Synthesis system hydrogen-to-nitrogen ratio
Thread Content
Dear seniors: I have just started working at the synthesis tower, and the issue that gives me the most trouble is the hydrogen-to-nitrogen ratio. The master explained it to me several times, but I still didn’t understand it; I’m too embarrassed to ask again. Our plant’s synthesis process features two systems that operate in parallel with twin towers; the fresh gas coming from the purification stage is already prepared at a 3:1 ratio. To prevent an insufficient hydrogen-to-nitrogen ratio in our synthesis system, we also add some nitrogen to the fresh gas in order to maintain this ratio. I’d like some help figuring out how to calculate the hydrogen-to-nitrogen ratio of 3:1. Sometimes the technician says it should be kept at 2.8, but what determines that value of 2.8? It’s really frustrating. I earnestly ask those with more experience to give me some advice!
3H2 + N2 = 2NH3. 3 moles of hydrogen and 1 mole of nitrogen produce 2 moles of ammonia. The molar ratio in a gas is roughly equal to its volume ratio, hence the ratio of 3:1. However, the pressure in ammonia synthesis is over 20 megapascals. Hydrogen and nitrogen have different compression ratios; for the same volume and under the same compression, the volume of hydrogen after compression is greater than that of nitrogen, so 2.8 is sufficient
You can take a look at the explanation of the reaction mechanism; the reaction rate is controlled by the adsorption process of nitrogen atoms. Therefore, in actual production, the nitrogen concentration is kept slightly higher. These are all conclusions drawn from experiments and practical production.
The main consideration is the stability of the N-N triple bond, which makes it difficult to break apart; by increasing the N2 content, the total number of active N-N bonds is increased
The ratio of fresh gas to recycled gas is generally kept at 3:1; the range for the recycled gas varies and is typically related to the pressure under which the synthesis takes place, the amount of inert gas present in the recycled gas, as well as the volume of gas released