gas or city gas result in differences?

The hydrogen-to-nitrogen ratio is determined based on the optimal hydrogen and nitrogen concentrations required for the best reaction with the catalyst being used, as well as the amount of inert gases present. The hydrogen in our cycle is kept at no less than 60%, with a target range of 61–63%. A high hydrogen ratio disrupts the reaction equilibrium more easily than a low one, leading to overpressure in the system. In some small ammonia synthesis plants, incomplete gas purification results in the presence of small amounts of oxides in the gas fed into the reactor; when the hydrogen ratio is high, this can also cause catalyst hydrogen poisoning.