Why are the controls for H2/C and H2/N2 different?
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In methanol synthesis, the H2/C ratio of the fresh gas is controlled between 2.05 and 2.15; in other words, the H2 content is slightly higher than that at equilibrium. In ammonia synthesis, the H2/N2 ratio of the fresh gas (makeup gas) is controlled between 2.6 and 2.7, meaning that the H2 content is slightly lower than that at equilibrium. Why is the H2 content controlled differently?
As I understand it, a slightly higher hydrogen content in methanol synthesis helps to protect the synthesis catalyst and reduces the formation of side reactions； In an ammonia synthesis production system, if the hydrogen content is high, the hydrogen content in the unreacted gas exiting the synthesis tower will be very high, resulting in increased waste through the emission of purge gas.
My understanding is as follows: 1. The ammonia content at the outlet of the ammonia synthesis tower is around 14%, while the methanol content at the outlet of the methanol synthesis tower is around 6–7%; thus, the one-pass conversion rate for ammonia is higher than that for methanol. If the conditions for ammonia synthesis become more severe, for example when the H2/N2 ratio increases, the pressure in the system rises at a faster rate than the pressure in the methanol system. From the perspectives of safety and practical operational requirements, the H2/N2 and H2/C ratios can be determined in this way ； 2. Ammonia synthesis involves few side reactions, while methanol synthesis has many. A higher H2/C ratio than the theoretical value can reduce the occurrence of side reactions and simultaneously promote the forward reaction.
These are two different reaction bases; differing conditions result in different required feed gases.