What is the Purification Process of Ammonia Synthesis?

The crude feedstock gas, derived from materials like coal and natural gas, undergoes purification to remove impurities, excluding hydrogen and nitrogen. This involves transformation processes, desulfurization, decarbonization, and gas refining.

1. Carbon Monoxide Conversion:

Raw gas contains CO (approximately 12%~40%) that needs removal for ammonia synthesis, requiring multi-step conversion. High-temperature conversion transforms most CO to CO2 and H2; low-temperature conversion reduces CO content to around 0.3%.

2. Desulfurization and Decarbonization:

Crude raw gas contains sulfur and carbon oxides, detrimental to catalysts. Desulfurization protects catalysts; methods include physical or chemical absorption, like Rectisol and Selexol.

3. Gas Refining:

After CO conversion and CO2 removal, residual CO and CO2 remain. To prevent catalyst poisoning, total CO and CO2 content must be below 10 cm3/m3 (volume fraction). Final gas purification employs methods like deep cryogenic separation (liquid nitrogen wash) and methanation. Deep cryogenic separation, typically combined with air separation and low-temperature methanol wash, achieves hydrogen-nitrogen mixtures containing ≤100 cm3/m3 inert gases. Methanation involves the reaction of CO, CO2, and H2 to form CH4 and H2O, reducing carbon oxides to ≤10 cm3/m3. However, this method consumes H2 and increases CH4 content. Methanation reactions are as follows:

CO + 3H2 → CH4 + H2O (ΔH = -206.2 kJ/mol)

CO2 + 4H2 → CH4 + 2H2O (ΔH = -165.1 kJ/mol)